diff --git a/.github/workflows/deploy.yaml b/.github/workflows/deploy.yaml new file mode 100644 index 000000000..809fcc961 --- /dev/null +++ b/.github/workflows/deploy.yaml @@ -0,0 +1,39 @@ +on: + push: + branches: + - main + - release/* +jobs: + deploy: + runs-on: "macos-15" + permissions: + contents: read + deployments: write + name: Deploy to Cloudflare Pages + steps: + - name: Checkout + uses: actions/checkout@v3 + - name: Setup node.js + uses: actions/setup-node@v4 + with: + node-version: "20" + - name: Install npm packages + run: npm install + - name: Setup Xcode + uses: maxim-lobanov/setup-xcode@v1 + with: + xcode-version: "16.0" + - name: Get swift version + run: swift --version + - name: Build + run: xcrun docc convert swift-6.docc --output-path ./docs --transform-for-static-hosting --experimental-enable-custom-templates + - name: Copy Redirects + run: cp swift-6.docc/_redirects ./docs/_redirects + - name: Publish + uses: cloudflare/pages-action@v1 + with: + apiToken: ${{ secrets.CLOUDFLARE_API_TOKEN }} + accountId: ${{ secrets.CLOUDFLARE_ACCOUNT_ID }} + projectName: swift-programming-lang + directory: ./docs + gitHubToken: ${{ secrets.GITHUB_TOKEN }} diff --git a/.gitignore b/.gitignore index e43b0f988..e20239029 100644 --- a/.gitignore +++ b/.gitignore @@ -1 +1,6 @@ .DS_Store +node_modules +package-lock.json +**/.docc-build +docs + diff --git a/LICENSE b/LICENSE new file mode 100644 index 000000000..261eeb9e9 --- /dev/null +++ b/LICENSE @@ -0,0 +1,201 @@ + Apache License + Version 2.0, January 2004 + http://www.apache.org/licenses/ + + TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION + + 1. 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We also recommend that a + file or class name and description of purpose be included on the + same "printed page" as the copyright notice for easier + identification within third-party archives. + + Copyright [yyyy] [name of copyright owner] + + Licensed under the Apache License, Version 2.0 (the "License"); + you may not use this file except in compliance with the License. + You may obtain a copy of the License at + + http://www.apache.org/licenses/LICENSE-2.0 + + Unless required by applicable law or agreed to in writing, software + distributed under the License is distributed on an "AS IS" BASIS, + WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + See the License for the specific language governing permissions and + limitations under the License. diff --git a/README.md b/README.md index 33ce6d48a..9de3d2a7f 100755 --- a/README.md +++ b/README.md @@ -1,153 +1,193 @@ -《The Swift Programming Language》in Chinese -============================================= +# The Swift Programming Language (Simplified Chinese Version by SwiftGG) -中文版 Apple 官方 Swift 教程《The Swift Programming Language》 +[English Version](#english-version) | [中文版本](#中文版本) -[英文原版在线版](https://docs.swift.org/swift-book/) +## English Version -# 在线阅读 +This repository contains the source for *The Swift Programming Language* +(sometimes abbreviated as TSPL), +which is published on [docs.swift.org][published] +and built using [Swift-DocC][docc]. -使用 GitBook 制作，可以在 [GitBook](https://swiftgg.gitbook.io/swift/) 网站阅读。 +This repository includes the latest version of TSPL, as well as Simplified Chinese translations corresponding to different Swift versions. -# 当前阶段 +## Building -- 更新到 Swift 5.7，2022-06-06 -- 更新到 Swift 5.6，2022-03-14 -- 更新到 Swift 5.5，2021-06-07 -- 更新到 Swift 5.4，2021-04-26 -- 更新到 Swift 5.3，2020-09-16 -- 更新到 Swift 5.2，2020-02-15 -- 更新到 Swift 5.1，2019-11-11 -- 更新到 Swift 5.0，2019-04-05 -- 更新到 Swift 4.5，2019-03-16 -- 更新到 Swift 4.2，2019-01-29 -- 更新到 Swift 4.1，2018-04-12，感谢 [@Mylittleswift](https://github.com/Mylittleswift) -- 更新到 Swift 3.0，2016-09-23 +Clone this repository and run `docc preview swift-6.docc` +in this repository's root directory. +After running DocC, open the link that `docc` outputs +to display a local preview in your browser. -# 贡献力量 +## Current Status -如果想做出贡献的话，你可以： +- Latest English version of The Swift Programming Language. [Latest commit: 11a2b29][11a2b29] +- Corresponding Simplified Chinese translations (in progress and archived): + - Swift 6 beta (currently in translation) + - Swift 5.x and earlier versions (available for reading on [GitBook][legacy-documentations]) -- 参与翻译 -- 帮忙校对，挑错别字、病句等等 -- 提出修改建议 -- 提出术语翻译建议 +## How to Contribute -# 翻译建议 +1. Fork this repository to your account. Claim a translation task labeled as `Swift x translation` in the issues. Create a branch in your forked repository corresponding to the issue, setting the source branch to the current Swift version being translated (e.g., swift-6-beta-translation). -如果你有兴趣参与项目，请仔细阅读说明： +2. Install the `docc` command-line tool by either downloading the toolchain from Swift.org or installing Xcode. -排版格式和流程说明： +> Note: +> +> If you installed DocC by downloading a toolchain from Swift.org, +> `docc` is located in `usr/bin/`, +> relative to the installation path of the toolchain. +> Make sure your shell's `PATH` environment variable +> includes that directory. +> +> If you installed DocC by downloading Xcode, +> run `xcrun docc preview swift-6.docc` instead. -- 翻译排版格式要求参考 SwiftGG [排版指南](https://github.com/SwiftGGTeam/translation/blob/master/SwiftGG%20排版指南.md) -- Pull Request 发起方式参考 SwiftGG [Pull Request 说明](https://github.com/SwiftGGTeam/translation/blob/master/%E7%BF%BB%E8%AF%91%E6%B5%81%E7%A8%8B%E6%A6%82%E8%BF%B0%E5%8F%8APR%E8%AF%B4%E6%98%8E.md#%E5%A6%82%E4%BD%95%E5%8F%91%E8%B5%B7-pull-request) +3. Replace the content of the original Markdown file with your Chinese translation, following the terminology table below and the [SwiftGG style guide][swiftgg-style-guide]. Submit your translation through a Pull Request. Once verified by SwiftGG members, it will be merged into the current translation branch. -原版文档差异比较： +## Contributors -在翻译时可以通过 Calibre 软件对 [document 目录下](https://github.com/SwiftGGTeam/the-swift-programming-language-in-chinese/tree/gh-pages/document) 不同版本的文档进行 diff，检查待更新部分。 +We extend our heartfelt thanks to all our contributors. You can find the [list of contributors here][contributors]. -diff 操作如下： +## 中文版本 -将最新文档加入到 Calibre 中，点击 **Edit Book**，然后在编辑界面选择 **File** -> **Compare to other book** 检查各模块的更新内容，详见 [链接](https://manual.calibre-ebook.com/diff.html)。 +本仓库包含 *The Swift Programming Language* (缩写为 TSPL) 的源代码， +该文档发布在 [docs.swift.org][published] 上， +并使用 [Swift-DocC][docc] 构建。 -其他说明： +本仓库包括 TSPL 的最新版本，以及对应不同 Swift 版本的简体中文翻译。 -- 相关术语请严格按照术语表来翻译，如果有问题可以发 Issue 大家一起讨论 -- 使用 Markdown 进行翻译，文件名必须使用英文 -- 翻译后的文档请放到 source 文件夹下的对应章节中，然后 Pull Request 即可，我们会用 GitBook 编译成网页 -- 有其他任何问题都欢迎发 Issue +## 构建 -# 术语表 +克隆此仓库并在仓库根目录运行 `docc preview swift-6.docc`。 -翻译术语的时候请参考这个对照表： +运行 DocC 后，打开 `docc` 输出的链接，即可在浏览器中显示本地预览。 -| 术语 | 备选翻译 | +## 当前状态 + +- The Swift Programming Language 的最新英文版本。[最新提交: fe0121d][https://github.com/swiftlang/swift-book/commit/fe0121d1f2d86d6139c2b424d45a7889b82ff5e2] +- 对应的简体中文翻译 (进行中和已归档)： + - Swift 6 beta (当前正在翻译) + - Swift 5.x 及更早版本 (可在 [GitBook][legacy-documentations] 上阅读) + +## 如何贡献 + +1. 首先，将此仓库 fork 到您的账户。在 issues 中认领标记为 `Swift x translation` 的翻译任务。在您 fork 的仓库中创建与 issue 对应的分支，将源分支设置为当前正在翻译的 Swift 版本 (例如，swift-6-beta-translation)。 + +2. 通过从 Swift.org 下载 toolchain 或安装 Xcode 来安装 `docc` 命令行工具。 + +> 注意： +> +> 如果您通过从 Swift.org 下载 toolchain 安装了 DocC， +> `docc` 位于 toolchain 安装路径下的 `usr/bin/` 目录中。 +> 确保您的 shell 的 `PATH` 环境变量包含该目录。 +> +> 如果您通过下载 Xcode 安装了 DocC， +> 请运行 `xcrun docc preview swift-6.docc`。 + +3. 按照下面的术语表和 [SwiftGG 排版指南][swiftgg-style-guide] 将原始 Markdown 文件的内容替换为您的中文翻译。通过 Pull Request 提交您的翻译。经 SwiftGG 成员验证后，将合并到当前的翻译分支。 + +## 参与成员 + +我们衷心感谢所有的参与成员。您可以在[这里][contributors]找到参与成员列表。 + +## Terminology Table（术语表） + +| Term | Suggest Transition | | --- | --- | -| result builder | 结果构造器 | -| property wrapper | 属性包装器（[翻译相关讨论](https://github.com/SwiftGGTeam/the-swift-programming-language-in-chinese/issues/982#issuecomment-536244784)） | -| projected value | 被呈现值 | -| wrapped value | 被包装值 | +| alias | 别名 | +| array | 数组 | | argument | 实参 | -| parameter | 形参 | -| variadic parameters| 可变参数 | +| assertion | 断言 | | associated type | 关联类型 | -| range | 区间 | -| type property | 类型属性 | -| unary operator | 一元运算符 | -| binary operator | 二元运算符 | -| ternary operator | 三元运算符 | -| labeled statement | 具名语句 | -| conform protocol | 遵循协议 | +| associated value | 关联值 | +| attribute | 特性或者属性，根据上下文 | +| automatic reference counting | 自动引用计数 | | availability-condition | 可用性条件 | -| fallthrough | 贯穿 | +| base class | 基类 | +| binary operator | 二元运算符 | +| boxed protocol type | 封装协议类型 | | branch statement | 分支语句 | -| control transfer statement | 控制传递语句 | -| type annotation | 类型注解 | -| type identifier | 类型标识符 | -| metatype type | 元类型 | -| protocol composition type | 复合协议类型 | -| associated value | 关联值 | -| raw value | 原始值 | +| Class Hierarchy | 类层次结构 | +| closure | 闭包 | +| collection | 集合 | | computed property | 计算属性 | -| stored property | 存储属性 | -| operator | 运算符 | -| playground | 不翻译 | -| array | 数组 | +| conditional compilation | 条件编译 | +| conform protocol | 遵循协议 | +| control transfer statement | 控制传递语句 | +| convenience initializer | 便利构造器 | +| convention | 约定 | +| decompose | 分解 | +| deinitialization | 析构过程 | +| deinitializer | 析构器 | +| designated initializer | 指定构造器 | | dictionary | 字典 | -| list | 列表 | -| statement | 语句 | +| downcast | 向下转型 | +| enumeration | 枚举 | | expression | 表达式 | -| optional | 可选 | +| extension | 扩展 | +| fallback (value) | 后备值 ?? 回退值 | +| fallthrough | 贯穿 | +| first-class | 一等 | +| function | 函数 | +| generic | 泛型 | +| getter | 不翻译 | | implicitly unwrapped optional | 隐式解包可选值 | -| optional binding | 可选绑定 | -| optional chaining | 可选链 | -| collection | 集合 | -| convention | 约定 | +| inheritance | 继承 | +| initialization | 构造过程 | +| initializer | 构造器 | | iterate | 迭代 | +| labeled statement | 具名语句 | +| list | 列表 | +| literal (value) | 字面量 | +| metatype type | 元类型 | +| method | 方法 | | nest | 嵌套 | -| inheritance | 继承 | +| nil-coalescing | 不译 | +| note | 注意 | +| opaque type | 不透明类型 | +| operator | 运算符 | +| optional | 可选 | +| optional binding | 可选绑定 | +| optional chaining | 可选链 | | override | 重写 | -| base class | 基类 | -| designated initializer | 指定构造器 | -| convenience initializer | 便利构造器 | -| automatic reference counting | 自动引用计数 | +| parameter | 形参 | +| playground | 不翻译 | +| projected value | 被呈现值 | +| property | 属性 | +| property wrapper | 属性包装器 | +| protocol | 协议 | +| protocol composition type | 复合协议类型 | +| result builder | 结果构造器 | +| range | 区间 | +| raw value | 原始值 | +| runtime | 运行时 | +| scope | 作用域 | +| setter | 不翻译 | +| statement | 语句 | +| stored property | 存储属性 | +| string interpolation | 字符串插值 | +| structure | 结构体 | +| subscript | 下标 | +| ternary operator | 三元运算符 | +| tuple | 元组 | +| type alias | 类型别名 | +| type annotation | 类型注解 | +| type identifier | 类型标识符 | +| type property | 类型属性 | | type inference | 类型推断 | | type casting | 类型转换 | +| unary operator | 一元运算符 | | unwrapped | 解包 | +| variadic parameters| 可变参数 | | wrapped | 包装 | -| note | 注意 | -| closure | 闭包 | -| tuple | 元组 | -| first-class | 一等 | -| deinitializer | 析构器 | -| initializer | 构造器 | -| initialization | 构造过程 | -| deinitialization | 析构过程 | -| getter | 不翻译 | -| setter | 不翻译 | -| subscript | 下标 | -| property | 属性 | -| attribute | 特性或者属性，根据上下文 | -| method | 方法 | -| enumeration | 枚举 | -| structure | 结构体 | -| protocol | 协议 | -| extension | 扩展 | -| generic | 泛型 | -| literal value | 字面量 | -| alias | 别名 | -| assertion | 断言 | -| conditional compilation | 条件编译 | -| opaque type | 不透明类型 | -| function | 函数 | -| runtime | 运行时 | - -# 贡献者 - -[贡献者列表](https://github.com/SwiftGGTeam/the-swift-programming-language-in-chinese/blob/gh-pages/source/contributors.md)，感谢大家！ - - - -# 协议 -和 [苹果官方文档](https://swift.org/documentation/) 协议一致：[Creative Commons Attribution 4.0 International (CC BY 4.0) License](https://creativecommons.org/licenses/by/4.0/)。 +| wrapped value | 被包装值 | +| superclass | 父类 | +| subclass | 子类 | + +[published]: https://docs.swift.org/swift-book/documentation/the-swift-programming-language/ +[docc]: https://github.com/apple/swift-docc +[11a2b29]: https://github.com/swiftlang/swift-book/commit/11a2b29983e9401c179d6269c9becc1256b11bc6 +[legacy-documentations]: https://swiftgg.gitbook.io/swift/ +[swiftgg-style-guide]: https://github.com/SwiftGGTeam/translation/blob/master/SwiftGG%20排版指南.md +[contributors]: https://github.com/SwiftGGTeam/the-swift-programming-language-in-chinese/blob/gh-pages/source/contributors.md diff --git a/TSPL.docc/Assets/CollectionTypes_intro_2x.png b/TSPL.docc/Assets/CollectionTypes_intro@2x.png similarity index 100% rename from TSPL.docc/Assets/CollectionTypes_intro_2x.png rename to TSPL.docc/Assets/CollectionTypes_intro@2x.png diff --git a/TSPL.docc/GuidedTour/AboutSwift.md b/TSPL.docc/GuidedTour/AboutSwift.md index 424548159..ab4c7ac02 100644 --- a/TSPL.docc/GuidedTour/AboutSwift.md +++ b/TSPL.docc/GuidedTour/AboutSwift.md @@ -45,12 +45,6 @@ Swift continues to evolve with thoughtful new features and powerful capabilities The goals for Swift are ambitious. We can’t wait to see what you create with it. -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - @@ -742,12 +747,6 @@ it doesn't allow the access. on performance and memory usage. --> -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - The first argument to this attribute -indicates the macros role: +indicates the macro's role: - term Peer macros: Write `peer` as the first argument to this attribute. @@ -1593,7 +1593,7 @@ s.$x.wrapper // WrapperWithProjection value ### resultBuilder -Apply this attribute to a class, structure, enumeration +Apply this attribute to a class, structure, or enumeration to use that type as a result builder. A *result builder* is a type that builds a nested data structure step by step. @@ -2625,12 +2625,6 @@ see . > *balanced-token* → Any identifier, keyword, literal, or operator \ > *balanced-token* → Any punctuation except **`(`**, **`)`**, **`[`**, **`]`**, **`{`**, or **`}`** -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - `; + + // 检查文件是否已经包含了注释 + const commentRegex = //; + if (commentRegex.test(content)) { + // 更新已存在的注释 + content = content.replace(commentRegex, newComment); + console.log(`Updated existing comment: ${filePath}`); + } else { + // 添加新的注释 + content = `${newComment}\n\n${content}`; + console.log(`Added new comment: ${filePath}`); + } + + fs.writeFileSync(filePath, content); + } else { + console.log(`No matching URL found for: ${filePath}`); + } +} + +// 开始处理 +traverseDirectory(rootDir); diff --git a/package.json b/package.json new file mode 100644 index 000000000..bb6dc3cf8 --- /dev/null +++ b/package.json @@ -0,0 +1,18 @@ +{ + "name": "the-swift-programming-language-in-chinese", + "version": "1.0.0", + "description": "《The Swift Programming Language》in Chinese =============================================", + "main": "add_issue.js", + "scripts": { + "test": "echo \"Error: no test specified\" && exit 1" + }, + "keywords": [], + "author": "", + "license": "ISC", + "dependencies": { + "axios": "^1.7.3" + }, + "devDependencies": { + "wrangler": "^3.91.0" + } +} diff --git a/swift-6-beta.docc/GuidedTour/AboutSwift.md b/swift-6-beta.docc/GuidedTour/AboutSwift.md deleted file mode 100644 index 424548159..000000000 --- a/swift-6-beta.docc/GuidedTour/AboutSwift.md +++ /dev/null @@ -1,62 +0,0 @@ -# About Swift - -Understand the high-level goals of the language. - -Swift is a fantastic way to write software -for phones, tablets, desktops, servers, -or anything else that runs code. -It's a safe and fast programming language -that combines the best in modern language thinking -with wisdom from a diverse open source community. - -Swift is friendly to new programmers, -without sacrificing the power and flexibility -that experienced programmers need. -It's an industrial-quality programming language -that's as expressive and enjoyable as a scripting language. -The compiler is optimized for performance -and the language is optimized for development, -without compromising on either. - -Swift defines away large classes of common programming errors -by adopting modern programming patterns: - -- Variables are always initialized before use. -- Array indices are checked for out-of-bounds errors. -- Integers are checked for overflow. -- Optionals ensure that `nil` values are handled explicitly. -- Memory is managed automatically. -- Error handling allows controlled recovery from unexpected failures. - -Swift code is compiled and optimized to get the most out of modern hardware. -The syntax and standard library have been designed -based on the guiding principle that -the obvious way to write your code should also perform the best. -Its combination of safety and speed make Swift an excellent choice for -everything from "Hello, world!" to an entire operating system. - -Swift combines a modern, lightweight syntax -that's familiar for developers coming from other popular languages -with powerful features like type inference and pattern matching, -allowing complex ideas to be expressed in a clear and concise manner. -As a result, code is easier to read, write, and maintain. - -Swift continues to evolve with thoughtful new features and powerful capabilities. -The goals for Swift are ambitious. -We can’t wait to see what you create with it. - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/GuidedTour/Compatibility.md b/swift-6-beta.docc/GuidedTour/Compatibility.md deleted file mode 100644 index 434d2c758..000000000 --- a/swift-6-beta.docc/GuidedTour/Compatibility.md +++ /dev/null @@ -1,60 +0,0 @@ -# Version Compatibility - -Learn what functionality is available in older language modes. - -This book describes Swift 6, -the default version of Swift that's included in Xcode 16. -You can use the Swift 6 compiler to build code -that's written in Swift 6, Swift 5, Swift 4.2, or Swift 4. - -When you use the Swift 6 compiler -to build code that uses the Swift 5 language mode, -you can use the new features from Swift 6 --- -they're enabled either by default or by an upcoming feature flag. -However, to enable strict concurrency checking, -you need to upgrade to the Swift 6 language mode. - -In addition, -when you use Xcode 15.3 to build Swift 4 and Swift 4.2 code, -most Swift 5 functionality is still available. -That said, -the following changes are available only to code -that uses the Swift 5 language mode: - -- Functions that return an opaque type require the Swift 5.1 runtime. -- The `try?` expression doesn't introduce an extra level of optionality - to expressions that already return optionals. -- Large integer literal initialization expressions are inferred - to be of the correct integer type. - For example, `UInt64(0xffff_ffff_ffff_ffff)` evaluates to the correct value - rather than overflowing. - -Concurrency requires the Swift 5 language mode -and a version of the Swift standard library -that provides the corresponding concurrency types. -On Apple platforms, set a deployment target -of at least iOS 13, macOS 10.15, tvOS 13, watchOS 6, or visionOS 1. - -A target written in Swift 6 can depend on -a target that's written in Swift 5, Swift 4.2 or Swift 4, -and vice versa. -This means, if you have a large project -that's divided into multiple frameworks, -you can migrate your code to a newer language version -one framework at a time. - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/LanguageGuide/AdvancedOperators.md b/swift-6-beta.docc/LanguageGuide/AdvancedOperators.md deleted file mode 100644 index 8486199b0..000000000 --- a/swift-6-beta.docc/LanguageGuide/AdvancedOperators.md +++ /dev/null @@ -1,1578 +0,0 @@ -# Advanced Operators - -Define custom operators, perform bitwise operations, and use builder syntax. - -In addition to the operators described in , -Swift provides several advanced operators that perform more complex value manipulation. -These include all of the bitwise and bit shifting operators you will be familiar with -from C and Objective-C. - -Unlike arithmetic operators in C, -arithmetic operators in Swift don't overflow by default. -Overflow behavior is trapped and reported as an error. -To opt in to overflow behavior, -use Swift's second set of arithmetic operators that overflow by default, -such as the overflow addition operator (`&+`). -All of these overflow operators begin with an ampersand (`&`). - -When you define your own structures, classes, and enumerations, -it can be useful to provide your own implementations of -the standard Swift operators for these custom types. -Swift makes it easy to provide tailored implementations of these operators -and to determine exactly what their behavior should be for each type you create. - -You're not limited to the predefined operators. -Swift gives you the freedom to define your own custom -infix, prefix, postfix, and assignment operators, -with custom precedence and associativity values. -These operators can be used and adopted in your code like any of the predefined operators, -and you can even extend existing types to support the custom operators you define. - -## Bitwise Operators - -*Bitwise operators* enable you to manipulate -the individual raw data bits within a data structure. -They're often used in low-level programming, -such as graphics programming and device driver creation. -Bitwise operators can also be useful when you work with raw data from external sources, -such as encoding and decoding data for communication over a custom protocol. - -Swift supports all of the bitwise operators found in C, as described below. - -### Bitwise NOT Operator - -The *bitwise NOT operator* (`~`) inverts all bits in a number: - -![](bitwiseNOT) - -The bitwise NOT operator is a prefix operator, -and appears immediately before the value it operates on, -without any white space: - -```swift -let initialBits: UInt8 = 0b00001111 -let invertedBits = ~initialBits // equals 11110000 -``` - - - -`UInt8` integers have eight bits -and can store any value between `0` and `255`. -This example initializes a `UInt8` integer with the binary value `00001111`, -which has its first four bits set to `0`, -and its second four bits set to `1`. -This is equivalent to a decimal value of `15`. - - - -The bitwise NOT operator is then used to create a new constant called `invertedBits`, -which is equal to `initialBits`, -but with all of the bits inverted. -Zeros become ones, and ones become zeros. -The value of `invertedBits` is `11110000`, -which is equal to an unsigned decimal value of `240`. - -### Bitwise AND Operator - -The *bitwise AND operator* (`&`) combines the bits of two numbers. -It returns a new number whose bits are set to `1` -only if the bits were equal to `1` in *both* input numbers: - -![](bitwiseAND) - -In the example below, -the values of `firstSixBits` and `lastSixBits` -both have four middle bits equal to `1`. -The bitwise AND operator combines them to make the number `00111100`, -which is equal to an unsigned decimal value of `60`: - -```swift -let firstSixBits: UInt8 = 0b11111100 -let lastSixBits: UInt8 = 0b00111111 -let middleFourBits = firstSixBits & lastSixBits // equals 00111100 -``` - - - -### Bitwise OR Operator - -The *bitwise OR operator* (`|`) compares the bits of two numbers. -The operator returns a new number whose bits are set to `1` -if the bits are equal to `1` in *either* input number: - -![](bitwiseOR) - - - -In the example below, -the values of `someBits` and `moreBits` have different bits set to `1`. -The bitwise OR operator combines them to make the number `11111110`, -which equals an unsigned decimal of `254`: - -```swift -let someBits: UInt8 = 0b10110010 -let moreBits: UInt8 = 0b01011110 -let combinedbits = someBits | moreBits // equals 11111110 -``` - - - -### Bitwise XOR Operator - -The *bitwise XOR operator*, or “exclusive OR operator” (`^`), -compares the bits of two numbers. -The operator returns a new number whose bits are set to `1` -where the input bits are different -and are set to `0` where the input bits are the same: - -![](bitwiseXOR) - -In the example below, -the values of `firstBits` and `otherBits` each have a bit set to `1` -in a location that the other does not. -The bitwise XOR operator sets both of these bits to `1` in its output value. -All of the other bits in `firstBits` and `otherBits` match -and are set to `0` in the output value: - -```swift -let firstBits: UInt8 = 0b00010100 -let otherBits: UInt8 = 0b00000101 -let outputBits = firstBits ^ otherBits // equals 00010001 -``` - - - -### Bitwise Left and Right Shift Operators - -The *bitwise left shift operator* (`<<`) -and *bitwise right shift operator* (`>>`) -move all bits in a number to the left or the right by a certain number of places, -according to the rules defined below. - -Bitwise left and right shifts have the effect of -multiplying or dividing an integer by a factor of two. -Shifting an integer's bits to the left by one position doubles its value, -whereas shifting it to the right by one position halves its value. - - - -#### Shifting Behavior for Unsigned Integers - -The bit-shifting behavior for unsigned integers is as follows: - -1. Existing bits are moved to the left or right by the requested number of places. -2. Any bits that are moved beyond the bounds of the integer's storage are discarded. -3. Zeros are inserted in the spaces left behind - after the original bits are moved to the left or right. - -This approach is known as a *logical shift*. - -The illustration below shows the results of `11111111 << 1` -(which is `11111111` shifted to the left by `1` place), -and `11111111 >> 1` -(which is `11111111` shifted to the right by `1` place). -Green numbers are shifted, -gray numbers are discarded, -and pink zeros are inserted: - -![](bitshiftUnsigned) - -Here's how bit shifting looks in Swift code: - -```swift -let shiftBits: UInt8 = 4 // 00000100 in binary -shiftBits << 1 // 00001000 -shiftBits << 2 // 00010000 -shiftBits << 5 // 10000000 -shiftBits << 6 // 00000000 -shiftBits >> 2 // 00000001 -``` - - - - - -You can use bit shifting to encode and decode values within other data types: - -```swift -let pink: UInt32 = 0xCC6699 -let redComponent = (pink & 0xFF0000) >> 16 // redComponent is 0xCC, or 204 -let greenComponent = (pink & 0x00FF00) >> 8 // greenComponent is 0x66, or 102 -let blueComponent = pink & 0x0000FF // blueComponent is 0x99, or 153 -``` - - - -This example uses a `UInt32` constant called `pink` to store a -Cascading Style Sheets color value for the color pink. -The CSS color value `#CC6699` is written as -`0xCC6699` in Swift's hexadecimal number representation. -This color is then decomposed into its -red (`CC`), green (`66`), and blue (`99`) components -by the bitwise AND operator (`&`) and the bitwise right shift operator (`>>`). - -The red component is obtained by performing a bitwise AND -between the numbers `0xCC6699` and `0xFF0000`. -The zeros in `0xFF0000` effectively “mask” the second and third bytes of `0xCC6699`, -causing the `6699` to be ignored and leaving `0xCC0000` as the result. - -This number is then shifted 16 places to the right (`>> 16`). -Each pair of characters in a hexadecimal number uses 8 bits, -so a move 16 places to the right will convert `0xCC0000` into `0x0000CC`. -This is the same as `0xCC`, which has a decimal value of `204`. - -Similarly, the green component is obtained by performing a bitwise AND -between the numbers `0xCC6699` and `0x00FF00`, -which gives an output value of `0x006600`. -This output value is then shifted eight places to the right, -giving a value of `0x66`, which has a decimal value of `102`. - -Finally, the blue component is obtained by performing a bitwise AND -between the numbers `0xCC6699` and `0x0000FF`, -which gives an output value of `0x000099`. -Because `0x000099` already equals `0x99`, -which has a decimal value of `153`, -this value is used without shifting it to the right, - -#### Shifting Behavior for Signed Integers - -The shifting behavior is more complex for signed integers than for unsigned integers, -because of the way signed integers are represented in binary. -(The examples below are based on 8-bit signed integers for simplicity, -but the same principles apply for signed integers of any size.) - -Signed integers use their first bit (known as the *sign bit*) -to indicate whether the integer is positive or negative. -A sign bit of `0` means positive, and a sign bit of `1` means negative. - -The remaining bits (known as the *value bits*) store the actual value. -Positive numbers are stored in exactly the same way as for unsigned integers, -counting upwards from `0`. -Here's how the bits inside an `Int8` look for the number `4`: - -![](bitshiftSignedFour) - -The sign bit is `0` (meaning “positive”), -and the seven value bits are just the number `4`, -written in binary notation. - -Negative numbers, however, are stored differently. -They're stored by subtracting their absolute value from `2` to the power of `n`, -where `n` is the number of value bits. -An eight-bit number has seven value bits, -so this means `2` to the power of `7`, or `128`. - -Here's how the bits inside an `Int8` look for the number `-4`: - -![](bitshiftSignedMinusFour) - -This time, the sign bit is `1` (meaning “negative”), -and the seven value bits have a binary value of `124` (which is `128 - 4`): - -![](bitshiftSignedMinusFourValue) - -This encoding for negative numbers is known as a *two's complement* representation. -It may seem an unusual way to represent negative numbers, -but it has several advantages. - -First, you can add `-1` to `-4`, -simply by performing a standard binary addition of all eight bits -(including the sign bit), -and discarding anything that doesn't fit in the eight bits once you're done: - -![](bitshiftSignedAddition) - -Second, the two's complement representation also lets you -shift the bits of negative numbers to the left and right like positive numbers, -and still end up doubling them for every shift you make to the left, -or halving them for every shift you make to the right. -To achieve this, an extra rule is used when signed integers are shifted to the right: -When you shift signed integers to the right, -apply the same rules as for unsigned integers, -but fill any empty bits on the left with the *sign bit*, -rather than with a zero. - -![](bitshiftSigned) - -This action ensures that signed integers have the same sign after they're shifted to the right, -and is known as an *arithmetic shift*. - -Because of the special way that positive and negative numbers are stored, -shifting either of them to the right moves them closer to zero. -Keeping the sign bit the same during this shift means that -negative integers remain negative as their value moves closer to zero. - -## Overflow Operators - -If you try to insert a number into an integer constant or variable -that can't hold that value, -by default Swift reports an error rather than allowing an invalid value to be created. -This behavior gives extra safety when you work with numbers that are too large or too small. - -For example, the `Int16` integer type can hold -any signed integer between `-32768` and `32767`. -Trying to set an `Int16` constant or variable to a number outside of this range -causes an error: - -```swift -var potentialOverflow = Int16.max -// potentialOverflow equals 32767, which is the maximum value an Int16 can hold -potentialOverflow += 1 -// this causes an error -``` - - - -Providing error handling when values get too large or too small -gives you much more flexibility when coding for boundary value conditions. - -However, when you specifically want an overflow condition -to truncate the number of available bits, -you can opt in to this behavior rather than triggering an error. -Swift provides three arithmetic *overflow operators* that opt in to -the overflow behavior for integer calculations. -These operators all begin with an ampersand (`&`): - -- Overflow addition (`&+`) -- Overflow subtraction (`&-`) -- Overflow multiplication (`&*`) - -### Value Overflow - -Numbers can overflow in both the positive and negative direction. - -Here's an example of what happens when -an unsigned integer is allowed to overflow in the positive direction, -using the overflow addition operator (`&+`): - -```swift -var unsignedOverflow = UInt8.max -// unsignedOverflow equals 255, which is the maximum value a UInt8 can hold -unsignedOverflow = unsignedOverflow &+ 1 -// unsignedOverflow is now equal to 0 -``` - - - -The variable `unsignedOverflow` is initialized with the maximum value a `UInt8` can hold -(`255`, or `11111111` in binary). -It's then incremented by `1` using the overflow addition operator (`&+`). -This pushes its binary representation just over the size that a `UInt8` can hold, -causing it to overflow beyond its bounds, -as shown in the diagram below. -The value that remains within the bounds of the `UInt8` -after the overflow addition is `00000000`, or zero. - -![](overflowAddition) - -Something similar happens when -an unsigned integer is allowed to overflow in the negative direction. -Here's an example using the overflow subtraction operator (`&-`): - -```swift -var unsignedOverflow = UInt8.min -// unsignedOverflow equals 0, which is the minimum value a UInt8 can hold -unsignedOverflow = unsignedOverflow &- 1 -// unsignedOverflow is now equal to 255 -``` - - - -The minimum value that a `UInt8` can hold is zero, -or `00000000` in binary. -If you subtract `1` from `00000000` using the overflow subtraction operator (`&-`), -the number will overflow and wrap around to `11111111`, -or `255` in decimal. - -![](overflowUnsignedSubtraction) - -Overflow also occurs for signed integers. -All addition and subtraction for signed integers is performed in bitwise fashion, -with the sign bit included as part of the numbers being added or subtracted, -as described in . - -```swift -var signedOverflow = Int8.min -// signedOverflow equals -128, which is the minimum value an Int8 can hold -signedOverflow = signedOverflow &- 1 -// signedOverflow is now equal to 127 -``` - - - -The minimum value that an `Int8` can hold is `-128`, -or `10000000` in binary. -Subtracting `1` from this binary number with the overflow operator -gives a binary value of `01111111`, -which toggles the sign bit and gives positive `127`, -the maximum positive value that an `Int8` can hold. - -![](overflowSignedSubtraction) - -For both signed and unsigned integers, -overflow in the positive direction -wraps around from the maximum valid integer value back to the minimum, -and overflow in the negative direction -wraps around from the minimum value to the maximum. - -## Precedence and Associativity - -Operator *precedence* gives some operators higher priority than others; -these operators are applied first. - -Operator *associativity* defines how operators of the same precedence -are grouped together --- -either grouped from the left, or grouped from the right. -Think of it as meaning “they associate with the expression to their left,” -or “they associate with the expression to their right.” - -It's important to consider -each operator's precedence and associativity -when working out the order in which a compound expression will be calculated. -For example, -operator precedence explains why the following expression equals `17`. - -```swift -2 + 3 % 4 * 5 -// this equals 17 -``` - - - - - -If you read strictly from left to right, -you might expect the expression to be calculated as follows: - -- `2` plus `3` equals `5` -- `5` remainder `4` equals `1` -- `1` times `5` equals `5` - -However, the actual answer is `17`, not `5`. -Higher-precedence operators are evaluated before lower-precedence ones. -In Swift, as in C, -the remainder operator (`%`) and the multiplication operator (`*`) -have a higher precedence than the addition operator (`+`). -As a result, they're both evaluated before the addition is considered. - -However, remainder and multiplication have the *same* precedence as each other. -To work out the exact evaluation order to use, -you also need to consider their associativity. -Remainder and multiplication both associate with the expression to their left. -Think of this as adding implicit parentheses around these parts of the expression, -starting from their left: - -```swift -2 + ((3 % 4) * 5) -``` - - - - - -`(3 % 4)` is `3`, so this is equivalent to: - -```swift -2 + (3 * 5) -``` - - - - - -`(3 * 5)` is `15`, so this is equivalent to: - -```swift -2 + 15 -``` - - - - - -This calculation yields the final answer of `17`. - -For information about the operators provided by the Swift standard library, -including a complete list of the operator precedence groups and associativity settings, -see [Operator Declarations](https://developer.apple.com/documentation/swift/operator_declarations). - -> Note: Swift's operator precedences and associativity rules are simpler and more predictable -> than those found in C and Objective-C. -> However, this means that they aren't exactly the same as in C-based languages. -> Be careful to ensure that operator interactions still behave in the way you intend -> when porting existing code to Swift. - -## Operator Methods - -Classes and structures can provide their own implementations of existing operators. -This is known as *overloading* the existing operators. - -The example below shows how to implement -the arithmetic addition operator (`+`) for a custom structure. -The arithmetic addition operator is a binary operator -because it operates on two targets -and it's an infix operator because it appears between those two targets. - -The example defines a `Vector2D` structure for -a two-dimensional position vector `(x, y)`, -followed by a definition of an *operator method* -to add together instances of the `Vector2D` structure: - -```swift -struct Vector2D { - var x = 0.0, y = 0.0 -} - -extension Vector2D { - static func + (left: Vector2D, right: Vector2D) -> Vector2D { - return Vector2D(x: left.x + right.x, y: left.y + right.y) - } -} -``` - - - -The operator method is defined as a type method on `Vector2D`, -with a method name that matches the operator to be overloaded (`+`). -Because addition isn't part of the essential behavior for a vector, -the type method is defined in an extension of `Vector2D` -rather than in the main structure declaration of `Vector2D`. -Because the arithmetic addition operator is a binary operator, -this operator method takes two input parameters of type `Vector2D` -and returns a single output value, also of type `Vector2D`. - -In this implementation, the input parameters are named `left` and `right` -to represent the `Vector2D` instances that will be on -the left side and right side of the `+` operator. -The method returns a new `Vector2D` instance, -whose `x` and `y` properties are -initialized with the sum of the `x` and `y` properties from -the two `Vector2D` instances that are added together. - -The type method -can be used as an infix operator between existing `Vector2D` instances: - -```swift -let vector = Vector2D(x: 3.0, y: 1.0) -let anotherVector = Vector2D(x: 2.0, y: 4.0) -let combinedVector = vector + anotherVector -// combinedVector is a Vector2D instance with values of (5.0, 5.0) -``` - - - -This example adds together the vectors `(3.0, 1.0)` and `(2.0, 4.0)` -to make the vector `(5.0, 5.0)`, as illustrated below. - -![](vectorAddition) - -### Prefix and Postfix Operators - -The example shown above demonstrates a custom implementation of a binary infix operator. -Classes and structures can also provide implementations -of the standard *unary operators*. -Unary operators operate on a single target. -They're *prefix* if they precede their target (such as `-a`) -and *postfix* operators if they follow their target (such as `b!`). - -You implement a prefix or postfix unary operator by writing -the `prefix` or `postfix` modifier -before the `func` keyword when declaring the operator method: - -```swift -extension Vector2D { - static prefix func - (vector: Vector2D) -> Vector2D { - return Vector2D(x: -vector.x, y: -vector.y) - } -} -``` - - - -The example above implements the unary minus operator -(`-a`) for `Vector2D` instances. -The unary minus operator is a prefix operator, -and so this method has to be qualified with the `prefix` modifier. - -For simple numeric values, the unary minus operator converts -positive numbers into their negative equivalent and vice versa. -The corresponding implementation for `Vector2D` instances -performs this operation on both the `x` and `y` properties: - -```swift -let positive = Vector2D(x: 3.0, y: 4.0) -let negative = -positive -// negative is a Vector2D instance with values of (-3.0, -4.0) -let alsoPositive = -negative -// alsoPositive is a Vector2D instance with values of (3.0, 4.0) -``` - - - -### Compound Assignment Operators - -*Compound assignment operators* combine assignment (`=`) with another operation. -For example, the addition assignment operator (`+=`) -combines addition and assignment into a single operation. -You mark a compound assignment operator's left input parameter type as `inout`, -because the parameter's value will be modified directly from within the operator method. - -The example below implements -an addition assignment operator method for `Vector2D` instances: - -```swift -extension Vector2D { - static func += (left: inout Vector2D, right: Vector2D) { - left = left + right - } -} -``` - - - -Because an addition operator was defined earlier, -you don't need to reimplement the addition process here. -Instead, the addition assignment operator method -takes advantage of the existing addition operator method, -and uses it to set the left value to be the left value plus the right value: - -```swift -var original = Vector2D(x: 1.0, y: 2.0) -let vectorToAdd = Vector2D(x: 3.0, y: 4.0) -original += vectorToAdd -// original now has values of (4.0, 6.0) -``` - - - -> Note: It isn't possible to overload the default -> assignment operator (`=`). -> Only the compound assignment operators can be overloaded. -> Similarly, the ternary conditional operator -> (`a ? b : c`) can't be overloaded. - - - -### Equivalence Operators - -By default, custom classes and structures don't have an implementation of -the *equivalence operators*, -known as the *equal to* operator (`==`) and *not equal to* operator (`!=`). -You usually implement the `==` operator, -and use the Swift standard library's default implementation of the `!=` operator -that negates the result of the `==` operator. -There are two ways to implement the `==` operator: -You can implement it yourself, -or for many types, you can ask Swift to synthesize -an implementation for you. -In both cases, -you add conformance to the Swift standard library's `Equatable` protocol. - -You provide an implementation of the `==` operator -in the same way as you implement other infix operators: - -```swift -extension Vector2D: Equatable { - static func == (left: Vector2D, right: Vector2D) -> Bool { - return (left.x == right.x) && (left.y == right.y) - } -} -``` - - - -The example above implements an `==` operator -to check whether two `Vector2D` instances have equivalent values. -In the context of `Vector2D`, -it makes sense to consider “equal” as meaning -“both instances have the same `x` values and `y` values”, -and so this is the logic used by the operator implementation. - -You can now use this operator to check whether two `Vector2D` instances are equivalent: - -```swift -let twoThree = Vector2D(x: 2.0, y: 3.0) -let anotherTwoThree = Vector2D(x: 2.0, y: 3.0) -if twoThree == anotherTwoThree { - print("These two vectors are equivalent.") -} -// Prints "These two vectors are equivalent." -``` - - - -In many simple cases, you can ask Swift -to provide synthesized implementations of the equivalence operators for you, -as described in . - -## Custom Operators - -You can declare and implement your own *custom operators* in addition to -the standard operators provided by Swift. -For a list of characters that can be used to define custom operators, -see . - -New operators are declared at a global level using the `operator` keyword, -and are marked with the `prefix`, `infix` or `postfix` modifiers: - -```swift -prefix operator +++ -``` - - - -The example above defines a new prefix operator called `+++`. -This operator doesn't have an existing meaning in Swift, -and so it's given its own custom meaning below in the specific context of -working with `Vector2D` instances. For the purposes of this example, -`+++` is treated as a new “prefix doubling” operator. -It doubles the `x` and `y` values of a `Vector2D` instance, -by adding the vector to itself with the addition assignment operator defined earlier. -To implement the `+++` operator, -you add a type method called `+++` to `Vector2D` as follows: - -```swift -extension Vector2D { - static prefix func +++ (vector: inout Vector2D) -> Vector2D { - vector += vector - return vector - } -} - -var toBeDoubled = Vector2D(x: 1.0, y: 4.0) -let afterDoubling = +++toBeDoubled -// toBeDoubled now has values of (2.0, 8.0) -// afterDoubling also has values of (2.0, 8.0) -``` - - - -### Precedence for Custom Infix Operators - -Custom infix operators each belong to a precedence group. -A precedence group specifies an operator's precedence relative -to other infix operators, as well as the operator's associativity. -See for an explanation of -how these characteristics affect an infix operator's interaction -with other infix operators. - -A custom infix operator that isn't explicitly placed into a precedence group is -given a default precedence group with a precedence immediately higher -than the precedence of the ternary conditional operator. - -The following example defines a new custom infix operator called `+-`, -which belongs to the precedence group `AdditionPrecedence`: - -```swift -infix operator +-: AdditionPrecedence -extension Vector2D { - static func +- (left: Vector2D, right: Vector2D) -> Vector2D { - return Vector2D(x: left.x + right.x, y: left.y - right.y) - } -} -let firstVector = Vector2D(x: 1.0, y: 2.0) -let secondVector = Vector2D(x: 3.0, y: 4.0) -let plusMinusVector = firstVector +- secondVector -// plusMinusVector is a Vector2D instance with values of (4.0, -2.0) -``` - - - -This operator adds together the `x` values of two vectors, -and subtracts the `y` value of the second vector from the first. -Because it's in essence an “additive” operator, -it has been given the same precedence group -as additive infix operators such as `+` and `-`. -For information about the operators provided by the Swift standard library, -including a complete list of the operator precedence groups and associativity settings, -see [Operator Declarations](https://developer.apple.com/documentation/swift/operator_declarations). -For more information about precedence groups and to see the syntax for -defining your own operators and precedence groups, -see . - -> Note: You don't specify a precedence when defining a prefix or postfix operator. -> However, if you apply both a prefix and a postfix operator to the same operand, -> the postfix operator is applied first. - - - -## Result Builders - -A *result builder* is a type you define -that adds syntax for creating nested data, -like a list or tree, -in a natural, declarative way. -The code that uses the result builder -can include ordinary Swift syntax, like `if` and `for`, -to handle conditional or repeated pieces of data. - -The code below defines a few types for drawing on a single line -using stars and text. - -```swift -protocol Drawable { - func draw() -> String -} -struct Line: Drawable { - var elements: [Drawable] - func draw() -> String { - return elements.map { $0.draw() }.joined(separator: "") - } -} -struct Text: Drawable { - var content: String - init(_ content: String) { self.content = content } - func draw() -> String { return content } -} -struct Space: Drawable { - func draw() -> String { return " " } -} -struct Stars: Drawable { - var length: Int - func draw() -> String { return String(repeating: "*", count: length) } -} -struct AllCaps: Drawable { - var content: Drawable - func draw() -> String { return content.draw().uppercased() } -} -``` - - - -The `Drawable` protocol defines the requirement -for something that can be drawn, like a line or shape: -The type must implement a `draw()` method. -The `Line` structure represents a single-line drawing, -and it serves the top-level container for most drawings. -To draw a `Line`, -the structure calls `draw()` on each of the line's components, -and then concatenates the resulting strings into a single string. -The `Text` structure wraps a string to make it part of a drawing. -The `AllCaps` structure wraps and modifies another drawing, -converting any text in the drawing to uppercase. - -It's possible to make a drawing with these types -by calling their initializers: - -```swift -let name: String? = "Ravi Patel" -let manualDrawing = Line(elements: [ - Stars(length: 3), - Text("Hello"), - Space(), - AllCaps(content: Text((name ?? "World") + "!")), - Stars(length: 2), -]) -print(manualDrawing.draw()) -// Prints "***Hello RAVI PATEL!**" -``` - - - -This code works, but it's a little awkward. -The deeply nested parentheses after `AllCaps` are hard to read. -The fallback logic to use "World" when `name` is `nil` -has to be done inline using the `??` operator, -which would be difficult with anything more complex. -If you needed to include switches or `for` loops -to build up part of the drawing, there's no way to do that. -A result builder lets you rewrite code like this -so that it looks like normal Swift code. - -To define a result builder, -you write the `@resultBuilder` attribute on a type declaration. -For example, this code defines a result builder called `DrawingBuilder`, -which lets you use a declarative syntax to describe a drawing: - -```swift -@resultBuilder -struct DrawingBuilder { - static func buildBlock(_ components: Drawable...) -> Drawable { - return Line(elements: components) - } - static func buildEither(first: Drawable) -> Drawable { - return first - } - static func buildEither(second: Drawable) -> Drawable { - return second - } -} -``` - - - -The `DrawingBuilder` structure defines three methods -that implement parts of the result builder syntax. -The `buildBlock(_:)` method adds support for -writing a series of lines in a block of code. -It combines the components in that block into a `Line`. -The `buildEither(first:)` and `buildEither(second:)` methods -add support for `if`-`else`. - -You can apply the `@DrawingBuilder` attribute to a function's parameter, -which turns a closure passed to the function -into the value that the result builder creates from that closure. -For example: - -```swift -func draw(@DrawingBuilder content: () -> Drawable) -> Drawable { - return content() -} -func caps(@DrawingBuilder content: () -> Drawable) -> Drawable { - return AllCaps(content: content()) -} - -func makeGreeting(for name: String? = nil) -> Drawable { - let greeting = draw { - Stars(length: 3) - Text("Hello") - Space() - caps { - if let name = name { - Text(name + "!") - } else { - Text("World!") - } - } - Stars(length: 2) - } - return greeting -} -let genericGreeting = makeGreeting() -print(genericGreeting.draw()) -// Prints "***Hello WORLD!**" - -let personalGreeting = makeGreeting(for: "Ravi Patel") -print(personalGreeting.draw()) -// Prints "***Hello RAVI PATEL!**" -``` - - - -The `makeGreeting(for:)` function takes a `name` parameter -and uses it to draw a personalized greeting. -The `draw(_:)` and `caps(_:)` functions -both take a single closure as their argument, -which is marked with the `@DrawingBuilder` attribute. -When you call those functions, -you use the special syntax that `DrawingBuilder` defines. -Swift transforms that declarative description of a drawing -into a series of calls to the methods on `DrawingBuilder` -to build up the value that's passed as the function argument. -For example, -Swift transforms the call to `caps(_:)` in that example -into code like the following: - -```swift -let capsDrawing = caps { - let partialDrawing: Drawable - if let name = name { - let text = Text(name + "!") - partialDrawing = DrawingBuilder.buildEither(first: text) - } else { - let text = Text("World!") - partialDrawing = DrawingBuilder.buildEither(second: text) - } - return partialDrawing -} -``` - - - -Swift transforms the `if`-`else` block into -calls to the `buildEither(first:)` and `buildEither(second:)` methods. -Although you don't call these methods in your own code, -showing the result of the transformation -makes it easier to see how Swift transforms your code -when you use the `DrawingBuilder` syntax. - -To add support for writing `for` loops in the special drawing syntax, -add a `buildArray(_:)` method. - -```swift -extension DrawingBuilder { - static func buildArray(_ components: [Drawable]) -> Drawable { - return Line(elements: components) - } -} -let manyStars = draw { - Text("Stars:") - for length in 1...3 { - Space() - Stars(length: length) - } -} -``` - - - -In the code above, the `for` loop creates an array of drawings, -and the `buildArray(_:)` method turns that array into a `Line`. - -For a complete list of how Swift transforms builder syntax -into calls to the builder type's methods, -see . - - - - - - - - - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/LanguageGuide/AutomaticReferenceCounting.md b/swift-6-beta.docc/LanguageGuide/AutomaticReferenceCounting.md deleted file mode 100644 index e343cfc52..000000000 --- a/swift-6-beta.docc/LanguageGuide/AutomaticReferenceCounting.md +++ /dev/null @@ -1,1529 +0,0 @@ -# Automatic Reference Counting - -Model the lifetime of objects and their relationships. - -Swift uses *Automatic Reference Counting* (ARC) -to track and manage your app's memory usage. -In most cases, this means that memory management “just works” in Swift, -and you don't need to think about memory management yourself. -ARC automatically frees up the memory used by class instances -when those instances are no longer needed. - -However, in a few cases ARC requires more information -about the relationships between parts of your code -in order to manage memory for you. -This chapter describes those situations -and shows how you enable ARC to manage all of your app's memory. -Using ARC in Swift is very similar to the approach described in -[Transitioning to ARC Release Notes](https://developer.apple.com/library/content/releasenotes/ObjectiveC/RN-TransitioningToARC/Introduction/Introduction.html) -for using ARC with Objective-C. - -Reference counting applies only to instances of classes. -Structures and enumerations are value types, not reference types, -and aren't stored and passed by reference. - -## How ARC Works - -Every time you create a new instance of a class, -ARC allocates a chunk of memory to store information about that instance. -This memory holds information about the type of the instance, -together with the values of any stored properties associated with that instance. - -Additionally, when an instance is no longer needed, -ARC frees up the memory used by that instance -so that the memory can be used for other purposes instead. -This ensures that class instances don't take up space in memory -when they're no longer needed. - -However, if ARC were to deallocate an instance that was still in use, -it would no longer be possible to access that instance's properties, -or call that instance's methods. -Indeed, if you tried to access the instance, your app would most likely crash. - -To make sure that instances don't disappear while they're still needed, -ARC tracks how many properties, constants, and variables -are currently referring to each class instance. -ARC will not deallocate an instance -as long as at least one active reference to that instance still exists. - -To make this possible, -whenever you assign a class instance to a property, constant, or variable, -that property, constant, or variable makes a *strong reference* to the instance. -The reference is called a "strong" reference because -it keeps a firm hold on that instance, -and doesn't allow it to be deallocated for as long as that strong reference remains. - -## ARC in Action - -Here's an example of how Automatic Reference Counting works. -This example starts with a simple class called `Person`, -which defines a stored constant property called `name`: - -```swift -class Person { - let name: String - init(name: String) { - self.name = name - print("\(name) is being initialized") - } - deinit { - print("\(name) is being deinitialized") - } -} -``` - - - -The `Person` class has an initializer that sets the instance's `name` property -and prints a message to indicate that initialization is underway. -The `Person` class also has a deinitializer -that prints a message when an instance of the class is deallocated. - -The next code snippet defines three variables of type `Person?`, -which are used to set up multiple references to a new `Person` instance -in subsequent code snippets. -Because these variables are of an optional type (`Person?`, not `Person`), -they're automatically initialized with a value of `nil`, -and don't currently reference a `Person` instance. - -```swift -var reference1: Person? -var reference2: Person? -var reference3: Person? -``` - - - -You can now create a new `Person` instance -and assign it to one of these three variables: - -```swift -reference1 = Person(name: "John Appleseed") -// Prints "John Appleseed is being initialized" -``` - - - -Note that the message `"John Appleseed is being initialized"` is printed -at the point that you call the `Person` class's initializer. -This confirms that initialization has taken place. - -Because the new `Person` instance has been assigned to the `reference1` variable, -there's now a strong reference from `reference1` to the new `Person` instance. -Because there's at least one strong reference, -ARC makes sure that this `Person` is kept in memory and isn't deallocated. - -If you assign the same `Person` instance to two more variables, -two more strong references to that instance are established: - -```swift -reference2 = reference1 -reference3 = reference1 -``` - - - -There are now *three* strong references to this single `Person` instance. - -If you break two of these strong references (including the original reference) -by assigning `nil` to two of the variables, -a single strong reference remains, -and the `Person` instance isn't deallocated: - -```swift -reference1 = nil -reference2 = nil -``` - - - -ARC doesn't deallocate the `Person` instance until -the third and final strong reference is broken, -at which point it's clear that you are no longer using the `Person` instance: - -```swift -reference3 = nil -// Prints "John Appleseed is being deinitialized" -``` - - - -## Strong Reference Cycles Between Class Instances - -In the examples above, -ARC is able to track the number of references to the new `Person` instance you create -and to deallocate that `Person` instance when it's no longer needed. - -However, it's possible to write code in which an instance of a class -*never* gets to a point where it has zero strong references. -This can happen if two class instances hold a strong reference to each other, -such that each instance keeps the other alive. -This is known as a *strong reference cycle*. - -You resolve strong reference cycles -by defining some of the relationships between classes -as weak or unowned references instead of as strong references. -This process is described in -. -However, before you learn how to resolve a strong reference cycle, -it's useful to understand how such a cycle is caused. - -Here's an example of how a strong reference cycle can be created by accident. -This example defines two classes called `Person` and `Apartment`, -which model a block of apartments and its residents: - -```swift -class Person { - let name: String - init(name: String) { self.name = name } - var apartment: Apartment? - deinit { print("\(name) is being deinitialized") } -} - -class Apartment { - let unit: String - init(unit: String) { self.unit = unit } - var tenant: Person? - deinit { print("Apartment \(unit) is being deinitialized") } -} -``` - - - -Every `Person` instance has a `name` property of type `String` -and an optional `apartment` property that's initially `nil`. -The `apartment` property is optional, because a person may not always have an apartment. - -Similarly, every `Apartment` instance has a `unit` property of type `String` -and has an optional `tenant` property that's initially `nil`. -The tenant property is optional because an apartment may not always have a tenant. - -Both of these classes also define a deinitializer, -which prints the fact that an instance of that class is being deinitialized. -This enables you to see whether -instances of `Person` and `Apartment` are being deallocated as expected. - -This next code snippet defines two variables of optional type -called `john` and `unit4A`, -which will be set to a specific `Apartment` and `Person` instance below. -Both of these variables have an initial value of `nil`, by virtue of being optional: - -```swift -var john: Person? -var unit4A: Apartment? -``` - - - -You can now create a specific `Person` instance and `Apartment` instance -and assign these new instances to the `john` and `unit4A` variables: - -```swift -john = Person(name: "John Appleseed") -unit4A = Apartment(unit: "4A") -``` - - - -Here's how the strong references look after creating and assigning these two instances. -The `john` variable now has a strong reference to the new `Person` instance, -and the `unit4A` variable has a strong reference to the new `Apartment` instance: - -![](referenceCycle01) - -You can now link the two instances together -so that the person has an apartment, and the apartment has a tenant. -Note that an exclamation point (`!`) is used to unwrap and access -the instances stored inside the `john` and `unit4A` optional variables, -so that the properties of those instances can be set: - -```swift -john!.apartment = unit4A -unit4A!.tenant = john -``` - - - -Here's how the strong references look after you link the two instances together: - -![](referenceCycle02) - -Unfortunately, linking these two instances creates -a strong reference cycle between them. -The `Person` instance now has a strong reference to the `Apartment` instance, -and the `Apartment` instance has a strong reference to the `Person` instance. -Therefore, when you break the strong references held by -the `john` and `unit4A` variables, -the reference counts don't drop to zero, -and the instances aren't deallocated by ARC: - -```swift -john = nil -unit4A = nil -``` - - - -Note that neither deinitializer was called -when you set these two variables to `nil`. -The strong reference cycle prevents the `Person` and `Apartment` instances -from ever being deallocated, causing a memory leak in your app. - -Here's how the strong references look after you set -the `john` and `unit4A` variables to `nil`: - -![](referenceCycle03) - -The strong references between the `Person` instance -and the `Apartment` instance remain and can't be broken. - -## Resolving Strong Reference Cycles Between Class Instances - -Swift provides two ways to resolve strong reference cycles -when you work with properties of class type: -weak references and unowned references. - -Weak and unowned references enable one instance in a reference cycle -to refer to the other instance *without* keeping a strong hold on it. -The instances can then refer to each other without creating a strong reference cycle. - -Use a weak reference when the other instance has a shorter lifetime --- -that is, when the other instance can be deallocated first. -In the `Apartment` example above, -it's appropriate for an apartment to be able to have -no tenant at some point in its lifetime, -and so a weak reference is an appropriate way to break the reference cycle in this case. -In contrast, use an unowned reference when the other instance -has the same lifetime or a longer lifetime. - - - -### Weak References - -A *weak reference* is a reference that doesn't keep a strong hold -on the instance it refers to, -and so doesn't stop ARC from disposing of the referenced instance. -This behavior prevents the reference from becoming part of a strong reference cycle. -You indicate a weak reference by placing the `weak` keyword -before a property or variable declaration. - -Because a weak reference doesn't keep a strong hold on the instance it refers to, -it's possible for that instance to be deallocated -while the weak reference is still referring to it. -Therefore, ARC automatically sets a weak reference to `nil` -when the instance that it refers to is deallocated. -And, because weak references need to allow -their value to be changed to `nil` at runtime, -they're always declared as variables, rather than constants, of an optional type. - -You can check for the existence of a value in the weak reference, -just like any other optional value, -and you will never end up with -a reference to an invalid instance that no longer exists. - -> Note: Property observers aren't called -> when ARC sets a weak reference to `nil`. - - - -The example below is identical to the `Person` and `Apartment` example from above, -with one important difference. -This time around, the `Apartment` type's `tenant` property -is declared as a weak reference: - -```swift -class Person { - let name: String - init(name: String) { self.name = name } - var apartment: Apartment? - deinit { print("\(name) is being deinitialized") } -} - -class Apartment { - let unit: String - init(unit: String) { self.unit = unit } - weak var tenant: Person? - deinit { print("Apartment \(unit) is being deinitialized") } -} -``` - - - -The strong references from the two variables (`john` and `unit4A`) -and the links between the two instances are created as before: - -```swift -var john: Person? -var unit4A: Apartment? - -john = Person(name: "John Appleseed") -unit4A = Apartment(unit: "4A") - -john!.apartment = unit4A -unit4A!.tenant = john -``` - - - -Here's how the references look now that you've linked the two instances together: - -![](weakReference01) - -The `Person` instance still has a strong reference to the `Apartment` instance, -but the `Apartment` instance now has a *weak* reference to the `Person` instance. -This means that when you break the strong reference held by -the `john` variable by setting it to `nil`, -there are no more strong references to the `Person` instance: - -```swift -john = nil -// Prints "John Appleseed is being deinitialized" -``` - - - -Because there are no more strong references to the `Person` instance, -it's deallocated -and the `tenant` property is set to `nil`: - -![](weakReference02) - -The only remaining strong reference to the `Apartment` instance -is from the `unit4A` variable. -If you break *that* strong reference, -there are no more strong references to the `Apartment` instance: - -```swift -unit4A = nil -// Prints "Apartment 4A is being deinitialized" -``` - - - -Because there are no more strong references to the `Apartment` instance, -it too is deallocated: - -![](weakReference03) - -> Note: In systems that use garbage collection, -> weak pointers are sometimes used to implement a simple caching mechanism -> because objects with no strong references are deallocated -> only when memory pressure triggers garbage collection. -> However, with ARC, values are deallocated -> as soon as their last strong reference is removed, -> making weak references unsuitable for such a purpose. - -### Unowned References - -Like a weak reference, -an *unowned reference* doesn't keep -a strong hold on the instance it refers to. -Unlike a weak reference, however, -an unowned reference is used when the other instance -has the same lifetime or a longer lifetime. -You indicate an unowned reference by placing the `unowned` keyword -before a property or variable declaration. - -Unlike a weak reference, -an unowned reference is expected to always have a value. -As a result, -marking a value as unowned doesn't make it optional, -and ARC never sets an unowned reference's value to `nil`. - - - -> Important: Use an unowned reference only when you are sure that -> the reference *always* refers to an instance that hasn't been deallocated. -> -> If you try to access the value of an unowned reference -> after that instance has been deallocated, -> you'll get a runtime error. - - - -The following example defines two classes, `Customer` and `CreditCard`, -which model a bank customer and a possible credit card for that customer. -These two classes each store an instance of the other class as a property. -This relationship has the potential to create a strong reference cycle. - -The relationship between `Customer` and `CreditCard` is slightly different from -the relationship between `Apartment` and `Person` -seen in the weak reference example above. -In this data model, a customer may or may not have a credit card, -but a credit card will *always* be associated with a customer. -A `CreditCard` instance never outlives the `Customer` that it refers to. -To represent this, the `Customer` class has an optional `card` property, -but the `CreditCard` class has an unowned (and non-optional) `customer` property. - -Furthermore, a new `CreditCard` instance can *only* be created -by passing a `number` value and a `customer` instance -to a custom `CreditCard` initializer. -This ensures that a `CreditCard` instance always has -a `customer` instance associated with it when the `CreditCard` instance is created. - -Because a credit card will always have a customer, -you define its `customer` property as an unowned reference, -to avoid a strong reference cycle: - -```swift -class Customer { - let name: String - var card: CreditCard? - init(name: String) { - self.name = name - } - deinit { print("\(name) is being deinitialized") } -} - -class CreditCard { - let number: UInt64 - unowned let customer: Customer - init(number: UInt64, customer: Customer) { - self.number = number - self.customer = customer - } - deinit { print("Card #\(number) is being deinitialized") } -} -``` - - - -> Note: The `number` property of the `CreditCard` class is defined with -> a type of `UInt64` rather than `Int`, -> to ensure that the `number` property's capacity is large enough to store -> a 16-digit card number on both 32-bit and 64-bit systems. - -This next code snippet defines an optional `Customer` variable called `john`, -which will be used to store a reference to a specific customer. -This variable has an initial value of nil, by virtue of being optional: - -```swift -var john: Customer? -``` - - - -You can now create a `Customer` instance, -and use it to initialize and assign a new `CreditCard` instance -as that customer's `card` property: - -```swift -john = Customer(name: "John Appleseed") -john!.card = CreditCard(number: 1234_5678_9012_3456, customer: john!) -``` - - - -Here's how the references look, now that you've linked the two instances: - -![](unownedReference01) - -The `Customer` instance now has a strong reference to the `CreditCard` instance, -and the `CreditCard` instance has an unowned reference to the `Customer` instance. - -Because of the unowned `customer` reference, -when you break the strong reference held by the `john` variable, -there are no more strong references to the `Customer` instance: - -![](unownedReference02) - -Because there are no more strong references to the `Customer` instance, -it's deallocated. -After this happens, -there are no more strong references to the `CreditCard` instance, -and it too is deallocated: - -```swift -john = nil -// Prints "John Appleseed is being deinitialized" -// Prints "Card #1234567890123456 is being deinitialized" -``` - - - -The final code snippet above shows that -the deinitializers for the `Customer` instance and `CreditCard` instance -both print their “deinitialized” messages -after the `john` variable is set to `nil`. - -> Note: The examples above show how to use *safe* unowned references. -> Swift also provides *unsafe* unowned references for cases where -> you need to disable runtime safety checks --- -> for example, for performance reasons. -> As with all unsafe operations, -> you take on the responsibility for checking that code for safety. -> -> You indicate an unsafe unowned reference by writing `unowned(unsafe)`. -> If you try to access an unsafe unowned reference -> after the instance that it refers to is deallocated, -> your program will try to access the memory location -> where the instance used to be, -> which is an unsafe operation. - - - -### Unowned Optional References - -You can mark an optional reference to a class as unowned. -In terms of the ARC ownership model, -an unowned optional reference and a weak reference -can both be used in the same contexts. -The difference is that when you use an unowned optional reference, -you're responsible for making sure it always -refers to a valid object or is set to `nil`. - -Here's an example that keeps track of the courses -offered by a particular department at a school: - -```swift -class Department { - var name: String - var courses: [Course] - init(name: String) { - self.name = name - self.courses = [] - } -} - -class Course { - var name: String - unowned var department: Department - unowned var nextCourse: Course? - init(name: String, in department: Department) { - self.name = name - self.department = department - self.nextCourse = nil - } -} -``` - - - -`Department` maintains a strong reference -to each course that the department offers. -In the ARC ownership model, a department owns its courses. -`Course` has two unowned references, -one to the department -and one to the next course a student should take; -a course doesn't own either of these objects. -Every course is part of some department -so the `department` property isn't an optional. -However, -because some courses don't have a recommended follow-on course, -the `nextCourse` property is an optional. - -Here's an example of using these classes: - -```swift -let department = Department(name: "Horticulture") - -let intro = Course(name: "Survey of Plants", in: department) -let intermediate = Course(name: "Growing Common Herbs", in: department) -let advanced = Course(name: "Caring for Tropical Plants", in: department) - -intro.nextCourse = intermediate -intermediate.nextCourse = advanced -department.courses = [intro, intermediate, advanced] -``` - - - -The code above creates a department and its three courses. -The intro and intermediate courses both have a suggested next course -stored in their `nextCourse` property, -which maintains an unowned optional reference to -the course a student should take after completing this one. - -![](unownedOptionalReference) - -An unowned optional reference doesn't keep a strong hold -on the instance of the class that it wraps, -and so it doesn't prevent ARC from deallocating the instance. -It behaves the same as an unowned reference does under ARC, -except that an unowned optional reference can be `nil`. - -Like non-optional unowned references, -you're responsible for ensuring that `nextCourse` -always refers to a course that hasn't been deallocated. -In this case, for example, -when you delete a course from `department.courses` -you also need to remove any references to it -that other courses might have. - -> Note: The underlying type of an optional value is `Optional`, -> which is an enumeration in the Swift standard library. -> However, optionals are an exception to the rule that -> value types can't be marked with `unowned`. -> -> The optional that wraps the class -> doesn't use reference counting, -> so you don't need to maintain a strong reference to the optional. - - - -### Unowned References and Implicitly Unwrapped Optional Properties - -The examples for weak and unowned references above -cover two of the more common scenarios -in which it's necessary to break a strong reference cycle. - -The `Person` and `Apartment` example shows -a situation where two properties, both of which are allowed to be `nil`, -have the potential to cause a strong reference cycle. -This scenario is best resolved with a weak reference. - -The `Customer` and `CreditCard` example -shows a situation where one property that's allowed to be `nil` -and another property that can't be `nil` -have the potential to cause a strong reference cycle. -This scenario is best resolved with an unowned reference. - -However, there's a third scenario, -in which *both* properties should always have a value, -and neither property should ever be `nil` once initialization is complete. -In this scenario, it's useful to combine an unowned property on one class -with an implicitly unwrapped optional property on the other class. - -This enables both properties to be accessed directly -(without optional unwrapping) once initialization is complete, -while still avoiding a reference cycle. -This section shows you how to set up such a relationship. - -The example below defines two classes, `Country` and `City`, -each of which stores an instance of the other class as a property. -In this data model, every country must always have a capital city, -and every city must always belong to a country. -To represent this, the `Country` class has a `capitalCity` property, -and the `City` class has a `country` property: - -```swift -class Country { - let name: String - var capitalCity: City! - init(name: String, capitalName: String) { - self.name = name - self.capitalCity = City(name: capitalName, country: self) - } -} - -class City { - let name: String - unowned let country: Country - init(name: String, country: Country) { - self.name = name - self.country = country - } -} -``` - - - -To set up the interdependency between the two classes, -the initializer for `City` takes a `Country` instance, -and stores this instance in its `country` property. - -The initializer for `City` is called from within the initializer for `Country`. -However, the initializer for `Country` can't pass `self` to the `City` initializer -until a new `Country` instance is fully initialized, -as described in . - -To cope with this requirement, -you declare the `capitalCity` property of `Country` as -an implicitly unwrapped optional property, -indicated by the exclamation point at the end of its type annotation (`City!`). -This means that the `capitalCity` property has a default value of `nil`, -like any other optional, -but can be accessed without the need to unwrap its value -as described in . - -Because `capitalCity` has a default `nil` value, -a new `Country` instance is considered fully initialized -as soon as the `Country` instance sets its `name` property within its initializer. -This means that the `Country` initializer can start to reference and pass around -the implicit `self` property as soon as the `name` property is set. -The `Country` initializer can therefore pass `self` as one of the parameters for -the `City` initializer when the `Country` initializer is setting -its own `capitalCity` property. - -All of this means that you can create the `Country` and `City` instances -in a single statement, without creating a strong reference cycle, -and the `capitalCity` property can be accessed directly, -without needing to use an exclamation point to unwrap its optional value: - -```swift -var country = Country(name: "Canada", capitalName: "Ottawa") -print("\(country.name)'s capital city is called \(country.capitalCity.name)") -// Prints "Canada's capital city is called Ottawa" -``` - - - -In the example above, the use of an implicitly unwrapped optional -means that all of the two-phase class initializer requirements are satisfied. -The `capitalCity` property can be used and accessed like a non-optional value -once initialization is complete, -while still avoiding a strong reference cycle. - -## Strong Reference Cycles for Closures - -You saw above how a strong reference cycle can be created -when two class instance properties hold a strong reference to each other. -You also saw how to use weak and unowned references to break these strong reference cycles. - -A strong reference cycle can also occur -if you assign a closure to a property of a class instance, -and the body of that closure captures the instance. -This capture might occur because the closure's body accesses a property of the instance, -such as `self.someProperty`, -or because the closure calls a method on the instance, -such as `self.someMethod()`. -In either case, these accesses cause the closure to “capture” `self`, -creating a strong reference cycle. - -This strong reference cycle occurs because closures, like classes, are *reference types*. -When you assign a closure to a property, -you are assigning a *reference* to that closure. -In essence, it's the same problem as above --- -two strong references are keeping each other alive. -However, rather than two class instances, -this time it's a class instance and a closure that are keeping each other alive. - -Swift provides an elegant solution to this problem, -known as a *closure capture list*. -However, before you learn how to break a strong reference cycle with a closure capture list, -it's useful to understand how such a cycle can be caused. - -The example below shows how you can create a strong reference cycle -when using a closure that references `self`. -This example defines a class called `HTMLElement`, -which provides a simple model for an individual element within an HTML document: - -```swift -class HTMLElement { - - let name: String - let text: String? - - lazy var asHTML: () -> String = { - if let text = self.text { - return "<\(self.name)>\(text)" - } else { - return "<\(self.name) />" - } - } - - init(name: String, text: String? = nil) { - self.name = name - self.text = text - } - - deinit { - print("\(name) is being deinitialized") - } - -} -``` - - - -The `HTMLElement` class defines a `name` property, -which indicates the name of the element, -such as `"h1"` for a heading element, -`"p"` for a paragraph element, -or `"br"` for a line break element. -`HTMLElement` also defines an optional `text` property, -which you can set to a string that represents -the text to be rendered within that HTML element. - -In addition to these two simple properties, -the `HTMLElement` class defines a lazy property called `asHTML`. -This property references a closure that combines `name` and `text` -into an HTML string fragment. -The `asHTML` property is of type `() -> String`, -or “a function that takes no parameters, and returns a `String` value”. - -By default, the `asHTML` property is assigned a closure that returns -a string representation of an HTML tag. -This tag contains the optional `text` value if it exists, -or no text content if `text` doesn't exist. -For a paragraph element, the closure would return `"

some text

"` or `"

"`, -depending on whether the `text` property equals `"some text"` or `nil`. - -The `asHTML` property is named and used somewhat like an instance method. -However, because `asHTML` is a closure property rather than an instance method, -you can replace the default value of the `asHTML` property with a custom closure, -if you want to change the HTML rendering for a particular HTML element. - -For example, the `asHTML` property could be set to a closure -that defaults to some text if the `text` property is `nil`, -in order to prevent the representation from returning an empty HTML tag: - -```swift -let heading = HTMLElement(name: "h1") -let defaultText = "some default text" -heading.asHTML = { - return "<\(heading.name)>\(heading.text ?? defaultText)" -} -print(heading.asHTML()) -// Prints "

some default text

" -``` - - - -> Note: The `asHTML` property is declared as a lazy property, -> because it's only needed if and when the element actually needs to be rendered -> as a string value for some HTML output target. -> The fact that `asHTML` is a lazy property means that you can refer to `self` -> within the default closure, -> because the lazy property will not be accessed until -> after initialization has been completed and `self` is known to exist. - -The `HTMLElement` class provides a single initializer, -which takes a `name` argument and (if desired) a `text` argument -to initialize a new element. -The class also defines a deinitializer, -which prints a message to show when an `HTMLElement` instance is deallocated. - -Here's how you use the `HTMLElement` class to create and print a new instance: - -```swift -var paragraph: HTMLElement? = HTMLElement(name: "p", text: "hello, world") -print(paragraph!.asHTML()) -// Prints "

hello, world

" -``` - - - -> Note: The `paragraph` variable above is defined as an *optional* `HTMLElement`, -> so that it can be set to `nil` below to demonstrate -> the presence of a strong reference cycle. - -Unfortunately, the `HTMLElement` class, as written above, -creates a strong reference cycle between -an `HTMLElement` instance and the closure used for its default `asHTML` value. -Here's how the cycle looks: - -![](closureReferenceCycle01) - -The instance's `asHTML` property holds a strong reference to its closure. -However, because the closure refers to `self` within its body -(as a way to reference `self.name` and `self.text`), -the closure *captures* self, -which means that it holds a strong reference back to the `HTMLElement` instance. -A strong reference cycle is created between the two. -(For more information about capturing values in a closure, -see .) - -> Note: Even though the closure refers to `self` multiple times, -> it only captures one strong reference to the `HTMLElement` instance. - -If you set the `paragraph` variable to `nil` -and break its strong reference to the `HTMLElement` instance, -the strong reference cycle prevents deallocating -both the `HTMLElement` instance and its closure: - -```swift -paragraph = nil -``` - - - -Note that the message in the `HTMLElement` deinitializer isn't printed, -which shows that the `HTMLElement` instance isn't deallocated. - -## Resolving Strong Reference Cycles for Closures - -You resolve a strong reference cycle between a closure and a class instance -by defining a *capture list* as part of the closure's definition. -A capture list defines the rules to use when capturing one or more reference types -within the closure's body. -As with strong reference cycles between two class instances, -you declare each captured reference to be a weak or unowned reference -rather than a strong reference. -The appropriate choice of weak or unowned depends on -the relationships between the different parts of your code. - -> Note: Swift requires you to write `self.someProperty` or `self.someMethod()` -> (rather than just `someProperty` or `someMethod()`) -> whenever you refer to a member of `self` within a closure. -> This helps you remember that it's possible to capture `self` by accident. - -### Defining a Capture List - -Each item in a capture list is a pairing of the `weak` or `unowned` keyword -with a reference to a class instance (such as `self`) -or a variable initialized with some value (such as `delegate = self.delegate`). -These pairings are written within a pair of square braces, separated by commas. - -Place the capture list before a closure's parameter list and return type -if they're provided: - -```swift -lazy var someClosure = { - [unowned self, weak delegate = self.delegate] - (index: Int, stringToProcess: String) -> String in - // closure body goes here -} -``` - - - -If a closure doesn't specify a parameter list or return type -because they can be inferred from context, -place the capture list at the very start of the closure, -followed by the `in` keyword: - -```swift -lazy var someClosure = { - [unowned self, weak delegate = self.delegate] in - // closure body goes here -} -``` - - - -### Weak and Unowned References - -Define a capture in a closure as an unowned reference -when the closure and the instance it captures will always refer to each other, -and will always be deallocated at the same time. - -Conversely, define a capture as a weak reference when the captured reference -may become `nil` at some point in the future. -Weak references are always of an optional type, -and automatically become `nil` when the instance they reference is deallocated. -This enables you to check for their existence within the closure's body. - - - -> Note: If the captured reference will never become `nil`, -> it should always be captured as an unowned reference, -> rather than a weak reference. - -An unowned reference is the appropriate capture method to use to resolve -the strong reference cycle in the `HTMLElement` example -from above. -Here's how you write the `HTMLElement` class to avoid the cycle: - -```swift -class HTMLElement { - - let name: String - let text: String? - - lazy var asHTML: () -> String = { - [unowned self] in - if let text = self.text { - return "<\(self.name)>\(text)" - } else { - return "<\(self.name) />" - } - } - - init(name: String, text: String? = nil) { - self.name = name - self.text = text - } - - deinit { - print("\(name) is being deinitialized") - } - -} -``` - - - -This implementation of `HTMLElement` is identical to the previous implementation, -apart from the addition of a capture list within the `asHTML` closure. -In this case, the capture list is `[unowned self]`, -which means “capture self as an unowned reference rather than a strong reference”. - -You can create and print an `HTMLElement` instance as before: - -```swift -var paragraph: HTMLElement? = HTMLElement(name: "p", text: "hello, world") -print(paragraph!.asHTML()) -// Prints "

hello, world

" -``` - - - -Here's how the references look with the capture list in place: - -![](closureReferenceCycle02) - -This time, the capture of `self` by the closure is an unowned reference, -and doesn't keep a strong hold on the `HTMLElement` instance it has captured. -If you set the strong reference from the `paragraph` variable to `nil`, -the `HTMLElement` instance is deallocated, -as can be seen from the printing of its deinitializer message in the example below: - -```swift -paragraph = nil -// Prints "p is being deinitialized" -``` - - - -For more information about capture lists, -see . - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/LanguageGuide/BasicOperators.md b/swift-6-beta.docc/LanguageGuide/BasicOperators.md deleted file mode 100644 index 6be2d17f7..000000000 --- a/swift-6-beta.docc/LanguageGuide/BasicOperators.md +++ /dev/null @@ -1,1248 +0,0 @@ -# Basic Operators - -Perform operations like assignment, arithmetic, and comparison. - -An *operator* is a special symbol or phrase that you use to -check, change, or combine values. -For example, the addition operator (`+`) adds two numbers, -as in `let i = 1 + 2`, -and the logical AND operator (`&&`) combines two Boolean values, -as in `if enteredDoorCode && passedRetinaScan`. - -Swift supports the operators you may already know from languages like C, -and improves several capabilities to eliminate common coding errors. -The assignment operator (`=`) doesn't return a value, -to prevent it from being mistakenly used when -the equal to operator (`==`) is intended. -Arithmetic operators (`+`, `-`, `*`, `/`, `%` and so forth) -detect and disallow value overflow, -to avoid unexpected results when working with numbers that become larger or smaller -than the allowed value range of the type that stores them. -You can opt in to value overflow behavior -by using Swift's overflow operators, -as described in . - -Swift also provides range operators that aren't found in C, -such as `a.. covers Swift's advanced operators, -and describes how to define your own custom operators -and implement the standard operators for your own custom types. - -## Terminology - -Operators are unary, binary, or ternary: - -- *Unary* operators operate on a single target (such as `-a`). - Unary *prefix* operators appear immediately before their target (such as `!b`), - and unary *postfix* operators appear immediately after their target (such as `c!`). -- *Binary* operators operate on two targets (such as `2 + 3`) - and are *infix* because they appear in between their two targets. -- *Ternary* operators operate on three targets. - Like C, Swift has only one ternary operator, - the ternary conditional operator (`a ? b : c`). - -The values that operators affect are *operands*. -In the expression `1 + 2`, the `+` symbol is an infix operator -and its two operands are the values `1` and `2`. - -## Assignment Operator - -The *assignment operator* (`a = b`) -initializes or updates the value of `a` with the value of `b`: - -```swift -let b = 10 -var a = 5 -a = b -// a is now equal to 10 -``` - - - -If the right side of the assignment is a tuple with multiple values, -its elements can be decomposed into multiple constants or variables at once: - -```swift -let (x, y) = (1, 2) -// x is equal to 1, and y is equal to 2 -``` - - - - - - - -Unlike the assignment operator in C and Objective-C, -the assignment operator in Swift doesn't itself return a value. -The following statement isn't valid: - -```swift -if x = y { - // This isn't valid, because x = y doesn't return a value. -} -``` - - - -This feature prevents the assignment operator (`=`) from being used by accident -when the equal to operator (`==`) is actually intended. -By making `if x = y` invalid, -Swift helps you to avoid these kinds of errors in your code. - - - -## Arithmetic Operators - -Swift supports the four standard *arithmetic operators* for all number types: - -- Addition (`+`) -- Subtraction (`-`) -- Multiplication (`*`) -- Division (`/`) - -```swift -1 + 2 // equals 3 -5 - 3 // equals 2 -2 * 3 // equals 6 -10.0 / 2.5 // equals 4.0 -``` - - - -Unlike the arithmetic operators in C and Objective-C, -the Swift arithmetic operators don't allow values to overflow by default. -You can opt in to value overflow behavior by using Swift's overflow operators -(such as `a &+ b`). See . - -The addition operator is also supported for `String` concatenation: - -```swift -"hello, " + "world" // equals "hello, world" -``` - - - -### Remainder Operator - -The *remainder operator* (`a % b`) -works out how many multiples of `b` will fit inside `a` -and returns the value that's left over -(known as the *remainder*). - -> Note: The remainder operator (`%`) is also known as -> a *modulo operator* in other languages. -> However, its behavior in Swift for negative numbers means that, -> strictly speaking, it's a remainder rather than a modulo operation. - - - -Here's how the remainder operator works. -To calculate `9 % 4`, you first work out how many `4`s will fit inside `9`: - -![](remainderInteger) - -You can fit two `4`s inside `9`, and the remainder is `1` (shown in orange). - -In Swift, this would be written as: - -```swift -9 % 4 // equals 1 -``` - - - -To determine the answer for `a % b`, -the `%` operator calculates the following equation -and returns `remainder` as its output: - -`a` = (`b` x `some multiplier`) + `remainder` - -where `some multiplier` is the largest number of multiples of `b` -that will fit inside `a`. - -Inserting `9` and `4` into this equation yields: - -`9` = (`4` x `2`) + `1` - -The same method is applied when calculating the remainder for a negative value of `a`: - -```swift --9 % 4 // equals -1 -``` - - - -Inserting `-9` and `4` into the equation yields: - -`-9` = (`4` x `-2`) + `-1` - -giving a remainder value of `-1`. - -The sign of `b` is ignored for negative values of `b`. -This means that `a % b` and `a % -b` always give the same answer. - -### Unary Minus Operator - -The sign of a numeric value can be toggled using a prefixed `-`, -known as the *unary minus operator*: - -```swift -let three = 3 -let minusThree = -three // minusThree equals -3 -let plusThree = -minusThree // plusThree equals 3, or "minus minus three" -``` - - - -The unary minus operator (`-`) is prepended directly before the value it operates on, -without any white space. - -### Unary Plus Operator - -The *unary plus operator* (`+`) simply returns -the value it operates on, without any change: - -```swift -let minusSix = -6 -let alsoMinusSix = +minusSix // alsoMinusSix equals -6 -``` - - - -Although the unary plus operator doesn't actually do anything, -you can use it to provide symmetry in your code for positive numbers -when also using the unary minus operator for negative numbers. - -## Compound Assignment Operators - -Like C, Swift provides *compound assignment operators* that combine assignment (`=`) with another operation. -One example is the *addition assignment operator* (`+=`): - -```swift -var a = 1 -a += 2 -// a is now equal to 3 -``` - - - -The expression `a += 2` is shorthand for `a = a + 2`. -Effectively, the addition and the assignment are combined into one operator -that performs both tasks at the same time. - -> Note: The compound assignment operators don't return a value. -> For example, you can't write `let b = a += 2`. - -For information about the operators provided by the Swift standard library, -see [Operator Declarations](https://developer.apple.com/documentation/swift/operator_declarations). - -## Comparison Operators - -Swift supports the following comparison operators: - -- Equal to (`a == b`) -- Not equal to (`a != b`) -- Greater than (`a > b`) -- Less than (`a < b`) -- Greater than or equal to (`a >= b`) -- Less than or equal to (`a <= b`) - -> Note: Swift also provides two *identity operators* (`===` and `!==`), -> which you use to test whether two object references both refer to the same object instance. -> For more information, see . - -Each of the comparison operators returns a `Bool` value to indicate whether or not the statement is true: - -```swift -1 == 1 // true because 1 is equal to 1 -2 != 1 // true because 2 isn't equal to 1 -2 > 1 // true because 2 is greater than 1 -1 < 2 // true because 1 is less than 2 -1 >= 1 // true because 1 is greater than or equal to 1 -2 <= 1 // false because 2 isn't less than or equal to 1 -``` - - - -Comparison operators are often used in conditional statements, -such as the `if` statement: - -```swift -let name = "world" -if name == "world" { - print("hello, world") -} else { - print("I'm sorry \(name), but I don't recognize you") -} -// Prints "hello, world", because name is indeed equal to "world". -``` - - - -For more about the `if` statement, see . - -You can compare -two tuples if they have the same type and the same number of values. -Tuples are compared from left to right, -one value at a time, -until the comparison finds two values -that aren't equal. -Those two values are compared, -and the result of that comparison -determines the overall result of the tuple comparison. -If all the elements are equal, -then the tuples themselves are equal. -For example: - -```swift -(1, "zebra") < (2, "apple") // true because 1 is less than 2; "zebra" and "apple" aren't compared -(3, "apple") < (3, "bird") // true because 3 is equal to 3, and "apple" is less than "bird" -(4, "dog") == (4, "dog") // true because 4 is equal to 4, and "dog" is equal to "dog" -``` - - - -In the example above, -you can see the left-to-right comparison behavior on the first line. -Because `1` is less than `2`, -`(1, "zebra")` is considered less than `(2, "apple")`, -regardless of any other values in the tuples. -It doesn't matter that `"zebra"` isn't less than `"apple"`, -because the comparison is already determined by the tuples' first elements. -However, -when the tuples' first elements are the same, -their second elements *are* compared --- -this is what happens on the second and third line. - -Tuples can be compared with a given operator only if the operator -can be applied to each value in the respective tuples. For example, -as demonstrated in the code below, you can compare -two tuples of type `(String, Int)` because -both `String` and `Int` values can be compared -using the `<` operator. In contrast, -two tuples of type `(String, Bool)` can't be compared -with the `<` operator because the `<` operator can't be applied to -`Bool` values. - -```swift -("blue", -1) < ("purple", 1) // OK, evaluates to true -("blue", false) < ("purple", true) // Error because < can't compare Boolean values -``` - - - - - -> Note: The Swift standard library includes tuple comparison operators -> for tuples with fewer than seven elements. -> To compare tuples with seven or more elements, -> you must implement the comparison operators yourself. - - - -## Ternary Conditional Operator - -The *ternary conditional operator* is a special operator with three parts, -which takes the form `question ? answer1 : answer2`. -It's a shortcut for evaluating one of two expressions -based on whether `question` is true or false. -If `question` is true, it evaluates `answer1` and returns its value; -otherwise, it evaluates `answer2` and returns its value. - -The ternary conditional operator is shorthand for the code below: - -```swift -if question { - answer1 -} else { - answer2 -} -``` - - - - - -Here's an example, which calculates the height for a table row. -The row height should be 50 points taller than the content height -if the row has a header, and 20 points taller if the row doesn't have a header: - -```swift -let contentHeight = 40 -let hasHeader = true -let rowHeight = contentHeight + (hasHeader ? 50 : 20) -// rowHeight is equal to 90 -``` - - - -The example above is shorthand for the code below: - -```swift -let contentHeight = 40 -let hasHeader = true -let rowHeight: Int -if hasHeader { - rowHeight = contentHeight + 50 -} else { - rowHeight = contentHeight + 20 -} -// rowHeight is equal to 90 -``` - - - -The first example's use of the ternary conditional operator means that -`rowHeight` can be set to the correct value on a single line of code, -which is more concise than the code used in the second example. - -The ternary conditional operator provides -an efficient shorthand for deciding which of two expressions to consider. -Use the ternary conditional operator with care, however. -Its conciseness can lead to hard-to-read code if overused. -Avoid combining multiple instances of the ternary conditional operator into one compound statement. - -## Nil-Coalescing Operator - -The *nil-coalescing operator* (`a ?? b`) -unwraps an optional `a` if it contains a value, -or returns a default value `b` if `a` is `nil`. -The expression `a` is always of an optional type. -The expression `b` must match the type that's stored inside `a`. - -The nil-coalescing operator is shorthand for the code below: - -```swift -a != nil ? a! : b -``` - - - -The code above uses the ternary conditional operator and forced unwrapping (`a!`) -to access the value wrapped inside `a` when `a` isn't `nil`, -and to return `b` otherwise. -The nil-coalescing operator provides a more elegant way to encapsulate -this conditional checking and unwrapping in a concise and readable form. - -> Note: If the value of `a` is non-`nil`, -> the value of `b` isn't evaluated. -> This is known as *short-circuit evaluation*. - -The example below uses the nil-coalescing operator to choose between -a default color name and an optional user-defined color name: - -```swift -let defaultColorName = "red" -var userDefinedColorName: String? // defaults to nil - -var colorNameToUse = userDefinedColorName ?? defaultColorName -// userDefinedColorName is nil, so colorNameToUse is set to the default of "red" -``` - - - -The `userDefinedColorName` variable is defined as an optional `String`, -with a default value of `nil`. -Because `userDefinedColorName` is of an optional type, -you can use the nil-coalescing operator to consider its value. -In the example above, the operator is used to determine -an initial value for a `String` variable called `colorNameToUse`. -Because `userDefinedColorName` is `nil`, -the expression `userDefinedColorName ?? defaultColorName` returns -the value of `defaultColorName`, or `"red"`. - -If you assign a non-`nil` value to `userDefinedColorName` -and perform the nil-coalescing operator check again, -the value wrapped inside `userDefinedColorName` is used instead of the default: - -```swift -userDefinedColorName = "green" -colorNameToUse = userDefinedColorName ?? defaultColorName -// userDefinedColorName isn't nil, so colorNameToUse is set to "green" -``` - - - -## Range Operators - -Swift includes several *range operators*, -which are shortcuts for expressing a range of values. - -### Closed Range Operator - -The *closed range operator* (`a...b`) -defines a range that runs from `a` to `b`, -and includes the values `a` and `b`. -The value of `a` must not be greater than `b`. - - - - - - - -The closed range operator is useful when iterating over a range -in which you want all of the values to be used, -such as with a `for`-`in` loop: - -```swift -for index in 1...5 { - print("\(index) times 5 is \(index * 5)") -} -// 1 times 5 is 5 -// 2 times 5 is 10 -// 3 times 5 is 15 -// 4 times 5 is 20 -// 5 times 5 is 25 -``` - - - -For more about `for`-`in` loops, see . - -### Half-Open Range Operator - -The *half-open range operator* (`a.. let range = 1..<2 - >> print(type(of: range)) - << Range - ``` ---> - - - - - -Half-open ranges are particularly useful when you work with -zero-based lists such as arrays, -where it's useful to count up to (but not including) the length of the list: - -```swift -let names = ["Anna", "Alex", "Brian", "Jack"] -let count = names.count -for i in 0.. let names = ["Anna", "Alex", "Brian", "Jack"] - -> let count = names.count - >> assert(count == 4) - -> for i in 0.. - -Note that the array contains four items, -but `0... - -### One-Sided Ranges - -The closed range operator -has an alternative form for ranges that continue -as far as possible in one direction --- -for example, -a range that includes all the elements of an array -from index 2 to the end of the array. -In these cases, you can omit the value -from one side of the range operator. -This kind of range is called a *one-sided range* -because the operator has a value on only one side. -For example: - -```swift -for name in names[2...] { - print(name) -} -// Brian -// Jack - -for name in names[...2] { - print(name) -} -// Anna -// Alex -// Brian -``` - - - -The half-open range operator also has -a one-sided form that's written -with only its final value. -Just like when you include a value on both sides, -the final value isn't part of the range. -For example: - -```swift -for name in names[..<2] { - print(name) -} -// Anna -// Alex -``` - - - -One-sided ranges can be used in other contexts, -not just in subscripts. -You can't iterate over a one-sided range -that omits a first value, -because it isn't clear where iteration should begin. -You *can* iterate over a one-sided range that omits its final value; -however, because the range continues indefinitely, -make sure you add an explicit end condition for the loop. -You can also check whether a one-sided range contains a particular value, -as shown in the code below. - -```swift -let range = ...5 -range.contains(7) // false -range.contains(4) // true -range.contains(-1) // true -``` - - - -## Logical Operators - -*Logical operators* modify or combine -the Boolean logic values `true` and `false`. -Swift supports the three standard logical operators found in C-based languages: - -- Logical NOT (`!a`) -- Logical AND (`a && b`) -- Logical OR (`a || b`) - -### Logical NOT Operator - -The *logical NOT operator* (`!a`) inverts a Boolean value so that `true` becomes `false`, -and `false` becomes `true`. - -The logical NOT operator is a prefix operator, -and appears immediately before the value it operates on, -without any white space. -It can be read as “not `a`”, as seen in the following example: - -```swift -let allowedEntry = false -if !allowedEntry { - print("ACCESS DENIED") -} -// Prints "ACCESS DENIED" -``` - - - -The phrase `if !allowedEntry` can be read as “if not allowed entry.” -The subsequent line is only executed if “not allowed entry” is true; -that is, if `allowedEntry` is `false`. - -As in this example, -careful choice of Boolean constant and variable names -can help to keep code readable and concise, -while avoiding double negatives or confusing logic statements. - -### Logical AND Operator - -The *logical AND operator* (`a && b`) creates logical expressions -where both values must be `true` for the overall expression to also be `true`. - -If either value is `false`, -the overall expression will also be `false`. -In fact, if the *first* value is `false`, -the second value won't even be evaluated, -because it can't possibly make the overall expression equate to `true`. -This is known as *short-circuit evaluation*. - -This example considers two `Bool` values -and only allows access if both values are `true`: - -```swift -let enteredDoorCode = true -let passedRetinaScan = false -if enteredDoorCode && passedRetinaScan { - print("Welcome!") -} else { - print("ACCESS DENIED") -} -// Prints "ACCESS DENIED" -``` - - - -### Logical OR Operator - -The *logical OR operator* -(`a || b`) is an infix operator made from two adjacent pipe characters. -You use it to create logical expressions in which -only *one* of the two values has to be `true` -for the overall expression to be `true`. - -Like the Logical AND operator above, -the Logical OR operator uses short-circuit evaluation to consider its expressions. -If the left side of a Logical OR expression is `true`, -the right side isn't evaluated, -because it can't change the outcome of the overall expression. - -In the example below, -the first `Bool` value (`hasDoorKey`) is `false`, -but the second value (`knowsOverridePassword`) is `true`. -Because one value is `true`, -the overall expression also evaluates to `true`, -and access is allowed: - -```swift -let hasDoorKey = false -let knowsOverridePassword = true -if hasDoorKey || knowsOverridePassword { - print("Welcome!") -} else { - print("ACCESS DENIED") -} -// Prints "Welcome!" -``` - - - -### Combining Logical Operators - -You can combine multiple logical operators to create longer compound expressions: - -```swift -if enteredDoorCode && passedRetinaScan || hasDoorKey || knowsOverridePassword { - print("Welcome!") -} else { - print("ACCESS DENIED") -} -// Prints "Welcome!" -``` - - - -This example uses multiple `&&` and `||` operators to create a longer compound expression. -However, the `&&` and `||` operators still operate on only two values, -so this is actually three smaller expressions chained together. -The example can be read as: - -If we've entered the correct door code and passed the retina scan, -or if we have a valid door key, -or if we know the emergency override password, -then allow access. - -Based on the values of `enteredDoorCode`, `passedRetinaScan`, and `hasDoorKey`, -the first two subexpressions are `false`. -However, the emergency override password is known, -so the overall compound expression still evaluates to `true`. - -> Note: The Swift logical operators `&&` and `||` are left-associative, -> meaning that compound expressions with multiple logical operators -> evaluate the leftmost subexpression first. - -### Explicit Parentheses - -It's sometimes useful to include parentheses when they're not strictly needed, -to make the intention of a complex expression easier to read. -In the door access example above, -it's useful to add parentheses around the first part of the compound expression -to make its intent explicit: - -```swift -if (enteredDoorCode && passedRetinaScan) || hasDoorKey || knowsOverridePassword { - print("Welcome!") -} else { - print("ACCESS DENIED") -} -// Prints "Welcome!" -``` - - - -The parentheses make it clear that the first two values -are considered as part of a separate possible state in the overall logic. -The output of the compound expression doesn't change, -but the overall intention is clearer to the reader. -Readability is always preferred over brevity; -use parentheses where they help to make your intentions clear. - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/LanguageGuide/ClassesAndStructures.md b/swift-6-beta.docc/LanguageGuide/ClassesAndStructures.md deleted file mode 100644 index a124732ed..000000000 --- a/swift-6-beta.docc/LanguageGuide/ClassesAndStructures.md +++ /dev/null @@ -1,727 +0,0 @@ -# Structures and Classes - -Model custom types that encapsulate data. - -*Structures* and *classes* are general-purpose, -flexible constructs that become the building blocks of your program's code. -You define properties and methods to add functionality to your structures and classes -using the same syntax you use to define constants, variables, and functions. - -Unlike other programming languages, -Swift doesn't require you to create separate interface and implementation files -for custom structures and classes. -In Swift, you define a structure or class in a single file, -and the external interface to that class or structure is -automatically made available for other code to use. - -> Note: An instance of a class is traditionally known as an *object*. -> However, Swift structures and classes -> are much closer in functionality than in other languages, -> and much of this chapter describes functionality that applies to -> instances of *either* a class or a structure type. -> Because of this, the more general term *instance* is used. - -## Comparing Structures and Classes - -Structures and classes in Swift have many things in common. -Both can: - -- Define properties to store values -- Define methods to provide functionality -- Define subscripts to provide access to their values using subscript syntax -- Define initializers to set up their initial state -- Be extended to expand their functionality beyond a default implementation -- Conform to protocols to provide standard functionality of a certain kind - -For more information, see -, , , , -, and . - -Classes have additional capabilities that structures don't have: - -- Inheritance enables one class to inherit the characteristics of another. -- Type casting enables you to check and interpret the type of a class instance at runtime. -- Deinitializers enable an instance of a class to free up any resources it has assigned. -- Reference counting allows more than one reference to a class instance. - -For more information, see -, , , -and . - -The additional capabilities that classes support -come at the cost of increased complexity. -As a general guideline, -prefer structures because they're easier to reason about, -and use classes when they're appropriate or necessary. -In practice, this means most of the custom types you define -will be structures and enumerations. -For a more detailed comparison, -see [Choosing Between Structures and Classes](https://developer.apple.com/documentation/swift/choosing_between_structures_and_classes). - -> Note: Classes and actors share many of the same characteristics and behaviors. -> For information about actors, see . - -### Definition Syntax - -Structures and classes have a similar definition syntax. -You introduce structures with the `struct` keyword -and classes with the `class` keyword. -Both place their entire definition within a pair of braces: - -```swift -struct SomeStructure { - // structure definition goes here -} -class SomeClass { - // class definition goes here -} -``` - - - -> Note: Whenever you define a new structure or class, -> you define a new Swift type. -> Give types `UpperCamelCase` names -> (such as `SomeStructure` and `SomeClass` here) -> to match the capitalization of standard Swift types -> (such as `String`, `Int`, and `Bool`). -> Give properties and methods `lowerCamelCase` names -> (such as `frameRate` and `incrementCount`) -> to differentiate them from type names. - -Here's an example of a structure definition and a class definition: - -```swift -struct Resolution { - var width = 0 - var height = 0 -} -class VideoMode { - var resolution = Resolution() - var interlaced = false - var frameRate = 0.0 - var name: String? -} -``` - - - -The example above defines a new structure called `Resolution`, -to describe a pixel-based display resolution. -This structure has two stored properties called `width` and `height`. -Stored properties are constants or variables that are bundled up and stored -as part of the structure or class. -These two properties are inferred to be of type `Int` -by setting them to an initial integer value of `0`. - -The example above also defines a new class called `VideoMode`, -to describe a specific video mode for video display. -This class has four variable stored properties. -The first, `resolution`, is initialized with a new `Resolution` structure instance, -which infers a property type of `Resolution`. -For the other three properties, -new `VideoMode` instances will be initialized with -an `interlaced` setting of `false` (meaning “noninterlaced video”), -a playback frame rate of `0.0`, -and an optional `String` value called `name`. -The `name` property is automatically given a default value of `nil`, -or “no `name` value”, because it's of an optional type. - -### Structure and Class Instances - -The `Resolution` structure definition and the `VideoMode` class definition -only describe what a `Resolution` or `VideoMode` will look like. -They themselves don't describe a specific resolution or video mode. -To do that, you need to create an instance of the structure or class. - -The syntax for creating instances is very similar for both structures and classes: - -```swift -let someResolution = Resolution() -let someVideoMode = VideoMode() -``` - - - -Structures and classes both use initializer syntax for new instances. -The simplest form of initializer syntax uses the type name of the class or structure -followed by empty parentheses, such as `Resolution()` or `VideoMode()`. -This creates a new instance of the class or structure, -with any properties initialized to their default values. -Class and structure initialization is described in more detail -in . - - - -### Accessing Properties - -You can access the properties of an instance using *dot syntax*. -In dot syntax, you write the property name immediately after the instance name, -separated by a period (`.`), without any spaces: - -```swift -print("The width of someResolution is \(someResolution.width)") -// Prints "The width of someResolution is 0" -``` - - - -In this example, -`someResolution.width` refers to the `width` property of `someResolution`, -and returns its default initial value of `0`. - -You can drill down into subproperties, -such as the `width` property in the `resolution` property of a `VideoMode`: - -```swift -print("The width of someVideoMode is \(someVideoMode.resolution.width)") -// Prints "The width of someVideoMode is 0" -``` - - - -You can also use dot syntax to assign a new value to a variable property: - -```swift -someVideoMode.resolution.width = 1280 -print("The width of someVideoMode is now \(someVideoMode.resolution.width)") -// Prints "The width of someVideoMode is now 1280" -``` - - - -### Memberwise Initializers for Structure Types - -All structures have an automatically generated *memberwise initializer*, -which you can use to initialize the member properties of new structure instances. -Initial values for the properties of the new instance -can be passed to the memberwise initializer by name: - -```swift -let vga = Resolution(width: 640, height: 480) -``` - - - -Unlike structures, class instances don't receive a default memberwise initializer. -Initializers are described in more detail in . - - - -## Structures and Enumerations Are Value Types - -A *value type* is a type whose value is *copied* -when it's assigned to a variable or constant, -or when it's passed to a function. - - - -You've actually been using value types extensively throughout the previous chapters. -In fact, all of the basic types in Swift --- -integers, floating-point numbers, Booleans, strings, arrays and dictionaries --- -are value types, and are implemented as structures behind the scenes. - -All structures and enumerations are value types in Swift. -This means that any structure and enumeration instances you create --- -and any value types they have as properties --- -are always copied when they're passed around in your code. - -> Note: Collections defined by the Swift standard library -> like arrays, dictionaries, and strings -> use an optimization to reduce the performance cost of copying. -> Instead of making a copy immediately, -> these collections share the memory where the elements are stored -> between the original instance and any copies. -> If one of the copies of the collection is modified, -> the elements are copied just before the modification. -> The behavior you see in your code -> is always as if a copy took place immediately. - -Consider this example, which uses the `Resolution` structure from the previous example: - -```swift -let hd = Resolution(width: 1920, height: 1080) -var cinema = hd -``` - - - -This example declares a constant called `hd` -and sets it to a `Resolution` instance initialized with -the width and height of full HD video -(1920 pixels wide by 1080 pixels high). - -It then declares a variable called `cinema` -and sets it to the current value of `hd`. -Because `Resolution` is a structure, -a *copy* of the existing instance is made, -and this new copy is assigned to `cinema`. -Even though `hd` and `cinema` now have the same width and height, -they're two completely different instances behind the scenes. - -Next, the `width` property of `cinema` is amended to be -the width of the slightly wider 2K standard used for digital cinema projection -(2048 pixels wide and 1080 pixels high): - -```swift -cinema.width = 2048 -``` - - - -Checking the `width` property of `cinema` -shows that it has indeed changed to be `2048`: - -```swift -print("cinema is now \(cinema.width) pixels wide") -// Prints "cinema is now 2048 pixels wide" -``` - - - -However, the `width` property of the original `hd` instance -still has the old value of `1920`: - -```swift -print("hd is still \(hd.width) pixels wide") -// Prints "hd is still 1920 pixels wide" -``` - - - -When `cinema` was given the current value of `hd`, -the *values* stored in `hd` were copied into the new `cinema` instance. -The end result was two completely separate instances -that contained the same numeric values. -However, because they're separate instances, -setting the width of `cinema` to `2048` -doesn't affect the width stored in `hd`, -as shown in the figure below: - -![](sharedStateStruct) - -The same behavior applies to enumerations: - -```swift -enum CompassPoint { - case north, south, east, west - mutating func turnNorth() { - self = .north - } -} -var currentDirection = CompassPoint.west -let rememberedDirection = currentDirection -currentDirection.turnNorth() - -print("The current direction is \(currentDirection)") -print("The remembered direction is \(rememberedDirection)") -// Prints "The current direction is north" -// Prints "The remembered direction is west" -``` - - - -When `rememberedDirection` is assigned the value of `currentDirection`, -it's actually set to a copy of that value. -Changing the value of `currentDirection` thereafter doesn't affect -the copy of the original value that was stored in `rememberedDirection`. - - - -## Classes Are Reference Types - -Unlike value types, *reference types* are *not* copied -when they're assigned to a variable or constant, -or when they're passed to a function. -Rather than a copy, a reference to the same existing instance is used. - -Here's an example, using the `VideoMode` class defined above: - -```swift -let tenEighty = VideoMode() -tenEighty.resolution = hd -tenEighty.interlaced = true -tenEighty.name = "1080i" -tenEighty.frameRate = 25.0 -``` - - - -This example declares a new constant called `tenEighty` -and sets it to refer to a new instance of the `VideoMode` class. -The video mode is assigned a copy of the HD resolution of `1920` by `1080` from before. -It's set to be interlaced, -its name is set to `"1080i"`, -and its frame rate is set to `25.0` frames per second. - -Next, `tenEighty` is assigned to a new constant, called `alsoTenEighty`, -and the frame rate of `alsoTenEighty` is modified: - -```swift -let alsoTenEighty = tenEighty -alsoTenEighty.frameRate = 30.0 -``` - - - -Because classes are reference types, -`tenEighty` and `alsoTenEighty` actually both refer to the *same* `VideoMode` instance. -Effectively, they're just two different names for the same single instance, -as shown in the figure below: - -![](sharedStateClass) - -Checking the `frameRate` property of `tenEighty` -shows that it correctly reports the new frame rate of `30.0` -from the underlying `VideoMode` instance: - -```swift -print("The frameRate property of tenEighty is now \(tenEighty.frameRate)") -// Prints "The frameRate property of tenEighty is now 30.0" -``` - - - -This example also shows how reference types can be harder to reason about. -If `tenEighty` and `alsoTenEighty` were far apart in your program's code, -it could be difficult to find all the ways that the video mode is changed. -Wherever you use `tenEighty`, -you also have to think about the code that uses `alsoTenEighty`, -and vice versa. -In contrast, value types are easier to reason about -because all of the code that interacts with the same value -is close together in your source files. - -Note that `tenEighty` and `alsoTenEighty` are declared as *constants*, -rather than variables. -However, you can still change `tenEighty.frameRate` and `alsoTenEighty.frameRate` because -the values of the `tenEighty` and `alsoTenEighty` constants themselves don't actually change. -`tenEighty` and `alsoTenEighty` themselves don't “store” the `VideoMode` instance --- -instead, they both *refer* to a `VideoMode` instance behind the scenes. -It's the `frameRate` property of the underlying `VideoMode` that's changed, -not the values of the constant references to that `VideoMode`. - - - - - -### Identity Operators - -Because classes are reference types, -it's possible for multiple constants and variables to refer to -the same single instance of a class behind the scenes. -(The same isn't true for structures and enumerations, -because they're always copied when they're assigned to a constant or variable, -or passed to a function.) - - - - - -It can sometimes be useful to find out whether two constants or variables refer to -exactly the same instance of a class. -To enable this, Swift provides two identity operators: - -- Identical to (`===`) -- Not identical to (`!==`) - -Use these operators to check whether two constants or variables refer to the same single instance: - -```swift -if tenEighty === alsoTenEighty { - print("tenEighty and alsoTenEighty refer to the same VideoMode instance.") -} -// Prints "tenEighty and alsoTenEighty refer to the same VideoMode instance." -``` - - - -Note that *identical to* (represented by three equal signs, or `===`) -doesn't mean the same thing as *equal to* (represented by two equal signs, or `==`). -*Identical to* means that -two constants or variables of class type refer to exactly the same class instance. -*Equal to* means that -two instances are considered equal or equivalent in value, -for some appropriate meaning of *equal*, as defined by the type's designer. - -When you define your own custom structures and classes, -it's your responsibility to decide what qualifies as two instances being equal. -The process of defining your own implementations of the `==` and `!=` operators -is described in . - - - - - - - -### Pointers - -If you have experience with C, C++, or Objective-C, -you may know that these languages use *pointers* to refer to addresses in memory. -A Swift constant or variable that refers to an instance of some reference type -is similar to a pointer in C, -but isn't a direct pointer to an address in memory, -and doesn't require you to write an asterisk (`*`) -to indicate that you are creating a reference. -Instead, these references are defined like any other constant or variable in Swift. -The Swift standard library provides pointer and buffer types -that you can use if you need to interact with pointers directly --- -see [Manual Memory Management](https://developer.apple.com/documentation/swift/swift_standard_library/manual_memory_management). - - - - - - - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/LanguageGuide/Closures.md b/swift-6-beta.docc/LanguageGuide/Closures.md deleted file mode 100644 index e354fdae5..000000000 --- a/swift-6-beta.docc/LanguageGuide/Closures.md +++ /dev/null @@ -1,1350 +0,0 @@ -# Closures - -Group code that executes together, without creating a named function. - -*Closures* are self-contained blocks of functionality -that can be passed around and used in your code. -Closures in Swift are similar -to closures, anonymous functions, lambdas, and blocks -in other programming languages. - -Closures can capture and store references to any constants and variables -from the context in which they're defined. -This is known as *closing over* those constants and variables. -Swift handles all of the memory management of capturing for you. - -> Note: Don't worry if you aren't familiar with the concept of capturing. -> It's explained in detail below in . - -Global and nested functions, as introduced in , -are actually special cases of closures. -Closures take one of three forms: - -- Global functions are closures that have a name - and don't capture any values. -- Nested functions are closures that have a name - and can capture values from their enclosing function. -- Closure expressions are unnamed closures written in a lightweight syntax - that can capture values from their surrounding context. - -Swift's closure expressions have a clean, clear style, -with optimizations that encourage brief, clutter-free syntax in common scenarios. -These optimizations include: - -- Inferring parameter and return value types from context -- Implicit returns from single-expression closures -- Shorthand argument names -- Trailing closure syntax - -## Closure Expressions - -Nested functions, as introduced in , -are a convenient means of naming and defining self-contained blocks of code -as part of a larger function. -However, it's sometimes useful to write shorter versions of function-like constructs -without a full declaration and name. -This is particularly true when you work with functions or methods that take functions -as one or more of their arguments. - -*Closure expressions* are a way to write inline closures in a brief, focused syntax. -Closure expressions provide several syntax optimizations -for writing closures in a shortened form without loss of clarity or intent. -The closure expression examples below illustrate these optimizations -by refining a single example of the `sorted(by:)` method over several iterations, -each of which expresses the same functionality in a more succinct way. - -### The Sorted Method - -Swift's standard library provides a method called `sorted(by:)`, -which sorts an array of values of a known type, -based on the output of a sorting closure that you provide. -Once it completes the sorting process, -the `sorted(by:)` method returns a new array of the same type and size as the old one, -with its elements in the correct sorted order. -The original array isn't modified by the `sorted(by:)` method. - -The closure expression examples below use the `sorted(by:)` method -to sort an array of `String` values in reverse alphabetical order. -Here's the initial array to be sorted: - -```swift -let names = ["Chris", "Alex", "Ewa", "Barry", "Daniella"] -``` - - - -The `sorted(by:)` method accepts a closure that takes two arguments -of the same type as the array's contents, -and returns a `Bool` value to say whether the first value should appear -before or after the second value once the values are sorted. -The sorting closure needs to return `true` -if the first value should appear *before* the second value, -and `false` otherwise. - -This example is sorting an array of `String` values, -and so the sorting closure needs to be a function of type `(String, String) -> Bool`. - -One way to provide the sorting closure is to write a normal function of the correct type, -and to pass it in as an argument to the `sorted(by:)` method: - -```swift -func backward(_ s1: String, _ s2: String) -> Bool { - return s1 > s2 -} -var reversedNames = names.sorted(by: backward) -// reversedNames is equal to ["Ewa", "Daniella", "Chris", "Barry", "Alex"] -``` - - - -If the first string (`s1`) is greater than the second string (`s2`), -the `backward(_:_:)` function will return `true`, -indicating that `s1` should appear before `s2` in the sorted array. -For characters in strings, -“greater than” means “appears later in the alphabet than”. -This means that the letter `"B"` is “greater than” the letter `"A"`, -and the string `"Tom"` is greater than the string `"Tim"`. -This gives a reverse alphabetical sort, -with `"Barry"` being placed before `"Alex"`, and so on. - -However, this is a rather long-winded way to write -what is essentially a single-expression function (`a > b`). -In this example, it would be preferable to write the sorting closure inline, -using closure expression syntax. - -### Closure Expression Syntax - -Closure expression syntax has the following general form: - -```swift -{ (<#parameters#>) -> <#return type#> in - <#statements#> -} -``` - -The *parameters* in closure expression syntax -can be in-out parameters, -but they can't have a default value. -Variadic parameters can be used if you name the variadic parameter. -Tuples can also be used as parameter types and return types. - -The example below shows a closure expression version of the `backward(_:_:)` function -from above: - -```swift -reversedNames = names.sorted(by: { (s1: String, s2: String) -> Bool in - return s1 > s2 -}) -``` - - - -Note that the declaration of parameters and return type for this inline closure -is identical to the declaration from the `backward(_:_:)` function. -In both cases, it's written as `(s1: String, s2: String) -> Bool`. -However, for the inline closure expression, -the parameters and return type are written *inside* the curly braces, -not outside of them. - -The start of the closure's body is introduced by the `in` keyword. -This keyword indicates that -the definition of the closure's parameters and return type has finished, -and the body of the closure is about to begin. - -Because the body of the closure is so short, -it can even be written on a single line: - -```swift -reversedNames = names.sorted(by: { (s1: String, s2: String) -> Bool in return s1 > s2 } ) -``` - - - -This illustrates that the overall call to the `sorted(by:)` method has remained the same. -A pair of parentheses still wrap the entire argument for the method. -However, that argument is now an inline closure. - -### Inferring Type From Context - -Because the sorting closure is passed as an argument to a method, -Swift can infer the types of its parameters -and the type of the value it returns. -The `sorted(by:)` method is being called on an array of strings, -so its argument must be a function of type `(String, String) -> Bool`. -This means that the `(String, String)` and `Bool` types don't need to be written -as part of the closure expression's definition. -Because all of the types can be inferred, -the return arrow (`->`) and the parentheses around the names of the parameters -can also be omitted: - -```swift -reversedNames = names.sorted(by: { s1, s2 in return s1 > s2 } ) -``` - - - -It's always possible to infer the parameter types and return type -when passing a closure to a function or method as an inline closure expression. -As a result, you never need to write an inline closure in its fullest form -when the closure is used as a function or method argument. - -Nonetheless, you can still make the types explicit if you wish, -and doing so is encouraged if it avoids ambiguity for readers of your code. -In the case of the `sorted(by:)` method, -the purpose of the closure is clear from the fact that sorting is taking place, -and it's safe for a reader to assume that -the closure is likely to be working with `String` values, -because it's assisting with the sorting of an array of strings. - -### Implicit Returns from Single-Expression Closures - -Single-expression closures can implicitly return the result of their single expression -by omitting the `return` keyword from their declaration, -as in this version of the previous example: - -```swift -reversedNames = names.sorted(by: { s1, s2 in s1 > s2 } ) -``` - - - -Here, the function type of the `sorted(by:)` method's argument -makes it clear that a `Bool` value must be returned by the closure. -Because the closure's body contains a single expression (`s1 > s2`) -that returns a `Bool` value, -there's no ambiguity, and the `return` keyword can be omitted. - -### Shorthand Argument Names - -Swift automatically provides shorthand argument names to inline closures, -which can be used to refer to the values of the closure's arguments -by the names `$0`, `$1`, `$2`, and so on. - -If you use these shorthand argument names within your closure expression, -you can omit the closure's argument list from its definition. -The type of the shorthand argument names -is inferred from the expected function type, -and the highest numbered shorthand argument you use -determines the number of arguments that the closure takes. -The `in` keyword can also be omitted, -because the closure expression is made up entirely of its body: - -```swift -reversedNames = names.sorted(by: { $0 > $1 } ) -``` - - - -Here, `$0` and `$1` refer to the closure's first and second `String` arguments. -Because `$1` is the shorthand argument with highest number, -the closure is understood to take two arguments. -Because the `sorted(by:)` function here expects a closure -whose arguments are both strings, -the shorthand arguments `$0` and `$1` are both of type `String`. - - - -### Operator Methods - -There's actually an even *shorter* way to write the closure expression above. -Swift's `String` type defines its string-specific implementation of -the greater-than operator (`>`) -as a method that has two parameters of type `String`, -and returns a value of type `Bool`. -This exactly matches the method type needed by the `sorted(by:)` method. -Therefore, you can simply pass in the greater-than operator, -and Swift will infer that you want to use its string-specific implementation: - -```swift -reversedNames = names.sorted(by: >) -``` - - - -For more about operator methods, see . - -## Trailing Closures - -If you need to pass a closure expression to a function as the function's final argument -and the closure expression is long, -it can be useful to write it as a *trailing closure* instead. -You write a trailing closure after the function call's parentheses, -even though the trailing closure is still an argument to the function. -When you use the trailing closure syntax, -you don't write the argument label for the first closure -as part of the function call. -A function call can include multiple trailing closures; -however, the first few examples below use a single trailing closure. - -```swift -func someFunctionThatTakesAClosure(closure: () -> Void) { - // function body goes here -} - -// Here's how you call this function without using a trailing closure: - -someFunctionThatTakesAClosure(closure: { - // closure's body goes here -}) - -// Here's how you call this function with a trailing closure instead: - -someFunctionThatTakesAClosure() { - // trailing closure's body goes here -} -``` - - - -The string-sorting closure from the section above -can be written outside of the `sorted(by:)` method's parentheses as a trailing closure: - -```swift -reversedNames = names.sorted() { $0 > $1 } -``` - - - -If a closure expression is provided as the function's or method's only argument -and you provide that expression as a trailing closure, -you don't need to write a pair of parentheses `()` -after the function or method's name when you call the function: - -```swift -reversedNames = names.sorted { $0 > $1 } -``` - - - -Trailing closures are most useful when the closure is sufficiently long that -it isn't possible to write it inline on a single line. -As an example, Swift's `Array` type has a `map(_:)` method, -which takes a closure expression as its single argument. -The closure is called once for each item in the array, -and returns an alternative mapped value (possibly of some other type) for that item. -You specify -the nature of the mapping and the type of the returned value -by writing code in the closure that you pass to `map(_:)`. - -After applying the provided closure to each array element, -the `map(_:)` method returns a new array containing all of the new mapped values, -in the same order as their corresponding values in the original array. - -Here's how you can use the `map(_:)` method with a trailing closure -to convert an array of `Int` values into an array of `String` values. -The array `[16, 58, 510]` is used to create the new array -`["OneSix", "FiveEight", "FiveOneZero"]`: - -```swift -let digitNames = [ - 0: "Zero", 1: "One", 2: "Two", 3: "Three", 4: "Four", - 5: "Five", 6: "Six", 7: "Seven", 8: "Eight", 9: "Nine" -] -let numbers = [16, 58, 510] -``` - - - -The code above creates a dictionary of mappings between -the integer digits and English-language versions of their names. -It also defines an array of integers, ready to be converted into strings. - -You can now use the `numbers` array to create an array of `String` values, -by passing a closure expression to the array's `map(_:)` method as a trailing closure: - -```swift -let strings = numbers.map { (number) -> String in - var number = number - var output = "" - repeat { - output = digitNames[number % 10]! + output - number /= 10 - } while number > 0 - return output -} -// strings is inferred to be of type [String] -// its value is ["OneSix", "FiveEight", "FiveOneZero"] -``` - - - -The `map(_:)` method calls the closure expression once for each item in the array. -You don't need to specify the type of the closure's input parameter, `number`, -because the type can be inferred from the values in the array to be mapped. - -In this example, -the variable `number` is initialized with the value of the closure's `number` parameter, -so that the value can be modified within the closure body. -(The parameters to functions and closures are always constants.) -The closure expression also specifies a return type of `String`, -to indicate the type that will be stored in the mapped output array. - -The closure expression builds a string called `output` each time it's called. -It calculates the last digit of `number` by using the remainder operator (`number % 10`), -and uses this digit to look up an appropriate string in the `digitNames` dictionary. -The closure can be used to create a string representation of any integer greater than zero. - -> Note: The call to the `digitNames` dictionary's subscript -> is followed by an exclamation point (`!`), -> because dictionary subscripts return an optional value -> to indicate that the dictionary lookup can fail if the key doesn't exist. -> In the example above, it's guaranteed that `number % 10` -> will always be a valid subscript key for the `digitNames` dictionary, -> and so an exclamation point is used to force-unwrap the `String` value -> stored in the subscript's optional return value. - -The string retrieved from the `digitNames` dictionary -is added to the *front* of `output`, -effectively building a string version of the number in reverse. -(The expression `number % 10` gives a value of -`6` for `16`, `8` for `58`, and `0` for `510`.) - -The `number` variable is then divided by `10`. -Because it's an integer, it's rounded down during the division, -so `16` becomes `1`, `58` becomes `5`, and `510` becomes `51`. - -The process is repeated until `number` is equal to `0`, -at which point the `output` string is returned by the closure, -and is added to the output array by the `map(_:)` method. - -The use of trailing closure syntax in the example above -neatly encapsulates the closure's functionality -immediately after the function that closure supports, -without needing to wrap the entire closure within -the `map(_:)` method's outer parentheses. - -If a function takes multiple closures, -you omit the argument label for the first trailing closure -and you label the remaining trailing closures. -For example, -the function below loads a picture for a photo gallery: - -```swift -func loadPicture(from server: Server, completion: (Picture) -> Void, onFailure: () -> Void) { - if let picture = download("photo.jpg", from: server) { - completion(picture) - } else { - onFailure() - } -} -``` - - - -When you call this function to load a picture, -you provide two closures. -The first closure is a completion handler -that displays a picture after a successful download. -The second closure is an error handler -that displays an error to the user. - -```swift -loadPicture(from: someServer) { picture in - someView.currentPicture = picture -} onFailure: { - print("Couldn't download the next picture.") -} -``` - - - -In this example, -the `loadPicture(from:completion:onFailure:)` function -dispatches its network task into the background, -and calls one of the two completion handlers when the network task finishes. -Writing the function this way lets you cleanly separate the code -that's responsible for handling a network failure -from the code that updates the user interface after a successful download, -instead of using just one closure that handles both circumstances. - -> Note: Completion handlers can become hard to read, -> especially when you have to nest multiple handlers. -> An alternate approach is to use asynchronous code, -> as described in . - -## Capturing Values - -A closure can *capture* constants and variables -from the surrounding context in which it's defined. -The closure can then refer to and modify -the values of those constants and variables from within its body, -even if the original scope that defined the constants and variables no longer exists. - -In Swift, the simplest form of a closure that can capture values is a nested function, -written within the body of another function. -A nested function can capture any of its outer function's arguments -and can also capture any constants and variables defined within the outer function. - -Here's an example of a function called `makeIncrementer`, -which contains a nested function called `incrementer`. -The nested `incrementer()` function captures two values, -`runningTotal` and `amount`, -from its surrounding context. -After capturing these values, -`incrementer` is returned by `makeIncrementer` as a closure -that increments `runningTotal` by `amount` each time it's called. - -```swift -func makeIncrementer(forIncrement amount: Int) -> () -> Int { - var runningTotal = 0 - func incrementer() -> Int { - runningTotal += amount - return runningTotal - } - return incrementer -} -``` - - - -The return type of `makeIncrementer` is `() -> Int`. -This means that it returns a *function*, rather than a simple value. -The function it returns has no parameters, -and returns an `Int` value each time it's called. -To learn how functions can return other functions, -see . - -The `makeIncrementer(forIncrement:)` function defines an integer variable called `runningTotal`, -to store the current running total of the incrementer that will be returned. -This variable is initialized with a value of `0`. - -The `makeIncrementer(forIncrement:)` function has a single `Int` parameter -with an argument label of `forIncrement`, and a parameter name of `amount`. -The argument value passed to this parameter specifies -how much `runningTotal` should be incremented by -each time the returned incrementer function is called. -The `makeIncrementer` function defines a nested function called `incrementer`, -which performs the actual incrementing. -This function simply adds `amount` to `runningTotal`, and returns the result. - -When considered in isolation, -the nested `incrementer()` function might seem unusual: - -```swift -func incrementer() -> Int { - runningTotal += amount - return runningTotal -} -``` - - - -The `incrementer()` function doesn't have any parameters, -and yet it refers to `runningTotal` and `amount` from within its function body. -It does this by capturing a *reference* to `runningTotal` and `amount` -from the surrounding function and using them within its own function body. -Capturing by reference ensures that `runningTotal` and `amount` don't disappear -when the call to `makeIncrementer` ends, -and also ensures that `runningTotal` is available -the next time the `incrementer` function is called. - -> Note: As an optimization, -> Swift may instead capture and store a *copy* of a value -> if that value isn't mutated by a closure, -> and if the value isn't mutated after the closure is created. -> -> Swift also handles all memory management involved in disposing of -> variables when they're no longer needed. - -Here's an example of `makeIncrementer` in action: - -```swift -let incrementByTen = makeIncrementer(forIncrement: 10) -``` - - - -This example sets a constant called `incrementByTen` -to refer to an incrementer function that adds `10` to -its `runningTotal` variable each time it's called. -Calling the function multiple times shows this behavior in action: - -```swift -incrementByTen() -// returns a value of 10 -incrementByTen() -// returns a value of 20 -incrementByTen() -// returns a value of 30 -``` - - - - - -If you create a second incrementer, -it will have its own stored reference to a new, separate `runningTotal` variable: - -```swift -let incrementBySeven = makeIncrementer(forIncrement: 7) -incrementBySeven() -// returns a value of 7 -``` - - - -Calling the original incrementer (`incrementByTen`) again -continues to increment its own `runningTotal` variable, -and doesn't affect the variable captured by `incrementBySeven`: - -```swift -incrementByTen() -// returns a value of 40 -``` - - - -> Note: If you assign a closure to a property of a class instance, -> and the closure captures that instance by referring to the instance or its members, -> you will create a strong reference cycle between the closure and the instance. -> Swift uses *capture lists* to break these strong reference cycles. -> For more information, see . - -## Closures Are Reference Types - -In the example above, -`incrementBySeven` and `incrementByTen` are constants, -but the closures these constants refer to are still able to increment -the `runningTotal` variables that they have captured. -This is because functions and closures are *reference types*. - -Whenever you assign a function or a closure to a constant or a variable, -you are actually setting that constant or variable to be -a *reference* to the function or closure. -In the example above, -it's the choice of closure that `incrementByTen` *refers to* that's constant, -and not the contents of the closure itself. - -This also means that if you assign a closure to two different constants or variables, -both of those constants or variables refer to the same closure. - -```swift -let alsoIncrementByTen = incrementByTen -alsoIncrementByTen() -// returns a value of 50 - -incrementByTen() -// returns a value of 60 -``` - - - -The example above shows that calling `alsoIncrementByTen` -is the same as calling `incrementByTen`. -Because both of them refer to the same closure, -they both increment and return the same running total. - -## Escaping Closures - -A closure is said to *escape* a function -when the closure is passed as an argument to the function, -but is called after the function returns. -When you declare a function that takes a closure as one of its parameters, -you can write `@escaping` before the parameter's type -to indicate that the closure is allowed to escape. - -One way that a closure can escape -is by being stored in a variable that's defined outside the function. -As an example, -many functions that start an asynchronous operation -take a closure argument as a completion handler. -The function returns after it starts the operation, -but the closure isn't called until the operation is completed --- -the closure needs to escape, to be called later. -For example: - -```swift -var completionHandlers: [() -> Void] = [] -func someFunctionWithEscapingClosure(completionHandler: @escaping () -> Void) { - completionHandlers.append(completionHandler) -} -``` - - - -The `someFunctionWithEscapingClosure(_:)` function takes a closure as its argument -and adds it to an array that's declared outside the function. -If you didn't mark the parameter of this function with `@escaping`, -you would get a compile-time error. - -An escaping closure that refers to `self` -needs special consideration if `self` refers to an instance of a class. -Capturing `self` in an escaping closure -makes it easy to accidentally create a strong reference cycle. -For information about reference cycles, -see . - -Normally, a closure captures variables implicitly -by using them in the body of the closure, -but in this case you need to be explicit. -If you want to capture `self`, -write `self` explicitly when you use it, -or include `self` in the closure's capture list. -Writing `self` explicitly lets you express your intent, -and reminds you to confirm that there isn't a reference cycle. -For example, in the code below, -the closure passed to `someFunctionWithEscapingClosure(_:)` -refers to `self` explicitly. -In contrast, the closure passed to `someFunctionWithNonescapingClosure(_:)` -is a nonescaping closure, which means it can refer to `self` implicitly. - -```swift -func someFunctionWithNonescapingClosure(closure: () -> Void) { - closure() -} - -class SomeClass { - var x = 10 - func doSomething() { - someFunctionWithEscapingClosure { self.x = 100 } - someFunctionWithNonescapingClosure { x = 200 } - } -} - -let instance = SomeClass() -instance.doSomething() -print(instance.x) -// Prints "200" - -completionHandlers.first?() -print(instance.x) -// Prints "100" -``` - - - -Here's a version of `doSomething()` that captures `self` -by including it in the closure's capture list, -and then refers to `self` implicitly: - -```swift -class SomeOtherClass { - var x = 10 - func doSomething() { - someFunctionWithEscapingClosure { [self] in x = 100 } - someFunctionWithNonescapingClosure { x = 200 } - } -} -``` - - - -If `self` is an instance of a structure or an enumeration, -you can always refer to `self` implicitly. -However, -an escaping closure can't capture a mutable reference to `self` -when `self` is an instance of a structure or an enumeration. -Structures and enumerations don’t allow shared mutability, -as discussed in . - -```swift -struct SomeStruct { - var x = 10 - mutating func doSomething() { - someFunctionWithNonescapingClosure { x = 200 } // Ok - someFunctionWithEscapingClosure { x = 100 } // Error - } -} -``` - - - -The call to the `someFunctionWithEscapingClosure` function -in the example above is an error -because it's inside a mutating method, -so `self` is mutable. -That violates the rule that escaping closures can't capture -a mutable reference to `self` for structures. - - - - - -## Autoclosures - -An *autoclosure* is a closure that's automatically created -to wrap an expression that's being passed as an argument to a function. -It doesn't take any arguments, -and when it's called, it returns the value -of the expression that's wrapped inside of it. -This syntactic convenience lets you omit braces around a function's parameter -by writing a normal expression instead of an explicit closure. - -It's common to *call* functions that take autoclosures, -but it's not common to *implement* that kind of function. -For example, -the `assert(condition:message:file:line:)` function -takes an autoclosure for its `condition` and `message` parameters; -its `condition` parameter is evaluated only in debug builds -and its `message` parameter is evaluated only if `condition` is `false`. - -An autoclosure lets you delay evaluation, -because the code inside isn't run until you call the closure. -Delaying evaluation is useful for code -that has side effects or is computationally expensive, -because it lets you control when that code is evaluated. -The code below shows how a closure delays evaluation. - -```swift -var customersInLine = ["Chris", "Alex", "Ewa", "Barry", "Daniella"] -print(customersInLine.count) -// Prints "5" - -let customerProvider = { customersInLine.remove(at: 0) } -print(customersInLine.count) -// Prints "5" - -print("Now serving \(customerProvider())!") -// Prints "Now serving Chris!" -print(customersInLine.count) -// Prints "4" -``` - - - - - - - -Even though the first element of the `customersInLine` array is removed -by the code inside the closure, -the array element isn't removed until the closure is actually called. -If the closure is never called, -the expression inside the closure is never evaluated, -which means the array element is never removed. -Note that the type of `customerProvider` isn't `String` -but `() -> String` --- -a function with no parameters that returns a string. - -You get the same behavior of delayed evaluation -when you pass a closure as an argument to a function. - -```swift -// customersInLine is ["Alex", "Ewa", "Barry", "Daniella"] -func serve(customer customerProvider: () -> String) { - print("Now serving \(customerProvider())!") -} -serve(customer: { customersInLine.remove(at: 0) } ) -// Prints "Now serving Alex!" -``` - - - -The `serve(customer:)` function in the listing above -takes an explicit closure that returns a customer's name. -The version of `serve(customer:)` below -performs the same operation but, instead of taking an explicit closure, -it takes an autoclosure -by marking its parameter's type with the `@autoclosure` attribute. -Now you can call the function -as if it took a `String` argument instead of a closure. -The argument is automatically converted to a closure, -because the `customerProvider` parameter's type is marked -with the `@autoclosure` attribute. - -```swift -// customersInLine is ["Ewa", "Barry", "Daniella"] -func serve(customer customerProvider: @autoclosure () -> String) { - print("Now serving \(customerProvider())!") -} -serve(customer: customersInLine.remove(at: 0)) -// Prints "Now serving Ewa!" -``` - - - -> Note: Overusing autoclosures can make your code hard to understand. -> The context and function name should make it clear -> that evaluation is being deferred. - -If you want an autoclosure that's allowed to escape, -use both the `@autoclosure` and `@escaping` attributes. -The `@escaping` attribute is described above in . - -```swift -// customersInLine is ["Barry", "Daniella"] -var customerProviders: [() -> String] = [] -func collectCustomerProviders(_ customerProvider: @autoclosure @escaping () -> String) { - customerProviders.append(customerProvider) -} -collectCustomerProviders(customersInLine.remove(at: 0)) -collectCustomerProviders(customersInLine.remove(at: 0)) - -print("Collected \(customerProviders.count) closures.") -// Prints "Collected 2 closures." -for customerProvider in customerProviders { - print("Now serving \(customerProvider())!") -} -// Prints "Now serving Barry!" -// Prints "Now serving Daniella!" -``` - - - -In the code above, -instead of calling the closure passed to it -as its `customerProvider` argument, -the `collectCustomerProviders(_:)` function -appends the closure to the `customerProviders` array. -The array is declared outside the scope of the function, -which means the closures in the array can be executed after the function returns. -As a result, -the value of the `customerProvider` argument -must be allowed to escape the function's scope. - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/LanguageGuide/CollectionTypes.md b/swift-6-beta.docc/LanguageGuide/CollectionTypes.md deleted file mode 100644 index 3621dd1d1..000000000 --- a/swift-6-beta.docc/LanguageGuide/CollectionTypes.md +++ /dev/null @@ -1,1486 +0,0 @@ -# Collection Types - -Organize data using arrays, sets, and dictionaries. - -Swift provides three primary *collection types*, -known as arrays, sets, and dictionaries, -for storing collections of values. -Arrays are ordered collections of values. -Sets are unordered collections of unique values. -Dictionaries are unordered collections of key-value associations. - -![](CollectionTypes_intro) - -Arrays, sets, and dictionaries in Swift are always clear about -the types of values and keys that they can store. -This means that you can't insert a value of the wrong type -into a collection by mistake. -It also means you can be confident about the type of values -you will retrieve from a collection. - -> Note: Swift's array, set, and dictionary types are implemented as *generic collections*. -> For more about generic types and collections, see . - - - - - - - -## Mutability of Collections - -If you create an array, a set, or a dictionary, and assign it to a variable, -the collection that's created will be *mutable*. -This means that you can change (or *mutate*) the collection after it's created -by adding, removing, or changing items in the collection. -If you assign an array, a set, or a dictionary to a constant, -that collection is *immutable*, -and its size and contents can't be changed. - -> Note: It's good practice to create immutable collections -> in all cases where the collection doesn't need to change. -> Doing so makes it easier for you to reason about your code -> and enables the Swift compiler to optimize the performance of -> the collections you create. - -## Arrays - -An *array* stores values of the same type in an ordered list. -The same value can appear in an array multiple times at different positions. - -> Note: Swift's `Array` type is bridged to Foundation's `NSArray` class. -> -> For more information about using `Array` with Foundation and Cocoa, -> see [Bridging Between Array and NSArray](https://developer.apple.com/documentation/swift/array#2846730). - -### Array Type Shorthand Syntax - -The type of a Swift array is written in full as `Array`, -where `Element` is the type of values the array is allowed to store. -You can also write the type of an array in shorthand form as `[Element]`. -Although the two forms are functionally identical, -the shorthand form is preferred -and is used throughout this guide when referring to the type of an array. - -### Creating an Empty Array - -You can create an empty array of a certain type -using initializer syntax: - -```swift -var someInts: [Int] = [] -print("someInts is of type [Int] with \(someInts.count) items.") -// Prints "someInts is of type [Int] with 0 items." -``` - - - -Note that the type of the `someInts` variable is inferred to be `[Int]` -from the type of the initializer. - -Alternatively, if the context already provides type information, -such as a function argument or an already typed variable or constant, -you can create an empty array with an empty array literal, -which is written as `[]` -(an empty pair of square brackets): - -```swift -someInts.append(3) -// someInts now contains 1 value of type Int -someInts = [] -// someInts is now an empty array, but is still of type [Int] -``` - - - -### Creating an Array with a Default Value - -Swift's `Array` type also provides -an initializer for creating an array of a certain size -with all of its values set to the same default value. -You pass this initializer -a default value of the appropriate type (called `repeating`): -and the number of times that value is repeated in the new array (called `count`): - -```swift -var threeDoubles = Array(repeating: 0.0, count: 3) -// threeDoubles is of type [Double], and equals [0.0, 0.0, 0.0] -``` - - - -### Creating an Array by Adding Two Arrays Together - -You can create a new array by adding together two existing arrays with compatible types -with the addition operator (`+`). -The new array's type is inferred from the type of the two arrays you add together: - -```swift -var anotherThreeDoubles = Array(repeating: 2.5, count: 3) -// anotherThreeDoubles is of type [Double], and equals [2.5, 2.5, 2.5] - -var sixDoubles = threeDoubles + anotherThreeDoubles -// sixDoubles is inferred as [Double], and equals [0.0, 0.0, 0.0, 2.5, 2.5, 2.5] -``` - - - - - - - -### Creating an Array with an Array Literal - -You can also initialize an array with an *array literal*, -which is a shorthand way to write one or more values as an array collection. -An array literal is written as a list of values, separated by commas, -surrounded by a pair of square brackets: - -```swift -[<#value 1#>, <#value 2#>, <#value 3#>] -``` - -The example below creates an array called `shoppingList` to store `String` values: - -```swift -var shoppingList: [String] = ["Eggs", "Milk"] -// shoppingList has been initialized with two initial items -``` - - - -The `shoppingList` variable is declared as -“an array of string values”, written as `[String]`. -Because this particular array has specified a value type of `String`, -it's allowed to store `String` values only. -Here, the `shoppingList` array is initialized with two `String` values -(`"Eggs"` and `"Milk"`), written within an array literal. - -> Note: The `shoppingList` array is declared as a variable (with the `var` introducer) -> and not a constant (with the `let` introducer) -> because more items are added to the shopping list in the examples below. - -In this case, the array literal contains two `String` values and nothing else. -This matches the type of the `shoppingList` variable's declaration -(an array that can only contain `String` values), -and so the assignment of the array literal is permitted -as a way to initialize `shoppingList` with two initial items. - -Thanks to Swift's type inference, -you don't have to write the type of the array -if you're initializing it with an array literal containing values of the same type. -The initialization of `shoppingList` could have been written in a shorter form instead: - -```swift -var shoppingList = ["Eggs", "Milk"] -``` - - - -Because all values in the array literal are of the same type, -Swift can infer that `[String]` is -the correct type to use for the `shoppingList` variable. - -### Accessing and Modifying an Array - -You access and modify an array through its methods and properties, -or by using subscript syntax. - -To find out the number of items in an array, check its read-only `count` property: - -```swift -print("The shopping list contains \(shoppingList.count) items.") -// Prints "The shopping list contains 2 items." -``` - - - -Use the Boolean `isEmpty` property -as a shortcut for checking whether the `count` property is equal to `0`: - -```swift -if shoppingList.isEmpty { - print("The shopping list is empty.") -} else { - print("The shopping list isn't empty.") -} -// Prints "The shopping list isn't empty." -``` - - - -You can add a new item to the end of an array by calling the array's `append(_:)` method: - -```swift -shoppingList.append("Flour") -// shoppingList now contains 3 items, and someone is making pancakes -``` - - - -Alternatively, append an array of one or more compatible items -with the addition assignment operator (`+=`): - -```swift -shoppingList += ["Baking Powder"] -// shoppingList now contains 4 items -shoppingList += ["Chocolate Spread", "Cheese", "Butter"] -// shoppingList now contains 7 items -``` - - - -Retrieve a value from the array by using *subscript syntax*, -passing the index of the value you want to retrieve within square brackets -immediately after the name of the array: - -```swift -var firstItem = shoppingList[0] -// firstItem is equal to "Eggs" -``` - - - -> Note: The first item in the array has an index of `0`, not `1`. -> Arrays in Swift are always zero-indexed. - -You can use subscript syntax to change an existing value at a given index: - -```swift -shoppingList[0] = "Six eggs" -// the first item in the list is now equal to "Six eggs" rather than "Eggs" -``` - - - -When you use subscript syntax, -the index you specify needs to be valid. -For example, writing `shoppingList[shoppingList.count] = "Salt"` -to try to append an item to the end of the array -results in a runtime error. - - - -You can also use subscript syntax to change a range of values at once, -even if the replacement set of values has a different length than the range you are replacing. -The following example replaces `"Chocolate Spread"`, `"Cheese"`, and `"Butter"` -with `"Bananas"` and `"Apples"`: - -```swift -shoppingList[4...6] = ["Bananas", "Apples"] -// shoppingList now contains 6 items -``` - - - -To insert an item into the array at a specified index, -call the array's `insert(_:at:)` method: - -```swift -shoppingList.insert("Maple Syrup", at: 0) -// shoppingList now contains 7 items -// "Maple Syrup" is now the first item in the list -``` - - - -This call to the `insert(_:at:)` method inserts a new item with a value of `"Maple Syrup"` -at the very beginning of the shopping list, -indicated by an index of `0`. - -Similarly, you remove an item from the array with the `remove(at:)` method. -This method removes the item at the specified index and returns the removed item -(although you can ignore the returned value if you don't need it): - -```swift -let mapleSyrup = shoppingList.remove(at: 0) -// the item that was at index 0 has just been removed -// shoppingList now contains 6 items, and no Maple Syrup -// the mapleSyrup constant is now equal to the removed "Maple Syrup" string -``` - - - -> Note: If you try to access or modify a value for an index -> that's outside of an array's existing bounds, -> you will trigger a runtime error. -> You can check that an index is valid before using it -> by comparing it to the array's `count` property. -> The largest valid index in an array is `count - 1` -> because arrays are indexed from zero --- -> however, when `count` is `0` (meaning the array is empty), -> there are no valid indexes. - -Any gaps in an array are closed when an item is removed, -and so the value at index `0` is once again equal to `"Six eggs"`: - -```swift -firstItem = shoppingList[0] -// firstItem is now equal to "Six eggs" -``` - - - -If you want to remove the final item from an array, -use the `removeLast()` method rather than the `remove(at:)` method -to avoid the need to query the array's `count` property. -Like the `remove(at:)` method, `removeLast()` returns the removed item: - -```swift -let apples = shoppingList.removeLast() -// the last item in the array has just been removed -// shoppingList now contains 5 items, and no apples -// the apples constant is now equal to the removed "Apples" string -``` - - - -### Iterating Over an Array - -You can iterate over the entire set of values in an array with the `for`-`in` loop: - -```swift -for item in shoppingList { - print(item) -} -// Six eggs -// Milk -// Flour -// Baking Powder -// Bananas -``` - - - -If you need the integer index of each item as well as its value, -use the `enumerated()` method to iterate over the array instead. -For each item in the array, -the `enumerated()` method returns a tuple -composed of an integer and the item. -The integers start at zero and count up by one for each item; -if you enumerate over a whole array, -these integers match the items' indices. -You can decompose the tuple into temporary constants or variables -as part of the iteration: - -```swift -for (index, value) in shoppingList.enumerated() { - print("Item \(index + 1): \(value)") -} -// Item 1: Six eggs -// Item 2: Milk -// Item 3: Flour -// Item 4: Baking Powder -// Item 5: Bananas -``` - - - -For more about the `for`-`in` loop, see . - -## Sets - -A *set* stores distinct values of the same type -in a collection with no defined ordering. -You can use a set instead of an array when the order of items isn't important, -or when you need to ensure that an item only appears once. - -> Note: Swift's `Set` type is bridged to Foundation's `NSSet` class. -> -> For more information about using `Set` with Foundation and Cocoa, -> see [Bridging Between Set and NSSet](https://developer.apple.com/documentation/swift/set#2845530). - - - -### Hash Values for Set Types - -A type must be *hashable* in order to be stored in a set --- -that is, the type must provide a way to compute a *hash value* for itself. -A hash value is an `Int` value that's the same for all objects that compare equally, -such that if `a == b`, -the hash value of `a` is equal to the hash value of `b`. - -All of Swift's basic types (such as `String`, `Int`, `Double`, and `Bool`) -are hashable by default, and can be used as set value types or dictionary key types. -Enumeration case values without associated values -(as described in ) -are also hashable by default. - -> Note: You can use your own custom types as set value types or dictionary key types -> by making them conform to the `Hashable` protocol -> from the Swift standard library. -> For information about implementing the required `hash(into:)` method, -> see [`Hashable`](https://developer.apple.com/documentation/swift/hashable). -> For information about conforming to protocols, see . - -### Set Type Syntax - -The type of a Swift set is written as `Set`, -where `Element` is the type that the set is allowed to store. -Unlike arrays, sets don't have an equivalent shorthand form. - -### Creating and Initializing an Empty Set - -You can create an empty set of a certain type -using initializer syntax: - -```swift -var letters = Set() -print("letters is of type Set with \(letters.count) items.") -// Prints "letters is of type Set with 0 items." -``` - - - -> Note: The type of the `letters` variable is inferred to be `Set`, -> from the type of the initializer. - -Alternatively, if the context already provides type information, -such as a function argument or an already typed variable or constant, -you can create an empty set with an empty array literal: - -```swift -letters.insert("a") -// letters now contains 1 value of type Character -letters = [] -// letters is now an empty set, but is still of type Set -``` - - - -### Creating a Set with an Array Literal - -You can also initialize a set with an array literal, -as a shorthand way to write one or more values as a set collection. - -The example below creates a set called `favoriteGenres` to store `String` values: - -```swift -var favoriteGenres: Set = ["Rock", "Classical", "Hip hop"] -// favoriteGenres has been initialized with three initial items -``` - - - -The `favoriteGenres` variable is declared as -“a set of `String` values”, written as `Set`. -Because this particular set has specified a value type of `String`, -it's *only* allowed to store `String` values. -Here, the `favoriteGenres` set is initialized with three `String` values -(`"Rock"`, `"Classical"`, and `"Hip hop"`), written within an array literal. - -> Note: The `favoriteGenres` set is declared as a variable (with the `var` introducer) -> and not a constant (with the `let` introducer) -> because items are added and removed in the examples below. - -A set type can't be inferred from an array literal alone, -so the type `Set` must be explicitly declared. -However, because of Swift's type inference, -you don't have to write the type of the set's elements -if you're initializing it with an array literal -that contains values of just one type. -The initialization of `favoriteGenres` could have been written in a shorter form instead: - -```swift -var favoriteGenres: Set = ["Rock", "Classical", "Hip hop"] -``` - - - -Because all values in the array literal are of the same type, -Swift can infer that `Set` is -the correct type to use for the `favoriteGenres` variable. - -### Accessing and Modifying a Set - -You access and modify a set through its methods and properties. - -To find out the number of items in a set, -check its read-only `count` property: - -```swift -print("I have \(favoriteGenres.count) favorite music genres.") -// Prints "I have 3 favorite music genres." -``` - - - -Use the Boolean `isEmpty` property -as a shortcut for checking whether the `count` property is equal to `0`: - -```swift -if favoriteGenres.isEmpty { - print("As far as music goes, I'm not picky.") -} else { - print("I have particular music preferences.") -} -// Prints "I have particular music preferences." -``` - - - -You can add a new item into a set by calling the set's `insert(_:)` method: - -```swift -favoriteGenres.insert("Jazz") -// favoriteGenres now contains 4 items -``` - - - -You can remove an item from a set by calling the set's `remove(_:)` method, -which removes the item if it's a member of the set, -and returns the removed value, -or returns `nil` if the set didn't contain it. -Alternatively, all items in a set can be removed with its `removeAll()` method. - -```swift -if let removedGenre = favoriteGenres.remove("Rock") { - print("\(removedGenre)? I'm over it.") -} else { - print("I never much cared for that.") -} -// Prints "Rock? I'm over it." -``` - - - -To check whether a set contains a particular item, use the `contains(_:)` method. - -```swift -if favoriteGenres.contains("Funk") { - print("I get up on the good foot.") -} else { - print("It's too funky in here.") -} -// Prints "It's too funky in here." -``` - - - -### Iterating Over a Set - -You can iterate over the values in a set with a `for`-`in` loop. - -```swift -for genre in favoriteGenres { - print("\(genre)") -} -// Classical -// Jazz -// Hip hop -``` - - - -For more about the `for`-`in` loop, see . - -Swift's `Set` type doesn't have a defined ordering. -To iterate over the values of a set in a specific order, -use the `sorted()` method, -which returns the set's elements as an array -sorted using the `<` operator. - -```swift -for genre in favoriteGenres.sorted() { - print("\(genre)") -} -// Classical -// Hip hop -// Jazz -``` - - - -## Performing Set Operations - -You can efficiently perform fundamental set operations, -such as combining two sets together, -determining which values two sets have in common, -or determining whether two sets contain all, some, or none of the same values. - -### Fundamental Set Operations - -The illustration below depicts two sets --- `a` and `b` --- -with the results of various set operations represented by the shaded regions. - -![](setVennDiagram) - -- Use the `intersection(_:)` method to create a new set with only the values common to both sets. -- Use the `symmetricDifference(_:)` method to create a new set with values in either set, but not both. -- Use the `union(_:)` method to create a new set with all of the values in both sets. -- Use the `subtracting(_:)` method to create a new set with values not in the specified set. - -```swift -let oddDigits: Set = [1, 3, 5, 7, 9] -let evenDigits: Set = [0, 2, 4, 6, 8] -let singleDigitPrimeNumbers: Set = [2, 3, 5, 7] - -oddDigits.union(evenDigits).sorted() -// [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] -oddDigits.intersection(evenDigits).sorted() -// [] -oddDigits.subtracting(singleDigitPrimeNumbers).sorted() -// [1, 9] -oddDigits.symmetricDifference(singleDigitPrimeNumbers).sorted() -// [1, 2, 9] -``` - - - - - -### Set Membership and Equality - -The illustration below depicts three sets --- `a`, `b` and `c` --- -with overlapping regions representing elements shared among sets. -Set `a` is a *superset* of set `b`, -because `a` contains all elements in `b`. -Conversely, set `b` is a *subset* of set `a`, -because all elements in `b` are also contained by `a`. -Set `b` and set `c` are *disjoint* with one another, -because they share no elements in common. - -![](setEulerDiagram) - -- Use the “is equal” operator (`==`) to determine whether two sets contain all of the same values. -- Use the `isSubset(of:)` method to determine whether all of the values of a set are contained in the specified set. -- Use the `isSuperset(of:)` method to determine whether a set contains all of the values in a specified set. -- Use the `isStrictSubset(of:)` or `isStrictSuperset(of:)` methods to determine whether a set is a subset or superset, but not equal to, a specified set. -- Use the `isDisjoint(with:)` method to determine whether two sets have no values in common. - -```swift -let houseAnimals: Set = ["🐶", "🐱"] -let farmAnimals: Set = ["🐮", "🐔", "🐑", "🐶", "🐱"] -let cityAnimals: Set = ["🐦", "🐭"] - -houseAnimals.isSubset(of: farmAnimals) -// true -farmAnimals.isSuperset(of: houseAnimals) -// true -farmAnimals.isDisjoint(with: cityAnimals) -// true -``` - - - - - -## Dictionaries - -A *dictionary* stores associations between -keys of the same type and values of the same type -in a collection with no defined ordering. -Each value is associated with a unique *key*, -which acts as an identifier for that value within the dictionary. -Unlike items in an array, items in a dictionary don't have a specified order. -You use a dictionary when you need to look up values based on their identifier, -in much the same way that a real-world dictionary is used to look up -the definition for a particular word. - -> Note: Swift's `Dictionary` type is bridged to Foundation's `NSDictionary` class. -> -> For more information about using `Dictionary` with Foundation and Cocoa, -> see [Bridging Between Dictionary and NSDictionary](https://developer.apple.com/documentation/swift/dictionary#2846239). - -### Dictionary Type Shorthand Syntax - -The type of a Swift dictionary is written in full as `Dictionary`, -where `Key` is the type of value that can be used as a dictionary key, -and `Value` is the type of value that the dictionary stores for those keys. - -> Note: A dictionary `Key` type must conform to the `Hashable` protocol, -> like a set's value type. - -You can also write the type of a dictionary in shorthand form as `[Key: Value]`. -Although the two forms are functionally identical, -the shorthand form is preferred -and is used throughout this guide when referring to the type of a dictionary. - -### Creating an Empty Dictionary - -As with arrays, -you can create an empty `Dictionary` of a certain type by using initializer syntax: - -```swift -var namesOfIntegers: [Int: String] = [:] -// namesOfIntegers is an empty [Int: String] dictionary -``` - - - -This example creates an empty dictionary of type `[Int: String]` -to store human-readable names of integer values. -Its keys are of type `Int`, and its values are of type `String`. - -If the context already provides type information, -you can create an empty dictionary with an empty dictionary literal, -which is written as `[:]` -(a colon inside a pair of square brackets): - -```swift -namesOfIntegers[16] = "sixteen" -// namesOfIntegers now contains 1 key-value pair -namesOfIntegers = [:] -// namesOfIntegers is once again an empty dictionary of type [Int: String] -``` - - - -### Creating a Dictionary with a Dictionary Literal - -You can also initialize a dictionary with a *dictionary literal*, -which has a similar syntax to the array literal seen earlier. -A dictionary literal is a shorthand way to write -one or more key-value pairs as a `Dictionary` collection. - -A *key-value pair* is a combination of a key and a value. -In a dictionary literal, -the key and value in each key-value pair are separated by a colon. -The key-value pairs are written as a list, separated by commas, -surrounded by a pair of square brackets: - -```swift -[<#key 1#>: <#value 1#>, <#key 2#>: <#value 2#>, <#key 3#>: <#value 3#>] -``` - -The example below creates a dictionary to store the names of international airports. -In this dictionary, the keys are three-letter International Air Transport Association codes, -and the values are airport names: - -```swift -var airports: [String: String] = ["YYZ": "Toronto Pearson", "DUB": "Dublin"] -``` - - - -The `airports` dictionary is declared as having a type of `[String: String]`, -which means “a `Dictionary` whose keys are of type `String`, -and whose values are also of type `String`”. - -> Note: The `airports` dictionary is declared as a variable (with the `var` introducer), -> and not a constant (with the `let` introducer), -> because more airports are added to the dictionary in the examples below. - -The `airports` dictionary is initialized with -a dictionary literal containing two key-value pairs. -The first pair has a key of `"YYZ"` and a value of `"Toronto Pearson"`. -The second pair has a key of `"DUB"` and a value of `"Dublin"`. - -This dictionary literal contains two `String: String` pairs. -This key-value type matches the type of the `airports` variable declaration -(a dictionary with only `String` keys, and only `String` values), -and so the assignment of the dictionary literal is permitted -as a way to initialize the `airports` dictionary with two initial items. - -As with arrays, -you don't have to write the type of the dictionary -if you're initializing it with a dictionary literal whose keys and values have consistent types. -The initialization of `airports` could have been written in a shorter form instead: - -```swift -var airports = ["YYZ": "Toronto Pearson", "DUB": "Dublin"] -``` - - - -Because all keys in the literal are of the same type as each other, -and likewise all values are of the same type as each other, -Swift can infer that `[String: String]` is -the correct type to use for the `airports` dictionary. - -### Accessing and Modifying a Dictionary - -You access and modify a dictionary through its methods and properties, -or by using subscript syntax. - -As with an array, you find out the number of items in a `Dictionary` -by checking its read-only `count` property: - -```swift -print("The airports dictionary contains \(airports.count) items.") -// Prints "The airports dictionary contains 2 items." -``` - - - -Use the Boolean `isEmpty` property -as a shortcut for checking whether the `count` property is equal to `0`: - -```swift -if airports.isEmpty { - print("The airports dictionary is empty.") -} else { - print("The airports dictionary isn't empty.") -} -// Prints "The airports dictionary isn't empty." -``` - - - -You can add a new item to a dictionary with subscript syntax. -Use a new key of the appropriate type as the subscript index, -and assign a new value of the appropriate type: - -```swift -airports["LHR"] = "London" -// the airports dictionary now contains 3 items -``` - - - -You can also use subscript syntax to change the value associated with a particular key: - -```swift -airports["LHR"] = "London Heathrow" -// the value for "LHR" has been changed to "London Heathrow" -``` - - - -As an alternative to subscripting, -use a dictionary's `updateValue(_:forKey:)` method -to set or update the value for a particular key. -Like the subscript examples above, the `updateValue(_:forKey:)` method -sets a value for a key if none exists, -or updates the value if that key already exists. -Unlike a subscript, however, -the `updateValue(_:forKey:)` method returns the *old* value after performing an update. -This enables you to check whether or not an update took place. - -The `updateValue(_:forKey:)` method returns an optional value -of the dictionary's value type. -For a dictionary that stores `String` values, for example, -the method returns a value of type `String?`, -or “optional `String`”. -This optional value contains the old value for that key if one existed before the update, -or `nil` if no value existed: - -```swift -if let oldValue = airports.updateValue("Dublin Airport", forKey: "DUB") { - print("The old value for DUB was \(oldValue).") -} -// Prints "The old value for DUB was Dublin." -``` - - - -You can also use subscript syntax to retrieve a value from the dictionary for a particular key. -Because it's possible to request a key for which no value exists, -a dictionary's subscript returns an optional value of the dictionary's value type. -If the dictionary contains a value for the requested key, -the subscript returns an optional value containing the existing value for that key. -Otherwise, the subscript returns `nil`: - -```swift -if let airportName = airports["DUB"] { - print("The name of the airport is \(airportName).") -} else { - print("That airport isn't in the airports dictionary.") -} -// Prints "The name of the airport is Dublin Airport." -``` - - - -You can use subscript syntax to remove a key-value pair from a dictionary -by assigning a value of `nil` for that key: - -```swift -airports["APL"] = "Apple International" -// "Apple International" isn't the real airport for APL, so delete it -airports["APL"] = nil -// APL has now been removed from the dictionary -``` - - - -Alternatively, remove a key-value pair from a dictionary -with the `removeValue(forKey:)` method. -This method removes the key-value pair if it exists -and returns the removed value, -or returns `nil` if no value existed: - -```swift -if let removedValue = airports.removeValue(forKey: "DUB") { - print("The removed airport's name is \(removedValue).") -} else { - print("The airports dictionary doesn't contain a value for DUB.") -} -// Prints "The removed airport's name is Dublin Airport." -``` - - - -### Iterating Over a Dictionary - -You can iterate over the key-value pairs in a dictionary with a `for`-`in` loop. -Each item in the dictionary is returned as a `(key, value)` tuple, -and you can decompose the tuple's members into temporary constants or variables -as part of the iteration: - -```swift -for (airportCode, airportName) in airports { - print("\(airportCode): \(airportName)") -} -// LHR: London Heathrow -// YYZ: Toronto Pearson -``` - - - -For more about the `for`-`in` loop, see . - -You can also retrieve an iterable collection of a dictionary's keys or values -by accessing its `keys` and `values` properties: - -```swift -for airportCode in airports.keys { - print("Airport code: \(airportCode)") -} -// Airport code: LHR -// Airport code: YYZ - -for airportName in airports.values { - print("Airport name: \(airportName)") -} -// Airport name: London Heathrow -// Airport name: Toronto Pearson -``` - - - -If you need to use a dictionary's keys or values -with an API that takes an `Array` instance, initialize a new array -with the `keys` or `values` property: - -```swift -let airportCodes = [String](airports.keys) -// airportCodes is ["LHR", "YYZ"] - -let airportNames = [String](airports.values) -// airportNames is ["London Heathrow", "Toronto Pearson"] -``` - - - -Swift's `Dictionary` type doesn't have a defined ordering. -To iterate over the keys or values of a dictionary in a specific order, -use the `sorted()` method on its `keys` or `values` property. - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/LanguageGuide/Concurrency.md b/swift-6-beta.docc/LanguageGuide/Concurrency.md deleted file mode 100644 index f7bf01b71..000000000 --- a/swift-6-beta.docc/LanguageGuide/Concurrency.md +++ /dev/null @@ -1,1448 +0,0 @@ -# Concurrency - -Perform asynchronous operations. - -Swift has built-in support for writing asynchronous and parallel code -in a structured way. -*Asynchronous code* can be suspended and resumed later, -although only one piece of the program executes at a time. -Suspending and resuming code in your program -lets it continue to make progress -on short-term operations like updating its UI -while continuing to work on long-running operations -like fetching data over the network or parsing files. -*Parallel code* means multiple pieces of code run simultaneously --- -for example, a computer with a four-core processor -can run four pieces of code at the same time, -with each core carrying out one of the tasks. -A program that uses parallel and asynchronous code -carries out multiple operations at a time, -and it suspends operations that are waiting for an external system. - -The additional scheduling flexibility from parallel or asynchronous code -also comes with a cost of increased complexity. -Swift lets you express your intent -in a way that enables some compile-time checking --- -for example, you can use actors to safely access mutable state. -However, adding concurrency to slow or buggy code -isn't a guarantee that it will become fast or correct. -In fact, adding concurrency might even make your code harder to debug. -However, using Swift's language-level support for concurrency -in code that needs to be concurrent -means Swift can help you catch problems at compile time. - -The rest of this chapter uses the term *concurrency* -to refer to this common combination of asynchronous and parallel code. - -> Note: If you've written concurrent code before, -> you might be used to working with threads. -> The concurrency model in Swift is built on top of threads, -> but you don't interact with them directly. -> An asynchronous function in Swift -> can give up the thread that it's running on, -> which lets another asynchronous function run on that thread -> while the first function is blocked. -> When an asynchronous function resumes, -> Swift doesn't make any guarantee about which thread -> that function will run on. - -Although it's possible to write concurrent code -without using Swift's language support, -that code tends to be harder to read. -For example, the following code downloads a list of photo names, -downloads the first photo in that list, -and shows that photo to the user: - -```swift -listPhotos(inGallery: "Summer Vacation") { photoNames in - let sortedNames = photoNames.sorted() - let name = sortedNames[0] - downloadPhoto(named: name) { photo in - show(photo) - } -} -``` - - - -Even in this simple case, -because the code has to be written as a series of completion handlers, -you end up writing nested closures. -In this style, -more complex code with deep nesting can quickly become unwieldy. - -## Defining and Calling Asynchronous Functions - -An *asynchronous function* or *asynchronous method* -is a special kind of function or method -that can be suspended while it's partway through execution. -This is in contrast to ordinary, synchronous functions and methods, -which either run to completion, throw an error, or never return. -An asynchronous function or method still does one of those three things, -but it can also pause in the middle when it's waiting for something. -Inside the body of an asynchronous function or method, -you mark each of these places where execution can be suspended. - -To indicate that a function or method is asynchronous, -you write the `async` keyword in its declaration after its parameters, -similar to how you use `throws` to mark a throwing function. -If the function or method returns a value, -you write `async` before the return arrow (`->`). -For example, -here's how you might fetch the names of photos in a gallery: - -```swift -func listPhotos(inGallery name: String) async -> [String] { - let result = // ... some asynchronous networking code ... - return result -} -``` - - - -For a function or method that's both asynchronous and throwing, -you write `async` before `throws`. - - - -When calling an asynchronous method, -execution suspends until that method returns. -You write `await` in front of the call -to mark the possible suspension point. -This is like writing `try` when calling a throwing function, -to mark the possible change to the program's flow if there's an error. -Inside an asynchronous method, -the flow of execution is suspended *only* when you call another asynchronous method --- -suspension is never implicit or preemptive --- -which means every possible suspension point is marked with `await`. -Marking all of the possible suspension points in your code -helps make concurrent code easier to read and understand. - -For example, -the code below fetches the names of all the pictures in a gallery -and then shows the first picture: - -```swift -let photoNames = await listPhotos(inGallery: "Summer Vacation") -let sortedNames = photoNames.sorted() -let name = sortedNames[0] -let photo = await downloadPhoto(named: name) -show(photo) -``` - - - -Because the `listPhotos(inGallery:)` and `downloadPhoto(named:)` functions -both need to make network requests, -they could take a relatively long time to complete. -Making them both asynchronous by writing `async` before the return arrow -lets the rest of the app's code keep running -while this code waits for the picture to be ready. - -To understand the concurrent nature of the example above, -here's one possible order of execution: - -1. The code starts running from the first line - and runs up to the first `await`. - It calls the `listPhotos(inGallery:)` function - and suspends execution while it waits for that function to return. - -2. While this code's execution is suspended, - some other concurrent code in the same program runs. - For example, maybe a long-running background task - continues updating a list of new photo galleries. - That code also runs until the next suspension point, marked by `await`, - or until it completes. - -3. After `listPhotos(inGallery:)` returns, - this code continues execution starting at that point. - It assigns the value that was returned to `photoNames`. - -4. The lines that define `sortedNames` and `name` - are regular, synchronous code. - Because nothing is marked `await` on these lines, - there aren't any possible suspension points. - -5. The next `await` marks the call to the `downloadPhoto(named:)` function. - This code pauses execution again until that function returns, - giving other concurrent code an opportunity to run. - -6. After `downloadPhoto(named:)` returns, - its return value is assigned to `photo` - and then passed as an argument when calling `show(_:)`. - -The possible suspension points in your code marked with `await` -indicate that the current piece of code might pause execution -while waiting for the asynchronous function or method to return. -This is also called *yielding the thread* -because, behind the scenes, -Swift suspends the execution of your code on the current thread -and runs some other code on that thread instead. -Because code with `await` needs to be able to suspend execution, -only certain places in your program can call asynchronous functions or methods: - -- Code in the body of an asynchronous function, method, or property. - -- Code in the static `main()` method of - a structure, class, or enumeration that's marked with `@main`. - -- Code in an unstructured child task, - as shown in below. - - - -You can explicitly insert a suspension point -by calling the [`Task.yield()`][] method. - -[`Task.yield()`]: https://developer.apple.com/documentation/swift/task/3814840-yield - -```swift -func generateSlideshow(forGallery gallery: String) async { - let photos = await listPhotos(inGallery: gallery) - for photo in photos { - // ... render a few seconds of video for this photo ... - await Task.yield() - } -} -``` - -Assuming the code that renders video is synchronous, -it doesn't contain any suspension points. -The work to render video could also take a long time. -However, -you can periodically call `Task.yield()` -to explicitly add suspension points. -Structuring long-running code this way -lets Swift balance between making progress on this task, -and letting other tasks in your program make progress on their work. - -The [`Task.sleep(for:tolerance:clock:)`][] method -is useful when writing simple code -to learn how concurrency works. -This method suspends the current task for at least the given amount of time. -Here's a version of the `listPhotos(inGallery:)` function -that uses `sleep(for:tolerance:clock:)` to simulate waiting for a network operation: - -[`Task.sleep(for:tolerance:clock:)`]: https://developer.apple.com/documentation/swift/task/sleep(for:tolerance:clock:) - -```swift -func listPhotos(inGallery name: String) async throws -> [String] { - try await Task.sleep(for: .seconds(2)) - return ["IMG001", "IMG99", "IMG0404"] -} -``` - - - -The version of `listPhotos(inGallery:)` in the code above -is both asynchronous and throwing, -because the call to `Task.sleep(until:tolerance:clock:)` can throw an error. -When you call this version of `listPhotos(inGallery:)`, -you write both `try` and `await`: - -```swift -let photos = try await listPhotos(inGallery: "A Rainy Weekend") -``` - -Asynchronous functions have some similarities to throwing functions: -When you define an asynchronous or throwing function, -you mark it with `async` or `throws`, -and you mark calls to that function with `await` or `try`. -An asynchronous function can call another asynchronous function, -just like a throwing function can call another throwing function. - -However, there's a very important difference. -You can wrap throwing code in a `do`-`catch` block to handle errors, -or use `Result` to store the error for code elsewhere to handle it. -These approaches let you call throwing functions -from nonthrowing code. -For example: - -```swift -func availableRainyWeekendPhotos() -> Result<[String], Error> { - return Result { - try listDownloadedPhotos(inGallery: "A Rainy Weekend") - } -} -``` - -In contrast, -there's no safe way to wrap asynchronous code -so you can call it from synchronous code and wait for the result. -The Swift standard library intentionally omits this unsafe functionality --- -trying to implement it yourself can lead to -problems like subtle races, threading issues, and deadlocks. -When adding concurrent code to an existing project, -work from the top down. -Specifically, -start by converting the top-most layer of code to use concurrency, -and then start converting the functions and methods that it calls, -working through the project's architecture one layer at a time. -There's no way to take a bottom-up approach, -because synchronous code can't ever call asynchronous code. - - - -## Asynchronous Sequences - -The `listPhotos(inGallery:)` function in the previous section -asynchronously returns the whole array at once, -after all of the array's elements are ready. -Another approach -is to wait for one element of the collection at a time -using an *asynchronous sequence*. -Here's what iterating over an asynchronous sequence looks like: - -```swift -import Foundation - -let handle = FileHandle.standardInput -for try await line in handle.bytes.lines { - print(line) -} -``` - - - -Instead of using an ordinary `for`-`in` loop, -the example above writes `for` with `await` after it. -Like when you call an asynchronous function or method, -writing `await` indicates a possible suspension point. -A `for`-`await`-`in` loop potentially suspends execution -at the beginning of each iteration, -when it's waiting for the next element to be available. - - - -In the same way that you can use your own types in a `for`-`in` loop -by adding conformance to the [`Sequence`][] protocol, -you can use your own types in a `for`-`await`-`in` loop -by adding conformance to the [`AsyncSequence`] protocol. - -[`Sequence`]: https://developer.apple.com/documentation/swift/sequence -[`AsyncSequence`]: https://developer.apple.com/documentation/swift/asyncsequence - - - -## Calling Asynchronous Functions in Parallel - -Calling an asynchronous function with `await` -runs only one piece of code at a time. -While the asynchronous code is running, -the caller waits for that code to finish -before moving on to run the next line of code. -For example, -to fetch the first three photos from a gallery, -you could await three calls to the `downloadPhoto(named:)` function -as follows: - -```swift -let firstPhoto = await downloadPhoto(named: photoNames[0]) -let secondPhoto = await downloadPhoto(named: photoNames[1]) -let thirdPhoto = await downloadPhoto(named: photoNames[2]) - -let photos = [firstPhoto, secondPhoto, thirdPhoto] -show(photos) -``` - - - -This approach has an important drawback: -Although the download is asynchronous -and lets other work happen while it progresses, -only one call to `downloadPhoto(named:)` runs at a time. -Each photo downloads completely before the next one starts downloading. -However, there's no need for these operations to wait --- -each photo can download independently, or even at the same time. - -To call an asynchronous function -and let it run in parallel with code around it, -write `async` in front of `let` when you define a constant, -and then write `await` each time you use the constant. - -```swift -async let firstPhoto = downloadPhoto(named: photoNames[0]) -async let secondPhoto = downloadPhoto(named: photoNames[1]) -async let thirdPhoto = downloadPhoto(named: photoNames[2]) - -let photos = await [firstPhoto, secondPhoto, thirdPhoto] -show(photos) -``` - - - -In this example, -all three calls to `downloadPhoto(named:)` start -without waiting for the previous one to complete. -If there are enough system resources available, they can run at the same time. -None of these function calls are marked with `await` -because the code doesn't suspend to wait for the function's result. -Instead, execution continues -until the line where `photos` is defined --- -at that point, the program needs the results from these asynchronous calls, -so you write `await` to pause execution -until all three photos finish downloading. - -Here's how you can think about the differences between these two approaches: - -- Call asynchronous functions with `await` - when the code on the following lines depends on that function's result. - This creates work that is carried out sequentially. -- Call asynchronous functions with `async`-`let` - when you don't need the result until later in your code. - This creates work that can be carried out in parallel. -- Both `await` and `async`-`let` - allow other code to run while they're suspended. -- In both cases, you mark the possible suspension point with `await` - to indicate that execution will pause, if needed, - until an asynchronous function has returned. - -You can also mix both of these approaches in the same code. - -## Tasks and Task Groups - -A *task* is a unit of work -that can be run asynchronously as part of your program. -All asynchronous code runs as part of some task. -A task itself does only one thing at a time, -but when you create multiple tasks, -Swift can schedule them to run simultaneously. - -The `async`-`let` syntax described in the previous section -implicitly creates a child task --- -this syntax works well when you already know -what tasks your program needs to run. -You can also create a task group -(an instance of [`TaskGroup`][]) -and explicitly add child tasks to that group, -which gives you more control over priority and cancellation, -and lets you create a dynamic number of tasks. - -[`TaskGroup`]: https://developer.apple.com/documentation/swift/taskgroup - -Tasks are arranged in a hierarchy. -Each task in a given task group has the same parent task, -and each task can have child tasks. -Because of the explicit relationship between tasks and task groups, -this approach is called *structured concurrency*. -The explicit parent-child relationships between tasks has several advantages: - -- In a parent task, - you can't forget to wait for its child tasks to complete. - -- When setting a higher priority on a child task, - the parent task's priority is automatically escalated. - -- When a parent task is canceled, - each of its child tasks is also automatically canceled. - -- Task-local values propagate to child tasks efficiently and automatically. - -Here's another version of the code to download photos -that handles any number of photos: - -```swift -await withTaskGroup(of: Data.self) { group in - let photoNames = await listPhotos(inGallery: "Summer Vacation") - for name in photoNames { - group.addTask { - return await downloadPhoto(named: name) - } - } - - for await photo in group { - show(photo) - } -} -``` - -The code above creates a new task group, -and then creates child tasks -to download each photo in the gallery. -Swift runs as many of these tasks concurrently as conditions allow. -As soon a child task finishes downloading a photo, -that photo is displayed. -There's no guarantee about the order that child tasks complete, -so the photos from this gallery can be shown in any order. - -> Note: -> If the code to download a photo could throw an error, -> you would call `withThrowingTaskGroup(of:returning:body:)` instead. - -In the code listing above, -each photo is downloaded and then displayed, -so the task group doesn't return any results. -For a task group that returns a result, -you add code that accumulates its result -inside the closure you pass to `withTaskGroup(of:returning:body:)`. - -``` -let photos = await withTaskGroup(of: Data.self) { group in - let photoNames = await listPhotos(inGallery: "Summer Vacation") - for name in photoNames { - group.addTask { - return await downloadPhoto(named: name) - } - } - - var results: [Data] = [] - for await photo in group { - results.append(photo) - } - - return results -} -``` - -Like the previous example, -this example creates a child task for each photo to download it. -Unlike the previous example, -the `for`-`await`-`in` loop waits for the next child task to finish, -appends the result of that task to the array of results, -and then continues waiting until all child tasks have finished. -Finally, -the task group returns the array of downloaded photos -as its overall result. - - - -### Task Cancellation - -Swift concurrency uses a cooperative cancellation model. -Each task checks whether it has been canceled -at the appropriate points in its execution, -and responds to cancellation appropriately. -Depending on what work the task is doing, -responding to cancellation usually means one of the following: - -- Throwing an error like `CancellationError` -- Returning `nil` or an empty collection -- Returning the partially completed work - -Downloading pictures could take a long time -if the pictures are large or the network is slow. -To let the user stop this work, -without waiting for all of the tasks to complete, -the tasks need check for cancellation and stop running if they are canceled. -There are two ways a task can do this: -by calling the [`Task.checkCancellation()`][] type method, -or by reading the [`Task.isCancelled`][`Task.isCancelled` type] type property. -Calling `checkCancellation()` throws an error if the task is canceled; -a throwing task can propagate the error out of the task, -stopping all of the task's work. -This has the advantage of being simple to implement and understand. -For more flexibility, use the `isCancelled` property, -which lets you perform clean-up work as part of stopping the task, -like closing network connections and deleting temporary files. - -[`Task.checkCancellation()`]: https://developer.apple.com/documentation/swift/task/3814826-checkcancellation -[`Task.isCancelled` type]: https://developer.apple.com/documentation/swift/task/iscancelled-swift.type.property - -``` -let photos = await withTaskGroup(of: Optional.self) { group in - let photoNames = await listPhotos(inGallery: "Summer Vacation") - for name in photoNames { - let added = group.addTaskUnlessCancelled { - guard !Task.isCancelled else { return nil } - return await downloadPhoto(named: name) - } - guard added else { break } - } - - var results: [Data] = [] - for await photo in group { - if let photo { results.append(photo) } - } - return results -} -``` - -The code above makes several changes from the previous version: - -- Each task is added using the - [`TaskGroup.addTaskUnlessCancelled(priority:operation:)`][] method, - to avoid starting new work after cancellation. - -- After each call to `addTaskUnlessCancelled(priority:operation:)`, - the code confirms that the new child task was added. - If the group is canceled, the value of `added` is `false` --- - in that case, the code stops trying to download additional photos. - -- Each task checks for cancellation - before starting to download the photo. - If it has been canceled, the task returns `nil`. - -- At the end, - the task group skips `nil` values when collecting the results. - Handling cancellation by returning `nil` - means the task group can return a partial result --- - the photos that were already downloaded at the time of cancellation --- - instead of discarding that completed work. - -[`TaskGroup.addTaskUnlessCancelled(priority:operation:)`]: https://developer.apple.com/documentation/swift/taskgroup/addtaskunlesscancelled(priority:operation:) - -> Note: -> To check whether a task has been canceled from outside that task, -> use the [`Task.isCancelled`][`Task.isCancelled` instance] instance property -> instead of the type property. - -[`Task.isCancelled` instance]: https://developer.apple.com/documentation/swift/task/iscancelled-swift.property - -For work that needs immediate notification of cancellation, -use the [`Task.withTaskCancellationHandler(operation:onCancel:)`][] method. -For example: - -[`Task.withTaskCancellationHandler(operation:onCancel:)`]: https://developer.apple.com/documentation/swift/withtaskcancellationhandler(operation:oncancel:) - -```swift -let task = await Task.withTaskCancellationHandler { - // ... -} onCancel: { - print("Canceled!") -} - -// ... some time later... -task.cancel() // Prints "Canceled!" -``` - -When using a cancellation handler, -task cancellation is still cooperative: -The task either runs to completion -or checks for cancellation and stops early. -Because the task is still running when the cancellation handler starts, -avoid sharing state between the task and its cancellation handler, -which could create a race condition. - - - - - - - -### Unstructured Concurrency - -In addition to the structured approaches to concurrency -described in the previous sections, -Swift also supports unstructured concurrency. -Unlike tasks that are part of a task group, -an *unstructured task* doesn't have a parent task. -You have complete flexibility to manage unstructured tasks -in whatever way your program needs, -but you're also completely responsible for their correctness. -To create an unstructured task that runs on the current actor, -call the [`Task.init(priority:operation:)`](https://developer.apple.com/documentation/swift/task/3856790-init) initializer. -To create an unstructured task that's not part of the current actor, -known more specifically as a *detached task*, -call the [`Task.detached(priority:operation:)`](https://developer.apple.com/documentation/swift/task/3856786-detached) class method. -Both of these operations return a task that you can interact with --- -for example, to wait for its result or to cancel it. - -```swift -let newPhoto = // ... some photo data ... -let handle = Task { - return await add(newPhoto, toGalleryNamed: "Spring Adventures") -} -let result = await handle.value -``` - -For more information about managing detached tasks, -see [`Task`](https://developer.apple.com/documentation/swift/task). - - - -## Actors - -You can use tasks to break up your program into isolated, concurrent pieces. -Tasks are isolated from each other, -which is what makes it safe for them to run at the same time, -but sometimes you need to share some information between tasks. -Actors let you safely share information between concurrent code. - -Like classes, actors are reference types, -so the comparison of value types and reference types -in -applies to actors as well as classes. -Unlike classes, -actors allow only one task to access their mutable state at a time, -which makes it safe for code in multiple tasks -to interact with the same instance of an actor. -For example, here's an actor that records temperatures: - -```swift -actor TemperatureLogger { - let label: String - var measurements: [Int] - private(set) var max: Int - - init(label: String, measurement: Int) { - self.label = label - self.measurements = [measurement] - self.max = measurement - } -} -``` - - - -You introduce an actor with the `actor` keyword, -followed by its definition in a pair of braces. -The `TemperatureLogger` actor has properties -that other code outside the actor can access, -and restricts the `max` property so only code inside the actor -can update the maximum value. - -You create an instance of an actor -using the same initializer syntax as structures and classes. -When you access a property or method of an actor, -you use `await` to mark the potential suspension point. -For example: - -```swift -let logger = TemperatureLogger(label: "Outdoors", measurement: 25) -print(await logger.max) -// Prints "25" -``` - -In this example, -accessing `logger.max` is a possible suspension point. -Because the actor allows only one task at a time to access its mutable state, -if code from another task is already interacting with the logger, -this code suspends while it waits to access the property. - -In contrast, -code that's part of the actor doesn't write `await` -when accessing the actor's properties. -For example, -here's a method that updates a `TemperatureLogger` with a new temperature: - -```swift -extension TemperatureLogger { - func update(with measurement: Int) { - measurements.append(measurement) - if measurement > max { - max = measurement - } - } -} -``` - -The `update(with:)` method is already running on the actor, -so it doesn't mark its access to properties like `max` with `await`. -This method also shows one of the reasons -why actors allow only one task at a time to interact with their mutable state: -Some updates to an actor's state temporarily break invariants. -The `TemperatureLogger` actor keeps track of -a list of temperatures and a maximum temperature, -and it updates the maximum temperature when you record a new measurement. -In the middle of an update, -after appending the new measurement but before updating `max`, -the temperature logger is in a temporary inconsistent state. -Preventing multiple tasks from interacting with the same instance simultaneously -prevents problems like the following sequence of events: - -1. Your code calls the `update(with:)` method. - It updates the `measurements` array first. -2. Before your code can update `max`, - code elsewhere reads the maximum value and the array of temperatures. -3. Your code finishes its update by changing `max`. - -In this case, -the code running elsewhere would read incorrect information -because its access to the actor was interleaved -in the middle of the call to `update(with:)` -while the data was temporarily invalid. -You can prevent this problem when using Swift actors -because they only allow one operation on their state at a time, -and because that code can be interrupted -only in places where `await` marks a suspension point. -Because `update(with:)` doesn't contain any suspension points, -no other code can access the data in the middle of an update. - -If code outside the actor tries to access those properties directly, -like accessing a structure or class's properties, -you'll get a compile-time error. -For example: - -```swift -print(logger.max) // Error -``` - -Accessing `logger.max` without writing `await` fails because -the properties of an actor are part of that actor's isolated local state. -The code to access this property needs to run as part of the actor, -which is an asynchronous operation and requires writing `await`. -Swift guarantees that -only code running on an actor can access that actor's local state. -This guarantee is known as *actor isolation*. - -The following aspects of the Swift concurrency model -work together to make it easier to reason about shared mutable state: - -- Code in between possible suspension points runs sequentially, - without the possibility of interruption from other concurrent code. - -- Code that interacts with an actor's local state - runs only on that actor. - -- An actor runs only one piece of code at a time. - -Because of these guarantees, -code that doesn't include `await` and that's inside an actor -can make the updates without a risk of other places in your program -observing the temporarily invalid state. -For example, -the code below converts measured temperatures from Fahrenheit to Celsius: - -```swift -extension TemperatureLogger { - func convertFahrenheitToCelsius() { - measurements = measurements.map { measurement in - (measurement - 32) * 5 / 9 - } - } -} -``` - -The code above converts the array of measurements, one at a time. -While the map operation is in progress, -some temperatures are in Fahrenheit and others are in Celsius. -However, because none of the code includes `await`, -there are no potential suspension points in this method. -The state that this method modifies belongs to the actor, -which protects it against code reading or modifying it -except when that code runs on the actor. -This means there's no way for other code -to read a list of partially converted temperatures -while unit conversion is in progress. - -In addition to writing code in an actor -that protects temporary invalid state by omitting potential suspension points, -you can move that code into a synchronous method. -The `convertFahrenheitToCelsius()` method above is a synchronous method, -so it's guaranteed to *never* contain potential suspension points. -This function encapsulates the code -that temporarily makes the data model inconsistent, -and makes it easier for anyone reading the code -to recognize that no other code can run -before data consistency is restored by completing the work. -In the future, -if you try to add concurrent code to this function, -introducing a possible suspension point, -you'll get compile-time error instead of introducing a bug. - - - -## Sendable Types - -Tasks and actors let you divide a program -into pieces that can safely run concurrently. -Inside of a task or an instance of an actor, -the part of a program that contains mutable state, -like variables and properties, -is called a *concurrency domain*. -Some kinds of data can't be shared between concurrency domains, -because that data contains mutable state, -but it doesn't protect against overlapping access. - -A type that can be shared from one concurrency domain to another -is known as a *sendable* type. -For example, it can be passed as an argument when calling an actor method -or be returned as the result of a task. -The examples earlier in this chapter didn't discuss sendability -because those examples use simple value types -that are always safe to share -for the data being passed between concurrency domains. -In contrast, -some types aren't safe to pass across concurrency domains. -For example, a class that contains mutable properties -and doesn't serialize access to those properties -can produce unpredictable and incorrect results -when you pass instances of that class between different tasks. - -You mark a type as being sendable -by declaring conformance to the `Sendable` protocol. -That protocol doesn't have any code requirements, -but it does have semantic requirements that Swift enforces. -In general, there are three ways for a type to be sendable: - -- The type is a value type, - and its mutable state is made up of other sendable data --- - for example, a structure with stored properties that are sendable - or an enumeration with associated values that are sendable. -- The type doesn't have any mutable state, - and its immutable state is made up of other sendable data --- - for example, a structure or class that has only read-only properties. -- The type has code that ensures the safety of its mutable state, - like a class that's marked `@MainActor` - or a class that serializes access to its properties - on a particular thread or queue. - - - -For a detailed list of the semantic requirements, -see the [`Sendable`](https://developer.apple.com/documentation/swift/sendable) protocol reference. - -Some types are always sendable, -like structures that have only sendable properties -and enumerations that have only sendable associated values. -For example: - -```swift -struct TemperatureReading: Sendable { - var measurement: Int -} - -extension TemperatureLogger { - func addReading(from reading: TemperatureReading) { - measurements.append(reading.measurement) - } -} - -let logger = TemperatureLogger(label: "Tea kettle", measurement: 85) -let reading = TemperatureReading(measurement: 45) -await logger.addReading(from: reading) -``` - - - -Because `TemperatureReading` is a structure that has only sendable properties, -and the structure isn't marked `public` or `@usableFromInline`, -it's implicitly sendable. -Here's a version of the structure -where conformance to the `Sendable` protocol is implied: - -```swift -struct TemperatureReading { - var measurement: Int -} -``` - - - -To explicitly mark a type as not being sendable, -overriding an implicit conformance to the `Sendable` protocol, -use an extension: - -```swift -struct FileDescriptor { - let rawValue: CInt -} - -@available(*, unavailable) -extension FileDescriptor: Sendable { } -``` - - - -The code above shows part of a wrapper around POSIX file descriptors. -Even though interface for file descriptors uses integers -to identify and interact with open files, -and integer values are sendable, -a file descriptor isn't safe to send across concurrency domains. - - - -In the code above, -the `FileDescriptor` is a structure -that meets the criteria to be implicitly sendable. -However, the extension makes its conformance to `Sendable` unavailable, -preventing the type from being sendable. - - - - - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/LanguageGuide/ControlFlow.md b/swift-6-beta.docc/LanguageGuide/ControlFlow.md deleted file mode 100644 index 04ce7b924..000000000 --- a/swift-6-beta.docc/LanguageGuide/ControlFlow.md +++ /dev/null @@ -1,2276 +0,0 @@ -# Control Flow - -Structure code with branches, loops, and early exits. - -Swift provides a variety of control flow statements. -These include `while` loops to perform a task multiple times; -`if`, `guard`, and `switch` statements -to execute different branches of code based on certain conditions; -and statements such as `break` and `continue` -to transfer the flow of execution to another point in your code. -Swift provides a `for`-`in` loop that makes it easy to iterate over -arrays, dictionaries, ranges, strings, and other sequences. -Swift also provides `defer` statements, -which wrap code to be executed when leaving the current scope. - -Swift's `switch` statement is considerably more powerful -than its counterpart in many C-like languages. -Cases can match many different patterns, -including interval matches, tuples, and casts to a specific type. -Matched values in a `switch` case can be bound to temporary constants or variables -for use within the case's body, -and complex matching conditions can be expressed with a `where` clause for each case. - -## For-In Loops - -You use the `for`-`in` loop to iterate over a sequence, -such as items in an array, ranges of numbers, or characters in a string. - -This example uses a `for`-`in` loop to iterate over the items in an array: - -```swift -let names = ["Anna", "Alex", "Brian", "Jack"] -for name in names { - print("Hello, \(name)!") -} -// Hello, Anna! -// Hello, Alex! -// Hello, Brian! -// Hello, Jack! -``` - - - -You can also iterate over a dictionary to access its key-value pairs. -Each item in the dictionary is returned as a `(key, value)` tuple -when the dictionary is iterated, -and you can decompose the `(key, value)` tuple's members as explicitly named constants -for use within the body of the `for`-`in` loop. -In the code example below, the dictionary's keys are decomposed into a constant called `animalName`, -and the dictionary's values are decomposed into a constant called `legCount`. - -```swift -let numberOfLegs = ["spider": 8, "ant": 6, "cat": 4] -for (animalName, legCount) in numberOfLegs { - print("\(animalName)s have \(legCount) legs") -} -// cats have 4 legs -// ants have 6 legs -// spiders have 8 legs -``` - - - -The contents of a `Dictionary` are inherently unordered, -and iterating over them doesn't guarantee the order -in which they will be retrieved. -In particular, -the order you insert items into a `Dictionary` -doesn't define the order they're iterated. -For more about arrays and dictionaries, see . - - - -You can also use `for`-`in` loops with numeric ranges. -This example prints the first few entries in a five-times table: - -```swift -for index in 1...5 { - print("\(index) times 5 is \(index * 5)") -} -// 1 times 5 is 5 -// 2 times 5 is 10 -// 3 times 5 is 15 -// 4 times 5 is 20 -// 5 times 5 is 25 -``` - - - -The sequence being iterated over is -a range of numbers from `1` to `5`, inclusive, -as indicated by the use of the closed range operator (`...`). -The value of `index` is set to the first number in the range (`1`), -and the statements inside the loop are executed. -In this case, the loop contains only one statement, -which prints an entry from the five-times table for the current value of `index`. -After the statement is executed, -the value of `index` is updated to contain the second value in the range (`2`), -and the `print(_:separator:terminator:)` function is called again. -This process continues until the end of the range is reached. - -In the example above, `index` is a constant whose value is automatically set -at the start of each iteration of the loop. -As such, `index` doesn't have to be declared before it's used. -It's implicitly declared simply by its inclusion in the loop declaration, -without the need for a `let` declaration keyword. - -If you don't need each value from a sequence, -you can ignore the values by using an underscore in place of a variable name. - -```swift -let base = 3 -let power = 10 -var answer = 1 -for _ in 1...power { - answer *= base -} -print("\(base) to the power of \(power) is \(answer)") -// Prints "3 to the power of 10 is 59049" -``` - - - -The example above calculates the value of one number to the power of another -(in this case, `3` to the power of `10`). -It multiplies a starting value of `1` -(that is, `3` to the power of `0`) -by `3`, ten times, -using a closed range that starts with `1` and ends with `10`. -For this calculation, the individual counter values each time through the loop are unnecessary --- -the code simply executes the loop the correct number of times. -The underscore character (`_`) -used in place of a loop variable -causes the individual values to be ignored -and doesn't provide access to the current value during each iteration of the loop. - -In some situations, you might not want to use closed ranges, -which include both endpoints. -Consider drawing the tick marks for every minute on a watch face. -You want to draw `60` tick marks, starting with the `0` minute. -Use the half-open range operator (`..<`) to include the -lower bound but not the upper bound. -For more about ranges, see . - -```swift -let minutes = 60 -for tickMark in 0.. let minutes = 60 - >> var result: [Int] = [] - -> for tickMark in 0..> result.append(tickMark) - } - >> print(result.first!, result.last!, result.count) - << 0 59 60 - ``` ---> - -Some users might want fewer tick marks in their UI. -They could prefer one mark every `5` minutes instead. -Use the `stride(from:to:by:)` function to skip the unwanted marks. - -```swift -let minuteInterval = 5 -for tickMark in stride(from: 0, to: minutes, by: minuteInterval) { - // render the tick mark every 5 minutes (0, 5, 10, 15 ... 45, 50, 55) -} -``` - - - -Closed ranges are also available, by using `stride(from:through:by:)` instead: - -```swift -let hours = 12 -let hourInterval = 3 -for tickMark in stride(from: 3, through: hours, by: hourInterval) { - // render the tick mark every 3 hours (3, 6, 9, 12) -} -``` - - - -The examples above use a `for`-`in` loop to iterate -ranges, arrays, dictionaries, and strings. -However, you can use this syntax to iterate *any* collection, -including your own classes and collection types, -as long as those types conform to the [`Sequence`](https://developer.apple.com/documentation/swift/sequence) protocol. - - - -## While Loops - -A `while` loop performs a set of statements until a condition becomes `false`. -These kinds of loops are best used when -the number of iterations isn't known before the first iteration begins. -Swift provides two kinds of `while` loops: - -- `while` evaluates its condition at the start of each pass through the loop. -- `repeat`-`while` evaluates its condition at the end of each pass through the loop. - -### While - -A `while` loop starts by evaluating a single condition. -If the condition is `true`, -a set of statements is repeated until the condition becomes `false`. - -Here's the general form of a `while` loop: - -```swift -while <#condition#> { - <#statements#> -} -``` - -This example plays a simple game of *Snakes and Ladders* -(also known as *Chutes and Ladders*): - - - -![](snakesAndLadders) - -The rules of the game are as follows: - -- The board has 25 squares, and the aim is to land on or beyond square 25. -- The player's starting square is “square zero”, - which is just off the bottom-left corner of the board. -- Each turn, you roll a six-sided dice and move by that number of squares, - following the horizontal path indicated by the dotted arrow above. -- If your turn ends at the bottom of a ladder, you move up that ladder. -- If your turn ends at the head of a snake, you move down that snake. - -The game board is represented by an array of `Int` values. -Its size is based on a constant called `finalSquare`, -which is used to initialize the array -and also to check for a win condition later in the example. -Because the players start off the board, on "square zero", -the board is initialized with 26 zero `Int` values, not 25. - -```swift -let finalSquare = 25 -var board = [Int](repeating: 0, count: finalSquare + 1) -``` - - - -Some squares are then set to have more specific values for the snakes and ladders. -Squares with a ladder base have a positive number to move you up the board, -whereas squares with a snake head have a negative number to move you back down the board. - -```swift -board[03] = +08; board[06] = +11; board[09] = +09; board[10] = +02 -board[14] = -10; board[19] = -11; board[22] = -02; board[24] = -08 -``` - - - - - -Square 3 contains the bottom of a ladder that moves you up to square 11. -To represent this, `board[03]` is equal to `+08`, -which is equivalent to an integer value of `8` -(the difference between `3` and `11`). -To align the values and statements, -the unary plus operator (`+i`) is explicitly used with -the unary minus operator (`-i`) -and numbers lower than `10` are padded with zeros. -(Neither stylistic technique is strictly necessary, -but they lead to neater code.) - -```swift -var square = 0 -var diceRoll = 0 -while square < finalSquare { - // roll the dice - diceRoll += 1 - if diceRoll == 7 { diceRoll = 1 } - // move by the rolled amount - square += diceRoll - if square < board.count { - // if we're still on the board, move up or down for a snake or a ladder - square += board[square] - } -} -print("Game over!") -``` - - - -The example above uses a very simple approach to dice rolling. -Instead of generating a random number, -it starts with a `diceRoll` value of `0`. -Each time through the `while` loop, -`diceRoll` is incremented by one -and is then checked to see whether it has become too large. -Whenever this return value equals `7`, -the dice roll has become too large and is reset to a value of `1`. -The result is a sequence of `diceRoll` values that's always -`1`, `2`, `3`, `4`, `5`, `6`, `1`, `2` and so on. - -After rolling the dice, the player moves forward by `diceRoll` squares. -It's possible that the dice roll may have moved the player beyond square 25, -in which case the game is over. -To cope with this scenario, -the code checks that `square` is less than the `board` array's `count` property. -If `square` is valid, the value stored in `board[square]` is added -to the current `square` value -to move the player up or down any ladders or snakes. - -> Note: If this check isn't performed, -> `board[square]` might try to access a value outside the bounds of the `board` array, -> which would trigger a runtime error. - -The current `while` loop execution then ends, -and the loop's condition is checked to see if the loop should be executed again. -If the player has moved on or beyond square number `25`, -the loop's condition evaluates to `false` and the game ends. - -A `while` loop is appropriate in this case, -because the length of the game isn't clear at the start of the `while` loop. -Instead, the loop is executed until a particular condition is satisfied. - -### Repeat-While - -The other variation of the `while` loop, -known as the `repeat`-`while` loop, -performs a single pass through the loop block first, -*before* considering the loop's condition. -It then continues to repeat the loop until the condition is `false`. - -> Note: The `repeat`-`while` loop in Swift is analogous to -> a `do`-`while` loop in other languages. - -Here's the general form of a `repeat`-`while` loop: - -```swift -repeat { - <#statements#> -} while <#condition#> -``` - -Here's the *Snakes and Ladders* example again, -written as a `repeat`-`while` loop rather than a `while` loop. -The values of `finalSquare`, `board`, `square`, and `diceRoll` -are initialized in exactly the same way as with a `while` loop. - -```swift -let finalSquare = 25 -var board = [Int](repeating: 0, count: finalSquare + 1) -board[03] = +08; board[06] = +11; board[09] = +09; board[10] = +02 -board[14] = -10; board[19] = -11; board[22] = -02; board[24] = -08 -var square = 0 -var diceRoll = 0 -``` - - - -In this version of the game, -the *first* action in the loop is to check for a ladder or a snake. -No ladder on the board takes the player straight to square 25, -and so it isn't possible to win the game by moving up a ladder. -Therefore, it's safe to check for a snake or a ladder as the first action in the loop. - -At the start of the game, the player is on “square zero”. -`board[0]` always equals `0` and has no effect. - -```swift -repeat { - // move up or down for a snake or ladder - square += board[square] - // roll the dice - diceRoll += 1 - if diceRoll == 7 { diceRoll = 1 } - // move by the rolled amount - square += diceRoll -} while square < finalSquare -print("Game over!") -``` - - - -After the code checks for snakes and ladders, -the dice is rolled and the player is moved forward by `diceRoll` squares. -The current loop execution then ends. - -The loop's condition (`while square < finalSquare`) is the same as before, -but this time it's not evaluated until the *end* of the first run through the loop. -The structure of the `repeat`-`while` loop is better suited to this game -than the `while` loop in the previous example. -In the `repeat`-`while` loop above, -`square += board[square]` is always executed *immediately after* -the loop's `while` condition confirms that `square` is still on the board. -This behavior removes the need for the array bounds check -seen in the `while` loop version of the game described earlier. - -## Conditional Statements - -It's often useful to execute different pieces of code based on certain conditions. -You might want to run an extra piece of code when an error occurs, -or to display a message when a value becomes too high or too low. -To do this, you make parts of your code *conditional*. - -Swift provides two ways to add conditional branches to your code: -the `if` statement and the `switch` statement. -Typically, you use the `if` statement -to evaluate simple conditions with only a few possible outcomes. -The `switch` statement is better suited to -more complex conditions with multiple possible permutations -and is useful in situations where pattern matching can help select -an appropriate code branch to execute. - -### If - -In its simplest form, -the `if` statement has a single `if` condition. -It executes a set of statements only if that condition is `true`. - -```swift -var temperatureInFahrenheit = 30 -if temperatureInFahrenheit <= 32 { - print("It's very cold. Consider wearing a scarf.") -} -// Prints "It's very cold. Consider wearing a scarf." -``` - - - -The example above checks whether the temperature -is less than or equal to 32 degrees Fahrenheit -(the freezing point of water). -If it is, a message is printed. -Otherwise, no message is printed, -and code execution continues after the `if` statement's closing brace. - -The `if` statement can provide an alternative set of statements, -known as an *else clause*, -for situations when the `if` condition is `false`. -These statements are indicated by the `else` keyword. - -```swift -temperatureInFahrenheit = 40 -if temperatureInFahrenheit <= 32 { - print("It's very cold. Consider wearing a scarf.") -} else { - print("It's not that cold. Wear a T-shirt.") -} -// Prints "It's not that cold. Wear a T-shirt." -``` - - - -One of these two branches is always executed. -Because the temperature has increased to `40` degrees Fahrenheit, -it's no longer cold enough to advise wearing a scarf -and so the `else` branch is triggered instead. - -You can chain multiple `if` statements together -to consider additional clauses. - -```swift -temperatureInFahrenheit = 90 -if temperatureInFahrenheit <= 32 { - print("It's very cold. Consider wearing a scarf.") -} else if temperatureInFahrenheit >= 86 { - print("It's really warm. Don't forget to wear sunscreen.") -} else { - print("It's not that cold. Wear a T-shirt.") -} -// Prints "It's really warm. Don't forget to wear sunscreen." -``` - - - -Here, an additional `if` statement was added to respond to particularly warm temperatures. -The final `else` clause remains, -and it prints a response for any temperatures that aren't too warm or too cold. - -The final `else` clause is optional, however, -and can be excluded if the set of conditions doesn't need to be complete. - -```swift -temperatureInFahrenheit = 72 -if temperatureInFahrenheit <= 32 { - print("It's very cold. Consider wearing a scarf.") -} else if temperatureInFahrenheit >= 86 { - print("It's really warm. Don't forget to wear sunscreen.") -} -``` - - - -Because the temperature isn't cold enough to trigger the `if` condition -or warm enough to trigger the `else if` condition, -no message is printed. - -Swift provides a shorthand spelling of `if` -that you can use when setting values. -For example, -consider the following code: - -```swift -let temperatureInCelsius = 25 -let weatherAdvice: String - -if temperatureInCelsius <= 0 { - weatherAdvice = "It's very cold. Consider wearing a scarf." -} else if temperatureInCelsius >= 30 { - weatherAdvice = "It's really warm. Don't forget to wear sunscreen." -} else { - weatherAdvice = "It's not that cold. Wear a T-shirt." -} - -print(weatherAdvice) -// Prints "It's not that cold. Wear a T-shirt." -``` - -Here, each of the branches sets a value for the `weatherAdvice` constant, -which is printed after the `if` statement. - -Using the alternate syntax, -known as an `if` expression, -you can write this code more concisely: - -```swift -let weatherAdvice = if temperatureInCelsius <= 0 { - "It's very cold. Consider wearing a scarf." -} else if temperatureInCelsius >= 30 { - "It's really warm. Don't forget to wear sunscreen." -} else { - "It's not that cold. Wear a T-shirt." -} - -print(weatherAdvice) -// Prints "It's not that cold. Wear a T-shirt." -``` - -In this `if` expression version, -each branch contains a single value. -If a branch's condition is true, -then that branch's value is used as the value for the whole `if` expression -in the assignment of `weatherAdvice`. -Every `if` branch has a corresponding `else if` branch or `else` branch, -ensuring that one of the branches always matches -and that the `if` expression always produces a value, -regardless of which conditions are true. - -Because the syntax for the assignment starts outside the `if` expression, -there's no need to repeat `weatherAdvice =` inside each branch. -Instead, -each branch of the `if` expression -produces one of the three possible values for `weatherAdvice`, -and the assignment uses that value. - -All of the branches of an `if` expression -need to contain values of the same type. -Because Swift checks the type of each branch separately, -values like `nil` that can be used with more than one type -prevent Swift from determining the `if` expression's type automatically. -Instead, you need to specify the type explicitly --- -for example: - -```swift -let freezeWarning: String? = if temperatureInCelsius <= 0 { - "It's below freezing. Watch for ice!" -} else { - nil -} -``` - -In the code above, -one branch of the `if` expression has a string value -and the other branch has a `nil` value. -The `nil` value could be used as a value for any optional type, -so you have to explicitly write that `freezeWarning` is an optional string, -as described in . - -An alternate way to provide this type information -is to provide an explicit type for `nil`, -instead of providing an explicit type for `freezeWarning`: - -```swift -let freezeWarning = if temperatureInCelsius <= 0 { - "It's below freezing. Watch for ice!" -} else { - nil as String? -} -``` - -An `if` expression can respond to unexpected failures by throwing an error -or calling a function like `fatalError(_:file:line:)` that never returns. -For example: - -```swift -let weatherAdvice = if temperatureInCelsius > 100 { - throw TemperatureError.boiling -} else { - "It's a reasonable temperature." -} -``` - -In this example, -the `if` expression checks whether the forecast temperature -is hotter than 100° C --- the boiling point of water. -A temperature this hot causes the `if` expression to throw a `.boiling` error -instead of returning a textual summary. -Even though this `if` expression can throw an error, -you don't write `try` before it. -For information about working with errors, see . - -In addition to using `if` expressions -on the right-hand side of an assignment, -as shown in the examples above, -you can also use them as the value that a function or closure returns. - -### Switch - -A `switch` statement considers a value -and compares it against several possible matching patterns. -It then executes an appropriate block of code, -based on the first pattern that matches successfully. -A `switch` statement provides an alternative to the `if` statement -for responding to multiple potential states. - -In its simplest form, a `switch` statement compares a value against -one or more values of the same type. - -```swift -switch <#some value to consider#> { -case <#value 1#>: - <#respond to value 1#> -case <#value 2#>, - <#value 3#>: - <#respond to value 2 or 3#> -default: - <#otherwise, do something else#> -} -``` - -Every `switch` statement consists of multiple possible *cases*, -each of which begins with the `case` keyword. -In addition to comparing against specific values, -Swift provides several ways for each case to specify -more complex matching patterns. -These options are described later in this chapter. - -Like the body of an `if` statement, each `case` is a separate branch of code execution. -The `switch` statement determines which branch should be selected. -This procedure is known as *switching* on the value that's being considered. - -Every `switch` statement must be *exhaustive*. -That is, every possible value of the type being considered -must be matched by one of the `switch` cases. -If it's not appropriate to provide a case for every possible value, -you can define a default case to cover any values that aren't addressed explicitly. -This default case is indicated by the `default` keyword, -and must always appear last. - -This example uses a `switch` statement to consider -a single lowercase character called `someCharacter`: - -```swift -let someCharacter: Character = "z" -switch someCharacter { -case "a": - print("The first letter of the Latin alphabet") -case "z": - print("The last letter of the Latin alphabet") -default: - print("Some other character") -} -// Prints "The last letter of the Latin alphabet" -``` - - - -The `switch` statement's first case matches -the first letter of the English alphabet, `a`, -and its second case matches the last letter, `z`. -Because the `switch` must have a case for every possible character, -not just every alphabetic character, -this `switch` statement uses a `default` case -to match all characters other than `a` and `z`. -This provision ensures that the `switch` statement is exhaustive. - -Like `if` statements, -`switch` statements also have an expression form: - -```swift -let anotherCharacter: Character = "a" -let message = switch anotherCharacter { -case "a": - "The first letter of the Latin alphabet" -case "z": - "The last letter of the Latin alphabet" -default: - "Some other character" -} - -print(message) -// Prints "The first letter of the Latin alphabet" -``` - -In this example, -each case in the `switch` expression -contains the value for `message` -to be used when that case matches `anotherCharacter`. -Because `switch` is always exhaustive, -there is always a value to assign. - -As with `if` expressions, -you can throw an error -or call a function like `fatalError(_:file:line:)` that never returns -instead of providing a value for a given case. -You can use `switch` expressions -on the right-hand side of an assignment, -as shown in the example above, -and as the value that a function or closure returns. - -#### No Implicit Fallthrough - -In contrast with `switch` statements in C and Objective-C, -`switch` statements in Swift don't -fall through the bottom of each case and into the next one by default. -Instead, the entire `switch` statement finishes its execution -as soon as the first matching `switch` case is completed, -without requiring an explicit `break` statement. -This makes the `switch` statement safer and easier to use than the one in C -and avoids executing more than one `switch` case by mistake. - -> Note: Although `break` isn't required in Swift, -> you can use a `break` statement to match and ignore a particular case -> or to break out of a matched case before that case has completed its execution. -> For details, see . - -The body of each case *must* contain at least one executable statement. -It isn't valid to write the following code, because the first case is empty: - -```swift -let anotherCharacter: Character = "a" -switch anotherCharacter { -case "a": // Invalid, the case has an empty body -case "A": - print("The letter A") -default: - print("Not the letter A") -} -// This will report a compile-time error. -``` - - - -Unlike a `switch` statement in C, -this `switch` statement doesn't match both `"a"` and `"A"`. -Rather, it reports a compile-time error that `case "a":` -doesn't contain any executable statements. -This approach avoids accidental fallthrough from one case to another -and makes for safer code that's clearer in its intent. - -To make a `switch` with a single case that -matches both `"a"` and `"A"`, -combine the two values into a compound case, -separating the values with commas. - -```swift -let anotherCharacter: Character = "a" -switch anotherCharacter { -case "a", "A": - print("The letter A") -default: - print("Not the letter A") -} -// Prints "The letter A" -``` - - - -For readability, -a compound case can also be written over multiple lines. -For more information about compound cases, -see . - -> Note: To explicitly fall through at the end of a particular `switch` case, -> use the `fallthrough` keyword, -> as described in . - -#### Interval Matching - -Values in `switch` cases can be checked for their inclusion in an interval. -This example uses number intervals -to provide a natural-language count for numbers of any size: - - - -```swift -let approximateCount = 62 -let countedThings = "moons orbiting Saturn" -let naturalCount: String -switch approximateCount { -case 0: - naturalCount = "no" -case 1..<5: - naturalCount = "a few" -case 5..<12: - naturalCount = "several" -case 12..<100: - naturalCount = "dozens of" -case 100..<1000: - naturalCount = "hundreds of" -default: - naturalCount = "many" -} -print("There are \(naturalCount) \(countedThings).") -// Prints "There are dozens of moons orbiting Saturn." -``` - - - -In the above example, `approximateCount` is evaluated in a `switch` statement. -Each `case` compares that value to a number or interval. -Because the value of `approximateCount` falls between 12 and 100, -`naturalCount` is assigned the value `"dozens of"`, -and execution is transferred out of the `switch` statement. - -#### Tuples - -You can use tuples to test multiple values in the same `switch` statement. -Each element of the tuple can be tested against a different value or interval of values. -Alternatively, use the underscore character (`_`), -also known as the wildcard pattern, -to match any possible value. - -The example below takes an (x, y) point, -expressed as a simple tuple of type `(Int, Int)`, -and categorizes it on the graph that follows the example. - -```swift -let somePoint = (1, 1) -switch somePoint { -case (0, 0): - print("\(somePoint) is at the origin") -case (_, 0): - print("\(somePoint) is on the x-axis") -case (0, _): - print("\(somePoint) is on the y-axis") -case (-2...2, -2...2): - print("\(somePoint) is inside the box") -default: - print("\(somePoint) is outside of the box") -} -// Prints "(1, 1) is inside the box" -``` - - - -![](coordinateGraphSimple) - -The `switch` statement determines whether the point is -at the origin (0, 0), -on the red x-axis, -on the green y-axis, -inside the blue 4-by-4 box centered on the origin, -or outside of the box. - -Unlike C, Swift allows multiple `switch` cases to consider the same value or values. -In fact, the point (0, 0) could match all *four* of the cases in this example. -However, if multiple matches are possible, -the first matching case is always used. -The point (0, 0) would match `case (0, 0)` first, -and so all other matching cases would be ignored. - -#### Value Bindings - -A `switch` case can name the value or values it matches to temporary constants or variables, -for use in the body of the case. -This behavior is known as *value binding*, -because the values are bound to temporary constants or variables within the case's body. - -The example below takes an (x, y) point, -expressed as a tuple of type `(Int, Int)`, -and categorizes it on the graph that follows: - -```swift -let anotherPoint = (2, 0) -switch anotherPoint { -case (let x, 0): - print("on the x-axis with an x value of \(x)") -case (0, let y): - print("on the y-axis with a y value of \(y)") -case let (x, y): - print("somewhere else at (\(x), \(y))") -} -// Prints "on the x-axis with an x value of 2" -``` - - - -![](coordinateGraphMedium) - -The `switch` statement determines whether the point is -on the red x-axis, -on the green y-axis, -or elsewhere (on neither axis). - -The three `switch` cases declare placeholder constants `x` and `y`, -which temporarily take on one or both tuple values from `anotherPoint`. -The first case, `case (let x, 0)`, -matches any point with a `y` value of `0` -and assigns the point's `x` value to the temporary constant `x`. -Similarly, the second case, `case (0, let y)`, -matches any point with an `x` value of `0` -and assigns the point's `y` value to the temporary constant `y`. - -After the temporary constants are declared, -they can be used within the case's code block. -Here, they're used to print the categorization of the point. - -This `switch` statement doesn't have a `default` case. -The final case, `case let (x, y)`, -declares a tuple of two placeholder constants that can match any value. -Because `anotherPoint` is always a tuple of two values, -this case matches all possible remaining values, -and a `default` case isn't needed to make the `switch` statement exhaustive. - -#### Where - -A `switch` case can use a `where` clause to check for additional conditions. - -The example below categorizes an (x, y) point on the following graph: - -```swift -let yetAnotherPoint = (1, -1) -switch yetAnotherPoint { -case let (x, y) where x == y: - print("(\(x), \(y)) is on the line x == y") -case let (x, y) where x == -y: - print("(\(x), \(y)) is on the line x == -y") -case let (x, y): - print("(\(x), \(y)) is just some arbitrary point") -} -// Prints "(1, -1) is on the line x == -y" -``` - - - -![](coordinateGraphComplex) - -The `switch` statement determines whether the point is -on the green diagonal line where `x == y`, -on the purple diagonal line where `x == -y`, -or neither. - -The three `switch` cases declare placeholder constants `x` and `y`, -which temporarily take on the two tuple values from `yetAnotherPoint`. -These constants are used as part of a `where` clause, -to create a dynamic filter. -The `switch` case matches the current value of `point` -only if the `where` clause's condition evaluates to `true` for that value. - -As in the previous example, the final case matches all possible remaining values, -and so a `default` case isn't needed to make the `switch` statement exhaustive. - -#### Compound Cases - -Multiple switch cases that share the same body -can be combined by writing several patterns after `case`, -with a comma between each of the patterns. -If any of the patterns match, then the case is considered to match. -The patterns can be written over multiple lines if the list is long. -For example: - -```swift -let someCharacter: Character = "e" -switch someCharacter { -case "a", "e", "i", "o", "u": - print("\(someCharacter) is a vowel") -case "b", "c", "d", "f", "g", "h", "j", "k", "l", "m", - "n", "p", "q", "r", "s", "t", "v", "w", "x", "y", "z": - print("\(someCharacter) is a consonant") -default: - print("\(someCharacter) isn't a vowel or a consonant") -} -// Prints "e is a vowel" -``` - - - -The `switch` statement's first case matches -all five lowercase vowels in the English language. -Similarly, its second case matches all lowercase English consonants. -Finally, the `default` case matches any other character. - -Compound cases can also include value bindings. -All of the patterns of a compound case -have to include the same set of value bindings, -and each binding has to get a value of the same type -from all of the patterns in the compound case. -This ensures that, -no matter which part of the compound case matched, -the code in the body of the case -can always access a value for the bindings -and that the value always has the same type. - -```swift -let stillAnotherPoint = (9, 0) -switch stillAnotherPoint { -case (let distance, 0), (0, let distance): - print("On an axis, \(distance) from the origin") -default: - print("Not on an axis") -} -// Prints "On an axis, 9 from the origin" -``` - - - -The `case` above has two patterns: -`(let distance, 0)` matches points on the x-axis -and `(0, let distance)` matches points on the y-axis. -Both patterns include a binding for `distance` -and `distance` is an integer in both patterns --- -which means that the code in the body of the `case` -can always access a value for `distance`. - -## Control Transfer Statements - -*Control transfer statements* change the order in which your code is executed, -by transferring control from one piece of code to another. -Swift has five control transfer statements: - -- `continue` -- `break` -- `fallthrough` -- `return` -- `throw` - -The `continue`, `break`, and `fallthrough` statements are described below. -The `return` statement is described in , -and the `throw` statement is described in . - -### Continue - -The `continue` statement tells a loop to stop what it's doing -and start again at the beginning of the next iteration through the loop. -It says “I am done with the current loop iteration” -without leaving the loop altogether. - -The following example removes all vowels and spaces from a lowercase string -to create a cryptic puzzle phrase: - -```swift -let puzzleInput = "great minds think alike" -var puzzleOutput = "" -let charactersToRemove: [Character] = ["a", "e", "i", "o", "u", " "] -for character in puzzleInput { - if charactersToRemove.contains(character) { - continue - } - puzzleOutput.append(character) -} -print(puzzleOutput) -// Prints "grtmndsthnklk" -``` - - - -The code above calls the `continue` keyword whenever it matches a vowel or a space, -causing the current iteration of the loop to end immediately -and to jump straight to the start of the next iteration. - -### Break - -The `break` statement ends execution of an entire control flow statement immediately. -The `break` statement can be used inside a `switch` or loop statement -when you want to terminate the execution of the `switch` or loop statement -earlier than would otherwise be the case. - -#### Break in a Loop Statement - -When used inside a loop statement, -`break` ends the loop's execution immediately -and transfers control to the code after the loop's closing brace (`}`). -No further code from the current iteration of the loop is executed, -and no further iterations of the loop are started. - - - -#### Break in a Switch Statement - -When used inside a `switch` statement, -`break` causes the `switch` statement to end its execution immediately -and to transfer control to the code after -the `switch` statement's closing brace (`}`). - -This behavior can be used to match and ignore one or more cases in a `switch` statement. -Because Swift's `switch` statement is exhaustive -and doesn't allow empty cases, -it's sometimes necessary to deliberately match and ignore a case -in order to make your intentions explicit. -You do this by writing the `break` statement as the entire body of the case you want to ignore. -When that case is matched by the `switch` statement, -the `break` statement inside the case ends the `switch` statement's execution immediately. - -> Note: A `switch` case that contains only a comment is reported as a compile-time error. -> Comments aren't statements and don't cause a `switch` case to be ignored. -> Always use a `break` statement to ignore a `switch` case. - -The following example switches on a `Character` value -and determines whether it represents a number symbol in one of four languages. -For brevity, multiple values are covered in a single `switch` case. - -```swift -let numberSymbol: Character = "三" // Chinese symbol for the number 3 -var possibleIntegerValue: Int? -switch numberSymbol { -case "1", "١", "一", "๑": - possibleIntegerValue = 1 -case "2", "٢", "二", "๒": - possibleIntegerValue = 2 -case "3", "٣", "三", "๓": - possibleIntegerValue = 3 -case "4", "٤", "四", "๔": - possibleIntegerValue = 4 -default: - break -} -if let integerValue = possibleIntegerValue { - print("The integer value of \(numberSymbol) is \(integerValue).") -} else { - print("An integer value couldn't be found for \(numberSymbol).") -} -// Prints "The integer value of 三 is 3." -``` - - - -This example checks `numberSymbol` to determine whether it's -a Latin, Arabic, Chinese, or Thai symbol for -the numbers `1` to `4`. -If a match is found, -one of the `switch` statement's cases sets -an optional `Int?` variable called `possibleIntegerValue` -to an appropriate integer value. - -After the `switch` statement completes its execution, -the example uses optional binding to determine whether a value was found. -The `possibleIntegerValue` variable has an implicit initial value of `nil` -by virtue of being an optional type, -and so the optional binding will succeed only -if `possibleIntegerValue` was set to an actual value -by one of the `switch` statement's first four cases. - -Because it's not practical to list every possible `Character` value in the example above, -a `default` case handles any characters that aren't matched. -This `default` case doesn't need to perform any action, -and so it's written with a single `break` statement as its body. -As soon as the `default` case is matched, -the `break` statement ends the `switch` statement's execution, -and code execution continues from the `if let` statement. - -### Fallthrough - -In Swift, `switch` statements don't fall through the bottom of each case and into the next one. -That is, the entire `switch` statement completes its execution as soon as the first matching case is completed. -By contrast, C requires you to insert an explicit `break` statement -at the end of every `switch` case to prevent fallthrough. -Avoiding default fallthrough means that Swift `switch` statements are -much more concise and predictable than their counterparts in C, -and thus they avoid executing multiple `switch` cases by mistake. - -If you need C-style fallthrough behavior, -you can opt in to this behavior on a case-by-case basis with the `fallthrough` keyword. -The example below uses `fallthrough` to create a textual description of a number. - -```swift -let integerToDescribe = 5 -var description = "The number \(integerToDescribe) is" -switch integerToDescribe { -case 2, 3, 5, 7, 11, 13, 17, 19: - description += " a prime number, and also" - fallthrough -default: - description += " an integer." -} -print(description) -// Prints "The number 5 is a prime number, and also an integer." -``` - - - -This example declares a new `String` variable called `description` -and assigns it an initial value. -The function then considers the value of `integerToDescribe` using a `switch` statement. -If the value of `integerToDescribe` is one of the prime numbers in the list, -the function appends text to the end of `description`, -to note that the number is prime. -It then uses the `fallthrough` keyword to “fall into” the `default` case as well. -The `default` case adds some extra text to the end of the description, -and the `switch` statement is complete. - -Unless the value of `integerToDescribe` is in the list of known prime numbers, -it isn't matched by the first `switch` case at all. -Because there are no other specific cases, -`integerToDescribe` is matched by the `default` case. - -After the `switch` statement has finished executing, -the number's description is printed using the `print(_:separator:terminator:)` function. -In this example, -the number `5` is correctly identified as a prime number. - -> Note: The `fallthrough` keyword doesn't check the case conditions -> for the `switch` case that it causes execution to fall into. -> The `fallthrough` keyword simply causes code execution to move -> directly to the statements inside the next case (or `default` case) block, -> as in C's standard `switch` statement behavior. - -### Labeled Statements - -In Swift, you can nest loops and conditional statements -inside other loops and conditional statements -to create complex control flow structures. -However, loops and conditional statements can both use the `break` statement -to end their execution prematurely. -Therefore, it's sometimes useful to be explicit about -which loop or conditional statement you want a `break` statement to terminate. -Similarly, if you have multiple nested loops, -it can be useful to be explicit about which loop the `continue` statement -should affect. - -To achieve these aims, -you can mark a loop statement or conditional statement with a *statement label*. -With a conditional statement, -you can use a statement label with the `break` statement -to end the execution of the labeled statement. -With a loop statement, -you can use a statement label with the `break` or `continue` statement -to end or continue the execution of the labeled statement. - -A labeled statement is indicated by placing -a label on the same line as the statement's introducer keyword, followed by a colon. -Here's an example of this syntax for a `while` loop, -although the principle is the same for all loops and `switch` statements: - -```swift -<#label name#>: while <#condition#> { - <#statements#> -} -``` - -The following example uses the `break` and `continue` statements -with a labeled `while` loop for an adapted version of the *Snakes and Ladders* game -that you saw earlier in this chapter. -This time around, the game has an extra rule: - -- To win, you must land *exactly* on square 25. - -If a particular dice roll would take you beyond square 25, -you must roll again until you roll the exact number needed to land on square 25. - -The game board is the same as before. - -![](snakesAndLadders) - -The values of `finalSquare`, `board`, `square`, and `diceRoll` -are initialized in the same way as before: - -```swift -let finalSquare = 25 -var board = [Int](repeating: 0, count: finalSquare + 1) -board[03] = +08; board[06] = +11; board[09] = +09; board[10] = +02 -board[14] = -10; board[19] = -11; board[22] = -02; board[24] = -08 -var square = 0 -var diceRoll = 0 -``` - - - -This version of the game uses a `while` loop and a `switch` statement -to implement the game's logic. -The `while` loop has a statement label called `gameLoop` -to indicate that it's the main game loop for the Snakes and Ladders game. - -The `while` loop's condition is `while square != finalSquare`, -to reflect that you must land exactly on square 25. - -```swift -gameLoop: while square != finalSquare { - diceRoll += 1 - if diceRoll == 7 { diceRoll = 1 } - switch square + diceRoll { - case finalSquare: - // diceRoll will move us to the final square, so the game is over - break gameLoop - case let newSquare where newSquare > finalSquare: - // diceRoll will move us beyond the final square, so roll again - continue gameLoop - default: - // this is a valid move, so find out its effect - square += diceRoll - square += board[square] - } -} -print("Game over!") -``` - - - -The dice is rolled at the start of each loop. -Rather than moving the player immediately, -the loop uses a `switch` statement to consider the result of the move -and to determine whether the move is allowed: - -- If the dice roll will move the player onto the final square, - the game is over. - The `break gameLoop` statement transfers control to - the first line of code outside of the `while` loop, which ends the game. -- If the dice roll will move the player *beyond* the final square, - the move is invalid and the player needs to roll again. - The `continue gameLoop` statement ends the current `while` loop iteration - and begins the next iteration of the loop. -- In all other cases, the dice roll is a valid move. - The player moves forward by `diceRoll` squares, - and the game logic checks for any snakes and ladders. - The loop then ends, and control returns to the `while` condition - to decide whether another turn is required. - -> Note: If the `break` statement above didn't use the `gameLoop` label, -> it would break out of the `switch` statement, not the `while` statement. -> Using the `gameLoop` label makes it clear which control statement should be terminated. -> -> It isn't strictly necessary to use the `gameLoop` label -> when calling `continue gameLoop` to jump to the next iteration of the loop. -> There's only one loop in the game, -> and therefore no ambiguity as to which loop the `continue` statement will affect. -> However, there's no harm in using the `gameLoop` label with the `continue` statement. -> Doing so is consistent with the label's use alongside the `break` statement -> and helps make the game's logic clearer to read and understand. - -## Early Exit - -A `guard` statement, like an `if` statement, -executes statements depending on the Boolean value of an expression. -You use a `guard` statement to require that a condition must be true -in order for the code after the `guard` statement to be executed. -Unlike an `if` statement, -a `guard` statement always has an `else` clause --- -the code inside the `else` clause is executed if the condition isn't true. - -```swift -func greet(person: [String: String]) { - guard let name = person["name"] else { - return - } - - print("Hello \(name)!") - - guard let location = person["location"] else { - print("I hope the weather is nice near you.") - return - } - - print("I hope the weather is nice in \(location).") -} - -greet(person: ["name": "John"]) -// Prints "Hello John!" -// Prints "I hope the weather is nice near you." -greet(person: ["name": "Jane", "location": "Cupertino"]) -// Prints "Hello Jane!" -// Prints "I hope the weather is nice in Cupertino." -``` - - - -If the `guard` statement's condition is met, -code execution continues after the `guard` statement's closing brace. -Any variables or constants that were assigned values -using an optional binding as part of the condition -are available for the rest of the code block -that the `guard` statement appears in. - -If that condition isn't met, -the code inside the `else` branch is executed. -That branch must transfer control to exit the code block -in which the `guard` statement appears. -It can do this with a control transfer statement -such as `return`, `break`, `continue`, or `throw`, -or it can call a function or method -that doesn't return, such as `fatalError(_:file:line:)`. - -Using a `guard` statement for requirements -improves the readability of your code, -compared to doing the same check with an `if` statement. -It lets you write the code that's typically executed -without wrapping it in an `else` block, -and it lets you keep the code that handles a violated requirement -next to the requirement. - -## Deferred Actions - -Unlike control-flow constructs like `if` and `while`, -which let you control whether part of your code is executed -or how many times it gets executed, -`defer` controls *when* a piece of code is executed. -You use a `defer` block to write code that will be executed later, -when your program reaches the end of the current scope. -For example: - -```swift -var score = 1 -if score < 10 { - defer { - print(score) - } - score += 5 -} -// Prints "6" -``` - - - -In the example above, -the code inside of the `defer` block is executed -before exiting the body of the `if` statement. -First, the code in the `if` statement runs, -which increments `score` by five. -Then, before exiting the `if` statement's scope, -the deferred code is run, which prints `score`. - -The code inside of the `defer` always runs, -regardless of how the program exits that scope. -That includes code like an early exit from a function, -breaking out of a `for` loop, -or throwing an error. -This behavior makes `defer` useful for operations -where you need to guarantee a pair of actions happen --- -like manually allocating and freeing memory, -opening and closing low-level file descriptors, -and beginning and ending transactions in a database --- -because you can write both actions next to each other in your code. -For example, -the following code gives a temporary bonus to the score, -by adding and subtracting 100 inside a chunk of code: - -```swift -var score = 3 -if score < 100 { - score += 100 - defer { - score -= 100 - } - // Other code that uses the score with its bonus goes here. - print(score) -} -// Prints "103" -``` - - - -If you write more than one `defer` block in the same scope, -the first one you specify is the last one to run. - -```swift -if score < 10 { - defer { - print(score) - } - defer { - print("The score is:") - } - score += 5 -} -// Prints "The score is:" -// Prints "6" -``` - - - -If your program stops running --- -for example, because of a runtime error or a crash --- -deferred code doesn't execute. -However, deferred code does execute after an error is thrown; -for information about using `defer` with error handling, -see . - -## Checking API Availability - -Swift has built-in support for checking API availability, -which ensures that you don't accidentally use APIs that are unavailable -on a given deployment target. - -The compiler uses availability information in the SDK -to verify that all of the APIs used in your code -are available on the deployment target specified by your project. -Swift reports an error at compile time -if you try to use an API that isn't available. - -You use an *availability condition* in an `if` or `guard` statement -to conditionally execute a block of code, -depending on whether the APIs you want to use are available at runtime. -The compiler uses the information from the availability condition -when it verifies that the APIs in that block of code are available. - -```swift -if #available(iOS 10, macOS 10.12, *) { - // Use iOS 10 APIs on iOS, and use macOS 10.12 APIs on macOS -} else { - // Fall back to earlier iOS and macOS APIs -} -``` - - - -The availability condition above specifies that in iOS, -the body of the `if` statement executes only in iOS 10 and later; -in macOS, only in macOS 10.12 and later. -The last argument, `*`, is required and specifies that on any other platform, -the body of the `if` executes on the minimum deployment target specified by your target. - -In its general form, -the availability condition takes a list of platform names and versions. -You use platform names such as `iOS`, `macOS`, `watchOS`, `tvOS`, and `visionOS` --- -for the full list, see . -In addition to specifying major version numbers like iOS 8 or macOS 10.10, -you can specify minor versions numbers like iOS 11.2.6 and macOS 10.13.3. - -```swift -if #available(<#platform name#> <#version#>, <#...#>, *) { - <#statements to execute if the APIs are available#> -} else { - <#fallback statements to execute if the APIs are unavailable#> -} -``` - -When you use an availability condition with a `guard` statement, -it refines the availability information that’s used -for the rest of the code in that code block. - -```swift -@available(macOS 10.12, *) -struct ColorPreference { - var bestColor = "blue" -} - -func chooseBestColor() -> String { - guard #available(macOS 10.12, *) else { - return "gray" - } - let colors = ColorPreference() - return colors.bestColor -} -``` - - - -In the example above, -the `ColorPreference` structure requires macOS 10.12 or later. -The `chooseBestColor()` function begins with an availability guard. -If the platform version is too old to use `ColorPreference`, -it falls back to behavior that's always available. -After the `guard` statement, -you can use APIs that require macOS 10.12 or later. - -In addition to `#available`, -Swift also supports the opposite check using an unavailability condition. -For example, the following two checks do the same thing: - -```swift -if #available(iOS 10, *) { -} else { - // Fallback code -} - -if #unavailable(iOS 10) { - // Fallback code -} -``` - - - -Using the `#unavailable` form helps make your code more readable -when the check contains only fallback code. - - - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/LanguageGuide/Deinitialization.md b/swift-6-beta.docc/LanguageGuide/Deinitialization.md deleted file mode 100644 index db3a50de4..000000000 --- a/swift-6-beta.docc/LanguageGuide/Deinitialization.md +++ /dev/null @@ -1,259 +0,0 @@ -# Deinitialization - -Release resources that require custom cleanup. - -A *deinitializer* is called immediately before a class instance is deallocated. -You write deinitializers with the `deinit` keyword, -similar to how initializers are written with the `init` keyword. -Deinitializers are only available on class types. - -## How Deinitialization Works - -Swift automatically deallocates your instances when they're no longer needed, -to free up resources. -Swift handles the memory management of instances through -*automatic reference counting* (*ARC*), -as described in . -Typically you don't need to perform manual cleanup when your instances are deallocated. -However, when you are working with your own resources, -you might need to perform some additional cleanup yourself. -For example, if you create a custom class to open a file and write some data to it, -you might need to close the file before the class instance is deallocated. - -Class definitions can have at most one deinitializer per class. -The deinitializer doesn't take any parameters -and is written without parentheses: - -```swift -deinit { - // perform the deinitialization -} -``` - - - -Deinitializers are called automatically, just before instance deallocation takes place. -You aren't allowed to call a deinitializer yourself. -Superclass deinitializers are inherited by their subclasses, -and the superclass deinitializer is called automatically at the end of -a subclass deinitializer implementation. -Superclass deinitializers are always called, -even if a subclass doesn't provide its own deinitializer. - -Because an instance isn't deallocated until after its deinitializer is called, -a deinitializer can access all properties of the instance it's called on -and can modify its behavior based on those properties -(such as looking up the name of a file that needs to be closed). - -## Deinitializers in Action - -Here's an example of a deinitializer in action. -This example defines two new types, `Bank` and `Player`, for a simple game. -The `Bank` class manages a made-up currency, -which can never have more than 10,000 coins in circulation. -There can only ever be one `Bank` in the game, -and so the `Bank` is implemented as a class with type properties and methods -to store and manage its current state: - -```swift -class Bank { - static var coinsInBank = 10_000 - static func distribute(coins numberOfCoinsRequested: Int) -> Int { - let numberOfCoinsToVend = min(numberOfCoinsRequested, coinsInBank) - coinsInBank -= numberOfCoinsToVend - return numberOfCoinsToVend - } - static func receive(coins: Int) { - coinsInBank += coins - } -} -``` - - - -`Bank` keeps track of the current number of coins it holds with its `coinsInBank` property. -It also offers two methods --- `distribute(coins:)` and `receive(coins:)` --- -to handle the distribution and collection of coins. - -The `distribute(coins:)` method checks that there are enough coins in the bank before distributing them. -If there aren't enough coins, -`Bank` returns a smaller number than the number that was requested -(and returns zero if no coins are left in the bank). -It returns an integer value to indicate the actual number of coins that were provided. - -The `receive(coins:)` method simply adds the received number of coins back into the bank's coin store. - -The `Player` class describes a player in the game. -Each player has a certain number of coins stored in their purse at any time. -This is represented by the player's `coinsInPurse` property: - -```swift -class Player { - var coinsInPurse: Int - init(coins: Int) { - coinsInPurse = Bank.distribute(coins: coins) - } - func win(coins: Int) { - coinsInPurse += Bank.distribute(coins: coins) - } - deinit { - Bank.receive(coins: coinsInPurse) - } -} -``` - - - -Each `Player` instance is initialized with a starting allowance of -a specified number of coins from the bank during initialization, -although a `Player` instance may receive fewer than that number -if not enough coins are available. - -The `Player` class defines a `win(coins:)` method, -which retrieves a certain number of coins from the bank -and adds them to the player's purse. -The `Player` class also implements a deinitializer, -which is called just before a `Player` instance is deallocated. -Here, the deinitializer simply returns all of the player's coins to the bank: - -```swift -var playerOne: Player? = Player(coins: 100) -print("A new player has joined the game with \(playerOne!.coinsInPurse) coins") -// Prints "A new player has joined the game with 100 coins" -print("There are now \(Bank.coinsInBank) coins left in the bank") -// Prints "There are now 9900 coins left in the bank" -``` - - - -A new `Player` instance is created, with a request for 100 coins if they're available. -This `Player` instance is stored in an optional `Player` variable called `playerOne`. -An optional variable is used here, because players can leave the game at any point. -The optional lets you track whether there's currently a player in the game. - -Because `playerOne` is an optional, it's qualified with an exclamation point (`!`) -when its `coinsInPurse` property is accessed to print its default number of coins, -and whenever its `win(coins:)` method is called: - -```swift -playerOne!.win(coins: 2_000) -print("PlayerOne won 2000 coins & now has \(playerOne!.coinsInPurse) coins") -// Prints "PlayerOne won 2000 coins & now has 2100 coins" -print("The bank now only has \(Bank.coinsInBank) coins left") -// Prints "The bank now only has 7900 coins left" -``` - - - -Here, the player has won 2,000 coins. -The player's purse now contains 2,100 coins, -and the bank has only 7,900 coins left. - -```swift -playerOne = nil -print("PlayerOne has left the game") -// Prints "PlayerOne has left the game" -print("The bank now has \(Bank.coinsInBank) coins") -// Prints "The bank now has 10000 coins" -``` - - - -The player has now left the game. -This is indicated by setting the optional `playerOne` variable to `nil`, -meaning “no `Player` instance.” -At the point that this happens, -the `playerOne` variable's reference to the `Player` instance is broken. -No other properties or variables are still referring to the `Player` instance, -and so it's deallocated in order to free up its memory. -Just before this happens, its deinitializer is called automatically, -and its coins are returned to the bank. - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/LanguageGuide/Enumerations.md b/swift-6-beta.docc/LanguageGuide/Enumerations.md deleted file mode 100644 index 5bc81a849..000000000 --- a/swift-6-beta.docc/LanguageGuide/Enumerations.md +++ /dev/null @@ -1,847 +0,0 @@ -# Enumerations - -Model custom types that define a list of possible values. - -An *enumeration* defines a common type for a group of related values -and enables you to work with those values in a type-safe way within your code. - -If you are familiar with C, -you will know that C enumerations assign related names to a set of integer values. -Enumerations in Swift are much more flexible, -and don't have to provide a value for each case of the enumeration. -If a value (known as a *raw* value) is provided for each enumeration case, -the value can be a string, a character, -or a value of any integer or floating-point type. - -Alternatively, enumeration cases can specify -associated values of *any* type to be stored along with each different case value, -much as unions or variants do in other languages. -You can define a common set of related cases as part of one enumeration, -each of which has a different set of values of appropriate types associated with it. - -Enumerations in Swift are first-class types in their own right. -They adopt many features traditionally supported only by classes, -such as computed properties to provide additional information about -the enumeration's current value, -and instance methods to provide functionality related to -the values the enumeration represents. -Enumerations can also define initializers to provide an initial case value; -can be extended to expand their functionality beyond their original implementation; -and can conform to protocols to provide standard functionality. - -For more about these capabilities, see -, , , -, and . - - - -## Enumeration Syntax - -You introduce enumerations with the `enum` keyword -and place their entire definition within a pair of braces: - -```swift -enum SomeEnumeration { - // enumeration definition goes here -} -``` - - - -Here's an example for the four main points of a compass: - -```swift -enum CompassPoint { - case north - case south - case east - case west -} -``` - - - -The values defined in an enumeration -(such as `north`, `south`, `east`, and `west`) -are its *enumeration cases*. -You use the `case` keyword to introduce new enumeration cases. - -> Note: Swift enumeration cases don't have an integer value set by default, -> unlike languages like C and Objective-C. -> In the `CompassPoint` example above, -> `north`, `south`, `east` and `west` -> don't implicitly equal -> `0`, `1`, `2` and `3`. -> Instead, the different enumeration cases are values in their own right, -> with an explicitly defined type of `CompassPoint`. - -Multiple cases can appear on a single line, separated by commas: - -```swift -enum Planet { - case mercury, venus, earth, mars, jupiter, saturn, uranus, neptune -} -``` - - - -Each enumeration definition defines a new type. -Like other types in Swift, their names -(such as `CompassPoint` and `Planet`) -start with a capital letter. -Give enumeration types singular rather than plural names, -so that they read as self-evident: - -```swift -var directionToHead = CompassPoint.west -``` - - - -The type of `directionToHead` is inferred -when it's initialized with one of the possible values of `CompassPoint`. -Once `directionToHead` is declared as a `CompassPoint`, -you can set it to a different `CompassPoint` value using a shorter dot syntax: - -```swift -directionToHead = .east -``` - - - -The type of `directionToHead` is already known, -and so you can drop the type when setting its value. -This makes for highly readable code when working with explicitly typed enumeration values. - -## Matching Enumeration Values with a Switch Statement - -You can match individual enumeration values with a `switch` statement: - -```swift -directionToHead = .south -switch directionToHead { -case .north: - print("Lots of planets have a north") -case .south: - print("Watch out for penguins") -case .east: - print("Where the sun rises") -case .west: - print("Where the skies are blue") -} -// Prints "Watch out for penguins" -``` - - - -You can read this code as: - -“Consider the value of `directionToHead`. -In the case where it equals `.north`, -print `"Lots of planets have a north"`. -In the case where it equals `.south`, -print `"Watch out for penguins"`.” - -…and so on. - -As described in , -a `switch` statement must be exhaustive when considering an enumeration's cases. -If the `case` for `.west` is omitted, -this code doesn't compile, -because it doesn't consider the complete list of `CompassPoint` cases. -Requiring exhaustiveness ensures that enumeration cases aren't accidentally omitted. - -When it isn't appropriate to provide a `case` for every enumeration case, -you can provide a `default` case to cover any cases that aren't addressed explicitly: - -```swift -let somePlanet = Planet.earth -switch somePlanet { -case .earth: - print("Mostly harmless") -default: - print("Not a safe place for humans") -} -// Prints "Mostly harmless" -``` - - - -## Iterating over Enumeration Cases - -For some enumerations, -it's useful to have a collection of all of that enumeration's cases. -You enable this by -writing `: CaseIterable` after the enumeration's name. -Swift exposes a collection of all the cases -as an `allCases` property of the enumeration type. -Here's an example: - -```swift -enum Beverage: CaseIterable { - case coffee, tea, juice -} -let numberOfChoices = Beverage.allCases.count -print("\(numberOfChoices) beverages available") -// Prints "3 beverages available" -``` - - - -In the example above, -you write `Beverage.allCases` to access a collection -that contains all of the cases of the `Beverage` enumeration. -You can use `allCases` like any other collection --- -the collection's elements are instances of the enumeration type, -so in this case they're `Beverage` values. -The example above counts how many cases there are, -and the example below uses a `for`-`in` loop to iterate over all the cases. - -```swift -for beverage in Beverage.allCases { - print(beverage) -} -// coffee -// tea -// juice -``` - - - -The syntax used in the examples above -marks the enumeration as conforming to the -[`CaseIterable`](https://developer.apple.com/documentation/swift/caseiterable) protocol. -For information about protocols, see . - -## Associated Values - -The examples in the previous section show how the cases of an enumeration are -a defined (and typed) value in their own right. -You can set a constant or variable to `Planet.earth`, -and check for this value later. -However, it's sometimes useful to be able to store -values of other types alongside these case values. -This additional information is called an *associated value*, -and it varies each time you use that case as a value in your code. - -You can define Swift enumerations to store associated values of any given type, -and the value types can be different for each case of the enumeration if needed. -Enumerations similar to these are known as -*discriminated unions*, *tagged unions*, or *variants* -in other programming languages. - -For example, suppose an inventory tracking system needs to -track products by two different types of barcode. -Some products are labeled with 1D barcodes in UPC format, -which uses the numbers `0` to `9`. -Each barcode has a number system digit, -followed by five manufacturer code digits and five product code digits. -These are followed by a check digit to verify that the code has been scanned correctly: - -![](barcode_UPC) - -Other products are labeled with 2D barcodes in QR code format, -which can use any ISO 8859-1 character -and can encode a string up to 2,953 characters long: - -![](barcode_QR) - -It's convenient for an inventory tracking system to store UPC barcodes -as a tuple of four integers, -and QR code barcodes as a string of any length. - -In Swift, an enumeration to define product barcodes of either type might look like this: - -```swift -enum Barcode { - case upc(Int, Int, Int, Int) - case qrCode(String) -} -``` - - - -This can be read as: - -“Define an enumeration type called `Barcode`, -which can take either a value of `upc` -with an associated value of type (`Int`, `Int`, `Int`, `Int`), -or a value of `qrCode` with an associated value of type `String`.” - -This definition doesn't provide any actual `Int` or `String` values --- -it just defines the *type* of associated values -that `Barcode` constants and variables can store -when they're equal to `Barcode.upc` or `Barcode.qrCode`. - -You can then create new barcodes using either type: - -```swift -var productBarcode = Barcode.upc(8, 85909, 51226, 3) -``` - - - -This example creates a new variable called `productBarcode` -and assigns it a value of `Barcode.upc` -with an associated tuple value of `(8, 85909, 51226, 3)`. - -You can assign the same product a different type of barcode: - -```swift -productBarcode = .qrCode("ABCDEFGHIJKLMNOP") -``` - - - -At this point, -the original `Barcode.upc` and its integer values are replaced by -the new `Barcode.qrCode` and its string value. -Constants and variables of type `Barcode` can store either a `.upc` or a `.qrCode` -(together with their associated values), -but they can store only one of them at any given time. - -You can check the different barcode types using a switch statement, -similar to the example in -. -This time, however, -the associated values are extracted as part of the switch statement. -You extract each associated value as a constant (with the `let` prefix) -or a variable (with the `var` prefix) -for use within the `switch` case's body: - -```swift -switch productBarcode { -case .upc(let numberSystem, let manufacturer, let product, let check): - print("UPC: \(numberSystem), \(manufacturer), \(product), \(check).") -case .qrCode(let productCode): - print("QR code: \(productCode).") -} -// Prints "QR code: ABCDEFGHIJKLMNOP." -``` - - - -If all of the associated values for an enumeration case -are extracted as constants, or if all are extracted as variables, -you can place a single `let` or `var` annotation before the case name, for brevity: - -```swift -switch productBarcode { -case let .upc(numberSystem, manufacturer, product, check): - print("UPC : \(numberSystem), \(manufacturer), \(product), \(check).") -case let .qrCode(productCode): - print("QR code: \(productCode).") -} -// Prints "QR code: ABCDEFGHIJKLMNOP." -``` - - - -## Raw Values - -The barcode example in -shows how cases of an enumeration can declare that they store -associated values of different types. -As an alternative to associated values, -enumeration cases can come prepopulated with default values -(called *raw values*), -which are all of the same type. - -Here's an example that stores raw ASCII values alongside named enumeration cases: - -```swift -enum ASCIIControlCharacter: Character { - case tab = "\t" - case lineFeed = "\n" - case carriageReturn = "\r" -} -``` - - - -Here, the raw values for an enumeration called `ASCIIControlCharacter` -are defined to be of type `Character`, -and are set to some of the more common ASCII control characters. -`Character` values are described in . - -Raw values can be -strings, characters, or any of the integer or floating-point number types. -Each raw value must be unique within its enumeration declaration. - -> Note: Raw values are *not* the same as associated values. -> Raw values are set to prepopulated values -> when you first define the enumeration in your code, -> like the three ASCII codes above. -> The raw value for a particular enumeration case is always the same. -> Associated values are set when you create a new constant or variable -> based on one of the enumeration's cases, -> and can be different each time you do so. - -### Implicitly Assigned Raw Values - -When you're working with enumerations that store integer or string raw values, -you don't have to explicitly assign a raw value for each case. -When you don't, Swift automatically assigns the values for you. - -For example, when integers are used for raw values, -the implicit value for each case is one more than the previous case. -If the first case doesn't have a value set, its value is `0`. - -The enumeration below is a refinement of the earlier `Planet` enumeration, -with integer raw values to represent each planet's order from the sun: - -```swift -enum Planet: Int { - case mercury = 1, venus, earth, mars, jupiter, saturn, uranus, neptune -} -``` - - - -In the example above, -`Planet.mercury` has an explicit raw value of `1`, -`Planet.venus` has an implicit raw value of `2`, and so on. - -When strings are used for raw values, -the implicit value for each case is the text of that case's name. - -The enumeration below is a refinement of the earlier `CompassPoint` enumeration, -with string raw values to represent each direction's name: - -```swift -enum CompassPoint: String { - case north, south, east, west -} -``` - - - -In the example above, -`CompassPoint.south` has an implicit raw value of `"south"`, and so on. - -You access the raw value of an enumeration case with its `rawValue` property: - -```swift -let earthsOrder = Planet.earth.rawValue -// earthsOrder is 3 - -let sunsetDirection = CompassPoint.west.rawValue -// sunsetDirection is "west" -``` - - - -### Initializing from a Raw Value - -If you define an enumeration with a raw-value type, -the enumeration automatically receives an initializer -that takes a value of the raw value's type (as a parameter called `rawValue`) -and returns either an enumeration case or `nil`. -You can use this initializer to try to create a new instance of the enumeration. - -This example identifies Uranus from its raw value of `7`: - -```swift -let possiblePlanet = Planet(rawValue: 7) -// possiblePlanet is of type Planet? and equals Planet.uranus -``` - - - -Not all possible `Int` values will find a matching planet, however. -Because of this, the raw value initializer always returns an *optional* enumeration case. -In the example above, `possiblePlanet` is of type `Planet?`, -or “optional `Planet`.” - -> Note: The raw value initializer is a failable initializer, -> because not every raw value will return an enumeration case. -> For more information, see . - -If you try to find a planet with a position of `11`, -the optional `Planet` value returned by the raw value initializer will be `nil`: - -```swift -let positionToFind = 11 -if let somePlanet = Planet(rawValue: positionToFind) { - switch somePlanet { - case .earth: - print("Mostly harmless") - default: - print("Not a safe place for humans") - } -} else { - print("There isn't a planet at position \(positionToFind)") -} -// Prints "There isn't a planet at position 11" -``` - - - -This example uses optional binding to try to access a planet with a raw value of `11`. -The statement `if let somePlanet = Planet(rawValue: 11)` creates an optional `Planet`, -and sets `somePlanet` to the value of that optional `Planet` if it can be retrieved. -In this case, it isn't possible to retrieve a planet with a position of `11`, -and so the `else` branch is executed instead. - - - -## Recursive Enumerations - -A *recursive enumeration* is an enumeration -that has another instance of the enumeration -as the associated value for one or more of the enumeration cases. -You indicate that an enumeration case is recursive -by writing `indirect` before it, -which tells the compiler to insert the necessary layer of indirection. - -For example, here is an enumeration that stores simple arithmetic expressions: - -```swift -enum ArithmeticExpression { - case number(Int) - indirect case addition(ArithmeticExpression, ArithmeticExpression) - indirect case multiplication(ArithmeticExpression, ArithmeticExpression) -} -``` - - - -You can also write `indirect` before the beginning of the enumeration -to enable indirection for all of the enumeration's cases that have an associated value: - -```swift -indirect enum ArithmeticExpression { - case number(Int) - case addition(ArithmeticExpression, ArithmeticExpression) - case multiplication(ArithmeticExpression, ArithmeticExpression) -} -``` - - - -This enumeration can store three kinds of arithmetic expressions: -a plain number, -the addition of two expressions, -and the multiplication of two expressions. -The `addition` and `multiplication` cases have associated values -that are also arithmetic expressions --- -these associated values make it possible to nest expressions. -For example, the expression `(5 + 4) * 2` -has a number on the right-hand side of the multiplication -and another expression on the left-hand side of the multiplication. -Because the data is nested, -the enumeration used to store the data also needs to support nesting --- -this means the enumeration needs to be recursive. -The code below shows the `ArithmeticExpression` recursive enumeration -being created for `(5 + 4) * 2`: - -```swift -let five = ArithmeticExpression.number(5) -let four = ArithmeticExpression.number(4) -let sum = ArithmeticExpression.addition(five, four) -let product = ArithmeticExpression.multiplication(sum, ArithmeticExpression.number(2)) -``` - - - -A recursive function is a straightforward way -to work with data that has a recursive structure. -For example, here's a function that evaluates an arithmetic expression: - -```swift -func evaluate(_ expression: ArithmeticExpression) -> Int { - switch expression { - case let .number(value): - return value - case let .addition(left, right): - return evaluate(left) + evaluate(right) - case let .multiplication(left, right): - return evaluate(left) * evaluate(right) - } -} - -print(evaluate(product)) -// Prints "18" -``` - - - -This function evaluates a plain number -by simply returning the associated value. -It evaluates an addition or multiplication -by evaluating the expression on the left-hand side, -evaluating the expression on the right-hand side, -and then adding them or multiplying them. - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/LanguageGuide/ErrorHandling.md b/swift-6-beta.docc/LanguageGuide/ErrorHandling.md deleted file mode 100644 index db77cf9ae..000000000 --- a/swift-6-beta.docc/LanguageGuide/ErrorHandling.md +++ /dev/null @@ -1,999 +0,0 @@ -# Error Handling - -Respond to and recover from errors. - -*Error handling* is the process of responding to -and recovering from error conditions in your program. -Swift provides first-class support for -throwing, catching, propagating, and manipulating -recoverable errors at runtime. - -Some operations -aren't guaranteed to always complete execution or produce a useful output. -Optionals are used to represent the absence of a value, -but when an operation fails, -it's often useful to understand what caused the failure, -so that your code can respond accordingly. - -As an example, consider the task of reading and processing data from a file on disk. -There are a number of ways this task can fail, including -the file not existing at the specified path, -the file not having read permissions, or -the file not being encoded in a compatible format. -Distinguishing among these different situations -allows a program to resolve some errors -and to communicate to the user any errors it can't resolve. - -> Note: Error handling in Swift interoperates with error handling patterns -> that use the `NSError` class in Cocoa and Objective-C. -> For more information about this class, -> see [Handling Cocoa Errors in Swift](https://developer.apple.com/documentation/swift/cocoa_design_patterns/handling_cocoa_errors_in_swift). - -## Representing and Throwing Errors - -In Swift, errors are represented by -values of types that conform to the `Error` protocol. -This empty protocol indicates that a type -can be used for error handling. - -Swift enumerations are particularly well suited to modeling -a group of related error conditions, -with associated values allowing for additional information -about the nature of an error to be communicated. -For example, here's how you might represent the error conditions -of operating a vending machine inside a game: - -```swift -enum VendingMachineError: Error { - case invalidSelection - case insufficientFunds(coinsNeeded: Int) - case outOfStock -} -``` - - - -Throwing an error lets you indicate that something unexpected happened -and the normal flow of execution can't continue. -You use a `throw` statement to throw an error. -For example, -the following code throws an error to indicate -that five additional coins are needed by the vending machine: - -```swift -throw VendingMachineError.insufficientFunds(coinsNeeded: 5) -``` - - - -## Handling Errors - -When an error is thrown, -some surrounding piece of code must be responsible -for handling the error --- -for example, by correcting the problem, -trying an alternative approach, -or informing the user of the failure. - -There are four ways to handle errors in Swift. -You can propagate the error from a function to the code that calls that function, -handle the error using a `do`-`catch` statement, -handle the error as an optional value, -or assert that the error will not occur. -Each approach is described in a section below. - -When a function throws an error, -it changes the flow of your program, -so it's important that you can quickly identify places in your code that can throw errors. -To identify these places in your code, write the `try` keyword --- -or the `try?` or `try!` variation --- -before a piece of code that calls a function, method, or initializer that can throw an error. -These keywords are described in the sections below. - -> Note: Error handling in Swift resembles exception handling in other languages, -> with the use of the `try`, `catch` and `throw` keywords. -> Unlike exception handling in many languages --- -> including Objective-C --- -> error handling in Swift doesn't involve unwinding the call stack, -> a process that can be computationally expensive. -> As such, the performance characteristics -> of a `throw` statement -> are comparable to those of a `return` statement. - -### Propagating Errors Using Throwing Functions - -To indicate that a function, method, or initializer can throw an error, -you write the `throws` keyword in the function's declaration -after its parameters. -A function marked with `throws` is called a *throwing function*. -If the function specifies a return type, -you write the `throws` keyword before the return arrow (`->`). - - - -```swift -func canThrowErrors() throws -> String - -func cannotThrowErrors() -> String -``` - - - - - - - - - - - -A throwing function propagates errors that are thrown inside of it -to the scope from which it's called. - -> Note: Only throwing functions can propagate errors. -> Any errors thrown inside a nonthrowing function -> must be handled inside the function. - -In the example below, -the `VendingMachine` class has a `vend(itemNamed:)` method -that throws an appropriate `VendingMachineError` -if the requested item isn't available, -is out of stock, -or has a cost that exceeds the current deposited amount: - -```swift -struct Item { - var price: Int - var count: Int -} - -class VendingMachine { - var inventory = [ - "Candy Bar": Item(price: 12, count: 7), - "Chips": Item(price: 10, count: 4), - "Pretzels": Item(price: 7, count: 11) - ] - var coinsDeposited = 0 - - func vend(itemNamed name: String) throws { - guard let item = inventory[name] else { - throw VendingMachineError.invalidSelection - } - - guard item.count > 0 else { - throw VendingMachineError.outOfStock - } - - guard item.price <= coinsDeposited else { - throw VendingMachineError.insufficientFunds(coinsNeeded: item.price - coinsDeposited) - } - - coinsDeposited -= item.price - - var newItem = item - newItem.count -= 1 - inventory[name] = newItem - - print("Dispensing \(name)") - } -} -``` - - - -The implementation of the `vend(itemNamed:)` method -uses `guard` statements to exit the method early and throw appropriate errors -if any of the requirements for purchasing a snack aren't met. -Because a `throw` statement immediately transfers program control, -an item will be vended only if all of these requirements are met. - -Because the `vend(itemNamed:)` method propagates any errors it throws, -any code that calls this method must either handle the errors --- -using a `do`-`catch` statement, `try?`, or `try!` --- -or continue to propagate them. -For example, -the `buyFavoriteSnack(person:vendingMachine:)` in the example below -is also a throwing function, -and any errors that the `vend(itemNamed:)` method throws will -propagate up to the point where the `buyFavoriteSnack(person:vendingMachine:)` function is called. - -```swift -let favoriteSnacks = [ - "Alice": "Chips", - "Bob": "Licorice", - "Eve": "Pretzels", -] -func buyFavoriteSnack(person: String, vendingMachine: VendingMachine) throws { - let snackName = favoriteSnacks[person] ?? "Candy Bar" - try vendingMachine.vend(itemNamed: snackName) -} -``` - - - -In this example, -the `buyFavoriteSnack(person: vendingMachine:)` function looks up a given person's favorite snack -and tries to buy it for them by calling the `vend(itemNamed:)` method. -Because the `vend(itemNamed:)` method can throw an error, -it's called with the `try` keyword in front of it. - -Throwing initializers can propagate errors in the same way as throwing functions. -For example, -the initializer for the `PurchasedSnack` structure in the listing below -calls a throwing function as part of the initialization process, -and it handles any errors that it encounters by propagating them to its caller. - -```swift -struct PurchasedSnack { - let name: String - init(name: String, vendingMachine: VendingMachine) throws { - try vendingMachine.vend(itemNamed: name) - self.name = name - } -} -``` - - - -### Handling Errors Using Do-Catch - -You use a `do`-`catch` statement to handle errors -by running a block of code. -If an error is thrown by the code in the `do` clause, -it's matched against the `catch` clauses -to determine which one of them can handle the error. - -Here is the general form of a `do`-`catch` statement: - -```swift -do { - try <#expression#> - <#statements#> -} catch <#pattern 1#> { - <#statements#> -} catch <#pattern 2#> where <#condition#> { - <#statements#> -} catch <#pattern 3#>, <#pattern 4#> where <#condition#> { - <#statements#> -} catch { - <#statements#> -} -``` - -You write a pattern after `catch` to indicate what errors -that clause can handle. -If a `catch` clause doesn't have a pattern, -the clause matches any error -and binds the error to a local constant named `error`. -For more information about pattern matching, -see . - - - -For example, the following code matches against all three cases -of the `VendingMachineError` enumeration. - -```swift -var vendingMachine = VendingMachine() -vendingMachine.coinsDeposited = 8 -do { - try buyFavoriteSnack(person: "Alice", vendingMachine: vendingMachine) - print("Success! Yum.") -} catch VendingMachineError.invalidSelection { - print("Invalid Selection.") -} catch VendingMachineError.outOfStock { - print("Out of Stock.") -} catch VendingMachineError.insufficientFunds(let coinsNeeded) { - print("Insufficient funds. Please insert an additional \(coinsNeeded) coins.") -} catch { - print("Unexpected error: \(error).") -} -// Prints "Insufficient funds. Please insert an additional 2 coins." -``` - - - -In the above example, -the `buyFavoriteSnack(person:vendingMachine:)` function is called in a `try` expression, -because it can throw an error. -If an error is thrown, -execution immediately transfers to the `catch` clauses, -which decide whether to allow propagation to continue. -If no pattern is matched, the error gets caught by the final `catch` -clause and is bound to a local `error` constant. -If no error is thrown, -the remaining statements in the `do` statement are executed. - -The `catch` clauses don't have to handle every possible error -that the code in the `do` clause can throw. -If none of the `catch` clauses handle the error, -the error propagates to the surrounding scope. -However, the propagated error -must be handled by *some* surrounding scope. -In a nonthrowing function, -an enclosing `do`-`catch` statement -must handle the error. -In a throwing function, -either an enclosing `do`-`catch` statement -or the caller -must handle the error. -If the error propagates to the top-level scope -without being handled, -you'll get a runtime error. - -For example, the above example can be written so any -error that isn't a `VendingMachineError` is instead -caught by the calling function: - -```swift -func nourish(with item: String) throws { - do { - try vendingMachine.vend(itemNamed: item) - } catch is VendingMachineError { - print("Couldn't buy that from the vending machine.") - } -} - -do { - try nourish(with: "Beet-Flavored Chips") -} catch { - print("Unexpected non-vending-machine-related error: \(error)") -} -// Prints "Couldn't buy that from the vending machine." -``` - - - -In the `nourish(with:)` function, -if `vend(itemNamed:)` throws an error that's -one of the cases of the `VendingMachineError` enumeration, -`nourish(with:)` handles the error by printing a message. -Otherwise, -`nourish(with:)` propagates the error to its call site. -The error is then caught by the general `catch` clause. - -Another way to catch several related errors -is to list them after `catch`, separated by commas. -For example: - -```swift -func eat(item: String) throws { - do { - try vendingMachine.vend(itemNamed: item) - } catch VendingMachineError.invalidSelection, VendingMachineError.insufficientFunds, VendingMachineError.outOfStock { - print("Invalid selection, out of stock, or not enough money.") - } -} -``` - - - - - -The `eat(item:)` function lists the vending machine errors to catch, -and its error text corresponds to the items in that list. -If any of the three listed errors are thrown, -this `catch` clause handles them by printing a message. -Any other errors are propagated to the surrounding scope, -including any vending-machine errors that might be added later. - -### Converting Errors to Optional Values - -You use `try?` to handle an error by converting it to an optional value. -If an error is thrown while evaluating the `try?` expression, -the value of the expression is `nil`. -For example, -in the following code `x` and `y` have the same value and behavior: - -```swift -func someThrowingFunction() throws -> Int { - // ... -} - -let x = try? someThrowingFunction() - -let y: Int? -do { - y = try someThrowingFunction() -} catch { - y = nil -} -``` - - - -If `someThrowingFunction()` throws an error, -the value of `x` and `y` is `nil`. -Otherwise, the value of `x` and `y` is the value that the function returned. -Note that `x` and `y` are an optional of whatever type `someThrowingFunction()` returns. -Here the function returns an integer, so `x` and `y` are optional integers. - -Using `try?` lets you write concise error handling code -when you want to handle all errors in the same way. -For example, -the following code -uses several approaches to fetch data, -or returns `nil` if all of the approaches fail. - -```swift -func fetchData() -> Data? { - if let data = try? fetchDataFromDisk() { return data } - if let data = try? fetchDataFromServer() { return data } - return nil -} -``` - - - -### Disabling Error Propagation - -Sometimes you know a throwing function or method -won't, in fact, throw an error at runtime. -On those occasions, -you can write `try!` before the expression to disable error propagation -and wrap the call in a runtime assertion that no error will be thrown. -If an error actually is thrown, you'll get a runtime error. - -For example, the following code uses a `loadImage(atPath:)` function, -which loads the image resource at a given path -or throws an error if the image can't be loaded. -In this case, because the image is shipped with the application, -no error will be thrown at runtime, -so it's appropriate to disable error propagation. - -```swift -let photo = try! loadImage(atPath: "./Resources/John Appleseed.jpg") -``` - - - -## Specifying the Error Type - -All of the examples above use the most common kind of error handling, -where the errors that your code throws -can be values of any type that conforms to the `Error` protocol. -This approach matches the reality that -you don't know ahead of time every error that could happen -while the code is running, -especially when propagating errors thrown somewhere else. -It also reflects the fact that errors can change over time. -New versions of a library --- -including libraries that your dependencies use --- -can throw new errors, -and the rich complexity of real-world user configurations -can expose failure modes that weren't visible during development or testing. -The error handling code in the examples above -always includes a default case to handle errors -that don't have a specific `catch` clause. - -Most Swift code doesn't specify the type for the errors it throws. -However, -you might limit code to throwing errors of only one specific type -in the following special cases: - -- When running code on an embedded system - that doesn't support dynamic allocation of memory. - Throwing an instance of `any Error` or another boxed protocol type - requires allocating memory at runtime to store the error. - In contrast, - throwing an error of a specific type - lets Swift avoid heap allocation for errors. - -- When the errors are an implementation detail of some unit of code, - like a library, - and aren't part of the interface to that code. - Because the errors come from only the library, - and not from other dependencies or the library's clients, - you can make an exhaustive list of all possible failures. - And because these errors are an implementation detail of the library, - they're always handled within that library. - -- In code that only propagates errors described by generic parameters, - like a function that takes a closure argument - and propagates any errors from that closure. - For a comparison between propagating a specific error type - and using `rethrows`, - see . - -For example, -consider code that summarizes ratings -and uses the following error type: - -```swift -enum StatisticsError: Error { - case noRatings - case invalidRating(Int) -} -``` - -To specify that a function throws only `StatisticsError` values as its errors, -you write `throws(StatisticsError)` instead of only `throws` -when declaring the function. -This syntax is also called *typed throws* -because you write the error type after `throws` in the declaration. -For example, -the function below throws `StatisticsError` values as its errors. - -```swift -func summarize(_ ratings: [Int]) throws(StatisticsError) { - guard !ratings.isEmpty else { throw .noRatings } - - var counts = [1: 0, 2: 0, 3: 0] - for rating in ratings { - guard rating > 0 && rating <= 3 else { throw .invalidRating(rating) } - counts[rating]! += 1 - } - - print("*", counts[1]!, "-- **", counts[2]!, "-- ***", counts[3]!) -} -``` - -In the code above, -the `summarize(_:)` function summarizes a list of ratings -expressed on a scale of 1 to 3. -This function throws an instance of `StatisticsError` if the input isn't valid. -Both places in the code above that throw an error -omit the type of the error -because the function's error type is already defined. -You can use the short form, `throw .noRatings`, -instead of writing `throw StatisticsError.noRatings` -when throwing an error in a function like this. - -When you write a specific error type at the start of the function, -Swift checks that you don't throw any other errors. -For example, -if you tried to use `VendingMachineError` from examples earlier in this chapter -in the `summarize(_:)` function above, -that code would produce an error at compile time. - -You can call a function that uses typed throws -from within a regular throwing function: - -```swift -func someThrowingFunction() -> throws { - let ratings = [1, 2, 3, 2, 2, 1] - try summarize(ratings) -} -``` - -The code above doesn't specify an error type for `someThrowingFunction()`, -so it throws `any Error`. -You could also write the error type explicitly as `throws(any Error)`; -the code below is equivalent to the code above: - -```swift -func someThrowingFunction() -> throws(any Error) { - let ratings = [1, 2, 3, 2, 2, 1] - try summarize(ratings) -} -``` - -In this code, -`someThrowingFunction()` propagates any errors that `summarize(_:)` throws. -The errors from `summarize(_:)` are always `StatisticsError` values, -which is also a valid error for `someThrowingFunction()` to throw. - -Just like you can write a function that never returns -with a return type of `Never`, -you can write a function that never throws with `throws(Never)`: - -```swift -func nonThrowingFunction() throws(Never) { - // ... -} -``` -This function can't throw because -it's impossible to create a value of type `Never` to throw. - -In addition to specifying a function's error type, -you can also write a specific error type for a `do`-`catch` statement. -For example: - -```swift -let ratings = [] -do throws(StatisticsError) { - try summarize(ratings) -} catch { - switch error { - case .noRatings: - print("No ratings available") - case .invalidRating(let rating): - print("Invalid rating: \(rating)") - } -} -// Prints "No ratings available" -``` - -In this code, -writing `do throws(StatisticsError)` indicates that -the `do`-`catch` statement throws `StatisticsError` values as its errors. -Like other `do`-`catch` statements, -the `catch` clause can either handle every possible error -or propagate unhandled errors for some surrounding scope to handle. -This code handles all of the errors, -using a `switch` statement with one case for each enumeration value. -Like other `catch` clauses that don't have a pattern, -the clause matches any error -and binds the error to a local constant named `error`. -Because the `do`-`catch` statement throws `StatisticsError` values, -`error` is a value of type `StatisticsError`. - -The `catch` clause above uses a `switch` statement -to match and handle each possible error. -If you tried to add a new case to `StatisticsError` -without updating the error-handling code, -Swift would give you an error -because the `switch` statement wouldn't be exhaustive anymore. -For a library that catches all of its own errors, -you could use this approach to ensure any new errors -get corresponding new code to handle them. - -If a function or `do` block throws errors of only a single type, -Swift infers that this code is using typed throws. -Using this shorter syntax, -you could write the `do`-`catch` example above as follows: - -```swift -let ratings = [] -do { - try summarize(ratings) -} catch { - switch error { - case .noRatings: - print("No ratings available") - case .invalidRating(let rating): - print("Invalid rating: \(rating)") - } -} -// Prints "No ratings available" -``` - -Even though the `do`-`catch` block above -doesn't specify what type of error it throws, -Swift infers that it throws `StatisticsError`. -You can explicitly write `throws(any Error)` -to avoid letting Swift infer typed throws. - -## Specifying Cleanup Actions - -You use a `defer` statement to execute a set of statements -just before code execution leaves the current block of code. -This statement lets you do any necessary cleanup -that should be performed regardless -of *how* execution leaves the current block of code --- -whether it leaves because an error was thrown -or because of a statement such as `return` or `break`. -For example, you can use a `defer` statement -to ensure that file descriptors are closed -and manually allocated memory is freed. - -A `defer` statement defers execution until the current scope is exited. -This statement consists of the `defer` keyword and the statements to be executed later. -The deferred statements may not contain any code -that would transfer control out of the statements, -such as a `break` or a `return` statement, -or by throwing an error. -Deferred actions are executed in the reverse of -the order that they're written in your source code. -That is, the code in the first `defer` statement executes last, -the code in the second `defer` statement executes second to last, -and so on. -The last `defer` statement in source code order executes first. - -```swift -func processFile(filename: String) throws { - if exists(filename) { - let file = open(filename) - defer { - close(file) - } - while let line = try file.readline() { - // Work with the file. - } - // close(file) is called here, at the end of the scope. - } -} -``` - - - -The above example uses a `defer` statement -to ensure that the `open(_:)` function -has a corresponding call to `close(_:)`. - -You can use a `defer` statement -even when no error handling code is involved. -For more information, -see . - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/LanguageGuide/Functions.md b/swift-6-beta.docc/LanguageGuide/Functions.md deleted file mode 100644 index bf21b2c1e..000000000 --- a/swift-6-beta.docc/LanguageGuide/Functions.md +++ /dev/null @@ -1,1321 +0,0 @@ -# Functions - -Define and call functions, label their arguments, and use their return values. - -*Functions* are self-contained chunks of code that perform a specific task. -You give a function a name that identifies what it does, -and this name is used to “call” the function to perform its task when needed. - -Swift's unified function syntax is flexible enough to express anything from -a simple C-style function with no parameter names -to a complex Objective-C-style method -with names and argument labels for each parameter. -Parameters can provide default values to simplify function calls -and can be passed as in-out parameters, -which modify a passed variable once the function has completed its execution. - -Every function in Swift has a type, -consisting of the function's parameter types and return type. -You can use this type like any other type in Swift, -which makes it easy to pass functions as parameters to other functions, -and to return functions from functions. -Functions can also be written within other functions -to encapsulate useful functionality within a nested function scope. - -## Defining and Calling Functions - -When you define a function, -you can optionally define one or more named, typed values that the function takes as input, -known as *parameters*. -You can also optionally define -a type of value that the function will pass back as output when it's done, -known as its *return type*. - -Every function has a *function name*, -which describes the task that the function performs. -To use a function, you “call” that function with its name -and pass it input values (known as *arguments*) -that match the types of the function's parameters. -A function's arguments must always be provided in the same order -as the function's parameter list. - -The function in the example below is called `greet(person:)`, -because that's what it does --- -it takes a person's name as input and returns a greeting for that person. -To accomplish this, you define one input parameter --- -a `String` value called `person` --- -and a return type of `String`, -which will contain a greeting for that person: - -```swift -func greet(person: String) -> String { - let greeting = "Hello, " + person + "!" - return greeting -} -``` - - - -All of this information is rolled up into the function's *definition*, -which is prefixed with the `func` keyword. -You indicate the function's return type with the *return arrow* `->` -(a hyphen followed by a right angle bracket), -which is followed by the name of the type to return. - -The definition describes what the function does, -what it expects to receive, -and what it returns when it's done. -The definition makes it easy for the function to be called unambiguously -from elsewhere in your code: - -```swift -print(greet(person: "Anna")) -// Prints "Hello, Anna!" -print(greet(person: "Brian")) -// Prints "Hello, Brian!" -``` - - - -You call the `greet(person:)` function -by passing it a `String` value after the `person` argument label, -such as `greet(person: "Anna")`. -Because the function returns a `String` value, -`greet(person:)` can be wrapped in a call to the `print(_:separator:terminator:)` function -to print that string and see its return value, as shown above. - -> Note: The `print(_:separator:terminator:)` function -> doesn't have a label for its first argument, -> and its other arguments are optional because they have a default value. -> These variations on function syntax are discussed below -> in -> and . - -The body of the `greet(person:)` function starts by -defining a new `String` constant called `greeting` -and setting it to a simple greeting message. -This greeting is then passed back out of the function using the `return` keyword. -In the line of code that says `return greeting`, -the function finishes its execution and returns the current value of `greeting`. - -You can call the `greet(person:)` function multiple times with different input values. -The example above shows what happens if it's called with an input value of `"Anna"`, -and an input value of `"Brian"`. -The function returns a tailored greeting in each case. - -To make the body of this function shorter, -you can combine the message creation and the return statement into one line: - -```swift -func greetAgain(person: String) -> String { - return "Hello again, " + person + "!" -} -print(greetAgain(person: "Anna")) -// Prints "Hello again, Anna!" -``` - - - -## Function Parameters and Return Values - -Function parameters and return values are extremely flexible in Swift. -You can define anything from a simple utility function with a single unnamed parameter -to a complex function with expressive parameter names and different parameter options. - -### Functions Without Parameters - -Functions aren't required to define input parameters. -Here's a function with no input parameters, -which always returns the same `String` message whenever it's called: - -```swift -func sayHelloWorld() -> String { - return "hello, world" -} -print(sayHelloWorld()) -// Prints "hello, world" -``` - - - -The function definition still needs parentheses after the function's name, -even though it doesn't take any parameters. -The function name is also followed by -an empty pair of parentheses when the function is called. - -### Functions With Multiple Parameters - -Functions can have multiple input parameters, -which are written within the function's parentheses, separated by commas. - -This function takes a person's name -and whether they have already been greeted as input, -and returns an appropriate greeting for that person: - -```swift -func greet(person: String, alreadyGreeted: Bool) -> String { - if alreadyGreeted { - return greetAgain(person: person) - } else { - return greet(person: person) - } -} -print(greet(person: "Tim", alreadyGreeted: true)) -// Prints "Hello again, Tim!" -``` - - - -You call the `greet(person:alreadyGreeted:)` function -by passing it both a `String` argument value labeled `person` -and a `Bool` argument value labeled `alreadyGreeted` -in parentheses, separated by commas. -Note that this function is distinct from the `greet(person:)` function -shown in an earlier section. -Although both functions have names that begin with `greet`, -the `greet(person:alreadyGreeted:)` function takes two arguments -but the `greet(person:)` function takes only one. - -### Functions Without Return Values - -Functions aren't required to define a return type. -Here's a version of the `greet(person:)` function, -which prints its own `String` value rather than returning it: - -```swift -func greet(person: String) { - print("Hello, \(person)!") -} -greet(person: "Dave") -// Prints "Hello, Dave!" -``` - - - -Because it doesn't need to return a value, -the function's definition doesn't include the return arrow (`->`) -or a return type. - -> Note: Strictly speaking, this version of the `greet(person:)` function *does* still return a value, -> even though no return value is defined. -> Functions without a defined return type return a special value of type `Void`. -> This is simply an empty tuple, -> which is written as `()`. - -The return value of a function can be ignored when it's called: - -```swift -func printAndCount(string: String) -> Int { - print(string) - return string.count -} -func printWithoutCounting(string: String) { - let _ = printAndCount(string: string) -} -printAndCount(string: "hello, world") -// prints "hello, world" and returns a value of 12 -printWithoutCounting(string: "hello, world") -// prints "hello, world" but doesn't return a value -``` - - - - - -The first function, `printAndCount(string:)`, -prints a string, and then returns its character count as an `Int`. -The second function, `printWithoutCounting(string:)`, -calls the first function, but ignores its return value. -When the second function is called, -the message is still printed by the first function, -but the returned value isn't used. - -> Note: Return values can be ignored, -> but a function that says it will return a value must always do so. -> A function with a defined return type -> can't allow control to fall out of the bottom of the function -> without returning a value, -> and attempting to do so will result in a compile-time error. - - - -### Functions with Multiple Return Values - -You can use a tuple type as the return type for a function -to return multiple values as part of one compound return value. - -The example below defines a function called `minMax(array:)`, -which finds the smallest and largest numbers in an array of `Int` values: - -```swift -func minMax(array: [Int]) -> (min: Int, max: Int) { - var currentMin = array[0] - var currentMax = array[0] - for value in array[1.. currentMax { - currentMax = value - } - } - return (currentMin, currentMax) -} -``` - - - -The `minMax(array:)` function returns a tuple containing two `Int` values. -These values are labeled `min` and `max` -so that they can be accessed by name when querying the function's return value. - -The body of the `minMax(array:)` function starts by setting -two working variables called `currentMin` and `currentMax` -to the value of the first integer in the array. -The function then iterates over the remaining values in the array -and checks each value to see if it's smaller or larger than -the values of `currentMin` and `currentMax` respectively. -Finally, the overall minimum and maximum values are returned as -a tuple of two `Int` values. - -Because the tuple's member values are named as part of the function's return type, -they can be accessed with dot syntax to retrieve the minimum and maximum found values: - -```swift -let bounds = minMax(array: [8, -6, 2, 109, 3, 71]) -print("min is \(bounds.min) and max is \(bounds.max)") -// Prints "min is -6 and max is 109" -``` - - - -Note that the tuple's members don't need to be named -at the point that the tuple is returned from the function, -because their names are already specified as part of the function's return type. - -#### Optional Tuple Return Types - -If the tuple type to be returned from a function -has the potential to have “no value” for the entire tuple, -you can use an *optional* tuple return type to reflect the fact that -the entire tuple can be `nil`. -You write an optional tuple return type by placing a question mark -after the tuple type's closing parenthesis, -such as `(Int, Int)?` or `(String, Int, Bool)?`. - -> Note: An optional tuple type such as `(Int, Int)?` -> is different from a tuple that contains optional types -> such as `(Int?, Int?)`. -> With an optional tuple type, the entire tuple is optional, -> not just each individual value within the tuple. - -The `minMax(array:)` function above returns a tuple containing two `Int` values. -However, the function doesn't perform any safety checks on the array it's passed. -If the `array` argument contains an empty array, -the `minMax(array:)` function, as defined above, -will trigger a runtime error when attempting to access `array[0]`. - -To handle an empty array safely, -write the `minMax(array:)` function with an optional tuple return type -and return a value of `nil` when the array is empty: - -```swift -func minMax(array: [Int]) -> (min: Int, max: Int)? { - if array.isEmpty { return nil } - var currentMin = array[0] - var currentMax = array[0] - for value in array[1.. currentMax { - currentMax = value - } - } - return (currentMin, currentMax) -} -``` - - - -You can use optional binding to check whether this version of the `minMax(array:)` function -returns an actual tuple value or `nil`: - -```swift -if let bounds = minMax(array: [8, -6, 2, 109, 3, 71]) { - print("min is \(bounds.min) and max is \(bounds.max)") -} -// Prints "min is -6 and max is 109" -``` - - - -### Functions With an Implicit Return - -If the entire body of the function is a single expression, -the function implicitly returns that expression. -For example, -both functions below have the same behavior: - -```swift -func greeting(for person: String) -> String { - "Hello, " + person + "!" -} -print(greeting(for: "Dave")) -// Prints "Hello, Dave!" - -func anotherGreeting(for person: String) -> String { - return "Hello, " + person + "!" -} -print(anotherGreeting(for: "Dave")) -// Prints "Hello, Dave!" -``` - - - -The entire definition of the `greeting(for:)` function -is the greeting message that it returns, -which means it can use this shorter form. -The `anotherGreeting(for:)` function returns the same greeting message, -using the `return` keyword like a longer function. -Any function that you write as just one `return` line can omit the `return`. - -As you'll see in , -property getters can also use an implicit return. - -> Note: The code you write as an implicit return value -> needs to return some value. -> For example, -> you can't use `print(13)` -> as an implicit return value. -> However, you can use a function that never returns -> like `fatalError("Oh no!")` -> as an implicit return value, -> because Swift knows that the implicit return doesn't happen. - - - -## Function Argument Labels and Parameter Names - -Each function parameter has both an *argument label* -and a *parameter name*. -The argument label is used when calling the function; -each argument is written in the function call with its argument label before it. -The parameter name is used in the implementation of the function. -By default, parameters -use their parameter name as their argument label. - -```swift -func someFunction(firstParameterName: Int, secondParameterName: Int) { - // In the function body, firstParameterName and secondParameterName - // refer to the argument values for the first and second parameters. -} -someFunction(firstParameterName: 1, secondParameterName: 2) -``` - - - -All parameters must have unique names. -Although it's possible for multiple parameters -to have the same argument label, -unique argument labels help make your code more readable. - - - -### Specifying Argument Labels - -You write an argument label before the parameter name, -separated by a space: - -```swift -func someFunction(argumentLabel parameterName: Int) { - // In the function body, parameterName refers to the argument value - // for that parameter. -} -``` - - - -Here's a variation of the `greet(person:)` function -that takes a person's name and hometown -and returns a greeting: - -```swift -func greet(person: String, from hometown: String) -> String { - return "Hello \(person)! Glad you could visit from \(hometown)." -} -print(greet(person: "Bill", from: "Cupertino")) -// Prints "Hello Bill! Glad you could visit from Cupertino." -``` - - - -The use of argument labels can allow a function -to be called in an expressive, sentence-like manner, -while still providing a function body that's readable and clear in intent. - -### Omitting Argument Labels - -If you don't want an argument label for a parameter, -write an underscore (`_`) instead of an explicit argument label for that parameter. - -```swift -func someFunction(_ firstParameterName: Int, secondParameterName: Int) { - // In the function body, firstParameterName and secondParameterName - // refer to the argument values for the first and second parameters. -} -someFunction(1, secondParameterName: 2) -``` - - - -If a parameter has an argument label, -the argument *must* be labeled when you call the function. - -### Default Parameter Values - -You can define a *default value* for any parameter in a function -by assigning a value to the parameter after that parameter's type. -If a default value is defined, you can omit that parameter when calling the function. - -```swift -func someFunction(parameterWithoutDefault: Int, parameterWithDefault: Int = 12) { - // If you omit the second argument when calling this function, then - // the value of parameterWithDefault is 12 inside the function body. -} -someFunction(parameterWithoutDefault: 3, parameterWithDefault: 6) // parameterWithDefault is 6 -someFunction(parameterWithoutDefault: 4) // parameterWithDefault is 12 -``` - - - -Place parameters that don't have default values -at the beginning of a function's parameter list, -before the parameters that have default values. -Parameters that don't have default values -are usually more important to the function's meaning --- -writing them first makes it easier to recognize -that the same function is being called, -regardless of whether any default parameters are omitted. - -### Variadic Parameters - -A *variadic parameter* accepts zero or more values of a specified type. -You use a variadic parameter to specify that the parameter can be passed -a varying number of input values when the function is called. -Write variadic parameters by inserting three period characters (`...`) -after the parameter's type name. - -The values passed to a variadic parameter are made available within the function's body -as an array of the appropriate type. -For example, a variadic parameter with a name of `numbers` and a type of `Double...` -is made available within the function's body as -a constant array called `numbers` of type `[Double]`. - -The example below calculates the *arithmetic mean* -(also known as the *average*) for a list of numbers of any length: - -```swift -func arithmeticMean(_ numbers: Double...) -> Double { - var total: Double = 0 - for number in numbers { - total += number - } - return total / Double(numbers.count) -} -arithmeticMean(1, 2, 3, 4, 5) -// returns 3.0, which is the arithmetic mean of these five numbers -arithmeticMean(3, 8.25, 18.75) -// returns 10.0, which is the arithmetic mean of these three numbers -``` - - - - - -A function can have multiple variadic parameters. -The first parameter that comes after a variadic parameter -must have an argument label. -The argument label makes it unambiguous -which arguments are passed to the variadic parameter -and which arguments are passed to the parameters -that come after the variadic parameter. - - - - - -### In-Out Parameters - -Function parameters are constants by default. -Trying to change the value of a function parameter -from within the body of that function results in a compile-time error. -This means that you can't change the value of a parameter by mistake. -If you want a function to modify a parameter's value, -and you want those changes to persist after the function call has ended, -define that parameter as an *in-out parameter* instead. - -You write an in-out parameter by placing the `inout` keyword -right before a parameter's type. -An in-out parameter has a value that's passed *in* to the function, -is modified by the function, -and is passed back *out* of the function to replace the original value. -For a detailed discussion of the behavior of in-out parameters -and associated compiler optimizations, -see . - -You can only pass a variable as the argument for an in-out parameter. -You can't pass a constant or a literal value as the argument, -because constants and literals can't be modified. -You place an ampersand (`&`) directly before a variable's name -when you pass it as an argument to an in-out parameter, -to indicate that it can be modified by the function. - -> Note: In-out parameters can't have default values, -> and variadic parameters can't be marked as `inout`. - -Here's an example of a function called `swapTwoInts(_:_:)`, -which has two in-out integer parameters called `a` and `b`: - -```swift -func swapTwoInts(_ a: inout Int, _ b: inout Int) { - let temporaryA = a - a = b - b = temporaryA -} -``` - - - -The `swapTwoInts(_:_:)` function simply swaps the value of `b` into `a`, -and the value of `a` into `b`. -The function performs this swap by storing the value of `a` in -a temporary constant called `temporaryA`, assigning the value of `b` to `a`, -and then assigning `temporaryA` to `b`. - -You can call the `swapTwoInts(_:_:)` function with two variables of type `Int` -to swap their values. -Note that the names of `someInt` and `anotherInt` are prefixed with an ampersand -when they're passed to the `swapTwoInts(_:_:)` function: - -```swift -var someInt = 3 -var anotherInt = 107 -swapTwoInts(&someInt, &anotherInt) -print("someInt is now \(someInt), and anotherInt is now \(anotherInt)") -// Prints "someInt is now 107, and anotherInt is now 3" -``` - - - -The example above shows that -the original values of `someInt` and `anotherInt` -are modified by the `swapTwoInts(_:_:)` function, -even though they were originally defined outside of the function. - -> Note: In-out parameters aren't the same as returning a value from a function. -> The `swapTwoInts` example above doesn't define a return type or return a value, -> but it still modifies the values of `someInt` and `anotherInt`. -> In-out parameters are an alternative way for a function to have an effect -> outside of the scope of its function body. - - - -## Function Types - -Every function has a specific *function type*, -made up of the parameter types and the return type of the function. - -For example: - -```swift -func addTwoInts(_ a: Int, _ b: Int) -> Int { - return a + b -} -func multiplyTwoInts(_ a: Int, _ b: Int) -> Int { - return a * b -} -``` - - - -This example defines two simple mathematical functions -called `addTwoInts` and `multiplyTwoInts`. -These functions each take two `Int` values, -and return an `Int` value, which is the result of -performing an appropriate mathematical operation. - -The type of both of these functions is `(Int, Int) -> Int`. -This can be read as: - -“A function that has two parameters, both of type `Int`, -and that returns a value of type `Int`.” - -Here's another example, for a function with no parameters or return value: - -```swift -func printHelloWorld() { - print("hello, world") -} -``` - - - -The type of this function is `() -> Void`, -or “a function that has no parameters, and returns `Void`.” - -### Using Function Types - -You use function types just like any other types in Swift. -For example, you can define a constant or variable to be of a function type -and assign an appropriate function to that variable: - -```swift -var mathFunction: (Int, Int) -> Int = addTwoInts -``` - - - -This can be read as: - -“Define a variable called `mathFunction`, -which has a type of ‘a function that takes two `Int` values, -and returns an `Int` value.’ -Set this new variable to refer to the function called `addTwoInts`.” - -The `addTwoInts(_:_:)` function has the same type as the `mathFunction` variable, -and so this assignment is allowed by Swift's type-checker. - -You can now call the assigned function with the name `mathFunction`: - -```swift -print("Result: \(mathFunction(2, 3))") -// Prints "Result: 5" -``` - - - -A different function with the same matching type can be assigned to the same variable, -in the same way as for nonfunction types: - -```swift -mathFunction = multiplyTwoInts -print("Result: \(mathFunction(2, 3))") -// Prints "Result: 6" -``` - - - -As with any other type, -you can leave it to Swift to infer the function type -when you assign a function to a constant or variable: - -```swift -let anotherMathFunction = addTwoInts -// anotherMathFunction is inferred to be of type (Int, Int) -> Int -``` - - - - - -### Function Types as Parameter Types - -You can use a function type such as `(Int, Int) -> Int` -as a parameter type for another function. -This enables you to leave some aspects of a function's implementation -for the function's caller to provide when the function is called. - -Here's an example to print the results of the math functions from above: - -```swift -func printMathResult(_ mathFunction: (Int, Int) -> Int, _ a: Int, _ b: Int) { - print("Result: \(mathFunction(a, b))") -} -printMathResult(addTwoInts, 3, 5) -// Prints "Result: 8" -``` - - - -This example defines a function called `printMathResult(_:_:_:)`, which has three parameters. -The first parameter is called `mathFunction`, and is of type `(Int, Int) -> Int`. -You can pass any function of that type as the argument for this first parameter. -The second and third parameters are called `a` and `b`, and are both of type `Int`. -These are used as the two input values for the provided math function. - -When `printMathResult(_:_:_:)` is called, -it's passed the `addTwoInts(_:_:)` function, and the integer values `3` and `5`. -It calls the provided function with the values `3` and `5`, and prints the result of `8`. - -The role of `printMathResult(_:_:_:)` is to print the result of -a call to a math function of an appropriate type. -It doesn't matter what that function's implementation actually does --- -it matters only that the function is of the correct type. -This enables `printMathResult(_:_:_:)` to hand off some of its functionality -to the caller of the function in a type-safe way. - -### Function Types as Return Types - -You can use a function type as the return type of another function. -You do this by writing a complete function type -immediately after the return arrow (`->`) of the returning function. - -The next example defines two simple functions called `stepForward(_:)` and `stepBackward(_:)`. -The `stepForward(_:)` function returns a value one more than its input value, -and the `stepBackward(_:)` function returns a value one less than its input value. -Both functions have a type of `(Int) -> Int`: - -```swift -func stepForward(_ input: Int) -> Int { - return input + 1 -} -func stepBackward(_ input: Int) -> Int { - return input - 1 -} -``` - - - -Here's a function called `chooseStepFunction(backward:)`, -whose return type is `(Int) -> Int`. -The `chooseStepFunction(backward:)` function returns the `stepForward(_:)` function -or the `stepBackward(_:)` function based on a Boolean parameter called `backward`: - -```swift -func chooseStepFunction(backward: Bool) -> (Int) -> Int { - return backward ? stepBackward : stepForward -} -``` - - - -You can now use `chooseStepFunction(backward:)` to obtain a function -that will step in one direction or the other: - -```swift -var currentValue = 3 -let moveNearerToZero = chooseStepFunction(backward: currentValue > 0) -// moveNearerToZero now refers to the stepBackward() function -``` - - - -The example above determines whether a positive or negative step is needed -to move a variable called `currentValue` progressively closer to zero. -`currentValue` has an initial value of `3`, -which means that `currentValue > 0` returns `true`, -causing `chooseStepFunction(backward:)` to return the `stepBackward(_:)` function. -A reference to the returned function is stored in a constant called `moveNearerToZero`. - -Now that `moveNearerToZero` refers to the correct function, -it can be used to count to zero: - -```swift -print("Counting to zero:") -// Counting to zero: -while currentValue != 0 { - print("\(currentValue)... ") - currentValue = moveNearerToZero(currentValue) -} -print("zero!") -// 3... -// 2... -// 1... -// zero! -``` - - - -## Nested Functions - -All of the functions you have encountered so far in this chapter -have been examples of *global functions*, which are defined at a global scope. -You can also define functions inside the bodies of other functions, -known as *nested functions*. - -Nested functions are hidden from the outside world by default, -but can still be called and used by their enclosing function. -An enclosing function can also return one of its nested functions -to allow the nested function to be used in another scope. - -You can rewrite the `chooseStepFunction(backward:)` example above -to use and return nested functions: - -```swift -func chooseStepFunction(backward: Bool) -> (Int) -> Int { - func stepForward(input: Int) -> Int { return input + 1 } - func stepBackward(input: Int) -> Int { return input - 1 } - return backward ? stepBackward : stepForward -} -var currentValue = -4 -let moveNearerToZero = chooseStepFunction(backward: currentValue > 0) -// moveNearerToZero now refers to the nested stepForward() function -while currentValue != 0 { - print("\(currentValue)... ") - currentValue = moveNearerToZero(currentValue) -} -print("zero!") -// -4... -// -3... -// -2... -// -1... -// zero! -``` - - - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/LanguageGuide/Generics.md b/swift-6-beta.docc/LanguageGuide/Generics.md deleted file mode 100644 index 62fcb5fdf..000000000 --- a/swift-6-beta.docc/LanguageGuide/Generics.md +++ /dev/null @@ -1,1981 +0,0 @@ -# Generics - -Write code that works for multiple types and specify requirements for those types. - -*Generic code* enables you to write flexible, reusable functions and types -that can work with any type, subject to requirements that you define. -You can write code that avoids duplication -and expresses its intent in a clear, abstracted manner. - -Generics are one of the most powerful features of Swift, -and much of the Swift standard library is built with generic code. -In fact, you've been using generics throughout the *Language Guide*, -even if you didn't realize it. -For example, Swift's `Array` and `Dictionary` types -are both generic collections. -You can create an array that holds `Int` values, -or an array that holds `String` values, -or indeed an array for any other type that can be created in Swift. -Similarly, you can create a dictionary to store values of any specified type, -and there are no limitations on what that type can be. - -## The Problem That Generics Solve - -Here's a standard, nongeneric function called `swapTwoInts(_:_:)`, -which swaps two `Int` values: - -```swift -func swapTwoInts(_ a: inout Int, _ b: inout Int) { - let temporaryA = a - a = b - b = temporaryA -} -``` - - - -This function makes use of in-out parameters to swap the values of `a` and `b`, -as described in . - -The `swapTwoInts(_:_:)` function swaps the original value of `b` into `a`, -and the original value of `a` into `b`. -You can call this function to swap the values in two `Int` variables: - -```swift -var someInt = 3 -var anotherInt = 107 -swapTwoInts(&someInt, &anotherInt) -print("someInt is now \(someInt), and anotherInt is now \(anotherInt)") -// Prints "someInt is now 107, and anotherInt is now 3" -``` - - - -The `swapTwoInts(_:_:)` function is useful, but it can only be used with `Int` values. -If you want to swap two `String` values, -or two `Double` values, -you have to write more functions, -such as the `swapTwoStrings(_:_:)` and `swapTwoDoubles(_:_:)` functions shown below: - -```swift -func swapTwoStrings(_ a: inout String, _ b: inout String) { - let temporaryA = a - a = b - b = temporaryA -} - -func swapTwoDoubles(_ a: inout Double, _ b: inout Double) { - let temporaryA = a - a = b - b = temporaryA -} -``` - - - -You may have noticed that the bodies of -the `swapTwoInts(_:_:)`, `swapTwoStrings(_:_:)`, and `swapTwoDoubles(_:_:)` functions are identical. -The only difference is the type of the values that they accept -(`Int`, `String`, and `Double`). - -It's more useful, and considerably more flexible, -to write a single function that swaps two values of *any* type. -Generic code enables you to write such a function. -(A generic version of these functions is defined below.) - -> Note: In all three functions, -> the types of `a` and `b` must be the same. -> If `a` and `b` aren't of the same type, -> it isn't possible to swap their values. -> Swift is a type-safe language, -> and doesn't allow (for example) a variable of type `String` -> and a variable of type `Double` -> to swap values with each other. -> Attempting to do so results in a compile-time error. - -## Generic Functions - -*Generic functions* can work with any type. -Here's a generic version of the `swapTwoInts(_:_:)` function from above, -called `swapTwoValues(_:_:)`: - -```swift -func swapTwoValues(_ a: inout T, _ b: inout T) { - let temporaryA = a - a = b - b = temporaryA -} -``` - - - - - -The body of the `swapTwoValues(_:_:)` function -is identical to the body of the `swapTwoInts(_:_:)` function. -However, the first line of `swapTwoValues(_:_:)` -is slightly different from `swapTwoInts(_:_:)`. -Here's how the first lines compare: - -```swift -func swapTwoInts(_ a: inout Int, _ b: inout Int) -func swapTwoValues(_ a: inout T, _ b: inout T) -``` - - - -The generic version of the function -uses a *placeholder* type name (called `T`, in this case) -instead of an *actual* type name (such as `Int`, `String`, or `Double`). -The placeholder type name doesn't say anything about what `T` must be, -but it *does* say that both `a` and `b` must be of the same type `T`, -whatever `T` represents. -The actual type to use in place of `T` -is determined each time the `swapTwoValues(_:_:)` function is called. - -The other difference between a generic function and a nongeneric function -is that the generic function's name (`swapTwoValues(_:_:)`) -is followed by the placeholder type name (`T`) inside angle brackets (``). -The brackets tell Swift that `T` is a placeholder type name -within the `swapTwoValues(_:_:)` function definition. -Because `T` is a placeholder, Swift doesn't look for an actual type called `T`. - -The `swapTwoValues(_:_:)` function can now be called in the same way as `swapTwoInts`, -except that it can be passed two values of *any* type, -as long as both of those values are of the same type as each other. -Each time `swapTwoValues(_:_:)` is called, -the type to use for `T` is inferred from the types of values passed to the function. - -In the two examples below, `T` is inferred to be `Int` and `String` respectively: - -```swift -var someInt = 3 -var anotherInt = 107 -swapTwoValues(&someInt, &anotherInt) -// someInt is now 107, and anotherInt is now 3 - -var someString = "hello" -var anotherString = "world" -swapTwoValues(&someString, &anotherString) -// someString is now "world", and anotherString is now "hello" -``` - - - -> Note: The `swapTwoValues(_:_:)` function defined above is inspired by -> a generic function called `swap`, which is part of the Swift standard library, -> and is automatically made available for you to use in your apps. -> If you need the behavior of the `swapTwoValues(_:_:)` function in your own code, -> you can use Swift's existing `swap(_:_:)` function rather than providing your own implementation. - -## Type Parameters - -In the `swapTwoValues(_:_:)` example above, -the placeholder type `T` is an example of a *type parameter*. -Type parameters specify and name a placeholder type, -and are written immediately after the function's name, -between a pair of matching angle brackets (such as ``). - -Once you specify a type parameter, -you can use it to define the type of a function's parameters -(such as the `a` and `b` parameters of the `swapTwoValues(_:_:)` function), -or as the function's return type, -or as a type annotation within the body of the function. -In each case, the type parameter -is replaced with an *actual* type whenever the function is called. -(In the `swapTwoValues(_:_:)` example above, -`T` was replaced with `Int` the first time the function was called, -and was replaced with `String` the second time it was called.) - -You can provide more than one type parameter -by writing multiple type parameter names within the angle brackets, -separated by commas. - -## Naming Type Parameters - -In most cases, type parameters have descriptive names, -such as `Key` and `Value` in `Dictionary` -and `Element` in `Array`, -which tells the reader about the relationship between the type parameter -and the generic type or function it's used in. -However, when there isn't a meaningful relationship between them, -it's traditional to name them using single letters such as `T`, `U`, and `V`, -such as `T` in the `swapTwoValues(_:_:)` function above. - -> Note: Always give type parameters upper camel case names -> (such as `T` and `MyTypeParameter`) -> to indicate that they're a placeholder for a *type*, not a value. - -## Generic Types - -In addition to generic functions, -Swift enables you to define your own *generic types*. -These are custom classes, structures, and enumerations -that can work with *any* type, in a similar way to `Array` and `Dictionary`. - -This section shows you how to write a generic collection type called `Stack`. -A stack is an ordered set of values, similar to an array, -but with a more restricted set of operations than Swift's `Array` type. -An array allows new items to be inserted and removed at any location in the array. -A stack, however, allows new items to be appended only to the end of the collection -(known as *pushing* a new value on to the stack). -Similarly, a stack allows items to be removed only from the end of the collection -(known as *popping* a value off the stack). - -> Note: The concept of a stack is used by the `UINavigationController` class -> to model the view controllers in its navigation hierarchy. -> You call the `UINavigationController` class -> `pushViewController(_:animated:)` method to add (or push) -> a view controller on to the navigation stack, -> and its `popViewControllerAnimated(_:)` method to remove (or pop) -> a view controller from the navigation stack. -> A stack is a useful collection model whenever you need a strict -> “last in, first out” approach to managing a collection. - -The illustration below shows the push and pop behavior for a stack: - -![](stackPushPop) - -1. There are currently three values on the stack. -2. A fourth value is pushed onto the top of the stack. -3. The stack now holds four values, with the most recent one at the top. -4. The top item in the stack is popped. -5. After popping a value, the stack once again holds three values. - -Here's how to write a nongeneric version of a stack, -in this case for a stack of `Int` values: - -```swift -struct IntStack { - var items: [Int] = [] - mutating func push(_ item: Int) { - items.append(item) - } - mutating func pop() -> Int { - return items.removeLast() - } -} -``` - - - -This structure uses an `Array` property called `items` to store the values in the stack. -`Stack` provides two methods, `push` and `pop`, -to push and pop values on and off the stack. -These methods are marked as `mutating`, -because they need to modify (or *mutate*) the structure's `items` array. - -The `IntStack` type shown above can only be used with `Int` values, however. -It would be much more useful to define a *generic* `Stack` structure, -that can manage a stack of *any* type of value. - -Here's a generic version of the same code: - -```swift -struct Stack { - var items: [Element] = [] - mutating func push(_ item: Element) { - items.append(item) - } - mutating func pop() -> Element { - return items.removeLast() - } -} -``` - - - -Note how the generic version of `Stack` -is essentially the same as the nongeneric version, -but with a type parameter called `Element` -instead of an actual type of `Int`. -This type parameter is written within a pair of angle brackets (``) -immediately after the structure's name. - -`Element` defines a placeholder name for -a type to be provided later. -This future type can be referred to as `Element` -anywhere within the structure's definition. -In this case, `Element` is used as a placeholder in three places: - -- To create a property called `items`, - which is initialized with an empty array of values of type `Element` -- To specify that the `push(_:)` method has a single parameter called `item`, - which must be of type `Element` -- To specify that the value returned by the `pop()` method - will be a value of type `Element` - -Because it's a generic type, -`Stack` can be used to create a stack of *any* valid type in Swift, -in a similar manner to `Array` and `Dictionary`. - -You create a new `Stack` instance by writing -the type to be stored in the stack within angle brackets. -For example, to create a new stack of strings, -you write `Stack()`: - -```swift -var stackOfStrings = Stack() -stackOfStrings.push("uno") -stackOfStrings.push("dos") -stackOfStrings.push("tres") -stackOfStrings.push("cuatro") -// the stack now contains 4 strings -``` - - - -Here's how `stackOfStrings` looks after pushing these four values on to the stack: - -![](stackPushedFourStrings) - -Popping a value from the stack removes and returns the top value, `"cuatro"`: - -```swift -let fromTheTop = stackOfStrings.pop() -// fromTheTop is equal to "cuatro", and the stack now contains 3 strings -``` - - - -Here's how the stack looks after popping its top value: - -![](stackPoppedOneString) - -## Extending a Generic Type - -When you extend a generic type, -you don't provide a type parameter list as part of the extension's definition. -Instead, the type parameter list from the *original* type definition -is available within the body of the extension, -and the original type parameter names are used to refer to -the type parameters from the original definition. - -The following example extends the generic `Stack` type to add -a read-only computed property called `topItem`, -which returns the top item on the stack without popping it from the stack: - -```swift -extension Stack { - var topItem: Element? { - return items.isEmpty ? nil : items[items.count - 1] - } -} -``` - - - -The `topItem` property returns an optional value of type `Element`. -If the stack is empty, `topItem` returns `nil`; -if the stack isn't empty, `topItem` returns the final item in the `items` array. - -Note that this extension doesn't define a type parameter list. -Instead, the `Stack` type's existing type parameter name, `Element`, -is used within the extension to indicate the optional type of -the `topItem` computed property. - -The `topItem` computed property can now be used with any `Stack` instance -to access and query its top item without removing it. - -```swift -if let topItem = stackOfStrings.topItem { - print("The top item on the stack is \(topItem).") -} -// Prints "The top item on the stack is tres." -``` - - - -Extensions of a generic type can also include requirements -that instances of the extended type must satisfy -in order to gain the new functionality, -as discussed in below. - -## Type Constraints - -The `swapTwoValues(_:_:)` function and the `Stack` type can work with any type. -However, it's sometimes useful to enforce -certain *type constraints* on the types that can be used with -generic functions and generic types. -Type constraints specify that a type parameter must -inherit from a specific class, -or conform to a particular protocol or protocol composition. - -For example, -Swift's `Dictionary` type places a limitation on -the types that can be used as keys for a dictionary. -As described in , -the type of a dictionary's keys must be *hashable*. -That is, it must provide a way to make itself uniquely representable. -`Dictionary` needs its keys to be hashable so that it can -check whether it already contains a value for a particular key. -Without this requirement, `Dictionary` couldn't tell -whether it should insert or replace a value for a particular key, -nor would it be able to find a value for a given key that's already in the dictionary. - -This requirement is enforced by a type constraint on the key type for `Dictionary`, -which specifies that the key type must conform to the `Hashable` protocol, -a special protocol defined in the Swift standard library. -All of Swift's basic types (such as `String`, `Int`, `Double`, and `Bool`) -are hashable by default. -For information about -making your own custom types conform to the `Hashable` protocol, -see [Conforming to the Hashable Protocol](https://developer.apple.com/documentation/swift/hashable#2849490). - -You can define your own type constraints when creating custom generic types, -and these constraints provide much of the power of generic programming. -Abstract concepts like `Hashable` -characterize types in terms of their conceptual characteristics, -rather than their concrete type. - -### Type Constraint Syntax - -You write type constraints by placing a single class or protocol constraint -after a type parameter's name, separated by a colon, -as part of the type parameter list. -The basic syntax for type constraints on a generic function is shown below -(although the syntax is the same for generic types): - -```swift -func someFunction(someT: T, someU: U) { - // function body goes here -} -``` - - - -The hypothetical function above has two type parameters. -The first type parameter, `T`, has a type constraint -that requires `T` to be a subclass of `SomeClass`. -The second type parameter, `U`, has a type constraint -that requires `U` to conform to the protocol `SomeProtocol`. - -### Type Constraints in Action - -Here's a nongeneric function called `findIndex(ofString:in:)`, -which is given a `String` value to find -and an array of `String` values within which to find it. -The `findIndex(ofString:in:)` function returns an optional `Int` value, -which will be the index of the first matching string in the array if it's found, -or `nil` if the string can't be found: - -```swift -func findIndex(ofString valueToFind: String, in array: [String]) -> Int? { - for (index, value) in array.enumerated() { - if value == valueToFind { - return index - } - } - return nil -} -``` - - - -The `findIndex(ofString:in:)` function can be used to find a string value in an array of strings: - -```swift -let strings = ["cat", "dog", "llama", "parakeet", "terrapin"] -if let foundIndex = findIndex(ofString: "llama", in: strings) { - print("The index of llama is \(foundIndex)") -} -// Prints "The index of llama is 2" -``` - - - -The principle of finding the index of a value in an array isn't useful only for strings, however. -You can write the same functionality as a generic function -by replacing any mention of strings with values of some type `T` instead. - -Here's how you might expect a generic version of `findIndex(ofString:in:)`, -called `findIndex(of:in:)`, to be written. -Note that the return type of this function is still `Int?`, -because the function returns an optional index number, -not an optional value from the array. -Be warned, though --- this function doesn't compile, -for reasons explained after the example: - -```swift -func findIndex(of valueToFind: T, in array:[T]) -> Int? { - for (index, value) in array.enumerated() { - if value == valueToFind { - return index - } - } - return nil -} -``` - - - -This function doesn't compile as written above. -The problem lies with the equality check, “`if value == valueToFind`”. -Not every type in Swift can be compared with the equal to operator (`==`). -If you create your own class or structure to represent a complex data model, for example, -then the meaning of “equal to” for that class or structure -isn't something that Swift can guess for you. -Because of this, it isn't possible to guarantee that this code will work -for *every* possible type `T`, -and an appropriate error is reported when you try to compile the code. - -All is not lost, however. -The Swift standard library defines a protocol called `Equatable`, -which requires any conforming type to implement -the equal to operator (`==`) and the not equal to operator (`!=`) -to compare any two values of that type. -All of Swift's standard types automatically support the `Equatable` protocol. - - - -Any type that's `Equatable` can be used safely with the `findIndex(of:in:)` function, -because it's guaranteed to support the equal to operator. -To express this fact, you write a type constraint of `Equatable` -as part of the type parameter's definition when you define the function: - -```swift -func findIndex(of valueToFind: T, in array:[T]) -> Int? { - for (index, value) in array.enumerated() { - if value == valueToFind { - return index - } - } - return nil -} -``` - - - -The single type parameter for `findIndex(of:in:)` is written as `T: Equatable`, -which means “any type `T` that conforms to the `Equatable` protocol.” - -The `findIndex(of:in:)` function now compiles successfully -and can be used with any type that's `Equatable`, such as `Double` or `String`: - -```swift -let doubleIndex = findIndex(of: 9.3, in: [3.14159, 0.1, 0.25]) -// doubleIndex is an optional Int with no value, because 9.3 isn't in the array -let stringIndex = findIndex(of: "Andrea", in: ["Mike", "Malcolm", "Andrea"]) -// stringIndex is an optional Int containing a value of 2 -``` - - - - - - - -## Associated Types - -When defining a protocol, -it's sometimes useful to declare one or more associated types -as part of the protocol's definition. -An *associated type* gives a placeholder name -to a type that's used as part of the protocol. -The actual type to use for that associated type -isn't specified until the protocol is adopted. -Associated types are specified with the `associatedtype` keyword. - -### Associated Types in Action - -Here's an example of a protocol called `Container`, -which declares an associated type called `Item`: - -```swift -protocol Container { - associatedtype Item - mutating func append(_ item: Item) - var count: Int { get } - subscript(i: Int) -> Item { get } -} -``` - - - -The `Container` protocol defines three required capabilities -that any container must provide: - -- It must be possible to add a new item to the container with an `append(_:)` method. -- It must be possible to access a count of the items in the container - through a `count` property that returns an `Int` value. -- It must be possible to retrieve each item in the container with a subscript - that takes an `Int` index value. - -This protocol doesn't specify how the items in the container should be stored -or what type they're allowed to be. -The protocol only specifies the three bits of functionality -that any type must provide in order to be considered a `Container`. -A conforming type can provide additional functionality, -as long as it satisfies these three requirements. - -Any type that conforms to the `Container` protocol must be able to specify -the type of values it stores. -Specifically, it must ensure that only items of the right type -are added to the container, -and it must be clear about the type of the items returned by its subscript. - -To define these requirements, -the `Container` protocol needs a way to refer to -the type of the elements that a container will hold, -without knowing what that type is for a specific container. -The `Container` protocol needs to specify that -any value passed to the `append(_:)` method -must have the same type as the container's element type, -and that the value returned by the container's subscript -will be of the same type as the container's element type. - -To achieve this, -the `Container` protocol declares an associated type called `Item`, -written as `associatedtype Item`. -The protocol doesn't define what `Item` is --- -that information is left for any conforming type to provide. -Nonetheless, the `Item` alias provides a way to refer to -the type of the items in a `Container`, -and to define a type for use with the `append(_:)` method and subscript, -to ensure that the expected behavior of any `Container` is enforced. - -Here's a version of the nongeneric `IntStack` type -from above, -adapted to conform to the `Container` protocol: - -```swift -struct IntStack: Container { - // original IntStack implementation - var items: [Int] = [] - mutating func push(_ item: Int) { - items.append(item) - } - mutating func pop() -> Int { - return items.removeLast() - } - // conformance to the Container protocol - typealias Item = Int - mutating func append(_ item: Int) { - self.push(item) - } - var count: Int { - return items.count - } - subscript(i: Int) -> Int { - return items[i] - } -} -``` - - - -The `IntStack` type implements all three of the `Container` protocol's requirements, -and in each case wraps part of the `IntStack` type's existing functionality -to satisfy these requirements. - -Moreover, `IntStack` specifies that for this implementation of `Container`, -the appropriate `Item` to use is a type of `Int`. -The definition of `typealias Item = Int` turns the abstract type of `Item` -into a concrete type of `Int` for this implementation of the `Container` protocol. - -Thanks to Swift's type inference, -you don't actually need to declare a concrete `Item` of `Int` -as part of the definition of `IntStack`. -Because `IntStack` conforms to all of the requirements of the `Container` protocol, -Swift can infer the appropriate `Item` to use, -simply by looking at the type of the `append(_:)` method's `item` parameter -and the return type of the subscript. -Indeed, if you delete the `typealias Item = Int` line from the code above, -everything still works, because it's clear what type should be used for `Item`. - -You can also make the generic `Stack` type conform to the `Container` protocol: - -```swift -struct Stack: Container { - // original Stack implementation - var items: [Element] = [] - mutating func push(_ item: Element) { - items.append(item) - } - mutating func pop() -> Element { - return items.removeLast() - } - // conformance to the Container protocol - mutating func append(_ item: Element) { - self.push(item) - } - var count: Int { - return items.count - } - subscript(i: Int) -> Element { - return items[i] - } -} -``` - - - -This time, the type parameter `Element` is used as -the type of the `append(_:)` method's `item` parameter -and the return type of the subscript. -Swift can therefore infer that `Element` is the appropriate type to use -as the `Item` for this particular container. - -### Extending an Existing Type to Specify an Associated Type - -You can extend an existing type to add conformance to a protocol, -as described in . -This includes a protocol with an associated type. - -Swift's `Array` type already provides an `append(_:)` method, -a `count` property, and a subscript with an `Int` index to retrieve its elements. -These three capabilities match the requirements of the `Container` protocol. -This means that you can extend `Array` to conform to the `Container` protocol -simply by declaring that `Array` adopts the protocol. -You do this with an empty extension, -as described in : - -```swift -extension Array: Container {} -``` - - - -Array's existing `append(_:)` method and subscript enable Swift to infer -the appropriate type to use for `Item`, -just as for the generic `Stack` type above. -After defining this extension, you can use any `Array` as a `Container`. - -### Adding Constraints to an Associated Type - -You can add type constraints to an associated type in a protocol -to require that conforming types satisfy those constraints. -For example, -the following code defines a version of `Container` -that requires the items in the container to be equatable. - -```swift -protocol Container { - associatedtype Item: Equatable - mutating func append(_ item: Item) - var count: Int { get } - subscript(i: Int) -> Item { get } -} -``` - - - -To conform to this version of `Container`, -the container's `Item` type has to conform to the `Equatable` protocol. - -### Using a Protocol in Its Associated Type's Constraints - -A protocol can appear as part of its own requirements. -For example, -here's a protocol that refines the `Container` protocol, -adding the requirement of a `suffix(_:)` method. -The `suffix(_:)` method -returns a given number of elements from the end of the container, -storing them in an instance of the `Suffix` type. - -```swift -protocol SuffixableContainer: Container { - associatedtype Suffix: SuffixableContainer where Suffix.Item == Item - func suffix(_ size: Int) -> Suffix -} -``` - - - -In this protocol, -`Suffix` is an associated type, -like the `Item` type in the `Container` example above. -`Suffix` has two constraints: -It must conform to the `SuffixableContainer` protocol -(the protocol currently being defined), -and its `Item` type must be the same -as the container's `Item` type. -The constraint on `Item` is a generic `where` clause, -which is discussed in below. - -Here's an extension of the `Stack` type -from above -that adds conformance to the `SuffixableContainer` protocol: - -```swift -extension Stack: SuffixableContainer { - func suffix(_ size: Int) -> Stack { - var result = Stack() - for index in (count-size)..() -stackOfInts.append(10) -stackOfInts.append(20) -stackOfInts.append(30) -let suffix = stackOfInts.suffix(2) -// suffix contains 20 and 30 -``` - - - -In the example above, -the `Suffix` associated type for `Stack` is also `Stack`, -so the suffix operation on `Stack` returns another `Stack`. -Alternatively, -a type that conforms to `SuffixableContainer` -can have a `Suffix` type that's different from itself --- -meaning the suffix operation can return a different type. -For example, -here's an extension to the nongeneric `IntStack` type -that adds `SuffixableContainer` conformance, -using `Stack` as its suffix type instead of `IntStack`: - -```swift -extension IntStack: SuffixableContainer { - func suffix(_ size: Int) -> Stack { - var result = Stack() - for index in (count-size)... -} -``` - - - -## Generic Where Clauses - -Type constraints, as described in , -enable you to define requirements on the type parameters associated with -a generic function, subscript, or type. - -It can also be useful to define requirements for associated types. -You do this by defining a *generic where clause*. -A generic `where` clause enables you to require that -an associated type must conform to a certain protocol, -or that certain type parameters and associated types must be the same. -A generic `where` clause starts with the `where` keyword, -followed by constraints for associated types -or equality relationships between types and associated types. -You write a generic `where` clause right before the opening curly brace -of a type or function's body. - -The example below defines a generic function called `allItemsMatch`, -which checks to see if two `Container` instances contain -the same items in the same order. -The function returns a Boolean value of `true` if all items match -and a value of `false` if they don't. - -The two containers to be checked don't have to be -the same type of container (although they can be), -but they do have to hold the same type of items. -This requirement is expressed through a combination of type constraints -and a generic `where` clause: - -```swift -func allItemsMatch - (_ someContainer: C1, _ anotherContainer: C2) -> Bool - where C1.Item == C2.Item, C1.Item: Equatable { - - // Check that both containers contain the same number of items. - if someContainer.count != anotherContainer.count { - return false - } - - // Check each pair of items to see if they're equivalent. - for i in 0.. func allItemsMatch - (_ someContainer: C1, _ anotherContainer: C2) -> Bool - where C1.Item == C2.Item, C1.Item: Equatable { - --- - // Check that both containers contain the same number of items. - if someContainer.count != anotherContainer.count { - return false - } - --- - // Check each pair of items to see if they're equivalent. - for i in 0.. - -This function takes two arguments called -`someContainer` and `anotherContainer`. -The `someContainer` argument is of type `C1`, -and the `anotherContainer` argument is of type `C2`. -Both `C1` and `C2` are type parameters -for two container types to be determined when the function is called. - -The following requirements are placed on the function's two type parameters: - -- `C1` must conform to the `Container` protocol (written as `C1: Container`). -- `C2` must also conform to the `Container` protocol (written as `C2: Container`). -- The `Item` for `C1` must be the same as the `Item` for `C2` - (written as `C1.Item == C2.Item`). -- The `Item` for `C1` must conform to the `Equatable` protocol - (written as `C1.Item: Equatable`). - -The first and second requirements are defined in the function's type parameter list, -and the third and fourth requirements are defined in the function's generic `where` clause. - -These requirements mean: - -- `someContainer` is a container of type `C1`. -- `anotherContainer` is a container of type `C2`. -- `someContainer` and `anotherContainer` contain the same type of items. -- The items in `someContainer` can be checked with the not equal operator (`!=`) - to see if they're different from each other. - -The third and fourth requirements combine to mean that -the items in `anotherContainer` can *also* be checked with the `!=` operator, -because they're exactly the same type as the items in `someContainer`. - -These requirements enable the `allItemsMatch(_:_:)` function to compare the two containers, -even if they're of a different container type. - -The `allItemsMatch(_:_:)` function starts by checking that -both containers contain the same number of items. -If they contain a different number of items, there's no way that they can match, -and the function returns `false`. - -After making this check, the function iterates over all of the items in `someContainer` -with a `for`-`in` loop and the half-open range operator (`..<`). -For each item, the function checks whether the item from `someContainer` isn't equal to -the corresponding item in `anotherContainer`. -If the two items aren't equal, then the two containers don't match, -and the function returns `false`. - -If the loop finishes without finding a mismatch, -the two containers match, and the function returns `true`. - -Here's how the `allItemsMatch(_:_:)` function looks in action: - -```swift -var stackOfStrings = Stack() -stackOfStrings.push("uno") -stackOfStrings.push("dos") -stackOfStrings.push("tres") - -var arrayOfStrings = ["uno", "dos", "tres"] - -if allItemsMatch(stackOfStrings, arrayOfStrings) { - print("All items match.") -} else { - print("Not all items match.") -} -// Prints "All items match." -``` - - - -The example above creates a `Stack` instance to store `String` values, -and pushes three strings onto the stack. -The example also creates an `Array` instance initialized with -an array literal containing the same three strings as the stack. -Even though the stack and the array are of a different type, -they both conform to the `Container` protocol, -and both contain the same type of values. -You can therefore call the `allItemsMatch(_:_:)` function -with these two containers as its arguments. -In the example above, the `allItemsMatch(_:_:)` function correctly reports that -all of the items in the two containers match. - -## Extensions with a Generic Where Clause - -You can also use a generic `where` clause as part of an extension. -The example below -extends the generic `Stack` structure from the previous examples -to add an `isTop(_:)` method. - -```swift -extension Stack where Element: Equatable { - func isTop(_ item: Element) -> Bool { - guard let topItem = items.last else { - return false - } - return topItem == item - } -} -``` - - - -This new `isTop(_:)` method -first checks that the stack isn't empty, -and then compares the given item -against the stack's topmost item. -If you tried to do this without a generic `where` clause, -you would have a problem: -The implementation of `isTop(_:)` uses the `==` operator, -but the definition of `Stack` doesn't require -its items to be equatable, -so using the `==` operator results in a compile-time error. -Using a generic `where` clause -lets you add a new requirement to the extension, -so that the extension adds the `isTop(_:)` method -only when the items in the stack are equatable. - -Here's how the `isTop(_:)` method looks in action: - -```swift -if stackOfStrings.isTop("tres") { - print("Top element is tres.") -} else { - print("Top element is something else.") -} -// Prints "Top element is tres." -``` - - - -If you try to call the `isTop(_:)` method -on a stack whose elements aren't equatable, -you'll get a compile-time error. - -```swift -struct NotEquatable { } -var notEquatableStack = Stack() -let notEquatableValue = NotEquatable() -notEquatableStack.push(notEquatableValue) -notEquatableStack.isTop(notEquatableValue) // Error -``` - - - -You can use a generic `where` clause with extensions to a protocol. -The example below extends the `Container` protocol from the previous examples -to add a `startsWith(_:)` method. - -```swift -extension Container where Item: Equatable { - func startsWith(_ item: Item) -> Bool { - return count >= 1 && self[0] == item - } -} -``` - - - - - -The `startsWith(_:)` method -first makes sure that the container has at least one item, -and then it checks -whether the first item in the container matches the given item. -This new `startsWith(_:)` method -can be used with any type that conforms to the `Container` protocol, -including the stacks and arrays used above, -as long as the container's items are equatable. - -```swift -if [9, 9, 9].startsWith(42) { - print("Starts with 42.") -} else { - print("Starts with something else.") -} -// Prints "Starts with something else." -``` - - - -The generic `where` clause in the example above -requires `Item` to conform to a protocol, -but you can also write a generic `where` clauses that require `Item` -to be a specific type. -For example: - -```swift -extension Container where Item == Double { - func average() -> Double { - var sum = 0.0 - for index in 0.. extension Container where Item == Double { - func average() -> Double { - var sum = 0.0 - for index in 0.. print([1260.0, 1200.0, 98.6, 37.0].average()) - <- 648.9 - ``` ---> - -This example adds an `average()` method -to containers whose `Item` type is `Double`. -It iterates over the items in the container to add them up, -and divides by the container's count to compute the average. -It explicitly converts the count from `Int` to `Double` -to be able to do floating-point division. - -You can include multiple requirements in a generic `where` clause -that's part of an extension, -just like you can for a generic `where` clause that you write elsewhere. -Separate each requirement in the list with a comma. - - - -## Contextual Where Clauses - -You can write a generic `where` clause -as part of a declaration that doesn't have its own generic type constraints, -when you're already working in the context of generic types. -For example, -you can write a generic `where` clause -on a subscript of a generic type -or on a method in an extension to a generic type. -The `Container` structure is generic, -and the `where` clauses in the example below -specify what type constraints have to be satisfied -to make these new methods available on a container. - -```swift -extension Container { - func average() -> Double where Item == Int { - var sum = 0.0 - for index in 0.. Bool where Item: Equatable { - return count >= 1 && self[count-1] == item - } -} -let numbers = [1260, 1200, 98, 37] -print(numbers.average()) -// Prints "648.75" -print(numbers.endsWith(37)) -// Prints "true" -``` - - - -This example -adds an `average()` method to `Container` when the items are integers, -and it adds an `endsWith(_:)` method when the items are equatable. -Both functions include a generic `where` clause -that adds type constraints to the generic `Item` type parameter -from the original declaration of `Container`. - -If you want to write this code without using contextual `where` clauses, -you write two extensions, -one for each generic `where` clause. -The example above and the example below have the same behavior. - -```swift -extension Container where Item == Int { - func average() -> Double { - var sum = 0.0 - for index in 0.. Bool { - return count >= 1 && self[count-1] == item - } -} -``` - - - -In the version of this example that uses contextual `where` clauses, -the implementation of `average()` and `endsWith(_:)` -are both in the same extension -because each method's generic `where` clause -states the requirements that need to be satisfied -to make that method available. -Moving those requirements to the extensions' generic `where` clauses -makes the methods available in the same situations, -but requires one extension per requirement. - -## Associated Types with a Generic Where Clause - -You can include a generic `where` clause on an associated type. -For example, suppose you want to make a version of `Container` -that includes an iterator, -like what the `Sequence` protocol uses in the Swift standard library. -Here's how you write that: - -```swift -protocol Container { - associatedtype Item - mutating func append(_ item: Item) - var count: Int { get } - subscript(i: Int) -> Item { get } - - associatedtype Iterator: IteratorProtocol where Iterator.Element == Item - func makeIterator() -> Iterator -} -``` - - - - - -The generic `where` clause on `Iterator` requires that -the iterator must traverse over elements -of the same item type as the container's items, -regardless of the iterator's type. -The `makeIterator()` function provides access to a container's iterator. - - - -For a protocol that inherits from another protocol, -you add a constraint to an inherited associated type -by including the generic `where` clause in the protocol declaration. -For example, the following code -declares a `ComparableContainer` protocol -that requires `Item` to conform to `Comparable`: - -```swift -protocol ComparableContainer: Container where Item: Comparable { } -``` - - - - - - - -## Generic Subscripts - -Subscripts can be generic, -and they can include generic `where` clauses. -You write the placeholder type name inside angle brackets after `subscript`, -and you write a generic `where` clause right before the opening curly brace -of the subscript's body. -For example: - - - -```swift -extension Container { - subscript(indices: Indices) -> [Item] - where Indices.Iterator.Element == Int { - var result: [Item] = [] - for index in indices { - result.append(self[index]) - } - return result - } -} -``` - - - - - -This extension to the `Container` protocol -adds a subscript that takes a sequence of indices -and returns an array containing the items at each given index. -This generic subscript is constrained as follows: - -- The generic parameter `Indices` in angle brackets - has to be a type that conforms to the `Sequence` protocol - from the Swift standard library. -- The subscript takes a single parameter, `indices`, - which is an instance of that `Indices` type. -- The generic `where` clause requires - that the iterator for the sequence - must traverse over elements of type `Int`. - This ensures that the indices in the sequence - are the same type as the indices used for a container. - -Taken together, these constraints mean that -the value passed for the `indices` parameter -is a sequence of integers. - - - - - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/LanguageGuide/Inheritance.md b/swift-6-beta.docc/LanguageGuide/Inheritance.md deleted file mode 100644 index 11f52f4fa..000000000 --- a/swift-6-beta.docc/LanguageGuide/Inheritance.md +++ /dev/null @@ -1,624 +0,0 @@ -# Inheritance - -Subclass to add or override functionality. - -A class can *inherit* methods, properties, and other characteristics -from another class. -When one class inherits from another, -the inheriting class is known as a *subclass*, -and the class it inherits from is known as its *superclass*. -Inheritance is a fundamental behavior that differentiates classes -from other types in Swift. - -Classes in Swift can call and access -methods, properties, and subscripts belonging to their superclass -and can provide their own overriding versions of those methods, properties, and subscripts -to refine or modify their behavior. -Swift helps to ensure your overrides are correct -by checking that the override definition has a matching superclass definition. - -Classes can also add property observers to inherited properties -in order to be notified when the value of a property changes. -Property observers can be added to any property, -regardless of whether it was originally defined as a stored or computed property. - -## Defining a Base Class - -Any class that doesn't inherit from another class is known as a *base class*. - -> Note: Swift classes don't inherit from a universal base class. -> Classes you define without specifying a superclass -> automatically become base classes for you to build upon. - -The example below defines a base class called `Vehicle`. -This base class defines a stored property called `currentSpeed`, -with a default value of `0.0` (inferring a property type of `Double`). -The `currentSpeed` property's value is used by -a read-only computed `String` property called `description` -to create a description of the vehicle. - -The `Vehicle` base class also defines a method called `makeNoise`. -This method doesn't actually do anything for a base `Vehicle` instance, -but will be customized by subclasses of `Vehicle` later on: - -```swift -class Vehicle { - var currentSpeed = 0.0 - var description: String { - return "traveling at \(currentSpeed) miles per hour" - } - func makeNoise() { - // do nothing - an arbitrary vehicle doesn't necessarily make a noise - } -} -``` - - - -You create a new instance of `Vehicle` with *initializer syntax*, -which is written as a type name followed by empty parentheses: - -```swift -let someVehicle = Vehicle() -``` - - - -Having created a new `Vehicle` instance, -you can access its `description` property to print -a human-readable description of the vehicle's current speed: - -```swift -print("Vehicle: \(someVehicle.description)") -// Vehicle: traveling at 0.0 miles per hour -``` - - - -The `Vehicle` class defines common characteristics for an arbitrary vehicle, -but isn't much use in itself. -To make it more useful, -you need to refine it to describe more specific kinds of vehicles. - -## Subclassing - -*Subclassing* is the act of basing a new class on an existing class. -The subclass inherits characteristics from the existing class, which you can then refine. -You can also add new characteristics to the subclass. - -To indicate that a subclass has a superclass, -write the subclass name before the superclass name, -separated by a colon: - -```swift -class SomeSubclass: SomeSuperclass { - // subclass definition goes here -} -``` - - - -The following example defines a subclass called `Bicycle`, -with a superclass of `Vehicle`: - -```swift -class Bicycle: Vehicle { - var hasBasket = false -} -``` - - - -The new `Bicycle` class automatically gains all of the characteristics of `Vehicle`, -such as its `currentSpeed` and `description` properties and its `makeNoise()` method. - -In addition to the characteristics it inherits, -the `Bicycle` class defines a new stored property, -`hasBasket`, with a default value of `false` -(inferring a type of `Bool` for the property). - -By default, any new `Bicycle` instance you create will not have a basket. -You can set the `hasBasket` property to `true` for a particular `Bicycle` instance -after that instance is created: - -```swift -let bicycle = Bicycle() -bicycle.hasBasket = true -``` - - - -You can also modify the inherited `currentSpeed` property of a `Bicycle` instance, -and query the instance's inherited `description` property: - -```swift -bicycle.currentSpeed = 15.0 -print("Bicycle: \(bicycle.description)") -// Bicycle: traveling at 15.0 miles per hour -``` - - - -Subclasses can themselves be subclassed. -The next example creates a subclass of `Bicycle` for a two-seater bicycle -known as a “tandem”: - -```swift -class Tandem: Bicycle { - var currentNumberOfPassengers = 0 -} -``` - - - -`Tandem` inherits all of the properties and methods from `Bicycle`, -which in turn inherits all of the properties and methods from `Vehicle`. -The `Tandem` subclass also adds a new stored property called `currentNumberOfPassengers`, -with a default value of `0`. - -If you create an instance of `Tandem`, -you can work with any of its new and inherited properties, -and query the read-only `description` property it inherits from `Vehicle`: - -```swift -let tandem = Tandem() -tandem.hasBasket = true -tandem.currentNumberOfPassengers = 2 -tandem.currentSpeed = 22.0 -print("Tandem: \(tandem.description)") -// Tandem: traveling at 22.0 miles per hour -``` - - - -## Overriding - -A subclass can provide its own custom implementation of -an instance method, type method, instance property, type property, or subscript -that it would otherwise inherit from a superclass. -This is known as *overriding*. - -To override a characteristic that would otherwise be inherited, -you prefix your overriding definition with the `override` keyword. -Doing so clarifies that you intend to provide an override -and haven't provided a matching definition by mistake. -Overriding by accident can cause unexpected behavior, -and any overrides without the `override` keyword are -diagnosed as an error when your code is compiled. - -The `override` keyword also prompts the Swift compiler -to check that your overriding class's superclass (or one of its parents) -has a declaration that matches the one you provided for the override. -This check ensures that your overriding definition is correct. - -### Accessing Superclass Methods, Properties, and Subscripts - -When you provide a method, property, or subscript override for a subclass, -it's sometimes useful to use the existing superclass implementation -as part of your override. -For example, you can refine the behavior of that existing implementation, -or store a modified value in an existing inherited variable. - -Where this is appropriate, -you access the superclass version of a method, property, or subscript -by using the `super` prefix: - -- An overridden method named `someMethod()` can call the superclass version of `someMethod()` - by calling `super.someMethod()` within the overriding method implementation. -- An overridden property called `someProperty` can access the superclass version of `someProperty` - as `super.someProperty` within the overriding getter or setter implementation. -- An overridden subscript for `someIndex` can access the superclass version of the same subscript - as `super[someIndex]` from within the overriding subscript implementation. - -### Overriding Methods - -You can override an inherited instance or type method -to provide a tailored or alternative implementation of the method within your subclass. - -The following example defines a new subclass of `Vehicle` called `Train`, -which overrides the `makeNoise()` method that `Train` inherits from `Vehicle`: - -```swift -class Train: Vehicle { - override func makeNoise() { - print("Choo Choo") - } -} -``` - - - -If you create a new instance of `Train` and call its `makeNoise()` method, -you can see that the `Train` subclass version of the method is called: - -```swift -let train = Train() -train.makeNoise() -// Prints "Choo Choo" -``` - - - -### Overriding Properties - -You can override an inherited instance or type property -to provide your own custom getter and setter for that property, -or to add property observers to enable the overriding property -to observe when the underlying property value changes. - -#### Overriding Property Getters and Setters - -You can provide a custom getter (and setter, if appropriate) -to override *any* inherited property, -regardless of whether the inherited property is implemented as -a stored or computed property at source. -The stored or computed nature of an inherited property isn't known by a subclass --- -it only knows that the inherited property has a certain name and type. -You must always state both the name and the type of the property you are overriding, -to enable the compiler to check that your override matches -a superclass property with the same name and type. - -You can present an inherited read-only property as a read-write property -by providing both a getter and a setter in your subclass property override. -You can't, however, present an inherited read-write property as a read-only property. - -> Note: If you provide a setter as part of a property override, -> you must also provide a getter for that override. -> If you don't want to modify the inherited property's value within the overriding getter, -> you can simply pass through the inherited value -> by returning `super.someProperty` from the getter, -> where `someProperty` is the name of the property you are overriding. - -The following example defines a new class called `Car`, -which is a subclass of `Vehicle`. -The `Car` class introduces a new stored property called `gear`, -with a default integer value of `1`. -The `Car` class also overrides the `description` property it inherits from `Vehicle`, -to provide a custom description that includes the current gear: - -```swift -class Car: Vehicle { - var gear = 1 - override var description: String { - return super.description + " in gear \(gear)" - } -} -``` - - - -The override of the `description` property starts by calling `super.description`, -which returns the `Vehicle` class's `description` property. -The `Car` class's version of `description` then adds some extra text onto -the end of this description to provide information about the current gear. - -If you create an instance of the `Car` class -and set its `gear` and `currentSpeed` properties, -you can see that its `description` property returns -the tailored description defined within the `Car` class: - -```swift -let car = Car() -car.currentSpeed = 25.0 -car.gear = 3 -print("Car: \(car.description)") -// Car: traveling at 25.0 miles per hour in gear 3 -``` - - - -#### Overriding Property Observers - -You can use property overriding to add property observers to an inherited property. -This enables you to be notified when the value of an inherited property changes, -regardless of how that property was originally implemented. -For more information on property observers, see . - -> Note: You can't add property observers to -> inherited constant stored properties or inherited read-only computed properties. -> The value of these properties can't be set, -> and so it isn't appropriate to provide a `willSet` or `didSet` implementation -> as part of an override. -> -> Note also that you can't provide both -> an overriding setter and an overriding property observer for the same property. -> If you want to observe changes to a property's value, -> and you are already providing a custom setter for that property, -> you can simply observe any value changes from within the custom setter. - -The following example defines a new class called `AutomaticCar`, -which is a subclass of `Car`. -The `AutomaticCar` class represents a car with an automatic gearbox, -which automatically selects an appropriate gear to use based on the current speed: - -```swift -class AutomaticCar: Car { - override var currentSpeed: Double { - didSet { - gear = Int(currentSpeed / 10.0) + 1 - } - } -} -``` - - - -Whenever you set the `currentSpeed` property of an `AutomaticCar` instance, -the property's `didSet` observer sets the instance's `gear` property to -an appropriate choice of gear for the new speed. -Specifically, the property observer chooses a gear that's -the new `currentSpeed` value divided by `10`, -rounded down to the nearest integer, plus `1`. -A speed of `35.0` produces a gear of `4`: - -```swift -let automatic = AutomaticCar() -automatic.currentSpeed = 35.0 -print("AutomaticCar: \(automatic.description)") -// AutomaticCar: traveling at 35.0 miles per hour in gear 4 -``` - - - -## Preventing Overrides - -You can prevent a method, property, or subscript from being overridden -by marking it as *final*. -Do this by writing the `final` modifier before -the method, property, or subscript's introducer keyword -(such as `final var`, `final func`, `final class func`, and `final subscript`). - -Any attempt to override a final method, property, or subscript in a subclass -is reported as a compile-time error. -Methods, properties, or subscripts that you add to a class in an extension -can also be marked as final within the extension's definition. -For more information, see . - - - -You can mark an entire class as final by writing the `final` modifier -before the `class` keyword in its class definition (`final class`). -Any attempt to subclass a final class is reported as a compile-time error. - - - - - - - - - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/LanguageGuide/Initialization.md b/swift-6-beta.docc/LanguageGuide/Initialization.md deleted file mode 100644 index f9d6698a1..000000000 --- a/swift-6-beta.docc/LanguageGuide/Initialization.md +++ /dev/null @@ -1,2971 +0,0 @@ -# Initialization - -Set the initial values for a type's stored properties and perform one-time setup. - -*Initialization* is the process of preparing an instance of -a class, structure, or enumeration for use. -This process involves setting an initial value for each stored property on that instance -and performing any other setup or initialization that's required -before the new instance is ready for use. - -You implement this initialization process by defining *initializers*, -which are like special methods that can be called -to create a new instance of a particular type. -Unlike Objective-C initializers, Swift initializers don't return a value. -Their primary role is to ensure that new instances of a type -are correctly initialized before they're used for the first time. - -Instances of class types can also implement a *deinitializer*, -which performs any custom cleanup just before an instance of that class is deallocated. -For more information about deinitializers, see . - -## Setting Initial Values for Stored Properties - -Classes and structures *must* set all of their stored properties -to an appropriate initial value by the time -an instance of that class or structure is created. -Stored properties can't be left in an indeterminate state. - -You can set an initial value for a stored property within an initializer, -or by assigning a default property value as part of the property's definition. -These actions are described in the following sections. - -> Note: When you assign a default value to a stored property, -> or set its initial value within an initializer, -> the value of that property is set directly, -> without calling any property observers. - -### Initializers - -*Initializers* are called to create a new instance of a particular type. -In its simplest form, an initializer is like an instance method with no parameters, -written using the `init` keyword: - -```swift -init() { - // perform some initialization here -} -``` - - - -The example below defines a new structure called `Fahrenheit` -to store temperatures expressed in the Fahrenheit scale. -The `Fahrenheit` structure has one stored property, -`temperature`, which is of type `Double`: - -```swift -struct Fahrenheit { - var temperature: Double - init() { - temperature = 32.0 - } -} -var f = Fahrenheit() -print("The default temperature is \(f.temperature)° Fahrenheit") -// Prints "The default temperature is 32.0° Fahrenheit" -``` - - - -The structure defines a single initializer, `init`, with no parameters, -which initializes the stored temperature with a value of `32.0` -(the freezing point of water in degrees Fahrenheit). - -### Default Property Values - -You can set the initial value of a stored property from within an initializer, -as shown above. -Alternatively, specify a *default property value* -as part of the property's declaration. -You specify a default property value by assigning an initial value to the property -when it's defined. - -> Note: If a property always takes the same initial value, -> provide a default value rather than setting a value within an initializer. -> The end result is the same, -> but the default value ties the property's initialization more closely to its declaration. -> It makes for shorter, clearer initializers -> and enables you to infer the type of the property from its default value. -> The default value also makes it easier for you to take advantage of -> default initializers and initializer inheritance, -> as described later in this chapter. - -You can write the `Fahrenheit` structure from above in a simpler form -by providing a default value for its `temperature` property -at the point that the property is declared: - -```swift -struct Fahrenheit { - var temperature = 32.0 -} -``` - - - -## Customizing Initialization - -You can customize the initialization process -with input parameters and optional property types, -or by assigning constant properties during initialization, -as described in the following sections. - -### Initialization Parameters - -You can provide *initialization parameters* as part of an initializer's definition, -to define the types and names of values that customize the initialization process. -Initialization parameters have the same capabilities and syntax -as function and method parameters. - -The following example defines a structure called `Celsius`, -which stores temperatures expressed in degrees Celsius. -The `Celsius` structure implements two custom initializers called -`init(fromFahrenheit:)` and `init(fromKelvin:)`, -which initialize a new instance of the structure -with a value from a different temperature scale: - -```swift -struct Celsius { - var temperatureInCelsius: Double - init(fromFahrenheit fahrenheit: Double) { - temperatureInCelsius = (fahrenheit - 32.0) / 1.8 - } - init(fromKelvin kelvin: Double) { - temperatureInCelsius = kelvin - 273.15 - } -} -let boilingPointOfWater = Celsius(fromFahrenheit: 212.0) -// boilingPointOfWater.temperatureInCelsius is 100.0 -let freezingPointOfWater = Celsius(fromKelvin: 273.15) -// freezingPointOfWater.temperatureInCelsius is 0.0 -``` - - - -The first initializer has a single initialization parameter -with an argument label of `fromFahrenheit` and a parameter name of `fahrenheit`. -The second initializer has a single initialization parameter -with an argument label of `fromKelvin` and a parameter name of `kelvin`. -Both initializers convert their single argument into -the corresponding Celsius value -and store this value in a property called `temperatureInCelsius`. - - - -### Parameter Names and Argument Labels - -As with function and method parameters, -initialization parameters can have both a parameter name -for use within the initializer's body -and an argument label for use when calling the initializer. - -However, initializers don't have an identifying function name before their parentheses -in the way that functions and methods do. -Therefore, the names and types of an initializer's parameters -play a particularly important role in identifying which initializer should be called. -Because of this, Swift provides an automatic argument label -for *every* parameter in an initializer if you don't provide one. - -The following example defines a structure called `Color`, -with three constant properties called `red`, `green`, and `blue`. -These properties store a value between `0.0` and `1.0` -to indicate the amount of red, green, and blue in the color. - -`Color` provides an initializer with -three appropriately named parameters of type `Double` -for its red, green, and blue components. -`Color` also provides a second initializer with a single `white` parameter, -which is used to provide the same value for all three color components. - -```swift -struct Color { - let red, green, blue: Double - init(red: Double, green: Double, blue: Double) { - self.red = red - self.green = green - self.blue = blue - } - init(white: Double) { - red = white - green = white - blue = white - } -} -``` - - - -Both initializers can be used to create a new `Color` instance, -by providing named values for each initializer parameter: - -```swift -let magenta = Color(red: 1.0, green: 0.0, blue: 1.0) -let halfGray = Color(white: 0.5) -``` - - - -Note that it isn't possible to call these initializers -without using argument labels. -Argument labels must always be used in an initializer if they're defined, -and omitting them is a compile-time error: - -```swift -let veryGreen = Color(0.0, 1.0, 0.0) -// this reports a compile-time error - argument labels are required -``` - - - -### Initializer Parameters Without Argument Labels - -If you don't want to use an argument label for an initializer parameter, -write an underscore (`_`) instead of an explicit argument label for that parameter -to override the default behavior. - -Here's an expanded version of the `Celsius` example -from above, -with an additional initializer to create a new `Celsius` instance -from a `Double` value that's already in the Celsius scale: - -```swift -struct Celsius { - var temperatureInCelsius: Double - init(fromFahrenheit fahrenheit: Double) { - temperatureInCelsius = (fahrenheit - 32.0) / 1.8 - } - init(fromKelvin kelvin: Double) { - temperatureInCelsius = kelvin - 273.15 - } - init(_ celsius: Double) { - temperatureInCelsius = celsius - } -} -let bodyTemperature = Celsius(37.0) -// bodyTemperature.temperatureInCelsius is 37.0 -``` - - - -The initializer call `Celsius(37.0)` is clear in its intent -without the need for an argument label. -It's therefore appropriate to write this initializer as `init(_ celsius: Double)` -so that it can be called by providing an unnamed `Double` value. - -### Optional Property Types - -If your custom type has a stored property that's logically allowed to have “no value” --- -perhaps because its value can't be set during initialization, -or because it's allowed to have “no value” at some later point --- -declare the property with an *optional* type. -Properties of optional type are automatically initialized with a value of `nil`, -indicating that the property is deliberately intended to have “no value yet” -during initialization. - -The following example defines a class called `SurveyQuestion`, -with an optional `String` property called `response`: - -```swift -class SurveyQuestion { - var text: String - var response: String? - init(text: String) { - self.text = text - } - func ask() { - print(text) - } -} -let cheeseQuestion = SurveyQuestion(text: "Do you like cheese?") -cheeseQuestion.ask() -// Prints "Do you like cheese?" -cheeseQuestion.response = "Yes, I do like cheese." -``` - - - -The response to a survey question can't be known until it's asked, -and so the `response` property is declared with a type of `String?`, -or “optional `String`”. -It's automatically assigned a default value of `nil`, meaning “no string yet”, -when a new instance of `SurveyQuestion` is initialized. - -### Assigning Constant Properties During Initialization - -You can assign a value to a constant property -at any point during initialization, -as long as it's set to a definite value by the time initialization finishes. -Once a constant property is assigned a value, -it can't be further modified. - - - - - -> Note: For class instances, -> a constant property can be modified during initialization -> only by the class that introduces it. -> It can't be modified by a subclass. - -You can revise the `SurveyQuestion` example from above to use -a constant property rather than a variable property for the `text` property of the question, -to indicate that the question doesn't change once an instance of `SurveyQuestion` is created. -Even though the `text` property is now a constant, -it can still be set within the class's initializer: - -```swift -class SurveyQuestion { - let text: String - var response: String? - init(text: String) { - self.text = text - } - func ask() { - print(text) - } -} -let beetsQuestion = SurveyQuestion(text: "How about beets?") -beetsQuestion.ask() -// Prints "How about beets?" -beetsQuestion.response = "I also like beets. (But not with cheese.)" -``` - - - -## Default Initializers - -Swift provides a *default initializer* -for any structure or class -that provides default values for all of its properties -and doesn't provide at least one initializer itself. -The default initializer simply creates a new instance -with all of its properties set to their default values. - - - -This example defines a class called `ShoppingListItem`, -which encapsulates the name, quantity, and purchase state -of an item in a shopping list: - -```swift -class ShoppingListItem { - var name: String? - var quantity = 1 - var purchased = false -} -var item = ShoppingListItem() -``` - - - -Because all properties of the `ShoppingListItem` class have default values, -and because it's a base class with no superclass, -`ShoppingListItem` automatically gains a default initializer implementation -that creates a new instance with all of its properties set to their default values. -(The `name` property is an optional `String` property, -and so it automatically receives a default value of `nil`, -even though this value isn't written in the code.) -The example above uses the default initializer for the `ShoppingListItem` class -to create a new instance of the class with initializer syntax, -written as `ShoppingListItem()`, -and assigns this new instance to a variable called `item`. - -### Memberwise Initializers for Structure Types - -Structure types automatically receive a *memberwise initializer* -if they don't define any of their own custom initializers. -Unlike a default initializer, -the structure receives a memberwise initializer -even if it has stored properties that don't have default values. - - - -The memberwise initializer is a shorthand way -to initialize the member properties of new structure instances. -Initial values for the properties of the new instance -can be passed to the memberwise initializer by name. - -The example below defines a structure called `Size` -with two properties called `width` and `height`. -Both properties are inferred to be of type `Double` -by assigning a default value of `0.0`. - -The `Size` structure automatically receives an `init(width:height:)` -memberwise initializer, -which you can use to initialize a new `Size` instance: - -```swift -struct Size { - var width = 0.0, height = 0.0 -} -let twoByTwo = Size(width: 2.0, height: 2.0) -``` - - - -When you call a memberwise initializer, -you can omit values for any properties -that have default values. -In the example above, -the `Size` structure has a default value -for both its `height` and `width` properties. -You can omit either property or both properties, -and the initializer uses the default value for anything you omit. -For example: - -```swift -let zeroByTwo = Size(height: 2.0) -print(zeroByTwo.width, zeroByTwo.height) -// Prints "0.0 2.0" - -let zeroByZero = Size() -print(zeroByZero.width, zeroByZero.height) -// Prints "0.0 0.0" -``` - - - -## Initializer Delegation for Value Types - -Initializers can call other initializers to perform part of an instance's initialization. -This process, known as *initializer delegation*, -avoids duplicating code across multiple initializers. - -The rules for how initializer delegation works, -and for what forms of delegation are allowed, -are different for value types and class types. -Value types (structures and enumerations) don't support inheritance, -and so their initializer delegation process is relatively simple, -because they can only delegate to another initializer that they provide themselves. -Classes, however, can inherit from other classes, -as described in . -This means that classes have additional responsibilities for ensuring that -all stored properties they inherit are assigned a suitable value during initialization. -These responsibilities are described in - below. - -For value types, you use `self.init` to refer to other initializers -from the same value type when writing your own custom initializers. -You can call `self.init` only from within an initializer. - -Note that if you define a custom initializer for a value type, -you will no longer have access to the default initializer -(or the memberwise initializer, if it's a structure) for that type. -This constraint prevents a situation in which additional essential setup -provided in a more complex initializer -is accidentally circumvented by someone using one of the automatic initializers. - -> Note: If you want your custom value type to be initializable with -> the default initializer and memberwise initializer, -> and also with your own custom initializers, -> write your custom initializers in an extension -> rather than as part of the value type's original implementation. -> For more information, see . - -The following example defines a custom `Rect` structure to represent a geometric rectangle. -The example requires two supporting structures called `Size` and `Point`, -both of which provide default values of `0.0` for all of their properties: - -```swift -struct Size { - var width = 0.0, height = 0.0 -} -struct Point { - var x = 0.0, y = 0.0 -} -``` - - - -You can initialize the `Rect` structure below in one of three ways --- -by using its default zero-initialized `origin` and `size` property values, -by providing a specific origin point and size, -or by providing a specific center point and size. -These initialization options are represented by -three custom initializers that are part of the `Rect` structure's definition: - -```swift -struct Rect { - var origin = Point() - var size = Size() - init() {} - init(origin: Point, size: Size) { - self.origin = origin - self.size = size - } - init(center: Point, size: Size) { - let originX = center.x - (size.width / 2) - let originY = center.y - (size.height / 2) - self.init(origin: Point(x: originX, y: originY), size: size) - } -} -``` - - - -The first `Rect` initializer, `init()`, -is functionally the same as the default initializer that the structure would have received -if it didn't have its own custom initializers. -This initializer has an empty body, -represented by an empty pair of curly braces `{}`. -Calling this initializer returns a `Rect` instance whose -`origin` and `size` properties are both initialized with -the default values of `Point(x: 0.0, y: 0.0)` -and `Size(width: 0.0, height: 0.0)` -from their property definitions: - -```swift -let basicRect = Rect() -// basicRect's origin is (0.0, 0.0) and its size is (0.0, 0.0) -``` - - - -The second `Rect` initializer, `init(origin:size:)`, -is functionally the same as the memberwise initializer that the structure would have received -if it didn't have its own custom initializers. -This initializer simply assigns the `origin` and `size` argument values to -the appropriate stored properties: - -```swift -let originRect = Rect(origin: Point(x: 2.0, y: 2.0), - size: Size(width: 5.0, height: 5.0)) -// originRect's origin is (2.0, 2.0) and its size is (5.0, 5.0) -``` - - - -The third `Rect` initializer, `init(center:size:)`, is slightly more complex. -It starts by calculating an appropriate origin point based on -a `center` point and a `size` value. -It then calls (or *delegates*) to the `init(origin:size:)` initializer, -which stores the new origin and size values in the appropriate properties: - -```swift -let centerRect = Rect(center: Point(x: 4.0, y: 4.0), - size: Size(width: 3.0, height: 3.0)) -// centerRect's origin is (2.5, 2.5) and its size is (3.0, 3.0) -``` - - - -The `init(center:size:)` initializer could have assigned -the new values of `origin` and `size` to the appropriate properties itself. -However, it's more convenient (and clearer in intent) -for the `init(center:size:)` initializer to take advantage of an existing initializer -that already provides exactly that functionality. - -> Note: For an alternative way to write this example without defining -> the `init()` and `init(origin:size:)` initializers yourself, -> see . - -## Class Inheritance and Initialization - -All of a class's stored properties --- -including any properties the class inherits from its superclass --- -*must* be assigned an initial value during initialization. - -Swift defines two kinds of initializers for class types -to help ensure all stored properties receive an initial value. -These are known as designated initializers and convenience initializers. - -### Designated Initializers and Convenience Initializers - -*Designated initializers* are the primary initializers for a class. -A designated initializer fully initializes all properties introduced by that class -and calls an appropriate superclass initializer -to continue the initialization process up the superclass chain. - -Classes tend to have very few designated initializers, -and it's quite common for a class to have only one. -Designated initializers are “funnel” points through which initialization takes place, -and through which the initialization process continues up the superclass chain. - -Every class must have at least one designated initializer. -In some cases, this requirement is satisfied -by inheriting one or more designated initializers from a superclass, -as described in below. - -*Convenience initializers* are secondary, supporting initializers for a class. -You can define a convenience initializer to call a designated initializer -from the same class as the convenience initializer -with some of the designated initializer's parameters set to default values. -You can also define a convenience initializer to create -an instance of that class for a specific use case or input value type. - -You don't have to provide convenience initializers if your class doesn't require them. -Create convenience initializers whenever a shortcut to a common initialization pattern -will save time or make initialization of the class clearer in intent. - -### Syntax for Designated and Convenience Initializers - -Designated initializers for classes are written in the same way as -simple initializers for value types: - -```swift -init(<#parameters#>) { - <#statements#> -} -``` - -Convenience initializers are written in the same style, -but with the `convenience` modifier placed before the `init` keyword, -separated by a space: - -```swift -convenience init(<#parameters#>) { - <#statements#> -} -``` - -### Initializer Delegation for Class Types - -To simplify the relationships between designated and convenience initializers, -Swift applies the following three rules for delegation calls between initializers: - -- term **Rule 1**: - A designated initializer must call a designated initializer from its immediate superclass. - -- term **Rule 2**: - A convenience initializer must call another initializer from the *same* class. - -- term **Rule 3**: - A convenience initializer must ultimately call a designated initializer. - -A simple way to remember this is: - -- Designated initializers must always delegate *up*. -- Convenience initializers must always delegate *across*. - -These rules are illustrated in the figure below: - -![](initializerDelegation01) - -Here, the superclass has a single designated initializer and two convenience initializers. -One convenience initializer calls another convenience initializer, -which in turn calls the single designated initializer. -This satisfies rules 2 and 3 from above. -The superclass doesn't itself have a further superclass, and so rule 1 doesn't apply. - -The subclass in this figure has two designated initializers and one convenience initializer. -The convenience initializer must call one of the two designated initializers, -because it can only call another initializer from the same class. -This satisfies rules 2 and 3 from above. -Both designated initializers must call the single designated initializer -from the superclass, to satisfy rule 1 from above. - -> Note: These rules don't affect how users of your classes *create* instances of each class. -> Any initializer in the diagram above can be used to create -> a fully initialized instance of the class they belong to. -> The rules only affect how you write the implementation of the class's initializers. - -The figure below shows a more complex class hierarchy for four classes. -It illustrates how the designated initializers in this hierarchy -act as “funnel” points for class initialization, -simplifying the interrelationships among classes in the chain: - -![](initializerDelegation02) - -### Two-Phase Initialization - -Class initialization in Swift is a two-phase process. -In the first phase, each stored property is assigned an initial value -by the class that introduced it. -Once the initial state for every stored property has been determined, -the second phase begins, -and each class is given the opportunity to customize its stored properties further -before the new instance is considered ready for use. - -The use of a two-phase initialization process makes initialization safe, -while still giving complete flexibility to each class in a class hierarchy. -Two-phase initialization prevents property values -from being accessed before they're initialized, -and prevents property values from being set to a different value -by another initializer unexpectedly. - -> Note: Swift's two-phase initialization process is similar to initialization in Objective-C. -> The main difference is that during phase 1, -> Objective-C assigns zero or null values (such as `0` or `nil`) to every property. -> Swift's initialization flow is more flexible -> in that it lets you set custom initial values, -> and can cope with types for which `0` or `nil` isn't a valid default value. - -Swift's compiler performs four helpful safety-checks to make sure that -two-phase initialization is completed without error: - -- term **Safety check 1**: - A designated initializer must ensure that all of the properties introduced by its class - are initialized before it delegates up to a superclass initializer. - -As mentioned above, -the memory for an object is only considered fully initialized -once the initial state of all of its stored properties is known. -In order for this rule to be satisfied, a designated initializer must make sure that -all of its own properties are initialized before it hands off up the chain. - -- term **Safety check 2**: - A designated initializer must delegate up to a superclass initializer - before assigning a value to an inherited property. - If it doesn't, the new value the designated initializer assigns - will be overwritten by the superclass as part of its own initialization. - -- term **Safety check 3**: - A convenience initializer must delegate to another initializer - before assigning a value to *any* property - (including properties defined by the same class). - If it doesn't, the new value the convenience initializer assigns - will be overwritten by its own class's designated initializer. - -- term **Safety check 4**: - An initializer can't call any instance methods, - read the values of any instance properties, - or refer to `self` as a value - until after the first phase of initialization is complete. - -The class instance isn't fully valid until the first phase ends. -Properties can only be accessed, and methods can only be called, -once the class instance is known to be valid at the end of the first phase. - -Here's how two-phase initialization plays out, based on the four safety checks above: - -**Phase 1** - -- A designated or convenience initializer is called on a class. -- Memory for a new instance of that class is allocated. - The memory isn't yet initialized. -- A designated initializer for that class confirms that - all stored properties introduced by that class have a value. - The memory for these stored properties is now initialized. -- The designated initializer hands off to a superclass initializer to perform the same task - for its own stored properties. -- This continues up the class inheritance chain until the top of the chain is reached. -- Once the top of the chain is reached, - and the final class in the chain has ensured that all of its stored properties have a value, - the instance's memory is considered to be fully initialized, and phase 1 is complete. - -**Phase 2** - -- Working back down from the top of the chain, - each designated initializer in the chain has the option to customize the instance further. - Initializers are now able to access `self` - and can modify its properties, call its instance methods, and so on. -- Finally, any convenience initializers in the chain have the option - to customize the instance and to work with `self`. - -Here's how phase 1 looks for an initialization call for a hypothetical subclass and superclass: - -![](twoPhaseInitialization01) - -In this example, initialization begins with a call to -a convenience initializer on the subclass. -This convenience initializer can't yet modify any properties. -It delegates across to a designated initializer from the same class. - -The designated initializer makes sure that all of the subclass's properties have a value, -as per safety check 1. It then calls a designated initializer on its superclass -to continue the initialization up the chain. - -The superclass's designated initializer makes sure that -all of the superclass properties have a value. -There are no further superclasses to initialize, -and so no further delegation is needed. - -As soon as all properties of the superclass have an initial value, -its memory is considered fully initialized, and phase 1 is complete. - -Here's how phase 2 looks for the same initialization call: - -![](twoPhaseInitialization02) - -The superclass's designated initializer now has an opportunity -to customize the instance further -(although it doesn't have to). - -Once the superclass's designated initializer is finished, -the subclass's designated initializer can perform additional customization -(although again, it doesn't have to). - -Finally, once the subclass's designated initializer is finished, -the convenience initializer that was originally called -can perform additional customization. - -### Initializer Inheritance and Overriding - -Unlike subclasses in Objective-C, -Swift subclasses don't inherit their superclass initializers by default. -Swift's approach prevents a situation in which a simple initializer from a superclass -is inherited by a more specialized subclass -and is used to create a new instance of the subclass -that isn't fully or correctly initialized. - -> Note: Superclass initializers *are* inherited in certain circumstances, -> but only when it's safe and appropriate to do so. -> For more information, see below. - -If you want a custom subclass to present -one or more of the same initializers as its superclass, -you can provide a custom implementation of those initializers within the subclass. - -When you write a subclass initializer that matches a superclass *designated* initializer, -you are effectively providing an override of that designated initializer. -Therefore, you must write the `override` modifier before the subclass's initializer definition. -This is true even if you are overriding an automatically provided default initializer, -as described in . - -As with an overridden property, method or subscript, -the presence of the `override` modifier prompts Swift to check that -the superclass has a matching designated initializer to be overridden, -and validates that the parameters for your overriding initializer have been specified as intended. - -> Note: You always write the `override` modifier when overriding a superclass designated initializer, -> even if your subclass's implementation of the initializer is a convenience initializer. - - - - - -Conversely, if you write a subclass initializer that matches a superclass *convenience* initializer, -that superclass convenience initializer can never be called directly by your subclass, -as per the rules described above in . -Therefore, your subclass is not (strictly speaking) providing an override of the superclass initializer. -As a result, you don't write the `override` modifier when providing -a matching implementation of a superclass convenience initializer. - - - -The example below defines a base class called `Vehicle`. -This base class declares a stored property called `numberOfWheels`, -with a default `Int` value of `0`. -The `numberOfWheels` property is used by a computed property called `description` -to create a `String` description of the vehicle's characteristics: - -```swift -class Vehicle { - var numberOfWheels = 0 - var description: String { - return "\(numberOfWheels) wheel(s)" - } -} -``` - - - -The `Vehicle` class provides a default value for its only stored property, -and doesn't provide any custom initializers itself. -As a result, it automatically receives a default initializer, -as described in . -The default initializer (when available) is always a designated initializer for a class, -and can be used to create a new `Vehicle` instance with a `numberOfWheels` of `0`: - -```swift -let vehicle = Vehicle() -print("Vehicle: \(vehicle.description)") -// Vehicle: 0 wheel(s) -``` - - - -The next example defines a subclass of `Vehicle` called `Bicycle`: - -```swift -class Bicycle: Vehicle { - override init() { - super.init() - numberOfWheels = 2 - } -} -``` - - - -The `Bicycle` subclass defines a custom designated initializer, `init()`. -This designated initializer matches a designated initializer from the superclass of `Bicycle`, -and so the `Bicycle` version of this initializer is marked with the `override` modifier. - -The `init()` initializer for `Bicycle` starts by calling `super.init()`, -which calls the default initializer for the `Bicycle` class's superclass, `Vehicle`. -This ensures that the `numberOfWheels` inherited property is initialized by `Vehicle` -before `Bicycle` has the opportunity to modify the property. -After calling `super.init()`, -the original value of `numberOfWheels` is replaced with a new value of `2`. - -If you create an instance of `Bicycle`, -you can call its inherited `description` computed property -to see how its `numberOfWheels` property has been updated: - -```swift -let bicycle = Bicycle() -print("Bicycle: \(bicycle.description)") -// Bicycle: 2 wheel(s) -``` - - - -If a subclass initializer performs no customization -in phase 2 of the initialization process, -and the superclass has a synchronous, zero-argument designated initializer, -you can omit a call to `super.init()` -after assigning values to all of the subclass's stored properties. -If the superclass's initializer is asynchronous, -you need to write `await super.init()` explicitly. - -This example defines another subclass of `Vehicle`, called `Hoverboard`. -In its initializer, the `Hoverboard` class sets only its `color` property. -Instead of making an explicit call to `super.init()`, -this initializer relies on an implicit call to its superclass's initializer -to complete the process. - -```swift -class Hoverboard: Vehicle { - var color: String - init(color: String) { - self.color = color - // super.init() implicitly called here - } - override var description: String { - return "\(super.description) in a beautiful \(color)" - } -} -``` - - - -An instance of `Hoverboard` uses the default number of wheels -supplied by the `Vehicle` initializer. - -```swift -let hoverboard = Hoverboard(color: "silver") -print("Hoverboard: \(hoverboard.description)") -// Hoverboard: 0 wheel(s) in a beautiful silver -``` - - - -> Note: Subclasses can modify inherited variable properties during initialization, -> but can't modify inherited constant properties. - - - -### Automatic Initializer Inheritance - -As mentioned above, -subclasses don't inherit their superclass initializers by default. -However, superclass initializers *are* automatically inherited if certain conditions are met. -In practice, this means that -you don't need to write initializer overrides in many common scenarios, -and can inherit your superclass initializers with minimal effort whenever it's safe to do so. - -Assuming that you provide default values for any new properties you introduce in a subclass, -the following two rules apply: - -- term **Rule 1**: - If your subclass doesn't define any designated initializers, - it automatically inherits all of its superclass designated initializers. - -- term **Rule 2**: - If your subclass provides an implementation of - *all* of its superclass designated initializers --- - either by inheriting them as per rule 1, - or by providing a custom implementation as part of its definition --- - then it automatically inherits all of the superclass convenience initializers. - -These rules apply even if your subclass adds further convenience initializers. - -> Note: A subclass can implement a superclass designated initializer -> as a subclass convenience initializer as part of satisfying rule 2. - - - - - -### Designated and Convenience Initializers in Action - -The following example shows designated initializers, convenience initializers, -and automatic initializer inheritance in action. -This example defines a hierarchy of three classes called -`Food`, `RecipeIngredient`, and `ShoppingListItem`, -and demonstrates how their initializers interact. - -The base class in the hierarchy is called `Food`, -which is a simple class to encapsulate the name of a foodstuff. -The `Food` class introduces a single `String` property called `name` -and provides two initializers for creating `Food` instances: - -```swift -class Food { - var name: String - init(name: String) { - self.name = name - } - convenience init() { - self.init(name: "[Unnamed]") - } -} -``` - - - -The figure below shows the initializer chain for the `Food` class: - -![](initializersExample01) - -Classes don't have a default memberwise initializer, -and so the `Food` class provides a designated initializer -that takes a single argument called `name`. -This initializer can be used to create a new `Food` instance with a specific name: - -```swift -let namedMeat = Food(name: "Bacon") -// namedMeat's name is "Bacon" -``` - - - -The `init(name: String)` initializer from the `Food` class -is provided as a *designated* initializer, -because it ensures that all stored properties of -a new `Food` instance are fully initialized. -The `Food` class doesn't have a superclass, -and so the `init(name: String)` initializer doesn't need to call `super.init()` -to complete its initialization. - -The `Food` class also provides a *convenience* initializer, `init()`, with no arguments. -The `init()` initializer provides a default placeholder name for a new food -by delegating across to the `Food` class's `init(name: String)` with -a `name` value of `[Unnamed]`: - -```swift -let mysteryMeat = Food() -// mysteryMeat's name is "[Unnamed]" -``` - - - -The second class in the hierarchy is a subclass of `Food` called `RecipeIngredient`. -The `RecipeIngredient` class models an ingredient in a cooking recipe. -It introduces an `Int` property called `quantity` -(in addition to the `name` property it inherits from `Food`) -and defines two initializers for creating `RecipeIngredient` instances: - -```swift -class RecipeIngredient: Food { - var quantity: Int - init(name: String, quantity: Int) { - self.quantity = quantity - super.init(name: name) - } - override convenience init(name: String) { - self.init(name: name, quantity: 1) - } -} -``` - - - -The figure below shows the initializer chain for the `RecipeIngredient` class: - -![](initializersExample02) - -The `RecipeIngredient` class has a single designated initializer, -`init(name: String, quantity: Int)`, -which can be used to populate all of the properties of a new `RecipeIngredient` instance. -This initializer starts by assigning -the passed `quantity` argument to the `quantity` property, -which is the only new property introduced by `RecipeIngredient`. -After doing so, the initializer delegates up to -the `init(name: String)` initializer of the `Food` class. -This process satisfies safety check 1 -from above. - -`RecipeIngredient` also defines a convenience initializer, `init(name: String)`, -which is used to create a `RecipeIngredient` instance by name alone. -This convenience initializer assumes a quantity of `1` -for any `RecipeIngredient` instance that's created without an explicit quantity. -The definition of this convenience initializer makes -`RecipeIngredient` instances quicker and more convenient to create, -and avoids code duplication when creating -several single-quantity `RecipeIngredient` instances. -This convenience initializer simply delegates across to the class's designated initializer, -passing in a `quantity` value of `1`. - -The `init(name: String)` convenience initializer provided by `RecipeIngredient` -takes the same parameters as the `init(name: String)` *designated* initializer from `Food`. -Because this convenience initializer overrides a designated initializer from its superclass, -it must be marked with the `override` modifier -(as described in ). - -Even though `RecipeIngredient` provides -the `init(name: String)` initializer as a convenience initializer, -`RecipeIngredient` has nonetheless provided an implementation of -all of its superclass's designated initializers. -Therefore, `RecipeIngredient` automatically inherits -all of its superclass's convenience initializers too. - -In this example, the superclass for `RecipeIngredient` is `Food`, -which has a single convenience initializer called `init()`. -This initializer is therefore inherited by `RecipeIngredient`. -The inherited version of `init()` functions in exactly the same way as the `Food` version, -except that it delegates to the `RecipeIngredient` version of `init(name: String)` -rather than the `Food` version. - -All three of these initializers can be used to create new `RecipeIngredient` instances: - -```swift -let oneMysteryItem = RecipeIngredient() -let oneBacon = RecipeIngredient(name: "Bacon") -let sixEggs = RecipeIngredient(name: "Eggs", quantity: 6) -``` - - - -The third and final class in the hierarchy is -a subclass of `RecipeIngredient` called `ShoppingListItem`. -The `ShoppingListItem` class models a recipe ingredient as it appears in a shopping list. - -Every item in the shopping list starts out as “unpurchased”. -To represent this fact, -`ShoppingListItem` introduces a Boolean property called `purchased`, -with a default value of `false`. -`ShoppingListItem` also adds a computed `description` property, -which provides a textual description of a `ShoppingListItem` instance: - -```swift -class ShoppingListItem: RecipeIngredient { - var purchased = false - var description: String { - var output = "\(quantity) x \(name)" - output += purchased ? " ✔" : " ✘" - return output - } -} -``` - - - -> Note: `ShoppingListItem` doesn't define an initializer to provide -> an initial value for `purchased`, -> because items in a shopping list (as modeled here) always start out unpurchased. - -Because it provides a default value for all of the properties it introduces -and doesn't define any initializers itself, -`ShoppingListItem` automatically inherits -*all* of the designated and convenience initializers from its superclass. - -The figure below shows the overall initializer chain for all three classes: - -![](initializersExample03) - -You can use all three of the inherited initializers -to create a new `ShoppingListItem` instance: - -```swift -var breakfastList = [ - ShoppingListItem(), - ShoppingListItem(name: "Bacon"), - ShoppingListItem(name: "Eggs", quantity: 6), -] -breakfastList[0].name = "Orange juice" -breakfastList[0].purchased = true -for item in breakfastList { - print(item.description) -} -// 1 x Orange juice ✔ -// 1 x Bacon ✘ -// 6 x Eggs ✘ -``` - - - -Here, a new array called `breakfastList` is created from -an array literal containing three new `ShoppingListItem` instances. -The type of the array is inferred to be `[ShoppingListItem]`. -After the array is created, -the name of the `ShoppingListItem` at the start of the array -is changed from `"[Unnamed]"` to `"Orange juice"` -and it's marked as having been purchased. -Printing the description of each item in the array -shows that their default states have been set as expected. - - - - - - - -## Failable Initializers - -It's sometimes useful to define a class, structure, or enumeration -for which initialization can fail. -This failure might be triggered by invalid initialization parameter values, -the absence of a required external resource, -or some other condition that prevents initialization from succeeding. - -To cope with initialization conditions that can fail, -define one or more failable initializers as part of -a class, structure, or enumeration definition. -You write a failable initializer -by placing a question mark after the `init` keyword (`init?`). - -> Note: You can't define a failable and a nonfailable initializer -> with the same parameter types and names. - - - -A failable initializer creates an *optional* value of the type it initializes. -You write `return nil` within a failable initializer -to indicate a point at which initialization failure can be triggered. - -> Note: Strictly speaking, initializers don't return a value. -> Rather, their role is to ensure that `self` is fully and correctly initialized -> by the time that initialization ends. -> Although you write `return nil` to trigger an initialization failure, -> you don't use the `return` keyword to indicate initialization success. - -For instance, failable initializers are implemented for numeric type conversions. -To ensure conversion between numeric types maintains the value exactly, -use the `init(exactly:)` initializer. -If the type conversion can't maintain the value, -the initializer fails. - -```swift -let wholeNumber: Double = 12345.0 -let pi = 3.14159 - -if let valueMaintained = Int(exactly: wholeNumber) { - print("\(wholeNumber) conversion to Int maintains value of \(valueMaintained)") -} -// Prints "12345.0 conversion to Int maintains value of 12345" - -let valueChanged = Int(exactly: pi) -// valueChanged is of type Int?, not Int - -if valueChanged == nil { - print("\(pi) conversion to Int doesn't maintain value") -} -// Prints "3.14159 conversion to Int doesn't maintain value" -``` - - - -The example below defines a structure called `Animal`, -with a constant `String` property called `species`. -The `Animal` structure also defines a failable initializer -with a single parameter called `species`. -This initializer checks if the `species` value passed to the initializer is an empty string. -If an empty string is found, an initialization failure is triggered. -Otherwise, the `species` property's value is set, and initialization succeeds: - -```swift -struct Animal { - let species: String - init?(species: String) { - if species.isEmpty { return nil } - self.species = species - } -} -``` - - - -You can use this failable initializer to try to initialize a new `Animal` instance -and to check if initialization succeeded: - -```swift -let someCreature = Animal(species: "Giraffe") -// someCreature is of type Animal?, not Animal - -if let giraffe = someCreature { - print("An animal was initialized with a species of \(giraffe.species)") -} -// Prints "An animal was initialized with a species of Giraffe" -``` - - - -If you pass an empty string value to the failable initializer's `species` parameter, -the initializer triggers an initialization failure: - -```swift -let anonymousCreature = Animal(species: "") -// anonymousCreature is of type Animal?, not Animal - -if anonymousCreature == nil { - print("The anonymous creature couldn't be initialized") -} -// Prints "The anonymous creature couldn't be initialized" -``` - - - -> Note: Checking for an empty string value (such as `""` rather than `"Giraffe"`) -> isn't the same as checking for `nil` to indicate the absence of an *optional* `String` value. -> In the example above, an empty string (`""`) is a valid, non-optional `String`. -> However, it's not appropriate for an animal -> to have an empty string as the value of its `species` property. -> To model this restriction, -> the failable initializer triggers an initialization failure if an empty string is found. - -### Failable Initializers for Enumerations - -You can use a failable initializer to select an appropriate enumeration case -based on one or more parameters. -The initializer can then fail if the provided parameters -don't match an appropriate enumeration case. - -The example below defines an enumeration called `TemperatureUnit`, -with three possible states (`kelvin`, `celsius`, and `fahrenheit`). -A failable initializer is used to find an appropriate enumeration case -for a `Character` value representing a temperature symbol: - -```swift -enum TemperatureUnit { - case kelvin, celsius, fahrenheit - init?(symbol: Character) { - switch symbol { - case "K": - self = .kelvin - case "C": - self = .celsius - case "F": - self = .fahrenheit - default: - return nil - } - } -} -``` - - - -You can use this failable initializer to choose -an appropriate enumeration case for the three possible states -and to cause initialization to fail if the parameter doesn't match one of these -states: - -```swift -let fahrenheitUnit = TemperatureUnit(symbol: "F") -if fahrenheitUnit != nil { - print("This is a defined temperature unit, so initialization succeeded.") -} -// Prints "This is a defined temperature unit, so initialization succeeded." - -let unknownUnit = TemperatureUnit(symbol: "X") -if unknownUnit == nil { - print("This isn't a defined temperature unit, so initialization failed.") -} -// Prints "This isn't a defined temperature unit, so initialization failed." -``` - - - -### Failable Initializers for Enumerations with Raw Values - -Enumerations with raw values automatically receive a failable initializer, -`init?(rawValue:)`, -that takes a parameter called `rawValue` of the appropriate raw-value type -and selects a matching enumeration case if one is found, -or triggers an initialization failure if no matching value exists. - -You can rewrite the `TemperatureUnit` example from above -to use raw values of type `Character` -and to take advantage of the `init?(rawValue:)` initializer: - -```swift -enum TemperatureUnit: Character { - case kelvin = "K", celsius = "C", fahrenheit = "F" -} - -let fahrenheitUnit = TemperatureUnit(rawValue: "F") -if fahrenheitUnit != nil { - print("This is a defined temperature unit, so initialization succeeded.") -} -// Prints "This is a defined temperature unit, so initialization succeeded." - -let unknownUnit = TemperatureUnit(rawValue: "X") -if unknownUnit == nil { - print("This isn't a defined temperature unit, so initialization failed.") -} -// Prints "This isn't a defined temperature unit, so initialization failed." -``` - - - -### Propagation of Initialization Failure - -A failable initializer of a class, structure, or enumeration -can delegate across to another failable initializer from the same class, structure, or enumeration. -Similarly, a subclass failable initializer can delegate up to a superclass failable initializer. - -In either case, if you delegate to another initializer that causes initialization to fail, -the entire initialization process fails immediately, -and no further initialization code is executed. - - - - - - - -> Note: A failable initializer can also delegate to a nonfailable initializer. -> Use this approach if you need to add a potential failure state -> to an existing initialization process that doesn't otherwise fail. - -The example below defines a subclass of `Product` called `CartItem`. -The `CartItem` class models an item in an online shopping cart. -`CartItem` introduces a stored constant property called `quantity` -and ensures that this property always has a value of at least `1`: - -```swift -class Product { - let name: String - init?(name: String) { - if name.isEmpty { return nil } - self.name = name - } -} - -class CartItem: Product { - let quantity: Int - init?(name: String, quantity: Int) { - if quantity < 1 { return nil } - self.quantity = quantity - super.init(name: name) - } -} -``` - - - -The failable initializer for `CartItem` starts by -validating that it has received a `quantity` value of `1` or more. -If the `quantity` is invalid, -the entire initialization process fails immediately -and no further initialization code is executed. -Likewise, the failable initializer for `Product` -checks the `name` value, -and the initializer process fails immediately -if `name` is the empty string. - -If you create a `CartItem` instance with a nonempty name and a quantity of `1` or more, -initialization succeeds: - -```swift -if let twoSocks = CartItem(name: "sock", quantity: 2) { - print("Item: \(twoSocks.name), quantity: \(twoSocks.quantity)") -} -// Prints "Item: sock, quantity: 2" -``` - - - -If you try to create a `CartItem` instance with a `quantity` value of `0`, -the `CartItem` initializer causes initialization to fail: - -```swift -if let zeroShirts = CartItem(name: "shirt", quantity: 0) { - print("Item: \(zeroShirts.name), quantity: \(zeroShirts.quantity)") -} else { - print("Unable to initialize zero shirts") -} -// Prints "Unable to initialize zero shirts" -``` - - - -Similarly, if you try to create a `CartItem` instance with an empty `name` value, -the superclass `Product` initializer causes initialization to fail: - -```swift -if let oneUnnamed = CartItem(name: "", quantity: 1) { - print("Item: \(oneUnnamed.name), quantity: \(oneUnnamed.quantity)") -} else { - print("Unable to initialize one unnamed product") -} -// Prints "Unable to initialize one unnamed product" -``` - - - -### Overriding a Failable Initializer - -You can override a superclass failable initializer in a subclass, -just like any other initializer. -Alternatively, you can override a superclass failable initializer -with a subclass *nonfailable* initializer. -This enables you to define a subclass for which initialization can't fail, -even though initialization of the superclass is allowed to fail. - -Note that if you override a failable superclass initializer with a nonfailable subclass initializer, -the only way to delegate up to the superclass initializer -is to force-unwrap the result of the failable superclass initializer. - -> Note: You can override a failable initializer with a nonfailable initializer -> but not the other way around. - - - -The example below defines a class called `Document`. -This class models a document that can be initialized with -a `name` property that's either a nonempty string value or `nil`, -but can't be an empty string: - -```swift -class Document { - var name: String? - // this initializer creates a document with a nil name value - init() {} - // this initializer creates a document with a nonempty name value - init?(name: String) { - if name.isEmpty { return nil } - self.name = name - } -} -``` - - - -The next example defines a subclass of `Document` called `AutomaticallyNamedDocument`. -The `AutomaticallyNamedDocument` subclass overrides -both of the designated initializers introduced by `Document`. -These overrides ensure that an `AutomaticallyNamedDocument` instance has -an initial `name` value of `"[Untitled]"` -if the instance is initialized without a name, -or if an empty string is passed to the `init(name:)` initializer: - -```swift -class AutomaticallyNamedDocument: Document { - override init() { - super.init() - self.name = "[Untitled]" - } - override init(name: String) { - super.init() - if name.isEmpty { - self.name = "[Untitled]" - } else { - self.name = name - } - } -} -``` - - - -The `AutomaticallyNamedDocument` overrides its superclass's -failable `init?(name:)` initializer with a nonfailable `init(name:)` initializer. -Because `AutomaticallyNamedDocument` copes with the empty string case -in a different way than its superclass, -its initializer doesn't need to fail, -and so it provides a nonfailable version of the initializer instead. - -You can use forced unwrapping in an initializer -to call a failable initializer from the superclass -as part of the implementation of a subclass's nonfailable initializer. -For example, the `UntitledDocument` subclass below is always named `"[Untitled]"`, -and it uses the failable `init(name:)` initializer -from its superclass during initialization. - -```swift -class UntitledDocument: Document { - override init() { - super.init(name: "[Untitled]")! - } -} -``` - - - -In this case, if the `init(name:)` initializer of the superclass -were ever called with an empty string as the name, -the forced unwrapping operation would result in a runtime error. -However, because it's called with a string constant, -you can see that the initializer won't fail, -so no runtime error can occur in this case. - -### The init! Failable Initializer - -You typically define a failable initializer -that creates an optional instance of the appropriate type -by placing a question mark after the `init` keyword (`init?`). -Alternatively, you can define a failable initializer that creates -an implicitly unwrapped optional instance of the appropriate type. -Do this by placing an exclamation point after the `init` keyword (`init!`) -instead of a question mark. - -You can delegate from `init?` to `init!` and vice versa, -and you can override `init?` with `init!` and vice versa. -You can also delegate from `init` to `init!`, -although doing so will trigger an assertion -if the `init!` initializer causes initialization to fail. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -## Required Initializers - -Write the `required` modifier before the definition of a class initializer -to indicate that every subclass of the class must implement that initializer: - -```swift -class SomeClass { - required init() { - // initializer implementation goes here - } -} -``` - - - - - - - -You must also write the `required` modifier before -every subclass implementation of a required initializer, -to indicate that the initializer requirement applies to further subclasses in the chain. -You don't write the `override` modifier when overriding a required designated initializer: - -```swift -class SomeSubclass: SomeClass { - required init() { - // subclass implementation of the required initializer goes here - } -} -``` - - - - - -> Note: You don't have to provide an explicit implementation of a required initializer -> if you can satisfy the requirement with an inherited initializer. - - - - - - - -## Setting a Default Property Value with a Closure or Function - -If a stored property's default value requires some customization or setup, -you can use a closure or global function to provide -a customized default value for that property. -Whenever a new instance of the type that the property belongs to is initialized, -the closure or function is called, -and its return value is assigned as the property's default value. - -These kinds of closures or functions typically create -a temporary value of the same type as the property, -tailor that value to represent the desired initial state, -and then return that temporary value to be used as the property's default value. - -Here's a skeleton outline of how a closure can be used -to provide a default property value: - -```swift -class SomeClass { - let someProperty: SomeType = { - // create a default value for someProperty inside this closure - // someValue must be of the same type as SomeType - return someValue - }() -} -``` - - - -Note that the closure's end curly brace is followed by an empty pair of parentheses. -This tells Swift to execute the closure immediately. -If you omit these parentheses, -you are trying to assign the closure itself to the property, -and not the return value of the closure. - - - -> Note: If you use a closure to initialize a property, -> remember that the rest of the instance hasn't yet been initialized -> at the point that the closure is executed. -> This means that you can't access any other property values from within your closure, -> even if those properties have default values. -> You also can't use the implicit `self` property, -> or call any of the instance's methods. - -The example below defines a structure called `Chessboard`, -which models a board for the game of chess. -Chess is played on an 8 x 8 board, -with alternating black and white squares. - -![](chessBoard) - -To represent this game board, -the `Chessboard` structure has a single property called `boardColors`, -which is an array of 64 `Bool` values. -A value of `true` in the array represents a black square -and a value of `false` represents a white square. -The first item in the array represents the top left square on the board -and the last item in the array represents the bottom right square on the board. - -The `boardColors` array is initialized with a closure to set up its color values: - -```swift -struct Chessboard { - let boardColors: [Bool] = { - var temporaryBoard: [Bool] = [] - var isBlack = false - for i in 1...8 { - for j in 1...8 { - temporaryBoard.append(isBlack) - isBlack = !isBlack - } - isBlack = !isBlack - } - return temporaryBoard - }() - func squareIsBlackAt(row: Int, column: Int) -> Bool { - return boardColors[(row * 8) + column] - } -} -``` - - - -Whenever a new `Chessboard` instance is created, the closure is executed, -and the default value of `boardColors` is calculated and returned. -The closure in the example above calculates and sets -the appropriate color for each square on the board -in a temporary array called `temporaryBoard`, -and returns this temporary array as the closure's return value -once its setup is complete. -The returned array value is stored in `boardColors` -and can be queried with the `squareIsBlackAt(row:column:)` utility function: - -```swift -let board = Chessboard() -print(board.squareIsBlackAt(row: 0, column: 1)) -// Prints "true" -print(board.squareIsBlackAt(row: 7, column: 7)) -// Prints "false" -``` - - - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/LanguageGuide/Macros.md b/swift-6-beta.docc/LanguageGuide/Macros.md deleted file mode 100644 index a660838d8..000000000 --- a/swift-6-beta.docc/LanguageGuide/Macros.md +++ /dev/null @@ -1,782 +0,0 @@ -# Macros - -Use macros to generate code at compile time. - -Macros transform your source code when you compile it, -letting you avoid writing repetitive code by hand. -During compilation, -Swift expands any macros in your code before building your code as usual. - -![A diagram showing an overview of macro expansion. On the left, a stylized representation of Swift code. On the right, the same code with several lines added by the macro.](macro-expansion) - -Expanding a macro is always an additive operation: -Macros add new code, -but they never delete or modify existing code. - -Both the input to a macro and the output of macro expansion -are checked to ensure they're syntactically valid Swift code. -Likewise, the values you pass to a macro -and the values in code generated by a macro -are checked to ensure they have the correct types. -In addition, -if the macro's implementation encounters an error when expanding that macro, -the compiler treats this as a compilation error. -These guarantees make it easier to reason about code that uses macros, -and they make it easier to identify issues -like using a macro incorrectly -or a macro implementation that has a bug. - -Swift has two kinds of macros: - -- *Freestanding macros* appear on their own, - without being attached to a declaration. - -- *Attached macros* modify the declaration that they're attached to. - -You call attached and freestanding macros slightly differently, -but they both follow the same model for macro expansion, -and you implement them both using the same approach. -The following sections describe both kinds of macros in more detail. - -## Freestanding Macros - -To call a freestanding macro, -you write a number sign (`#`) before its name, -and you write any arguments to the macro in parentheses after its name. -For example: - -```swift -func myFunction() { - print("Currently running \(#function)") - #warning("Something's wrong") -} -``` - -In the first line, -`#function` calls the [`function()`][] macro from the Swift standard library. -When you compile this code, -Swift calls that macro's implementation, -which replaces `#function` with the name of the current function. -When you run this code and call `myFunction()`, -it prints "Currently running myFunction()". -In the second line, -`#warning` calls the [`warning(_:)`][] macro from the Swift standard library -to produce a custom compile-time warning. - -[`function()`]: https://developer.apple.com/documentation/swift/function() -[`warning(_:)`]: https://developer.apple.com/documentation/swift/warning(_:) - -Freestanding macros can produce a value, like `#function` does, -or they can perform an action at compile time, like `#warning` does. - - -## Attached Macros - -To call an attached macro, -you write an at sign (`@`) before its name, -and you write any arguments to the macro in parentheses after its name. - -Attached macros modify the declaration that they're attached to. -They add code to that declaration, -like defining a new method or adding conformance to a protocol. - -For example, consider the following code -that doesn't use macros: - -```swift -struct SundaeToppings: OptionSet { - let rawValue: Int - static let nuts = SundaeToppings(rawValue: 1 << 0) - static let cherry = SundaeToppings(rawValue: 1 << 1) - static let fudge = SundaeToppings(rawValue: 1 << 2) -} -``` - -In this code, -each of the options in the `SundaeToppings` option set -includes a call to the initializer, -which is repetitive and manual. -It would be easy to make a mistake when adding a new option, -like typing the wrong number at the end of the line. - -Here's a version of this code that uses a macro instead: - -```swift -@OptionSet -struct SundaeToppings { - private enum Options: Int { - case nuts - case cherry - case fudge - } -} -``` - -This version of `SundaeToppings` calls an `@OptionSet` macro. -The macro reads the list of cases in the private enumeration, -generates the list of constants for each option, -and adds a conformance to the [`OptionSet`][] protocol. - -[`OptionSet`]: https://developer.apple.com/documentation/swift/optionset - - - - -For comparison, -here's what the expanded version of the `@OptionSet` macro looks like. -You don't write this code, -and you would see it only if you specifically asked Swift -to show the macro's expansion. - -```swift -struct SundaeToppings { - private enum Options: Int { - case nuts - case cherry - case fudge - } - - typealias RawValue = Int - var rawValue: RawValue - init() { self.rawValue = 0 } - init(rawValue: RawValue) { self.rawValue = rawValue } - static let nuts: Self = Self(rawValue: 1 << Options.nuts.rawValue) - static let cherry: Self = Self(rawValue: 1 << Options.cherry.rawValue) - static let fudge: Self = Self(rawValue: 1 << Options.fudge.rawValue) -} -extension SundaeToppings: OptionSet { } -``` - -All of the code after the private enumeration -comes from the `@OptionSet` macro. -The version of `SundaeToppings` -that uses a macro to generate all of the static variables -is easier to read and easier to maintain -than the manually coded version, earlier. - -## Macro Declarations - -In most Swift code, -when you implement a symbol, like a function or type, -there's no separate declaration. -However, for macros, the declaration and implementation are separate. -A macro's declaration contains its name, -the parameters it takes, -where it can be used, -and what kind of code it generates. -A macro's implementation contains the code -that expands the macro by generating Swift code. - -You introduce a macro declaration with the `macro` keyword. -For example, -here's part of the declaration for -the `@OptionSet` macro used in the previous example: - -```swift -public macro OptionSet() = - #externalMacro(module: "SwiftMacros", type: "OptionSetMacro") -``` - -The first line -specifies the macro's name and its arguments --- -the name is `OptionSet`, and it doesn't take any arguments. -The second line -uses the [`externalMacro(module:type:)`][] macro from the Swift standard library -to tell Swift where the macro's implementation is located. -In this case, -the `SwiftMacros` module -contains a type named `OptionSetMacro`, -which implements the `@OptionSet` macro. - -[`externalMacro(module:type:)`]: https://developer.apple.com/documentation/swift/externalmacro(module:type:) - -Because `OptionSet` is an attached macro, -its name uses upper camel case, -like the names for structures and classes. -Freestanding macros have lower camel case names, -like the names for variables and functions. - -> Note: -> Macros are always declared as `public`. -> Because the code that declares a macro -> is in a different module from code that uses that macro, -> there isn't anywhere you could apply a nonpublic macro. - -A macro declaration defines the macro's *roles* --- -the places in source code where that macro can be called, -and the kinds of code the macro can generate. -Every macro has one or more roles, -which you write as part of the attributes -at the beginning of the macro declaration. -Here's a bit more of the declaration for `@OptionSet`, -including the attributes for its roles: - -```swift -@attached(member) -@attached(extension, conformances: OptionSet) -public macro OptionSet() = - #externalMacro(module: "SwiftMacros", type: "OptionSetMacro") -``` - -The `@attached` attribute appears twice in this declaration, -once for each macro role. -The first use, `@attached(member)`, indicates that the macro -adds new members to the type you apply it to. -The `@OptionSet` macro adds an `init(rawValue:)` initializer -that's required by the `OptionSet` protocol, -as well as some additional members. -The second use, `@attached(extension, conformances: OptionSet)`, -tells you that `@OptionSet` -adds conformance to the `OptionSet` protocol. -The `@OptionSet` macro -extends the type that you apply the macro to, -to add conformance to the `OptionSet` protocol. - -For a freestanding macro, -you write the `@freestanding` attribute to specify its role: - -``` -@freestanding(expression) -public macro line() -> T = - /* ... location of the macro implementation... */ -``` - - - -The `#line` macro above has the `expression` role. -An expression macro produces a value, -or performs a compile-time action like generating a warning. - -In addition to the macro's role, -a macro's declaration provides information about -the names of the symbols that the macro generates. -When a macro declaration provides a list of names, -it's guaranteed to produce only declarations that use those names, -which helps you understand and debug the generated code. -Here's the full declaration of `@OptionSet`: - -```swift -@attached(member, names: named(RawValue), named(rawValue), - named(`init`), arbitrary) -@attached(extension, conformances: OptionSet) -public macro OptionSet() = - #externalMacro(module: "SwiftMacros", type: "OptionSetMacro") -``` - -In the declaration above, -the `@attached(member)` macro includes arguments after the `names:` label -for each of the symbols that the `@OptionSet` macro generates. -The macro adds declarations for symbols named -`RawValue`, `rawValue`, and `init` --- -because those names are known ahead of time, -the macro declaration lists them explicitly. - -The macro declaration also includes `arbitrary` after the list of names, -allowing the macro to generate declarations -whose names aren't known until you use the macro. -For example, -when the `@OptionSet` macro is applied to the `SundaeToppings` above, -it generates type properties that correspond to the enumeration cases, -`nuts`, `cherry`, and `fudge`. - -For more information, -including a full list of macro roles, -see and -in . - -## Macro Expansion - -When building Swift code that uses macros, -the compiler calls the macros' implementation to expand them. - -![Diagram showing the four steps of expanding macros. The input is Swift source code. This becomes a tree, representing the code's structure. The macro implementation adds branches to the tree. The result is Swift source with additional code.](macro-expansion-full) - -Specifically, Swift expands macros in the following way: - -1. The compiler reads the code, - creating an in-memory representation of the syntax. - -1. The compiler sends part of the in-memory representation - to the macro implementation, - which expands the macro. - -1. The compiler replaces the macro call with its expanded form. - -1. The compiler continues with compilation, - using the expanded source code. - -To go through the specific steps, consider the following: - -``` -let magicNumber = #fourCharacterCode("ABCD") -``` - -The `#fourCharacterCode` macro takes a string that's four characters long -and returns an unsigned 32-bit integer -that corresponds to the ASCII values in the string joined together. -Some file formats use integers like this to identify data -because they're compact but still readable in a debugger. -The section below -shows how to implement this macro. - -To expand the macros in the code above, -the compiler reads the Swift file -and creates an in-memory representation of that code -known as an *abstract syntax tree*, or AST. -The AST makes the code's structure explicit, -which makes it easier to write code that interacts with that structure --- -like a compiler or a macro implementation. -Here's a representation of the AST for the code above, -slightly simplified by omitting some extra detail: - -![A tree diagram, with a constant as the root element. The constant has a name, magic number, and a value. The constant's value is a macro call. The macro call has a name, fourCharacterCode, and arguments. The argument is a string literal, ABCD.](macro-ast-original) - -The diagram above shows how the structure of this code -is represented in memory. -Each element in the AST -corresponds to a part of the source code. -The "Constant declaration" AST element -has two child elements under it, -which represent the two parts of a constant declaration: -its name and its value. -The "Macro call" element has child elements -that represent the macro's name -and the list of arguments being passed to the macro. - -As part of constructing this AST, -the compiler checks that the source code is valid Swift. -For example, `#fourCharacterCode` takes a single argument, -which must be a string. -If you tried to pass an integer argument, -or forgot the quotation mark (`"`) at the end of the string literal, -you'd get an error at this point in the process. - -The compiler finds the places in the code where you call a macro, -and loads the external binary that implements those macros. -For each macro call, -the compiler passes part of the AST to that macro's implementation. -Here's a representation of that partial AST: - -![A tree diagram, with a macro call as the root element. The macro call has a name, fourCharacterCode, and arguments. The argument is a string literal, ABCD.](macro-ast-input) - -The implementation of the `#fourCharacterCode` macro -reads this partial AST as its input when expanding the macro. -A macro's implementation -operates only on the partial AST that it receives as its input, -meaning a macro always expands the same way -regardless of what code comes before and after it. -This limitation helps make macro expansion easier to understand, -and helps your code build faster -because Swift can avoid expanding macros that haven't changed. - -Swift helps macro authors avoid accidentally reading other input -by restricting the code that implements macros: - -- The AST passed to a macro implementation - contains only the AST elements that represent the macro, - not any of the code that comes before or after it. - -- The macro implementation runs in a sandboxed environment - that prevents it from accessing the file system or the network. - -In addition to these safeguards, -the macro's author is responsible for not reading or modifying anything -outside of the macro's inputs. -For example, a macro's expansion must not depend on the current time of day. - -The implementation of `#fourCharacterCode` -generates a new AST containing the expanded code. -Here's what that code returns to the compiler: - -![A tree diagram with the integer literal 1145258561 of type UInt32.](macro-ast-output) - -When the compiler receives this expansion, -it replaces the AST element that contains the macro call -with the element that contains the macro's expansion. -After macro expansion, -the compiler checks again to ensure -the program is still syntactically valid Swift -and all the types are correct. -That produces a final AST that can be compiled as usual: - -![A tree diagram, with a constant as the root element. The constant has a name, magic number, and a value. The constant's value is the integer literal 1145258561 of type UInt32.](macro-ast-result) - -This AST corresponds to Swift code like this: - -``` -let magicNumber = 1145258561 as UInt32 -``` - -In this example, the input source code has only one macro, -but a real program could have several instances of the same macro -and several calls to different macros. -The compiler expands macros one at a time. - -If one macro appears inside another, -the outer macro is expanded first --- -this lets the outer macro modify the inner macro before it's expanded. - - - -## Implementing a Macro - -To implement a macro, you make two components: -A type that performs the macro expansion, -and a library that declares the macro to expose it as API. -These parts are built separately from code that uses the macro, -even if you're developing the macro and its clients together, -because the macro implementation runs -as part of building the macro's clients. - -To create a new macro using Swift Package Manager, -run `swift package init --type macro` --- -this creates several files, -including a template for a macro implementation and declaration. - -To add macros to an existing project, -edit the beginning of your `Package.swift` file as follows: - -- Set a Swift tools version of 5.9 or later in the `swift-tools-version` comment. -- Import the `CompilerPluginSupport` module. -- Include macOS 10.15 as a minimum deployment target in the `platforms` list. - -The code below shows the beginning of an example `Package.swift` file. - -```swift -// swift-tools-version: 5.9 - -import PackageDescription -import CompilerPluginSupport - -let package = Package( - name: "MyPackage", - platforms: [ .iOS(.v17), .macOS(.v13)], - // ... -) -``` - -Next, add a target for the macro implementation -and a target for the macro library -to your existing `Package.swift` file. -For example, -you can add something like the following, -changing the names to match your project: - -```swift -targets: [ - // Macro implementation that performs the source transformations. - .macro( - name: "MyProjectMacros", - dependencies: [ - .product(name: "SwiftSyntaxMacros", package: "swift-syntax"), - .product(name: "SwiftCompilerPlugin", package: "swift-syntax") - ] - ), - - // Library that exposes a macro as part of its API. - .target(name: "MyProject", dependencies: ["MyProjectMacros"]), -] -``` - -The code above defines two targets: -`MyProjectMacros` contains the implementation of the macros, -and `MyProject` makes those macros available. - -The implementation of a macro -uses the [SwiftSyntax][] module to interact with Swift code -in a structured way, using an AST. -If you created a new macro package with Swift Package Manager, -the generated `Package.swift` file -automatically includes a dependency on SwiftSyntax. -If you're adding macros to an existing project, -add a dependency on SwiftSyntax in your `Package.swift` file: - -[SwiftSyntax]: http://github.com/apple/swift-syntax/ - -```swift -dependencies: [ - .package(url: "https://github.com/apple/swift-syntax", from: "509.0.0") -], -``` - -Depending on your macro's role, -there's a corresponding protocol from SwiftSyntax -that the macro implementation conforms to. -For example, -consider `#fourCharacterCode` from the previous section. -Here's a structure that implements that macro: - -```swift -import SwiftSyntax -import SwiftSyntaxMacros - -public struct FourCharacterCode: ExpressionMacro { - public static func expansion( - of node: some FreestandingMacroExpansionSyntax, - in context: some MacroExpansionContext - ) throws -> ExprSyntax { - guard let argument = node.argumentList.first?.expression, - let segments = argument.as(StringLiteralExprSyntax.self)?.segments, - segments.count == 1, - case .stringSegment(let literalSegment)? = segments.first - else { - throw CustomError.message("Need a static string") - } - - let string = literalSegment.content.text - guard let result = fourCharacterCode(for: string) else { - throw CustomError.message("Invalid four-character code") - } - - return "\(raw: result) as UInt32" - } -} - -private func fourCharacterCode(for characters: String) -> UInt32? { - guard characters.count == 4 else { return nil } - - var result: UInt32 = 0 - for character in characters { - result = result << 8 - guard let asciiValue = character.asciiValue else { return nil } - result += UInt32(asciiValue) - } - return result -} -enum CustomError: Error { case message(String) } -``` - -If you're adding this macro to an existing Swift Package Manager project, -add a type that acts as the entry point for the macro target -and lists the macros that the target defines: - -```swift -import SwiftCompilerPlugin - -@main -struct MyProjectMacros: CompilerPlugin { - var providingMacros: [Macro.Type] = [FourCharacterCode.self] -} -``` - -The `#fourCharacterCode` macro -is a freestanding macro that produces an expression, -so the `FourCharacterCode` type that implements it -conforms to the `ExpressionMacro` protocol. -The `ExpressionMacro` protocol has one requirement, -an `expansion(of:in:)` method that expands the AST. -For the list of macro roles and their corresponding SwiftSyntax protocols, -see and -in . - -To expand the `#fourCharacterCode` macro, -Swift sends the AST for the code that uses this macro -to the library that contains the macro implementation. -Inside the library, Swift calls `FourCharacterCode.expansion(of:in:)`, -passing in the AST and the context as arguments to the method. -The implementation of `expansion(of:in:)` -finds the string that was passed as an argument to `#fourCharacterCode` -and calculates the corresponding 32-bit unsigned integer literal value. - -In the example above, -the first `guard` block extracts the string literal from the AST, -assigning that AST element to `literalSegment`. -The second `guard` block -calls the private `fourCharacterCode(for:)` function. -Both of these blocks throw an error if the macro is used incorrectly --- -the error message becomes a compiler error -at the malformed call site. -For example, -if you try to call the macro as `#fourCharacterCode("AB" + "CD")` -the compiler shows the error "Need a static string". - -The `expansion(of:in:)` method returns an instance of `ExprSyntax`, -a type from SwiftSyntax that represents an expression in an AST. -Because this type conforms to the `StringLiteralConvertible` protocol, -the macro implementation uses a string literal -as a lightweight syntax to create its result. -All of the SwiftSyntax types that you return from a macro implementation -conform to `StringLiteralConvertible`, -so you can use this approach when implementing any kind of macro. - - - - - - - - -## Developing and Debugging Macros - -Macros are well suited to development using tests: -They transform one AST into another AST -without depending on any external state, -and without making changes to any external state. -In addition, you can create syntax nodes from a string literal, -which simplifies setting up the input for a test. -You can also read the `description` property of an AST -to get a string to compare against an expected value. -For example, -here's a test of the `#fourCharacterCode` macro from previous sections: - -```swift -let source: SourceFileSyntax = - """ - let abcd = #fourCharacterCode("ABCD") - """ - -let file = BasicMacroExpansionContext.KnownSourceFile( - moduleName: "MyModule", - fullFilePath: "test.swift" -) - -let context = BasicMacroExpansionContext(sourceFiles: [source: file]) - -let transformedSF = source.expand( - macros:["fourCharacterCode": FourCharacterCode.self], - in: context -) - -let expectedDescription = - """ - let abcd = 1145258561 as UInt32 - """ - -precondition(transformedSF.description == expectedDescription) -``` - -The example above tests the macro using a precondition, -but you could use a testing framework instead. - - - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/LanguageGuide/MemorySafety.md b/swift-6-beta.docc/LanguageGuide/MemorySafety.md deleted file mode 100644 index 7644aeee9..000000000 --- a/swift-6-beta.docc/LanguageGuide/MemorySafety.md +++ /dev/null @@ -1,759 +0,0 @@ -# Memory Safety - -Structure your code to avoid conflicts when accessing memory. - -By default, Swift prevents unsafe behavior from happening in your code. -For example, -Swift ensures that variables are initialized before they're used, -memory isn't accessed after it's been deallocated, -and array indices are checked for out-of-bounds errors. - -Swift also makes sure that multiple accesses -to the same area of memory don't conflict, -by requiring code that modifies a location in memory -to have exclusive access to that memory. -Because Swift manages memory automatically, -most of the time you don't have to think about accessing memory at all. -However, -it's important to understand where potential conflicts can occur, -so you can avoid writing code that has conflicting access to memory. -If your code does contain conflicts, -you'll get a compile-time or runtime error. - - - -## Understanding Conflicting Access to Memory - -Access to memory happens in your code -when you do things like set the value of a variable -or pass an argument to a function. -For example, -the following code contains both a read access and a write access: - -```swift -// A write access to the memory where one is stored. -var one = 1 - -// A read access from the memory where one is stored. -print("We're number \(one)!") -``` - - - - - -A conflicting access to memory can occur -when different parts of your code are trying -to access the same location in memory at the same time. -Multiple accesses to a location in memory at the same time -can produce unpredictable or inconsistent behavior. -In Swift, there are ways to modify a value -that span several lines of code, -making it possible to attempt to access a value -in the middle of its own modification. - -You can see a similar problem -by thinking about how you update a budget -that's written on a piece of paper. -Updating the budget is a two-step process: -First you add the items' names and prices, -and then you change the total amount -to reflect the items currently on the list. -Before and after the update, -you can read any information from the budget -and get a correct answer, -as shown in the figure below. - -![](memory_shopping) - -While you're adding items to the budget, -it's in a temporary, invalid state -because the total amount hasn't been updated -to reflect the newly added items. -Reading the total amount -during the process of adding an item -gives you incorrect information. - -This example also demonstrates -a challenge you may encounter -when fixing conflicting access to memory: -There are sometimes multiple ways to fix the conflict -that produce different answers, -and it's not always obvious which answer is correct. -In this example, -depending on whether you wanted the original total amount -or the updated total amount, -either $5 or $320 could be the correct answer. -Before you can fix the conflicting access, -you have to determine what it was intended to do. - -> Note: If you've written concurrent or multithreaded code, -> conflicting access to memory might be a familiar problem. -> However, -> the conflicting access discussed here can happen -> on a single thread and -> *doesn't* involve concurrent or multithreaded code. -> -> If you have conflicting access to memory -> from within a single thread, -> Swift guarantees that you'll get an error -> at either compile time or runtime. -> For multithreaded code, -> use [Thread Sanitizer](https://developer.apple.com/documentation/xcode/diagnosing_memory_thread_and_crash_issues_early) -> to help detect conflicting access across threads. - - - -### Characteristics of Memory Access - -There are three characteristics of memory access -to consider in the context of conflicting access: -whether the access is a read or a write, -the duration of the access, -and the location in memory being accessed. -Specifically, -a conflict occurs if you have two accesses -that meet all of the following conditions: - -- At least one is a write access or a nonatomic access. -- They access the same location in memory. -- Their durations overlap. - -The difference between a read and write access -is usually obvious: -a write access changes the location in memory, -but a read access doesn't. -The location in memory -refers to what is being accessed --- -for example, a variable, constant, or property. -The duration of a memory access -is either instantaneous or long-term. - -An operation is *atomic* -if it uses only C atomic operations; -otherwise it's nonatomic. -For a list of those functions, see the `stdatomic(3)` man page. - - - -An access is *instantaneous* -if it's not possible for other code to run -after that access starts but before it ends. -By their nature, two instantaneous accesses can't happen at the same time. -Most memory access is instantaneous. -For example, -all the read and write accesses in the code listing below are instantaneous: - -```swift -func oneMore(than number: Int) -> Int { - return number + 1 -} - -var myNumber = 1 -myNumber = oneMore(than: myNumber) -print(myNumber) -// Prints "2" -``` - - - -However, -there are several ways to access memory, -called *long-term* accesses, -that span the execution of other code. -The difference between instantaneous access and long-term access -is that it’s possible for other code to run -after a long-term access starts but before it ends, -which is called *overlap*. -A long-term access can overlap -with other long-term accesses and instantaneous accesses. - -Overlapping accesses appear primarily in code that uses -in-out parameters in functions and methods -or mutating methods of a structure. -The specific kinds of Swift code that use long-term accesses -are discussed in the sections below. - -## Conflicting Access to In-Out Parameters - -A function has long-term write access -to all of its in-out parameters. -The write access for an in-out parameter starts -after all of the non-in-out parameters have been evaluated -and lasts for the entire duration of that function call. -If there are multiple in-out parameters, -the write accesses start in the same order as the parameters appear. - -One consequence of this long-term write access -is that you can't access the original -variable that was passed as in-out, -even if scoping rules and access control would otherwise permit it --- -any access to the original creates a conflict. -For example: - -```swift -var stepSize = 1 - -func increment(_ number: inout Int) { - number += stepSize -} - -increment(&stepSize) -// Error: conflicting accesses to stepSize -``` - - - -In the code above, -`stepSize` is a global variable, -and it's normally accessible from within `increment(_:)`. -However, -the read access to `stepSize` overlaps with -the write access to `number`. -As shown in the figure below, -both `number` and `stepSize` refer to the same location in memory. -The read and write accesses -refer to the same memory and they overlap, -producing a conflict. - -![](memory_increment) - -One way to solve this conflict -is to make an explicit copy of `stepSize`: - -```swift -// Make an explicit copy. -var copyOfStepSize = stepSize -increment(©OfStepSize) - -// Update the original. -stepSize = copyOfStepSize -// stepSize is now 2 -``` - - - -When you make a copy of `stepSize` before calling `increment(_:)`, -it's clear that the value of `copyOfStepSize` is incremented -by the current step size. -The read access ends before the write access starts, -so there isn't a conflict. - -Another consequence of long-term write access -to in-out parameters is that -passing a single variable -as the argument for multiple in-out parameters -of the same function -produces a conflict. -For example: - -```swift -func balance(_ x: inout Int, _ y: inout Int) { - let sum = x + y - x = sum / 2 - y = sum - x -} -var playerOneScore = 42 -var playerTwoScore = 30 -balance(&playerOneScore, &playerTwoScore) // OK -balance(&playerOneScore, &playerOneScore) -// Error: conflicting accesses to playerOneScore -``` - - - -The `balance(_:_:)` function above -modifies its two parameters -to divide the total value evenly between them. -Calling it with `playerOneScore` and `playerTwoScore` as arguments -doesn't produce a conflict --- -there are two write accesses that overlap in time, -but they access different locations in memory. -In contrast, -passing `playerOneScore` as the value for both parameters -produces a conflict -because it tries to perform two write accesses -to the same location in memory at the same time. - -> Note: Because operators are functions, -> they can also have long-term accesses to their in-out parameters. -> For example, if `balance(_:_:)` was an operator function named `<^>`, -> writing `playerOneScore <^> playerOneScore` -> would result in the same conflict -> as `balance(&playerOneScore, &playerOneScore)`. - -## Conflicting Access to self in Methods - - - - - -A mutating method on a structure has write access to `self` -for the duration of the method call. -For example, consider a game where each player -has a health amount, which decreases when taking damage, -and an energy amount, which decreases when using special abilities. - -```swift -struct Player { - var name: String - var health: Int - var energy: Int - - static let maxHealth = 10 - mutating func restoreHealth() { - health = Player.maxHealth - } -} -``` - - - -In the `restoreHealth()` method above, -a write access to `self` starts at the beginning of the method -and lasts until the method returns. -In this case, there's no other code -inside `restoreHealth()` -that could have an overlapping access to the properties of a `Player` instance. -The `shareHealth(with:)` method below -takes another `Player` instance as an in-out parameter, -creating the possibility of overlapping accesses. - -```swift -extension Player { - mutating func shareHealth(with teammate: inout Player) { - balance(&teammate.health, &health) - } -} - -var oscar = Player(name: "Oscar", health: 10, energy: 10) -var maria = Player(name: "Maria", health: 5, energy: 10) -oscar.shareHealth(with: &maria) // OK -``` - - - -In the example above, -calling the `shareHealth(with:)` method -for Oscar's player to share health with Maria's player -doesn't cause a conflict. -There's a write access to `oscar` during the method call -because `oscar` is the value of `self` in a mutating method, -and there's a write access to `maria` -for the same duration -because `maria` was passed as an in-out parameter. -As shown in the figure below, -they access different locations in memory. -Even though the two write accesses overlap in time, -they don't conflict. - -![](memory_share_health_maria) - -However, -if you pass `oscar` as the argument to `shareHealth(with:)`, -there's a conflict: - -```swift -oscar.shareHealth(with: &oscar) -// Error: conflicting accesses to oscar -``` - - - -The mutating method needs write access to `self` -for the duration of the method, -and the in-out parameter needs write access to `teammate` -for the same duration. -Within the method, -both `self` and `teammate` refer to -the same location in memory --- -as shown in the figure below. -The two write accesses -refer to the same memory and they overlap, -producing a conflict. - -![](memory_share_health_oscar) - -## Conflicting Access to Properties - -Types like structures, tuples, and enumerations -are made up of individual constituent values, -such as the properties of a structure or the elements of a tuple. -Because these are value types, mutating any piece of the value -mutates the whole value, -meaning read or write access to one of the properties -requires read or write access to the whole value. -For example, -overlapping write accesses to the elements of a tuple -produces a conflict: - -```swift -var playerInformation = (health: 10, energy: 20) -balance(&playerInformation.health, &playerInformation.energy) -// Error: conflicting access to properties of playerInformation -``` - - - -In the example above, -calling `balance(_:_:)` on the elements of a tuple -produces a conflict -because there are overlapping write accesses to `playerInformation`. -Both `playerInformation.health` and `playerInformation.energy` -are passed as in-out parameters, -which means `balance(_:_:)` needs write access to them -for the duration of the function call. -In both cases, a write access to the tuple element -requires a write access to the entire tuple. -This means there are two write accesses to `playerInformation` -with durations that overlap, -causing a conflict. - -The code below shows that the same error appears -for overlapping write accesses -to the properties of a structure -that's stored in a global variable. - -```swift -var holly = Player(name: "Holly", health: 10, energy: 10) -balance(&holly.health, &holly.energy) // Error -``` - - - -In practice, -most access to the properties of a structure -can overlap safely. -For example, -if the variable `holly` in the example above -is changed to a local variable instead of a global variable, -the compiler can prove that overlapping access -to stored properties of the structure is safe: - -```swift -func someFunction() { - var oscar = Player(name: "Oscar", health: 10, energy: 10) - balance(&oscar.health, &oscar.energy) // OK -} -``` - - - -In the example above, -Oscar's health and energy are passed -as the two in-out parameters to `balance(_:_:)`. -The compiler can prove that memory safety is preserved -because the two stored properties don't interact in any way. - -The restriction against -overlapping access to properties of a structure -isn't always necessary to preserve memory safety. -Memory safety is the desired guarantee, -but exclusive access is a stricter requirement than memory safety --- -which means some code preserves memory safety, -even though it violates exclusive access to memory. -Swift allows this memory-safe code if the compiler can prove -that the nonexclusive access to memory is still safe. -Specifically, it can prove -that overlapping access to properties of a structure is safe -if the following conditions apply: - -- You're accessing only stored properties of an instance, - not computed properties or class properties. -- The structure is the value of a local variable, - not a global variable. -- The structure is either not captured by any closures, - or it's captured only by nonescaping closures. - -If the compiler can't prove the access is safe, -it doesn't allow the access. - - - - - - - - - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/LanguageGuide/Methods.md b/swift-6-beta.docc/LanguageGuide/Methods.md deleted file mode 100644 index a7eb7e92d..000000000 --- a/swift-6-beta.docc/LanguageGuide/Methods.md +++ /dev/null @@ -1,667 +0,0 @@ -# Methods - -Define and call functions that are part of an instance or type. - -*Methods* are functions that are associated with a particular type. -Classes, structures, and enumerations can all define instance methods, -which encapsulate specific tasks and functionality for working with an instance of a given type. -Classes, structures, and enumerations can also define type methods, -which are associated with the type itself. -Type methods are similar to class methods in Objective-C. - -The fact that structures and enumerations can define methods in Swift -is a major difference from C and Objective-C. -In Objective-C, classes are the only types that can define methods. -In Swift, you can choose whether to define a class, structure, or enumeration, -and still have the flexibility to define methods on the type you create. - -## Instance Methods - -*Instance methods* are functions that belong to instances of -a particular class, structure, or enumeration. -They support the functionality of those instances, -either by providing ways to access and modify instance properties, -or by providing functionality related to the instance's purpose. -Instance methods have exactly the same syntax as functions, -as described in . - -You write an instance method within the opening and closing braces of the type it belongs to. -An instance method has implicit access to all other instance methods and properties of that type. -An instance method can be called only on a specific instance of the type it belongs to. -It can't be called in isolation without an existing instance. - -Here's an example that defines a simple `Counter` class, -which can be used to count the number of times an action occurs: - -```swift -class Counter { - var count = 0 - func increment() { - count += 1 - } - func increment(by amount: Int) { - count += amount - } - func reset() { - count = 0 - } -} -``` - - - -The `Counter` class defines three instance methods: - -- `increment()` increments the counter by `1`. -- `increment(by: Int)` increments the counter by a specified integer amount. -- `reset()` resets the counter to zero. - -The `Counter` class also declares a variable property, `count`, -to keep track of the current counter value. - -You call instance methods with the same dot syntax as properties: - -```swift -let counter = Counter() -// the initial counter value is 0 -counter.increment() -// the counter's value is now 1 -counter.increment(by: 5) -// the counter's value is now 6 -counter.reset() -// the counter's value is now 0 -``` - - - -Function parameters can have both a name (for use within the function's body) -and an argument label (for use when calling the function), -as described in . -The same is true for method parameters, -because methods are just functions that are associated with a type. - -### The self Property - -Every instance of a type has an implicit property called `self`, -which is exactly equivalent to the instance itself. -You use the `self` property to refer to the current instance -within its own instance methods. - -The `increment()` method in the example above could have been written like this: - -```swift -func increment() { - self.count += 1 -} -``` - - - - - -In practice, you don't need to write `self` in your code very often. -If you don't explicitly write `self`, -Swift assumes that you are referring to a property or method of the current instance -whenever you use a known property or method name within a method. -This assumption is demonstrated by the use of `count` (rather than `self.count`) -inside the three instance methods for `Counter`. - -The main exception to this rule occurs when a parameter name for an instance method -has the same name as a property of that instance. -In this situation, the parameter name takes precedence, -and it becomes necessary to refer to the property in a more qualified way. -You use the `self` property to -distinguish between the parameter name and the property name. - -Here, `self` disambiguates between -a method parameter called `x` and an instance property that's also called `x`: - -```swift -struct Point { - var x = 0.0, y = 0.0 - func isToTheRightOf(x: Double) -> Bool { - return self.x > x - } -} -let somePoint = Point(x: 4.0, y: 5.0) -if somePoint.isToTheRightOf(x: 1.0) { - print("This point is to the right of the line where x == 1.0") -} -// Prints "This point is to the right of the line where x == 1.0" -``` - - - -Without the `self` prefix, -Swift would assume that both uses of `x` referred to the method parameter called `x`. - -### Modifying Value Types from Within Instance Methods - -Structures and enumerations are *value types*. -By default, the properties of a value type can't be modified from within its instance methods. - - - -However, if you need to modify the properties of your structure or enumeration -within a particular method, -you can opt in to *mutating* behavior for that method. -The method can then mutate (that is, change) -its properties from within the method, -and any changes that it makes are written back to the original structure when the method ends. -The method can also assign a completely new instance to its implicit `self` property, -and this new instance will replace the existing one when the method ends. - -You can opt in to this behavior by placing the `mutating` keyword -before the `func` keyword for that method: - -```swift -struct Point { - var x = 0.0, y = 0.0 - mutating func moveBy(x deltaX: Double, y deltaY: Double) { - x += deltaX - y += deltaY - } -} -var somePoint = Point(x: 1.0, y: 1.0) -somePoint.moveBy(x: 2.0, y: 3.0) -print("The point is now at (\(somePoint.x), \(somePoint.y))") -// Prints "The point is now at (3.0, 4.0)" -``` - - - -The `Point` structure above defines a mutating `moveBy(x:y:)` method, -which moves a `Point` instance by a certain amount. -Instead of returning a new point, -this method actually modifies the point on which it's called. -The `mutating` keyword is added to its definition -to enable it to modify its properties. - -Note that you can't call a mutating method on a constant of structure type, -because its properties can't be changed, even if they're variable properties, -as described in : - -```swift -let fixedPoint = Point(x: 3.0, y: 3.0) -fixedPoint.moveBy(x: 2.0, y: 3.0) -// this will report an error -``` - - - - - -### Assigning to self Within a Mutating Method - -Mutating methods can assign an entirely new instance to the implicit `self` property. -The `Point` example shown above could have been written in the following way instead: - -```swift -struct Point { - var x = 0.0, y = 0.0 - mutating func moveBy(x deltaX: Double, y deltaY: Double) { - self = Point(x: x + deltaX, y: y + deltaY) - } -} -``` - - - -This version of the mutating `moveBy(x:y:)` method creates a new structure -whose `x` and `y` values are set to the target location. -The end result of calling this alternative version of the method -will be exactly the same as for calling the earlier version. - -Mutating methods for enumerations can set the implicit `self` parameter to be -a different case from the same enumeration: - -```swift -enum TriStateSwitch { - case off, low, high - mutating func next() { - switch self { - case .off: - self = .low - case .low: - self = .high - case .high: - self = .off - } - } -} -var ovenLight = TriStateSwitch.low -ovenLight.next() -// ovenLight is now equal to .high -ovenLight.next() -// ovenLight is now equal to .off -``` - - - -This example defines an enumeration for a three-state switch. -The switch cycles between three different power states -(`off`, `low` and `high`) -every time its `next()` method is called. - -## Type Methods - -Instance methods, as described above, -are methods that you call on an instance of a particular type. -You can also define methods that are called on the type itself. -These kinds of methods are called *type methods*. -You indicate type methods by writing -the `static` keyword before the method's `func` keyword. -Classes can use the `class` keyword instead, -to allow subclasses to override the superclass’s implementation of that method. - -> Note: In Objective-C, you can define type-level methods only for Objective-C classes. -> In Swift, you can define type-level methods for all classes, structures, and enumerations. -> Each type method is explicitly scoped to the type it supports. - -Type methods are called with dot syntax, like instance methods. -However, you call type methods on the type, not on an instance of that type. -Here's how you call a type method on a class called `SomeClass`: - -```swift -class SomeClass { - class func someTypeMethod() { - // type method implementation goes here - } -} -SomeClass.someTypeMethod() -``` - - - -Within the body of a type method, -the implicit `self` property refers to the type itself, -rather than an instance of that type. -This means that you can use `self` to disambiguate between -type properties and type method parameters, -just as you do for instance properties and instance method parameters. - -More generally, any unqualified method and property names that you use -within the body of a type method will refer to other type-level methods and properties. -A type method can call another type method with the other method's name, -without needing to prefix it with the type name. -Similarly, type methods on structures and enumerations can access type properties -by using the type property's name without a type name prefix. - -The example below defines a structure called `LevelTracker`, -which tracks a player's progress through the different levels or stages of a game. -It's a single-player game, -but can store information for multiple players on a single device. - -All of the game's levels (apart from level one) are locked when the game is first played. -Every time a player finishes a level, -that level is unlocked for all players on the device. -The `LevelTracker` structure uses type properties and methods -to keep track of which levels of the game have been unlocked. -It also tracks the current level for an individual player. - -```swift -struct LevelTracker { - static var highestUnlockedLevel = 1 - var currentLevel = 1 - - static func unlock(_ level: Int) { - if level > highestUnlockedLevel { highestUnlockedLevel = level } - } - - static func isUnlocked(_ level: Int) -> Bool { - return level <= highestUnlockedLevel - } - - @discardableResult - mutating func advance(to level: Int) -> Bool { - if LevelTracker.isUnlocked(level) { - currentLevel = level - return true - } else { - return false - } - } -} -``` - - - -The `LevelTracker` structure keeps track of the highest level that any player has unlocked. -This value is stored in a type property called `highestUnlockedLevel`. - -`LevelTracker` also defines two type functions to work with -the `highestUnlockedLevel` property. -The first is a type function called `unlock(_:)`, -which updates the value of `highestUnlockedLevel` whenever a new level is unlocked. -The second is a convenience type function called `isUnlocked(_:)`, -which returns `true` if a particular level number is already unlocked. -(Note that these type methods can access the `highestUnlockedLevel` type property -without your needing to write it as `LevelTracker.highestUnlockedLevel`.) - -In addition to its type property and type methods, -`LevelTracker` tracks an individual player's progress through the game. -It uses an instance property called `currentLevel` to track -the level that a player is currently playing. - -To help manage the `currentLevel` property, -`LevelTracker` defines an instance method called `advance(to:)`. -Before updating `currentLevel`, -this method checks whether the requested new level is already unlocked. -The `advance(to:)` method returns a Boolean value to indicate -whether or not it was actually able to set `currentLevel`. -Because it's not necessarily a mistake for -code that calls the `advance(to:)` method -to ignore the return value, -this function is marked with the `@discardableResult` attribute. -For more information about this attribute, -see . - -The `LevelTracker` structure is used with the `Player` class, shown below, -to track and update the progress of an individual player: - -```swift -class Player { - var tracker = LevelTracker() - let playerName: String - func complete(level: Int) { - LevelTracker.unlock(level + 1) - tracker.advance(to: level + 1) - } - init(name: String) { - playerName = name - } -} -``` - - - -The `Player` class creates a new instance of `LevelTracker` -to track that player's progress. -It also provides a method called `complete(level:)`, -which is called whenever a player completes a particular level. -This method unlocks the next level for all players -and updates the player's progress to move them to the next level. -(The Boolean return value of `advance(to:)` is ignored, -because the level is known to have been unlocked -by the call to `LevelTracker.unlock(_:)` on the previous line.) - -You can create an instance of the `Player` class for a new player, -and see what happens when the player completes level one: - -```swift -var player = Player(name: "Argyrios") -player.complete(level: 1) -print("highest unlocked level is now \(LevelTracker.highestUnlockedLevel)") -// Prints "highest unlocked level is now 2" -``` - - - -If you create a second player, whom you try to move to a level -that's not yet unlocked by any player in the game, -the attempt to set the player's current level fails: - -```swift -player = Player(name: "Beto") -if player.tracker.advance(to: 6) { - print("player is now on level 6") -} else { - print("level 6 hasn't yet been unlocked") -} -// Prints "level 6 hasn't yet been unlocked" -``` - - - - - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/LanguageGuide/NestedTypes.md b/swift-6-beta.docc/LanguageGuide/NestedTypes.md deleted file mode 100644 index d653a4c7a..000000000 --- a/swift-6-beta.docc/LanguageGuide/NestedTypes.md +++ /dev/null @@ -1,209 +0,0 @@ -# Nested Types - -Define types inside the scope of another type. - -Enumerations are often created to support a specific class or structure's functionality. -Similarly, it can be convenient to define utility structures -purely for use within the context of a more complex type, -and protocols that are normally used in conjunction with a specific type. -To accomplish this, Swift enables you to define *nested types*, -whereby you nest supporting types like enumerations, structures, and protocols -within the definition of the type they support. - -To nest a type within another type, -write its definition within the outer braces of the type it supports. -Types can be nested to as many levels as are required. - -## Nested Types in Action - -The example below defines a structure called `BlackjackCard`, -which models a playing card as used in the game of Blackjack. -The `BlackjackCard` structure contains two nested enumeration types -called `Suit` and `Rank`. - -In Blackjack, the Ace cards have a value of either one or eleven. -This feature is represented by a structure called `Values`, -which is nested within the `Rank` enumeration: - -```swift -struct BlackjackCard { - - // nested Suit enumeration - enum Suit: Character { - case spades = "♠", hearts = "♡", diamonds = "♢", clubs = "♣" - } - - // nested Rank enumeration - enum Rank: Int { - case two = 2, three, four, five, six, seven, eight, nine, ten - case jack, queen, king, ace - struct Values { - let first: Int, second: Int? - } - var values: Values { - switch self { - case .ace: - return Values(first: 1, second: 11) - case .jack, .queen, .king: - return Values(first: 10, second: nil) - default: - return Values(first: self.rawValue, second: nil) - } - } - } - - // BlackjackCard properties and methods - let rank: Rank, suit: Suit - var description: String { - var output = "suit is \(suit.rawValue)," - output += " value is \(rank.values.first)" - if let second = rank.values.second { - output += " or \(second)" - } - return output - } -} -``` - - - -The `Suit` enumeration describes the four common playing card suits, -together with a raw `Character` value to represent their symbol. - -The `Rank` enumeration describes the thirteen possible playing card ranks, -together with a raw `Int` value to represent their face value. -(This raw `Int` value isn't used for the Jack, Queen, King, and Ace cards.) - -As mentioned above, the `Rank` enumeration defines -a further nested structure of its own, called `Values`. -This structure encapsulates the fact that most cards have one value, -but the Ace card has two values. -The `Values` structure defines two properties to represent this: - -- `first`, of type `Int` -- `second`, of type `Int?`, or “optional `Int`” - -`Rank` also defines a computed property, `values`, -which returns an instance of the `Values` structure. -This computed property considers the rank of the card -and initializes a new `Values` instance with appropriate values based on its rank. -It uses special values for `jack`, `queen`, `king`, and `ace`. -For the numeric cards, it uses the rank's raw `Int` value. - -The `BlackjackCard` structure itself has two properties --- `rank` and `suit`. -It also defines a computed property called `description`, -which uses the values stored in `rank` and `suit` to build -a description of the name and value of the card. -The `description` property uses optional binding to check whether there's -a second value to display, and if so, -inserts additional description detail for that second value. - -Because `BlackjackCard` is a structure with no custom initializers, -it has an implicit memberwise initializer, -as described in . -You can use this initializer to initialize a new constant called `theAceOfSpades`: - -```swift -let theAceOfSpades = BlackjackCard(rank: .ace, suit: .spades) -print("theAceOfSpades: \(theAceOfSpades.description)") -// Prints "theAceOfSpades: suit is ♠, value is 1 or 11" -``` - - - -Even though `Rank` and `Suit` are nested within `BlackjackCard`, -their type can be inferred from context, -and so the initialization of this instance is able to refer to the enumeration cases -by their case names (`.ace` and `.spades`) alone. -In the example above, the `description` property correctly reports that -the Ace of Spades has a value of `1` or `11`. - -## Referring to Nested Types - -To use a nested type outside of its definition context, -prefix its name with the name of the type it's nested within: - -```swift -let heartsSymbol = BlackjackCard.Suit.hearts.rawValue -// heartsSymbol is "♡" -``` - - - -For the example above, -this enables the names of `Suit`, `Rank`, and `Values` to be kept deliberately short, -because their names are naturally qualified by the context in which they're defined. - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/LanguageGuide/OpaqueTypes.md b/swift-6-beta.docc/LanguageGuide/OpaqueTypes.md deleted file mode 100644 index 34539fe1b..000000000 --- a/swift-6-beta.docc/LanguageGuide/OpaqueTypes.md +++ /dev/null @@ -1,939 +0,0 @@ -# Opaque and Boxed Protocol Types - -Hide implementation details about a value's type. - -Swift provides two ways to hide details about a value's type: -opaque types and boxed protocol types. -Hiding type information -is useful at boundaries between -a module and code that calls into the module, -because the underlying type of the return value can remain private. - -A function or method that returns an opaque type -hides its return value's type information. -Instead of providing a concrete type as the function's return type, -the return value is described in terms of the protocols it supports. -Opaque types preserve type identity --- -the compiler has access to the type information, -but clients of the module don't. - -A boxed protocol type can store an instance of any type -that conforms to the given protocol. -Boxed protocol types don't preserve type identity --- -the value's specific type isn't known until runtime, -and it can change over time as different values are stored. - -## The Problem That Opaque Types Solve - -For example, -suppose you're writing a module that draws ASCII art shapes. -The basic characteristic of an ASCII art shape -is a `draw()` function that returns the string representation of that shape, -which you can use as the requirement for the `Shape` protocol: - -```swift -protocol Shape { - func draw() -> String -} - -struct Triangle: Shape { - var size: Int - func draw() -> String { - var result: [String] = [] - for length in 1...size { - result.append(String(repeating: "*", count: length)) - } - return result.joined(separator: "\n") - } -} -let smallTriangle = Triangle(size: 3) -print(smallTriangle.draw()) -// * -// ** -// *** -``` - - - -You could use generics to implement operations like flipping a shape vertically, -as shown in the code below. -However, there's an important limitation to this approach: -The flipped result exposes the exact generic types -that were used to create it. - -```swift -struct FlippedShape: Shape { - var shape: T - func draw() -> String { - let lines = shape.draw().split(separator: "\n") - return lines.reversed().joined(separator: "\n") - } -} -let flippedTriangle = FlippedShape(shape: smallTriangle) -print(flippedTriangle.draw()) -// *** -// ** -// * -``` - - - -This approach to defining a `JoinedShape` structure -that joins two shapes together vertically, like the code below shows, -results in types like `JoinedShape>` -from joining a triangle with a flipped triangle. - -```swift -struct JoinedShape: Shape { - var top: T - var bottom: U - func draw() -> String { - return top.draw() + "\n" + bottom.draw() - } -} -let joinedTriangles = JoinedShape(top: smallTriangle, bottom: flippedTriangle) -print(joinedTriangles.draw()) -// * -// ** -// *** -// *** -// ** -// * -``` - - - -Exposing detailed information about the creation of a shape -allows types that aren't meant to be -part of the ASCII art module's public interface -to leak out because of the need to state the full return type. -The code inside the module -could build up the same shape in a variety of ways, -and other code outside the module -that uses the shape shouldn't have to account for -the implementation details about the list of transformations. -Wrapper types like `JoinedShape` and `FlippedShape` -don't matter to the module's users, -and they shouldn't be visible. -The module's public interface -consists of operations like joining and flipping a shape, -and those operations return another `Shape` value. - -## Returning an Opaque Type - -You can think of an opaque type like being the reverse of a generic type. -Generic types let the code that calls a function -pick the type for that function's parameters and return value -in a way that's abstracted away from the function implementation. -For example, the function in the following code -returns a type that depends on its caller: - -```swift -func max(_ x: T, _ y: T) -> T where T: Comparable { ... } -``` - - - -The code that calls `max(_:_:)` chooses the values for `x` and `y`, -and the type of those values determines the concrete type of `T`. -The calling code can use any type -that conforms to the `Comparable` protocol. -The code inside the function is written in a general way -so it can handle whatever type the caller provides. -The implementation of `max(_:_:)` uses only functionality -that all `Comparable` types share. - -Those roles are reversed for a function with an opaque return type. -An opaque type lets the function implementation -pick the type for the value it returns -in a way that's abstracted away from the code that calls the function. -For example, the function in the following example returns a trapezoid -without exposing the underlying type of that shape. - -```swift -struct Square: Shape { - var size: Int - func draw() -> String { - let line = String(repeating: "*", count: size) - let result = Array(repeating: line, count: size) - return result.joined(separator: "\n") - } -} - -func makeTrapezoid() -> some Shape { - let top = Triangle(size: 2) - let middle = Square(size: 2) - let bottom = FlippedShape(shape: top) - let trapezoid = JoinedShape( - top: top, - bottom: JoinedShape(top: middle, bottom: bottom) - ) - return trapezoid -} -let trapezoid = makeTrapezoid() -print(trapezoid.draw()) -// * -// ** -// ** -// ** -// ** -// * -``` - - - -The `makeTrapezoid()` function in this example -declares its return type as `some Shape`; -as a result, the function -returns a value of some given type that conforms to the `Shape` protocol, -without specifying any particular concrete type. -Writing `makeTrapezoid()` this way lets it express -the fundamental aspect of its public interface --- -the value it returns is a shape --- -without making the specific types that the shape is made from -a part of its public interface. -This implementation uses two triangles and a square, -but the function could be rewritten to draw a trapezoid -in a variety of other ways -without changing its return type. - -This example highlights the way that an opaque return type -is like the reverse of a generic type. -The code inside `makeTrapezoid()` can return any type it needs to, -as long as that type conforms to the `Shape` protocol, -like the calling code does for a generic function. -The code that calls the function needs to be written in a general way, -like the implementation of a generic function, -so that it can work with any `Shape` value -that's returned by `makeTrapezoid()`. - -You can also combine opaque return types with generics. -The functions in the following code both return a value -of some type that conforms to the `Shape` protocol. - -```swift -func flip(_ shape: T) -> some Shape { - return FlippedShape(shape: shape) -} -func join(_ top: T, _ bottom: U) -> some Shape { - JoinedShape(top: top, bottom: bottom) -} - -let opaqueJoinedTriangles = join(smallTriangle, flip(smallTriangle)) -print(opaqueJoinedTriangles.draw()) -// * -// ** -// *** -// *** -// ** -// * -``` - - - -The value of `opaqueJoinedTriangles` in this example -is the same as `joinedTriangles` in the generics example -in the section earlier in this chapter. -However, unlike the value in that example, -`flip(_:)` and `join(_:_:)` wrap the underlying types -that the generic shape operations return -in an opaque return type, -which prevents those types from being visible. -Both functions are generic because the types they rely on are generic, -and the type parameters to the function -pass along the type information needed by `FlippedShape` and `JoinedShape`. - -If a function with an opaque return type -returns from multiple places, -all of the possible return values must have the same type. -For a generic function, -that return type can use the function's generic type parameters, -but it must still be a single type. -For example, -here's an *invalid* version of the shape-flipping function -that includes a special case for squares: - -```swift -func invalidFlip(_ shape: T) -> some Shape { - if shape is Square { - return shape // Error: return types don't match - } - return FlippedShape(shape: shape) // Error: return types don't match -} -``` - - - -If you call this function with a `Square`, it returns a `Square`; -otherwise, it returns a `FlippedShape`. -This violates the requirement to return values of only one type -and makes `invalidFlip(_:)` invalid code. -One way to fix `invalidFlip(_:)` is to move the special case for squares -into the implementation of `FlippedShape`, -which lets this function always return a `FlippedShape` value: - -```swift -struct FlippedShape: Shape { - var shape: T - func draw() -> String { - if shape is Square { - return shape.draw() - } - let lines = shape.draw().split(separator: "\n") - return lines.reversed().joined(separator: "\n") - } -} -``` - - - - - -The requirement to always return a single type -doesn't prevent you from using generics in an opaque return type. -Here's an example of a function that incorporates its type parameter -into the underlying type of the value it returns: - -```swift -func `repeat`(shape: T, count: Int) -> some Collection { - return Array(repeating: shape, count: count) -} -``` - - - -In this case, -the underlying type of the return value -varies depending on `T`: -Whatever shape is passed it, -`repeat(shape:count:)` creates and returns an array of that shape. -Nevertheless, -the return value always has the same underlying type of `[T]`, -so it follows the requirement that functions with opaque return types -must return values of only a single type. - -## Boxed Protocol Types - -A boxed protocol type is also sometimes called an *existential type*, -which comes from the phrase -"there exists a type *T* such that *T* conforms to the protocol". -To make a boxed protocol type, -write `any` before the name of a protocol. -Here's an example: - -```swift -struct VerticalShapes: Shape { - var shapes: [any Shape] - func draw() -> String { - return shapes.map { $0.draw() }.joined(separator: "\n\n") - } -} - -let largeTriangle = Triangle(size: 5) -let largeSquare = Square(size: 5) -let vertical = VerticalShapes(shapes: [largeTriangle, largeSquare]) -print(vertical.draw()) -``` - - - -In the example above, -`VerticalShapes` declares the type of `shapes` as `[any Shape]` --- -an array of boxed `Shape` elements. -Each element in the array can be a different type, -and each of those types must conform to the `Shape` protocol. -To support this runtime flexibility, -Swift adds a level of indirection when necessary --- -this indirection is called a *box*, -and it has a performance cost. - -Within the `VerticalShapes` type, -the code can use methods, properties, and subscripts -that are required by the `Shape` protocol. -For example, the `draw()` method of `VerticalShapes` -calls the `draw()` method on each element of the array. -This method is available because `Shape` requires a `draw()` method. -In contrast, -trying to access the `size` property of the triangle, -or any other properties or methods that aren't required by `Shape`, -produces an error. - -Contrast the three types you could use for `shapes`: - -- Using generics, - by writing `struct VerticalShapes` and `var shapes: [S]`, - makes an array whose elements are some specific shape type, - and where the identity of that specific type - is visible to any code that interacts with the array. - -- Using an opaque type, - by writing `var shapes: [some Shape]`, - makes an array whose elements are some specific shape type, - and where that specific type's identity is hidden. - -- Using a boxed protocol type, - by writing `var shapes: [any Shape]`, - makes an array that can store elements of different types, - and where those types' identities are hidden. - -In this case, -a boxed protocol type is the only approach -that lets callers of `VerticalShapes` mix different kinds of shapes together. - -You can use an `as` cast -when you know the underlying type of a boxed value. -For example: - -```swift -if let downcastTriangle = vertical.shapes[0] as? Triangle { - print(downcastTriangle.size) -} -// Prints "5" -``` - -For more information, see . - -## Differences Between Opaque Types and Boxed Protocol Types - -Returning an opaque type looks very similar -to using a boxed protocol type as the return type of a function, -but these two kinds of return type differ in -whether they preserve type identity. -An opaque type refers to one specific type, -although the caller of the function isn't able to see which type; -a boxed protocol type can refer to any type that conforms to the protocol. -Generally speaking, -boxed protocol types give you more flexibility -about the underlying types of the values they store, -and opaque types let you make stronger guarantees -about those underlying types. - -For example, -here's a version of `flip(_:)` -that uses a boxed protocol type as its return type -instead of an opaque return type: - -```swift -func protoFlip(_ shape: T) -> Shape { - return FlippedShape(shape: shape) -} -``` - - - -This version of `protoFlip(_:)` -has the same body as `flip(_:)`, -and it always returns a value of the same type. -Unlike `flip(_:)`, -the value that `protoFlip(_:)` returns isn't required -to always have the same type --- -it just has to conform to the `Shape` protocol. -Put another way, -`protoFlip(_:)` makes a much looser API contract with its caller -than `flip(_:)` makes. -It reserves the flexibility to return values of multiple types: - -```swift -func protoFlip(_ shape: T) -> Shape { - if shape is Square { - return shape - } - - return FlippedShape(shape: shape) -} -``` - - - -The revised version of the code returns -an instance of `Square` or an instance of `FlippedShape`, -depending on what shape is passed in. -Two flipped shapes returned by this function -might have completely different types. -Other valid versions of this function could return values of different types -when flipping multiple instances of the same shape. -The less specific return type information from `protoFlip(_:)` means that -many operations that depend on type information -aren't available on the returned value. -For example, it's not possible to write an `==` operator -comparing results returned by this function. - -```swift -let protoFlippedTriangle = protoFlip(smallTriangle) -let sameThing = protoFlip(smallTriangle) -protoFlippedTriangle == sameThing // Error -``` - - - -The error on the last line of the example occurs for several reasons. -The immediate issue is that the `Shape` doesn't include an `==` operator -as part of its protocol requirements. -If you try adding one, the next issue you'll encounter -is that the `==` operator needs to know -the types of its left-hand and right-hand arguments. -This sort of operator usually takes arguments of type `Self`, -matching whatever concrete type adopts the protocol, -but adding a `Self` requirement to the protocol -doesn't allow for the type erasure that happens -when you use the protocol as a type. - -Using a boxed protocol type as the return type for a function -gives you the flexibility to return any type that conforms to the protocol. -However, the cost of that flexibility -is that some operations aren't possible on the returned values. -The example shows how the `==` operator isn't available --- -it depends on specific type information -that isn't preserved by using a boxed protocol type. - -Another problem with this approach is that the shape transformations don't nest. -The result of flipping a triangle is a value of type `Shape`, -and the `protoFlip(_:)` function takes an argument -of some type that conforms to the `Shape` protocol. -However, a value of a boxed protocol type doesn't conform to that protocol; -the value returned by `protoFlip(_:)` doesn't conform to `Shape`. -This means code like `protoFlip(protoFlip(smallTriangle))` -that applies multiple transformations is invalid -because the flipped shape isn't a valid argument to `protoFlip(_:)`. - -In contrast, -opaque types preserve the identity of the underlying type. -Swift can infer associated types, -which lets you use an opaque return value -in places where a boxed protocol type can't be used as a return value. -For example, -here's a version of the `Container` protocol from : - -```swift -protocol Container { - associatedtype Item - var count: Int { get } - subscript(i: Int) -> Item { get } -} -extension Array: Container { } -``` - - - -You can't use `Container` as the return type of a function -because that protocol has an associated type. -You also can't use it as constraint in a generic return type -because there isn't enough information outside the function body -to infer what the generic type needs to be. - -```swift -// Error: Protocol with associated types can't be used as a return type. -func makeProtocolContainer(item: T) -> Container { - return [item] -} - -// Error: Not enough information to infer C. -func makeProtocolContainer(item: T) -> C { - return [item] -} -``` - - - -Using the opaque type `some Container` as a return type -expresses the desired API contract --- the function returns a container, -but declines to specify the container's type: - -```swift -func makeOpaqueContainer(item: T) -> some Container { - return [item] -} -let opaqueContainer = makeOpaqueContainer(item: 12) -let twelve = opaqueContainer[0] -print(type(of: twelve)) -// Prints "Int" -``` - - - -The type of `twelve` is inferred to be `Int`, -which illustrates the fact that type inference works with opaque types. -In the implementation of `makeOpaqueContainer(item:)`, -the underlying type of the opaque container is `[T]`. -In this case, `T` is `Int`, -so the return value is an array of integers -and the `Item` associated type is inferred to be `Int`. -The subscript on `Container` returns `Item`, -which means that the type of `twelve` is also inferred to be `Int`. - - - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - - diff --git a/swift-6-beta.docc/LanguageGuide/OptionalChaining.md b/swift-6-beta.docc/LanguageGuide/OptionalChaining.md deleted file mode 100644 index 3dc115533..000000000 --- a/swift-6-beta.docc/LanguageGuide/OptionalChaining.md +++ /dev/null @@ -1,900 +0,0 @@ -# Optional Chaining - -Access members of an optional value without unwrapping. - -*Optional chaining* is a process for querying and calling -properties, methods, and subscripts on an optional that might currently be `nil`. -If the optional contains a value, -the property, method, or subscript call succeeds; -if the optional is `nil`, the property, method, or subscript call returns `nil`. -Multiple queries can be chained together, -and the entire chain fails gracefully if any link in the chain is `nil`. - -> Note: Optional chaining in Swift is similar to messaging `nil` in Objective-C, -> but in a way that works for any type, and that can be checked for success or failure. - -## Optional Chaining as an Alternative to Forced Unwrapping - -You specify optional chaining by placing a question mark (`?`) -after the optional value on which you wish to call a property, method or subscript -if the optional is non-`nil`. -This is very similar to placing an exclamation point (`!`) -after an optional value to force the unwrapping of its value. -The main difference is that optional chaining fails gracefully when the optional is `nil`, -whereas forced unwrapping triggers a runtime error when the optional is `nil`. - -To reflect the fact that optional chaining can be called on a `nil` value, -the result of an optional chaining call is always an optional value, -even if the property, method, or subscript you are querying returns a non-optional value. -You can use this optional return value to check whether -the optional chaining call was successful -(the returned optional contains a value), -or didn't succeed due to a `nil` value in the chain -(the returned optional value is `nil`). - -Specifically, the result of an optional chaining call -is of the same type as the expected return value, but wrapped in an optional. -A property that normally returns an `Int` will return an `Int?` -when accessed through optional chaining. - -The next several code snippets demonstrate -how optional chaining differs from forced unwrapping -and enables you to check for success. - -First, two classes called `Person` and `Residence` are defined: - -```swift -class Person { - var residence: Residence? -} - -class Residence { - var numberOfRooms = 1 -} -``` - - - -`Residence` instances have a single `Int` property called `numberOfRooms`, -with a default value of `1`. -`Person` instances have an optional `residence` property of type `Residence?`. - -If you create a new `Person` instance, -its `residence` property is default initialized to `nil`, -by virtue of being optional. -In the code below, `john` has a `residence` property value of `nil`: - -```swift -let john = Person() -``` - - - -If you try to access the `numberOfRooms` property of this person's `residence`, -by placing an exclamation point after `residence` to force the unwrapping of its value, -you trigger a runtime error, -because there's no `residence` value to unwrap: - -```swift -let roomCount = john.residence!.numberOfRooms -// this triggers a runtime error -``` - - - -The code above succeeds when `john.residence` has a non-`nil` value -and will set `roomCount` to an `Int` value containing the appropriate number of rooms. -However, this code always triggers a runtime error when `residence` is `nil`, -as illustrated above. - -Optional chaining provides an alternative way to access the value of `numberOfRooms`. -To use optional chaining, use a question mark in place of the exclamation point: - -```swift -if let roomCount = john.residence?.numberOfRooms { - print("John's residence has \(roomCount) room(s).") -} else { - print("Unable to retrieve the number of rooms.") -} -// Prints "Unable to retrieve the number of rooms." -``` - - - -This tells Swift to “chain” on the optional `residence` property -and to retrieve the value of `numberOfRooms` if `residence` exists. - -Because the attempt to access `numberOfRooms` has the potential to fail, -the optional chaining attempt returns a value of type `Int?`, or “optional `Int`”. -When `residence` is `nil`, as in the example above, -this optional `Int` will also be `nil`, -to reflect the fact that it was not possible to access `numberOfRooms`. -The optional `Int` is accessed through optional binding -to unwrap the integer and assign the non-optional value -to the `roomCount` constant. - -Note that this is true even though `numberOfRooms` is a non-optional `Int`. -The fact that it's queried through an optional chain -means that the call to `numberOfRooms` -will always return an `Int?` instead of an `Int`. - -You can assign a `Residence` instance to `john.residence`, -so that it no longer has a `nil` value: - -```swift -john.residence = Residence() -``` - - - -`john.residence` now contains an actual `Residence` instance, rather than `nil`. -If you try to access `numberOfRooms` with the same optional chaining as before, -it will now return an `Int?` that contains -the default `numberOfRooms` value of `1`: - -```swift -if let roomCount = john.residence?.numberOfRooms { - print("John's residence has \(roomCount) room(s).") -} else { - print("Unable to retrieve the number of rooms.") -} -// Prints "John's residence has 1 room(s)." -``` - - - -## Defining Model Classes for Optional Chaining - -You can use optional chaining with calls to properties, methods, and subscripts -that are more than one level deep. -This enables you to drill down into subproperties -within complex models of interrelated types, -and to check whether it's possible to access -properties, methods, and subscripts on those subproperties. - -The code snippets below define four model classes -for use in several subsequent examples, -including examples of multilevel optional chaining. -These classes expand upon the `Person` and `Residence` model from above -by adding a `Room` and `Address` class, -with associated properties, methods, and subscripts. - -The `Person` class is defined in the same way as before: - -```swift -class Person { - var residence: Residence? -} -``` - - - -The `Residence` class is more complex than before. -This time, the `Residence` class defines a variable property called `rooms`, -which is initialized with an empty array of type `[Room]`: - -```swift -class Residence { - var rooms: [Room] = [] - var numberOfRooms: Int { - return rooms.count - } - subscript(i: Int) -> Room { - get { - return rooms[i] - } - set { - rooms[i] = newValue - } - } - func printNumberOfRooms() { - print("The number of rooms is \(numberOfRooms)") - } - var address: Address? -} -``` - - - -Because this version of `Residence` stores an array of `Room` instances, -its `numberOfRooms` property is implemented as a computed property, -not a stored property. -The computed `numberOfRooms` property simply returns -the value of the `count` property from the `rooms` array. - -As a shortcut to accessing its `rooms` array, -this version of `Residence` provides a read-write subscript that provides access to -the room at the requested index in the `rooms` array. - -This version of `Residence` also provides a method called `printNumberOfRooms`, -which simply prints the number of rooms in the residence. - -Finally, `Residence` defines an optional property called `address`, -with a type of `Address?`. -The `Address` class type for this property is defined below. - -The `Room` class used for the `rooms` array is -a simple class with one property called `name`, -and an initializer to set that property to a suitable room name: - -```swift -class Room { - let name: String - init(name: String) { self.name = name } -} -``` - - - -The final class in this model is called `Address`. -This class has three optional properties of type `String?`. -The first two properties, `buildingName` and `buildingNumber`, -are alternative ways to identify a particular building as part of an address. -The third property, `street`, is used to name the street for that address: - -```swift -class Address { - var buildingName: String? - var buildingNumber: String? - var street: String? - func buildingIdentifier() -> String? { - if let buildingNumber = buildingNumber, let street = street { - return "\(buildingNumber) \(street)" - } else if buildingName != nil { - return buildingName - } else { - return nil - } - } -} -``` - - - -The `Address` class also provides a method called `buildingIdentifier()`, -which has a return type of `String?`. -This method checks the properties of the address -and returns `buildingName` if it has a value, -or `buildingNumber` concatenated with `street` if both have values, -or `nil` otherwise. - -## Accessing Properties Through Optional Chaining - -As demonstrated in , -you can use optional chaining to access a property on an optional value, -and to check if that property access is successful. - -Use the classes defined above to create a new `Person` instance, -and try to access its `numberOfRooms` property as before: - -```swift -let john = Person() -if let roomCount = john.residence?.numberOfRooms { - print("John's residence has \(roomCount) room(s).") -} else { - print("Unable to retrieve the number of rooms.") -} -// Prints "Unable to retrieve the number of rooms." -``` - - - -Because `john.residence` is `nil`, -this optional chaining call fails in the same way as before. - -You can also attempt to set a property's value through optional chaining: - -```swift -let someAddress = Address() -someAddress.buildingNumber = "29" -someAddress.street = "Acacia Road" -john.residence?.address = someAddress -``` - - - -In this example, -the attempt to set the `address` property of `john.residence` will fail, -because `john.residence` is currently `nil`. - -The assignment is part of the optional chaining, -which means none of the code on the right-hand side of the `=` operator -is evaluated. -In the previous example, -it's not easy to see that `someAddress` is never evaluated, -because accessing a constant doesn't have any side effects. -The listing below does the same assignment, -but it uses a function to create the address. -The function prints "Function was called" before returning a value, -which lets you see -whether the right-hand side of the `=` operator was evaluated. - -```swift -func createAddress() -> Address { - print("Function was called.") - - let someAddress = Address() - someAddress.buildingNumber = "29" - someAddress.street = "Acacia Road" - - return someAddress -} -john.residence?.address = createAddress() -``` - - - -You can tell that the `createAddress()` function isn't called, -because nothing is printed. - -## Calling Methods Through Optional Chaining - -You can use optional chaining to call a method on an optional value, -and to check whether that method call is successful. -You can do this even if that method doesn't define a return value. - -The `printNumberOfRooms()` method on the `Residence` class -prints the current value of `numberOfRooms`. -Here's how the method looks: - -```swift -func printNumberOfRooms() { - print("The number of rooms is \(numberOfRooms)") -} -``` - - - -This method doesn't specify a return type. -However, functions and methods with no return type have an implicit return type of `Void`, -as described in . -This means that they return a value of `()`, or an empty tuple. - -If you call this method on an optional value with optional chaining, -the method's return type will be `Void?`, not `Void`, -because return values are always of an optional type when called through optional chaining. -This enables you to use an `if` statement -to check whether it was possible to call the `printNumberOfRooms()` method, -even though the method doesn't itself define a return value. -Compare the return value from the `printNumberOfRooms` call against `nil` -to see if the method call was successful: - -```swift -if john.residence?.printNumberOfRooms() != nil { - print("It was possible to print the number of rooms.") -} else { - print("It was not possible to print the number of rooms.") -} -// Prints "It was not possible to print the number of rooms." -``` - - - -The same is true if you attempt to set a property through optional chaining. -The example above in -attempts to set an `address` value for `john.residence`, -even though the `residence` property is `nil`. -Any attempt to set a property through optional chaining returns a value of type `Void?`, -which enables you to compare against `nil` to see if the property was set successfully: - -```swift -if (john.residence?.address = someAddress) != nil { - print("It was possible to set the address.") -} else { - print("It was not possible to set the address.") -} -// Prints "It was not possible to set the address." -``` - - - -## Accessing Subscripts Through Optional Chaining - -You can use optional chaining to try to retrieve and set -a value from a subscript on an optional value, -and to check whether that subscript call is successful. - -> Note: When you access a subscript on an optional value through optional chaining, -> you place the question mark *before* the subscript's brackets, not after. -> The optional chaining question mark always follows immediately after -> the part of the expression that's optional. - -The example below tries to retrieve the name of -the first room in the `rooms` array of the `john.residence` property -using the subscript defined on the `Residence` class. -Because `john.residence` is currently `nil`, -the subscript call fails: - -```swift -if let firstRoomName = john.residence?[0].name { - print("The first room name is \(firstRoomName).") -} else { - print("Unable to retrieve the first room name.") -} -// Prints "Unable to retrieve the first room name." -``` - - - -The optional chaining question mark in this subscript call -is placed immediately after `john.residence`, before the subscript brackets, -because `john.residence` is the optional value -on which optional chaining is being attempted. - -Similarly, you can try to set a new value through a subscript with optional chaining: - -```swift -john.residence?[0] = Room(name: "Bathroom") -``` - - - -This subscript setting attempt also fails, because `residence` is currently `nil`. - -If you create and assign an actual `Residence` instance to `john.residence`, -with one or more `Room` instances in its `rooms` array, -you can use the `Residence` subscript to access -the actual items in the `rooms` array through optional chaining: - -```swift -let johnsHouse = Residence() -johnsHouse.rooms.append(Room(name: "Living Room")) -johnsHouse.rooms.append(Room(name: "Kitchen")) -john.residence = johnsHouse - -if let firstRoomName = john.residence?[0].name { - print("The first room name is \(firstRoomName).") -} else { - print("Unable to retrieve the first room name.") -} -// Prints "The first room name is Living Room." -``` - - - -### Accessing Subscripts of Optional Type - -If a subscript returns a value of optional type --- -such as the key subscript of Swift's `Dictionary` type --- -place a question mark *after* the subscript's closing bracket -to chain on its optional return value: - -```swift -var testScores = ["Dave": [86, 82, 84], "Bev": [79, 94, 81]] -testScores["Dave"]?[0] = 91 -testScores["Bev"]?[0] += 1 -testScores["Brian"]?[0] = 72 -// the "Dave" array is now [91, 82, 84] and the "Bev" array is now [80, 94, 81] -``` - - - -The example above defines a dictionary called `testScores`, -which contains two key-value pairs that map a `String` key to an array of `Int` values. -The example uses optional chaining to set the first item in the `"Dave"` array to `91`; -to increment the first item in the `"Bev"` array by `1`; -and to try to set the first item in an array for a key of `"Brian"`. -The first two calls succeed, because the `testScores` dictionary -contains keys for `"Dave"` and `"Bev"`. -The third call fails, because the `testScores` dictionary -doesn't contain a key for `"Brian"`. - -## Linking Multiple Levels of Chaining - -You can link together multiple levels of optional chaining -to drill down to properties, methods, and subscripts deeper within a model. -However, multiple levels of optional chaining -don't add more levels of optionality to the returned value. - -To put it another way: - -- If the type you are trying to retrieve isn't optional, - it will become optional because of the optional chaining. -- If the type you are trying to retrieve is *already* optional, - it will not become *more* optional because of the chaining. - -Therefore: - -- If you try to retrieve an `Int` value through optional chaining, - an `Int?` is always returned, - no matter how many levels of chaining are used. -- Similarly, if you try to retrieve an `Int?` value through optional chaining, - an `Int?` is always returned, - no matter how many levels of chaining are used. - -The example below tries to access the `street` property of the `address` property -of the `residence` property of `john`. -There are *two* levels of optional chaining in use here, -to chain through the `residence` and `address` properties, -both of which are of optional type: - -```swift -if let johnsStreet = john.residence?.address?.street { - print("John's street name is \(johnsStreet).") -} else { - print("Unable to retrieve the address.") -} -// Prints "Unable to retrieve the address." -``` - - - -The value of `john.residence` currently contains a valid `Residence` instance. -However, the value of `john.residence.address` is currently `nil`. -Because of this, the call to `john.residence?.address?.street` fails. - -Note that in the example above, -you are trying to retrieve the value of the `street` property. -The type of this property is `String?`. -The return value of `john.residence?.address?.street` is therefore also `String?`, -even though two levels of optional chaining are applied in addition to -the underlying optional type of the property. - -If you set an actual `Address` instance as the value for `john.residence.address`, -and set an actual value for the address's `street` property, -you can access the value of the `street` property through multilevel optional chaining: - -```swift -let johnsAddress = Address() -johnsAddress.buildingName = "The Larches" -johnsAddress.street = "Laurel Street" -john.residence?.address = johnsAddress - -if let johnsStreet = john.residence?.address?.street { - print("John's street name is \(johnsStreet).") -} else { - print("Unable to retrieve the address.") -} -// Prints "John's street name is Laurel Street." -``` - - - -In this example, -the attempt to set the `address` property of `john.residence` will succeed, -because the value of `john.residence` -currently contains a valid `Residence` instance. - -## Chaining on Methods with Optional Return Values - -The previous example shows how to retrieve the value of -a property of optional type through optional chaining. -You can also use optional chaining to call a method that returns a value of optional type, -and to chain on that method's return value if needed. - -The example below calls the `Address` class's `buildingIdentifier()` method -through optional chaining. This method returns a value of type `String?`. -As described above, the ultimate return type of this method call after optional chaining -is also `String?`: - -```swift -if let buildingIdentifier = john.residence?.address?.buildingIdentifier() { - print("John's building identifier is \(buildingIdentifier).") -} -// Prints "John's building identifier is The Larches." -``` - - - -If you want to perform further optional chaining on this method's return value, -place the optional chaining question mark *after* the method's parentheses: - -```swift -if let beginsWithThe = - john.residence?.address?.buildingIdentifier()?.hasPrefix("The") { - if beginsWithThe { - print("John's building identifier begins with \"The\".") - } else { - print("John's building identifier doesn't begin with \"The\".") - } -} -// Prints "John's building identifier begins with "The"." -``` - - - -> Note: In the example above, -> you place the optional chaining question mark *after* the parentheses, -> because the optional value you are chaining on is -> the `buildingIdentifier()` method's return value, -> and not the `buildingIdentifier()` method itself. - - - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/LanguageGuide/Protocols.md b/swift-6-beta.docc/LanguageGuide/Protocols.md deleted file mode 100644 index 5e875ddbb..000000000 --- a/swift-6-beta.docc/LanguageGuide/Protocols.md +++ /dev/null @@ -1,2502 +0,0 @@ -# Protocols - -Define requirements that conforming types must implement. - -A *protocol* defines a blueprint of -methods, properties, and other requirements -that suit a particular task or piece of functionality. -The protocol can then be *adopted* by a class, structure, or enumeration -to provide an actual implementation of those requirements. -Any type that satisfies the requirements of a protocol is said to -*conform* to that protocol. - -In addition to specifying requirements that conforming types must implement, -you can extend a protocol to implement some of these requirements -or to implement additional functionality that conforming types can take advantage of. - - - -## Protocol Syntax - -You define protocols in a very similar way to classes, structures, and enumerations: - -```swift -protocol SomeProtocol { - // protocol definition goes here -} -``` - - - -Custom types state that they adopt a particular protocol -by placing the protocol's name after the type's name, -separated by a colon, as part of their definition. -Multiple protocols can be listed, and are separated by commas: - -```swift -struct SomeStructure: FirstProtocol, AnotherProtocol { - // structure definition goes here -} -``` - - - -If a class has a superclass, list the superclass name -before any protocols it adopts, followed by a comma: - -```swift -class SomeClass: SomeSuperclass, FirstProtocol, AnotherProtocol { - // class definition goes here -} -``` - - - -> Note: Because protocols are types, -> begin their names with a capital letter -> (such as `FullyNamed` and `RandomNumberGenerator`) -> to match the names of other types in Swift -> (such as `Int`, `String`, and `Double`). - -## Property Requirements - -A protocol can require any conforming type to provide -an instance property or type property with a particular name and type. -The protocol doesn't specify whether the property should be -a stored property or a computed property --- -it only specifies the required property name and type. -The protocol also specifies whether each property must be gettable -or gettable *and* settable. - -If a protocol requires a property to be gettable and settable, -that property requirement can't be fulfilled by -a constant stored property or a read-only computed property. -If the protocol only requires a property to be gettable, -the requirement can be satisfied by any kind of property, -and it's valid for the property to be also settable -if this is useful for your own code. - -Property requirements are always declared as variable properties, -prefixed with the `var` keyword. -Gettable and settable properties are indicated by writing -`{ get set }` after their type declaration, -and gettable properties are indicated by writing `{ get }`. - -```swift -protocol SomeProtocol { - var mustBeSettable: Int { get set } - var doesNotNeedToBeSettable: Int { get } -} -``` - - - -Always prefix type property requirements with the `static` keyword -when you define them in a protocol. -This rule pertains even though type property requirements can be prefixed with -the `class` or `static` keyword when implemented by a class: - -```swift -protocol AnotherProtocol { - static var someTypeProperty: Int { get set } -} -``` - - - -Here's an example of a protocol with a single instance property requirement: - -```swift -protocol FullyNamed { - var fullName: String { get } -} -``` - - - -The `FullyNamed` protocol requires a conforming type to provide a fully qualified name. -The protocol doesn't specify anything else about the nature of the conforming type --- -it only specifies that the type must be able to provide a full name for itself. -The protocol states that any `FullyNamed` type must have -a gettable instance property called `fullName`, which is of type `String`. - -Here's an example of a simple structure that adopts and conforms to -the `FullyNamed` protocol: - -```swift -struct Person: FullyNamed { - var fullName: String -} -let john = Person(fullName: "John Appleseed") -// john.fullName is "John Appleseed" -``` - - - -This example defines a structure called `Person`, -which represents a specific named person. -It states that it adopts the `FullyNamed` protocol -as part of the first line of its definition. - -Each instance of `Person` has a single stored property called `fullName`, -which is of type `String`. -This matches the single requirement of the `FullyNamed` protocol, -and means that `Person` has correctly conformed to the protocol. -(Swift reports an error at compile time if a protocol requirement isn't fulfilled.) - -Here's a more complex class, which also adopts and conforms to the `FullyNamed` protocol: - -```swift -class Starship: FullyNamed { - var prefix: String? - var name: String - init(name: String, prefix: String? = nil) { - self.name = name - self.prefix = prefix - } - var fullName: String { - return (prefix != nil ? prefix! + " " : "") + name - } -} -var ncc1701 = Starship(name: "Enterprise", prefix: "USS") -// ncc1701.fullName is "USS Enterprise" -``` - - - -This class implements the `fullName` property requirement as -a computed read-only property for a starship. -Each `Starship` class instance stores a mandatory `name` and an optional `prefix`. -The `fullName` property uses the `prefix` value if it exists, -and prepends it to the beginning of `name` to create a full name for the starship. - - - -## Method Requirements - -Protocols can require specific instance methods and type methods -to be implemented by conforming types. -These methods are written as part of the protocol's definition -in exactly the same way as for normal instance and type methods, -but without curly braces or a method body. -Variadic parameters are allowed, subject to the same rules as for normal methods. -Default values, however, can't be specified for method parameters within a protocol's definition. - -As with type property requirements, -you always prefix type method requirements with the `static` keyword -when they're defined in a protocol. -This is true even though type method requirements are prefixed with -the `class` or `static` keyword when implemented by a class: - -```swift -protocol SomeProtocol { - static func someTypeMethod() -} -``` - - - -The following example defines a protocol with a single instance method requirement: - -```swift -protocol RandomNumberGenerator { - func random() -> Double -} -``` - - - -This protocol, `RandomNumberGenerator`, requires any conforming type -to have an instance method called `random`, -which returns a `Double` value whenever it's called. -Although it's not specified as part of the protocol, -it's assumed that this value will be -a number from `0.0` up to (but not including) `1.0`. - -The `RandomNumberGenerator` protocol doesn't make any assumptions -about how each random number will be generated --- -it simply requires the generator to provide a standard way -to generate a new random number. - -Here's an implementation of a class that adopts and conforms to -the `RandomNumberGenerator` protocol. -This class implements a pseudorandom number generator algorithm known as -a *linear congruential generator*: - -```swift -class LinearCongruentialGenerator: RandomNumberGenerator { - var lastRandom = 42.0 - let m = 139968.0 - let a = 3877.0 - let c = 29573.0 - func random() -> Double { - lastRandom = ((lastRandom * a + c) - .truncatingRemainder(dividingBy:m)) - return lastRandom / m - } -} -let generator = LinearCongruentialGenerator() -print("Here's a random number: \(generator.random())") -// Prints "Here's a random number: 0.3746499199817101" -print("And another one: \(generator.random())") -// Prints "And another one: 0.729023776863283" -``` - - - -## Mutating Method Requirements - -It's sometimes necessary for a method to modify (or *mutate*) the instance it belongs to. -For instance methods on value types (that is, structures and enumerations) -you place the `mutating` keyword before a method's `func` keyword -to indicate that the method is allowed to modify the instance it belongs to -and any properties of that instance. -This process is described in . - -If you define a protocol instance method requirement -that's intended to mutate instances of any type that adopts the protocol, -mark the method with the `mutating` keyword -as part of the protocol's definition. -This enables structures and enumerations to adopt the protocol -and satisfy that method requirement. - -> Note: If you mark a protocol instance method requirement as `mutating`, -> you don't need to write the `mutating` keyword when writing -> an implementation of that method for a class. -> The `mutating` keyword is only used by structures and enumerations. - -The example below defines a protocol called `Togglable`, -which defines a single instance method requirement called `toggle`. -As its name suggests, the `toggle()` method is intended to -toggle or invert the state of any conforming type, -typically by modifying a property of that type. - -The `toggle()` method is marked with the `mutating` keyword -as part of the `Togglable` protocol definition, -to indicate that the method is expected to mutate the state of a conforming instance -when it's called: - -```swift -protocol Togglable { - mutating func toggle() -} -``` - - - -If you implement the `Togglable` protocol for a structure or enumeration, -that structure or enumeration can conform to the protocol -by providing an implementation of the `toggle()` method -that's also marked as `mutating`. - -The example below defines an enumeration called `OnOffSwitch`. -This enumeration toggles between two states, -indicated by the enumeration cases `on` and `off`. -The enumeration's `toggle` implementation is marked as `mutating`, -to match the `Togglable` protocol's requirements: - -```swift -enum OnOffSwitch: Togglable { - case off, on - mutating func toggle() { - switch self { - case .off: - self = .on - case .on: - self = .off - } - } -} -var lightSwitch = OnOffSwitch.off -lightSwitch.toggle() -// lightSwitch is now equal to .on -``` - - - -## Initializer Requirements - -Protocols can require specific initializers -to be implemented by conforming types. -You write these initializers as part of the protocol's definition -in exactly the same way as for normal initializers, -but without curly braces or an initializer body: - -```swift -protocol SomeProtocol { - init(someParameter: Int) -} -``` - - - -### Class Implementations of Protocol Initializer Requirements - -You can implement a protocol initializer requirement on a conforming class -as either a designated initializer or a convenience initializer. -In both cases, -you must mark the initializer implementation with the `required` modifier: - -```swift -class SomeClass: SomeProtocol { - required init(someParameter: Int) { - // initializer implementation goes here - } -} -``` - - - - - -The use of the `required` modifier ensures that -you provide an explicit or inherited implementation of the initializer requirement -on all subclasses of the conforming class, -such that they also conform to the protocol. - -For more information on required initializers, -see . - - - - - -> Note: You don't need to mark protocol initializer implementations with the `required` modifier -> on classes that are marked with the `final` modifier, -> because final classes can't subclassed. -> For more about the `final` modifier, see . - - - -If a subclass overrides a designated initializer from a superclass, -and also implements a matching initializer requirement from a protocol, -mark the initializer implementation with both the `required` and `override` modifiers: - -```swift -protocol SomeProtocol { - init() -} - -class SomeSuperClass { - init() { - // initializer implementation goes here - } -} - -class SomeSubClass: SomeSuperClass, SomeProtocol { - // "required" from SomeProtocol conformance; "override" from SomeSuperClass - required override init() { - // initializer implementation goes here - } -} -``` - - - -### Failable Initializer Requirements - -Protocols can define failable initializer requirements for conforming types, -as defined in . - -A failable initializer requirement can be satisfied by -a failable or nonfailable initializer on a conforming type. -A nonfailable initializer requirement can be satisfied by -a nonfailable initializer or an implicitly unwrapped failable initializer. - - - - - - - - - - - - - - - - - -## Protocols as Types - -Protocols don't actually implement any functionality themselves. -Regardless, you can use a protocol as a type in your code. - -The most common way to use a protocol as a type -is to use a protocol as a generic constraint. -Code with generic constraints can work with -any type that conforms to the protocol, -and the specific type is chosen by the code that uses the API. -For example, -when you call a function that takes an argument -and that argument's type is generic, -the caller chooses the type. - -Code with an opaque type -works with some type that conforms to the protocol. -The underlying type is known at compile time, -and the API implementation chooses that type, -but that type's identity is hidden from clients of the API. -Using an opaque type lets you prevent implementation details of an API -from leaking through the layer of abstraction --- -for example, by hiding the specific return type from a function, -and only guaranteeing that the value conforms to a given protocol. - -Code with a boxed protocol type -works with any type, chosen at runtime, that conforms to the protocol. -To support this runtime flexibility, -Swift adds a level of indirection when necessary --- -known as a *box*, -which has a performance cost. -Because of this flexibility, -Swift doesn't know the underlying type at compile time, -which means you can access only the members -that are required by the protocol. -Accessing any other APIs on the underlying type -requires casting at runtime. - -For information about using protocols as generic constraints, -see . -For information about opaque types, and boxed protocol types, -see . - - - -## Delegation - -*Delegation* is a design pattern that enables -a class or structure to hand off (or *delegate*) -some of its responsibilities to an instance of another type. -This design pattern is implemented by defining -a protocol that encapsulates the delegated responsibilities, -such that a conforming type (known as a delegate) -is guaranteed to provide the functionality that has been delegated. -Delegation can be used to respond to a particular action, -or to retrieve data from an external source without needing to know -the underlying type of that source. - -The example below defines a dice game -and a nested protocol for a delegate -that tracks the game's progress: - -```swift -class DiceGame { - let sides: Int - let generator = LinearCongruentialGenerator() - weak var delegate: Delegate? - - init(sides: Int) { - self.sides = sides - } - - func roll() -> Int { - return Int(generator.random() * Double(sides)) + 1 - } - - func play(rounds: Int) { - delegate?.gameDidStart(self) - for round in 1...rounds { - let player1 = roll() - let player2 = roll() - if player1 == player2 { - delegate?.game(self, didEndRound: round, winner: nil) - } else if player1 > player2 { - delegate?.game(self, didEndRound: round, winner: 1) - } else { - delegate?.game(self, didEndRound: round, winner: 2) - } - } - delegate?.gameDidEnd(self) - } - - protocol Delegate: AnyObject { - func gameDidStart(_ game: DiceGame) - func game(_ game: DiceGame, didEndRound round: Int, winner: Int?) - func gameDidEnd(_ game: DiceGame) - } -} -``` - -The `DiceGame` class implements a game where -each player takes a turn rolling dice, -and the player who rolls the highest number wins the round. -It uses a linear congruential generator -from the example earlier in the chapter, -to generate random numbers for dice rolls. - -The `DiceGame.Delegate` protocol can be adopted -to track the progress of a dice game. -Because the `DiceGame.Delegate` protocol -is always used in the context of a dice game, -it's nested inside of the `DiceGame` class. -Protocols can be nested -inside of type declarations like structures and classes, -as long as the outer declaration isn't generic. -For information about nesting types, see . - -To prevent strong reference cycles, -delegates are declared as weak references. -For information about weak references, -see . -Marking the protocol as class-only -lets the `DiceGame` class -declare that its delegate must use a weak reference. -A class-only protocol -is marked by its inheritance from `AnyObject`, -as discussed in . - -`DiceGame.Delegate` provides three methods for tracking the progress of a game. -These three methods are incorporated into the game logic -in the `play(rounds:)` method above. -The `DiceGame` class calls its delegate methods when -a new game starts, a new turn begins, or the game ends. - -Because the `delegate` property is an *optional* `DiceGame.Delegate`, -the `play(rounds:)` method uses optional chaining each time it calls a method on the delegate, -as discussed in . -If the `delegate` property is nil, -these delegate calls are ignored. -If the `delegate` property is non-nil, -the delegate methods are called, -and are passed the `DiceGame` instance as a parameter. - -This next example shows a class called `DiceGameTracker`, -which adopts the `DiceGame.Delegate` protocol: - -```swift -class DiceGameTracker: DiceGame.Delegate { - var playerScore1 = 0 - var playerScore2 = 0 - func gameDidStart(_ game: DiceGame) { - print("Started a new game") - playerScore1 = 0 - playerScore2 = 0 - } - func game(_ game: DiceGame, didEndRound round: Int, winner: Int?) { - switch winner { - case 1: - playerScore1 += 1 - print("Player 1 won round \(round)") - case 2: playerScore2 += 1 - print("Player 2 won round \(round)") - default: - print("The round was a draw") - } - } - func gameDidEnd(_ game: DiceGame) { - if playerScore1 == playerScore2 { - print("The game ended in a draw.") - } else if playerScore1 > playerScore2 { - print("Player 1 won!") - } else { - print("Player 2 won!") - } - } -} -``` - -The `DiceGameTracker` class implements all three methods -that are required by the `DiceGame.Delegate` protocol. -It uses these methods to zero out both players' scores -at the start of a new game, -to update their scores at the end of each round, -and to announce a winner at the end of the game. - -Here's how `DiceGame` and `DiceGameTracker` look in action: - -```swift -let tracker = DiceGameTracker() -let game = DiceGame(sides: 6) -game.delegate = tracker -game.play(rounds: 3) -// Started a new game -// Player 2 won round 1 -// Player 2 won round 2 -// Player 1 won round 3 -// Player 2 won! -``` - -## Adding Protocol Conformance with an Extension - -You can extend an existing type to adopt and conform to a new protocol, -even if you don't have access to the source code for the existing type. -Extensions can add new properties, methods, and subscripts to an existing type, -and are therefore able to add any requirements that a protocol may demand. -For more about extensions, see . - -> Note: Existing instances of a type automatically adopt and conform to a protocol -> when that conformance is added to the instance's type in an extension. - -For example, this protocol, called `TextRepresentable`, can be implemented by -any type that has a way to be represented as text. -This might be a description of itself, or a text version of its current state: - -```swift -protocol TextRepresentable { - var textualDescription: String { get } -} -``` - - - -The `Dice` class from above can be extended to adopt and conform to `TextRepresentable`: - - - -```swift -extension Dice: TextRepresentable { - var textualDescription: String { - return "A \(sides)-sided dice" - } -} -``` - - - -This extension adopts the new protocol in exactly the same way -as if `Dice` had provided it in its original implementation. -The protocol name is provided after the type name, separated by a colon, -and an implementation of all requirements of the protocol -is provided within the extension's curly braces. - -Any `Dice` instance can now be treated as `TextRepresentable`: - -```swift -let d12 = Dice(sides: 12, generator: LinearCongruentialGenerator()) -print(d12.textualDescription) -// Prints "A 12-sided dice" -``` - - - -Similarly, the `SnakesAndLadders` game class can be extended to -adopt and conform to the `TextRepresentable` protocol: - -```swift -extension SnakesAndLadders: TextRepresentable { - var textualDescription: String { - return "A game of Snakes and Ladders with \(finalSquare) squares" - } -} -print(game.textualDescription) -// Prints "A game of Snakes and Ladders with 25 squares" -``` - - - -### Conditionally Conforming to a Protocol - -A generic type may be able to satisfy the requirements of a protocol -only under certain conditions, -such as when the type's generic parameter conforms to the protocol. -You can make a generic type conditionally conform to a protocol -by listing constraints when extending the type. -Write these constraints after the name of the protocol you're adopting -by writing a generic `where` clause. -For more about generic `where` clauses, see . - -The following extension -makes `Array` instances conform to the `TextRepresentable` protocol -whenever they store elements of a type that conforms to `TextRepresentable`. - -```swift -extension Array: TextRepresentable where Element: TextRepresentable { - var textualDescription: String { - let itemsAsText = self.map { $0.textualDescription } - return "[" + itemsAsText.joined(separator: ", ") + "]" - } -} -let myDice = [d6, d12] -print(myDice.textualDescription) -// Prints "[A 6-sided dice, A 12-sided dice]" -``` - - - -### Declaring Protocol Adoption with an Extension - -If a type already conforms to all of the requirements of a protocol, -but hasn't yet stated that it adopts that protocol, -you can make it adopt the protocol with an empty extension: - -```swift -struct Hamster { - var name: String - var textualDescription: String { - return "A hamster named \(name)" - } -} -extension Hamster: TextRepresentable {} -``` - - - -Instances of `Hamster` can now be used wherever `TextRepresentable` is the required type: - -```swift -let simonTheHamster = Hamster(name: "Simon") -let somethingTextRepresentable: TextRepresentable = simonTheHamster -print(somethingTextRepresentable.textualDescription) -// Prints "A hamster named Simon" -``` - - - -> Note: Types don't automatically adopt a protocol just by satisfying its requirements. -> They must always explicitly declare their adoption of the protocol. - -## Adopting a Protocol Using a Synthesized Implementation - -Swift can automatically provide the protocol conformance -for `Equatable`, `Hashable`, and `Comparable` -in many simple cases. -Using this synthesized implementation -means you don't have to write repetitive boilerplate code -to implement the protocol requirements yourself. - - - -Swift provides a synthesized implementation of `Equatable` -for the following kinds of custom types: - -- Structures that have only stored properties that conform to the `Equatable` protocol -- Enumerations that have only associated types that conform to the `Equatable` protocol -- Enumerations that have no associated types - -To receive a synthesized implementation of `==`, -declare conformance to `Equatable` -in the file that contains the original declaration, -without implementing an `==` operator yourself. -The `Equatable` protocol provides a default implementation of `!=`. - -The example below defines a `Vector3D` structure -for a three-dimensional position vector `(x, y, z)`, -similar to the `Vector2D` structure. -Because the `x`, `y`, and `z` properties are all of an `Equatable` type, -`Vector3D` receives synthesized implementations -of the equivalence operators. - -```swift -struct Vector3D: Equatable { - var x = 0.0, y = 0.0, z = 0.0 -} - -let twoThreeFour = Vector3D(x: 2.0, y: 3.0, z: 4.0) -let anotherTwoThreeFour = Vector3D(x: 2.0, y: 3.0, z: 4.0) -if twoThreeFour == anotherTwoThreeFour { - print("These two vectors are also equivalent.") -} -// Prints "These two vectors are also equivalent." -``` - - - - - -Swift provides a synthesized implementation of `Hashable` -for the following kinds of custom types: - -- Structures that have only stored properties that conform to the `Hashable` protocol -- Enumerations that have only associated types that conform to the `Hashable` protocol -- Enumerations that have no associated types - -To receive a synthesized implementation of `hash(into:)`, -declare conformance to `Hashable` -in the file that contains the original declaration, -without implementing a `hash(into:)` method yourself. - -Swift provides a synthesized implementation of `Comparable` -for enumerations that don't have a raw value. -If the enumeration has associated types, -they must all conform to the `Comparable` protocol. -To receive a synthesized implementation of `<`, -declare conformance to `Comparable` -in the file that contains the original enumeration declaration, -without implementing a `<` operator yourself. -The `Comparable` protocol's default implementation -of `<=`, `>`, and `>=` provides the remaining comparison operators. - -The example below defines a `SkillLevel` enumeration -with cases for beginners, intermediates, and experts. -Experts are additionally ranked by the number of stars they have. - -```swift -enum SkillLevel: Comparable { - case beginner - case intermediate - case expert(stars: Int) -} -var levels = [SkillLevel.intermediate, SkillLevel.beginner, - SkillLevel.expert(stars: 5), SkillLevel.expert(stars: 3)] -for level in levels.sorted() { - print(level) -} -// Prints "beginner" -// Prints "intermediate" -// Prints "expert(stars: 3)" -// Prints "expert(stars: 5)" -``` - - - - - - - -## Collections of Protocol Types - -A protocol can be used as the type to be stored in -a collection such as an array or a dictionary, -as mentioned in . -This example creates an array of `TextRepresentable` things: - -```swift -let things: [TextRepresentable] = [game, d12, simonTheHamster] -``` - - - -It's now possible to iterate over the items in the array, -and print each item's textual description: - -```swift -for thing in things { - print(thing.textualDescription) -} -// A game of Snakes and Ladders with 25 squares -// A 12-sided dice -// A hamster named Simon -``` - - - -Note that the `thing` constant is of type `TextRepresentable`. -It's not of type `Dice`, or `DiceGame`, or `Hamster`, -even if the actual instance behind the scenes is of one of those types. -Nonetheless, because it's of type `TextRepresentable`, -and anything that's `TextRepresentable` is known to have a `textualDescription` property, -it's safe to access `thing.textualDescription` each time through the loop. - -## Protocol Inheritance - -A protocol can *inherit* one or more other protocols -and can add further requirements on top of the requirements it inherits. -The syntax for protocol inheritance is similar to the syntax for class inheritance, -but with the option to list multiple inherited protocols, separated by commas: - -```swift -protocol InheritingProtocol: SomeProtocol, AnotherProtocol { - // protocol definition goes here -} -``` - - - -Here's an example of a protocol that inherits -the `TextRepresentable` protocol from above: - -```swift -protocol PrettyTextRepresentable: TextRepresentable { - var prettyTextualDescription: String { get } -} -``` - - - -This example defines a new protocol, `PrettyTextRepresentable`, -which inherits from `TextRepresentable`. -Anything that adopts `PrettyTextRepresentable` must satisfy all of the requirements -enforced by `TextRepresentable`, -*plus* the additional requirements enforced by `PrettyTextRepresentable`. -In this example, `PrettyTextRepresentable` adds a single requirement -to provide a gettable property called `prettyTextualDescription` that returns a `String`. - -The `SnakesAndLadders` class can be extended to adopt and conform to `PrettyTextRepresentable`: - -```swift -extension SnakesAndLadders: PrettyTextRepresentable { - var prettyTextualDescription: String { - var output = textualDescription + ":\n" - for index in 1...finalSquare { - switch board[index] { - case let ladder where ladder > 0: - output += "▲ " - case let snake where snake < 0: - output += "▼ " - default: - output += "○ " - } - } - return output - } -} -``` - - - -This extension states that it adopts the `PrettyTextRepresentable` protocol -and provides an implementation of the `prettyTextualDescription` property -for the `SnakesAndLadders` type. -Anything that's `PrettyTextRepresentable` must also be `TextRepresentable`, -and so the implementation of `prettyTextualDescription` starts -by accessing the `textualDescription` property -from the `TextRepresentable` protocol to begin an output string. -It appends a colon and a line break, -and uses this as the start of its pretty text representation. -It then iterates through the array of board squares, -and appends a geometric shape to represent the contents of each square: - -- If the square's value is greater than `0`, it's the base of a ladder, - and is represented by `▲`. -- If the square's value is less than `0`, it's the head of a snake, - and is represented by `▼`. -- Otherwise, the square's value is `0`, and it's a “free” square, - represented by `○`. - -The `prettyTextualDescription` property can now be used to print a pretty text description -of any `SnakesAndLadders` instance: - -```swift -print(game.prettyTextualDescription) -// A game of Snakes and Ladders with 25 squares: -// ○ ○ ▲ ○ ○ ▲ ○ ○ ▲ ▲ ○ ○ ○ ▼ ○ ○ ○ ○ ▼ ○ ○ ▼ ○ ▼ ○ -``` - - - -## Class-Only Protocols - -You can limit protocol adoption to class types (and not structures or enumerations) -by adding the `AnyObject` protocol to a protocol's inheritance list. - -```swift -protocol SomeClassOnlyProtocol: AnyObject, SomeInheritedProtocol { - // class-only protocol definition goes here -} -``` - - - -In the example above, `SomeClassOnlyProtocol` can only be adopted by class types. -It's a compile-time error to write a structure or enumeration definition -that tries to adopt `SomeClassOnlyProtocol`. - -> Note: Use a class-only protocol when the behavior defined by that protocol's requirements -> assumes or requires that a conforming type has -> reference semantics rather than value semantics. -> For more about reference and value semantics, -> see -> and . - - - - - -## Protocol Composition - -It can be useful to require a type to conform to multiple protocols at the same time. -You can combine multiple protocols into a single requirement -with a *protocol composition*. -Protocol compositions behave as if you -defined a temporary local protocol that has the combined requirements -of all protocols in the composition. -Protocol compositions don't define any new protocol types. - -Protocol compositions have the form `SomeProtocol & AnotherProtocol`. -You can list as many protocols as you need, -separating them with ampersands (`&`). -In addition to its list of protocols, -a protocol composition can also contain one class type, -which you can use to specify a required superclass. - -Here's an example that combines two protocols called `Named` and `Aged` -into a single protocol composition requirement on a function parameter: - -```swift -protocol Named { - var name: String { get } -} -protocol Aged { - var age: Int { get } -} -struct Person: Named, Aged { - var name: String - var age: Int -} -func wishHappyBirthday(to celebrator: Named & Aged) { - print("Happy birthday, \(celebrator.name), you're \(celebrator.age)!") -} -let birthdayPerson = Person(name: "Malcolm", age: 21) -wishHappyBirthday(to: birthdayPerson) -// Prints "Happy birthday, Malcolm, you're 21!" -``` - - - -In this example, -the `Named` protocol -has a single requirement for a gettable `String` property called `name`. -The `Aged` protocol -has a single requirement for a gettable `Int` property called `age`. -Both protocols are adopted by a structure called `Person`. - -The example also defines a `wishHappyBirthday(to:)` function. -The type of the `celebrator` parameter is `Named & Aged`, -which means “any type that conforms to both the `Named` and `Aged` protocols.” -It doesn't matter which specific type is passed to the function, -as long as it conforms to both of the required protocols. - -The example then creates a new `Person` instance called `birthdayPerson` -and passes this new instance to the `wishHappyBirthday(to:)` function. -Because `Person` conforms to both protocols, this call is valid, -and the `wishHappyBirthday(to:)` function can print its birthday greeting. - -Here's an example that combines -the `Named` protocol from the previous example -with a `Location` class: - -```swift -class Location { - var latitude: Double - var longitude: Double - init(latitude: Double, longitude: Double) { - self.latitude = latitude - self.longitude = longitude - } -} -class City: Location, Named { - var name: String - init(name: String, latitude: Double, longitude: Double) { - self.name = name - super.init(latitude: latitude, longitude: longitude) - } -} -func beginConcert(in location: Location & Named) { - print("Hello, \(location.name)!") -} - -let seattle = City(name: "Seattle", latitude: 47.6, longitude: -122.3) -beginConcert(in: seattle) -// Prints "Hello, Seattle!" -``` - - - -The `beginConcert(in:)` function takes -a parameter of type `Location & Named`, -which means "any type that's a subclass of `Location` -and that conforms to the `Named` protocol." -In this case, `City` satisfies both requirements. - -Passing `birthdayPerson` to the `beginConcert(in:)` function -is invalid because `Person` isn't a subclass of `Location`. -Likewise, -if you made a subclass of `Location` -that didn't conform to the `Named` protocol, -calling `beginConcert(in:)` with an instance of that type -is also invalid. - -## Checking for Protocol Conformance - -You can use the `is` and `as` operators described in -to check for protocol conformance, and to cast to a specific protocol. -Checking for and casting to a protocol -follows exactly the same syntax as checking for and casting to a type: - -- The `is` operator returns `true` if an instance conforms to a protocol - and returns `false` if it doesn't. -- The `as?` version of the downcast operator returns - an optional value of the protocol's type, - and this value is `nil` if the instance doesn't conform to that protocol. -- The `as!` version of the downcast operator forces the downcast to the protocol type - and triggers a runtime error if the downcast doesn't succeed. - -This example defines a protocol called `HasArea`, -with a single property requirement of a gettable `Double` property called `area`: - -```swift -protocol HasArea { - var area: Double { get } -} -``` - - - -Here are two classes, `Circle` and `Country`, -both of which conform to the `HasArea` protocol: - -```swift -class Circle: HasArea { - let pi = 3.1415927 - var radius: Double - var area: Double { return pi * radius * radius } - init(radius: Double) { self.radius = radius } -} -class Country: HasArea { - var area: Double - init(area: Double) { self.area = area } -} -``` - - - -The `Circle` class implements the `area` property requirement -as a computed property, based on a stored `radius` property. -The `Country` class implements the `area` requirement directly as a stored property. -Both classes correctly conform to the `HasArea` protocol. - -Here's a class called `Animal`, which doesn't conform to the `HasArea` protocol: - -```swift -class Animal { - var legs: Int - init(legs: Int) { self.legs = legs } -} -``` - - - -The `Circle`, `Country` and `Animal` classes don't have a shared base class. -Nonetheless, they're all classes, and so instances of all three types -can be used to initialize an array that stores values of type `AnyObject`: - -```swift -let objects: [AnyObject] = [ - Circle(radius: 2.0), - Country(area: 243_610), - Animal(legs: 4) -] -``` - - - -The `objects` array is initialized with an array literal containing -a `Circle` instance with a radius of 2 units; -a `Country` instance initialized with -the surface area of the United Kingdom in square kilometers; -and an `Animal` instance with four legs. - -The `objects` array can now be iterated, -and each object in the array can be checked to see if -it conforms to the `HasArea` protocol: - -```swift -for object in objects { - if let objectWithArea = object as? HasArea { - print("Area is \(objectWithArea.area)") - } else { - print("Something that doesn't have an area") - } -} -// Area is 12.5663708 -// Area is 243610.0 -// Something that doesn't have an area -``` - - - -Whenever an object in the array conforms to the `HasArea` protocol, -the optional value returned by the `as?` operator is unwrapped with optional binding -into a constant called `objectWithArea`. -The `objectWithArea` constant is known to be of type `HasArea`, -and so its `area` property can be accessed and printed in a type-safe way. - -Note that the underlying objects aren't changed by the casting process. -They continue to be a `Circle`, a `Country` and an `Animal`. -However, at the point that they're stored in the `objectWithArea` constant, -they're only known to be of type `HasArea`, -and so only their `area` property can be accessed. - - - - - -## Optional Protocol Requirements - - - - - -You can define *optional requirements* for protocols. -These requirements don't have to be implemented by types that conform to the protocol. -Optional requirements are prefixed by the `optional` modifier -as part of the protocol's definition. -Optional requirements are available so that you can write code -that interoperates with Objective-C. -Both the protocol and the optional requirement -must be marked with the `@objc` attribute. -Note that `@objc` protocols can be adopted only by classes, -not by structures or enumerations. - -When you use a method or property in an optional requirement, -its type automatically becomes an optional. -For example, -a method of type `(Int) -> String` becomes `((Int) -> String)?`. -Note that the entire function type -is wrapped in the optional, -not the method's return value. - -An optional protocol requirement can be called with optional chaining, -to account for the possibility that the requirement was not implemented -by a type that conforms to the protocol. -You check for an implementation of an optional method -by writing a question mark after the name of the method when it's called, -such as `someOptionalMethod?(someArgument)`. -For information on optional chaining, see . - -The following example defines an integer-counting class called `Counter`, -which uses an external data source to provide its increment amount. -This data source is defined by the `CounterDataSource` protocol, -which has two optional requirements: - -```swift -@objc protocol CounterDataSource { - @objc optional func increment(forCount count: Int) -> Int - @objc optional var fixedIncrement: Int { get } -} -``` - - - -The `CounterDataSource` protocol defines -an optional method requirement called `increment(forCount:)` -and an optional property requirement called `fixedIncrement`. -These requirements define two different ways for data sources to provide -an appropriate increment amount for a `Counter` instance. - -> Note: Strictly speaking, you can write a custom class -> that conforms to `CounterDataSource` without implementing -> *either* protocol requirement. -> They're both optional, after all. -> Although technically allowed, this wouldn't make for a very good data source. - -The `Counter` class, defined below, -has an optional `dataSource` property of type `CounterDataSource?`: - -```swift -class Counter { - var count = 0 - var dataSource: CounterDataSource? - func increment() { - if let amount = dataSource?.increment?(forCount: count) { - count += amount - } else if let amount = dataSource?.fixedIncrement { - count += amount - } - } -} -``` - - - -The `Counter` class stores its current value in a variable property called `count`. -The `Counter` class also defines a method called `increment`, -which increments the `count` property every time the method is called. - -The `increment()` method first tries to retrieve an increment amount -by looking for an implementation of the `increment(forCount:)` method on its data source. -The `increment()` method uses optional chaining to try to call `increment(forCount:)`, -and passes the current `count` value as the method's single argument. - -Note that *two* levels of optional chaining are at play here. -First, it's possible that `dataSource` may be `nil`, -and so `dataSource` has a question mark after its name to indicate that -`increment(forCount:)` should be called only if `dataSource` isn't `nil`. -Second, even if `dataSource` *does* exist, -there's no guarantee that it implements `increment(forCount:)`, -because it's an optional requirement. -Here, the possibility that `increment(forCount:)` might not be implemented -is also handled by optional chaining. -The call to `increment(forCount:)` happens -only if `increment(forCount:)` exists --- -that is, if it isn't `nil`. -This is why `increment(forCount:)` is also written with a question mark after its name. - -Because the call to `increment(forCount:)` can fail for either of these two reasons, -the call returns an *optional* `Int` value. -This is true even though `increment(forCount:)` is defined as returning -a non-optional `Int` value in the definition of `CounterDataSource`. -Even though there are two optional chaining operations, -one after another, -the result is still wrapped in a single optional. -For more information about using multiple optional chaining operations, -see . - -After calling `increment(forCount:)`, the optional `Int` that it returns -is unwrapped into a constant called `amount`, using optional binding. -If the optional `Int` does contain a value --- -that is, if the delegate and method both exist, -and the method returned a value --- -the unwrapped `amount` is added onto the stored `count` property, -and incrementation is complete. - -If it's *not* possible to retrieve a value from the `increment(forCount:)` method --- -either because `dataSource` is nil, -or because the data source doesn't implement `increment(forCount:)` --- -then the `increment()` method tries to retrieve a value -from the data source's `fixedIncrement` property instead. -The `fixedIncrement` property is also an optional requirement, -so its value is an optional `Int` value, -even though `fixedIncrement` is defined as a non-optional `Int` property -as part of the `CounterDataSource` protocol definition. - -Here's a simple `CounterDataSource` implementation where the data source -returns a constant value of `3` every time it's queried. -It does this by implementing the optional `fixedIncrement` property requirement: - -```swift -class ThreeSource: NSObject, CounterDataSource { - let fixedIncrement = 3 -} -``` - - - -You can use an instance of `ThreeSource` as the data source for a new `Counter` instance: - -```swift -var counter = Counter() -counter.dataSource = ThreeSource() -for _ in 1...4 { - counter.increment() - print(counter.count) -} -// 3 -// 6 -// 9 -// 12 -``` - - - -The code above creates a new `Counter` instance; -sets its data source to be a new `ThreeSource` instance; -and calls the counter's `increment()` method four times. -As expected, the counter's `count` property increases by three -each time `increment()` is called. - -Here's a more complex data source called `TowardsZeroSource`, -which makes a `Counter` instance count up or down towards zero -from its current `count` value: - -```swift -class TowardsZeroSource: NSObject, CounterDataSource { - func increment(forCount count: Int) -> Int { - if count == 0 { - return 0 - } else if count < 0 { - return 1 - } else { - return -1 - } - } -} -``` - - - -The `TowardsZeroSource` class implements -the optional `increment(forCount:)` method from the `CounterDataSource` protocol -and uses the `count` argument value to work out which direction to count in. -If `count` is already zero, the method returns `0` -to indicate that no further counting should take place. - -You can use an instance of `TowardsZeroSource` with the existing `Counter` instance -to count from `-4` to zero. -Once the counter reaches zero, no more counting takes place: - -```swift -counter.count = -4 -counter.dataSource = TowardsZeroSource() -for _ in 1...5 { - counter.increment() - print(counter.count) -} -// -3 -// -2 -// -1 -// 0 -// 0 -``` - - - -## Protocol Extensions - -Protocols can be extended to provide method, -initializer, subscript, and computed property implementations -to conforming types. -This allows you to define behavior on protocols themselves, -rather than in each type's individual conformance or in a global function. - -For example, the `RandomNumberGenerator` protocol can be extended -to provide a `randomBool()` method, -which uses the result of the required `random()` method -to return a random `Bool` value: - -```swift -extension RandomNumberGenerator { - func randomBool() -> Bool { - return random() > 0.5 - } -} -``` - - - -By creating an extension on the protocol, -all conforming types automatically gain this method implementation -without any additional modification. - -```swift -let generator = LinearCongruentialGenerator() -print("Here's a random number: \(generator.random())") -// Prints "Here's a random number: 0.3746499199817101" -print("And here's a random Boolean: \(generator.randomBool())") -// Prints "And here's a random Boolean: true" -``` - - - - - -Protocol extensions can add implementations to conforming types -but can't make a protocol extend or inherit from another protocol. -Protocol inheritance is always specified in the protocol declaration itself. - -### Providing Default Implementations - -You can use protocol extensions to provide a default implementation -to any method or computed property requirement of that protocol. -If a conforming type provides its own implementation of a required method or property, -that implementation will be used instead of the one provided by the extension. - -> Note: Protocol requirements with default implementations provided by extensions -> are distinct from optional protocol requirements. -> Although conforming types don't have to provide their own implementation of either, -> requirements with default implementations can be called without optional chaining. - -For example, the `PrettyTextRepresentable` protocol, -which inherits the `TextRepresentable` protocol -can provide a default implementation of its required `prettyTextualDescription` property -to simply return the result of accessing the `textualDescription` property: - -```swift -extension PrettyTextRepresentable { - var prettyTextualDescription: String { - return textualDescription - } -} -``` - - - - - - - - - - - - - - - -### Adding Constraints to Protocol Extensions - -When you define a protocol extension, -you can specify constraints that conforming types -must satisfy before the methods and properties of the extension are available. -You write these constraints after the name of the protocol you're extending -by writing a generic `where` clause. -For more about generic `where` clauses, see . - -For example, -you can define an extension to the `Collection` protocol -that applies to any collection whose elements conform -to the `Equatable` protocol. -By constraining a collection's elements to the `Equatable` protocol, -a part of the Swift standard library, -you can use the `==` and `!=` operators to check for equality and inequality between two elements. - -```swift -extension Collection where Element: Equatable { - func allEqual() -> Bool { - for element in self { - if element != self.first { - return false - } - } - return true - } -} -``` - - - -The `allEqual()` method returns `true` -only if all the elements in the collection are equal. - -Consider two arrays of integers, -one where all the elements are the same, -and one where they aren't: - -```swift -let equalNumbers = [100, 100, 100, 100, 100] -let differentNumbers = [100, 100, 200, 100, 200] -``` - - - -Because arrays conform to `Collection` -and integers conform to `Equatable`, -`equalNumbers` and `differentNumbers` can use the `allEqual()` method: - -```swift -print(equalNumbers.allEqual()) -// Prints "true" -print(differentNumbers.allEqual()) -// Prints "false" -``` - - - -> Note: If a conforming type satisfies the requirements for multiple constrained extensions -> that provide implementations for the same method or property, -> Swift uses the implementation corresponding to the most specialized constraints. - - - - - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/LanguageGuide/StringsAndCharacters.md b/swift-6-beta.docc/LanguageGuide/StringsAndCharacters.md deleted file mode 100644 index 6ba6a60d4..000000000 --- a/swift-6-beta.docc/LanguageGuide/StringsAndCharacters.md +++ /dev/null @@ -1,1802 +0,0 @@ -# Strings and Characters - -Store and manipulate text. - -A *string* is a series of characters, -such as `"hello, world"` or `"albatross"`. -Swift strings are represented by the `String` type. -The contents of a `String` can be accessed in various ways, -including as a collection of `Character` values. - -Swift's `String` and `Character` types provide -a fast, Unicode-compliant way to work with text in your code. -The syntax for string creation and manipulation is lightweight and readable, -with a string literal syntax that's similar to C. -String concatenation is as simple as -combining two strings with the `+` operator, -and string mutability is managed by choosing between a constant or a variable, -just like any other value in Swift. -You can also use strings to insert -constants, variables, literals, and expressions into longer strings, -in a process known as string interpolation. -This makes it easy to create custom string values for display, storage, and printing. - -Despite this simplicity of syntax, -Swift's `String` type is a fast, modern string implementation. -Every string is composed of encoding-independent Unicode characters, -and provides support for accessing those characters in various Unicode representations. - -> Note: Swift's `String` type is bridged with Foundation's `NSString` class. -> Foundation also extends `String` to expose methods defined by `NSString`. -> This means, if you import Foundation, -> you can access those `NSString` methods on `String` without casting. -> -> For more information about using `String` with Foundation and Cocoa, -> see [Bridging Between String and NSString](https://developer.apple.com/documentation/swift/string#2919514). - -## String Literals - -You can include predefined `String` values within your code as *string literals*. -A string literal is a sequence of characters -surrounded by double quotation marks (`"`). - -Use a string literal as an initial value for a constant or variable: - -```swift -let someString = "Some string literal value" -``` - - - -Note that Swift infers a type of `String` for the `someString` constant -because it's initialized with a string literal value. - -### Multiline String Literals - -If you need a string that spans several lines, -use a multiline string literal --- -a sequence of characters -surrounded by three double quotation marks: - - - -```swift -let quotation = """ -The White Rabbit put on his spectacles. "Where shall I begin, -please your Majesty?" he asked. - -"Begin at the beginning," the King said gravely, "and go on -till you come to the end; then stop." -""" -``` - - - -A multiline string literal includes all of the lines between -its opening and closing quotation marks. -The string begins on the first line after the opening quotation marks (`"""`) -and ends on the line before the closing quotation marks, -which means that neither of the strings below -start or end with a line break: - -```swift -let singleLineString = "These are the same." -let multilineString = """ -These are the same. -""" -``` - - - -When your source code includes a line break -inside of a multiline string literal, -that line break also appears in the string's value. -If you want to use line breaks -to make your source code easier to read, -but you don't want the line breaks to be part of the string's value, -write a backslash (`\`) at the end of those lines: - -```swift -let softWrappedQuotation = """ -The White Rabbit put on his spectacles. "Where shall I begin, \ -please your Majesty?" he asked. - -"Begin at the beginning," the King said gravely, "and go on \ -till you come to the end; then stop." -""" -``` - - - -To make a multiline string literal that begins or ends with a line feed, -write a blank line as the first or last line. -For example: - -```swift -let lineBreaks = """ - -This string starts with a line break. -It also ends with a line break. - -""" -``` - - - - - -A multiline string can be indented to match the surrounding code. -The whitespace before the closing quotation marks (`"""`) -tells Swift what whitespace to ignore before all of the other lines. -However, if you write whitespace at the beginning of a line -in addition to what's before the closing quotation marks, -that whitespace *is* included. - -![](multilineStringWhitespace) - - - - - -In the example above, -even though the entire multiline string literal is indented, -the first and last lines in the string don't begin with any whitespace. -The middle line has more indentation than the closing quotation marks, -so it starts with that extra four-space indentation. - -### Special Characters in String Literals - -String literals can include the following special characters: - -- The escaped special characters `\0` (null character), `\\` (backslash), - `\t` (horizontal tab), `\n` (line feed), `\r` (carriage return), - `\"` (double quotation mark) and `\'` (single quotation mark) -- An arbitrary Unicode scalar value, written as `\u{`*n*`}`, - where *n* is a 1--8 digit hexadecimal number - (Unicode is discussed in below) - - - -The code below shows four examples of these special characters. -The `wiseWords` constant contains two escaped double quotation marks. -The `dollarSign`, `blackHeart`, and `sparklingHeart` constants -demonstrate the Unicode scalar format: - -```swift -let wiseWords = "\"Imagination is more important than knowledge\" - Einstein" -// "Imagination is more important than knowledge" - Einstein -let dollarSign = "\u{24}" // $, Unicode scalar U+0024 -let blackHeart = "\u{2665}" // ♥, Unicode scalar U+2665 -let sparklingHeart = "\u{1F496}" // 💖, Unicode scalar U+1F496 -``` - - - -Because multiline string literals use three double quotation marks instead of just one, -you can include a double quotation mark (`"`) inside of a multiline string literal -without escaping it. -To include the text `"""` in a multiline string, -escape at least one of the quotation marks. -For example: - -```swift -let threeDoubleQuotationMarks = """ -Escaping the first quotation mark \""" -Escaping all three quotation marks \"\"\" -""" -``` - - - -### Extended String Delimiters - -You can place a string literal within *extended delimiters* -to include special characters in a string -without invoking their effect. -You place your string within quotation marks (`"`) -and surround that with number signs (`#`). -For example, printing the string literal `#"Line 1\nLine 2"#` -prints the line feed escape sequence (`\n`) -rather than printing the string across two lines. - -If you need the special effects of a character in a string literal, -match the number of number signs within the string -following the escape character (`\`). -For example, if your string is `#"Line 1\nLine 2"#` -and you want to break the line, -you can use `#"Line 1\#nLine 2"#` instead. -Similarly, `###"Line1\###nLine2"###` also breaks the line. - -String literals created using extended delimiters can also be multiline string literals. -You can use extended delimiters to include the text `"""` in a multiline string, -overriding the default behavior that ends the literal. For example: - -```swift -let threeMoreDoubleQuotationMarks = #""" -Here are three more double quotes: """ -"""# -``` - - - -## Initializing an Empty String - -To create an empty `String` value as the starting point -for building a longer string, -either assign an empty string literal to a variable -or initialize a new `String` instance with initializer syntax: - -```swift -var emptyString = "" // empty string literal -var anotherEmptyString = String() // initializer syntax -// these two strings are both empty, and are equivalent to each other -``` - - - -Find out whether a `String` value is empty -by checking its Boolean `isEmpty` property: - -```swift -if emptyString.isEmpty { - print("Nothing to see here") -} -// Prints "Nothing to see here" -``` - - - - - -## String Mutability - -You indicate whether a particular `String` can be modified (or *mutated*) -by assigning it to a variable (in which case it can be modified), -or to a constant (in which case it can't be modified): - -```swift -var variableString = "Horse" -variableString += " and carriage" -// variableString is now "Horse and carriage" - -let constantString = "Highlander" -constantString += " and another Highlander" -// this reports a compile-time error - a constant string cannot be modified -``` - - - - - -> Note: This approach is different from string mutation in Objective-C and Cocoa, -> where you choose between two classes (`NSString` and `NSMutableString`) -> to indicate whether a string can be mutated. - -## Strings Are Value Types - -Swift's `String` type is a *value type*. -If you create a new `String` value, -that `String` value is *copied* when it's passed to a function or method, -or when it's assigned to a constant or variable. -In each case, a new copy of the existing `String` value is created, -and the new copy is passed or assigned, not the original version. -Value types are described in . - -Swift's copy-by-default `String` behavior ensures that -when a function or method passes you a `String` value, -it's clear that you own that exact `String` value, -regardless of where it came from. -You can be confident that the string you are passed won't be modified -unless you modify it yourself. - -Behind the scenes, Swift's compiler optimizes string usage -so that actual copying takes place only when absolutely necessary. -This means you always get great performance -when working with strings as value types. - -## Working with Characters - -You can access the individual `Character` values for a `String` -by iterating over the string with a `for`-`in` loop: - -```swift -for character in "Dog!🐶" { - print(character) -} -// D -// o -// g -// ! -// 🐶 -``` - - - -The `for`-`in` loop is described in . - -Alternatively, you can create a stand-alone `Character` constant or variable -from a single-character string literal by providing a `Character` type annotation: - -```swift -let exclamationMark: Character = "!" -``` - - - -`String` values can be constructed by passing an array of `Character` values -as an argument to its initializer: - -```swift -let catCharacters: [Character] = ["C", "a", "t", "!", "🐱"] -let catString = String(catCharacters) -print(catString) -// Prints "Cat!🐱" -``` - - - -## Concatenating Strings and Characters - -`String` values can be added together (or *concatenated*) -with the addition operator (`+`) to create a new `String` value: - -```swift -let string1 = "hello" -let string2 = " there" -var welcome = string1 + string2 -// welcome now equals "hello there" -``` - - - -You can also append a `String` value to an existing `String` variable -with the addition assignment operator (`+=`): - -```swift -var instruction = "look over" -instruction += string2 -// instruction now equals "look over there" -``` - - - -You can append a `Character` value to a `String` variable -with the `String` type's `append()` method: - -```swift -let exclamationMark: Character = "!" -welcome.append(exclamationMark) -// welcome now equals "hello there!" -``` - - - -> Note: You can't append a `String` or `Character` to an existing `Character` variable, -> because a `Character` value must contain a single character only. - -If you're using multiline string literals -to build up the lines of a longer string, -you want every line in the string to end with a line break, -including the last line. -For example: - -```swift -let badStart = """ - one - two - """ -let end = """ - three - """ -print(badStart + end) -// Prints two lines: -// one -// twothree - -let goodStart = """ - one - two - - """ -print(goodStart + end) -// Prints three lines: -// one -// two -// three -``` - - - -In the code above, -concatenating `badStart` with `end` -produces a two-line string, -which isn't the desired result. -Because the last line of `badStart` -doesn't end with a line break, -that line gets combined with the first line of `end`. -In contrast, -both lines of `goodStart` end with a line break, -so when it's combined with `end` -the result has three lines, -as expected. - -## String Interpolation - -*String interpolation* is a way to construct a new `String` value -from a mix of constants, variables, literals, and expressions -by including their values inside a string literal. -You can use string interpolation -in both single-line and multiline string literals. -Each item that you insert into the string literal is wrapped in -a pair of parentheses, prefixed by a backslash (`\`): - -```swift -let multiplier = 3 -let message = "\(multiplier) times 2.5 is \(Double(multiplier) * 2.5)" -// message is "3 times 2.5 is 7.5" -``` - - - -In the example above, -the value of `multiplier` is inserted into a string literal as `\(multiplier)`. -This placeholder is replaced with the actual value of `multiplier` -when the string interpolation is evaluated to create an actual string. - -The value of `multiplier` is also part of a larger expression later in the string. -This expression calculates the value of `Double(multiplier) * 2.5` -and inserts the result (`7.5`) into the string. -In this case, the expression is written as `\(Double(multiplier) * 2.5)` -when it's included inside the string literal. - -You can use extended string delimiters to create strings containing -characters that would otherwise be treated as a string interpolation. -For example: - -```swift -print(#"Write an interpolated string in Swift using \(multiplier)."#) -// Prints "Write an interpolated string in Swift using \(multiplier)." -``` - - - -To use string interpolation -inside a string that uses extended delimiters, -match the number of number signs after the backslash -to the number of number signs at the beginning and end of the string. -For example: - -```swift -print(#"6 times 7 is \#(6 * 7)."#) -// Prints "6 times 7 is 42." -``` - - - -> Note: The expressions you write inside parentheses within an interpolated string -> can't contain an unescaped backslash (`\`), a carriage return, or a line feed. -> However, they can contain other string literals. - -## Unicode - -*Unicode* is an international standard for -encoding, representing, and processing text in different writing systems. -It enables you to represent almost any character from any language in a standardized form, -and to read and write those characters to and from an external source -such as a text file or web page. -Swift's `String` and `Character` types are fully Unicode-compliant, -as described in this section. - -### Unicode Scalar Values - -Behind the scenes, -Swift's native `String` type is built from *Unicode scalar values*. -A Unicode scalar value is a unique 21-bit number for a character or modifier, -such as `U+0061` for `LATIN SMALL LETTER A` (`"a"`), -or `U+1F425` for `FRONT-FACING BABY CHICK` (`"🐥"`). - -Note that not all 21-bit Unicode scalar values are assigned to a character --- -some scalars are reserved for future assignment or for use in UTF-16 encoding. -Scalar values that have been assigned to a character typically also have a name, -such as `LATIN SMALL LETTER A` and `FRONT-FACING BABY CHICK` in the examples above. - -### Extended Grapheme Clusters - -Every instance of Swift's `Character` type represents -a single *extended grapheme cluster*. -An extended grapheme cluster is a sequence of one or more Unicode scalars -that (when combined) produce a single human-readable character. - -Here's an example. -The letter `é` can be represented as the single Unicode scalar `é` -(`LATIN SMALL LETTER E WITH ACUTE`, or `U+00E9`). -However, the same letter can also be represented as a *pair* of scalars --- -a standard letter `e` (`LATIN SMALL LETTER E`, or `U+0065`), -followed by the `COMBINING ACUTE ACCENT` scalar (`U+0301`). -The `COMBINING ACUTE ACCENT` scalar is graphically applied to the scalar that precedes it, -turning an `e` into an `é` when it's rendered by -a Unicode-aware text-rendering system. - -In both cases, the letter `é` is represented as a single Swift `Character` value -that represents an extended grapheme cluster. -In the first case, the cluster contains a single scalar; -in the second case, it's a cluster of two scalars: - -```swift -let eAcute: Character = "\u{E9}" // é -let combinedEAcute: Character = "\u{65}\u{301}" // e followed by ́ -// eAcute is é, combinedEAcute is é -``` - - - -Extended grapheme clusters are a flexible way to represent -many complex script characters as a single `Character` value. -For example, Hangul syllables from the Korean alphabet -can be represented as either a precomposed or decomposed sequence. -Both of these representations qualify as a single `Character` value in Swift: - -```swift -let precomposed: Character = "\u{D55C}" // 한 -let decomposed: Character = "\u{1112}\u{1161}\u{11AB}" // ᄒ, ᅡ, ᆫ -// precomposed is 한, decomposed is 한 -``` - - - -Extended grapheme clusters enable -scalars for enclosing marks (such as `COMBINING ENCLOSING CIRCLE`, or `U+20DD`) -to enclose other Unicode scalars as part of a single `Character` value: - -```swift -let enclosedEAcute: Character = "\u{E9}\u{20DD}" -// enclosedEAcute is é⃝ -``` - - - -Unicode scalars for regional indicator symbols -can be combined in pairs to make a single `Character` value, -such as this combination of `REGIONAL INDICATOR SYMBOL LETTER U` (`U+1F1FA`) -and `REGIONAL INDICATOR SYMBOL LETTER S` (`U+1F1F8`): - -```swift -let regionalIndicatorForUS: Character = "\u{1F1FA}\u{1F1F8}" -// regionalIndicatorForUS is 🇺🇸 -``` - - - -## Counting Characters - -To retrieve a count of the `Character` values in a string, -use the `count` property of the string: - -```swift -let unusualMenagerie = "Koala 🐨, Snail 🐌, Penguin 🐧, Dromedary 🐪" -print("unusualMenagerie has \(unusualMenagerie.count) characters") -// Prints "unusualMenagerie has 40 characters" -``` - - - -Note that Swift's use of extended grapheme clusters for `Character` values -means that string concatenation and modification may not always affect -a string's character count. - -For example, if you initialize a new string with the four-character word `cafe`, -and then append a `COMBINING ACUTE ACCENT` (`U+0301`) to the end of the string, -the resulting string will still have a character count of `4`, -with a fourth character of `é`, not `e`: - -```swift -var word = "cafe" -print("the number of characters in \(word) is \(word.count)") -// Prints "the number of characters in cafe is 4" - -word += "\u{301}" // COMBINING ACUTE ACCENT, U+0301 - -print("the number of characters in \(word) is \(word.count)") -// Prints "the number of characters in café is 4" -``` - - - -> Note: Extended grapheme clusters can be composed of multiple Unicode scalars. -> This means that different characters --- -> and different representations of the same character --- -> can require different amounts of memory to store. -> Because of this, characters in Swift don't each take up -> the same amount of memory within a string's representation. -> As a result, the number of characters in a string can't be calculated -> without iterating through the string to determine -> its extended grapheme cluster boundaries. -> If you are working with particularly long string values, -> be aware that the `count` property -> must iterate over the Unicode scalars in the entire string -> in order to determine the characters for that string. -> -> The count of the characters returned by the `count` property -> isn't always the same as the `length` property of -> an `NSString` that contains the same characters. -> The length of an `NSString` is based on -> the number of 16-bit code units within the string's UTF-16 representation -> and not the number of Unicode extended grapheme clusters within the string. - -## Accessing and Modifying a String - -You access and modify a string through its methods and properties, -or by using subscript syntax. - -### String Indices - -Each `String` value has an associated *index type*, -`String.Index`, -which corresponds to the position of each `Character` in the string. - -As mentioned above, -different characters can require different amounts of memory to store, -so in order to determine which `Character` is at a particular position, -you must iterate over each Unicode scalar from the start or end of that `String`. -For this reason, Swift strings can't be indexed by integer values. - -Use the `startIndex` property to access -the position of the first `Character` of a `String`. -The `endIndex` property is the position after the last character in a `String`. -As a result, -the `endIndex` property isn't a valid argument to a string's subscript. -If a `String` is empty, `startIndex` and `endIndex` are equal. - -You access the indices before and after a given index -using the `index(before:)` and `index(after:)` methods of `String`. -To access an index farther away from the given index, -you can use the `index(_:offsetBy:)` method -instead of calling one of these methods multiple times. - -You can use subscript syntax to access -the `Character` at a particular `String` index. - -```swift -let greeting = "Guten Tag!" -greeting[greeting.startIndex] -// G -greeting[greeting.index(before: greeting.endIndex)] -// ! -greeting[greeting.index(after: greeting.startIndex)] -// u -let index = greeting.index(greeting.startIndex, offsetBy: 7) -greeting[index] -// a -``` - - - -Attempting to access an index outside of a string's range -or a `Character` at an index outside of a string's range -will trigger a runtime error. - -```swift -greeting[greeting.endIndex] // Error -greeting.index(after: greeting.endIndex) // Error -``` - - - - - -Use the `indices` property to access all of the -indices of individual characters in a string. - -```swift -for index in greeting.indices { - print("\(greeting[index]) ", terminator: "") -} -// Prints "G u t e n T a g ! " -``` - - - - - -> Note: You can use the `startIndex` and `endIndex` properties -> and the `index(before:)`, `index(after:)`, and `index(_:offsetBy:)` methods -> on any type that conforms to the `Collection` protocol. -> This includes `String`, as shown here, -> as well as collection types such as `Array`, `Dictionary`, and `Set`. - -### Inserting and Removing - -To insert a single character into a string at a specified index, -use the `insert(_:at:)` method, -and to insert the contents of another string at a specified index, -use the `insert(contentsOf:at:)` method. - -```swift -var welcome = "hello" -welcome.insert("!", at: welcome.endIndex) -// welcome now equals "hello!" - -welcome.insert(contentsOf: " there", at: welcome.index(before: welcome.endIndex)) -// welcome now equals "hello there!" -``` - - - -To remove a single character from a string at a specified index, -use the `remove(at:)` method, -and to remove a substring at a specified range, -use the `removeSubrange(_:)` method: - -```swift -welcome.remove(at: welcome.index(before: welcome.endIndex)) -// welcome now equals "hello there" - -let range = welcome.index(welcome.endIndex, offsetBy: -6).. welcome.remove(at: welcome.index(before: welcome.endIndex)) - /> welcome now equals \"\(welcome)\" - let range = welcome.index(welcome.endIndex, offsetBy: -6).. welcome.removeSubrange(range) - /> welcome now equals \"\(welcome)\" - - - - -> Note: You can use the `insert(_:at:)`, `insert(contentsOf:at:)`, -> `remove(at:)`, and `removeSubrange(_:)` methods -> on any type that conforms to the `RangeReplaceableCollection` protocol. -> This includes `String`, as shown here, -> as well as collection types such as `Array`, `Dictionary`, and `Set`. - -## Substrings - -When you get a substring from a string --- -for example, using a subscript or a method like `prefix(_:)` --- -the result is an instance -of [`Substring`](https://developer.apple.com/documentation/swift/substring), -not another string. -Substrings in Swift have most of the same methods as strings, -which means you can work with substrings -the same way you work with strings. -However, unlike strings, -you use substrings for only a short amount of time -while performing actions on a string. -When you're ready to store the result for a longer time, -you convert the substring to an instance of `String`. -For example: - -```swift -let greeting = "Hello, world!" -let index = greeting.firstIndex(of: ",") ?? greeting.endIndex -let beginning = greeting[.. let greeting = "Hello, world!" - -> let index = greeting.firstIndex(of: ",") ?? greeting.endIndex - -> let beginning = greeting[.. beginning is \"\(beginning)\" - let newString = String(beginning) - ``` ---> - -Like strings, each substring has a region of memory -where the characters that make up the substring are stored. -The difference between strings and substrings -is that, as a performance optimization, -a substring can reuse part of the memory -that's used to store the original string, -or part of the memory that's used to store another substring. -(Strings have a similar optimization, -but if two strings share memory, they're equal.) -This performance optimization means -you don't have to pay the performance cost of copying memory -until you modify either the string or substring. -As mentioned above, -substrings aren't suitable for long-term storage --- -because they reuse the storage of the original string, -the entire original string must be kept in memory -as long as any of its substrings are being used. - -In the example above, -`greeting` is a string, -which means it has a region of memory -where the characters that make up the string are stored. -Because -`beginning` is a substring of `greeting`, -it reuses the memory that `greeting` uses. -In contrast, -`newString` is a string --- -when it's created from the substring, -it has its own storage. -The figure below shows these relationships: - - - -![](stringSubstring) - -> Note: Both `String` and `Substring` conform to the -> [`StringProtocol`](https://developer.apple.com/documentation/swift/stringprotocol) protocol, -> which means it's often convenient for string-manipulation functions -> to accept a `StringProtocol` value. -> You can call such functions with either a `String` or `Substring` value. - -## Comparing Strings - -Swift provides three ways to compare textual values: -string and character equality, prefix equality, and suffix equality. - -### String and Character Equality - -String and character equality is checked with the “equal to” operator (`==`) -and the “not equal to” operator (`!=`), -as described in : - -```swift -let quotation = "We're a lot alike, you and I." -let sameQuotation = "We're a lot alike, you and I." -if quotation == sameQuotation { - print("These two strings are considered equal") -} -// Prints "These two strings are considered equal" -``` - - - -Two `String` values (or two `Character` values) are considered equal if -their extended grapheme clusters are *canonically equivalent*. -Extended grapheme clusters are canonically equivalent if they have -the same linguistic meaning and appearance, -even if they're composed from different Unicode scalars behind the scenes. - - - - - -For example, `LATIN SMALL LETTER E WITH ACUTE` (`U+00E9`) -is canonically equivalent to `LATIN SMALL LETTER E` (`U+0065`) -followed by `COMBINING ACUTE ACCENT` (`U+0301`). -Both of these extended grapheme clusters are valid ways to represent the character `é`, -and so they're considered to be canonically equivalent: - -```swift -// "Voulez-vous un café?" using LATIN SMALL LETTER E WITH ACUTE -let eAcuteQuestion = "Voulez-vous un caf\u{E9}?" - -// "Voulez-vous un café?" using LATIN SMALL LETTER E and COMBINING ACUTE ACCENT -let combinedEAcuteQuestion = "Voulez-vous un caf\u{65}\u{301}?" - -if eAcuteQuestion == combinedEAcuteQuestion { - print("These two strings are considered equal") -} -// Prints "These two strings are considered equal" -``` - - - -Conversely, `LATIN CAPITAL LETTER A` (`U+0041`, or `"A"`), -as used in English, is *not* equivalent to -`CYRILLIC CAPITAL LETTER A` (`U+0410`, or `"А"`), -as used in Russian. -The characters are visually similar, -but don't have the same linguistic meaning: - -```swift -let latinCapitalLetterA: Character = "\u{41}" - -let cyrillicCapitalLetterA: Character = "\u{0410}" - -if latinCapitalLetterA != cyrillicCapitalLetterA { - print("These two characters aren't equivalent.") -} -// Prints "These two characters aren't equivalent." -``` - - - -> Note: String and character comparisons in Swift aren't locale-sensitive. - - - -### Prefix and Suffix Equality - -To check whether a string has a particular string prefix or suffix, -call the string's `hasPrefix(_:)` and `hasSuffix(_:)` methods, -both of which take a single argument of type `String` and return a Boolean value. - - - - - -The examples below consider an array of strings representing -the scene locations from the first two acts of Shakespeare's *Romeo and Juliet*: - -```swift -let romeoAndJuliet = [ - "Act 1 Scene 1: Verona, A public place", - "Act 1 Scene 2: Capulet's mansion", - "Act 1 Scene 3: A room in Capulet's mansion", - "Act 1 Scene 4: A street outside Capulet's mansion", - "Act 1 Scene 5: The Great Hall in Capulet's mansion", - "Act 2 Scene 1: Outside Capulet's mansion", - "Act 2 Scene 2: Capulet's orchard", - "Act 2 Scene 3: Outside Friar Lawrence's cell", - "Act 2 Scene 4: A street in Verona", - "Act 2 Scene 5: Capulet's mansion", - "Act 2 Scene 6: Friar Lawrence's cell" -] -``` - - - -You can use the `hasPrefix(_:)` method with the `romeoAndJuliet` array -to count the number of scenes in Act 1 of the play: - -```swift -var act1SceneCount = 0 -for scene in romeoAndJuliet { - if scene.hasPrefix("Act 1 ") { - act1SceneCount += 1 - } -} -print("There are \(act1SceneCount) scenes in Act 1") -// Prints "There are 5 scenes in Act 1" -``` - - - -Similarly, use the `hasSuffix(_:)` method to count the number of scenes -that take place in or around Capulet's mansion and Friar Lawrence's cell: - -```swift -var mansionCount = 0 -var cellCount = 0 -for scene in romeoAndJuliet { - if scene.hasSuffix("Capulet's mansion") { - mansionCount += 1 - } else if scene.hasSuffix("Friar Lawrence's cell") { - cellCount += 1 - } -} -print("\(mansionCount) mansion scenes; \(cellCount) cell scenes") -// Prints "6 mansion scenes; 2 cell scenes" -``` - - - -> Note: The `hasPrefix(_:)` and `hasSuffix(_:)` methods -> perform a character-by-character canonical equivalence comparison between -> the extended grapheme clusters in each string, -> as described in . - -## Unicode Representations of Strings - -When a Unicode string is written to a text file or some other storage, -the Unicode scalars in that string are encoded in one of -several Unicode-defined *encoding forms*. -Each form encodes the string in small chunks known as *code units*. -These include the UTF-8 encoding form (which encodes a string as 8-bit code units), -the UTF-16 encoding form (which encodes a string as 16-bit code units), -and the UTF-32 encoding form (which encodes a string as 32-bit code units). - -Swift provides several different ways to access Unicode representations of strings. -You can iterate over the string with a `for`-`in` statement, -to access its individual `Character` values as Unicode extended grapheme clusters. -This process is described in . - -Alternatively, access a `String` value -in one of three other Unicode-compliant representations: - -- A collection of UTF-8 code units (accessed with the string's `utf8` property) -- A collection of UTF-16 code units (accessed with the string's `utf16` property) -- A collection of 21-bit Unicode scalar values, - equivalent to the string's UTF-32 encoding form - (accessed with the string's `unicodeScalars` property) - -Each example below shows a different representation of the following string, -which is made up of the characters `D`, `o`, `g`, -`‼` (`DOUBLE EXCLAMATION MARK`, or Unicode scalar `U+203C`), -and the 🐶 character (`DOG FACE`, or Unicode scalar `U+1F436`): - -```swift -let dogString = "Dog‼🐶" -``` - - - -### UTF-8 Representation - -You can access a UTF-8 representation of a `String` -by iterating over its `utf8` property. -This property is of type `String.UTF8View`, -which is a collection of unsigned 8-bit (`UInt8`) values, -one for each byte in the string's UTF-8 representation: - -![](UTF8) - -```swift -for codeUnit in dogString.utf8 { - print("\(codeUnit) ", terminator: "") -} -print("") -// Prints "68 111 103 226 128 188 240 159 144 182 " -``` - - - - - -In the example above, the first three decimal `codeUnit` values -(`68`, `111`, `103`) -represent the characters `D`, `o`, and `g`, -whose UTF-8 representation is the same as their ASCII representation. -The next three decimal `codeUnit` values -(`226`, `128`, `188`) -are a three-byte UTF-8 representation of the `DOUBLE EXCLAMATION MARK` character. -The last four `codeUnit` values (`240`, `159`, `144`, `182`) -are a four-byte UTF-8 representation of the `DOG FACE` character. - - - - - -### UTF-16 Representation - -You can access a UTF-16 representation of a `String` -by iterating over its `utf16` property. -This property is of type `String.UTF16View`, -which is a collection of unsigned 16-bit (`UInt16`) values, -one for each 16-bit code unit in the string's UTF-16 representation: - -![](UTF16) - -```swift -for codeUnit in dogString.utf16 { - print("\(codeUnit) ", terminator: "") -} -print("") -// Prints "68 111 103 8252 55357 56374 " -``` - - - - - -Again, the first three `codeUnit` values -(`68`, `111`, `103`) -represent the characters `D`, `o`, and `g`, -whose UTF-16 code units have the same values as in the string's UTF-8 representation -(because these Unicode scalars represent ASCII characters). - -The fourth `codeUnit` value (`8252`) is a decimal equivalent of -the hexadecimal value `203C`, -which represents the Unicode scalar `U+203C` -for the `DOUBLE EXCLAMATION MARK` character. -This character can be represented as a single code unit in UTF-16. - -The fifth and sixth `codeUnit` values (`55357` and `56374`) -are a UTF-16 surrogate pair representation of the `DOG FACE` character. -These values are a high-surrogate value of `U+D83D` (decimal value `55357`) -and a low-surrogate value of `U+DC36` (decimal value `56374`). - -### Unicode Scalar Representation - -You can access a Unicode scalar representation of a `String` value -by iterating over its `unicodeScalars` property. -This property is of type `UnicodeScalarView`, -which is a collection of values of type `UnicodeScalar`. - -Each `UnicodeScalar` has a `value` property that returns -the scalar's 21-bit value, represented within a `UInt32` value: - -![](UnicodeScalar) - -```swift -for scalar in dogString.unicodeScalars { - print("\(scalar.value) ", terminator: "") -} -print("") -// Prints "68 111 103 8252 128054 " -``` - - - - - -The `value` properties for the first three `UnicodeScalar` values -(`68`, `111`, `103`) -once again represent the characters `D`, `o`, and `g`. - -The fourth `codeUnit` value (`8252`) is again a decimal equivalent of -the hexadecimal value `203C`, -which represents the Unicode scalar `U+203C` -for the `DOUBLE EXCLAMATION MARK` character. - -The `value` property of the fifth and final `UnicodeScalar`, `128054`, -is a decimal equivalent of the hexadecimal value `1F436`, -which represents the Unicode scalar `U+1F436` for the `DOG FACE` character. - -As an alternative to querying their `value` properties, -each `UnicodeScalar` value can also be used to construct a new `String` value, -such as with string interpolation: - -```swift -for scalar in dogString.unicodeScalars { - print("\(scalar) ") -} -// D -// o -// g -// ‼ -// 🐶 -``` - - - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/LanguageGuide/Subscripts.md b/swift-6-beta.docc/LanguageGuide/Subscripts.md deleted file mode 100644 index a1f9a19c8..000000000 --- a/swift-6-beta.docc/LanguageGuide/Subscripts.md +++ /dev/null @@ -1,439 +0,0 @@ -# Subscripts - -Access the elements of a collection. - -Classes, structures, and enumerations can define *subscripts*, -which are shortcuts for accessing the member elements of a collection, list, or sequence. -You use subscripts to set and retrieve values by index without needing -separate methods for setting and retrieval. -For example, you access elements in an `Array` instance as `someArray[index]` -and elements in a `Dictionary` instance as `someDictionary[key]`. - -You can define multiple subscripts for a single type, -and the appropriate subscript overload to use is selected -based on the type of index value you pass to the subscript. -Subscripts aren't limited to a single dimension, -and you can define subscripts with multiple input parameters -to suit your custom type's needs. - - - -## Subscript Syntax - -Subscripts enable you to query instances of a type -by writing one or more values in square brackets after the instance name. -Their syntax is similar to both instance method syntax and computed property syntax. -You write subscript definitions with the `subscript` keyword, -and specify one or more input parameters and a return type, -in the same way as instance methods. -Unlike instance methods, subscripts can be read-write or read-only. -This behavior is communicated by a getter and setter -in the same way as for computed properties: - -```swift -subscript(index: Int) -> Int { - get { - // Return an appropriate subscript value here. - } - set(newValue) { - // Perform a suitable setting action here. - } -} -``` - - - -The type of `newValue` is the same as the return value of the subscript. -As with computed properties, you can choose not to specify -the setter's `(newValue)` parameter. -A default parameter called `newValue` is provided to your setter -if you don't provide one yourself. - -As with read-only computed properties, -you can simplify the declaration of a read-only subscript -by removing the `get` keyword and its braces: - -```swift -subscript(index: Int) -> Int { - // Return an appropriate subscript value here. -} -``` - - - -Here's an example of a read-only subscript implementation, -which defines a `TimesTable` structure to represent an *n*-times-table of integers: - -```swift -struct TimesTable { - let multiplier: Int - subscript(index: Int) -> Int { - return multiplier * index - } -} -let threeTimesTable = TimesTable(multiplier: 3) -print("six times three is \(threeTimesTable[6])") -// Prints "six times three is 18" -``` - - - -In this example, a new instance of `TimesTable` is created -to represent the three-times-table. -This is indicated by passing a value of `3` to the structure's `initializer` -as the value to use for the instance's `multiplier` parameter. - -You can query the `threeTimesTable` instance by calling its subscript, -as shown in the call to `threeTimesTable[6]`. -This requests the sixth entry in the three-times-table, -which returns a value of `18`, or `3` times `6`. - -> Note: An *n*-times-table is based on a fixed mathematical rule. -> It isn't appropriate to set `threeTimesTable[someIndex]` to a new value, -> and so the subscript for `TimesTable` is defined as a read-only subscript. - -## Subscript Usage - -The exact meaning of “subscript” depends on the context in which it's used. -Subscripts are typically used as a shortcut for accessing -the member elements in a collection, list, or sequence. -You are free to implement subscripts in the most appropriate way for -your particular class or structure's functionality. - -For example, Swift's `Dictionary` type implements a subscript -to set and retrieve the values stored in a `Dictionary` instance. -You can set a value in a dictionary -by providing a key of the dictionary's key type within subscript brackets, -and assigning a value of the dictionary's value type to the subscript: - -```swift -var numberOfLegs = ["spider": 8, "ant": 6, "cat": 4] -numberOfLegs["bird"] = 2 -``` - - - -The example above defines a variable called `numberOfLegs` -and initializes it with a dictionary literal containing three key-value pairs. -The type of the `numberOfLegs` dictionary is inferred to be `[String: Int]`. -After creating the dictionary, -this example uses subscript assignment to add -a `String` key of `"bird"` and an `Int` value of `2` to the dictionary. - -For more information about `Dictionary` subscripting, -see . - -> Note: Swift's `Dictionary` type implements its key-value subscripting -> as a subscript that takes and returns an *optional* type. -> For the `numberOfLegs` dictionary above, -> the key-value subscript takes and returns a value of type `Int?`, -> or “optional int”. -> The `Dictionary` type uses an optional subscript type to model the fact that -> not every key will have a value, and to give a way to delete a value for a key -> by assigning a `nil` value for that key. - -## Subscript Options - -Subscripts can take any number of input parameters, -and these input parameters can be of any type. -Subscripts can also return a value of any type. - -Like functions, -subscripts can take a varying number of parameters -and provide default values for their parameters, -as discussed in -and . -However, unlike functions, -subscripts can't use in-out parameters. - - - -A class or structure can provide as many subscript implementations as it needs, -and the appropriate subscript to be used will be inferred based on -the types of the value or values that are contained within the subscript brackets -at the point that the subscript is used. -This definition of multiple subscripts is known as *subscript overloading*. - -While it's most common for a subscript to take a single parameter, -you can also define a subscript with multiple parameters -if it's appropriate for your type. -The following example defines a `Matrix` structure, -which represents a two-dimensional matrix of `Double` values. -The `Matrix` structure's subscript takes two integer parameters: - -```swift -struct Matrix { - let rows: Int, columns: Int - var grid: [Double] - init(rows: Int, columns: Int) { - self.rows = rows - self.columns = columns - grid = Array(repeating: 0.0, count: rows * columns) - } - func indexIsValid(row: Int, column: Int) -> Bool { - return row >= 0 && row < rows && column >= 0 && column < columns - } - subscript(row: Int, column: Int) -> Double { - get { - assert(indexIsValid(row: row, column: column), "Index out of range") - return grid[(row * columns) + column] - } - set { - assert(indexIsValid(row: row, column: column), "Index out of range") - grid[(row * columns) + column] = newValue - } - } -} -``` - - - -`Matrix` provides an initializer that takes two parameters called `rows` and `columns`, -and creates an array that's large enough to store `rows * columns` values of type `Double`. -Each position in the matrix is given an initial value of `0.0`. -To achieve this, the array's size, and an initial cell value of `0.0`, -are passed to an array initializer that creates and initializes a new array of the correct size. -This initializer is described in more detail -in . - -You can construct a new `Matrix` instance by passing -an appropriate row and column count to its initializer: - -```swift -var matrix = Matrix(rows: 2, columns: 2) -``` - - - -The example above creates a new `Matrix` instance with two rows and two columns. -The `grid` array for this `Matrix` instance -is effectively a flattened version of the matrix, -as read from top left to bottom right: - -![](subscriptMatrix01) - -Values in the matrix can be set by passing row and column values into the subscript, -separated by a comma: - -```swift -matrix[0, 1] = 1.5 -matrix[1, 0] = 3.2 -``` - - - -These two statements call the subscript's setter to set -a value of `1.5` in the top right position of the matrix -(where `row` is `0` and `column` is `1`), -and `3.2` in the bottom left position -(where `row` is `1` and `column` is `0`): - -![](subscriptMatrix02) - -The `Matrix` subscript's getter and setter both contain an assertion -to check that the subscript's `row` and `column` values are valid. -To assist with these assertions, -`Matrix` includes a convenience method called `indexIsValid(row:column:)`, -which checks whether the requested `row` and `column` -are inside the bounds of the matrix: - -```swift -func indexIsValid(row: Int, column: Int) -> Bool { - return row >= 0 && row < rows && column >= 0 && column < columns -} -``` - - - -An assertion is triggered if you try to access a subscript -that's outside of the matrix bounds: - -```swift -let someValue = matrix[2, 2] -// This triggers an assert, because [2, 2] is outside of the matrix bounds. -``` - - - -## Type Subscripts - -Instance subscripts, as described above, -are subscripts that you call on an instance of a particular type. -You can also define subscripts that are called on the type itself. -This kind of subscript is called a *type subscript*. -You indicate a type subscript -by writing the `static` keyword before the `subscript` keyword. -Classes can use the `class` keyword instead, -to allow subclasses to override the superclass’s implementation of that subscript. -The example below shows how you define and call a type subscript: - -```swift -enum Planet: Int { - case mercury = 1, venus, earth, mars, jupiter, saturn, uranus, neptune - static subscript(n: Int) -> Planet { - return Planet(rawValue: n)! - } -} -let mars = Planet[4] -print(mars) -``` - - - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/LanguageGuide/TheBasics.md b/swift-6-beta.docc/LanguageGuide/TheBasics.md deleted file mode 100644 index 655afce70..000000000 --- a/swift-6-beta.docc/LanguageGuide/TheBasics.md +++ /dev/null @@ -1,2164 +0,0 @@ -# The Basics - -Work with common kinds of data and write basic syntax. - -Swift provides many fundamental data types, -including `Int` for integers, -`Double` for floating-point values, -`Bool` for Boolean values, -and `String` for text. -Swift also provides powerful versions of the three primary collection types, -`Array`, `Set`, and `Dictionary`, -as described in . - -Swift uses variables to store and refer to values by an identifying name. -Swift also makes extensive use of variables whose values can't be changed. -These are known as constants, and are used throughout Swift to make code safer and clearer in intent -when you work with values that don't need to change. - -In addition to familiar types, -Swift introduces advanced types such as tuples. -Tuples enable you to create and pass around groupings of values. -You can use a tuple to return multiple values from a function as a single compound value. - -Swift also introduces optional types, -which handle the absence of a value. -Optionals say either “there *is* a value, and it equals *x*” -or “there *isn't* a value at all”. - -Swift is a *type-safe* language, -which means the language helps you to be clear about the types of values your code can work with. -If part of your code requires a `String`, -type safety prevents you from passing it an `Int` by mistake. -Likewise, type safety prevents you from -accidentally passing an optional `String` -to a piece of code that requires a non-optional `String`. -Type safety helps you catch and fix errors as early as possible in the development process. - -## Constants and Variables - -Constants and variables associate a name -(such as `maximumNumberOfLoginAttempts` or `welcomeMessage`) -with a value of a particular type -(such as the number `10` or the string `"Hello"`). -The value of a *constant* can't be changed once it's set, -whereas a *variable* can be set to a different value in the future. - -### Declaring Constants and Variables - -Constants and variables must be declared before they're used. -You declare constants with the `let` keyword -and variables with the `var` keyword. -Here's an example of how constants and variables can be used -to track the number of login attempts a user has made: - -```swift -let maximumNumberOfLoginAttempts = 10 -var currentLoginAttempt = 0 -``` - - - -This code can be read as: - -“Declare a new constant called `maximumNumberOfLoginAttempts`, -and give it a value of `10`. -Then, declare a new variable called `currentLoginAttempt`, -and give it an initial value of `0`.” - -In this example, -the maximum number of allowed login attempts is declared as a constant, -because the maximum value never changes. -The current login attempt counter is declared as a variable, -because this value must be incremented after each failed login attempt. - -If a stored value in your code won't change, -always declare it as a constant with the `let` keyword. -Use variables only for storing values that change. - -When you declare a constant or a variable, -you can give it a value as part of that declaration, -like the examples above. -Alternatively, -you can assign its initial value later in the program, -as long as it's guaranteed to have a value -before the first time you read from it. - -```swift -var environment = "development" -let maximumNumberOfLoginAttempts: Int -// maximumNumberOfLoginAttempts has no value yet. - -if environment == "development" { - maximumNumberOfLoginAttempts = 100 -} else { - maximumNumberOfLoginAttempts = 10 -} -// Now maximumNumberOfLoginAttempts has a value, and can be read. -``` - - - -In this example, -the maximum number of login attempts is constant, -and its value depends on the environment. -In the development environment, -it has a value of 100; -in any other environment, its value is 10. -Both branches of the `if` statement -initialize `maximumNumberOfLoginAttempts` with some value, -guaranteeing that the constant always gets a value. -For information about how Swift checks your code -when you set an initial value this way, -see . - -You can declare multiple constants or multiple variables on a single line, -separated by commas: - -```swift -var x = 0.0, y = 0.0, z = 0.0 -``` - - - -### Type Annotations - -You can provide a *type annotation* when you declare a constant or variable, -to be clear about the kind of values the constant or variable can store. -Write a type annotation by placing a colon after the constant or variable name, -followed by a space, followed by the name of the type to use. - -This example provides a type annotation for a variable called `welcomeMessage`, -to indicate that the variable can store `String` values: - -```swift -var welcomeMessage: String -``` - - - -The colon in the declaration means “…of type…,” -so the code above can be read as: - -“Declare a variable called `welcomeMessage` that's of type `String`.” - -The phrase “of type `String`” means “can store any `String` value.” -Think of it as meaning “the type of thing” (or “the kind of thing”) that can be stored. - -The `welcomeMessage` variable can now be set to any string value without error: - -```swift -welcomeMessage = "Hello" -``` - - - -You can define multiple related variables of the same type on a single line, -separated by commas, with a single type annotation after the final variable name: - -```swift -var red, green, blue: Double -``` - - - -> Note: It's rare that you need to write type annotations in practice. -> If you provide an initial value for a constant or variable at the point that it's defined, -> Swift can almost always infer the type to be used for that constant or variable, -> as described in . -> In the `welcomeMessage` example above, no initial value is provided, -> and so the type of the `welcomeMessage` variable is specified with a type annotation -> rather than being inferred from an initial value. - -### Naming Constants and Variables - -Constant and variable names can contain almost any character, -including Unicode characters: - -```swift -let π = 3.14159 -let 你好 = "你好世界" -let 🐶🐮 = "dogcow" -``` - - - -Constant and variable names can't contain -whitespace characters, mathematical symbols, arrows, private-use Unicode scalar values, -or line- and box-drawing characters. -Nor can they begin with a number, -although numbers may be included elsewhere within the name. - -Once you've declared a constant or variable of a certain type, -you can't declare it again with the same name, -or change it to store values of a different type. -Nor can you change a constant into a variable -or a variable into a constant. - -> Note: If you need to give a constant or variable the same name as a reserved Swift keyword, -> surround the keyword with backticks (`` ` ``) when using it as a name. -> However, avoid using keywords as names unless you have absolutely no choice. - -You can change the value of an existing variable to another value of a compatible type. -In this example, the value of `friendlyWelcome` is changed from -`"Hello!"` to `"Bonjour!"`: - -```swift -var friendlyWelcome = "Hello!" -friendlyWelcome = "Bonjour!" -// friendlyWelcome is now "Bonjour!" -``` - - - -Unlike a variable, the value of a constant can't be changed after it's set. -Attempting to do so is reported as an error when your code is compiled: - -```swift -let languageName = "Swift" -languageName = "Swift++" -// This is a compile-time error: languageName cannot be changed. -``` - - - -### Printing Constants and Variables - -You can print the current value of a constant or variable with the `print(_:separator:terminator:)` function: - -```swift -print(friendlyWelcome) -// Prints "Bonjour!" -``` - - - -The `print(_:separator:terminator:)` function -is a global function that prints one or more values -to an appropriate output. -In Xcode, for example, -the `print(_:separator:terminator:)` function prints its output in Xcode's “console” pane. -The `separator` and `terminator` parameter have default values, -so you can omit them when you call this function. -By default, the function terminates the line it prints by adding a line break. -To print a value without a line break after it, -pass an empty string as the terminator --- for example, -`print(someValue, terminator: "")`. -For information about parameters with default values, -see . - - - - - - - -Swift uses *string interpolation* to include the name of a constant or variable -as a placeholder in a longer string, -and to prompt Swift to replace it with the current value of that constant or variable. -Wrap the name in parentheses and escape it with a backslash before the opening parenthesis: - -```swift -print("The current value of friendlyWelcome is \(friendlyWelcome)") -// Prints "The current value of friendlyWelcome is Bonjour!" -``` - - - -> Note: All options you can use with string interpolation -> are described in . - -## Comments - -Use comments to include nonexecutable text in your code, -as a note or reminder to yourself. -Comments are ignored by the Swift compiler when your code is compiled. - -Comments in Swift are very similar to comments in C. -Single-line comments begin with two forward-slashes (`//`): - -```swift -// This is a comment. -``` - - - -Multiline comments start with a forward-slash followed by an asterisk (`/*`) -and end with an asterisk followed by a forward-slash (`*/`): - -```swift -/* This is also a comment -but is written over multiple lines. */ -``` - - - -Unlike multiline comments in C, -multiline comments in Swift can be nested inside other multiline comments. -You write nested comments by starting a multiline comment block -and then starting a second multiline comment within the first block. -The second block is then closed, followed by the first block: - -```swift -/* This is the start of the first multiline comment. - /* This is the second, nested multiline comment. */ -This is the end of the first multiline comment. */ -``` - - - -Nested multiline comments enable you to comment out large blocks of code quickly and easily, -even if the code already contains multiline comments. - -## Semicolons - -Unlike many other languages, -Swift doesn't require you to write a semicolon (`;`) after each statement in your code, -although you can do so if you wish. -However, semicolons *are* required -if you want to write multiple separate statements on a single line: - -```swift -let cat = "🐱"; print(cat) -// Prints "🐱" -``` - - - -## Integers - -*Integers* are whole numbers with no fractional component, -such as `42` and `-23`. -Integers are either *signed* (positive, zero, or negative) -or *unsigned* (positive or zero). - -Swift provides signed and unsigned integers in 8, 16, 32, and 64 bit forms. -These integers follow a naming convention similar to C, -in that an 8-bit unsigned integer is of type `UInt8`, -and a 32-bit signed integer is of type `Int32`. -Like all types in Swift, these integer types have capitalized names. - -### Integer Bounds - -You can access the minimum and maximum values of each integer type -with its `min` and `max` properties: - -```swift -let minValue = UInt8.min // minValue is equal to 0, and is of type UInt8 -let maxValue = UInt8.max // maxValue is equal to 255, and is of type UInt8 -``` - - - -The values of these properties are of the appropriate-sized number type -(such as `UInt8` in the example above) -and can therefore be used in expressions alongside other values of the same type. - -### Int - -In most cases, you don't need to pick a specific size of integer to use in your code. -Swift provides an additional integer type, `Int`, -which has the same size as the current platform's native word size: - -- On a 32-bit platform, `Int` is the same size as `Int32`. -- On a 64-bit platform, `Int` is the same size as `Int64`. - -Unless you need to work with a specific size of integer, -always use `Int` for integer values in your code. -This aids code consistency and interoperability. -Even on 32-bit platforms, `Int` can store any value between `-2,147,483,648` and `2,147,483,647`, -and is large enough for many integer ranges. - -### UInt - -Swift also provides an unsigned integer type, `UInt`, -which has the same size as the current platform's native word size: - -- On a 32-bit platform, `UInt` is the same size as `UInt32`. -- On a 64-bit platform, `UInt` is the same size as `UInt64`. - -> Note: Use `UInt` only when you specifically need -> an unsigned integer type with the same size as the platform's native word size. -> If this isn't the case, `Int` is preferred, -> even when the values to be stored are known to be nonnegative. -> A consistent use of `Int` for integer values aids code interoperability, -> avoids the need to convert between different number types, -> and matches integer type inference, as described in . - -## Floating-Point Numbers - -*Floating-point numbers* are numbers with a fractional component, -such as `3.14159`, `0.1`, and `-273.15`. - -Floating-point types can represent a much wider range of values than integer types, -and can store numbers that are much larger or smaller than can be stored in an `Int`. -Swift provides two signed floating-point number types: - -- `Double` represents a 64-bit floating-point number. -- `Float` represents a 32-bit floating-point number. - -> Note: `Double` has a precision of at least 15 decimal digits, -> whereas the precision of `Float` can be as little as 6 decimal digits. -> The appropriate floating-point type to use depends on the nature and range of -> values you need to work with in your code. -> In situations where either type would be appropriate, `Double` is preferred. - - - - - -## Type Safety and Type Inference - -Swift is a *type-safe* language. -A type safe language encourages you to be clear about -the types of values your code can work with. -If part of your code requires a `String`, you can't pass it an `Int` by mistake. - -Because Swift is type safe, -it performs *type checks* when compiling your code -and flags any mismatched types as errors. -This enables you to catch and fix errors as early as possible in the development process. - -Type-checking helps you avoid errors when you're working with different types of values. -However, this doesn't mean that you have to specify the type of -every constant and variable that you declare. -If you don't specify the type of value you need, -Swift uses *type inference* to work out the appropriate type. -Type inference enables a compiler to -deduce the type of a particular expression automatically when it compiles your code, -simply by examining the values you provide. - -Because of type inference, Swift requires far fewer type declarations -than languages such as C or Objective-C. -Constants and variables are still explicitly typed, -but much of the work of specifying their type is done for you. - -Type inference is particularly useful -when you declare a constant or variable with an initial value. -This is often done by assigning a *literal value* (or *literal*) -to the constant or variable at the point that you declare it. -(A literal value is a value that appears directly in your source code, -such as `42` and `3.14159` in the examples below.) - -For example, if you assign a literal value of `42` to a new constant -without saying what type it is, -Swift infers that you want the constant to be an `Int`, -because you have initialized it with a number that looks like an integer: - -```swift -let meaningOfLife = 42 -// meaningOfLife is inferred to be of type Int -``` - - - -Likewise, if you don't specify a type for a floating-point literal, -Swift infers that you want to create a `Double`: - -```swift -let pi = 3.14159 -// pi is inferred to be of type Double -``` - - - -Swift always chooses `Double` (rather than `Float`) -when inferring the type of floating-point numbers. - -If you combine integer and floating-point literals in an expression, -a type of `Double` will be inferred from the context: - -```swift -let anotherPi = 3 + 0.14159 -// anotherPi is also inferred to be of type Double -``` - - - -The literal value of `3` has no explicit type in and of itself, -and so an appropriate output type of `Double` is inferred -from the presence of a floating-point literal as part of the addition. - -## Numeric Literals - -Integer literals can be written as: - -- A *decimal* number, with no prefix -- A *binary* number, with a `0b` prefix -- An *octal* number, with a `0o` prefix -- A *hexadecimal* number, with a `0x` prefix - -All of these integer literals have a decimal value of `17`: - -```swift -let decimalInteger = 17 -let binaryInteger = 0b10001 // 17 in binary notation -let octalInteger = 0o21 // 17 in octal notation -let hexadecimalInteger = 0x11 // 17 in hexadecimal notation -``` - - - -Floating-point literals can be decimal (with no prefix), -or hexadecimal (with a `0x` prefix). -They must always have a number (or hexadecimal number) on both sides of the decimal point. -Decimal floats can also have an optional *exponent*, -indicated by an uppercase or lowercase `e`; -hexadecimal floats must have an exponent, -indicated by an uppercase or lowercase `p`. - - - - - -For decimal numbers with an exponent of `x`, -the base number is multiplied by 10ˣ: - -- `1.25e2` means 1.25 x 10², or `125.0`. -- `1.25e-2` means 1.25 x 10⁻², or `0.0125`. - -For hexadecimal numbers with an exponent of `x`, -the base number is multiplied by 2ˣ: - -- `0xFp2` means 15 x 2², or `60.0`. -- `0xFp-2` means 15 x 2⁻², or `3.75`. - -All of these floating-point literals have a decimal value of `12.1875`: - -```swift -let decimalDouble = 12.1875 -let exponentDouble = 1.21875e1 -let hexadecimalDouble = 0xC.3p0 -``` - - - -Numeric literals can contain extra formatting to make them easier to read. -Both integers and floats can be padded with extra zeros -and can contain underscores to help with readability. -Neither type of formatting affects the underlying value of the literal: - -```swift -let paddedDouble = 000123.456 -let oneMillion = 1_000_000 -let justOverOneMillion = 1_000_000.000_000_1 -``` - - - -## Numeric Type Conversion - -Use the `Int` type for all general-purpose integer constants and variables in your code, -even if they're known to be nonnegative. -Using the default integer type in everyday situations means that -integer constants and variables are immediately interoperable in your code -and will match the inferred type for integer literal values. - -Use other integer types only when they're specifically needed for the task at hand, -because of explicitly sized data from an external source, -or for performance, memory usage, or other necessary optimization. -Using explicitly sized types in these situations -helps to catch any accidental value overflows -and implicitly documents the nature of the data being used. - -### Integer Conversion - -The range of numbers that can be stored in an integer constant or variable -is different for each numeric type. -An `Int8` constant or variable can store numbers between `-128` and `127`, -whereas a `UInt8` constant or variable can store numbers between `0` and `255`. -A number that won't fit into a constant or variable of a sized integer type -is reported as an error when your code is compiled: - -```swift -let cannotBeNegative: UInt8 = -1 -// UInt8 can't store negative numbers, and so this will report an error -let tooBig: Int8 = Int8.max + 1 -// Int8 can't store a number larger than its maximum value, -// and so this will also report an error -``` - - - -Because each numeric type can store a different range of values, -you must opt in to numeric type conversion on a case-by-case basis. -This opt-in approach prevents hidden conversion errors -and helps make type conversion intentions explicit in your code. - -To convert one specific number type to another, -you initialize a new number of the desired type with the existing value. -In the example below, -the constant `twoThousand` is of type `UInt16`, -whereas the constant `one` is of type `UInt8`. -They can't be added together directly, -because they're not of the same type. -Instead, this example calls `UInt16(one)` to create -a new `UInt16` initialized with the value of `one`, -and uses this value in place of the original: - -```swift -let twoThousand: UInt16 = 2_000 -let one: UInt8 = 1 -let twoThousandAndOne = twoThousand + UInt16(one) -``` - - - -Because both sides of the addition are now of type `UInt16`, -the addition is allowed. -The output constant (`twoThousandAndOne`) is inferred to be of type `UInt16`, -because it's the sum of two `UInt16` values. - -`SomeType(ofInitialValue)` is the default way to call the initializer of a Swift type -and pass in an initial value. -Behind the scenes, `UInt16` has an initializer that accepts a `UInt8` value, -and so this initializer is used to make a new `UInt16` from an existing `UInt8`. -You can't pass in *any* type here, however --- -it has to be a type for which `UInt16` provides an initializer. -Extending existing types to provide initializers that accept new types -(including your own type definitions) -is covered in . - -### Integer and Floating-Point Conversion - -Conversions between integer and floating-point numeric types must be made explicit: - -```swift -let three = 3 -let pointOneFourOneFiveNine = 0.14159 -let pi = Double(three) + pointOneFourOneFiveNine -// pi equals 3.14159, and is inferred to be of type Double -``` - - - -Here, the value of the constant `three` is used to create a new value of type `Double`, -so that both sides of the addition are of the same type. -Without this conversion in place, the addition would not be allowed. - -Floating-point to integer conversion must also be made explicit. -An integer type can be initialized with a `Double` or `Float` value: - -```swift -let integerPi = Int(pi) -// integerPi equals 3, and is inferred to be of type Int -``` - - - -Floating-point values are always truncated when used to initialize a new integer value in this way. -This means that `4.75` becomes `4`, and `-3.9` becomes `-3`. - -> Note: The rules for combining numeric constants and variables are different from -> the rules for numeric literals. -> The literal value `3` can be added directly to the literal value `0.14159`, -> because number literals don't have an explicit type in and of themselves. -> Their type is inferred only at the point that they're evaluated by the compiler. - - - -## Type Aliases - -*Type aliases* define an alternative name for an existing type. -You define type aliases with the `typealias` keyword. - -Type aliases are useful when you want to refer to an existing type -by a name that's contextually more appropriate, -such as when working with data of a specific size from an external source: - -```swift -typealias AudioSample = UInt16 -``` - - - -Once you define a type alias, -you can use the alias anywhere you might use the original name: - -```swift -var maxAmplitudeFound = AudioSample.min -// maxAmplitudeFound is now 0 -``` - - - -Here, `AudioSample` is defined as an alias for `UInt16`. -Because it's an alias, -the call to `AudioSample.min` actually calls `UInt16.min`, -which provides an initial value of `0` for the `maxAmplitudeFound` variable. - -## Booleans - -Swift has a basic *Boolean* type, called `Bool`. -Boolean values are referred to as *logical*, -because they can only ever be true or false. -Swift provides two Boolean constant values, -`true` and `false`: - -```swift -let orangesAreOrange = true -let turnipsAreDelicious = false -``` - - - -The types of `orangesAreOrange` and `turnipsAreDelicious` -have been inferred as `Bool` from the fact that -they were initialized with Boolean literal values. -As with `Int` and `Double` above, -you don't need to declare constants or variables as `Bool` -if you set them to `true` or `false` as soon as you create them. -Type inference helps make Swift code more concise and readable -when it initializes constants or variables with other values whose type is already known. - -Boolean values are particularly useful when you work with conditional statements -such as the `if` statement: - -```swift -if turnipsAreDelicious { - print("Mmm, tasty turnips!") -} else { - print("Eww, turnips are horrible.") -} -// Prints "Eww, turnips are horrible." -``` - - - -Conditional statements such as the `if` statement are covered in more detail in . - -Swift's type safety prevents non-Boolean values from being substituted for `Bool`. -The following example reports a compile-time error: - -```swift -let i = 1 -if i { - // this example will not compile, and will report an error -} -``` - - - -However, the alternative example below is valid: - -```swift -let i = 1 -if i == 1 { - // this example will compile successfully -} -``` - - - -The result of the `i == 1` comparison is of type `Bool`, -and so this second example passes the type-check. -Comparisons like `i == 1` are discussed in . - -As with other examples of type safety in Swift, -this approach avoids accidental errors -and ensures that the intention of a particular section of code is always clear. - -## Tuples - -*Tuples* group multiple values into a single compound value. -The values within a tuple can be of any type -and don't have to be of the same type as each other. - -In this example, `(404, "Not Found")` is a tuple that describes an *HTTP status code*. -An HTTP status code is a special value returned by a web server whenever you request a web page. -A status code of `404 Not Found` is returned if you request a webpage that doesn't exist. - -```swift -let http404Error = (404, "Not Found") -// http404Error is of type (Int, String), and equals (404, "Not Found") -``` - - - -The `(404, "Not Found")` tuple groups together an `Int` and a `String` -to give the HTTP status code two separate values: -a number and a human-readable description. -It can be described as “a tuple of type `(Int, String)`”. - -You can create tuples from any permutation of types, -and they can contain as many different types as you like. -There's nothing stopping you from having -a tuple of type `(Int, Int, Int)`, or `(String, Bool)`, -or indeed any other permutation you require. - -You can *decompose* a tuple's contents into separate constants or variables, -which you then access as usual: - -```swift -let (statusCode, statusMessage) = http404Error -print("The status code is \(statusCode)") -// Prints "The status code is 404" -print("The status message is \(statusMessage)") -// Prints "The status message is Not Found" -``` - - - -If you only need some of the tuple's values, -ignore parts of the tuple with an underscore (`_`) -when you decompose the tuple: - -```swift -let (justTheStatusCode, _) = http404Error -print("The status code is \(justTheStatusCode)") -// Prints "The status code is 404" -``` - - - -Alternatively, -access the individual element values in a tuple using index numbers starting at zero: - -```swift -print("The status code is \(http404Error.0)") -// Prints "The status code is 404" -print("The status message is \(http404Error.1)") -// Prints "The status message is Not Found" -``` - - - -You can name the individual elements in a tuple when the tuple is defined: - -```swift -let http200Status = (statusCode: 200, description: "OK") -``` - - - -If you name the elements in a tuple, -you can use the element names to access the values of those elements: - -```swift -print("The status code is \(http200Status.statusCode)") -// Prints "The status code is 200" -print("The status message is \(http200Status.description)") -// Prints "The status message is OK" -``` - - - -Tuples are particularly useful as the return values of functions. -A function that tries to retrieve a web page might return the `(Int, String)` tuple type -to describe the success or failure of the page retrieval. -By returning a tuple with two distinct values, -each of a different type, -the function provides more useful information about its outcome -than if it could only return a single value of a single type. -For more information, see . - -> Note: Tuples are useful for simple groups of related values. -> They're not suited to the creation of complex data structures. -> If your data structure is likely to be more complex, -> model it as a class or structure, rather than as a tuple. -> For more information, see . - -## Optionals - -You use *optionals* in situations where a value may be absent. -An optional represents two possibilities: -Either there *is* a value of a specified type, -and you can unwrap the optional to access that value, -or there *isn't* a value at all. - -As an example of a value that might be missing, -Swift's `Int` type has an initializer -that tries to convert a `String` value into an `Int` value. -However, only some strings can be converted into integers. -The string `"123"` can be converted into the numeric value `123`, -but the string `"hello, world"` doesn't have a corresponding numeric value. -The example below uses the initializer to try to convert a `String` into an `Int`: - -```swift -let possibleNumber = "123" -let convertedNumber = Int(possibleNumber) -// The type of convertedNumber is "optional Int" -``` - - - -Because the initializer in the code above might fail, -it returns an *optional* `Int`, rather than an `Int`. - -To write an optional type, -you write a question mark (`?`) -after the name of the type that the optional contains --- -for example, the type of an optional `Int` is `Int?`. -An optional `Int` always contains -either some `Int` value or no value at all. -It can't contain anything else, like a `Bool` or `String` value. - -### nil - -You set an optional variable to a valueless state -by assigning it the special value `nil`: - -```swift -var serverResponseCode: Int? = 404 -// serverResponseCode contains an actual Int value of 404 -serverResponseCode = nil -// serverResponseCode now contains no value -``` - - - -If you define an optional variable without providing a default value, -the variable is automatically set to `nil`: - -```swift -var surveyAnswer: String? -// surveyAnswer is automatically set to nil -``` - - - -You can use an `if` statement to find out whether an optional contains a value -by comparing the optional against `nil`. -You perform this comparison with the “equal to” operator (`==`) -or the “not equal to” operator (`!=`). - -If an optional has a value, it's considered as “not equal to” `nil`: - -```swift -let possibleNumber = "123" -let convertedNumber = Int(possibleNumber) - -if convertedNumber != nil { - print("convertedNumber contains some integer value.") -} -// Prints "convertedNumber contains some integer value." -``` - - - -You can't use `nil` with non-optional constants or variables. -If a constant or variable in your code needs to work with -the absence of a value under certain conditions, -declare it as an optional value of the appropriate type. -A constant or variable that's declared as a non-optional value -is guaranteed to never contain a `nil` value. -If you try to assign `nil` to a non-optional value, -you'll get a compile-time error. - -This separation of optional and non-optional values -lets you explicitly mark what information can be missing, -and makes it easier to write code that handle missing values. -You can't accidentally treat an optional as if it were non-optional -because this mistake produces an error at compile time. -After you unwrap the value, -none of the other code that works with that value needs to check for `nil`, -so there's no need to repeatedly check the same value -in different parts of your code. - -When you access an optional value, -your code always handles both the `nil` and non-`nil` case. -There are several things you can do when a value is missing, -as described in the following sections: - -- Skip the code that operates on the value when it's `nil`. - -- Propagate the `nil` value, - by returning `nil` - or using the `?.` operator described in . - -- Provide a fallback value, using the `??` operator. - -- Stop program execution, using the `!` operator. - -> Note: -> In Objective-C, `nil` is a pointer to a nonexistent object. -> In Swift, `nil` isn't a pointer --- it's the absence of a value of a certain type. -> Optionals of *any* type can be set to `nil`, not just object types. - -### Optional Binding - -You use optional binding to find out whether an optional contains a value, -and if so, to make that value available as a temporary constant or variable. -Optional binding can be used with `if`, `guard`, and `while` statements -to check for a value inside an optional, -and to extract that value into a constant or variable, -as part of a single action. -For more information about `if`, `guard`, and `while` statements, -see . - -Write an optional binding for an `if` statement as follows: - -```swift -if let <#constantName#> = <#someOptional#> { - <#statements#> -} -``` - -You can rewrite the `possibleNumber` example from -the section -to use optional binding rather than forced unwrapping: - -```swift -if let actualNumber = Int(possibleNumber) { - print("The string \"\(possibleNumber)\" has an integer value of \(actualNumber)") -} else { - print("The string \"\(possibleNumber)\" couldn't be converted to an integer") -} -// Prints "The string "123" has an integer value of 123" -``` - - - -This code can be read as: - -“If the optional `Int` returned by `Int(possibleNumber)` contains a value, -set a new constant called `actualNumber` to the value contained in the optional.” - -If the conversion is successful, -the `actualNumber` constant becomes available for use within -the first branch of the `if` statement. -It has already been initialized with the value contained within the optional, -and has the corresponding non-optional type. -In this case, the type of `possibleNumber` is `Int?`, -so the type of `actualNumber` is `Int`. - -If you don't need to refer to the original, optional constant or variable -after accessing the value it contains, -you can use the same name for the new constant or variable: - -```swift -let myNumber = Int(possibleNumber) -// Here, myNumber is an optional integer -if let myNumber = myNumber { - // Here, myNumber is a non-optional integer - print("My number is \(myNumber)") -} -// Prints "My number is 123" -``` - - - -This code starts by checking whether `myNumber` contains a value, -just like the code in the previous example. -If `myNumber` has a value, -the value of a new constant named `myNumber` is set to that value. -Inside the body of the `if` statement, -writing `myNumber` refers to that new non-optional constant. -Writing `myNumber` before or after the `if` statement -refers to the original optional integer constant. - -Because this kind of code is so common, -you can use a shorter spelling to unwrap an optional value: -Write just the name of the constant or variable that you're unwrapping. -The new, unwrapped constant or variable -implicitly uses the same name as the optional value. - -```swift -if let myNumber { - print("My number is \(myNumber)") -} -// Prints "My number is 123" -``` - - - -You can use both constants and variables with optional binding. -If you wanted to manipulate the value of `myNumber` -within the first branch of the `if` statement, -you could write `if var myNumber` instead, -and the value contained within the optional -would be made available as a variable rather than a constant. -Changes you make to `myNumber` inside the body of the `if` statement -apply only to that local variable, -*not* to the original, optional constant or variable that you unwrapped. - -You can include as many optional bindings and Boolean conditions -in a single `if` statement as you need to, -separated by commas. -If any of the values in the optional bindings are `nil` -or any Boolean condition evaluates to `false`, -the whole `if` statement's condition -is considered to be `false`. -The following `if` statements are equivalent: - -```swift -if let firstNumber = Int("4"), let secondNumber = Int("42"), firstNumber < secondNumber && secondNumber < 100 { - print("\(firstNumber) < \(secondNumber) < 100") -} -// Prints "4 < 42 < 100" - -if let firstNumber = Int("4") { - if let secondNumber = Int("42") { - if firstNumber < secondNumber && secondNumber < 100 { - print("\(firstNumber) < \(secondNumber) < 100") - } - } -} -// Prints "4 < 42 < 100" -``` - - - - - -Constants and variables created with optional binding in an `if` statement -are available only within the body of the `if` statement. -In contrast, the constants and variables created with a `guard` statement -are available in the lines of code that follow the `guard` statement, -as described in . - -### Providing a Fallback Value - -Another way to handle a missing value is to supply -a default value using the nil-coalescing operator (`??`). -If the optional on the left of the `??` isn't `nil`, -that value is unwrapped and used. -Otherwise, the value on the right of `??` is used. -For example, -the code below greets someone by name if one is specified, -and uses a generic greeting when the name is `nil`. - -```swift -let name: String? = nil -let greeting = "Hello, " + (name ?? "friend") + "!" -print(greeting) -// Prints "Hello, friend!" -``` - - - -For more information about using `??` to provide a fallback value, -see . - -### Force Unwrapping - -When `nil` represents an unrecoverable failure, -such as a programmer error or corrupted state, -you can access the underlying value -by adding an exclamation mark (`!`) to the end of the optional's name. -This is known as *force unwrapping* the optional's value. -When you force unwrap a non-`nil` value, -the result is its unwrapped value. -Force unwrapping a `nil` value triggers a runtime error. - -The `!` is, effectively, a shorter spelling of [`fatalError(_:file:line:)`][]. -For example, the code below shows two equivalent approaches: - -[`fatalError(_:file:line:)`]: https://developer.apple.com/documentation/swift/fatalerror(_:file:line:) - -```swift -let possibleNumber = "123" -let convertedNumber = Int(possibleNumber) - -let number = convertedNumber! - -guard let number = convertedNumber else { - fatalError("The number was invalid") -} -``` - -Both versions of the code above depend on `convertedNumber` -always containing a value. -Writing that requirement as part of the code, -using either of the approaches above, -lets your code check that the requirement is true at runtime. - -For more information about enforcing data requirements -and checking assumptions at runtime, -see . - -### Implicitly Unwrapped Optionals - -As described above, -optionals indicate that a constant or variable is allowed to have “no value”. -Optionals can be checked with an `if` statement to see if a value exists, -and can be conditionally unwrapped with optional binding -to access the optional's value if it does exist. - -Sometimes it's clear from a program's structure that an optional will *always* have a value, -after that value is first set. -In these cases, it's useful to remove the need -to check and unwrap the optional's value every time it's accessed, -because it can be safely assumed to have a value all of the time. - -These kinds of optionals are defined as *implicitly unwrapped optionals*. -You write an implicitly unwrapped optional by placing an exclamation point (`String!`) -rather than a question mark (`String?`) after the type that you want to make optional. -Rather than placing an exclamation point after the optional's name when you use it, -you place an exclamation point after the optional's type when you declare it. - -Implicitly unwrapped optionals are useful when -an optional's value is confirmed to exist immediately after the optional is first defined -and can definitely be assumed to exist at every point thereafter. -The primary use of implicitly unwrapped optionals in Swift is during class initialization, -as described in . - -Don't use an implicitly unwrapped optional when there's a possibility of -a variable becoming `nil` at a later point. -Always use a normal optional type if you need to check for a `nil` value -during the lifetime of a variable. - -An implicitly unwrapped optional is a normal optional behind the scenes, -but can also be used like a non-optional value, -without the need to unwrap the optional value each time it's accessed. -The following example shows the difference in behavior between -an optional string and an implicitly unwrapped optional string -when accessing their wrapped value as an explicit `String`: - -```swift -let possibleString: String? = "An optional string." -let forcedString: String = possibleString! // Requires explicit unwrapping - -let assumedString: String! = "An implicitly unwrapped optional string." -let implicitString: String = assumedString // Unwrapped automatically -``` - - - -You can think of an implicitly unwrapped optional as -giving permission for the optional to be force-unwrapped if needed. -When you use an implicitly unwrapped optional value, -Swift first tries to use it as an ordinary optional value; -if it can't be used as an optional, Swift force-unwraps the value. -In the code above, -the optional value `assumedString` is force-unwrapped -before assigning its value to `implicitString` -because `implicitString` has an explicit, non-optional type of `String`. -In code below, -`optionalString` doesn't have an explicit type -so it's an ordinary optional. - -```swift -let optionalString = assumedString -// The type of optionalString is "String?" and assumedString isn't force-unwrapped. -``` - - - -If an implicitly unwrapped optional is `nil` and you try to access its wrapped value, -you'll trigger a runtime error. -The result is exactly the same as if you write an exclamation point -to force unwrap a normal optional that doesn't contain a value. - -You can check whether an implicitly unwrapped optional is `nil` -the same way you check a normal optional: - -```swift -if assumedString != nil { - print(assumedString!) -} -// Prints "An implicitly unwrapped optional string." -``` - - - -You can also use an implicitly unwrapped optional with optional binding, -to check and unwrap its value in a single statement: - -```swift -if let definiteString = assumedString { - print(definiteString) -} -// Prints "An implicitly unwrapped optional string." -``` - - - -## Error Handling - -You use *error handling* to respond to error conditions -your program may encounter during execution. - -In contrast to optionals, -which can use the presence or absence of a value -to communicate success or failure of a function, -error handling allows you to determine the underlying cause of failure, -and, if necessary, propagate the error to another part of your program. - -When a function encounters an error condition, it *throws* an error. -That function's caller can then *catch* the error and respond appropriately. - -```swift -func canThrowAnError() throws { - // this function may or may not throw an error -} -``` - - - -A function indicates that it can throw an error -by including the `throws` keyword in its declaration. -When you call a function that can throw an error, -you prepend the `try` keyword to the expression. - -Swift automatically propagates errors out of their current scope -until they're handled by a `catch` clause. - -```swift -do { - try canThrowAnError() - // no error was thrown -} catch { - // an error was thrown -} -``` - - - -A `do` statement creates a new containing scope, -which allows errors to be propagated to one or more `catch` clauses. - -Here's an example of how error handling can be used -to respond to different error conditions: - -```swift -func makeASandwich() throws { - // ... -} - -do { - try makeASandwich() - eatASandwich() -} catch SandwichError.outOfCleanDishes { - washDishes() -} catch SandwichError.missingIngredients(let ingredients) { - buyGroceries(ingredients) -} -``` - - - -In this example, the `makeASandwich()` function will throw an error -if no clean dishes are available -or if any ingredients are missing. -Because `makeASandwich()` can throw an error, -the function call is wrapped in a `try` expression. -By wrapping the function call in a `do` statement, -any errors that are thrown will be propagated -to the provided `catch` clauses. - -If no error is thrown, the `eatASandwich()` function is called. -If an error is thrown and it matches the `SandwichError.outOfCleanDishes` case, -then the `washDishes()` function will be called. -If an error is thrown and it matches the `SandwichError.missingIngredients` case, -then the `buyGroceries(_:)` function is called -with the associated `[String]` value captured by the `catch` pattern. - -Throwing, catching, and propagating errors is covered in greater detail in -. - -## Assertions and Preconditions - -*Assertions* and *preconditions* -are checks that happen at runtime. -You use them to make sure an essential condition is satisfied -before executing any further code. -If the Boolean condition in the assertion or precondition -evaluates to `true`, -code execution continues as usual. -If the condition evaluates to `false`, -the current state of the program is invalid; -code execution ends, and your app is terminated. - -You use assertions and preconditions -to express the assumptions you make -and the expectations you have -while coding, -so you can include them as part of your code. -Assertions help you find mistakes and incorrect assumptions during development, -and preconditions help you detect issues in production. - -In addition to verifying your expectations at runtime, -assertions and preconditions also become a useful form of documentation -within the code. -Unlike the error conditions discussed in above, -assertions and preconditions aren't used -for recoverable or expected errors. -Because a failed assertion or precondition -indicates an invalid program state, -there's no way to catch a failed assertion. -Recovering from an invalid state is impossible. -When an assertion fails, -at least one piece of the program's data is invalid --- -but you don't know why it's invalid -or whether an additional state is also invalid. - -Using assertions and preconditions -isn't a substitute for designing your code in such a way -that invalid conditions are unlikely to arise. -However, -using them to enforce valid data and state -causes your app to terminate more predictably -if an invalid state occurs, -and helps make the problem easier to debug. -When assumptions aren't checked, -you might not notice this kind problem until much later -when code elsewhere starts failing visibly, -and after user data has been silently corrupted. -Stopping execution as soon as an invalid state is detected -also helps limit the damage caused by that invalid state. - -The difference between assertions and preconditions is in when they're checked: -Assertions are checked only in debug builds, -but preconditions are checked in both debug and production builds. -In production builds, -the condition inside an assertion isn't evaluated. -This means you can use as many assertions as you want -during your development process, -without impacting performance in production. - -### Debugging with Assertions - - - -You write an assertion by calling the -[`assert(_:_:file:line:)`](https://developer.apple.com/documentation/swift/1541112-assert) function -from the Swift standard library. -You pass this function an expression that evaluates to `true` or `false` -and a message to display if the result of the condition is `false`. -For example: - -```swift -let age = -3 -assert(age >= 0, "A person's age can't be less than zero.") -// This assertion fails because -3 isn't >= 0. -``` - - - -In this example, code execution continues if `age >= 0` evaluates to `true`, -that is, if the value of `age` is nonnegative. -If the value of `age` is negative, as in the code above, -then `age >= 0` evaluates to `false`, -and the assertion fails, terminating the application. - -You can omit the assertion message --- -for example, when it would just repeat the condition as prose. - -```swift -assert(age >= 0) -``` - - - - - -If the code already checks the condition, -you use the -[`assertionFailure(_:file:line:)`](https://developer.apple.com/documentation/swift/1539616-assertionfailure) function -to indicate that an assertion has failed. -For example: - -```swift -if age > 10 { - print("You can ride the roller-coaster or the ferris wheel.") -} else if age >= 0 { - print("You can ride the ferris wheel.") -} else { - assertionFailure("A person's age can't be less than zero.") -} -``` - - - -### Enforcing Preconditions - -Use a precondition whenever a condition has the potential to be false, -but must *definitely* be true for your code to continue execution. -For example, use a precondition to check that a subscript isn't out of bounds, -or to check that a function has been passed a valid value. - -You write a precondition by calling the -[`precondition(_:_:file:line:)`](https://developer.apple.com/documentation/swift/1540960-precondition) function. -You pass this function an expression that evaluates to `true` or `false` -and a message to display if the result of the condition is `false`. -For example: - -```swift -// In the implementation of a subscript... -precondition(index > 0, "Index must be greater than zero.") -``` - - - -You can also call the -[`preconditionFailure(_:file:line:)`](https://developer.apple.com/documentation/swift/1539374-preconditionfailure) function -to indicate that a failure has occurred --- -for example, if the default case of a switch was taken, -but all valid input data should have been handled -by one of the switch's other cases. - -> Note: If you compile in unchecked mode (`-Ounchecked`), -> preconditions aren't checked. -> The compiler assumes that preconditions are always true, -> and it optimizes your code accordingly. -> However, the `fatalError(_:file:line:)` function always halts execution, -> regardless of optimization settings. -> -> You can use the `fatalError(_:file:line:)` function -> during prototyping and early development -> to create stubs for functionality that hasn't been implemented yet, -> by writing `fatalError("Unimplemented")` as the stub implementation. -> Because fatal errors are never optimized out, -> unlike assertions or preconditions, -> you can be sure that execution always halts -> if it encounters a stub implementation. - - - - - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/ReferenceManual/AboutTheLanguageReference.md b/swift-6-beta.docc/ReferenceManual/AboutTheLanguageReference.md deleted file mode 100644 index acb552346..000000000 --- a/swift-6-beta.docc/ReferenceManual/AboutTheLanguageReference.md +++ /dev/null @@ -1,65 +0,0 @@ -# About the Language Reference - -Read the notation that the formal grammar uses. - -This part of the book describes the formal grammar of the Swift programming language. -The grammar described here is intended to help you understand the language in more -detail, rather than to allow you to directly implement a parser or compiler. - -The Swift language is relatively small, because many common types, functions, and operators -that appear virtually everywhere in Swift code -are actually defined in the Swift standard library. Although these types, functions, -and operators aren't part of the Swift language itself, -they're used extensively in the discussions and code examples in this part of the book. - -## How to Read the Grammar - -The notation used to describe the formal grammar of the Swift programming language -follows a few conventions: - -- An arrow (→) is used to mark grammar productions and can be read as "can consist of." -- Syntactic categories are indicated by *italic* text and appear on both sides - of a grammar production rule. -- Literal words and punctuation are indicated by **`boldface constant width`** text - and appear only on the right-hand side of a grammar production rule. -- Alternative grammar productions are separated by vertical - bars (|). When alternative productions are too long to read easily, - they're broken into multiple grammar production rules on new lines. -- In a few cases, regular font text is used to describe the right-hand side - of a grammar production rule. -- Optional syntactic categories and literals are marked by a trailing - question mark, *?*. - -As an example, the grammar of a getter-setter block is defined as follows: - -> Grammar of a getter-setter block: -> -> *getter-setter-block* → **`{`** *getter-clause* *setter-clause*_?_ **`}`** | **`{`** *setter-clause* *getter-clause* **`}`** - -This definition indicates that a getter-setter block can consist of a getter clause -followed by an optional setter clause, enclosed in braces, -*or* a setter clause followed by a getter clause, enclosed in braces. -The grammar production above is equivalent to the following two productions, -where the alternatives are spelled out explicitly: - -> Grammar of a getter-setter block: -> -> -> *getter-setter-block* → **`{`** *getter-clause* *setter-clause*_?_ **`}`** \ -> *getter-setter-block* → **`{`** *setter-clause* *getter-clause* **`}`** - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/ReferenceManual/Attributes.md b/swift-6-beta.docc/ReferenceManual/Attributes.md deleted file mode 100644 index c0689c769..000000000 --- a/swift-6-beta.docc/ReferenceManual/Attributes.md +++ /dev/null @@ -1,2642 +0,0 @@ -# Attributes - -Add information to declarations and types. - -There are two kinds of attributes in Swift --- -those that apply to declarations and those that apply to types. -An attribute provides additional information about the declaration or type. -For example, -the `discardableResult` attribute on a function declaration indicates that, -although the function returns a value, -the compiler shouldn't generate a warning if the return value is unused. - -You specify an attribute by writing the `@` symbol followed by the attribute's name -and any arguments that the attribute accepts: - -```swift -@<#attribute name#> -@<#attribute name#>(<#attribute arguments#>) -``` - -Some declaration attributes accept arguments -that specify more information about the attribute -and how it applies to a particular declaration. -These *attribute arguments* are enclosed in parentheses, -and their format is defined by the attribute they belong to. - -Attached macros and property wrappers also use attribute syntax. -For information about how macros expand, -see . -For information about property wrappers, -see . - -## Declaration Attributes - -You can apply a declaration attribute to declarations only. - -### attached - -Apply the `attached` attribute to a macro declaration. -The arguments to this attribute indicate the macro's role. -For a macro that has multiple roles, -apply the `attached` macro multiple times, once for each role. - - - -The first argument to this attribute -indicates the macros role: - -- term Peer macros: - Write `peer` as the first argument to this attribute. - The type that implements the macro conforms to the `PeerMacro` protocol. - These macros produce new declarations - in the same scope as the declaration - that the macro is attached to. - For example, - applying a peer macro to a method of a structure - can define additional methods and properties on that structure. - -- term Member macros: - Write `member` as the first argument to this attribute. - The type that implements the macro conforms to the `MemberMacro` protocol. - These macros produce new declarations - that are members of the type or extension - that the macro is attached to. - For example, - applying a member macro to a structure declaration - can define additional methods and properties on that structure. - -- term Member attribute: - Write `memberAttribute` as the first argument to this attribute. - The type that implements the macro conforms to the `MemberAttributeMacro` protocol. - These macros add attributes to members of the type or extension - that the macro is attached to. - -- term Accessor macros: - Write `accessor` as the first argument to this attribute. - The type that implements the macro conforms to the `AccessorMacro` protocol. - These macros add accessors to the stored property they're attached to, - turning it into a computed property. - -- term Extension macros: - Write `extension` as the first argument to this attribute. - The type that implements the macro conforms to the `ExtensionMacro` protocol. - These macros can add protocol conformance, - a `where` clause, - and new declarations that are members of the type the macro is attached to. - If the macro adds protocol conformances, - include the `conformances:` argument and specify those protocols. - The conformance list contains protocol names, - type aliases that refer to conformance list items, - or protocol compositions of conformance list items. - An extension macro on a nested type - expands to an extension at the top level of that file. - You can't write an extension macro - on an extension, a type alias, or a type that's nested inside a function, - or use an extension macro to add an extension that has a peer macro. - -The peer, member, and accessor macro roles require a `names:` argument, -listing the names of the symbols that the macro generates. -The extension macro role also requires a `names:` argument -if the macro adds declarations inside the extension. -When a macro declaration includes the `names:` argument, -the macro implementation must generate -only symbol with names that match that list. -That said, -a macro need not generate a symbol for every listed name. -The value for that argument is a list of one or more of the following: - -- `named(<#name#>)` - where *name* is that fixed symbol name, - for a name that's known in advance. - -- `overloaded` - for a name that's the same as an existing symbol. - -- `prefixed(<#prefix#>)` - where *prefix* is prepended to the symbol name, - for a name that starts with a fixed string. - -- `suffixed(<#suffix#>)` - where *suffix* is appended to the symbol name, - for a name that ends with a fixed string. - -- `arbitrary` - for a name that can't be determined until macro expansion. - -As a special case, -you can write `prefixed($)` -for a macro that behaves similar to a property wrapper. - - -### available - -Apply this attribute to indicate a declaration's life cycle -relative to certain Swift language versions -or certain platforms and operating system versions. - -The `available` attribute always appears -with a list of two or more comma-separated attribute arguments. -These arguments begin with one of the following platform or language names: - -- `iOS` -- `iOSApplicationExtension` -- `macOS` -- `macOSApplicationExtension` -- `macCatalyst` -- `macCatalystApplicationExtension` -- `watchOS` -- `watchOSApplicationExtension` -- `tvOS` -- `tvOSApplicationExtension` -- `visionOS` -- `visionOSApplicationExtension` -- `swift` - - - - - -You can also use an asterisk (`*`) to indicate the -availability of the declaration on all of the platform names listed above. -An `available` attribute -that specifies availability using a Swift version number -can't use the asterisk. - -The remaining arguments can appear in any order -and specify additional information about the declaration's life cycle, -including important milestones. - -- The `unavailable` argument indicates that the declaration - isn't available on the specified platform. - This argument can't be used when specifying Swift version availability. -- The `introduced` argument indicates the first version - of the specified platform or language in which the declaration was introduced. - It has the following form: - - ```swift - introduced: <#version number#> - ``` - The *version number* consists of one to three positive integers, - separated by periods. -- The `deprecated` argument indicates the first version - of the specified platform or language in which the declaration was deprecated. - It has the following form: - - ```swift - deprecated: <#version number#> - ``` - The optional *version number* consists of one to three positive integers, - separated by periods. - Omitting the version number indicates that the declaration is currently deprecated, - without giving any information about when the deprecation occurred. - If you omit the version number, omit the colon (`:`) as well. -- The `obsoleted` argument indicates the first version - of the specified platform or language in which the declaration was obsoleted. - When a declaration is obsoleted, - it's removed from the specified platform or language and can no longer be used. - It has the following form: - - ```swift - obsoleted: <#version number#> - ``` - The *version number* consists of one to three positive integers, separated by periods. -- The `message` argument provides a textual message that the compiler displays - when emitting a warning or error about the use of a deprecated or obsoleted declaration. - It has the following form: - - ```swift - message: <#message#> - ``` - The *message* consists of a string literal. -- The `renamed` argument provides a textual message - that indicates the new name for a declaration that's been renamed. - The compiler displays the new name - when emitting an error about the use of a renamed declaration. - It has the following form: - - ```swift - renamed: <#new name#> - ``` - The *new name* consists of a string literal. - - You can apply the `available` attribute - with the `renamed` and `unavailable` arguments - to a type alias declaration, as shown below, - to indicate that the name of a declaration changed - between releases of a framework or library. - This combination results in a compile-time error - that the declaration has been renamed. - - ```swift - // First release - protocol MyProtocol { - // protocol definition - } - ``` - - - - - ```swift - // Subsequent release renames MyProtocol - protocol MyRenamedProtocol { - // protocol definition - } - - @available(*, unavailable, renamed: "MyRenamedProtocol") - typealias MyProtocol = MyRenamedProtocol - ``` - - - - -You can apply multiple `available` attributes on a single declaration -to specify the declaration's availability on different platforms -and different versions of Swift. -The declaration that the `available` attribute applies to -is ignored if the attribute specifies -a platform or language version that doesn't match the current target. -If you use multiple `available` attributes, -the effective availability is the combination of -the platform and Swift availabilities. - - - -If an `available` attribute only specifies an `introduced` argument -in addition to a platform or language name argument, -you can use the following shorthand syntax instead: - -```swift -@available(<#platform name#> <#version number#>, *) -@available(swift <#version number#>) -``` - -The shorthand syntax for `available` attributes -concisely expresses availability for multiple platforms. -Although the two forms are functionally equivalent, -the shorthand form is preferred whenever possible. - -```swift -@available(iOS 10.0, macOS 10.12, *) -class MyClass { - // class definition -} -``` - - - -An `available` attribute -that specifies availability using a Swift version number -can't additionally specify a declaration's platform availability. -Instead, use separate `available` attributes to specify a Swift -version availability and one or more platform availabilities. - -```swift -@available(swift 3.0.2) -@available(macOS 10.12, *) -struct MyStruct { - // struct definition -} -``` - - - -### backDeployed - -Apply this attribute to a function, method, subscript, or computed property -to include a copy of the symbol's implementation -in programs that call or access the symbol. -You use this attribute to annotate symbols that ship as part of a platform, -like the APIs that are included with an operating system. -This attribute marks symbols that can be made available retroactively -by including a copy of their implementation in programs that access them. -Copying the implementation is also known as *emitting into the client*. - -This attribute takes a `before:` argument, -specifying the first version of platforms that provide this symbol. -These platform versions have the same meaning -as the platform version you specify for the `available` attribute. -Unlike the `available` attribute, -the list can't contain an asterisk (`*`) to refer to all versions. -For example, consider the following code: - -```swift -@available(iOS 16, *) -@backDeployed(before: iOS 17) -func someFunction() { /* ... */ } -``` - -In the example above, -the iOS SDK provides `someFunction()` starting in iOS 17. -In addition, -the SDK makes `someFunction()` available on iOS 16 using back deployment. - -When compiling code that calls this function, -Swift inserts a layer of indirection that finds the function's implementation. -If the code is run using a version of the SDK that includes this function, -the SDK's implementation is used. -Otherwise, the copy included in the caller is used. -In the example above, -calling `someFunction()` uses the implementation from the SDK -when running on iOS 17 or later, -and when running on iOS 16 -it uses the copy of `someFunction()` that's included in the caller. - -> Note: -> When the caller's minimum deployment target -> is the same as or greater than -> the first version of the SDK that includes the symbol, -> the compiler can optimize away the runtime check -> and call the SDK's implementation directly. -> In this case, -> if you access the back-deployed symbol directly, -> the compiler can also omit -> the copy of the symbol's implementation from the client. - - - -Functions, methods, subscripts, and computed properties -that meet the following criteria can be back deployed: - -- The declaration is `public` or `@usableFromInline`. -- For class instance methods and class type methods, - the method is marked `final` and isn't marked `@objc`. -- The implementation satisfies the requirements for an inlinable function, - described in . - -### discardableResult - -Apply this attribute to a function or method declaration -to suppress the compiler warning -when the function or method that returns a value -is called without using its result. - -### dynamicCallable - -Apply this attribute to a class, structure, enumeration, or protocol -to treat instances of the type as callable functions. -The type must implement either a `dynamicallyCall(withArguments:)` method, -a `dynamicallyCall(withKeywordArguments:)` method, -or both. - -You can call an instance of a dynamically callable type -as if it's a function that takes any number of arguments. - -```swift -@dynamicCallable -struct TelephoneExchange { - func dynamicallyCall(withArguments phoneNumber: [Int]) { - if phoneNumber == [4, 1, 1] { - print("Get Swift help on forums.swift.org") - } else { - print("Unrecognized number") - } - } -} - -let dial = TelephoneExchange() - -// Use a dynamic method call. -dial(4, 1, 1) -// Prints "Get Swift help on forums.swift.org" - -dial(8, 6, 7, 5, 3, 0, 9) -// Prints "Unrecognized number" - -// Call the underlying method directly. -dial.dynamicallyCall(withArguments: [4, 1, 1]) -``` - - - -The declaration of the `dynamicallyCall(withArguments:)` method -must have a single parameter that conforms to the -[`ExpressibleByArrayLiteral`](https://developer.apple.com/documentation/swift/expressiblebyarrayliteral) -protocol --- like `[Int]` in the example above. -The return type can be any type. - -You can include labels in a dynamic method call -if you implement the `dynamicallyCall(withKeywordArguments:)` method. - -```swift -@dynamicCallable -struct Repeater { - func dynamicallyCall(withKeywordArguments pairs: KeyValuePairs) -> String { - return pairs - .map { label, count in - repeatElement(label, count: count).joined(separator: " ") - } - .joined(separator: "\n") - } -} - -let repeatLabels = Repeater() -print(repeatLabels(a: 1, b: 2, c: 3, b: 2, a: 1)) -// a -// b b -// c c c -// b b -// a -``` - - - -The declaration of the `dynamicallyCall(withKeywordArguments:)` method -must have a single parameter that conforms to the -[`ExpressibleByDictionaryLiteral`](https://developer.apple.com/documentation/swift/expressiblebydictionaryliteral) -protocol, -and the return type can be any type. -The parameter's [`Key`](https://developer.apple.com/documentation/swift/expressiblebydictionaryliteral/2294108-key) -must be -[`ExpressibleByStringLiteral`](https://developer.apple.com/documentation/swift/expressiblebystringliteral). -The previous example uses [`KeyValuePairs`](https://developer.apple.com/documentation/swift/keyvaluepairs) -as the parameter type -so that callers can include duplicate parameter labels --- -`a` and `b` appear multiple times in the call to `repeat`. - -If you implement both `dynamicallyCall` methods, -`dynamicallyCall(withKeywordArguments:)` is called -when the method call includes keyword arguments. -In all other cases, `dynamicallyCall(withArguments:)` is called. - -You can only call a dynamically callable instance -with arguments and a return value that match the types you specify -in one of your `dynamicallyCall` method implementations. -The call in the following example doesn't compile because -there isn't an implementation of `dynamicallyCall(withArguments:)` -that takes `KeyValuePairs`. - -```swift -repeatLabels(a: "four") // Error -``` - - - -### dynamicMemberLookup - -Apply this attribute to a class, structure, enumeration, or protocol -to enable members to be looked up by name at runtime. -The type must implement a `subscript(dynamicMember:)` subscript. - -In an explicit member expression, -if there isn't a corresponding declaration for the named member, -the expression is understood as a call to -the type's `subscript(dynamicMember:)` subscript, -passing information about the member as the argument. -The subscript can accept a parameter that's either a key path or a member name; -if you implement both subscripts, -the subscript that takes key path argument is used. - -An implementation of `subscript(dynamicMember:)` -can accept key paths using an argument of type -[`KeyPath`](https://developer.apple.com/documentation/swift/keypath), -[`WritableKeyPath`](https://developer.apple.com/documentation/swift/writablekeypath), -or [`ReferenceWritableKeyPath`](https://developer.apple.com/documentation/swift/referencewritablekeypath). -It can accept member names using an argument of a type that conforms to the -[`ExpressibleByStringLiteral`](https://developer.apple.com/documentation/swift/expressiblebystringliteral) protocol --- -in most cases, `String`. -The subscript's return type can be any type. - -Dynamic member lookup by member name -can be used to create a wrapper type around data -that can't be type checked at compile time, -such as when bridging data from other languages into Swift. -For example: - -```swift -@dynamicMemberLookup -struct DynamicStruct { - let dictionary = ["someDynamicMember": 325, - "someOtherMember": 787] - subscript(dynamicMember member: String) -> Int { - return dictionary[member] ?? 1054 - } -} -let s = DynamicStruct() - -// Use dynamic member lookup. -let dynamic = s.someDynamicMember -print(dynamic) -// Prints "325" - -// Call the underlying subscript directly. -let equivalent = s[dynamicMember: "someDynamicMember"] -print(dynamic == equivalent) -// Prints "true" -``` - - - -Dynamic member lookup by key path -can be used to implement a wrapper type -in a way that supports compile-time type checking. -For example: - -```swift -struct Point { var x, y: Int } - -@dynamicMemberLookup -struct PassthroughWrapper { - var value: Value - subscript(dynamicMember member: KeyPath) -> T { - get { return value[keyPath: member] } - } -} - -let point = Point(x: 381, y: 431) -let wrapper = PassthroughWrapper(value: point) -print(wrapper.x) -``` - - - -### freestanding - -Apply the `freestanding` attribute -to the declaration of a freestanding macro. - - - -### frozen - -Apply this attribute to a structure or enumeration declaration -to restrict the kinds of changes you can make to the type. -This attribute is allowed only when compiling in library evolution mode. -Future versions of the library can't change the declaration -by adding, removing, or reordering -an enumeration's cases -or a structure's stored instance properties. -These changes are allowed on nonfrozen types, -but they break ABI compatibility for frozen types. - -> Note: When the compiler isn't in library evolution mode, -> all structures and enumerations are implicitly frozen, -> and this attribute is ignored. - - - - - - - -In library evolution mode, -code that interacts with members of nonfrozen structures and enumerations -is compiled in a way that allows it to continue working without recompiling -even if a future version of the library -adds, removes, or reorders some of that type's members. -The compiler makes this possible using techniques like -looking up information at runtime -and adding a layer of indirection. -Marking a structure or enumeration as frozen -gives up this flexibility to gain performance: -Future versions of the library can make only limited changes to the type, -but the compiler can make additional optimizations -in code that interacts with the type's members. - -Frozen types, -the types of the stored properties of frozen structures, -and the associated values of frozen enumeration cases -must be public or marked with the `usableFromInline` attribute. -The properties of a frozen structure can't have property observers, -and expressions that provide the initial value for stored instance properties -must follow the same restrictions as inlinable functions, -as discussed in . - - - -To enable library evolution mode on the command line, -pass the `-enable-library-evolution` option to the Swift compiler. -To enable it in Xcode, -set the "Build Libraries for Distribution" build setting -(`BUILD_LIBRARY_FOR_DISTRIBUTION`) to Yes, -as described in [Xcode Help](https://help.apple.com/xcode/mac/current/#/dev04b3a04ba). - - - -A switch statement over a frozen enumeration doesn't require a `default` case, -as discussed in . -Including a `default` or `@unknown default` case -when switching over a frozen enumeration -produces a warning because that code is never executed. - - - - - - - -### GKInspectable - -Apply this attribute to expose a custom GameplayKit component property -to the SpriteKit editor UI. -Applying this attribute also implies the `objc` attribute. - - - -### inlinable - -Apply this attribute to a -function, method, computed property, subscript, -convenience initializer, or deinitializer declaration -to expose that declaration's implementation -as part of the module's public interface. -The compiler is allowed to replace calls to an inlinable symbol -with a copy of the symbol's implementation at the call site. - -Inlinable code -can interact with `open` and `public` symbols declared in any module, -and it can interact with `internal` symbols -declared in the same module -that are marked with the `usableFromInline` attribute. -Inlinable code can't interact with `private` or `fileprivate` symbols. - -This attribute can't be applied -to declarations that are nested inside functions -or to `fileprivate` or `private` declarations. -Functions and closures that are defined inside an inlinable function -are implicitly inlinable, -even though they can't be marked with this attribute. - - - - - - - -### main - -Apply this attribute to a structure, class, or enumeration declaration -to indicate that it contains the top-level entry point for program flow. -The type must provide a `main` type function -that doesn't take any arguments and returns `Void`. -For example: - -```swift -@main -struct MyTopLevel { - static func main() { - // Top-level code goes here - } -} -``` - - - -Another way to describe the requirements of the `main` attribute -is that the type you write this attribute on -must satisfy the same requirements -as types that conform to the following hypothetical protocol: - -```swift -protocol ProvidesMain { - static func main() throws -} -``` - - - -The Swift code you compile to make an executable -can contain at most one top-level entry point, -as discussed in . - - - - - - - -### nonobjc - -Apply this attribute to a -method, property, subscript, or initializer declaration -to suppress an implicit `objc` attribute. -The `nonobjc` attribute tells the compiler -to make the declaration unavailable in Objective-C code, -even though it's possible to represent it in Objective-C. - -Applying this attribute to an extension -has the same effect as -applying it to every member of that extension -that isn't explicitly marked with the `objc` attribute. - -You use the `nonobjc` attribute to resolve circularity -for bridging methods in a class marked with the `objc` attribute, -and to allow overloading of methods and initializers -in a class marked with the `objc` attribute. - -A method marked with the `nonobjc` attribute -can't override a method marked with the `objc` attribute. -However, a method marked with the `objc` attribute -can override a method marked with the `nonobjc` attribute. -Similarly, a method marked with the `nonobjc` attribute -can't satisfy a protocol requirement -for a method marked with the `objc` attribute. - -### NSApplicationMain - -> Deprecated: -> This attribute is deprecated; -> use the attribute instead. -> In Swift 6, -> using this attribute will be an error. - -Apply this attribute to a class -to indicate that it's the app delegate. -Using this attribute is equivalent to calling the -`NSApplicationMain(_:_:)` function. - -If you don't use this attribute, -supply a `main.swift` file with code at the top level -that calls the `NSApplicationMain(_:_:)` function as follows: - -```swift -import AppKit -NSApplicationMain(CommandLine.argc, CommandLine.unsafeArgv) -``` - - - -The Swift code you compile to make an executable -can contain at most one top-level entry point, -as discussed in . - -### NSCopying - -Apply this attribute to a stored variable property of a class. -This attribute causes the property's setter to be synthesized with a *copy* -of the property's value --- returned by the `copyWithZone(_:)` method --- instead of the -value of the property itself. -The type of the property must conform to the `NSCopying` protocol. - -The `NSCopying` attribute behaves in a way similar to the Objective-C `copy` -property attribute. - - - -### NSManaged - -Apply this attribute to an instance method or stored variable property -of a class that inherits from `NSManagedObject` -to indicate that Core Data dynamically provides its implementation at runtime, -based on the associated entity description. -For a property marked with the `NSManaged` attribute, -Core Data also provides the storage at runtime. -Applying this attribute also implies the `objc` attribute. - -### objc - -Apply this attribute to any declaration that can be represented in Objective-C --- -for example, nonnested classes, protocols, -nongeneric enumerations (constrained to integer raw-value types), -properties and methods (including getters and setters) of classes, -protocols and optional members of a protocol, -initializers, and subscripts. -The `objc` attribute tells the compiler -that a declaration is available to use in Objective-C code. - -Applying this attribute to an extension -has the same effect as -applying it to every member of that extension -that isn't explicitly marked with the `nonobjc` attribute. - -The compiler implicitly adds the `objc` attribute -to subclasses of any class defined in Objective-C. -However, the subclass must not be generic, -and must not inherit from any generic classes. -You can explicitly add the `objc` attribute -to a subclass that meets these criteria, -to specify its Objective-C name as discussed below. -Protocols that are marked with the `objc` attribute can't inherit -from protocols that aren't marked with this attribute. - -The `objc` attribute is also implicitly added in the following cases: - -- The declaration is an override in a subclass, - and the superclass's declaration has the `objc` attribute. -- The declaration satisfies a requirement - from a protocol that has the `objc` attribute. -- The declaration has the `IBAction`, `IBSegueAction`, `IBOutlet`, - `IBDesignable`, `IBInspectable`, - `NSManaged`, or `GKInspectable` attribute. - -If you apply the `objc` attribute to an enumeration, -each enumeration case is exposed to Objective-C code -as the concatenation of the enumeration name and the case name. -The first letter of the case name is capitalized. -For example, a case named `venus` in a Swift `Planet` enumeration -is exposed to Objective-C code as a case named `PlanetVenus`. - -The `objc` attribute optionally accepts a single attribute argument, -which consists of an identifier. -The identifier specifies the name to be exposed to Objective-C -for the entity that the `objc` attribute applies to. -You can use this argument to name -classes, enumerations, enumeration cases, protocols, -methods, getters, setters, and initializers. -If you specify the Objective-C name -for a class, protocol, or enumeration, -include a three-letter prefix on the name, -as described in [Conventions](https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/Conventions/Conventions.html#//apple_ref/doc/uid/TP40011210-CH10-SW1) -in [Programming with Objective-C](https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/Introduction/Introduction.html#//apple_ref/doc/uid/TP40011210). -The example below exposes -the getter for the `enabled` property of the `ExampleClass` -to Objective-C code as `isEnabled` -rather than just as the name of the property itself. - -```swift -class ExampleClass: NSObject { - @objc var enabled: Bool { - @objc(isEnabled) get { - // Return the appropriate value - } - } -} -``` - - - -For more information, see -[Importing Swift into Objective-C](https://developer.apple.com/documentation/swift/imported_c_and_objective-c_apis/importing_swift_into_objective-c). - -> Note: The argument to the `objc` attribute -> can also change the runtime name for that declaration. -> You use the runtime name when calling functions -> that interact with the Objective-C runtime, -> like [`NSClassFromString(_:)`](https://developer.apple.com/documentation/foundation/1395135-nsclassfromstring), -> and when specifying class names in an app's Info.plist file. -> If you specify a name by passing an argument, -> that name is used as the name in Objective-C code -> and as the runtime name. -> If you omit the argument, -> the name used in Objective-C code matches the name in Swift code, -> and the runtime name follows the normal Swift compiler convention -> of name mangling. - -### objcMembers - -Apply this attribute to a class declaration, -to implicitly apply the `objc` attribute -to all Objective-C compatible members of the class, -its extensions, its subclasses, and all of the extensions of its subclasses. - -Most code should use the `objc` attribute instead, -to expose only the declarations that are needed. -If you need to expose many declarations, -you can group them in an extension that has the `objc` attribute. -The `objcMembers` attribute is a convenience for -libraries that make heavy use of -the introspection facilities of the Objective-C runtime. -Applying the `objc` attribute when it isn't needed -can increase your binary size and adversely affect performance. - - - -### preconcurrency - -Apply this attribute to a declaration, -to suppress strict concurrency checking. -You can apply this attribute -to the following kinds of declarations: - -- Imports -- Structures, classes, and actors -- Enumerations and enumeration cases -- Protocols -- Variables and constants -- Subscripts -- Initializers -- Functions - -On an import declaration, -this attribute reduces the strictness of concurrency checking -for code that uses types from the imported module. -Specifically, -types from the imported module -that aren't explicitly marked as nonsendable -can be used in a context that requires sendable types. - -On other declarations, -this attribute reduces the strictness of concurrency checking -for code that uses the symbol being declared. -When you use this symbol in a scope that has minimal concurrency checking, -concurrency-related constraints specified by that symbol, -such as `Sendable` requirements or global actors, -aren't checked. - -You can use this attribute as follows, -to aid in migrating code to strict concurrency checking: - -1. Enable strict checking. -1. Annotate imports with the `preconcurrency` attribute - for modules that haven't enabled strict checking. -1. After migrating a module to strict checking, - remove the `preconcurrency` attribute. - The compiler warns you about - any places where the `preconcurrency` attribute on an import - no longer has an effect and should be removed. - -For other declarations, -add the `preconcurrency` attribute -when you add concurrency-related constraints to the declaration, -if you still have clients -that haven't migrated to strict checking. -Remove the `preconcurrency` attribute after all your clients have migrated. - -Declarations from Objective-C are always imported -as if they were marked with the `preconcurrency` attribute. - -### propertyWrapper - -Apply this attribute to a class, structure, or enumeration declaration -to use that type as a property wrapper. -When you apply this attribute to a type, -you create a custom attribute with the same name as the type. -Apply that new attribute to a property of a class, structure, or enumeration -to wrap access to the property through an instance of the wrapper type; -apply the attribute to a local stored variable declaration -to wrap access to the variable the same way. -Computed variables, global variables, and constants can't use property wrappers. - - - - - - - -The wrapper must define a `wrappedValue` instance property. -The *wrapped value* of the property -is the value that the getter and setter for this property expose. -In most cases, `wrappedValue` is a computed value, -but it can be a stored value instead. -The wrapper defines and manages -any underlying storage needed by its wrapped value. -The compiler synthesizes storage for the instance of the wrapper type -by prefixing the name of the wrapped property with an underscore (`_`) --- -for example, the wrapper for `someProperty` is stored as `_someProperty`. -The synthesized storage for the wrapper has an access control level of `private`. - -A property that has a property wrapper -can include `willSet` and `didSet` blocks, -but it can't override the compiler-synthesized `get` or `set` blocks. - -Swift provides two forms of syntactic sugar -for initialization of a property wrapper. -You can use assignment syntax in the definition of a wrapped value -to pass the expression on the right-hand side of the assignment -as the argument to the `wrappedValue` parameter -of the property wrapper's initializer. -You can also provide arguments to the attribute -when you apply it to a property, -and those arguments are passed to the property wrapper's initializer. -For example, in the code below, -`SomeStruct` calls each of the initializers that `SomeWrapper` defines. - -```swift -@propertyWrapper -struct SomeWrapper { - var wrappedValue: Int - var someValue: Double - init() { - self.wrappedValue = 100 - self.someValue = 12.3 - } - init(wrappedValue: Int) { - self.wrappedValue = wrappedValue - self.someValue = 45.6 - } - init(wrappedValue value: Int, custom: Double) { - self.wrappedValue = value - self.someValue = custom - } -} - -struct SomeStruct { - // Uses init() - @SomeWrapper var a: Int - - // Uses init(wrappedValue:) - @SomeWrapper var b = 10 - - // Both use init(wrappedValue:custom:) - @SomeWrapper(custom: 98.7) var c = 30 - @SomeWrapper(wrappedValue: 30, custom: 98.7) var d -} -``` - - - - - - - -The *projected value* for a wrapped property is a second value -that a property wrapper can use to expose additional functionality. -The author of a property wrapper type -is responsible for determining the meaning of its projected value -and defining the interface that the projected value exposes. -To project a value from a property wrapper, -define a `projectedValue` instance property on the wrapper type. -The compiler synthesizes an identifier for the projected value -by prefixing the name of the wrapped property with a dollar sign (`$`) --- -for example, the projected value for `someProperty` is `$someProperty`. -The projected value has the same access control level -as the original wrapped property. - -```swift -@propertyWrapper -struct WrapperWithProjection { - var wrappedValue: Int - var projectedValue: SomeProjection { - return SomeProjection(wrapper: self) - } -} -struct SomeProjection { - var wrapper: WrapperWithProjection -} - -struct SomeStruct { - @WrapperWithProjection var x = 123 -} -let s = SomeStruct() -s.x // Int value -s.$x // SomeProjection value -s.$x.wrapper // WrapperWithProjection value -``` - - - -### resultBuilder - -Apply this attribute to a class, structure, enumeration -to use that type as a result builder. -A *result builder* is a type -that builds a nested data structure step by step. -You use result builders to implement a domain-specific language (DSL) -for creating nested data structures in a natural, declarative way. -For an example of how to use the `resultBuilder` attribute, -see . - -#### Result-Building Methods - -A result builder implements static methods described below. -Because all of the result builder's functionality -is exposed through static methods, -you don't ever initialize an instance of that type. -A result builder must implement either the `buildBlock(_:)` method -or both the `buildPartialBlock(first:)` -and `buildPartialBlock(accumulated:next:)` methods. -The other methods --- -which enable additional functionality in the DSL --- -are optional. -The declaration of a result builder type -doesn't actually have to include any protocol conformance. - -The description of the static methods uses three types as placeholders. -The type `Expression` is a placeholder -for the type of the result builder's input, -`Component` is a placeholder for the type of a partial result, -and `FinalResult` is a placeholder -for the type of the result that the result builder produces. -You replace these types with the actual types that your result builder uses. -If your result-building methods -don't specify a type for `Expression` or `FinalResult`, -they default to being the same as `Component`. - -The block-building methods are as follows: - -- term `static func buildBlock(_ components: Component...) -> Component`: - Combines an array of partial results into a single partial result. - -- term `static func buildPartialBlock(first: Component) -> Component`: - Builds a partial result component from the first component. - Implement both this method and `buildPartialBlock(accumulated:next:)` - to support building blocks one component at a time. - Compared to `buildBlock(_:)`, - this approach reduces the need for generic overloads - that handle different numbers of arguments. - -- term `static func buildPartialBlock(accumulated: Component, next: Component) -> Component`: - Builds a partial result component - by combining an accumulated component with a new component. - Implement both this method and `buildPartialBlock(first:)` - to support building blocks one component at a time. - Compared to `buildBlock(_:)`, - this approach reduces the need for generic overloads - that handle different numbers of arguments. - -A result builder can implement all three of the block-building methods listed above; -in that case, availability determines which method is called. -By default, Swift calls the `buildPartialBlock(first:)` and `buildPartialBlock(accumulated:next:)` -methods. To make Swift call `buildBlock(_:)` instead, -mark the enclosing declaration as being available -before the availability you write on `buildPartialBlock(first:)` and -`buildPartialBlock(accumulated:next:)`. - -The additional result-building methods are as follows: - -- term `static func buildOptional(_ component: Component?) -> Component`: - Builds a partial result from a partial result that can be `nil`. - Implement this method to support `if` statements - that don’t include an `else` clause. - -- term `static func buildEither(first: Component) -> Component`: - Builds a partial result whose value varies depending on some condition. - Implement both this method and `buildEither(second:)` - to support `switch` statements - and `if` statements that include an `else` clause. - -- term `static func buildEither(second: Component) -> Component`: - Builds a partial result whose value varies depending on some condition. - Implement both this method and `buildEither(first:)` - to support `switch` statements - and `if` statements that include an `else` clause. - -- term `static func buildArray(_ components: [Component]) -> Component`: - Builds a partial result from an array of partial results. - Implement this method to support `for` loops. - -- term `static func buildExpression(_ expression: Expression) -> Component`: - Builds a partial result from an expression. - You can implement this method to perform preprocessing --- - for example, converting expressions to an internal type --- - or to provide additional information for type inference at use sites. - -- term `static func buildFinalResult(_ component: Component) -> FinalResult`: - Builds a final result from a partial result. - You can implement this method as part of a result builder - that uses a different type for partial and final results, - or to perform other postprocessing on a result before returning it. - -- term `static func buildLimitedAvailability(_ component: Component) -> Component`: - Builds a partial result that erases type information. - You can implement this method to prevent type information - from propagating outside a compiler-control statement - that performs an availability check. - -For example, the code below defines a simple result builder -that builds an array of integers. -This code defines `Component` and `Expression` as type aliases, -to make it easier to match the examples below to the list of methods above. - -```swift -@resultBuilder -struct ArrayBuilder { - typealias Component = [Int] - typealias Expression = Int - static func buildExpression(_ element: Expression) -> Component { - return [element] - } - static func buildOptional(_ component: Component?) -> Component { - guard let component = component else { return [] } - return component - } - static func buildEither(first component: Component) -> Component { - return component - } - static func buildEither(second component: Component) -> Component { - return component - } - static func buildArray(_ components: [Component]) -> Component { - return Array(components.joined()) - } - static func buildBlock(_ components: Component...) -> Component { - return Array(components.joined()) - } -} -``` - - - -#### Result Transformations - -The following syntactic transformations are applied recursively -to turn code that uses result-builder syntax -into code that calls the static methods of the result builder type: - -- If the result builder has a `buildExpression(_:)` method, - each expression becomes a call to that method. - This transformation is always first. - For example, the following declarations are equivalent: - - ```swift - @ArrayBuilder var builderNumber: [Int] { 10 } - var manualNumber = ArrayBuilder.buildExpression(10) - ``` - - - -- An assignment statement is transformed like an expression, - but is understood to evaluate to `()`. - You can define an overload of `buildExpression(_:)` - that takes an argument of type `()` to handle assignments specifically. -- A branch statement that checks an availability condition - becomes a call to the `buildLimitedAvailability(_:)` method, - if that method is implemented. - If you don't implement `buildLimitedAvailability(_:)`, - then branch statements that check availability - use the same transformations as other branch statements. - This transformation happens before the transformation into a call to - `buildEither(first:)`, `buildEither(second:)`, or `buildOptional(_:)`. - - You use the `buildLimitedAvailability(_:)` method to erase type information - that changes depending on which branch is taken. - For example, - the `buildEither(first:)` and `buildEither(second:)` methods below - use a generic type that captures type information about both branches. - - ```swift - protocol Drawable { - func draw() -> String - } - struct Text: Drawable { - var content: String - init(_ content: String) { self.content = content } - func draw() -> String { return content } - } - struct Line: Drawable { - var elements: [D] - func draw() -> String { - return elements.map { $0.draw() }.joined(separator: "") - } - } - struct DrawEither: Drawable { - var content: Drawable - func draw() -> String { return content.draw() } - } - - @resultBuilder - struct DrawingBuilder { - static func buildBlock(_ components: D...) -> Line { - return Line(elements: components) - } - static func buildEither(first: First) - -> DrawEither { - return DrawEither(content: first) - } - static func buildEither(second: Second) - -> DrawEither { - return DrawEither(content: second) - } - } - ``` - - - - However, this approach causes a problem in code that has availability checks: - - ```swift - @available(macOS 99, *) - struct FutureText: Drawable { - var content: String - init(_ content: String) { self.content = content } - func draw() -> String { return content } - } - @DrawingBuilder var brokenDrawing: Drawable { - if #available(macOS 99, *) { - FutureText("Inside.future") // Problem - } else { - Text("Inside.present") - } - } - // The type of brokenDrawing is Line, Line>> - ``` - - - - In the code above, - `FutureText` appears as part of the type of `brokenDrawing` - because it's one of the types in the `DrawEither` generic type. - This could cause your program to crash if `FutureText` - isn't available at runtime, - even in the case where that type is explicitly not being used. - - To solve this problem, - implement a `buildLimitedAvailability(_:)` method - to erase type information by returning a type that's always available. - For example, the code below builds an `AnyDrawable` value - from its availability check. - - ```swift - struct AnyDrawable: Drawable { - var content: Drawable - func draw() -> String { return content.draw() } - } - extension DrawingBuilder { - static func buildLimitedAvailability(_ content: some Drawable) -> AnyDrawable { - return AnyDrawable(content: content) - } - } - - @DrawingBuilder var typeErasedDrawing: Drawable { - if #available(macOS 99, *) { - FutureText("Inside.future") - } else { - Text("Inside.present") - } - } - // The type of typeErasedDrawing is Line>> - ``` - - - -- A branch statement becomes a series of nested calls to the - `buildEither(first:)` and `buildEither(second:)` methods. - The statements' conditions and cases are mapped onto - the leaf nodes of a binary tree, - and the statement becomes - a nested call to the `buildEither` methods - following the path to that leaf node from the root node. - - For example, if you write a switch statement that has three cases, - the compiler uses a binary tree with three leaf nodes. - Likewise, - because the path from the root node to the second case is - "second child" and then "first child", - that case becomes a nested call like - `buildEither(first: buildEither(second: ... ))`. - The following declarations are equivalent: - - ```swift - let someNumber = 19 - @ArrayBuilder var builderConditional: [Int] { - if someNumber < 12 { - 31 - } else if someNumber == 19 { - 32 - } else { - 33 - } - } - - var manualConditional: [Int] - if someNumber < 12 { - let partialResult = ArrayBuilder.buildExpression(31) - let outerPartialResult = ArrayBuilder.buildEither(first: partialResult) - manualConditional = ArrayBuilder.buildEither(first: outerPartialResult) - } else if someNumber == 19 { - let partialResult = ArrayBuilder.buildExpression(32) - let outerPartialResult = ArrayBuilder.buildEither(second: partialResult) - manualConditional = ArrayBuilder.buildEither(first: outerPartialResult) - } else { - let partialResult = ArrayBuilder.buildExpression(33) - manualConditional = ArrayBuilder.buildEither(second: partialResult) - } - ``` - - - -- A branch statement that might not produce a value, - like an `if` statement without an `else` clause, - becomes a call to `buildOptional(_:)`. - If the `if` statement's condition is satisfied, - its code block is transformed and passed as the argument; - otherwise, `buildOptional(_:)` is called with `nil` as its argument. - For example, the following declarations are equivalent: - - ```swift - @ArrayBuilder var builderOptional: [Int] { - if (someNumber % 2) == 1 { 20 } - } - - var partialResult: [Int]? = nil - if (someNumber % 2) == 1 { - partialResult = ArrayBuilder.buildExpression(20) - } - var manualOptional = ArrayBuilder.buildOptional(partialResult) - ``` - - - -- If the result builder implements - the `buildPartialBlock(first:)` - and `buildPartialBlock(accumulated:next:)` methods, - a code block or `do` statement becomes a call to those methods. - The first statement inside of the block - is transformed to become an argument - to the `buildPartialBlock(first:)` method, - and the remaining statements become nested calls - to the `buildPartialBlock(accumulated:next:)` method. - For example, the following declarations are equivalent: - - ```swift - struct DrawBoth: Drawable { - var first: First - var second: Second - func draw() -> String { return first.draw() + second.draw() } - } - - @resultBuilder - struct DrawingPartialBlockBuilder { - static func buildPartialBlock(first: D) -> D { - return first - } - static func buildPartialBlock( - accumulated: Accumulated, next: Next - ) -> DrawBoth { - return DrawBoth(first: accumulated, second: next) - } - } - - @DrawingPartialBlockBuilder var builderBlock: some Drawable { - Text("First") - Line(elements: [Text("Second"), Text("Third")]) - Text("Last") - } - - let partialResult1 = DrawingPartialBlockBuilder.buildPartialBlock(first: Text("first")) - let partialResult2 = DrawingPartialBlockBuilder.buildPartialBlock( - accumulated: partialResult1, - next: Line(elements: [Text("Second"), Text("Third")]) - ) - let manualResult = DrawingPartialBlockBuilder.buildPartialBlock( - accumulated: partialResult2, - next: Text("Last") - ) - ``` - - -- Otherwise, a code block or `do` statement - becomes a call to the `buildBlock(_:)` method. - Each of the statements inside of the block is transformed, - one at a time, - and they become the arguments to the `buildBlock(_:)` method. - For example, the following declarations are equivalent: - - ```swift - @ArrayBuilder var builderBlock: [Int] { - 100 - 200 - 300 - } - - var manualBlock = ArrayBuilder.buildBlock( - ArrayBuilder.buildExpression(100), - ArrayBuilder.buildExpression(200), - ArrayBuilder.buildExpression(300) - ) - ``` - - - -- A `for` loop becomes a temporary variable, a `for` loop, - and call to the `buildArray(_:)` method. - The new `for` loop iterates over the sequence - and appends each partial result to that array. - The temporary array is passed as the argument in the `buildArray(_:)` call. - For example, the following declarations are equivalent: - - ```swift - @ArrayBuilder var builderArray: [Int] { - for i in 5...7 { - 100 + i - } - } - - var temporary: [[Int]] = [] - for i in 5...7 { - let partialResult = ArrayBuilder.buildExpression(100 + i) - temporary.append(partialResult) - } - let manualArray = ArrayBuilder.buildArray(temporary) - ``` - - - -- If the result builder has a `buildFinalResult(_:)` method, - the final result becomes a call to that method. - This transformation is always last. - - - - - - - -Although the transformation behavior is described in terms of temporary variables, -using a result builder doesn't actually create any new declarations -that are visible from the rest of your code. - -You can't use -`break`, `continue`, `defer`, `guard`, or `return` statements, -`while` statements, -or `do`-`catch` statements -in the code that a result builder transforms. - -The transformation process doesn't change declarations in the code, -which lets you use temporary constants and variables -to build up expressions piece by piece. -It also doesn't change -`throw` statements, -compile-time diagnostic statements, -or closures that contain a `return` statement. - -Whenever possible, transformations are coalesced. -For example, the expression `4 + 5 * 6` becomes -`buildExpression(4 + 5 * 6)` rather multiple calls to that function. -Likewise, nested branch statements become -a single binary tree of calls to the `buildEither` methods. - - - -#### Custom Result-Builder Attributes - -Creating a result builder type creates a custom attribute with the same name. -You can apply that attribute in the following places: - -- On a function declaration, - the result builder builds the body of the function. -- On a variable or subscript declaration that includes a getter, - the result builder builds the body of the getter. -- On a parameter in a function declaration, - the result builder builds the body of a closure - that's passed as the corresponding argument. - -Applying a result builder attribute doesn't impact ABI compatibility. -Applying a result builder attribute to a parameter -makes that attribute part of the function's interface, -which can affect source compatibility. - -### requires_stored_property_inits - -Apply this attribute to a class declaration -to require all stored properties within the class -to provide default values as part of their definitions. -This attribute is inferred for any class -that inherits from `NSManagedObject`. - - - -### testable - -Apply this attribute to an `import` declaration -to import that module with changes to its access control -that simplify testing the module's code. -Entities in the imported module -that are marked with the `internal` access-level modifier -are imported as if they were declared with the `public` access-level modifier. -Classes and class members -that are marked with the `internal` or `public` access-level modifier -are imported as if they were declared with the `open` access-level modifier. -The imported module must be compiled with testing enabled. - -### UIApplicationMain - -> Deprecated: -> This attribute is deprecated; -> use the attribute instead. -> In Swift 6, -> using this attribute will be an error. - -Apply this attribute to a class -to indicate that it's the app delegate. -Using this attribute is equivalent to calling the -`UIApplicationMain` function and -passing this class's name as the name of the delegate class. - -If you don't use this attribute, -supply a `main.swift` file with code at the top level -that calls the [`UIApplicationMain(_:_:_:_:)`](https://developer.apple.com/documentation/uikit/1622933-uiapplicationmain) function. -For example, -if your app uses a custom subclass of `UIApplication` -as its principal class, -call the `UIApplicationMain(_:_:_:_:)` function -instead of using this attribute. - -The Swift code you compile to make an executable -can contain at most one top-level entry point, -as discussed in . - -### unchecked - -Apply this attribute to a protocol type -as part of a type declaration's list of adopted protocols -to turn off enforcement of that protocol's requirements. - -The only supported protocol is [`Sendable`](https://developer.apple.com/documentation/swift/sendable). - -### usableFromInline - -Apply this attribute to a -function, method, computed property, subscript, -initializer, or deinitializer declaration -to allow that symbol to be used in inlinable code -that's defined in the same module as the declaration. -The declaration must have the `internal` access-level modifier. -A structure or class marked `usableFromInline` -can use only types that are public or `usableFromInline` for its properties. -An enumeration marked `usableFromInline` -can use only types that are public or `usableFromInline` -for the raw values and associated values of its cases. - -Like the `public` access-level modifier, -this attribute -exposes the declaration as part of the module's public interface. -Unlike `public`, -the compiler doesn't allow declarations marked with `usableFromInline` -to be referenced by name in code outside the module, -even though the declaration's symbol is exported. -However, code outside the module might still be able -to interact with the declaration's symbol by using runtime behavior. - -Declarations marked with the `inlinable` attribute -are implicitly usable from inlinable code. -Although either `inlinable` or `usableFromInline` -can be applied to `internal` declarations, -applying both attributes is an error. - - - -### warn_unqualified_access - -Apply this attribute to a -top-level function, instance method, or class or static method -to trigger warnings when that function or method is used -without a preceding qualifier, -such as a module name, type name, or instance variable or constant. -Use this attribute to help discourage ambiguity between functions -with the same name that are accessible from the same scope. - -For example, -the Swift standard library includes both a top-level -[`min(_:_:)`](https://developer.apple.com/documentation/swift/1538339-min/) -function and a -[`min()`](https://developer.apple.com/documentation/swift/sequence/1641174-min) -method for sequences with comparable elements. -The sequence method is declared with the `warn_unqualified_access` attribute -to help reduce confusion -when attempting to use one or the other from within a `Sequence` extension. - -### Declaration Attributes Used by Interface Builder - -Interface Builder attributes are declaration attributes -used by Interface Builder to synchronize with Xcode. -Swift provides the following Interface Builder attributes: -`IBAction`, `IBSegueAction`, `IBOutlet`, -`IBDesignable`, and `IBInspectable`. -These attributes are conceptually the same as their -Objective-C counterparts. - - - -You apply the `IBOutlet` and `IBInspectable` attributes -to property declarations of a class. -You apply the `IBAction` and `IBSegueAction` attribute -to method declarations of a class -and the `IBDesignable` attribute to class declarations. - -Applying the `IBAction`, `IBSegueAction`, `IBOutlet`, -`IBDesignable`, or `IBInspectable` attribute -also implies the `objc` attribute. - -## Type Attributes - -You can apply type attributes to types only. - -### autoclosure - -Apply this attribute to delay the evaluation of an expression -by automatically wrapping that expression in a closure with no arguments. -You apply it to a parameter's type in a function or method declaration, -for a parameter whose type is a function type that takes no arguments -and that returns a value of the type of the expression. -For an example of how to use the `autoclosure` attribute, -see and . - -### convention - -Apply this attribute to the type of a function -to indicate its calling conventions. - -The `convention` attribute always appears with -one of the following arguments: - -- The `swift` argument indicates a Swift function reference. - This is the standard calling convention for function values in Swift. -- The `block` argument indicates an Objective-C compatible block reference. - The function value is represented as a reference to the block object, - which is an `id`-compatible Objective-C object that embeds its invocation - function within the object. - The invocation function uses the C calling convention. -- The `c` argument indicates a C function reference. - The function value carries no context and uses the C calling convention. - - - -With a few exceptions, -a function of any calling convention can be used -when a function any other calling convention is needed. -A nongeneric global function, -a local function that doesn't capture any local variables, -or a closure that doesn't capture any local variables -can be converted to the C calling convention. -Other Swift functions can't be converted to the C calling convention. -A function with the Objective-C block calling convention -can't be converted to the C calling convention. - -### escaping - -Apply this attribute to a parameter's type in a function or method declaration -to indicate that the parameter's value can be stored for later execution. -This means that the value is allowed to outlive the lifetime of the call. -Function type parameters with the `escaping` type attribute -require explicit use of `self.` for properties or methods. -For an example of how to use the `escaping` attribute, -see . - -### Sendable - -Apply this attribute to the type of a function -to indicate that the function or closure is sendable. -Applying this attribute to a function type -has the same meaning as conforming a non–function type -to the [`Sendable`](https://developer.apple.com/documentation/swift/sendable) protocol. - -This attribute is inferred on functions and closures -if the function or closure is used in a context -that expects a sendable value, -and the function or closure satisfies the requirements to be sendable. - -A sendable function type -is a subtype of the corresponding nonsendable function type. - -## Switch Case Attributes - -You can apply switch case attributes to switch cases only. - -### unknown - -Apply this attribute to a switch case -to indicate that it isn't expected to be matched -by any case of the enumeration that's known -at the time the code is compiled. -For an example of how to use the `unknown` attribute, -see . - -> Grammar of an attribute: -> -> *attribute* → **`@`** *attribute-name* *attribute-argument-clause*_?_ \ -> *attribute-name* → *identifier* \ -> *attribute-argument-clause* → **`(`** *balanced-tokens*_?_ **`)`** \ -> *attributes* → *attribute* *attributes*_?_ -> -> *balanced-tokens* → *balanced-token* *balanced-tokens*_?_ \ -> *balanced-token* → **`(`** *balanced-tokens*_?_ **`)`** \ -> *balanced-token* → **`[`** *balanced-tokens*_?_ **`]`** \ -> *balanced-token* → **`{`** *balanced-tokens*_?_ **`}`** \ -> *balanced-token* → Any identifier, keyword, literal, or operator \ -> *balanced-token* → Any punctuation except **`(`**, **`)`**, **`[`**, **`]`**, **`{`**, or **`}`** - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/ReferenceManual/Declarations.md b/swift-6-beta.docc/ReferenceManual/Declarations.md deleted file mode 100644 index 3ae12c65c..000000000 --- a/swift-6-beta.docc/ReferenceManual/Declarations.md +++ /dev/null @@ -1,4000 +0,0 @@ -# Declarations - -Introduce types, operators, variables, and other names and constructs. - -A *declaration* introduces a new name or construct into your program. -For example, you use declarations to introduce functions and methods, -to introduce variables and constants, -and to define enumeration, structure, class, and protocol types. -You can also use a declaration to extend the behavior -of an existing named type and to import symbols into your program that are declared elsewhere. - -In Swift, most declarations are also definitions in the sense that they're implemented -or initialized at the same time they're declared. That said, because protocols don't -implement their members, most protocol members are declarations only. For convenience -and because the distinction isn't that important in Swift, -the term *declaration* covers both declarations and definitions. - -> Grammar of a declaration: -> -> *declaration* → *import-declaration* \ -> *declaration* → *constant-declaration* \ -> *declaration* → *variable-declaration* \ -> *declaration* → *typealias-declaration* \ -> *declaration* → *function-declaration* \ -> *declaration* → *enum-declaration* \ -> *declaration* → *struct-declaration* \ -> *declaration* → *class-declaration* \ -> *declaration* → *actor-declaration* \ -> *declaration* → *protocol-declaration* \ -> *declaration* → *initializer-declaration* \ -> *declaration* → *deinitializer-declaration* \ -> *declaration* → *extension-declaration* \ -> *declaration* → *subscript-declaration* \ -> *declaration* → *macro-declaration* \ -> *declaration* → *operator-declaration* \ -> *declaration* → *precedence-group-declaration* \ - -## Top-Level Code - -The top-level code in a Swift source file consists of zero or more statements, -declarations, and expressions. -By default, variables, constants, and other named declarations that are declared -at the top-level of a source file are accessible to code -in every source file that's part of the same module. -You can override this default behavior -by marking the declaration with an access-level modifier, -as described in . - -There are two kinds of top-level code: -top-level declarations and executable top-level code. -Top-level declarations consist of only declarations, -and are allowed in all Swift source files. -Executable top-level code contains statements and expressions, -not just declarations, -and is allowed only as the top-level entry point for the program. - -The Swift code you compile to make an executable -can contain at most one of the following approaches -to mark the top-level entry point, -regardless of how the code is organized into files and modules: -the `main` attribute, -the `NSApplicationMain` attribute, -the `UIApplicationMain` attribute, -a `main.swift` file, -or a file that contains top-level executable code. - -> Grammar of a top-level declaration: -> -> *top-level-declaration* → *statements*_?_ - -## Code Blocks - -A *code block* is used by a variety of declarations and control structures -to group statements together. -It has the following form: - -```swift -{ - <#statements#> -} -``` - -The *statements* inside a code block include declarations, -expressions, and other kinds of statements and are executed in order -of their appearance in source code. - - - - - -> Grammar of a code block: -> -> *code-block* → **`{`** *statements*_?_ **`}`** - -## Import Declaration - -An *import declaration* lets you access symbols -that are declared outside the current file. -The basic form imports the entire module; -it consists of the `import` keyword followed by a module name: - -```swift -import <#module#> -``` - -Providing more detail limits which symbols are imported --- -you can specify a specific submodule -or a specific declaration within a module or submodule. -When this detailed form is used, -only the imported symbol -(and not the module that declares it) -is made available in the current scope. - -```swift -import <#import kind#> <#module#>.<#symbol name#> -import <#module#>.<#submodule#> -``` - - - -> Grammar of an import declaration: -> -> *import-declaration* → *attributes*_?_ **`import`** *import-kind*_?_ *import-path* -> -> *import-kind* → **`typealias`** | **`struct`** | **`class`** | **`enum`** | **`protocol`** | **`let`** | **`var`** | **`func`** \ -> *import-path* → *identifier* | *identifier* **`.`** *import-path* - -## Constant Declaration - -A *constant declaration* introduces a constant named value into your program. -Constant declarations are declared using the `let` keyword and have the following form: - -```swift -let <#constant name#>: <#type#> = <#expression#> -``` - -A constant declaration defines an immutable binding between the *constant name* -and the value of the initializer *expression*; -after the value of a constant is set, it can't be changed. -That said, if a constant is initialized with a class object, -the object itself can change, -but the binding between the constant name and the object it refers to can't. - -When a constant is declared at global scope, -it must be initialized with a value. -When a constant declaration occurs in the context of a function or method, -it can be initialized later, -as long as it's guaranteed to have a value set -before the first time its value is read. -If the compiler can prove that the constant's value is never read, -the constant isn't required to have a value set at all. -This analysis is called *definite initialization* --- -the compiler proves that a value is definitely set before being read. - -> Note: -> Definite initialization -> can't construct proofs that require domain knowledge, -> and its ability to track state across conditionals has a limit. -> If you can determine that constant always has a value set, -> but the compiler can't prove this is the case, -> try simplifying the code paths that set the value, -> or use a variable declaration instead. - - - -When a constant declaration occurs in the context of a class or structure -declaration, it's considered a *constant property*. -Constant declarations aren't computed properties and therefore don't have getters -or setters. - -If the *constant name* of a constant declaration is a tuple pattern, -the name of each item in the tuple is bound to the corresponding value -in the initializer *expression*. - -```swift -let (firstNumber, secondNumber) = (10, 42) -``` - - - -In this example, -`firstNumber` is a named constant for the value `10`, -and `secondNumber` is a named constant for the value `42`. -Both constants can now be used independently: - -```swift -print("The first number is \(firstNumber).") -// Prints "The first number is 10." -print("The second number is \(secondNumber).") -// Prints "The second number is 42." -``` - - - -The type annotation (`:` *type*) is optional in a constant declaration -when the type of the *constant name* can be inferred, -as described in . - -To declare a constant type property, -mark the declaration with the `static` declaration modifier. -A constant type property of a class is always implicitly final; -you can't mark it with the `class` or `final` declaration modifier -to allow or disallow overriding by subclasses. -Type properties are discussed in . - - - -For more information about constants and for guidance about when to use them, -see and . - -> Grammar of a constant declaration: -> -> *constant-declaration* → *attributes*_?_ *declaration-modifiers*_?_ **`let`** *pattern-initializer-list* -> -> *pattern-initializer-list* → *pattern-initializer* | *pattern-initializer* **`,`** *pattern-initializer-list* \ -> *pattern-initializer* → *pattern* *initializer*_?_ \ -> *initializer* → **`=`** *expression* - -## Variable Declaration - -A *variable declaration* introduces a variable named value into your program -and is declared using the `var` keyword. - -Variable declarations have several forms that declare different kinds -of named, mutable values, -including stored and computed variables and properties, -stored variable and property observers, and static variable properties. -The appropriate form to use depends on -the scope at which the variable is declared and the kind of variable you intend to declare. - -> Note: You can also declare properties in the context of a protocol declaration, -> as described in . - -You can override a property in a subclass by marking the subclass's property declaration -with the `override` declaration modifier, as described in . - -### Stored Variables and Stored Variable Properties - -The following form declares a stored variable or stored variable property: - -```swift -var <#variable name#>: <#type#> = <#expression#> -``` - -You define this form of a variable declaration at global scope, the local scope -of a function, or in the context of a class or structure declaration. -When a variable declaration of this form is declared at global scope or the local -scope of a function, it's referred to as a *stored variable*. -When it's declared in the context of a class or structure declaration, -it's referred to as a *stored variable property*. - -The initializer *expression* can't be present in a protocol declaration, -but in all other contexts, the initializer *expression* is optional. -That said, if no initializer *expression* is present, -the variable declaration must include an explicit type annotation (`:` *type*). - -As with constant declarations, -if a variable declaration omits the initializer *expression*, -the variable must have a value set before the first time it is read. -Also like constant declarations, -if the *variable name* is a tuple pattern, -the name of each item in the tuple is bound to the corresponding value -in the initializer *expression*. - -As their names suggest, the value of a stored variable or a stored variable property -is stored in memory. - -### Computed Variables and Computed Properties - -The following form declares a computed variable or computed property: - -```swift -var <#variable name#>: <#type#> { - get { - <#statements#> - } - set(<#setter name#>) { - <#statements#> - } -} -``` - -You define this form of a variable declaration at global scope, the local scope -of a function, or in the context of a class, structure, enumeration, or extension declaration. -When a variable declaration of this form is declared at global scope or the local -scope of a function, it's referred to as a *computed variable*. -When it's declared in the context of a class, -structure, or extension declaration, -it's referred to as a *computed property*. - -The getter is used to read the value, -and the setter is used to write the value. -The setter clause is optional, -and when only a getter is needed, you can omit both clauses and simply -return the requested value directly, -as described in . -But if you provide a setter clause, you must also provide a getter clause. - -The *setter name* and enclosing parentheses is optional. -If you provide a setter name, it's used as the name of the parameter to the setter. -If you don't provide a setter name, the default parameter name to the setter is `newValue`, -as described in . - -Unlike stored named values and stored variable properties, -the value of a computed named value or a computed property isn't stored in memory. - -For more information and to see examples of computed properties, -see . - -### Stored Variable Observers and Property Observers - -You can also declare a stored variable or property with `willSet` and `didSet` observers. -A stored variable or property declared with observers has the following form: - -```swift -var <#variable name#>: <#type#> = <#expression#> { - willSet(<#setter name#>) { - <#statements#> - } - didSet(<#setter name#>) { - <#statements#> - } -} -``` - -You define this form of a variable declaration at global scope, the local scope -of a function, or in the context of a class or structure declaration. -When a variable declaration of this form is declared at global scope or the local -scope of a function, the observers are referred to as *stored variable observers*. -When it's declared in the context of a class or structure declaration, -the observers are referred to as *property observers*. - -You can add property observers to any stored property. You can also add property -observers to any inherited property (whether stored or computed) by overriding -the property within a subclass, as described in . - -The initializer *expression* is optional in the context of a class or structure declaration, -but required elsewhere. The *type* annotation is optional -when the type can be inferred from the initializer *expression*. -This expression is evaluated the first time you read the property's value. -If you overwrite the property's initial value without reading it, -this expression is evaluated before the first time you write to the property. - - - -The `willSet` and `didSet` observers provide a way to observe (and to respond appropriately) -when the value of a variable or property is being set. -The observers aren't called when the variable or property -is first initialized. -Instead, they're called only when the value is set outside of an initialization context. - -A `willSet` observer is called just before the value of the variable or property -is set. The new value is passed to the `willSet` observer as a constant, -and therefore it can't be changed in the implementation of the `willSet` clause. -The `didSet` observer is called immediately after the new value is set. In contrast -to the `willSet` observer, the old value of the variable or property -is passed to the `didSet` observer in case you still need access to it. That said, -if you assign a value to a variable or property within its own `didSet` observer clause, -that new value that you assign will replace the one that was just set and passed to -the `willSet` observer. - -The *setter name* and enclosing parentheses in the `willSet` and `didSet` clauses are optional. -If you provide setter names, -they're used as the parameter names to the `willSet` and `didSet` observers. -If you don't provide setter names, -the default parameter name to the `willSet` observer is `newValue` -and the default parameter name to the `didSet` observer is `oldValue`. - -The `didSet` clause is optional when you provide a `willSet` clause. -Likewise, the `willSet` clause is optional when you provide a `didSet` clause. - -If the body of the `didSet` observer refers to the old value, -the getter is called before the observer, -to make the old value available. -Otherwise, the new value is stored without calling the superclass's getter. -The example below shows a computed property that's defined by the superclass -and overridden by its subclasses to add an observer. - -```swift -class Superclass { - private var xValue = 12 - var x: Int { - get { print("Getter was called"); return xValue } - set { print("Setter was called"); xValue = newValue } - } -} - -// This subclass doesn't refer to oldValue in its observer, so the -// superclass's getter is called only once to print the value. -class New: Superclass { - override var x: Int { - didSet { print("New value \(x)") } - } -} -let new = New() -new.x = 100 -// Prints "Setter was called" -// Prints "Getter was called" -// Prints "New value 100" - -// This subclass refers to oldValue in its observer, so the superclass's -// getter is called once before the setter, and again to print the value. -class NewAndOld: Superclass { - override var x: Int { - didSet { print("Old value \(oldValue) - new value \(x)") } - } -} -let newAndOld = NewAndOld() -newAndOld.x = 200 -// Prints "Getter was called" -// Prints "Setter was called" -// Prints "Getter was called" -// Prints "Old value 12 - new value 200" -``` - - - -For more information and to see an example of how to use property observers, -see . - - - -### Type Variable Properties - -To declare a type variable property, -mark the declaration with the `static` declaration modifier. -Classes can mark type computed properties with the `class` declaration modifier instead -to allow subclasses to override the superclass’s implementation. -Type properties are discussed in . - -> Grammar of a variable declaration: -> -> *variable-declaration* → *variable-declaration-head* *pattern-initializer-list* \ -> *variable-declaration* → *variable-declaration-head* *variable-name* *type-annotation* *code-block* \ -> *variable-declaration* → *variable-declaration-head* *variable-name* *type-annotation* *getter-setter-block* \ -> *variable-declaration* → *variable-declaration-head* *variable-name* *type-annotation* *getter-setter-keyword-block* \ -> *variable-declaration* → *variable-declaration-head* *variable-name* *initializer* *willSet-didSet-block* \ -> *variable-declaration* → *variable-declaration-head* *variable-name* *type-annotation* *initializer*_?_ *willSet-didSet-block* -> -> *variable-declaration-head* → *attributes*_?_ *declaration-modifiers*_?_ **`var`** \ -> *variable-name* → *identifier* -> -> *getter-setter-block* → *code-block* \ -> *getter-setter-block* → **`{`** *getter-clause* *setter-clause*_?_ **`}`** \ -> *getter-setter-block* → **`{`** *setter-clause* *getter-clause* **`}`** \ -> *getter-clause* → *attributes*_?_ *mutation-modifier*_?_ **`get`** *code-block* \ -> *setter-clause* → *attributes*_?_ *mutation-modifier*_?_ **`set`** *setter-name*_?_ *code-block* \ -> *setter-name* → **`(`** *identifier* **`)`** -> -> *getter-setter-keyword-block* → **`{`** *getter-keyword-clause* *setter-keyword-clause*_?_ **`}`** \ -> *getter-setter-keyword-block* → **`{`** *setter-keyword-clause* *getter-keyword-clause* **`}`** \ -> *getter-keyword-clause* → *attributes*_?_ *mutation-modifier*_?_ **`get`** \ -> *setter-keyword-clause* → *attributes*_?_ *mutation-modifier*_?_ **`set`** -> -> *willSet-didSet-block* → **`{`** *willSet-clause* *didSet-clause*_?_ **`}`** \ -> *willSet-didSet-block* → **`{`** *didSet-clause* *willSet-clause*_?_ **`}`** \ -> *willSet-clause* → *attributes*_?_ **`willSet`** *setter-name*_?_ *code-block* \ -> *didSet-clause* → *attributes*_?_ **`didSet`** *setter-name*_?_ *code-block* - - - -## Type Alias Declaration - -A *type alias declaration* introduces a named alias of an existing type into your program. -Type alias declarations are declared using the `typealias` keyword and have the following form: - -```swift -typealias <#name#> = <#existing type#> -``` - -After a type alias is declared, the aliased *name* can be used -instead of the *existing type* everywhere in your program. -The *existing type* can be a named type or a compound type. -Type aliases don't create new types; -they simply allow a name to refer to an existing type. - -A type alias declaration can use generic parameters -to give a name to an existing generic type. The type alias -can provide concrete types for some or all of the generic parameters -of the existing type. -For example: - -```swift -typealias StringDictionary = Dictionary - -// The following dictionaries have the same type. -var dictionary1: StringDictionary = [:] -var dictionary2: Dictionary = [:] -``` - - - -When a type alias is declared with generic parameters, the constraints on those -parameters must match exactly the constraints on the existing type's generic parameters. -For example: - -```swift -typealias DictionaryOfInts = Dictionary -``` - - - -Because the type alias and the existing type can be used interchangeably, -the type alias can't introduce additional generic constraints. - -A type alias can forward an existing type's generic parameters -by omitting all generic parameters from the declaration. -For example, -the `Diccionario` type alias declared here -has the same generic parameters and constraints as `Dictionary`. - -```swift -typealias Diccionario = Dictionary -``` - - - - - - - -Inside a protocol declaration, -a type alias can give a shorter and more convenient name -to a type that's used frequently. -For example: - -```swift -protocol Sequence { - associatedtype Iterator: IteratorProtocol - typealias Element = Iterator.Element -} - -func sum(_ sequence: T) -> Int where T.Element == Int { - // ... -} -``` - - - -Without this type alias, -the `sum` function would have to refer to the associated type -as `T.Iterator.Element` instead of `T.Element`. - -See also . - -> Grammar of a type alias declaration: -> -> *typealias-declaration* → *attributes*_?_ *access-level-modifier*_?_ **`typealias`** *typealias-name* *generic-parameter-clause*_?_ *typealias-assignment* \ -> *typealias-name* → *identifier* \ -> *typealias-assignment* → **`=`** *type* - - - -## Function Declaration - -A *function declaration* introduces a function or method into your program. -A function declared in the context of class, structure, enumeration, or protocol -is referred to as a *method*. -Function declarations are declared using the `func` keyword and have the following form: - -```swift -func <#function name#>(<#parameters#>) -> <#return type#> { - <#statements#> -} -``` - -If the function has a return type of `Void`, -the return type can be omitted as follows: - -```swift -func <#function name#>(<#parameters#>) { - <#statements#> -} -``` - -The type of each parameter must be included --- -it can't be inferred. -If you write `inout` in front of a parameter's type, -the parameter can be modified inside the scope of the function. -In-out parameters are discussed in detail -in , below. - -A function declaration whose *statements* -include only a single expression -is understood to return the value of that expression. -This implicit return syntax is considered -only when the expression's type and the function's return type -aren't `Void` -and aren't an enumeration like `Never` that doesn't have any cases. - - - -Functions can return multiple values using a tuple type -as the return type of the function. - - - -A function definition can appear inside another function declaration. -This kind of function is known as a *nested function*. - -A nested function is nonescaping if it captures -a value that's guaranteed to never escape --- -such as an in-out parameter --- -or passed as a nonescaping function argument. -Otherwise, the nested function is an escaping function. - -For a discussion of nested functions, -see . - -### Parameter Names - -Function parameters are a comma-separated list -where each parameter has one of several forms. -The order of arguments in a function call -must match the order of parameters in the function's declaration. -The simplest entry in a parameter list has the following form: - -```swift -<#parameter name#>: <#parameter type#> -``` - -A parameter has a name, -which is used within the function body, -as well as an argument label, -which is used when calling the function or method. -By default, -parameter names are also used as argument labels. -For example: - -```swift -func f(x: Int, y: Int) -> Int { return x + y } -f(x: 1, y: 2) // both x and y are labeled -``` - - - - - -You can override the default behavior for argument labels -with one of the following forms: - -```swift -<#argument label#> <#parameter name#>: <#parameter type#> -_ <#parameter name#>: <#parameter type#> -``` - -A name before the parameter name -gives the parameter an explicit argument label, -which can be different from the parameter name. -The corresponding argument must use the given argument label -in function or method calls. - -An underscore (`_`) before a parameter name -suppresses the argument label. -The corresponding argument must have no label in function or method calls. - -```swift -func repeatGreeting(_ greeting: String, count n: Int) { /* Greet n times */ } -repeatGreeting("Hello, world!", count: 2) // count is labeled, greeting is not -``` - - - -### Parameter Modifiers - -A *parameter modifier* changes how an argument is passed to the function. - -```swift -<#argument label#> <#parameter name#>: <#parameter modifier#> <#parameter type#> -``` - -To use a parameter modifier, -write `inout`, `borrowing`, or `consuming` -before the argument's type. - -```swift -func someFunction(a: inout A, b: consuming B, c: C) { ... } -``` - -#### In-Out Parameters - -By default, function arguments in Swift are passed by value: -Any changes made within the function are not visible in the caller. -To make an in-out parameter instead, -you apply the `inout` parameter modifier. - - -```swift -func someFunction(a: inout Int) { - a += 1 -} -``` - -When calling a function that includes in-out parameters, -the in-out argument must be prefixed with an ampersand (`&`) -to mark that the function call can change the argument's value. - -```swift -var x = 7 -someFunction(&x) -print(x) // Prints "8" -``` - -In-out parameters are passed as follows: - -1. When the function is called, - the value of the argument is copied. -2. In the body of the function, - the copy is modified. -3. When the function returns, - the copy's value is assigned to the original argument. - -This behavior is known as *copy-in copy-out* -or *call by value result*. -For example, -when a computed property or a property with observers -is passed as an in-out parameter, -its getter is called as part of the function call -and its setter is called as part of the function return. - -As an optimization, -when the argument is a value stored at a physical address in memory, -the same memory location is used both inside and outside the function body. -The optimized behavior is known as *call by reference*; -it satisfies all of the requirements -of the copy-in copy-out model -while removing the overhead of copying. -Write your code using the model given by copy-in copy-out, -without depending on the call-by-reference optimization, -so that it behaves correctly with or without the optimization. - -Within a function, don't access a value that was passed as an in-out argument, -even if the original value is available in the current scope. -Accessing the original is a simultaneous access of the value, -which violates memory exclusivity. - -```swift -var someValue: Int -func someFunction(a: inout Int) { - a += someValue -} - -// Error: This causes a runtime exclusivity violation -someFunction(&someValue) -``` - -For the same reason, -you can't pass the same value to multiple in-out parameters. - -```swift -var someValue: Int -func someFunction(a: inout Int, b: inout Int) { - a += b - b += 1 -} - -// Error: Cannot pass the same value to multiple in-out parameters -someFunction(&someValue, &someValue) -``` - -For more information about memory safety and memory exclusivity, -see . - - - -A closure or nested function -that captures an in-out parameter must be nonescaping. -If you need to capture an in-out parameter -without mutating it, -use a capture list to explicitly capture the parameter immutably. - -```swift -func someFunction(a: inout Int) -> () -> Int { - return { [a] in return a + 1 } -} -``` - - - -If you need to capture and mutate an in-out parameter, -use an explicit local copy, -such as in multithreaded code that ensures -all mutation has finished before the function returns. - -```swift -func multithreadedFunction(queue: DispatchQueue, x: inout Int) { - // Make a local copy and manually copy it back. - var localX = x - defer { x = localX } - - // Operate on localX asynchronously, then wait before returning. - queue.async { someMutatingOperation(&localX) } - queue.sync {} -} -``` - - - -For more discussion and examples of in-out parameters, -see . - - - -#### Borrowing and Consuming Parameters - -By default, Swift uses a set of rules -to automatically manage object lifetime across function calls, -copying values when required. -The default rules are designed to minimize overhead in most cases --- -if you want more specific control, -you can apply the `borrowing` or `consuming` parameter modifier. -In this case, -use `copy` to explicitly mark copy operations. - -Regardless of whether you use the default rules, -Swift guarantees that object lifetime and -ownership are correctly managed in all cases. -These parameter modifiers impact only the relative efficiency -of particular usage patterns, not correctness. - - - -The `borrowing` modifier indicates that the function -does not keep the parameter's value. -In this case, the caller maintains ownership of the object -and the responsibility for the object's lifetime. -Using `borrowing` minimizes overhead when the function -uses the object only transiently. - -```swift -// `isLessThan` does not keep either argument -func isLessThan(lhs: borrowing A, rhs: borrowing A) -> Bool { - ... -} -``` - -If the function needs to keep the parameter's value -for example, by storing it in a global variable --- -you use `copy` to explicitly copy that value. - -```swift -// As above, but this `isLessThan` also wants to record the smallest value -func isLessThan(lhs: borrowing A, rhs: borrowing A) -> Bool { - if lhs < storedValue { - storedValue = copy lhs - } else if rhs < storedValue { - storedValue = copy rhs - } - return lhs < rhs -} -``` - -Conversely, -the `consuming` parameter modifier indicates -that the function takes ownership of the value, -accepting responsibility for either storing or destroying it -before the function returns. - -```swift -// `store` keeps its argument, so mark it `consuming` -func store(a: consuming A) { - someGlobalVariable = a -} -``` - -Using `consuming` minimizes overhead when the caller no longer -needs to use the object after the function call. - -```swift -// Usually, this is the last thing you do with a value -store(a: value) -``` - -If you keep using a copyable object after the function call, -the compiler automatically makes a copy of that object -before the function call. - -```swift -// The compiler inserts an implicit copy here -store(a: someValue) // This function consumes someValue -print(someValue) // This uses the copy of someValue -``` - -Unlike `inout`, neither `borrowing` nor -`consuming` parameters require any special -notation when you call the function: - -```swift -func someFunction(a: borrowing A, b: consuming B) { ... } - -someFunction(a: someA, b: someB) -``` - -The explicit use of either `borrowing` or `consuming` -indicates your intention to more tightly control -the overhead of runtime ownership management. -Because copies can cause unexpected runtime ownership -operations, -parameters marked with either of these -modifiers cannot be copied unless you -use an explicit `copy` keyword: - -```swift -func borrowingFunction1(a: borrowing A) { - // Error: Cannot implicitly copy a - // This assignment requires a copy because - // `a` is only borrowed from the caller. - someGlobalVariable = a -} - -func borrowingFunction2(a: borrowing A) { - // OK: Explicit copying works - someGlobalVariable = copy a -} - -func consumingFunction1(a: consuming A) { - // Error: Cannot implicitly copy a - // This assignment requires a copy because - // of the following `print` - someGlobalVariable = a - print(a) -} - -func consumingFunction2(a: consuming A) { - // OK: Explicit copying works regardless - someGlobalVariable = copy a - print(a) -} - -func consumingFunction3(a: consuming A) { - // OK: No copy needed here because this is the last use - someGlobalVariable = a -} -``` - - - - -### Special Kinds of Parameters - -Parameters can be ignored, -take a variable number of values, -and provide default values -using the following forms: - -```swift -_ : <#parameter type#> -<#parameter name#>: <#parameter type#>... -<#parameter name#>: <#parameter type#> = <#default argument value#> -``` - -An underscore (`_`) parameter -is explicitly ignored and can't be accessed within the body of the function. - -A parameter with a base type name followed immediately by three dots (`...`) -is understood as a variadic parameter. -A parameter that immediately follows a variadic parameter -must have an argument label. -A function can have multiple variadic parameters. -A variadic parameter is treated as an array that contains elements of the base type name. -For example, the variadic parameter `Int...` is treated as `[Int]`. -For an example that uses a variadic parameter, -see . - -A parameter with an equal sign (`=`) and an expression after its type -is understood to have a default value of the given expression. -The given expression is evaluated when the function is called. -If the parameter is omitted when calling the function, -the default value is used instead. - -```swift -func f(x: Int = 42) -> Int { return x } -f() // Valid, uses default value -f(x: 7) // Valid, uses the value provided -f(7) // Invalid, missing argument label -``` - - - - - - - -### Special Kinds of Methods - -Methods on an enumeration or a structure -that modify `self` must be marked with the `mutating` declaration modifier. - -Methods that override a superclass method -must be marked with the `override` declaration modifier. -It's a compile-time error to override a method without the `override` modifier -or to use the `override` modifier on a method -that doesn't override a superclass method. - -Methods associated with a type -rather than an instance of a type -must be marked with the `static` declaration modifier for enumerations and structures, -or with either the `static` or `class` declaration modifier for classes. -A class type method marked with the `class` declaration modifier -can be overridden by a subclass implementation; -a class type method marked with `class final` or `static` can't be overridden. - - - - - -### Methods with Special Names - -Several methods that have special names -enable syntactic sugar for function call syntax. -If a type defines one of these methods, -instances of the type can be used in function call syntax. -The function call is understood to be a call to -one of the specially named methods on that instance. - -A class, structure, or enumeration type -can support function call syntax -by defining a `dynamicallyCall(withArguments:)` method -or a `dynamicallyCall(withKeywordArguments:)` method, -as described in , -or by defining a call-as-function method, as described below. -If the type defines -both a call-as-function method -and one of the methods used by the `dynamicCallable` attribute, -the compiler gives preference to the call-as-function method -in circumstances where either method could be used. - -The name of a call-as-function method is `callAsFunction()`, -or another name that begins with `callAsFunction(` -and adds labeled or unlabeled arguments --- -for example, `callAsFunction(_:_:)` and `callAsFunction(something:)` -are also valid call-as-function method names. - - - -The following function calls are equivalent: - -```swift -struct CallableStruct { - var value: Int - func callAsFunction(_ number: Int, scale: Int) { - print(scale * (number + value)) - } -} -let callable = CallableStruct(value: 100) -callable(4, scale: 2) -callable.callAsFunction(4, scale: 2) -// Both function calls print 208. -``` - - - -The call-as-function methods -and the methods from the `dynamicCallable` attribute -make different trade-offs between -how much information you encode into the type system -and how much dynamic behavior is possible at runtime. -When you declare a call-as-function method, -you specify the number of arguments, -and each argument's type and label. -The `dynamicCallable` attribute's methods specify only the type -used to hold the array of arguments. - -Defining a call-as-function method, -or a method from the `dynamicCallable` attribute, -doesn't let you use an instance of that type -as if it were a function in any context other than a function call expression. -For example: - -```swift -let someFunction1: (Int, Int) -> Void = callable(_:scale:) // Error -let someFunction2: (Int, Int) -> Void = callable.callAsFunction(_:scale:) -``` - - - -The `subscript(dynamicMember:)` subscript -enables syntactic sugar for member lookup, -as described in . - -### Throwing Functions and Methods - -Functions and methods that can throw an error must be marked with the `throws` keyword. -These functions and methods are known as *throwing functions* -and *throwing methods*. -They have the following form: - -```swift -func <#function name#>(<#parameters#>) throws -> <#return type#> { - <#statements#> -} -``` - -A function that throws a specific error type has the following form: - -```swift -func <#function name#>(<#parameters#>) throws(<#error type#>) -> <#return type#> { - <#statements#> -} -``` - -Calls to a throwing function or method must be wrapped in a `try` or `try!` expression -(that is, in the scope of a `try` or `try!` operator). - -A function's type includes whether it can throw an error -and what type of error it throws. -This subtype relationship means, for example, you can use a nonthrowing function -in a context where a throwing one is expected. -For more information about the type of a throwing function, -see . -For examples of working with errors that have explicit types, -see . - -You can't overload a function based only on whether the function can throw an error. -That said, -you can overload a function based on whether a function *parameter* can throw an error. - -A throwing method can't override a nonthrowing method, -and a throwing method can't satisfy a protocol requirement for a nonthrowing method. -That said, a nonthrowing method can override a throwing method, -and a nonthrowing method can satisfy a protocol requirement for a throwing method. - -### Rethrowing Functions and Methods - -A function or method can be declared with the `rethrows` keyword -to indicate that it throws an error only if one of its function parameters throws an error. -These functions and methods are known as *rethrowing functions* -and *rethrowing methods*. -Rethrowing functions and methods -must have at least one throwing function parameter. - -```swift -func someFunction(callback: () throws -> Void) rethrows { - try callback() -} -``` - - - -A rethrowing function or method can contain a `throw` statement -only inside a `catch` clause. -This lets you call the throwing function inside a `do`-`catch` statement -and handle errors in the `catch` clause by throwing a different error. -In addition, -the `catch` clause must handle -only errors thrown by one of the rethrowing function's -throwing parameters. -For example, the following is invalid -because the `catch` clause would handle -the error thrown by `alwaysThrows()`. - -```swift -func alwaysThrows() throws { - throw SomeError.error -} -func someFunction(callback: () throws -> Void) rethrows { - do { - try callback() - try alwaysThrows() // Invalid, alwaysThrows() isn't a throwing parameter - } catch { - throw AnotherError.error - } -} -``` - - - - - -A throwing method can't override a rethrowing method, -and a throwing method can't satisfy a protocol requirement for a rethrowing method. -That said, a rethrowing method can override a throwing method, -and a rethrowing method can satisfy a protocol requirement for a throwing method. - -An alternative to rethrowing is throwing a specific error type in generic code. -For example: - -```swift -func someFunction(callback: () throws(E) -> Void) throws(E) { - try callback() -} -``` - -This approach to propagating an error -preserves type information about the error. -However, unlike marking a function `rethrows`, -this approach doesn't prevent the function -from throwing an error of the same type. - - - -### Asynchronous Functions and Methods - -Functions and methods that run asynchronously must be marked with the `async` keyword. -These functions and methods are known as *asynchronous functions* -and *asynchronous methods*. -They have the following form: - -```swift -func <#function name#>(<#parameters#>) async -> <#return type#> { - <#statements#> -} -``` - -Calls to an asynchronous function or method -must be wrapped in an `await` expression --- -that is, they must be in the scope of an `await` operator. - -The `async` keyword is part of the function's type, -and synchronous functions are subtypes of asynchronous functions. -As a result, you can use a synchronous function -in a context where an asynchronous function is expected. -For example, -you can override an asynchronous method with a synchronous method, -and a synchronous method can satisfy a protocol requirement -that requires an asynchronous method. - -You can overload a function based on whether or not the function is asynchronous. -At the call site, context determines which overload is used: -In an asynchronous context, the asynchronous function is used, -and in a synchronous context, the synchronous function is used. - -An asynchronous method can't override a synchronous method, -and an asynchronous method can't satisfy a protocol requirement for a synchronous method. -That said, a synchronous method can override an asynchronous method, -and a synchronous method can satisfy a protocol requirement for an asynchronous method. - - - -### Functions that Never Return - -Swift defines a [`Never`][] type, -which indicates that a function or method doesn't return to its caller. -Functions and methods with the `Never` return type are called *nonreturning*. -Nonreturning functions and methods either cause an irrecoverable error -or begin a sequence of work that continues indefinitely. -This means that -code that would otherwise run immediately after the call is never executed. -Throwing and rethrowing functions can transfer program control -to an appropriate `catch` block, even when they're nonreturning. - -[`Never`]: https://developer.apple.com/documentation/swift/never - -A nonreturning function or method can be called to conclude the `else` clause -of a guard statement, -as discussed in . - -You can override a nonreturning method, -but the new method must preserve its return type and nonreturning behavior. - -> Grammar of a function declaration: -> -> *function-declaration* → *function-head* *function-name* *generic-parameter-clause*_?_ *function-signature* *generic-where-clause*_?_ *function-body*_?_ -> -> *function-head* → *attributes*_?_ *declaration-modifiers*_?_ **`func`** \ -> *function-name* → *identifier* | *operator* -> -> *function-signature* → *parameter-clause* **`async`**_?_ *throws-clause*_?_ *function-result*_?_ \ -> *function-signature* → *parameter-clause* **`async`**_?_ **`rethrows`** *function-result*_?_ \ -> *function-result* → **`->`** *attributes*_?_ *type* \ -> *function-body* → *code-block* -> -> *parameter-clause* → **`(`** **`)`** | **`(`** *parameter-list* **`)`** \ -> *parameter-list* → *parameter* | *parameter* **`,`** *parameter-list* \ -> *parameter* → *external-parameter-name*_?_ *local-parameter-name* *parameter-type-annotation* *default-argument-clause*_?_ \ -> *parameter* → *external-parameter-name*_?_ *local-parameter-name* *parameter-type-annotation* \ -> *parameter* → *external-parameter-name*_?_ *local-parameter-name* *parameter-type-annotation* **`...`** -> -> *external-parameter-name* → *identifier* \ -> *local-parameter-name* → *identifier* \ -> *parameter-type-annotation* → **`:`** *attributes*_?_ *parameter-modifier*_?_ *type* \ -> *parameter-modifier* → **`inout`** | **`borrowing`** | **`consuming`** -> *default-argument-clause* → **`=`** *expression* - - - -## Enumeration Declaration - -An *enumeration declaration* introduces a named enumeration type into your program. - -Enumeration declarations have two basic forms and are declared using the `enum` keyword. -The body of an enumeration declared using either form contains -zero or more values --- called *enumeration cases* --- -and any number of declarations, -including computed properties, -instance methods, type methods, initializers, type aliases, -and even other enumeration, structure, class, and actor declarations. -Enumeration declarations can't contain deinitializer or protocol declarations. - -Enumeration types can adopt any number of protocols, but can’t inherit from classes, -structures, or other enumerations. - -Unlike classes and structures, -enumeration types don't have an implicitly provided default initializer; -all initializers must be declared explicitly. Initializers can delegate -to other initializers in the enumeration, but the initialization process is complete -only after an initializer assigns one of the enumeration cases to `self`. - -Like structures but unlike classes, enumerations are value types; -instances of an enumeration are copied when assigned to -variables or constants, or when passed as arguments to a function call. -For information about value types, -see . - -You can extend the behavior of an enumeration type with an extension declaration, -as discussed in . - -### Enumerations with Cases of Any Type - -The following form declares an enumeration type that contains -enumeration cases of any type: - -```swift -enum <#enumeration name#>: <#adopted protocols#> { - case <#enumeration case 1#> - case <#enumeration case 2#>(<#associated value types#>) -} -``` - -Enumerations declared in this form are sometimes called *discriminated unions* -in other programming languages. - -In this form, each case block consists of the `case` keyword -followed by one or more enumeration cases, separated by commas. -The name of each case must be unique. -Each case can also specify that it stores values of a given type. -These types are specified in the *associated value types* tuple, -immediately following the name of the case. - -Enumeration cases that store associated values can be used as functions -that create instances of the enumeration with the specified associated values. -And just like functions, -you can get a reference to an enumeration case and apply it later in your code. - -```swift -enum Number { - case integer(Int) - case real(Double) -} -let f = Number.integer -// f is a function of type (Int) -> Number - -// Apply f to create an array of Number instances with integer values -let evenInts: [Number] = [0, 2, 4, 6].map(f) -``` - - - - - -For more information and to see examples of cases with associated value types, -see . - -#### Enumerations with Indirection - -Enumerations can have a recursive structure, -that is, they can have cases with associated values -that are instances of the enumeration type itself. -However, instances of enumeration types have value semantics, -which means they have a fixed layout in memory. -To support recursion, -the compiler must insert a layer of indirection. - -To enable indirection for a particular enumeration case, -mark it with the `indirect` declaration modifier. -An indirect case must have an associated value. - - - -```swift -enum Tree { - case empty - indirect case node(value: T, left: Tree, right: Tree) -} -``` - - - -To enable indirection for all the cases of an enumeration -that have an associated value, -mark the entire enumeration with the `indirect` modifier --- -this is convenient when the enumeration contains many cases -that would each need to be marked with the `indirect` modifier. - -An enumeration that's marked with the `indirect` modifier -can contain a mixture of cases that have associated values and cases those that don't. -That said, -it can't contain any cases that are also marked with the `indirect` modifier. - - - - - - - -### Enumerations with Cases of a Raw-Value Type - -The following form declares an enumeration type that contains -enumeration cases of the same basic type: - -```swift -enum <#enumeration name#>: <#raw-value type#>, <#adopted protocols#> { - case <#enumeration case 1#> = <#raw value 1#> - case <#enumeration case 2#> = <#raw value 2#> -} -``` - -In this form, each case block consists of the `case` keyword, -followed by one or more enumeration cases, separated by commas. -Unlike the cases in the first form, each case has an underlying -value, called a *raw value*, of the same basic type. -The type of these values is specified in the *raw-value type* and must represent an -integer, floating-point number, string, or single character. -In particular, the *raw-value type* must conform to the `Equatable` protocol -and one of the following protocols: -`ExpressibleByIntegerLiteral` for integer literals, -`ExpressibleByFloatLiteral` for floating-point literals, -`ExpressibleByStringLiteral` for string literals that contain any number of characters, -and `ExpressibleByUnicodeScalarLiteral` -or `ExpressibleByExtendedGraphemeClusterLiteral` for string literals -that contain only a single character. -Each case must have a unique name and be assigned a unique raw value. - - - -If the raw-value type is specified as `Int` -and you don't assign a value to the cases explicitly, -they're implicitly assigned the values `0`, `1`, `2`, and so on. -Each unassigned case of type `Int` is implicitly assigned a raw value -that's automatically incremented from the raw value of the previous case. - -```swift -enum ExampleEnum: Int { - case a, b, c = 5, d -} -``` - - - -In the above example, the raw value of `ExampleEnum.a` is `0` and the value of -`ExampleEnum.b` is `1`. And because the value of `ExampleEnum.c` is -explicitly set to `5`, the value of `ExampleEnum.d` is automatically incremented -from `5` and is therefore `6`. - -If the raw-value type is specified as `String` -and you don't assign values to the cases explicitly, -each unassigned case is implicitly assigned a string with the same text as the name of that case. - -```swift -enum GamePlayMode: String { - case cooperative, individual, competitive -} -``` - - - -In the above example, the raw value of `GamePlayMode.cooperative` is `"cooperative"`, -the raw value of `GamePlayMode.individual` is `"individual"`, -and the raw value of `GamePlayMode.competitive` is `"competitive"`. - -Enumerations that have cases of a raw-value type implicitly conform to the -`RawRepresentable` protocol, defined in the Swift standard library. -As a result, they have a `rawValue` property -and a failable initializer with the signature `init?(rawValue: RawValue)`. -You can use the `rawValue` property to access the raw value of an enumeration case, -as in `ExampleEnum.b.rawValue`. -You can also use a raw value to find a corresponding case, if there is one, -by calling the enumeration's failable initializer, -as in `ExampleEnum(rawValue: 5)`, which returns an optional case. -For more information and to see examples of cases with raw-value types, -see . - -### Accessing Enumeration Cases - -To reference the case of an enumeration type, use dot (`.`) syntax, -as in `EnumerationType.enumerationCase`. When the enumeration type can be inferred -from context, you can omit it (the dot is still required), -as described in -and . - -To check the values of enumeration cases, use a `switch` statement, -as shown in . -The enumeration type is pattern-matched against the enumeration case patterns -in the case blocks of the `switch` statement, -as described in . - - - - - - - -> Grammar of an enumeration declaration: -> -> *enum-declaration* → *attributes*_?_ *access-level-modifier*_?_ *union-style-enum* \ -> *enum-declaration* → *attributes*_?_ *access-level-modifier*_?_ *raw-value-style-enum* -> -> *union-style-enum* → **`indirect`**_?_ **`enum`** *enum-name* *generic-parameter-clause*_?_ *type-inheritance-clause*_?_ *generic-where-clause*_?_ **`{`** *union-style-enum-members*_?_ **`}`** \ -> *union-style-enum-members* → *union-style-enum-member* *union-style-enum-members*_?_ \ -> *union-style-enum-member* → *declaration* | *union-style-enum-case-clause* | *compiler-control-statement* \ -> *union-style-enum-case-clause* → *attributes*_?_ **`indirect`**_?_ **`case`** *union-style-enum-case-list* \ -> *union-style-enum-case-list* → *union-style-enum-case* | *union-style-enum-case* **`,`** *union-style-enum-case-list* \ -> *union-style-enum-case* → *enum-case-name* *tuple-type*_?_ \ -> *enum-name* → *identifier* \ -> *enum-case-name* → *identifier* -> -> *raw-value-style-enum* → **`enum`** *enum-name* *generic-parameter-clause*_?_ *type-inheritance-clause* *generic-where-clause*_?_ **`{`** *raw-value-style-enum-members* **`}`** \ -> *raw-value-style-enum-members* → *raw-value-style-enum-member* *raw-value-style-enum-members*_?_ \ -> *raw-value-style-enum-member* → *declaration* | *raw-value-style-enum-case-clause* | *compiler-control-statement* \ -> *raw-value-style-enum-case-clause* → *attributes*_?_ **`case`** *raw-value-style-enum-case-list* \ -> *raw-value-style-enum-case-list* → *raw-value-style-enum-case* | *raw-value-style-enum-case* **`,`** *raw-value-style-enum-case-list* \ -> *raw-value-style-enum-case* → *enum-case-name* *raw-value-assignment*_?_ \ -> *raw-value-assignment* → **`=`** *raw-value-literal* \ -> *raw-value-literal* → *numeric-literal* | *static-string-literal* | *boolean-literal* - - - -## Structure Declaration - -A *structure declaration* introduces a named structure type into your program. -Structure declarations are declared using the `struct` keyword and have the following form: - -```swift -struct <#structure name#>: <#adopted protocols#> { - <#declarations#> -} -``` - -The body of a structure contains zero or more *declarations*. -These *declarations* can include both stored and computed properties, -type properties, instance methods, type methods, initializers, subscripts, -type aliases, and even other structure, class, actor, and enumeration declarations. -Structure declarations can't contain deinitializer or protocol declarations. -For a discussion and several examples of structures -that include various kinds of declarations, -see . - -Structure types can adopt any number of protocols, -but can't inherit from classes, enumerations, or other structures. - -There are three ways to create an instance of a previously declared structure: - -- Call one of the initializers declared within the structure, - as described in . -- If no initializers are declared, - call the structure's memberwise initializer, - as described in . -- If no initializers are declared, - and all properties of the structure declaration were given initial values, - call the structure's default initializer, - as described in . - -The process of initializing a structure's declared properties -is described in . - -Properties of a structure instance can be accessed using dot (`.`) syntax, -as described in . - -Structures are value types; instances of a structure are copied when assigned to -variables or constants, or when passed as arguments to a function call. -For information about value types, -see . - -You can extend the behavior of a structure type with an extension declaration, -as discussed in . - -> Grammar of a structure declaration: -> -> *struct-declaration* → *attributes*_?_ *access-level-modifier*_?_ **`struct`** *struct-name* *generic-parameter-clause*_?_ *type-inheritance-clause*_?_ *generic-where-clause*_?_ *struct-body* \ -> *struct-name* → *identifier* \ -> *struct-body* → **`{`** *struct-members*_?_ **`}`** -> -> *struct-members* → *struct-member* *struct-members*_?_ \ -> *struct-member* → *declaration* | *compiler-control-statement* - -## Class Declaration - -A *class declaration* introduces a named class type into your program. -Class declarations are declared using the `class` keyword and have the following form: - -```swift -class <#class name#>: <#superclass#>, <#adopted protocols#> { - <#declarations#> -} -``` - -The body of a class contains zero or more *declarations*. -These *declarations* can include both stored and computed properties, -instance methods, type methods, initializers, -a single deinitializer, subscripts, type aliases, -and even other class, structure, actor, and enumeration declarations. -Class declarations can't contain protocol declarations. -For a discussion and several examples of classes -that include various kinds of declarations, -see . - -A class type can inherit from only one parent class, its *superclass*, -but can adopt any number of protocols. -The *superclass* appears first after the *class name* and colon, -followed by any *adopted protocols*. -Generic classes can inherit from other generic and nongeneric classes, -but a nongeneric class can inherit only from other nongeneric classes. -When you write the name of a generic superclass class after the colon, -you must include the full name of that generic class, -including its generic parameter clause. - -As discussed in , -classes can have designated and convenience initializers. -The designated initializer of a class must initialize all of the class's -declared properties and it must do so before calling any of its superclass's -designated initializers. - -A class can override properties, methods, subscripts, and initializers of its superclass. -Overridden properties, methods, subscripts, -and designated initializers must be marked with the `override` declaration modifier. - - - -To require that subclasses implement a superclass's initializer, -mark the superclass's initializer with the `required` declaration modifier. -The subclass's implementation of that initializer -must also be marked with the `required` declaration modifier. - -Although properties and methods declared in the *superclass* are inherited by -the current class, designated initializers declared in the *superclass* are only -inherited when the subclass meets the conditions described in -. -Swift classes don't inherit from a universal base class. - -There are two ways to create an instance of a previously declared class: - -- Call one of the initializers declared within the class, - as described in . -- If no initializers are declared, - and all properties of the class declaration were given initial values, - call the class's default initializer, - as described in . - -Access properties of a class instance with dot (`.`) syntax, -as described in . - -Classes are reference types; instances of a class are referred to, rather than copied, -when assigned to variables or constants, or when passed as arguments to a function call. -For information about reference types, -see . - -You can extend the behavior of a class type with an extension declaration, -as discussed in . - -> Grammar of a class declaration: -> -> *class-declaration* → *attributes*_?_ *access-level-modifier*_?_ **`final`**_?_ **`class`** *class-name* *generic-parameter-clause*_?_ *type-inheritance-clause*_?_ *generic-where-clause*_?_ *class-body* \ -> *class-declaration* → *attributes*_?_ **`final`** *access-level-modifier*_?_ **`class`** *class-name* *generic-parameter-clause*_?_ *type-inheritance-clause*_?_ *generic-where-clause*_?_ *class-body* \ -> *class-name* → *identifier* \ -> *class-body* → **`{`** *class-members*_?_ **`}`** -> -> *class-members* → *class-member* *class-members*_?_ \ -> *class-member* → *declaration* | *compiler-control-statement* - -## Actor Declaration - -An *actor declaration* introduces a named actor type into your program. -Actor declarations are declared using the `actor` keyword and have the following form: - -```swift -actor <#actor name#>: <#adopted protocols#> { - <#declarations#> -} -``` - -The body of an actor contains zero or more *declarations*. -These *declarations* can include both stored and computed properties, -instance methods, type methods, initializers, -a single deinitializer, subscripts, type aliases, -and even other class, structure, and enumeration declarations. -For a discussion and several examples of actors -that include various kinds of declarations, -see . - -Actor types can adopt any number of protocols, -but can't inherit from classes, enumerations, structures, or other actors. -However, an actor that is marked with the `@objc` attribute -implicitly conforms to the `NSObjectProtocol` protocol -and is exposed to the Objective-C runtime as a subtype of `NSObject`. - -There are two ways to create an instance of a previously declared actor: - -- Call one of the initializers declared within the actor, - as described in . -- If no initializers are declared, - and all properties of the actor declaration were given initial values, - call the actor's default initializer, - as described in . - -By default, members of an actor are isolated to that actor. -Code, such as the body of a method or the getter for a property, -is executed on that actor. -Code within the actor can interact with them synchronously -because that code is already running on the same actor, -but code outside the actor must mark them with `await` -to indicate that this code is asynchronously running code on another actor. -Key paths can't refer to isolated members of an actor. -Actor-isolated stored properties can be passed as in-out parameters -to synchronous functions, -but not to asynchronous functions. - -Actors can also have nonisolated members, -whose declarations are marked with the `nonisolated` keyword. -A nonisolated member executes like code outside of the actor: -It can't interact with any of the actor's isolated state, -and callers don't mark it with `await` when using it. - -Members of an actor can be marked with the `@objc` attribute -only if they are nonisolated or asynchronous. - -The process of initializing an actor's declared properties -is described in . - -Properties of an actor instance can be accessed using dot (`.`) syntax, -as described in . - -Actors are reference types; instances of an actor are referred to, rather than copied, -when assigned to variables or constants, or when passed as arguments to a function call. -For information about reference types, -see . - -You can extend the behavior of an actor type with an extension declaration, -as discussed in . - - - -> Grammar of an actor declaration: -> -> *actor-declaration* → *attributes*_?_ *access-level-modifier*_?_ **`actor`** *actor-name* *generic-parameter-clause*_?_ *type-inheritance-clause*_?_ *generic-where-clause*_?_ *actor-body* \ -> *actor-name* → *identifier* \ -> *actor-body* → **`{`** *actor-members*_?_ **`}`** -> -> *actor-members* → *actor-member* *actor-members*_?_ \ -> *actor-member* → *declaration* | *compiler-control-statement* - -## Protocol Declaration - -A *protocol declaration* introduces a named protocol type into your program. -Protocol declarations are declared -using the `protocol` keyword and have the following form: - -```swift -protocol <#protocol name#>: <#inherited protocols#> { - <#protocol member declarations#> -} -``` - -Protocol declarations can appear at global scope, -or nested inside a nongeneric type or nongeneric function. - -The body of a protocol contains zero or more *protocol member declarations*, -which describe the conformance requirements that any type adopting the protocol must fulfill. -In particular, a protocol can declare that conforming types must -implement certain properties, methods, initializers, and subscripts. -Protocols can also declare special kinds of type aliases, -called *associated types*, that can specify relationships -among the various declarations of the protocol. -Protocol declarations can't contain -class, structure, enumeration, or other protocol declarations. -The *protocol member declarations* are discussed in detail below. - -Protocol types can inherit from any number of other protocols. -When a protocol type inherits from other protocols, -the set of requirements from those other protocols are aggregated, -and any type that inherits from the current protocol must conform to all those requirements. -For an example of how to use protocol inheritance, -see . - -> Note: You can also aggregate the conformance requirements of multiple -> protocols using protocol composition types, -> as described in -> and . - -You can add protocol conformance to a previously declared type -by adopting the protocol in an extension declaration of that type. -In the extension, you must implement all of the adopted protocol's -requirements. If the type already implements all of the requirements, -you can leave the body of the extension declaration empty. - -By default, types that conform to a protocol must implement all -properties, methods, and subscripts declared in the protocol. -That said, you can mark these protocol member declarations with the `optional` declaration modifier -to specify that their implementation by a conforming type is optional. -The `optional` modifier can be applied -only to members that are marked with the `objc` attribute, -and only to members of protocols that are marked -with the `objc` attribute. As a result, only class types can adopt and conform -to a protocol that contains optional member requirements. -For more information about how to use the `optional` declaration modifier -and for guidance about how to access optional protocol members --- -for example, when you're not sure whether a conforming type implements them --- -see . - - - -The cases of an enumeration can satisfy -protocol requirements for type members. -Specifically, -an enumeration case without any associated values -satisfies a protocol requirement for -a get-only type variable of type `Self`, -and an enumeration case with associated values -satisfies a protocol requirement for a function that returns `Self` -whose parameters and their argument labels -match the case's associated values. -For example: - -```swift -protocol SomeProtocol { - static var someValue: Self { get } - static func someFunction(x: Int) -> Self -} -enum MyEnum: SomeProtocol { - case someValue - case someFunction(x: Int) -} -``` - - - -To restrict the adoption of a protocol to class types only, -include the `AnyObject` protocol in the *inherited protocols* -list after the colon. -For example, the following protocol can be adopted only by class types: - -```swift -protocol SomeProtocol: AnyObject { - /* Protocol members go here */ -} -``` - - - -Any protocol that inherits from a protocol that's marked with the `AnyObject` requirement -can likewise be adopted only by class types. - -> Note: If a protocol is marked with the `objc` attribute, -> the `AnyObject` requirement is implicitly applied to that protocol; -> there’s no need to mark the protocol with the `AnyObject` requirement explicitly. - -Protocols are named types, and thus they can appear in all the same places -in your code as other named types, as discussed in . -However, -you can't construct an instance of a protocol, -because protocols don't actually provide the implementations for the requirements -they specify. - -You can use protocols to declare which methods a delegate of a class or structure -should implement, as described in . - -> Grammar of a protocol declaration: -> -> *protocol-declaration* → *attributes*_?_ *access-level-modifier*_?_ **`protocol`** *protocol-name* *type-inheritance-clause*_?_ *generic-where-clause*_?_ *protocol-body* \ -> *protocol-name* → *identifier* \ -> *protocol-body* → **`{`** *protocol-members*_?_ **`}`** -> -> *protocol-members* → *protocol-member* *protocol-members*_?_ \ -> *protocol-member* → *protocol-member-declaration* | *compiler-control-statement* -> -> *protocol-member-declaration* → *protocol-property-declaration* \ -> *protocol-member-declaration* → *protocol-method-declaration* \ -> *protocol-member-declaration* → *protocol-initializer-declaration* \ -> *protocol-member-declaration* → *protocol-subscript-declaration* \ -> *protocol-member-declaration* → *protocol-associated-type-declaration* \ -> *protocol-member-declaration* → *typealias-declaration* - -### Protocol Property Declaration - -Protocols declare that conforming types must implement a property -by including a *protocol property declaration* -in the body of the protocol declaration. -Protocol property declarations have a special form of a variable -declaration: - -```swift -var <#property name#>: <#type#> { get set } -``` - -As with other protocol member declarations, these property declarations -declare only the getter and setter requirements for types -that conform to the protocol. As a result, you don't implement the getter or setter -directly in the protocol in which it's declared. - -The getter and setter requirements can be satisfied by a conforming type in a variety of ways. -If a property declaration includes both the `get` and `set` keywords, -a conforming type can implement it with a stored variable property -or a computed property that's both readable and writeable -(that is, one that implements both a getter and a setter). However, -that property declaration can't be implemented as a constant property -or a read-only computed property. If a property declaration includes -only the `get` keyword, it can be implemented as any kind of property. -For examples of conforming types that implement the property requirements of a protocol, -see . - -To declare a type property requirement in a protocol declaration, -mark the property declaration with the `static` keyword. -Structures and enumerations that conform to the protocol -declare the property with the `static` keyword, -and classes that conform to the protocol -declare the property with either the `static` or `class` keyword. -Extensions that add protocol conformance to a structure, enumeration, or class -use the same keyword as the type they extend uses. -Extensions that provide a default implementation for a type property requirement -use the `static` keyword. - - - - - -See also . - -> Grammar of a protocol property declaration: -> -> *protocol-property-declaration* → *variable-declaration-head* *variable-name* *type-annotation* *getter-setter-keyword-block* - -### Protocol Method Declaration - -Protocols declare that conforming types must implement a method -by including a protocol method declaration in the body of the protocol declaration. -Protocol method declarations have the same form as -function declarations, with two exceptions: They don't include a function body, -and you can't provide any default parameter values as part of the function declaration. -For examples of conforming types that implement the method requirements of a protocol, -see . - -To declare a class or static method requirement in a protocol declaration, -mark the method declaration with the `static` declaration modifier. -Structures and enumerations that conform to the protocol -declare the method with the `static` keyword, -and classes that conform to the protocol -declare the method with either the `static` or `class` keyword. -Extensions that add protocol conformance to a structure, enumeration, or class -use the same keyword as the type they extend uses. -Extensions that provide a default implementation for a type method requirement -use the `static` keyword. - -See also . - - - -> Grammar of a protocol method declaration: -> -> *protocol-method-declaration* → *function-head* *function-name* *generic-parameter-clause*_?_ *function-signature* *generic-where-clause*_?_ - -### Protocol Initializer Declaration - -Protocols declare that conforming types must implement an initializer -by including a protocol initializer declaration in the body of the protocol declaration. -Protocol initializer declarations have the same form as -initializer declarations, except they don't include the initializer's body. - -A conforming type can satisfy a nonfailable protocol initializer requirement -by implementing a nonfailable initializer or an `init!` failable initializer. -A conforming type can satisfy a failable protocol initializer requirement -by implementing any kind of initializer. - -When a class implements an initializer to satisfy a protocol's initializer requirement, -the initializer must be marked with the `required` declaration modifier -if the class isn't already marked with the `final` declaration modifier. - -See also . - -> Grammar of a protocol initializer declaration: -> -> *protocol-initializer-declaration* → *initializer-head* *generic-parameter-clause*_?_ *parameter-clause* *throws-clause*_?_ *generic-where-clause*_?_ \ -> *protocol-initializer-declaration* → *initializer-head* *generic-parameter-clause*_?_ *parameter-clause* **`rethrows`** *generic-where-clause*_?_ - -### Protocol Subscript Declaration - -Protocols declare that conforming types must implement a subscript -by including a protocol subscript declaration in the body of the protocol declaration. -Protocol subscript declarations have a special form of a subscript declaration: - -```swift -subscript (<#parameters#>) -> <#return type#> { get set } -``` - -Subscript declarations only declare the minimum getter and setter implementation -requirements for types that conform to the protocol. -If the subscript declaration includes both the `get` and `set` keywords, -a conforming type must implement both a getter and a setter clause. -If the subscript declaration includes only the `get` keyword, -a conforming type must implement *at least* a getter clause -and optionally can implement a setter clause. - -To declare a static subscript requirement in a protocol declaration, -mark the subscript declaration with the `static` declaration modifier. -Structures and enumerations that conform to the protocol -declare the subscript with the `static` keyword, -and classes that conform to the protocol -declare the subscript with either the `static` or `class` keyword. -Extensions that add protocol conformance to a structure, enumeration, or class -use the same keyword as the type they extend uses. -Extensions that provide a default implementation for a static subscript requirement -use the `static` keyword. - -See also . - -> Grammar of a protocol subscript declaration: -> -> *protocol-subscript-declaration* → *subscript-head* *subscript-result* *generic-where-clause*_?_ *getter-setter-keyword-block* - -### Protocol Associated Type Declaration - -Protocols declare associated types using the `associatedtype` keyword. -An associated type provides an alias for a type -that's used as part of a protocol's declaration. -Associated types are similar to type parameters in generic parameter clauses, -but they're associated with `Self` in the protocol in which they're declared. -In that context, `Self` refers to the eventual type that conforms to the protocol. -For more information and examples, -see . - -You use a generic `where` clause in a protocol declaration -to add constraints to an associated types inherited from another protocol, -without redeclaring the associated types. -For example, the declarations of `SubProtocol` below are equivalent: - -```swift -protocol SomeProtocol { - associatedtype SomeType -} - -protocol SubProtocolA: SomeProtocol { - // This syntax produces a warning. - associatedtype SomeType: Equatable -} - -// This syntax is preferred. -protocol SubProtocolB: SomeProtocol where SomeType: Equatable { } -``` - - - - - - - -See also . - -> Grammar of a protocol associated type declaration: -> -> *protocol-associated-type-declaration* → *attributes*_?_ *access-level-modifier*_?_ **`associatedtype`** *typealias-name* *type-inheritance-clause*_?_ *typealias-assignment*_?_ *generic-where-clause*_?_ - -## Initializer Declaration - -An *initializer declaration* introduces an initializer for a class, -structure, or enumeration into your program. -Initializer declarations are declared using the `init` keyword and have -two basic forms. - -Structure, enumeration, and class types can have any number of initializers, -but the rules and associated behavior for class initializers are different. -Unlike structures and enumerations, classes have two kinds of initializers: -designated initializers and convenience initializers, -as described in . - -The following form declares initializers for structures, enumerations, -and designated initializers of classes: - -```swift -init(<#parameters#>) { - <#statements#> -} -``` - -A designated initializer of a class initializes -all of the class's properties directly. It can't call any other initializers -of the same class, and if the class has a superclass, it must call one of -the superclass's designated initializers. -If the class inherits any properties from its superclass, one of the -superclass's designated initializers must be called before any of these -properties can be set or modified in the current class. - -Designated initializers can be declared in the context of a class declaration only -and therefore can't be added to a class using an extension declaration. - -Initializers in structures and enumerations can call other declared initializers -to delegate part or all of the initialization process. - -To declare convenience initializers for a class, -mark the initializer declaration with the `convenience` declaration modifier. - -```swift -convenience init(<#parameters#>) { - <#statements#> -} -``` - -Convenience initializers can delegate the initialization process to another -convenience initializer or to one of the class's designated initializers. -That said, the initialization processes must end with a call to a designated -initializer that ultimately initializes the class's properties. -Convenience initializers can't call a superclass's initializers. - -You can mark designated and convenience initializers with the `required` -declaration modifier to require that every subclass implement the initializer. -A subclass’s implementation of that initializer -must also be marked with the `required` declaration modifier. - -By default, initializers declared in a superclass -aren't inherited by subclasses. -That said, if a subclass initializes all of its stored properties with default values -and doesn't define any initializers of its own, -it inherits all of the superclass's initializers. -If the subclass overrides all of the superclass’s designated initializers, -it inherits the superclass’s convenience initializers. - -As with methods, properties, and subscripts, -you need to mark overridden designated initializers with the `override` declaration modifier. - -> Note: If you mark an initializer with the `required` declaration modifier, -> you don't also mark the initializer with the `override` modifier -> when you override the required initializer in a subclass. - -Just like functions and methods, initializers can throw or rethrow errors. -And just like functions and methods, -you use the `throws` or `rethrows` keyword after an initializer's parameters -to indicate the appropriate behavior. -Likewise, initializers can be asynchronous, -and you use the `async` keyword to indicate this. - -To see examples of initializers in various type declarations, -see . - -### Failable Initializers - -A *failable initializer* is a type of initializer that produces an optional instance -or an implicitly unwrapped optional instance of the type the initializer is declared on. -As a result, a failable initializer can return `nil` to indicate that initialization failed. - -To declare a failable initializer that produces an optional instance, -append a question mark to the `init` keyword in the initializer declaration (`init?`). -To declare a failable initializer that produces an implicitly unwrapped optional instance, -append an exclamation point instead (`init!`). The example below shows an `init?` -failable initializer that produces an optional instance of a structure. - -```swift -struct SomeStruct { - let property: String - // produces an optional instance of 'SomeStruct' - init?(input: String) { - if input.isEmpty { - // discard 'self' and return 'nil' - return nil - } - property = input - } -} -``` - - - -You call an `init?` failable initializer in the same way that you call a nonfailable initializer, -except that you must deal with the optionality of the result. - -```swift -if let actualInstance = SomeStruct(input: "Hello") { - // do something with the instance of 'SomeStruct' -} else { - // initialization of 'SomeStruct' failed and the initializer returned 'nil' -} -``` - - - -A failable initializer can return `nil` -at any point in the implementation of the initializer's body. - -A failable initializer can delegate to any kind of initializer. -A nonfailable initializer can delegate to another nonfailable initializer -or to an `init!` failable initializer. -A nonfailable initializer can delegate to an `init?` failable initializer -by force-unwrapping the result of the superclass's initializer --- -for example, by writing `super.init()!`. - -Initialization failure propagates through initializer delegation. -Specifically, -if a failable initializer delegates to an initializer that fails and returns `nil`, -then the initializer that delegated also fails and implicitly returns `nil`. -If a nonfailable initializer delegates to an `init!` failable initializer that fails and returns `nil`, -then a runtime error is raised -(as if you used the `!` operator to unwrap an optional that has a `nil` value). - -A failable designated initializer can be overridden in a subclass -by any kind of designated initializer. -A nonfailable designated initializer can be overridden in a subclass -by a nonfailable designated initializer only. - -For more information and to see examples of failable initializers, -see . - -> Grammar of an initializer declaration: -> -> *initializer-declaration* → *initializer-head* *generic-parameter-clause*_?_ *parameter-clause* **`async`**_?_ *throws-clause*_?_ *generic-where-clause*_?_ *initializer-body* \ -> *initializer-declaration* → *initializer-head* *generic-parameter-clause*_?_ *parameter-clause* **`async`**_?_ **`rethrows`** *generic-where-clause*_?_ *initializer-body* \ -> *initializer-head* → *attributes*_?_ *declaration-modifiers*_?_ **`init`** \ -> *initializer-head* → *attributes*_?_ *declaration-modifiers*_?_ **`init`** **`?`** \ -> *initializer-head* → *attributes*_?_ *declaration-modifiers*_?_ **`init`** **`!`** \ -> *initializer-body* → *code-block* - -## Deinitializer Declaration - -A *deinitializer declaration* declares a deinitializer for a class type. -Deinitializers take no parameters and have the following form: - -```swift -deinit { - <#statements#> -} -``` - -A deinitializer is called automatically when there are no longer any references -to a class object, just before the class object is deallocated. -A deinitializer can be declared only in the body of a class declaration --- -but not in an extension of a class --- -and each class can have at most one. - -A subclass inherits its superclass's deinitializer, -which is implicitly called just before the subclass object is deallocated. -The subclass object isn't deallocated until all deinitializers in its inheritance chain -have finished executing. - -Deinitializers aren't called directly. - -For an example of how to use a deinitializer in a class declaration, -see . - -> Grammar of a deinitializer declaration: -> -> *deinitializer-declaration* → *attributes*_?_ **`deinit`** *code-block* - -## Extension Declaration - -An *extension declaration* allows you to extend -the behavior of existing types. -Extension declarations are declared using the `extension` keyword -and have the following form: - -```swift -extension <#type name#> where <#requirements#> { - <#declarations#> -} -``` - -The body of an extension declaration contains zero or more *declarations*. -These *declarations* can include computed properties, computed type properties, -instance methods, type methods, initializers, subscript declarations, -and even class, structure, and enumeration declarations. -Extension declarations can't contain deinitializer or protocol declarations, -stored properties, property observers, or other extension declarations. -Declarations in a protocol extension can't be marked `final`. -For a discussion and several examples of extensions that include various kinds of declarations, -see . - -If the *type name* is a class, structure, or enumeration type, -the extension extends that type. -If the *type name* is a protocol type, -the extension extends all types that conform to that protocol. - -Extension declarations that extend a generic type -or a protocol with associated types -can include *requirements*. -If an instance of the extended type -or of a type that conforms to the extended protocol -satisfies the *requirements*, -the instance gains the behavior specified in the declaration. - -Extension declarations can contain initializer declarations. That said, -if the type you're extending is defined in another module, -an initializer declaration must delegate to an initializer already defined in that module -to ensure members of that type are properly initialized. - -Properties, methods, and initializers of an existing type -can't be overridden in an extension of that type. - -Extension declarations can add protocol conformance to an existing -class, structure, or enumeration type by specifying *adopted protocols*: - -```swift -extension <#type name#>: <#adopted protocols#> where <#requirements#> { - <#declarations#> -} -``` - -Extension declarations can't add class inheritance to an existing class, -and therefore you can specify only a list of protocols after the *type name* and colon. - -### Conditional Conformance - -You can extend a generic type -to conditionally conform to a protocol, -so that instances of the type conform to the protocol -only when certain requirements are met. -You add conditional conformance to a protocol -by including *requirements* in an extension declaration. - -#### Overridden Requirements Aren't Used in Some Generic Contexts - -In some generic contexts, -types that get behavior from conditional conformance to a protocol -don't always use the specialized implementations -of that protocol's requirements. -To illustrate this behavior, -the following example defines two protocols -and a generic type that conditionally conforms to both protocols. - - - -```swift -protocol Loggable { - func log() -} -extension Loggable { - func log() { - print(self) - } -} - -protocol TitledLoggable: Loggable { - static var logTitle: String { get } -} -extension TitledLoggable { - func log() { - print("\(Self.logTitle): \(self)") - } -} - -struct Pair: CustomStringConvertible { - let first: T - let second: T - var description: String { - return "(\(first), \(second))" - } -} - -extension Pair: Loggable where T: Loggable { } -extension Pair: TitledLoggable where T: TitledLoggable { - static var logTitle: String { - return "Pair of '\(T.logTitle)'" - } -} - -extension String: TitledLoggable { - static var logTitle: String { - return "String" - } -} -``` - - - -The `Pair` structure conforms to `Loggable` and `TitledLoggable` -whenever its generic type conforms to `Loggable` or `TitledLoggable`, respectively. -In the example below, -`oneAndTwo` is an instance of `Pair`, -which conforms to `TitledLoggable` -because `String` conforms to `TitledLoggable`. -When the `log()` method is called on `oneAndTwo` directly, -the specialized version containing the title string is used. - -```swift -let oneAndTwo = Pair(first: "one", second: "two") -oneAndTwo.log() -// Prints "Pair of 'String': (one, two)" -``` - - - -However, when `oneAndTwo` is used in a generic context -or as an instance of the `Loggable` protocol, -the specialized version isn't used. -Swift picks which implementation of `log()` to call -by consulting only the minimum requirements that `Pair` needs to conform to `Loggable`. -For this reason, -the default implementation provided by the `Loggable` protocol is used instead. - -```swift -func doSomething(with x: T) { - x.log() -} -doSomething(with: oneAndTwo) -// Prints "(one, two)" -``` - - - -When `log()` is called on the instance that's passed to `doSomething(_:)`, -the customized title is omitted from the logged string. - -### Protocol Conformance Must Not Be Redundant - -A concrete type can conform to a particular protocol only once. -Swift marks redundant protocol conformances as an error. -You're likely to encounter this kind of error -in two kinds of situations. -The first situation is when -you explicitly conform to the same protocol multiple times, -but with different requirements. -The second situation is when -you implicitly inherit from the same protocol multiple times. -These situations are discussed in the sections below. - -#### Resolving Explicit Redundancy - -Multiple extensions on a concrete type -can't add conformance to the same protocol, -even if the extensions' requirements are mutually exclusive. -This restriction is demonstrated in the example below. -Two extension declarations attempt to add conditional conformance -to the `Serializable` protocol, -one for arrays with `Int` elements, -and one for arrays with `String` elements. - -```swift -protocol Serializable { - func serialize() -> Any -} - -extension Array: Serializable where Element == Int { - func serialize() -> Any { - // implementation - } -} -extension Array: Serializable where Element == String { - func serialize() -> Any { - // implementation - } -} -// Error: redundant conformance of 'Array' to protocol 'Serializable' -``` - - - -If you need to add conditional conformance based on multiple concrete types, -create a new protocol that each type can conform to -and use that protocol as the requirement when declaring conditional conformance. - -```swift -protocol SerializableInArray { } -extension Int: SerializableInArray { } -extension String: SerializableInArray { } - -extension Array: Serializable where Element: SerializableInArray { - func serialize() -> Any { - // implementation - } -} -``` - - - -#### Resolving Implicit Redundancy - -When a concrete type conditionally conforms to a protocol, -that type implicitly conforms to any parent protocols -with the same requirements. - -If you need a type to conditionally conform to two protocols -that inherit from a single parent, -explicitly declare conformance to the parent protocol. -This avoids implicitly conforming to the parent protocol twice -with different requirements. - -The following example explicitly declares -the conditional conformance of `Array` to `Loggable` -to avoid a conflict when declaring its conditional conformance -to both `TitledLoggable` and the new `MarkedLoggable` protocol. - -```swift -protocol MarkedLoggable: Loggable { - func markAndLog() -} - -extension MarkedLoggable { - func markAndLog() { - print("----------") - log() - } -} - -extension Array: Loggable where Element: Loggable { } -extension Array: TitledLoggable where Element: TitledLoggable { - static var logTitle: String { - return "Array of '\(Element.logTitle)'" - } -} -extension Array: MarkedLoggable where Element: MarkedLoggable { } -``` - - - -Without the extension -to explicitly declare conditional conformance to `Loggable`, -the other `Array` extensions would implicitly create these declarations, -resulting in an error: - -```swift -extension Array: Loggable where Element: TitledLoggable { } -extension Array: Loggable where Element: MarkedLoggable { } -// Error: redundant conformance of 'Array' to protocol 'Loggable' -``` - - - - - - - - - -> Grammar of an extension declaration: -> -> *extension-declaration* → *attributes*_?_ *access-level-modifier*_?_ **`extension`** *type-identifier* *type-inheritance-clause*_?_ *generic-where-clause*_?_ *extension-body* \ -> *extension-body* → **`{`** *extension-members*_?_ **`}`** -> -> *extension-members* → *extension-member* *extension-members*_?_ \ -> *extension-member* → *declaration* | *compiler-control-statement* - -## Subscript Declaration - -A *subscript* declaration allows you to add subscripting support for objects -of a particular type and are typically used to provide a convenient syntax -for accessing the elements in a collection, list, or sequence. -Subscript declarations are declared using the `subscript` keyword -and have the following form: - -```swift -subscript (<#parameters#>) -> <#return type#> { - get { - <#statements#> - } - set(<#setter name#>) { - <#statements#> - } -} -``` - -Subscript declarations can appear only in the context of a class, structure, -enumeration, extension, or protocol declaration. - -The *parameters* specify one or more indexes used to access elements of the corresponding type -in a subscript expression (for example, the `i` in the expression `object[i]`). -Although the indexes used to access the elements can be of any type, -each parameter must include a type annotation to specify the type of each index. -The *return type* specifies the type of the element being accessed. - -As with computed properties, -subscript declarations support reading and writing the value of the accessed elements. -The getter is used to read the value, -and the setter is used to write the value. -The setter clause is optional, -and when only a getter is needed, you can omit both clauses and simply -return the requested value directly. -That said, if you provide a setter clause, you must also provide a getter clause. - -The *setter name* and enclosing parentheses are optional. -If you provide a setter name, it's used as the name of the parameter to the setter. -If you don't provide a setter name, the default parameter name to the setter is `value`. -The type of the parameter to the setter is the same as the *return type*. - -You can overload a subscript declaration in the type in which it's declared, -as long as the *parameters* or the *return type* differ from the one you're overloading. -You can also override a subscript declaration inherited from a superclass. When you do so, -you must mark the overridden subscript declaration with the `override` declaration modifier. - -Subscript parameters follow the same rules as function parameters, -with two exceptions. -By default, the parameters used in subscripting don't have argument labels, -unlike functions, methods, and initializers. -However, you can provide explicit argument labels -using the same syntax that functions, methods, and initializers use. -In addition, subscripts can't have in-out parameters. -A subscript parameter can have a default value, -using the syntax described in . - -You can also declare subscripts in the context of a protocol declaration, -as described in . - -For more information about subscripting and to see examples of subscript declarations, -see . - -### Type Subscript Declarations - -To declare a subscript that's exposed by the type, -rather than by instances of the type, -mark the subscript declaration with the `static` declaration modifier. -Classes can mark type computed properties with the `class` declaration modifier instead -to allow subclasses to override the superclass’s implementation. -In a class declaration, -the `static` keyword has the same effect as marking the declaration -with both the `class` and `final` declaration modifiers. - - - -> Grammar of a subscript declaration: -> -> *subscript-declaration* → *subscript-head* *subscript-result* *generic-where-clause*_?_ *code-block* \ -> *subscript-declaration* → *subscript-head* *subscript-result* *generic-where-clause*_?_ *getter-setter-block* \ -> *subscript-declaration* → *subscript-head* *subscript-result* *generic-where-clause*_?_ *getter-setter-keyword-block* \ -> *subscript-head* → *attributes*_?_ *declaration-modifiers*_?_ **`subscript`** *generic-parameter-clause*_?_ *parameter-clause* \ -> *subscript-result* → **`->`** *attributes*_?_ *type* - -## Macro Declaration - -A *macro declaration* introduces a new macro. -It begins with the `macro` keyword -and has the following form: - -```swift -macro <#name#> = <#macro implementation#> -``` - -The *macro implementation* is another macro, -and indicates the location of the code that performs this macro's expansion. -The code that performs macro expansion is a separate Swift program, -that uses the [SwiftSyntax][] module to interact with Swift code. -Call the `externalMacro(module:type:)` macro from the Swift standard library, -passing in the name of a type that contains the macro's implementation, -and the name of the module that contains that type. - -[SwiftSyntax]: http://github.com/apple/swift-syntax/ - -Macros can be overloaded, -following the same model used by functions. -A macro declaration appears only at file scope. - -For an overview of macros in Swift, see . - -> Grammar of a macro declaration: -> -> *macro-declaration* → *macro-head* *identifier* *generic-parameter-clause*_?_ *macro-signature* *macro-definition*_?_ *generic-where-clause* \ -> *macro-head* → *attributes*_?_ *declaration-modifiers*_?_ **`macro`** \ -> *macro-signature* → *parameter-clause* *macro-function-signature-result*_?_ \ -> *macro-function-signature-result* → **`->`** *type* \ -> *macro-definition* → **`=`** *expression* - -## Operator Declaration - -An *operator declaration* introduces a new infix, prefix, -or postfix operator into your program -and is declared using the `operator` keyword. - -You can declare operators of three different fixities: -infix, prefix, and postfix. -The *fixity* of an operator specifies the relative position of an operator -to its operands. - -There are three basic forms of an operator declaration, -one for each fixity. -The fixity of the operator is specified by marking the operator declaration with the -`infix`, `prefix`, or `postfix` declaration modifier before the `operator` keyword. -In each form, the name of the operator can contain only the operator characters -defined in . - -The following form declares a new infix operator: - -```swift -infix operator <#operator name#>: <#precedence group#> -``` - -An *infix operator* is a binary operator that's written between its two operands, -such as the familiar addition operator (`+`) in the expression `1 + 2`. - -Infix operators can optionally specify a precedence group. -If you omit the precedence group for an operator, -Swift uses the default precedence group, `DefaultPrecedence`, -which specifies a precedence just higher than `TernaryPrecedence`. -For more information, see . - -The following form declares a new prefix operator: - -```swift -prefix operator <#operator name#> -``` - -A *prefix operator* is a unary operator that's written immediately before its operand, -such as the prefix logical NOT operator (`!`) in the expression `!a`. - -Prefix operators declarations don't specify a precedence level. -Prefix operators are nonassociative. - -The following form declares a new postfix operator: - -```swift -postfix operator <#operator name#> -``` - -A *postfix operator* is a unary operator that's written immediately after its operand, -such as the postfix forced-unwrap operator (`!`) in the expression `a!`. - -As with prefix operators, postfix operator declarations don't specify a precedence level. -Postfix operators are nonassociative. - -After declaring a new operator, -you implement it by declaring a static method that has the same name as the operator. -The static method is a member of -one of the types whose values the operator takes as an argument --- -for example, an operator that multiplies a `Double` by an `Int` -is implemented as a static method on either the `Double` or `Int` structure. -If you're implementing a prefix or postfix operator, -you must also mark that method declaration with the corresponding `prefix` or `postfix` -declaration modifier. -To see an example of how to create and implement a new operator, -see . - -> Grammar of an operator declaration: -> -> *operator-declaration* → *prefix-operator-declaration* | *postfix-operator-declaration* | *infix-operator-declaration* -> -> *prefix-operator-declaration* → **`prefix`** **`operator`** *operator* \ -> *postfix-operator-declaration* → **`postfix`** **`operator`** *operator* \ -> *infix-operator-declaration* → **`infix`** **`operator`** *operator* *infix-operator-group*_?_ -> -> *infix-operator-group* → **`:`** *precedence-group-name* - -## Precedence Group Declaration - -A *precedence group declaration* introduces -a new grouping for infix operator precedence into your program. -The precedence of an operator specifies how tightly the operator -binds to its operands, in the absence of grouping parentheses. - -A precedence group declaration has the following form: - -```swift -precedencegroup <#precedence group name#> { - higherThan: <#lower group names#> - lowerThan: <#higher group names#> - associativity: <#associativity#> - assignment: <#assignment#> -} -``` - -The *lower group names* and *higher group names* lists specify -the new precedence group's relation to existing precedence groups. -The `lowerThan` precedence group attribute may only be used -to refer to precedence groups declared outside of the current module. -When two operators compete with each other for their operands, -such as in the expression `2 + 3 * 5`, -the operator with the higher relative precedence -binds more tightly to its operands. - -> Note: Precedence groups related to each other -> using *lower group names* and *higher group names* -> must fit into a single relational hierarchy, -> but they *don't* have to form a linear hierarchy. -> This means it's possible to have precedence groups -> with undefined relative precedence. -> Operators from those precedence groups -> can't be used next to each other without grouping parentheses. - -Swift defines numerous precedence groups to go along -with the operators provided by the Swift standard library. -For example, the addition (`+`) and subtraction (`-`) operators -belong to the `AdditionPrecedence` group, -and the multiplication (`*`) and division (`/`) operators -belong to the `MultiplicationPrecedence` group. -For a complete list of precedence groups -provided by the Swift standard library, -see [Operator Declarations](https://developer.apple.com/documentation/swift/operator_declarations). - -The *associativity* of an operator specifies how a sequence of operators -with the same precedence level are grouped together in the absence of grouping parentheses. -You specify the associativity of an operator by writing -one of the context-sensitive keywords `left`, `right`, or `none` --- -if your omit the associativity, the default is `none`. -Operators that are left-associative group left-to-right. -For example, -the subtraction operator (`-`) is left-associative, -so the expression `4 - 5 - 6` is grouped as `(4 - 5) - 6` -and evaluates to `-7`. -Operators that are right-associative group right-to-left, -and operators that are specified with an associativity of `none` -don't associate at all. -Nonassociative operators of the same precedence level -can't appear adjacent to each to other. -For example, -the `<` operator has an associativity of `none`, -which means `1 < 2 < 3` isn't a valid expression. - -The *assignment* of a precedence group specifies the precedence of an operator -when used in an operation that includes optional chaining. -When set to `true`, an operator in the corresponding precedence group -uses the same grouping rules during optional chaining -as the assignment operators from the Swift standard library. -Otherwise, when set to `false` or omitted, -operators in the precedence group follows the same optional chaining rules -as operators that don't perform assignment. - -> Grammar of a precedence group declaration: -> -> *precedence-group-declaration* → **`precedencegroup`** *precedence-group-name* **`{`** *precedence-group-attributes*_?_ **`}`** -> -> *precedence-group-attributes* → *precedence-group-attribute* *precedence-group-attributes*_?_ \ -> *precedence-group-attribute* → *precedence-group-relation* \ -> *precedence-group-attribute* → *precedence-group-assignment* \ -> *precedence-group-attribute* → *precedence-group-associativity* -> -> *precedence-group-relation* → **`higherThan`** **`:`** *precedence-group-names* \ -> *precedence-group-relation* → **`lowerThan`** **`:`** *precedence-group-names* -> -> *precedence-group-assignment* → **`assignment`** **`:`** *boolean-literal* -> -> *precedence-group-associativity* → **`associativity`** **`:`** **`left`** \ -> *precedence-group-associativity* → **`associativity`** **`:`** **`right`** \ -> *precedence-group-associativity* → **`associativity`** **`:`** **`none`** -> -> *precedence-group-names* → *precedence-group-name* | *precedence-group-name* **`,`** *precedence-group-names* \ -> *precedence-group-name* → *identifier* - -## Declaration Modifiers - -*Declaration modifiers* are keywords or context-sensitive keywords that modify the behavior -or meaning of a declaration. You specify a declaration modifier by writing the appropriate -keyword or context-sensitive keyword between a declaration's attributes (if any) and the keyword -that introduces the declaration. - -- term `class`: - Apply this modifier to a member of a class - to indicate that the member is a member of the class itself, - rather than a member of instances of the class. - Members of a superclass that have this modifier - and don't have the `final` modifier - can be overridden by subclasses. - -- term `dynamic`: - Apply this modifier to any member of a class that can be represented by Objective-C. - When you mark a member declaration with the `dynamic` modifier, - access to that member is always dynamically dispatched using the Objective-C runtime. - Access to that member is never inlined or devirtualized by the compiler. - - Because declarations marked with the `dynamic` modifier are dispatched - using the Objective-C runtime, they must be marked with the - `objc` attribute. - -- term `final`: - Apply this modifier to a class or to a property, method, - or subscript member of a class. It's applied to a class to indicate that the class - can't be subclassed. It's applied to a property, method, or subscript of a class - to indicate that a class member can't be overridden in any subclass. - For an example of how to use the `final` attribute, - see . - -- term `lazy`: - Apply this modifier to a stored variable property of a class or structure - to indicate that the property's initial value is calculated and stored at most - once, when the property is first accessed. - For an example of how to use the `lazy` modifier, - see . - -- term `optional`: - Apply this modifier to a protocol's property, method, - or subscript members to indicate that a conforming type isn't required - to implement those members. - - You can apply the `optional` modifier only to protocols that are marked - with the `objc` attribute. As a result, only class types can adopt and conform - to a protocol that contains optional member requirements. - For more information about how to use the `optional` modifier - and for guidance about how to access optional protocol members --- - for example, when you're not sure whether a conforming type implements them --- - see . - - - -- term `required`: - Apply this modifier to a designated or convenience initializer - of a class to indicate that every subclass must implement that initializer. - The subclass's implementation of that initializer - must also be marked with the `required` modifier. - -- term `static`: - Apply this modifier to a member of a structure, class, enumeration, or protocol - to indicate that the member is a member of the type, - rather than a member of instances of that type. - In the scope of a class declaration, - writing the `static` modifier on a member declaration - has the same effect as writing the `class` and `final` modifiers - on that member declaration. - However, constant type properties of a class are an exception: - `static` has its normal, nonclass meaning there - because you can't write `class` or `final` on those declarations. - -- term `unowned`: - Apply this modifier to a stored variable, constant, or stored property - to indicate that the variable or property has an unowned reference - to the object stored as its value. - If you try to access the variable or property - after the object has been deallocated, - a runtime error is raised. - Like a weak reference, - the type of the property or value must be a class type; - unlike a weak reference, - the type is non-optional. - For an example and more information about the `unowned` modifier, - see . - -- term `unowned(safe)`: - An explicit spelling of `unowned`. - -- term `unowned(unsafe)`: - Apply this modifier to a stored variable, constant, or stored property - to indicate that the variable or property has an unowned reference - to the object stored as its value. - If you try to access the variable or property - after the object has been deallocated, - you'll access the memory at the location where the object used to be, - which is a memory-unsafe operation. - Like a weak reference, - the type of the property or value must be a class type; - unlike a weak reference, - the type is non-optional. - For an example and more information about the `unowned` modifier, - see . - -- term `weak`: - Apply this modifier to a stored variable or stored variable property - to indicate that the variable or property has a weak reference to the - object stored as its value. The type of the variable or property - must be an optional class type. - If you access the variable or property - after the object has been deallocated, - its value is `nil`. - For an example and more information about the `weak` modifier, - see . - -### Access Control Levels - -Swift provides five levels of access control: open, public, internal, file private, and private. -You can mark a declaration with one of the access-level modifiers below -to specify the declaration's access level. -Access control is discussed in detail in . - -- term `open`: - Apply this modifier to a declaration to indicate the declaration can be accessed and subclassed - by code in the same module as the declaration. - Declarations marked with the `open` access-level modifier can also be accessed and subclassed - by code in a module that imports the module that contains that declaration. - -- term `public`: - Apply this modifier to a declaration to indicate the declaration can be accessed and subclassed - by code in the same module as the declaration. - Declarations marked with the `public` access-level modifier can also be accessed (but not subclassed) - by code in a module that imports the module that contains that declaration. - -- term `package`: - Apply this modifier to a declaration - to indicate that the declaration can be accessed - only by code in the same package as the declaration. - A package is a unit of code distribution - that you define in the build system you're using. - When the build system compiles code, - it specifies the package name - by passing the `-package-name` flag to the Swift compiler. - Two modules are part of the same package - if the build system specifies the same package name when building them. - -- term `internal`: - Apply this modifier to a declaration to indicate the declaration can be accessed - only by code in the same module as the declaration. - By default, - most declarations are implicitly marked with the `internal` access-level modifier. - -- term `fileprivate`: - Apply this modifier to a declaration to indicate the declaration can be accessed - only by code in the same source file as the declaration. - -- term `private`: - Apply this modifier to a declaration to indicate the declaration can be accessed - only by code within the declaration's immediate enclosing scope. - -For the purpose of access control, -extensions behave as follows: - -- If there are multiple extensions in the same file, - and those extensions all extend the same type, - then all of those extensions have the same access-control scope. - The extensions and the type they extend can be in different files. - -- If there are extensions in the same file as the type they extend, - the extensions have the same access-control scope as the type they extend. - -- Private members declared in a type's declaration - can be accessed from extensions to that type. - Private members declared in one extension - can be accessed from other extensions - and from the extended type's declaration. - -Each access-level modifier above optionally accepts a single argument, -which consists of the `set` keyword enclosed in parentheses --- -for example, `private(set)`. -Use this form of an access-level modifier when you want to specify an access level -for the setter of a variable or subscript that's less than or equal -to the access level of the variable or subscript itself, -as discussed in . - -> Grammar of a declaration modifier: -> -> *declaration-modifier* → **`class`** | **`convenience`** | **`dynamic`** | **`final`** | **`infix`** | **`lazy`** | **`optional`** | **`override`** | **`postfix`** | **`prefix`** | **`required`** | **`static`** | **`unowned`** | **`unowned`** **`(`** **`safe`** **`)`** | **`unowned`** **`(`** **`unsafe`** **`)`** | **`weak`** \ -> *declaration-modifier* → *access-level-modifier* \ -> *declaration-modifier* → *mutation-modifier* \ -> *declaration-modifier* → *actor-isolation-modifier* \ -> *declaration-modifiers* → *declaration-modifier* *declaration-modifiers*_?_ -> -> *access-level-modifier* → **`private`** | **`private`** **`(`** **`set`** **`)`** \ -> *access-level-modifier* → **`fileprivate`** | **`fileprivate`** **`(`** **`set`** **`)`** \ -> *access-level-modifier* → **`internal`** | **`internal`** **`(`** **`set`** **`)`** \ -> *access-level-modifier* → **`package`** | **`package`** **`(`** **`set`** **`)`** \ -> *access-level-modifier* → **`public`** | **`public`** **`(`** **`set`** **`)`** \ -> *access-level-modifier* → **`open`** | **`open`** **`(`** **`set`** **`)`** -> -> *mutation-modifier* → **`mutating`** | **`nonmutating`** -> -> *actor-isolation-modifier* → **`nonisolated`** - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/ReferenceManual/Expressions.md b/swift-6-beta.docc/ReferenceManual/Expressions.md deleted file mode 100644 index eac7bc81a..000000000 --- a/swift-6-beta.docc/ReferenceManual/Expressions.md +++ /dev/null @@ -1,3268 +0,0 @@ -# Expressions - -Access, modify, and assign values. - -In Swift, there are four kinds of expressions: -prefix expressions, infix expressions, primary expressions, and postfix expressions. -Evaluating an expression returns a value, -causes a side effect, or both. - -Prefix and infix expressions let you -apply operators to smaller expressions. -Primary expressions are conceptually the simplest kind of expression, -and they provide a way to access values. -Postfix expressions, -like prefix and infix expressions, -let you build up more complex expressions -using postfixes such as function calls and member access. -Each kind of expression is described in detail -in the sections below. - -> Grammar of an expression: -> -> *expression* → *try-operator*_?_ *await-operator*_?_ *prefix-expression* *infix-expressions*_?_ \ - -## Prefix Expressions - -*Prefix expressions* combine -an optional prefix operator with an expression. -Prefix operators take one argument, -the expression that follows them. - -For information about the behavior of these operators, -see and . - -For information about the operators provided by the Swift standard library, -see [Operator Declarations](https://developer.apple.com/documentation/swift/operator_declarations). - -> Grammar of a prefix expression: -> -> *prefix-expression* → *prefix-operator*_?_ *postfix-expression* \ -> *prefix-expression* → *in-out-expression* - -### In-Out Expression - -An *in-out expression* marks a variable -that's being passed -as an in-out argument to a function call expression. - -```swift -&<#expression#> -``` - -For more information about in-out parameters and to see an example, -see . - -In-out expressions are also used -when providing a non-pointer argument -in a context where a pointer is needed, -as described in . - -> Grammar of an in-out expression: -> -> *in-out-expression* → **`&`** *primary-expression* - -### Try Operator - -A *try expression* consists of the `try` operator -followed by an expression that can throw an error. -It has the following form: - -```swift -try <#expression#> -``` - -The value of a `try` expression is the value of the *expression*. - -An *optional-try expression* consists of the `try?` operator -followed by an expression that can throw an error. -It has the following form: - -```swift -try? <#expression#> -``` - -If the *expression* doesn't throw an error, -the value of the optional-try expression -is an optional containing the value of the *expression*. -Otherwise, the value of the optional-try expression is `nil`. - -A *forced-try expression* consists of the `try!` operator -followed by an expression that can throw an error. -It has the following form: - -```swift -try! <#expression#> -``` - -The value of a forced-try expression is the value of the *expression*. -If the *expression* throws an error, -a runtime error is produced. - -When the expression on the left-hand side of an infix operator -is marked with `try`, `try?`, or `try!`, -that operator applies to the whole infix expression. -That said, you can use parentheses to be explicit about the scope of the operator's application. - -```swift -// try applies to both function calls -sum = try someThrowingFunction() + anotherThrowingFunction() - -// try applies to both function calls -sum = try (someThrowingFunction() + anotherThrowingFunction()) - -// Error: try applies only to the first function call -sum = (try someThrowingFunction()) + anotherThrowingFunction() -``` - - - -A `try` expression can't appear on the right-hand side of an infix operator, -unless the infix operator is the assignment operator -or the `try` expression is enclosed in parentheses. - - - -If an expression includes both the `try` and `await` operator, -the `try` operator must appear first. - - - -For more information and to see examples of how to use `try`, `try?`, and `try!`, -see . - -> Grammar of a try expression: -> -> *try-operator* → **`try`** | **`try`** **`?`** | **`try`** **`!`** - -### Await Operator - -An *await expression* consists of the `await` operator -followed by an expression that uses the result of an asynchronous operation. -It has the following form: - -```swift -await <#expression#> -``` - -The value of an `await` expression is the value of the *expression*. - -An expression marked with `await` is called a *potential suspension point*. -Execution of an asynchronous function can be suspended -at each expression that's marked with `await`. -In addition, -execution of concurrent code is never suspended at any other point. -This means code between potential suspension points -can safely update state that requires temporarily breaking invariants, -provided that it completes the update -before the next potential suspension point. - -An `await` expression can appear only within an asynchronous context, -such as the trailing closure passed to the `async(priority:operation:)` function. -It can't appear in the body of a `defer` statement, -or in an autoclosure of synchronous function type. - -When the expression on the left-hand side of an infix operator -is marked with the `await` operator, -that operator applies to the whole infix expression. -That said, you can use parentheses -to be explicit about the scope of the operator's application. - -```swift -// await applies to both function calls -sum = await someAsyncFunction() + anotherAsyncFunction() - -// await applies to both function calls -sum = await (someAsyncFunction() + anotherAsyncFunction()) - -// Error: await applies only to the first function call -sum = (await someAsyncFunction()) + anotherAsyncFunction() -``` - - - -An `await` expression can't appear on the right-hand side of an infix operator, -unless the infix operator is the assignment operator -or the `await` expression is enclosed in parentheses. - - - -If an expression includes both the `await` and `try` operator, -the `try` operator must appear first. - - - -> Grammar of an await expression: -> -> *await-operator* → **`await`** - -## Infix Expressions - -*Infix expressions* combine -an infix binary operator with the expression that it takes -as its left- and right-hand arguments. -It has the following form: - -```swift -<#left-hand argument#> <#operator#> <#right-hand argument#> -``` - -For information about the behavior of these operators, -see and . - -For information about the operators provided by the Swift standard library, -see [Operator Declarations](https://developer.apple.com/documentation/swift/operator_declarations). - - - -> Note: At parse time, -> an expression made up of infix operators is represented -> as a flat list. -> This list is transformed into a tree -> by applying operator precedence. -> For example, the expression `2 + 3 * 5` -> is initially understood as a flat list of five items, -> `2`, `+`, `3`, `*`, and `5`. -> This process transforms it into the tree (2 + (3 * 5)). - -> Grammar of an infix expression: -> -> *infix-expression* → *infix-operator* *prefix-expression* \ -> *infix-expression* → *assignment-operator* *try-operator*_?_ *await-operator*_?_ *prefix-expression* \ -> *infix-expression* → *conditional-operator* *try-operator*_?_ *await-operator*_?_ *prefix-expression* \ -> *infix-expression* → *type-casting-operator* \ -> *infix-expressions* → *infix-expression* *infix-expressions*_?_ - -### Assignment Operator - -The *assignment operator* sets a new value -for a given expression. -It has the following form: - -```swift -<#expression#> = <#value#> -``` - -The value of the *expression* -is set to the value obtained by evaluating the *value*. -If the *expression* is a tuple, -the *value* must be a tuple -with the same number of elements. -(Nested tuples are allowed.) -Assignment is performed from each part of the *value* -to the corresponding part of the *expression*. -For example: - -```swift -(a, _, (b, c)) = ("test", 9.45, (12, 3)) -// a is "test", b is 12, c is 3, and 9.45 is ignored -``` - - - -The assignment operator doesn't return any value. - -> Grammar of an assignment operator: -> -> *assignment-operator* → **`=`** - -### Ternary Conditional Operator - -The *ternary conditional operator* evaluates to one of two given values -based on the value of a condition. -It has the following form: - -```swift -<#condition#> ? <#expression used if true#> : <#expression used if false#> -``` - -If the *condition* evaluates to `true`, -the conditional operator evaluates the first expression -and returns its value. -Otherwise, it evaluates the second expression -and returns its value. -The unused expression isn't evaluated. - -For an example that uses the ternary conditional operator, -see . - -> Grammar of a conditional operator: -> -> *conditional-operator* → **`?`** *expression* **`:`** - -### Type-Casting Operators - -There are four type-casting operators: -the `is` operator, -the `as` operator, -the `as?` operator, -and the `as!` operator. - -They have the following form: - -```swift -<#expression#> is <#type#> -<#expression#> as <#type#> -<#expression#> as? <#type#> -<#expression#> as! <#type#> -``` - -The `is` operator checks at runtime whether the *expression* -can be cast to the specified *type*. -It returns `true` if the *expression* can be cast to the specified *type*; -otherwise, it returns `false`. - - - - - -The `as` operator performs a cast -when it's known at compile time -that the cast always succeeds, -such as upcasting or bridging. -Upcasting lets you use an expression as an instance of its type's supertype, -without using an intermediate variable. -The following approaches are equivalent: - -```swift -func f(_ any: Any) { print("Function for Any") } -func f(_ int: Int) { print("Function for Int") } -let x = 10 -f(x) -// Prints "Function for Int" - -let y: Any = x -f(y) -// Prints "Function for Any" - -f(x as Any) -// Prints "Function for Any" -``` - - - -Bridging lets you use an expression of -a Swift standard library type such as `String` -as its corresponding Foundation type such as `NSString` -without needing to create a new instance. -For more information on bridging, -see [Working with Foundation Types](https://developer.apple.com/documentation/swift/imported_c_and_objective_c_apis/working_with_foundation_types). - -The `as?` operator -performs a conditional cast of the *expression* -to the specified *type*. -The `as?` operator returns an optional of the specified *type*. -At runtime, if the cast succeeds, -the value of *expression* is wrapped in an optional and returned; -otherwise, the value returned is `nil`. -If casting to the specified *type* -is guaranteed to fail or is guaranteed to succeed, -a compile-time error is raised. - -The `as!` operator performs a forced cast of the *expression* to the specified *type*. -The `as!` operator returns a value of the specified *type*, not an optional type. -If the cast fails, a runtime error is raised. -The behavior of `x as! T` is the same as the behavior of `(x as? T)!`. - -For more information about type casting -and to see examples that use the type-casting operators, -see . - -> Grammar of a type-casting operator: -> -> *type-casting-operator* → **`is`** *type* \ -> *type-casting-operator* → **`as`** *type* \ -> *type-casting-operator* → **`as`** **`?`** *type* \ -> *type-casting-operator* → **`as`** **`!`** *type* - -## Primary Expressions - -*Primary expressions* -are the most basic kind of expression. -They can be used as expressions on their own, -and they can be combined with other tokens -to make prefix expressions, infix expressions, and postfix expressions. - -> Grammar of a primary expression: -> -> *primary-expression* → *identifier* *generic-argument-clause*_?_ \ -> *primary-expression* → *literal-expression* \ -> *primary-expression* → *self-expression* \ -> *primary-expression* → *superclass-expression* \ -> *primary-expression* → *conditional-expression* \ -> *primary-expression* → *closure-expression* \ -> *primary-expression* → *parenthesized-expression* \ -> *primary-expression* → *tuple-expression* \ -> *primary-expression* → *implicit-member-expression* \ -> *primary-expression* → *wildcard-expression* \ -> *primary-expression* → *macro-expansion-expression* \ -> *primary-expression* → *key-path-expression* \ -> *primary-expression* → *selector-expression* \ -> *primary-expression* → *key-path-string-expression* - - - - - -### Literal Expression - -A *literal expression* consists of -either an ordinary literal (such as a string or a number), -an array or dictionary literal, -or a playground literal. - -> Note: -> Prior to Swift 5.9, -> the following special literals were recognized: -> `#column`, -> `#dsohandle`, -> `#fileID`, -> `#filePath`, -> `#file`, -> `#function`, -> and `#line`. -> These are now implemented as macros in the Swift standard library: -> [`column()`](https://developer.apple.com/documentation/swift/column()), -> [`dsohandle()`](https://developer.apple.com/documentation/swift/dsohandle()), -> [`fileID()`](https://developer.apple.com/documentation/swift/fileID()), -> [`filePath()`](https://developer.apple.com/documentation/swift/filePath()), -> [`file()`](https://developer.apple.com/documentation/swift/file()), -> [`function()`](https://developer.apple.com/documentation/swift/function()), -> and [`line()`](https://developer.apple.com/documentation/swift/line()). - - - -An *array literal* is -an ordered collection of values. -It has the following form: - -```swift -[<#value 1#>, <#value 2#>, <#...#>] -``` - -The last expression in the array can be followed by an optional comma. -The value of an array literal has type `[T]`, -where `T` is the type of the expressions inside it. -If there are expressions of multiple types, -`T` is their closest common supertype. -Empty array literals are written using an empty -pair of square brackets and can be used to create an empty array of a specified type. - -```swift -var emptyArray: [Double] = [] -``` - - - -A *dictionary literal* is -an unordered collection of key-value pairs. -It has the following form: - -```swift -[<#key 1#>: <#value 1#>, <#key 2#>: <#value 2#>, <#...#>] -``` - -The last expression in the dictionary can be followed by an optional comma. -The value of a dictionary literal has type `[Key: Value]`, -where `Key` is the type of its key expressions -and `Value` is the type of its value expressions. -If there are expressions of multiple types, -`Key` and `Value` are the closest common supertype -for their respective values. -An empty dictionary literal is written as -a colon inside a pair of brackets (`[:]`) -to distinguish it from an empty array literal. -You can use an empty dictionary literal to create an empty dictionary literal -of specified key and value types. - -```swift -var emptyDictionary: [String: Double] = [:] -``` - - - -A *playground literal* -is used by Xcode to create an interactive representation -of a color, file, or image within the program editor. -Playground literals in plain text outside of Xcode -are represented using a special literal syntax. - -For information on using playground literals in Xcode, -see [Add a color, file, or image literal](https://help.apple.com/xcode/mac/current/#/dev4c60242fc) -in Xcode Help. - -> Grammar of a literal expression: -> -> *literal-expression* → *literal* \ -> *literal-expression* → *array-literal* | *dictionary-literal* | *playground-literal* -> -> *array-literal* → **`[`** *array-literal-items*_?_ **`]`** \ -> *array-literal-items* → *array-literal-item* **`,`**_?_ | *array-literal-item* **`,`** *array-literal-items* \ -> *array-literal-item* → *expression* -> -> *dictionary-literal* → **`[`** *dictionary-literal-items* **`]`** | **`[`** **`:`** **`]`** \ -> *dictionary-literal-items* → *dictionary-literal-item* **`,`**_?_ | *dictionary-literal-item* **`,`** *dictionary-literal-items* \ -> *dictionary-literal-item* → *expression* **`:`** *expression* -> -> *playground-literal* → **`#colorLiteral`** **`(`** **`red`** **`:`** *expression* **`,`** **`green`** **`:`** *expression* **`,`** **`blue`** **`:`** *expression* **`,`** **`alpha`** **`:`** *expression* **`)`** \ -> *playground-literal* → **`#fileLiteral`** **`(`** **`resourceName`** **`:`** *expression* **`)`** \ -> *playground-literal* → **`#imageLiteral`** **`(`** **`resourceName`** **`:`** *expression* **`)`** - -### Self Expression - -The `self` expression is an explicit reference to the current type -or instance of the type in which it occurs. -It has the following forms: - -```swift -self -self.<#member name#> -self[<#subscript index#>] -self(<#initializer arguments#>) -self.init(<#initializer arguments#>) -``` - - - -In an initializer, subscript, or instance method, `self` refers to the current -instance of the type in which it occurs. In a type method, -`self` refers to the current type in which it occurs. - -The `self` expression is used to specify scope when accessing members, -providing disambiguation when there's -another variable of the same name in scope, -such as a function parameter. -For example: - -```swift -class SomeClass { - var greeting: String - init(greeting: String) { - self.greeting = greeting - } -} -``` - - - -In a mutating method of a value type, -you can assign a new instance of that value type to `self`. -For example: - -```swift -struct Point { - var x = 0.0, y = 0.0 - mutating func moveBy(x deltaX: Double, y deltaY: Double) { - self = Point(x: x + deltaX, y: y + deltaY) - } -} -``` - - - - - -> Grammar of a self expression: -> -> *self-expression* → **`self`** | *self-method-expression* | *self-subscript-expression* | *self-initializer-expression* -> -> *self-method-expression* → **`self`** **`.`** *identifier* \ -> *self-subscript-expression* → **`self`** **`[`** *function-call-argument-list* **`]`** \ -> *self-initializer-expression* → **`self`** **`.`** **`init`** - -### Superclass Expression - -A *superclass expression* lets a class -interact with its superclass. -It has one of the following forms: - -```swift -super.<#member name#> -super[<#subscript index#>] -super.init(<#initializer arguments#>) -``` - -The first form is used to access a member of the superclass. -The second form is used to access the superclass's subscript implementation. -The third form is used to access an initializer of the superclass. - -Subclasses can use a superclass expression -in their implementation of members, subscripting, and initializers -to make use of the implementation in their superclass. - -> Grammar of a superclass expression: -> -> *superclass-expression* → *superclass-method-expression* | *superclass-subscript-expression* | *superclass-initializer-expression* -> -> *superclass-method-expression* → **`super`** **`.`** *identifier* \ -> *superclass-subscript-expression* → **`super`** **`[`** *function-call-argument-list* **`]`** \ -> *superclass-initializer-expression* → **`super`** **`.`** **`init`** - -### Conditional Expression - -A *conditional expression* evaluates to one of several given values -based on the value of a condition. -It has one the following forms: - -```swift -if <#condition 1#> { - <#expression used if condition 1 is true#> -} else if <#condition 2#> { - <#expression used if condition 2 is true#> -} else { - <#expression used if both conditions are false#> -} - -switch <#expression#> { -case <#pattern 1#>: - <#expression 1#> -case <#pattern 2#> where <#condition#>: - <#expression 2#> -default: - <#expression 3#> -} -``` - -A conditional expression -has the same behavior and syntax as an `if` statement or a `switch` statement, -except for the differences that the paragraphs below describe. - -A conditional expression appears only in the following contexts: - - - As the value assigned to a variable. - - As the initial value in a variable or constant declaration. - - As the error thrown by a `throw` expression. - - As the value returned by a function, closure, or property getter. - - As the value inside a branch of a conditional expression. - -The branches of a conditional expression are exhaustive, -ensuring that the expression always produces a value -regardless of the condition. -This means each `if` branch needs a corresponding `else` branch. - -Each branch contains either a single expression, -which is used as the value for the conditional expression -when that branch's conditional is true, -a `throw` statement, -or a call to a function that never returns. - -Each branch must produce a value of the same type. -Because type checking of each branch is independent, -you sometimes need to specify the value's type explicitly, -like when branches include different kinds of literals, -or when a branch's value is `nil`. -When you need to provide this information, -add a type annotation to the variable that the result is assigned to, -or add an `as` cast to the branches' values. - -```swift -let number: Double = if someCondition { 10 } else { 12.34 } -let number = if someCondition { 10 as Double } else { 12.34 } -``` - -Inside a result builder, -conditional expressions can appear -only as the initial value of a variable or constant. -This behavior means when you write `if` or `switch` in a result builder --- -outside of a variable or constant declaration --- -that code is understood as a branch statement -and one of the result builder's methods transforms that code. - -Don't put a conditional expression in a `try` expression, -even if one of the branches of a conditional expression is throwing. - -> Grammar of a conditional expression: -> -> *conditional-expression* → *if-expression* | *switch-expression* -> -> *if-expression* → **`if`** *condition-list* **`{`** *statement* **`}`** *if-expression-tail* \ -> *if-expression-tail* → **`else`** *if-expression* \ -> *if-expression-tail* → **`else`** **`{`** *statement* **`}`** -> -> *switch-expression* → **`switch`** *expression* **`{`** *switch-expression-cases* **`}`** \ -> *switch-expression-cases* → *switch-expression-case* *switch-expression-cases*_?_ \ -> *switch-expression-case* → *case-label* *statement* \ -> *switch-expression-case* → *default-label* *statement* - -### Closure Expression - -A *closure expression* creates a closure, -also known as a *lambda* or an *anonymous function* -in other programming languages. -Like a function declaration, -a closure contains statements, -and it captures constants and variables from its enclosing scope. -It has the following form: - -```swift -{ (<#parameters#>) -> <#return type#> in - <#statements#> -} -``` - -The *parameters* have the same form -as the parameters in a function declaration, -as described in . - -Writing `throws` or `async` in a closure expression -explicitly marks a closure as throwing or asynchronous. - -```swift -{ (<#parameters#>) async throws -> <#return type#> in - <#statements#> -} -``` - -If the body of a closure includes a `throws` statement or a `try` expression -that isn't nested inside of a `do` statement with exhaustive error handling, -the closure is understood to be throwing. -If a throwing closure throws errors of only a single type, -the closure is understood as throwing that error type; -otherwise, it's understood as throwing `any Error`. -Likewise, if the body includes an `await` expression, -it's understood to be asynchronous. - -There are several special forms -that allow closures to be written more concisely: - - - -- A closure can omit the types - of its parameters, its return type, or both. - If you omit the parameter names and both types, - omit the `in` keyword before the statements. - If the omitted types can't be inferred, - a compile-time error is raised. -- A closure may omit names for its parameters. - Its parameters are then implicitly named - `$` followed by their position: - `$0`, `$1`, `$2`, and so on. -- A closure that consists of only a single expression - is understood to return the value of that expression. - The contents of this expression are also considered - when performing type inference on the surrounding expression. - -The following closure expressions are equivalent: - -```swift -myFunction { (x: Int, y: Int) -> Int in - return x + y -} - -myFunction { x, y in - return x + y -} - -myFunction { return $0 + $1 } - -myFunction { $0 + $1 } -``` - - - -For information about passing a closure as an argument to a function, -see . - -Closure expressions can be used -without being stored in a variable or constant, -such as when you immediately use a closure as part of a function call. -The closure expressions passed to `myFunction` in code above are -examples of this kind of immediate use. -As a result, -whether a closure expression is escaping or nonescaping depends -on the surrounding context of the expression. -A closure expression is nonescaping -if it's called immediately -or passed as a nonescaping function argument. -Otherwise, the closure expression is escaping. - -For more information about escaping closures, see . - -#### Capture Lists - -By default, a closure expression captures -constants and variables from its surrounding scope -with strong references to those values. -You can use a *capture list* to explicitly control -how values are captured in a closure. - -A capture list is written as a comma-separated list of expressions -surrounded by square brackets, -before the list of parameters. -If you use a capture list, you must also use the `in` keyword, -even if you omit the parameter names, parameter types, and return type. - -The entries in the capture list are initialized -when the closure is created. -For each entry in the capture list, -a constant is initialized -to the value of the constant or variable that has the same name -in the surrounding scope. -For example in the code below, -`a` is included in the capture list but `b` is not, -which gives them different behavior. - -```swift -var a = 0 -var b = 0 -let closure = { [a] in - print(a, b) -} - -a = 10 -b = 10 -closure() -// Prints "0 10" -``` - - - -There are two different things named `a`, -the variable in the surrounding scope -and the constant in the closure's scope, -but only one variable named `b`. -The `a` in the inner scope is initialized -with the value of the `a` in the outer scope -when the closure is created, -but their values aren't connected in any special way. -This means that a change to the value of `a` in the outer scope -doesn't affect the value of `a` in the inner scope, -nor does a change to `a` inside the closure -affect the value of `a` outside the closure. -In contrast, there's only one variable named `b` --- -the `b` in the outer scope --- -so changes from inside or outside the closure are visible in both places. - - - -This distinction isn't visible -when the captured variable's type has reference semantics. -For example, -there are two things named `x` in the code below, -a variable in the outer scope and a constant in the inner scope, -but they both refer to the same object -because of reference semantics. - -```swift -class SimpleClass { - var value: Int = 0 -} -var x = SimpleClass() -var y = SimpleClass() -let closure = { [x] in - print(x.value, y.value) -} - -x.value = 10 -y.value = 10 -closure() -// Prints "10 10" -``` - - - - - - - - - -If the type of the expression's value is a class, -you can mark the expression in a capture list -with `weak` or `unowned` to capture a weak or unowned reference -to the expression's value. - -```swift -myFunction { print(self.title) } // implicit strong capture -myFunction { [self] in print(self.title) } // explicit strong capture -myFunction { [weak self] in print(self!.title) } // weak capture -myFunction { [unowned self] in print(self.title) } // unowned capture -``` - - - -You can also bind an arbitrary expression -to a named value in a capture list. -The expression is evaluated when the closure is created, -and the value is captured with the specified strength. -For example: - -```swift -// Weak capture of "self.parent" as "parent" -myFunction { [weak parent = self.parent] in print(parent!.title) } -``` - - - -For more information and examples of closure expressions, -see . -For more information and examples of capture lists, -see . - - - -> Grammar of a closure expression: -> -> *closure-expression* → **`{`** *attributes*_?_ *closure-signature*_?_ *statements*_?_ **`}`** -> -> *closure-signature* → *capture-list*_?_ *closure-parameter-clause* **`async`**_?_ *throws-clause*_?_ *function-result*_?_ **`in`** \ -> *closure-signature* → *capture-list* **`in`** -> -> *closure-parameter-clause* → **`(`** **`)`** | **`(`** *closure-parameter-list* **`)`** | *identifier-list* \ -> *closure-parameter-list* → *closure-parameter* | *closure-parameter* **`,`** *closure-parameter-list* \ -> *closure-parameter* → *closure-parameter-name* *type-annotation*_?_ \ -> *closure-parameter* → *closure-parameter-name* *type-annotation* **`...`** \ -> *closure-parameter-name* → *identifier* -> -> *capture-list* → **`[`** *capture-list-items* **`]`** \ -> *capture-list-items* → *capture-list-item* | *capture-list-item* **`,`** *capture-list-items* \ -> *capture-list-item* → *capture-specifier*_?_ *identifier* \ -> *capture-list-item* → *capture-specifier*_?_ *identifier* **`=`** *expression* \ -> *capture-list-item* → *capture-specifier*_?_ *self-expression* \ -> *capture-specifier* → **`weak`** | **`unowned`** | **`unowned(safe)`** | **`unowned(unsafe)`** - -### Implicit Member Expression - -An *implicit member expression* -is an abbreviated way to access a member of a type, -such as an enumeration case or a type method, -in a context where type inference -can determine the implied type. -It has the following form: - -```swift -.<#member name#> -``` - -For example: - -```swift -var x = MyEnumeration.someValue -x = .anotherValue -``` - - - -If the inferred type is an optional, -you can also use a member of the non-optional type -in an implicit member expression. - -```swift -var someOptional: MyEnumeration? = .someValue -``` - - - -Implicit member expressions can be followed by -a postfix operator or other postfix syntax listed in -. -This is called a *chained implicit member expression*. -Although it's common for all of the chained postfix expressions -to have the same type, -the only requirement is that the whole chained implicit member expression -needs to be convertible to the type implied by its context. -Specifically, -if the implied type is an optional -you can use a value of the non-optional type, -and if the implied type is a class type -you can use a value of one of its subclasses. -For example: - -```swift -class SomeClass { - static var shared = SomeClass() - static var sharedSubclass = SomeSubclass() - var a = AnotherClass() -} -class SomeSubclass: SomeClass { } -class AnotherClass { - static var s = SomeClass() - func f() -> SomeClass { return AnotherClass.s } -} -let x: SomeClass = .shared.a.f() -let y: SomeClass? = .shared -let z: SomeClass = .sharedSubclass -``` - - - -In the code above, -the type of `x` matches the type implied by its context exactly, -the type of `y` is convertible from `SomeClass` to `SomeClass?`, -and the type of `z` is convertible from `SomeSubclass` to `SomeClass`. - -> Grammar of an implicit member expression: -> -> *implicit-member-expression* → **`.`** *identifier* \ -> *implicit-member-expression* → **`.`** *identifier* **`.`** *postfix-expression* - - - - - -### Parenthesized Expression - -A *parenthesized expression* consists of -an expression surrounded by parentheses. -You can use parentheses to specify the precedence of operations -by explicitly grouping expressions. -Grouping parentheses don't change an expression's type --- -for example, the type of `(1)` is simply `Int`. - - - -> Grammar of a parenthesized expression: -> -> *parenthesized-expression* → **`(`** *expression* **`)`** - -### Tuple Expression - -A *tuple expression* consists of -a comma-separated list of expressions surrounded by parentheses. -Each expression can have an optional identifier before it, -separated by a colon (`:`). -It has the following form: - -```swift -(<#identifier 1#>: <#expression 1#>, <#identifier 2#>: <#expression 2#>, <#...#>) -``` - -Each identifier in a tuple expression must be unique -within the scope of the tuple expression. -In a nested tuple expression, -identifiers at the same level of nesting must be unique. -For example, -`(a: 10, a: 20)` is invalid -because the label `a` appears twice at the same level. -However, `(a: 10, b: (a: 1, x: 2))` is valid --- -although `a` appears twice, -it appears once in the outer tuple and once in the inner tuple. - - - -A tuple expression can contain zero expressions, -or it can contain two or more expressions. -A single expression inside parentheses is a parenthesized expression. - -> Note: Both an empty tuple expression and an empty tuple type -> are written `()` in Swift. -> Because `Void` is a type alias for `()`, -> you can use it to write an empty tuple type. -> However, like all type aliases, `Void` is always a type --- -> you can't use it to write an empty tuple expression. - -> Grammar of a tuple expression: -> -> *tuple-expression* → **`(`** **`)`** | **`(`** *tuple-element* **`,`** *tuple-element-list* **`)`** \ -> *tuple-element-list* → *tuple-element* | *tuple-element* **`,`** *tuple-element-list* \ -> *tuple-element* → *expression* | *identifier* **`:`** *expression* - -### Wildcard Expression - -A *wildcard expression* -is used to explicitly ignore a value during an assignment. -For example, in the following assignment -10 is assigned to `x` and 20 is ignored: - -```swift -(x, _) = (10, 20) -// x is 10, and 20 is ignored -``` - - - -> Grammar of a wildcard expression: -> -> *wildcard-expression* → **`_`** - -### Macro-Expansion Expression - -A *macro-expansion expression* consists of a macro name -followed by a comma-separated list of the macro's arguments in parentheses. -The macro is expanded at compile time. -Macro-expansion expressions have the following form: - -```swift -<#macro name#>(<#macro argument 1#>, <#macro argument 2#>) -``` - -A macro-expansion expression omits the parentheses after the macro's name -if the macro doesn't take any arguments. - -A macro-expansion expression can't appear as the default value for a parameter, -except the [`file()`][] and [`line()`][] macros from the Swift standard library. -When used as the default value of a function or method parameter, -these macros are evaluated using the source code location of the call site, -not the location where they appear in a function definition. - -[`file()`]: https://developer.apple.com/documentation/swift/file() -[`line()`]: https://developer.apple.com/documentation/swift/line() - -You use macro expressions to call freestanding macros. -To call an attached macro, -use the custom attribute syntax described in . -Both freestanding and attached macros expand as follows: - -1. Swift parses the source code - to produce an abstract syntax tree (AST). - -2. The macro implementation receives AST nodes as its input - and performs the transformations needed by that macro. - -3. The transformed AST nodes that the macro implementation produced - are added to the original AST. - -The expansion of each macro is independent and self-contained. -However, as a performance optimization, -Swift might start an external process that implements the macro -and reuse the same process to expand multiple macros. -When you implement a macro, -that code must not depend on what macros your code previously expanded, -or on any other external state like the current time. - -For nested macros and attached macros that have multiple roles, -the expansion process repeats. -Nested macro-expansion expressions expand from the outside in. -For example, in the code below -`outerMacro(_:)` expands first and the unexpanded call to `innerMacro(_:)` -appears in the abstract syntax tree -that `outerMacro(_:)` receives as its input. - -```swift -#outerMacro(12, #innerMacro(34), "some text") -``` - -An attached macro that has multiple roles expands once for each role. -Each expansion receives the same, original, AST as its input. -Swift forms the overall expansion -by collecting all of the generated AST nodes -and putting them in their corresponding places in the AST. - -For an overview of macros in Swift, see . - -> Grammar of a macro-expansion expression: -> -> *macro-expansion-expression* → **`#`** *identifier* *generic-argument-clause*_?_ *function-call-argument-clause*_?_ *trailing-closures*_?_ - -### Key-Path Expression - -A *key-path expression* -refers to a property or subscript of a type. -You use key-path expressions -in dynamic programming tasks, -such as key-value observing. -They have the following form: - -```swift -\<#type name#>.<#path#> -``` - -The *type name* is the name of a concrete type, -including any generic parameters, -such as `String`, `[Int]`, or `Set`. - -The *path* consists of -property names, subscripts, optional-chaining expressions, -and forced unwrapping expressions. -Each of these key-path components -can be repeated as many times as needed, -in any order. - -At compile time, a key-path expression -is replaced by an instance -of the [`KeyPath`](https://developer.apple.com/documentation/swift/keypath) class. - -To access a value using a key path, -pass the key path to the `subscript(keyPath:)` subscript, -which is available on all types. -For example: - - - -```swift -struct SomeStructure { - var someValue: Int -} - -let s = SomeStructure(someValue: 12) -let pathToProperty = \SomeStructure.someValue - -let value = s[keyPath: pathToProperty] -// value is 12 -``` - - - -The *type name* can be omitted -in contexts where type inference -can determine the implied type. -The following code uses `\.someProperty` -instead of `\SomeClass.someProperty`: - -```swift -class SomeClass: NSObject { - @objc dynamic var someProperty: Int - init(someProperty: Int) { - self.someProperty = someProperty - } -} - -let c = SomeClass(someProperty: 10) -c.observe(\.someProperty) { object, change in - // ... -} -``` - - - - - -The *path* can refer to `self` to create the identity key path (`\.self`). -The identity key path refers to a whole instance, -so you can use it to access and change all of the data stored in a variable -in a single step. -For example: - -```swift -var compoundValue = (a: 1, b: 2) -// Equivalent to compoundValue = (a: 10, b: 20) -compoundValue[keyPath: \.self] = (a: 10, b: 20) -``` - - - -The *path* can contain multiple property names, -separated by periods, -to refer to a property of a property's value. -This code uses the key path expression -`\OuterStructure.outer.someValue` -to access the `someValue` property -of the `OuterStructure` type's `outer` property: - -```swift -struct OuterStructure { - var outer: SomeStructure - init(someValue: Int) { - self.outer = SomeStructure(someValue: someValue) - } -} - -let nested = OuterStructure(someValue: 24) -let nestedKeyPath = \OuterStructure.outer.someValue - -let nestedValue = nested[keyPath: nestedKeyPath] -// nestedValue is 24 -``` - - - -The *path* can include subscripts using brackets, -as long as the subscript's parameter type conforms to the `Hashable` protocol. -This example uses a subscript in a key path -to access the second element of an array: - -```swift -let greetings = ["hello", "hola", "bonjour", "안녕"] -let myGreeting = greetings[keyPath: \[String].[1]] -// myGreeting is 'hola' -``` - - - - - -The value used in a subscript can be a named value or a literal. -Values are captured in key paths using value semantics. -The following code uses the variable `index` -in both a key-path expression and in a closure to access -the third element of the `greetings` array. -When `index` is modified, -the key-path expression still references the third element, -while the closure uses the new index. - -```swift -var index = 2 -let path = \[String].[index] -let fn: ([String]) -> String = { strings in strings[index] } - -print(greetings[keyPath: path]) -// Prints "bonjour" -print(fn(greetings)) -// Prints "bonjour" - -// Setting 'index' to a new value doesn't affect 'path' -index += 1 -print(greetings[keyPath: path]) -// Prints "bonjour" - -// Because 'fn' closes over 'index', it uses the new value -print(fn(greetings)) -// Prints "안녕" -``` - - - -The *path* can use optional chaining and forced unwrapping. -This code uses optional chaining in a key path -to access a property of an optional string: - -```swift -let firstGreeting: String? = greetings.first -print(firstGreeting?.count as Any) -// Prints "Optional(5)" - -// Do the same thing using a key path. -let count = greetings[keyPath: \[String].first?.count] -print(count as Any) -// Prints "Optional(5)" -``` - - - - - -You can mix and match components of key paths to access values -that are deeply nested within a type. -The following code accesses different values and properties -of a dictionary of arrays -by using key-path expressions -that combine these components. - -```swift -let interestingNumbers = ["prime": [2, 3, 5, 7, 11, 13, 17], - "triangular": [1, 3, 6, 10, 15, 21, 28], - "hexagonal": [1, 6, 15, 28, 45, 66, 91]] -print(interestingNumbers[keyPath: \[String: [Int]].["prime"]] as Any) -// Prints "Optional([2, 3, 5, 7, 11, 13, 17])" -print(interestingNumbers[keyPath: \[String: [Int]].["prime"]![0]]) -// Prints "2" -print(interestingNumbers[keyPath: \[String: [Int]].["hexagonal"]!.count]) -// Prints "7" -print(interestingNumbers[keyPath: \[String: [Int]].["hexagonal"]!.count.bitWidth]) -// Prints "64" -``` - - - -You can use a key path expression -in contexts where you would normally provide a function or closure. -Specifically, -you can use a key path expression -whose root type is `SomeType` -and whose path produces a value of type `Value`, -instead of a function or closure of type `(SomeType) -> Value`. - -```swift -struct Task { - var description: String - var completed: Bool -} -var toDoList = [ - Task(description: "Practice ping-pong.", completed: false), - Task(description: "Buy a pirate costume.", completed: true), - Task(description: "Visit Boston in the Fall.", completed: false), -] - -// Both approaches below are equivalent. -let descriptions = toDoList.filter(\.completed).map(\.description) -let descriptions2 = toDoList.filter { $0.completed }.map { $0.description } -``` - - - - - -Any side effects of a key path expression -are evaluated only at the point where the expression is evaluated. -For example, -if you make a function call inside a subscript in a key path expression, -the function is called only once as part of evaluating the expression, -not every time the key path is used. - -```swift -func makeIndex() -> Int { - print("Made an index") - return 0 -} -// The line below calls makeIndex(). -let taskKeyPath = \[Task][makeIndex()] -// Prints "Made an index" - -// Using taskKeyPath doesn't call makeIndex() again. -let someTask = toDoList[keyPath: taskKeyPath] -``` - - - -For more information about using key paths -in code that interacts with Objective-C APIs, -see [Using Objective-C Runtime Features in Swift](https://developer.apple.com/documentation/swift/using_objective_c_runtime_features_in_swift). -For information about key-value coding and key-value observing, -see [Key-Value Coding Programming Guide](https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/KeyValueCoding/index.html#//apple_ref/doc/uid/10000107i) -and [Key-Value Observing Programming Guide](https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/KeyValueObserving/KeyValueObserving.html#//apple_ref/doc/uid/10000177i). - -> Grammar of a key-path expression: -> -> *key-path-expression* → **`\`** *type*_?_ **`.`** *key-path-components* \ -> *key-path-components* → *key-path-component* | *key-path-component* **`.`** *key-path-components* \ -> *key-path-component* → *identifier* *key-path-postfixes*_?_ | *key-path-postfixes* -> -> *key-path-postfixes* → *key-path-postfix* *key-path-postfixes*_?_ \ -> *key-path-postfix* → **`?`** | **`!`** | **`self`** | **`[`** *function-call-argument-list* **`]`** - -### Selector Expression - -A selector expression lets you access the selector -used to refer to a method or to a property's -getter or setter in Objective-C. -It has the following form: - -```swift -#selector(<#method name#>) -#selector(getter: <#property name#>) -#selector(setter: <#property name#>) -``` - -The *method name* and *property name* must be a reference to a method or a property -that's available in the Objective-C runtime. -The value of a selector expression is an instance of the `Selector` type. -For example: - -```swift -class SomeClass: NSObject { - @objc let property: String - - @objc(doSomethingWithInt:) - func doSomething(_ x: Int) { } - - init(property: String) { - self.property = property - } -} -let selectorForMethod = #selector(SomeClass.doSomething(_:)) -let selectorForPropertyGetter = #selector(getter: SomeClass.property) -``` - - - -When creating a selector for a property's getter, -the *property name* can be a reference to a variable or constant property. -In contrast, when creating a selector for a property's setter, -the *property name* must be a reference to a variable property only. - -The *method name* can contain parentheses for grouping, -as well the `as` operator to disambiguate between methods that share a name -but have different type signatures. -For example: - -```swift -extension SomeClass { - @objc(doSomethingWithString:) - func doSomething(_ x: String) { } -} -let anotherSelector = #selector(SomeClass.doSomething(_:) as (SomeClass) -> (String) -> Void) -``` - - - -Because a selector is created at compile time, not at runtime, -the compiler can check that a method or property exists -and that they're exposed to the Objective-C runtime. - -> Note: Although the *method name* and the *property name* are expressions, -> they're never evaluated. - -For more information about using selectors -in Swift code that interacts with Objective-C APIs, -see [Using Objective-C Runtime Features in Swift](https://developer.apple.com/documentation/swift/using_objective_c_runtime_features_in_swift). - -> Grammar of a selector expression: -> -> *selector-expression* → **`#selector`** **`(`** *expression* **`)`** \ -> *selector-expression* → **`#selector`** **`(`** **`getter:`** *expression* **`)`** \ -> *selector-expression* → **`#selector`** **`(`** **`setter:`** *expression* **`)`** - - - -### Key-Path String Expression - -A key-path string expression lets you access the string -used to refer to a property in Objective-C, -for use in key-value coding and key-value observing APIs. -It has the following form: - -```swift -#keyPath(<#property name#>) -``` - -The *property name* must be a reference to a property -that's available in the Objective-C runtime. -At compile time, the key-path string expression is replaced by a string literal. -For example: - -```swift -class SomeClass: NSObject { - @objc var someProperty: Int - init(someProperty: Int) { - self.someProperty = someProperty - } -} - -let c = SomeClass(someProperty: 12) -let keyPath = #keyPath(SomeClass.someProperty) - -if let value = c.value(forKey: keyPath) { - print(value) -} -// Prints "12" -``` - - - -When you use a key-path string expression within a class, -you can refer to a property of that class -by writing just the property name, without the class name. - -```swift -extension SomeClass { - func getSomeKeyPath() -> String { - return #keyPath(someProperty) - } -} -print(keyPath == c.getSomeKeyPath()) -// Prints "true" -``` - - - -Because the key path string is created at compile time, not at runtime, -the compiler can check that the property exists -and that the property is exposed to the Objective-C runtime. - -For more information about using key paths -in Swift code that interacts with Objective-C APIs, -see [Using Objective-C Runtime Features in Swift](https://developer.apple.com/documentation/swift/using_objective_c_runtime_features_in_swift). -For information about key-value coding and key-value observing, -see [Key-Value Coding Programming Guide](https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/KeyValueCoding/index.html#//apple_ref/doc/uid/10000107i) -and [Key-Value Observing Programming Guide](https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/KeyValueObserving/KeyValueObserving.html#//apple_ref/doc/uid/10000177i). - -> Note: Although the *property name* is an expression, it's never evaluated. - -> Grammar of a key-path string expression: -> -> *key-path-string-expression* → **`#keyPath`** **`(`** *expression* **`)`** - -## Postfix Expressions - -*Postfix expressions* are formed -by applying a postfix operator or other postfix syntax -to an expression. -Syntactically, every primary expression is also a postfix expression. - -For information about the behavior of these operators, -see and . - -For information about the operators provided by the Swift standard library, -see [Operator Declarations](https://developer.apple.com/documentation/swift/operator_declarations). - -> Grammar of a postfix expression: -> -> *postfix-expression* → *primary-expression* \ -> *postfix-expression* → *postfix-expression* *postfix-operator* \ -> *postfix-expression* → *function-call-expression* \ -> *postfix-expression* → *initializer-expression* \ -> *postfix-expression* → *explicit-member-expression* \ -> *postfix-expression* → *postfix-self-expression* \ -> *postfix-expression* → *subscript-expression* \ -> *postfix-expression* → *forced-value-expression* \ -> *postfix-expression* → *optional-chaining-expression* - -### Function Call Expression - - - -A *function call expression* consists of a function name -followed by a comma-separated list of the function's arguments in parentheses. -Function call expressions have the following form: - -```swift -<#function name#>(<#argument value 1#>, <#argument value 2#>) -``` - -The *function name* can be any expression whose value is of a function type. - -If the function definition includes names for its parameters, -the function call must include names before its argument values, -separated by a colon (`:`). -This kind of function call expression has the following form: - -```swift -<#function name#>(<#argument name 1#>: <#argument value 1#>, <#argument name 2#>: <#argument value 2#>) -``` - -A function call expression can include trailing closures -in the form of closure expressions immediately after the closing parenthesis. -The trailing closures are understood as arguments to the function, -added after the last parenthesized argument. -The first closure expression is unlabeled; -any additional closure expressions are preceded by their argument labels. -The example below shows the equivalent version of function calls -that do and don't use trailing closure syntax: - -```swift -// someFunction takes an integer and a closure as its arguments -someFunction(x: x, f: { $0 == 13 }) -someFunction(x: x) { $0 == 13 } - -// anotherFunction takes an integer and two closures as its arguments -anotherFunction(x: x, f: { $0 == 13 }, g: { print(99) }) -anotherFunction(x: x) { $0 == 13 } g: { print(99) } -``` - - - - - -If the trailing closure is the function's only argument, -you can omit the parentheses. - -```swift -// someMethod takes a closure as its only argument -myData.someMethod() { $0 == 13 } -myData.someMethod { $0 == 13 } -``` - - - - - -To include the trailing closures in the arguments, -the compiler examines the function's parameters from left to right as follows: - -| Trailing Closure | Parameter | Action | -| ---------------- | --------- | ------ | -| Labeled | Labeled | If the labels are the same, the closure matches the parameter; otherwise, the parameter is skipped. | -| Labeled | Unlabeled | The parameter is skipped. | -| Unlabeled | Labeled or unlabeled | If the parameter structurally resembles a function type, as defined below, the closure matches the parameter; otherwise, the parameter is skipped. | - -The trailing closure is passed as the argument for the parameter that it matches. -Parameters that were skipped during the scanning process -don't have an argument passed to them --- -for example, they can use a default parameter. -After finding a match, scanning continues -with the next trailing closure and the next parameter. -At the end of the matching process, -all trailing closures must have a match. - -A parameter *structurally resembles* a function type -if the parameter isn't an in-out parameter, -and the parameter is one of the following: - -- A parameter whose type is a function type, - like `(Bool) -> Int` -- An autoclosure parameter - whose wrapped expression's type is a function type, - like `@autoclosure () -> ((Bool) -> Int)` -- A variadic parameter - whose array element type is a function type, - like `((Bool) -> Int)...` -- A parameter whose type is wrapped in one or more layers of optional, - like `Optional<(Bool) -> Int>` -- A parameter whose type combines these allowed types, - like `(Optional<(Bool) -> Int>)...` - -When a trailing closure is matched to a parameter -whose type structurally resembles a function type, but isn't a function, -the closure is wrapped as needed. -For example, if the parameter's type is an optional type, -the closure is wrapped in `Optional` automatically. - - - -To ease migration of code from versions of Swift prior to 5.3 --- -which performed this matching from right to left --- -the compiler checks both the left-to-right and right-to-left orderings. -If the scan directions produce different results, -the old right-to-left ordering is used -and the compiler generates a warning. -A future version of Swift will always use the left-to-right ordering. - -```swift -typealias Callback = (Int) -> Int -func someFunction(firstClosure: Callback? = nil, - secondClosure: Callback? = nil) { - let first = firstClosure?(10) - let second = secondClosure?(20) - print(first ?? "-", second ?? "-") -} - -someFunction() // Prints "- -" -someFunction { return $0 + 100 } // Ambiguous -someFunction { return $0 } secondClosure: { return $0 } // Prints "10 20" -``` - - - -In the example above, -the function call marked "Ambiguous" -prints "- 120" and produces a compiler warning on Swift 5.3. -A future version of Swift will print “110 -”. - - - -A class, structure, or enumeration type -can enable syntactic sugar for function call syntax -by declaring one of several methods, -as described in . - -#### Implicit Conversion to a Pointer Type - -In a function call expression, -if the argument and parameter have a different type, -the compiler tries to make their types match -by applying one of the implicit conversions in the following list: - -- `inout SomeType` can become - `UnsafePointer` or `UnsafeMutablePointer` -- `inout Array` can become - `UnsafePointer` or `UnsafeMutablePointer` -- `Array` can become `UnsafePointer` -- `String` can become `UnsafePointer` - -The following two function calls are equivalent: - -```swift -func unsafeFunction(pointer: UnsafePointer) { - // ... -} -var myNumber = 1234 - -unsafeFunction(pointer: &myNumber) -withUnsafePointer(to: myNumber) { unsafeFunction(pointer: $0) } -``` - - - -A pointer that's created by these implicit conversions -is valid only for the duration of the function call. -To avoid undefined behavior, -ensure that your code -never persists the pointer after the function call ends. - -> Note: When implicitly converting an array to an unsafe pointer, -> Swift ensures that the array's storage is contiguous -> by converting or copying the array as needed. -> For example, you can use this syntax -> with an array that was bridged to `Array` -> from an `NSArray` subclass that makes no API contract about its storage. -> If you need to guarantee that the array's storage is already contiguous, -> so the implicit conversion never needs to do this work, -> use `ContiguousArray` instead of `Array`. - -Using `&` instead of an explicit function like `withUnsafePointer(to:)` -can help make calls to low-level C functions more readable, -especially when the function takes several pointer arguments. -However, when calling functions from other Swift code, -avoid using `&` instead of using the unsafe APIs explicitly. - - - -> Grammar of a function call expression: -> -> *function-call-expression* → *postfix-expression* *function-call-argument-clause* \ -> *function-call-expression* → *postfix-expression* *function-call-argument-clause*_?_ *trailing-closures* -> -> *function-call-argument-clause* → **`(`** **`)`** | **`(`** *function-call-argument-list* **`)`** \ -> *function-call-argument-list* → *function-call-argument* | *function-call-argument* **`,`** *function-call-argument-list* \ -> *function-call-argument* → *expression* | *identifier* **`:`** *expression* \ -> *function-call-argument* → *operator* | *identifier* **`:`** *operator* -> -> *trailing-closures* → *closure-expression* *labeled-trailing-closures*_?_ \ -> *labeled-trailing-closures* → *labeled-trailing-closure* *labeled-trailing-closures*_?_ \ -> *labeled-trailing-closure* → *identifier* **`:`** *closure-expression* - -### Initializer Expression - -An *initializer expression* provides access -to a type's initializer. -It has the following form: - -```swift -<#expression#>.init(<#initializer arguments#>) -``` - -You use the initializer expression in a function call expression -to initialize a new instance of a type. -You also use an initializer expression -to delegate to the initializer of a superclass. - -```swift -class SomeSubClass: SomeSuperClass { - override init() { - // subclass initialization goes here - super.init() - } -} -``` - - - -Like a function, an initializer can be used as a value. -For example: - -```swift -// Type annotation is required because String has multiple initializers. -let initializer: (Int) -> String = String.init -let oneTwoThree = [1, 2, 3].map(initializer).reduce("", +) -print(oneTwoThree) -// Prints "123" -``` - - - -If you specify a type by name, -you can access the type's initializer without using an initializer expression. -In all other cases, you must use an initializer expression. - -```swift -let s1 = SomeType.init(data: 3) // Valid -let s2 = SomeType(data: 1) // Also valid - -let s3 = type(of: someValue).init(data: 7) // Valid -let s4 = type(of: someValue)(data: 5) // Error -``` - - - -> Grammar of an initializer expression: -> -> *initializer-expression* → *postfix-expression* **`.`** **`init`** \ -> *initializer-expression* → *postfix-expression* **`.`** **`init`** **`(`** *argument-names* **`)`** - -### Explicit Member Expression - -An *explicit member expression* allows access -to the members of a named type, a tuple, or a module. -It consists of a period (`.`) between the item -and the identifier of its member. - -```swift -<#expression#>.<#member name#> -``` - -The members of a named type are named -as part of the type's declaration or extension. -For example: - -```swift -class SomeClass { - var someProperty = 42 -} -let c = SomeClass() -let y = c.someProperty // Member access -``` - - - -The members of a tuple -are implicitly named using integers in the order they appear, -starting from zero. -For example: - -```swift -var t = (10, 20, 30) -t.0 = t.1 -// Now t is (20, 20, 30) -``` - - - -The members of a module access -the top-level declarations of that module. - -Types declared with the `dynamicMemberLookup` attribute -include members that are looked up at runtime, -as described in . - -To distinguish between methods or initializers -whose names differ only by the names of their arguments, -include the argument names in parentheses, -with each argument name followed by a colon (`:`). -Write an underscore (`_`) for an argument with no name. -To distinguish between overloaded methods, -use a type annotation. -For example: - -```swift -class SomeClass { - func someMethod(x: Int, y: Int) {} - func someMethod(x: Int, z: Int) {} - func overloadedMethod(x: Int, y: Int) {} - func overloadedMethod(x: Int, y: Bool) {} -} -let instance = SomeClass() - -let a = instance.someMethod // Ambiguous -let b = instance.someMethod(x:y:) // Unambiguous - -let d = instance.overloadedMethod // Ambiguous -let d = instance.overloadedMethod(x:y:) // Still ambiguous -let d: (Int, Bool) -> Void = instance.overloadedMethod(x:y:) // Unambiguous -``` - - - -If a period appears at the beginning of a line, -it's understood as part of an explicit member expression, -not as an implicit member expression. -For example, the following listing shows chained method calls -split over several lines: - -```swift -let x = [10, 3, 20, 15, 4] - .sorted() - .filter { $0 > 5 } - .map { $0 * 100 } -``` - - - -You can combine this multiline chained syntax -with compiler control statements -to control when each method is called. -For example, -the following code uses a different filtering rule on iOS: - -```swift -let numbers = [10, 20, 33, 43, 50] -#if os(iOS) - .filter { $0 < 40 } -#else - .filter { $0 > 25 } -#endif -``` - - - -Between `#if`, `#endif`, and other compilation directives, -the conditional compilation block can contain -an implicit member expression -followed by zero or more postfixes, -to form a postfix expression. -It can also contain -another conditional compilation block, -or a combination of these expressions and blocks. - -You can use this syntax anywhere that you can write -an explicit member expression, -not just in top-level code. - -In the conditional compilation block, -the branch for the `#if` compilation directive -must contain at least one expression. -The other branches can be empty. - - - - - - - -> Grammar of an explicit member expression: -> -> *explicit-member-expression* → *postfix-expression* **`.`** *decimal-digits* \ -> *explicit-member-expression* → *postfix-expression* **`.`** *identifier* *generic-argument-clause*_?_ \ -> *explicit-member-expression* → *postfix-expression* **`.`** *identifier* **`(`** *argument-names* **`)`** \ -> *explicit-member-expression* → *postfix-expression* *conditional-compilation-block* -> -> *argument-names* → *argument-name* *argument-names*_?_ \ -> *argument-name* → *identifier* **`:`** - - - - - -### Postfix Self Expression - -A postfix `self` expression consists of an expression or the name of a type, -immediately followed by `.self`. It has the following forms: - -```swift -<#expression#>.self -<#type#>.self -``` - -The first form evaluates to the value of the *expression*. -For example, `x.self` evaluates to `x`. - -The second form evaluates to the value of the *type*. Use this form -to access a type as a value. For example, -because `SomeClass.self` evaluates to the `SomeClass` type itself, -you can pass it to a function or method that accepts a type-level argument. - -> Grammar of a postfix self expression: -> -> *postfix-self-expression* → *postfix-expression* **`.`** **`self`** - -### Subscript Expression - -A *subscript expression* provides subscript access -using the getter and setter -of the corresponding subscript declaration. -It has the following form: - -```swift -<#expression#>[<#index expressions#>] -``` - -To evaluate the value of a subscript expression, -the subscript getter for the *expression*'s type is called -with the *index expressions* passed as the subscript parameters. -To set its value, -the subscript setter is called in the same way. - - - -For information about subscript declarations, -see . - -> Grammar of a subscript expression: -> -> *subscript-expression* → *postfix-expression* **`[`** *function-call-argument-list* **`]`** - - - -### Forced-Value Expression - -A *forced-value expression* unwraps an optional value -that you are certain isn't `nil`. -It has the following form: - -```swift -<#expression#>! -``` - -If the value of the *expression* isn't `nil`, -the optional value is unwrapped -and returned with the corresponding non-optional type. -Otherwise, a runtime error is raised. - -The unwrapped value of a forced-value expression can be modified, -either by mutating the value itself, -or by assigning to one of the value's members. -For example: - -```swift -var x: Int? = 0 -x! += 1 -// x is now 1 - -var someDictionary = ["a": [1, 2, 3], "b": [10, 20]] -someDictionary["a"]![0] = 100 -// someDictionary is now ["a": [100, 2, 3], "b": [10, 20]] -``` - - - -> Grammar of a forced-value expression: -> -> *forced-value-expression* → *postfix-expression* **`!`** - -### Optional-Chaining Expression - -An *optional-chaining expression* provides a simplified syntax -for using optional values in postfix expressions. -It has the following form: - -```swift -<#expression#>? -``` - -The postfix `?` operator makes an optional-chaining expression -from an expression without changing the expression's value. - -Optional-chaining expressions must appear within a postfix expression, -and they cause the postfix expression to be evaluated in a special way. -If the value of the optional-chaining expression is `nil`, -all of the other operations in the postfix expression are ignored -and the entire postfix expression evaluates to `nil`. -If the value of the optional-chaining expression isn't `nil`, -the value of the optional-chaining expression is unwrapped -and used to evaluate the rest of the postfix expression. -In either case, -the value of the postfix expression is still of an optional type. - -If a postfix expression that contains an optional-chaining expression -is nested inside other postfix expressions, -only the outermost expression returns an optional type. -In the example below, -when `c` isn't `nil`, -its value is unwrapped and used to evaluate `.property`, -the value of which is used to evaluate `.performAction()`. -The entire expression `c?.property.performAction()` -has a value of an optional type. - -```swift -var c: SomeClass? -var result: Bool? = c?.property.performAction() -``` - - - -The following example shows the behavior -of the example above -without using optional chaining. - -```swift -var result: Bool? -if let unwrappedC = c { - result = unwrappedC.property.performAction() -} -``` - - - -The unwrapped value of an optional-chaining expression can be modified, -either by mutating the value itself, -or by assigning to one of the value's members. -If the value of the optional-chaining expression is `nil`, -the expression on the right-hand side of the assignment operator -isn't evaluated. -For example: - -```swift -func someFunctionWithSideEffects() -> Int { - return 42 // No actual side effects. -} -var someDictionary = ["a": [1, 2, 3], "b": [10, 20]] - -someDictionary["not here"]?[0] = someFunctionWithSideEffects() -// someFunctionWithSideEffects isn't evaluated -// someDictionary is still ["a": [1, 2, 3], "b": [10, 20]] - -someDictionary["a"]?[0] = someFunctionWithSideEffects() -// someFunctionWithSideEffects is evaluated and returns 42 -// someDictionary is now ["a": [42, 2, 3], "b": [10, 20]] -``` - - - -> Grammar of an optional-chaining expression: -> -> *optional-chaining-expression* → *postfix-expression* **`?`** - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/ReferenceManual/GenericParametersAndArguments.md b/swift-6-beta.docc/ReferenceManual/GenericParametersAndArguments.md deleted file mode 100644 index 77bc2a213..000000000 --- a/swift-6-beta.docc/ReferenceManual/GenericParametersAndArguments.md +++ /dev/null @@ -1,307 +0,0 @@ -# Generic Parameters and Arguments - -Generalize declarations to abstract away concrete types. - -This chapter describes parameters and arguments for generic types, functions, and -initializers. When you declare a generic type, function, subscript, or initializer, -you specify the type parameters that the generic type, function, or initializer -can work with. These type parameters act as placeholders that -are replaced by actual concrete type arguments when an instance of a generic type is -created or a generic function or initializer is called. - -For an overview of generics in Swift, see . - - - -## Generic Parameter Clause - -A *generic parameter clause* specifies the type parameters of a generic -type or function, along with any associated constraints and requirements on those parameters. -A generic parameter clause is enclosed in angle brackets (<>) -and has the following form: - -```swift -<<#generic parameter list#>> -``` - -The *generic parameter list* is a comma-separated list of generic parameters, -each of which has the following form: - -```swift -<#type parameter#>: <#constraint#> -``` - -A generic parameter consists of a *type parameter* followed by -an optional *constraint*. A *type parameter* is simply the name -of a placeholder type -(for example, `T`, `U`, `V`, `Key`, `Value`, and so on). -You have access to the type parameters (and any of their associated types) in the rest of the -type, function, or initializer declaration, including in the signature of the function -or initializer. - -The *constraint* specifies that a type parameter inherits -from a specific class or conforms to a protocol or protocol composition. -For example, in the generic function below, the generic parameter `T: Comparable` -indicates that any type argument substituted -for the type parameter `T` must conform to the `Comparable` protocol. - -```swift -func simpleMax(_ x: T, _ y: T) -> T { - if x < y { - return y - } - return x -} -``` - - - -Because `Int` and `Double`, for example, both conform to the `Comparable` protocol, -this function accepts arguments of either type. In contrast with generic types, you don't -specify a generic argument clause when you use a generic function or initializer. -The type arguments are instead inferred from the type of the arguments passed -to the function or initializer. - -```swift -simpleMax(17, 42) // T is inferred to be Int -simpleMax(3.14159, 2.71828) // T is inferred to be Double -``` - - - - - -### Generic Where Clauses - -You can specify additional requirements on type parameters and their associated types -by including a generic `where` clause right before the opening curly brace -of a type or function's body. -A generic `where` clause consists of the `where` keyword, -followed by a comma-separated list of one or more *requirements*. - -```swift -where <#requirements#> -``` - -The *requirements* in a generic `where` clause specify that a type parameter inherits from -a class or conforms to a protocol or protocol composition. -Although the generic `where` clause provides syntactic -sugar for expressing simple constraints on type parameters -(for example, `` is equivalent to ` where T: Comparable` and so on), -you can use it to provide more complex constraints on type parameters -and their associated types. For example, -you can constrain the associated types of type parameters to conform to protocols. -For example, ` where S.Iterator.Element: Equatable` -specifies that `S` conforms to the `Sequence` protocol -and that the associated type `S.Iterator.Element` -conforms to the `Equatable` protocol. -This constraint ensures that each element of the sequence is equatable. - -You can also specify the requirement that two types be identical, -using the `==` operator. For example, -` where S1.Iterator.Element == S2.Iterator.Element` -expresses the constraints that `S1` and `S2` conform to the `Sequence` protocol -and that the elements of both sequences must be of the same type. - -Any type argument substituted for a type parameter must -meet all the constraints and requirements placed on the type parameter. - -A generic `where` clause can appear -as part of a declaration that includes type parameters, -or as part of a declaration -that's nested inside of a declaration that includes type parameters. -The generic `where` clause for a nested declaration -can still refer to the type parameters of the enclosing declaration; -however, -the requirements from that `where` clause -apply only to the declaration where it's written. - -If the enclosing declaration also has a `where` clause, -the requirements from both clauses are combined. -In the example below, `startsWithZero()` is available -only if `Element` conforms to both `SomeProtocol` and `Numeric`. - -```swift -extension Collection where Element: SomeProtocol { - func startsWithZero() -> Bool where Element: Numeric { - return first == .zero - } -} -``` - - - - - -You can overload a generic function or initializer by providing different -constraints, requirements, or both on the type parameters. -When you call an overloaded generic function or initializer, -the compiler uses these constraints to resolve which overloaded function -or initializer to invoke. - -For more information about generic `where` clauses and to see an example -of one in a generic function declaration, -see . - -> Grammar of a generic parameter clause: -> -> *generic-parameter-clause* → **`<`** *generic-parameter-list* **`>`** \ -> *generic-parameter-list* → *generic-parameter* | *generic-parameter* **`,`** *generic-parameter-list* \ -> *generic-parameter* → *type-name* \ -> *generic-parameter* → *type-name* **`:`** *type-identifier* \ -> *generic-parameter* → *type-name* **`:`** *protocol-composition-type* -> -> *generic-where-clause* → **`where`** *requirement-list* \ -> *requirement-list* → *requirement* | *requirement* **`,`** *requirement-list* \ -> *requirement* → *conformance-requirement* | *same-type-requirement* -> -> *conformance-requirement* → *type-identifier* **`:`** *type-identifier* \ -> *conformance-requirement* → *type-identifier* **`:`** *protocol-composition-type* \ -> *same-type-requirement* → *type-identifier* **`==`** *type* - - - -## Generic Argument Clause - -A *generic argument clause* specifies the type arguments of a generic -type. -A generic argument clause is enclosed in angle brackets (<>) -and has the following form: - -```swift -<<#generic argument list#>> -``` - -The *generic argument list* is a comma-separated list of type arguments. -A *type argument* is the name of an actual concrete type that replaces -a corresponding type parameter in the generic parameter clause of a generic type. -The result is a specialized version of that generic type. -The example below shows a simplified version of the Swift standard library's -generic dictionary type. - -```swift -struct Dictionary: Collection, ExpressibleByDictionaryLiteral { - /* ... */ -} -``` - - - -The specialized version of the generic `Dictionary` type, `Dictionary` -is formed by replacing the generic parameters `Key: Hashable` and `Value` -with the concrete type arguments `String` and `Int`. Each type argument must satisfy -all the constraints of the generic parameter it replaces, including any additional -requirements specified in a generic `where` clause. In the example above, -the `Key` type parameter is constrained to conform to the `Hashable` protocol -and therefore `String` must also conform to the `Hashable` protocol. - -You can also replace a type parameter with a type argument that's itself -a specialized version of a generic type (provided it satisfies the appropriate -constraints and requirements). For example, you can replace the type parameter -`Element` in `Array` with a specialized version of an array, `Array`, -to form an array whose elements are themselves arrays of integers. - -```swift -let arrayOfArrays: Array> = [[1, 2, 3], [4, 5, 6], [7, 8, 9]] -``` - - - -As mentioned in , -you don't use a generic argument clause to specify the type arguments -of a generic function or initializer. - -> Grammar of a generic argument clause: -> -> *generic-argument-clause* → **`<`** *generic-argument-list* **`>`** \ -> *generic-argument-list* → *generic-argument* | *generic-argument* **`,`** *generic-argument-list* \ -> *generic-argument* → *type* - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/ReferenceManual/LexicalStructure.md b/swift-6-beta.docc/ReferenceManual/LexicalStructure.md deleted file mode 100644 index ecdb695b4..000000000 --- a/swift-6-beta.docc/ReferenceManual/LexicalStructure.md +++ /dev/null @@ -1,1268 +0,0 @@ -# Lexical Structure - -Use the lowest-level components of the syntax. - -The *lexical structure* of Swift describes what sequence of characters -form valid tokens of the language. -These valid tokens form the lowest-level building blocks of the language -and are used to describe the rest of the language in subsequent chapters. -A token consists of an identifier, keyword, punctuation, literal, or operator. - -In most cases, tokens are generated from the characters of a Swift source file -by considering the longest possible substring from the input text, -within the constraints of the grammar that are specified below. -This behavior is referred to as *longest match* -or *maximal munch*. - -## Whitespace and Comments - -Whitespace has two uses: to separate tokens in the source file -and to distinguish between prefix, postfix, and infix operators -(see ), -but is otherwise ignored. -The following characters are considered whitespace: -space (U+0020), -line feed (U+000A), -carriage return (U+000D), -horizontal tab (U+0009), -vertical tab (U+000B), -form feed (U+000C) -and null (U+0000). - - - -Comments are treated as whitespace by the compiler. -Single line comments begin with `//` -and continue until a line feed (U+000A) or carriage return (U+000D). -Multiline comments begin with `/*` and end with `*/`. -Nesting multiline comments is allowed, -but the comment markers must be balanced. - -Comments can contain additional formatting and markup, -as described in [Markup Formatting Reference](https://developer.apple.com/library/content/documentation/Xcode/Reference/xcode_markup_formatting_ref/index.html). - -> Grammar of whitespace: -> -> *whitespace* → *whitespace-item* *whitespace*_?_ \ -> *whitespace-item* → *line-break* \ -> *whitespace-item* → *inline-space* \ -> *whitespace-item* → *comment* \ -> *whitespace-item* → *multiline-comment* \ -> *whitespace-item* → U+0000, U+000B, or U+000C -> -> *line-break* → U+000A \ -> *line-break* → U+000D \ -> *line-break* → U+000D followed by U+000A -> -> *inline-spaces* → *inline-space* *inline-spaces*_?_ \ -> *inline-space* → U+0009 or U+0020 -> -> *comment* → **`//`** *comment-text* *line-break* \ -> *multiline-comment* → **`/*`** *multiline-comment-text* **`*/`** -> -> *comment-text* → *comment-text-item* *comment-text*_?_ \ -> *comment-text-item* → Any Unicode scalar value except U+000A or U+000D -> -> *multiline-comment-text* → *multiline-comment-text-item* *multiline-comment-text*_?_ \ -> *multiline-comment-text-item* → *multiline-comment* \ -> *multiline-comment-text-item* → *comment-text-item* \ -> *multiline-comment-text-item* → Any Unicode scalar value except **`/*`** or **`*/`** - -## Identifiers - -*Identifiers* begin with -an uppercase or lowercase letter A through Z, -an underscore (`_`), -a noncombining alphanumeric Unicode character -in the Basic Multilingual Plane, -or a character outside the Basic Multilingual Plane -that isn't in a Private Use Area. -After the first character, -digits and combining Unicode characters are also allowed. - -Treat identifiers that begin with an underscore, -subscripts whose first argument label begins with an underscore, -and initializers whose first argument label begins with an underscore, -as internal, -even if their declaration has the `public` access-level modifier. -This convention lets framework authors mark part of an API -that clients must not interact with or depend on, -even though some limitation requires the declaration to be public. -In addition, -identifiers that begin with two underscores -are reserved for the Swift compiler and standard library. - -To use a reserved word as an identifier, -put a backtick (\`) before and after it. -For example, `class` isn't a valid identifier, -but `` `class` `` is valid. -The backticks aren't considered part of the identifier; -`` `x` `` and `x` have the same meaning. - - - -Inside a closure with no explicit parameter names, -the parameters are implicitly named `$0`, `$1`, `$2`, and so on. -These names are valid identifiers within the scope of the closure. - -The compiler synthesizes identifiers that begin with a dollar sign (`$`) -for properties that have a property wrapper projection. -Your code can interact with these identifiers, -but you can't declare identifiers with that prefix. -For more information, see the section -of the chapter. - - - - - -> Grammar of an identifier: -> -> *identifier* → *identifier-head* *identifier-characters*_?_ \ -> *identifier* → **`` ` ``** *identifier-head* *identifier-characters*_?_ **`` ` ``** \ -> *identifier* → *implicit-parameter-name* \ -> *identifier* → *property-wrapper-projection* \ -> *identifier-list* → *identifier* | *identifier* **`,`** *identifier-list* -> -> *identifier-head* → Upper- or lowercase letter A through Z \ -> *identifier-head* → **`_`** \ -> *identifier-head* → U+00A8, U+00AA, U+00AD, U+00AF, U+00B2–U+00B5, or U+00B7–U+00BA \ -> *identifier-head* → U+00BC–U+00BE, U+00C0–U+00D6, U+00D8–U+00F6, or U+00F8–U+00FF \ -> *identifier-head* → U+0100–U+02FF, U+0370–U+167F, U+1681–U+180D, or U+180F–U+1DBF \ -> *identifier-head* → U+1E00–U+1FFF \ -> *identifier-head* → U+200B–U+200D, U+202A–U+202E, U+203F–U+2040, U+2054, or U+2060–U+206F \ -> *identifier-head* → U+2070–U+20CF, U+2100–U+218F, U+2460–U+24FF, or U+2776–U+2793 \ -> *identifier-head* → U+2C00–U+2DFF or U+2E80–U+2FFF \ -> *identifier-head* → U+3004–U+3007, U+3021–U+302F, U+3031–U+303F, or U+3040–U+D7FF \ -> *identifier-head* → U+F900–U+FD3D, U+FD40–U+FDCF, U+FDF0–U+FE1F, or U+FE30–U+FE44 \ -> *identifier-head* → U+FE47–U+FFFD \ -> *identifier-head* → U+10000–U+1FFFD, U+20000–U+2FFFD, U+30000–U+3FFFD, or U+40000–U+4FFFD \ -> *identifier-head* → U+50000–U+5FFFD, U+60000–U+6FFFD, U+70000–U+7FFFD, or U+80000–U+8FFFD \ -> *identifier-head* → U+90000–U+9FFFD, U+A0000–U+AFFFD, U+B0000–U+BFFFD, or U+C0000–U+CFFFD \ -> *identifier-head* → U+D0000–U+DFFFD or U+E0000–U+EFFFD -> -> *identifier-character* → Digit 0 through 9 \ -> *identifier-character* → U+0300–U+036F, U+1DC0–U+1DFF, U+20D0–U+20FF, or U+FE20–U+FE2F \ -> *identifier-character* → *identifier-head* \ -> *identifier-characters* → *identifier-character* *identifier-characters*_?_ -> -> *implicit-parameter-name* → **`$`** *decimal-digits* \ -> *property-wrapper-projection* → **`$`** *identifier-characters* - -## Keywords and Punctuation - -The following keywords are reserved and can't be used as identifiers, -unless they're escaped with backticks, -as described above in . -Keywords other than `inout`, `var`, and `let` -can be used as parameter names -in a function declaration or function call -without being escaped with backticks. -When a member has the same name as a keyword, -references to that member don't need to be escaped with backticks, -except when there's ambiguity between referring to the member -and using the keyword --- -for example, `self`, `Type`, and `Protocol` -have special meaning in an explicit member expression, -so they must be escaped with backticks in that context. - - - - - - - - - - - -- Keywords used in declarations: - `associatedtype`, - `borrowing`, - `class`, - `consuming`, - `deinit`, - `enum`, - `extension`, - `fileprivate`, - `func`, - `import`, - `init`, - `inout`, - `internal`, - `let`, - `nonisolated`, - `open`, - `operator`, - `private`, - `precedencegroup`, - `protocol`, - `public`, - `rethrows`, - `static`, - `struct`, - `subscript`, - `typealias`, - and `var`. - - - -- Keywords used in statements: - `break`, - `case`, - `catch`, - `continue`, - `default`, - `defer`, - `do`, - `else`, - `fallthrough`, - `for`, - `guard`, - `if`, - `in`, - `repeat`, - `return`, - `throw`, - `switch`, - `where`, - and `while`. -- Keywords used in expressions and types: - `Any`, - `as`, - `await`, - `catch`, - `false`, - `is`, - `nil`, - `rethrows`, - `self`, - `Self`, - `super`, - `throw`, - `throws`, - `true`, - and `try`. -- Keywords used in patterns: - `_`. -- Keywords that begin with a number sign (`#`): - `#available`, - `#colorLiteral`, - `#else`, - `#elseif`, - `#endif`, - `#fileLiteral`, - `#if`, - `#imageLiteral`, - `#keyPath`, - `#selector`, - `#sourceLocation`, - `#unavailable`. - -> Note: -> Prior to Swift 5.9, -> the following keywords were reserved: -> `#column`, -> `#dsohandle`, -> `#error`, -> `#fileID`, -> `#filePath`, -> `#file`, -> `#function`, -> `#line`, -> and `#warning`. -> These are now implemented as macros in the Swift standard library: -> [`column`](https://developer.apple.com/documentation/swift/column()), -> [`dsohandle`](https://developer.apple.com/documentation/swift/dsohandle()), -> [`error(_:)`](https://developer.apple.com/documentation/swift/error(_:)), -> [`fileID`](https://developer.apple.com/documentation/swift/fileID()), -> [`filePath`](https://developer.apple.com/documentation/swift/filePath()), -> [`file`](https://developer.apple.com/documentation/swift/file()), -> [`function`](https://developer.apple.com/documentation/swift/function()), -> [`line`](https://developer.apple.com/documentation/swift/line()), -> and [`warning(_:)`](https://developer.apple.com/documentation/swift/warning(_:)). - - - - - - - -- Keywords reserved in particular contexts: - `associativity`, - `convenience`, - `didSet`, - `dynamic`, - `final`, - `get`, - `indirect`, - `infix`, - `lazy`, - `left`, - `mutating`, - `none`, - `nonmutating`, - `optional`, - `override`, - `package`, - `postfix`, - `precedence`, - `prefix`, - `Protocol`, - `required`, - `right`, - `set`, - `some`, - `Type`, - `unowned`, - `weak`, - and `willSet`. - Outside the context in which they appear in the grammar, - they can be used as identifiers. - - - -The following tokens are reserved as punctuation -and can't be used as custom operators: -`(`, `)`, `{`, `}`, `[`, `]`, -`.`, `,`, `:`, `;`, `=`, `@`, `#`, -`&` (as a prefix operator), `->`, `` ` ``, -`?`, and `!` (as a postfix operator). - -## Literals - -A *literal* is the source code representation of a value of a type, -such as a number or string. - -The following are examples of literals: - -```swift -42 // Integer literal -3.14159 // Floating-point literal -"Hello, world!" // String literal -/Hello, .*/ // Regular expression literal -true // Boolean literal -``` - - - - - -A literal doesn't have a type on its own. -Instead, a literal is parsed as having infinite precision and Swift's type inference -attempts to infer a type for the literal. For example, -in the declaration `let x: Int8 = 42`, -Swift uses the explicit type annotation (`: Int8`) to infer -that the type of the integer literal `42` is `Int8`. -If there isn't suitable type information available, -Swift infers that the literal's type is one of the default literal types -defined in the Swift standard library -and listed in the table below. -When specifying the type annotation for a literal value, -the annotation's type must be a type that can be instantiated from that literal value. -That is, the type must conform to the Swift standard library protocols -listed in the table below. - -| Literal | Default type | Protocol | -| ------- | ------------ | -------- | -| Integer | `Int` | `ExpressibleByIntegerLiteral` | -| Floating-point | `Double` | `ExpressibleByFloatLiteral` | -| String | `String` | `ExpressibleByStringLiteral`, `ExpressibleByUnicodeScalarLiteral` for string literals that contain only a single Unicode scalar, `ExpressibleByExtendedGraphemeClusterLiteral` for string literals that contain only a single extended grapheme cluster | -| Regular expression | `Regex` | None | -| Boolean | `Bool` | `ExpressibleByBooleanLiteral` | - -For example, in the declaration `let str = "Hello, world"`, -the default inferred type of the string -literal `"Hello, world"` is `String`. -Also, `Int8` conforms to the `ExpressibleByIntegerLiteral` protocol, -and therefore it can be used in the type annotation for the integer literal `42` -in the declaration `let x: Int8 = 42`. - - - -> Grammar of a literal: -> -> *literal* → *numeric-literal* | *string-literal* | *regular-expression-literal* | *boolean-literal* | *nil-literal* -> -> *numeric-literal* → **`-`**_?_ *integer-literal* | **`-`**_?_ *floating-point-literal* \ -> *boolean-literal* → **`true`** | **`false`** \ -> *nil-literal* → **`nil`** - -### Integer Literals - -*Integer literals* represent integer values of unspecified precision. -By default, integer literals are expressed in decimal; -you can specify an alternate base using a prefix. -Binary literals begin with `0b`, -octal literals begin with `0o`, -and hexadecimal literals begin with `0x`. - -Decimal literals contain the digits `0` through `9`. -Binary literals contain `0` and `1`, -octal literals contain `0` through `7`, -and hexadecimal literals contain `0` through `9` -as well as `A` through `F` in upper- or lowercase. - -Negative integers literals are expressed by prepending a minus sign (`-`) -to an integer literal, as in `-42`. - -Underscores (`_`) are allowed between digits for readability, -but they're ignored and therefore don't affect the value of the literal. -Integer literals can begin with leading zeros (`0`), -but they're likewise ignored and don't affect the base or value of the literal. - -Unless otherwise specified, -the default inferred type of an integer literal is the Swift standard library type `Int`. -The Swift standard library also defines types for various sizes of -signed and unsigned integers, -as described in . - - - - - -> Grammar of an integer literal: -> -> *integer-literal* → *binary-literal* \ -> *integer-literal* → *octal-literal* \ -> *integer-literal* → *decimal-literal* \ -> *integer-literal* → *hexadecimal-literal* -> -> *binary-literal* → **`0b`** *binary-digit* *binary-literal-characters*_?_ \ -> *binary-digit* → Digit 0 or 1 \ -> *binary-literal-character* → *binary-digit* | **`_`** \ -> *binary-literal-characters* → *binary-literal-character* *binary-literal-characters*_?_ -> -> *octal-literal* → **`0o`** *octal-digit* *octal-literal-characters*_?_ \ -> *octal-digit* → Digit 0 through 7 \ -> *octal-literal-character* → *octal-digit* | **`_`** \ -> *octal-literal-characters* → *octal-literal-character* *octal-literal-characters*_?_ -> -> *decimal-literal* → *decimal-digit* *decimal-literal-characters*_?_ \ -> *decimal-digit* → Digit 0 through 9 \ -> *decimal-digits* → *decimal-digit* *decimal-digits*_?_ \ -> *decimal-literal-character* → *decimal-digit* | **`_`** \ -> *decimal-literal-characters* → *decimal-literal-character* *decimal-literal-characters*_?_ -> -> *hexadecimal-literal* → **`0x`** *hexadecimal-digit* *hexadecimal-literal-characters*_?_ \ -> *hexadecimal-digit* → Digit 0 through 9, a through f, or A through F \ -> *hexadecimal-literal-character* → *hexadecimal-digit* | **`_`** \ -> *hexadecimal-literal-characters* → *hexadecimal-literal-character* *hexadecimal-literal-characters*_?_ - -### Floating-Point Literals - -*Floating-point literals* represent floating-point values of unspecified precision. - -By default, floating-point literals are expressed in decimal (with no prefix), -but they can also be expressed in hexadecimal (with a `0x` prefix). - -Decimal floating-point literals consist of a sequence of decimal digits -followed by either a decimal fraction, a decimal exponent, or both. -The decimal fraction consists of a decimal point (`.`) -followed by a sequence of decimal digits. -The exponent consists of an upper- or lowercase `e` prefix -followed by a sequence of decimal digits that indicates -what power of 10 the value preceding the `e` is multiplied by. -For example, `1.25e2` represents 1.25 x 10², -which evaluates to `125.0`. -Similarly, `1.25e-2` represents 1.25 x 10⁻², -which evaluates to `0.0125`. - -Hexadecimal floating-point literals consist of a `0x` prefix, -followed by an optional hexadecimal fraction, -followed by a hexadecimal exponent. -The hexadecimal fraction consists of a decimal point -followed by a sequence of hexadecimal digits. -The exponent consists of an upper- or lowercase `p` prefix -followed by a sequence of decimal digits that indicates -what power of 2 the value preceding the `p` is multiplied by. -For example, `0xFp2` represents 15 x 2², -which evaluates to `60`. -Similarly, `0xFp-2` represents 15 x 2⁻², -which evaluates to `3.75`. - -Negative floating-point literals are expressed by prepending a minus sign (`-`) -to a floating-point literal, as in `-42.5`. - -Underscores (`_`) are allowed between digits for readability, -but they're ignored and therefore don't affect the value of the literal. -Floating-point literals can begin with leading zeros (`0`), -but they're likewise ignored and don't affect the base or value of the literal. - -Unless otherwise specified, -the default inferred type of a floating-point literal is the Swift standard library type `Double`, -which represents a 64-bit floating-point number. -The Swift standard library also defines a `Float` type, -which represents a 32-bit floating-point number. - -> Grammar of a floating-point literal: -> -> *floating-point-literal* → *decimal-literal* *decimal-fraction*_?_ *decimal-exponent*_?_ \ -> *floating-point-literal* → *hexadecimal-literal* *hexadecimal-fraction*_?_ *hexadecimal-exponent* -> -> *decimal-fraction* → **`.`** *decimal-literal* \ -> *decimal-exponent* → *floating-point-e* *sign*_?_ *decimal-literal* -> -> *hexadecimal-fraction* → **`.`** *hexadecimal-digit* *hexadecimal-literal-characters*_?_ \ -> *hexadecimal-exponent* → *floating-point-p* *sign*_?_ *decimal-literal* -> -> *floating-point-e* → **`e`** | **`E`** \ -> *floating-point-p* → **`p`** | **`P`** \ -> *sign* → **`+`** | **`-`** - -### String Literals - -A string literal is a sequence of characters surrounded by quotation marks. -A single-line string literal is surrounded by double quotation marks -and has the following form: - -```swift -"<#characters#>" -``` - -String literals can't contain -an unescaped double quotation mark (`"`), -an unescaped backslash (`\`), -a carriage return, or a line feed. - -A multiline string literal is surrounded by three double quotation marks -and has the following form: - -```swift -""" -<#characters#> -""" -``` - -Unlike a single-line string literal, -a multiline string literal can contain -unescaped double quotation marks (`"`), carriage returns, and line feeds. -It can't contain three unescaped double quotation marks next to each other. - -The line break after the `"""` -that begins the multiline string literal -isn't part of the string. -The line break before the `"""` -that ends the literal is also not part of the string. -To make a multiline string literal -that begins or ends with a line feed, -write a blank line as its first or last line. - -A multiline string literal can be indented -using any combination of spaces and tabs; -this indentation isn't included in the string. -The `"""` that ends the literal -determines the indentation: -Every nonblank line in the literal must begin -with exactly the same indentation -that appears before the closing `"""`; -there's no conversion between tabs and spaces. -You can include additional spaces and tabs after that indentation; -those spaces and tabs appear in the string. - -Line breaks in a multiline string literal are -normalized to use the line feed character. -Even if your source file has a mix of carriage returns and line feeds, -all of the line breaks in the string will be the same. - -In a multiline string literal, -writing a backslash (`\`) at the end of a line -omits that line break from the string. -Any whitespace between the backslash and the line break -is also omitted. -You can use this syntax -to hard wrap a multiline string literal in your source code, -without changing the value of the resulting string. - -Special characters -can be included in string literals -of both the single-line and multiline forms -using the following escape sequences: - -- Null character (`\0`) -- Backslash (`\\`) -- Horizontal tab (`\t`) -- Line feed (`\n`) -- Carriage return (`\r`) -- Double quotation mark (`\"`) -- Single quotation mark (`\'`) -- Unicode scalar (`\u{`*n*`}`), - where *n* is a hexadecimal number - that has one to eight digits - - - -The value of an expression can be inserted into a string literal -by placing the expression in parentheses after a backslash (`\`). -The interpolated expression can contain a string literal, -but can't contain an unescaped backslash, -a carriage return, or a line feed. - -For example, all of the following string literals have the same value: - -```swift -"1 2 3" -"1 2 \("3")" -"1 2 \(3)" -"1 2 \(1 + 2)" -let x = 3; "1 2 \(x)" -``` - - - - - -A string delimited by extended delimiters is a sequence of characters -surrounded by quotation marks and a balanced set of one or more number signs (`#`). -A string delimited by extended delimiters has the following forms: - -```swift -#"<#characters#>"# - -#""" -<#characters#> -"""# -``` - -Special characters in a string delimited by extended delimiters -appear in the resulting string as normal characters -rather than as special characters. -You can use extended delimiters to create strings with characters -that would ordinarily have a special effect -such as generating a string interpolation, -starting an escape sequence, -or terminating the string. - -The following example shows a string literal -and a string delimited by extended delimiters -that create equivalent string values: - -```swift -let string = #"\(x) \ " \u{2603}"# -let escaped = "\\(x) \\ \" \\u{2603}" -print(string) -// Prints "\(x) \ " \u{2603}" -print(string == escaped) -// Prints "true" -``` - - - -If you use more than one number sign to form -a string delimited by extended delimiters, -don't place whitespace in between the number signs: - - - -```swift -print(###"Line 1\###nLine 2"###) // OK -print(# # #"Line 1\# # #nLine 2"# # #) // Error -``` - - - -Multiline string literals that you create using extended delimiters -have the same indentation requirements as regular multiline string literals. - -The default inferred type of a string literal is `String`. -For more information about the `String` type, -see -and [`String`](https://developer.apple.com/documentation/swift/string). - -String literals that are concatenated by the `+` operator -are concatenated at compile time. -For example, the values of `textA` and `textB` -in the example below are identical --- -no runtime concatenation is performed. - -```swift -let textA = "Hello " + "world" -let textB = "Hello world" -``` - - - -> Grammar of a string literal: -> -> *string-literal* → *static-string-literal* | *interpolated-string-literal* -> -> *string-literal-opening-delimiter* → *extended-string-literal-delimiter*_?_ **`"`** \ -> *string-literal-closing-delimiter* → **`"`** *extended-string-literal-delimiter*_?_ -> -> *static-string-literal* → *string-literal-opening-delimiter* *quoted-text*_?_ *string-literal-closing-delimiter* \ -> *static-string-literal* → *multiline-string-literal-opening-delimiter* *multiline-quoted-text*_?_ *multiline-string-literal-closing-delimiter* -> -> *multiline-string-literal-opening-delimiter* → *extended-string-literal-delimiter*_?_ **`"""`** \ -> *multiline-string-literal-closing-delimiter* → **`"""`** *extended-string-literal-delimiter*_?_ \ -> *extended-string-literal-delimiter* → **`#`** *extended-string-literal-delimiter*_?_ -> -> *quoted-text* → *quoted-text-item* *quoted-text*_?_ \ -> *quoted-text-item* → *escaped-character* \ -> *quoted-text-item* → Any Unicode scalar value except **`"`**, **`\`**, U+000A, or U+000D -> -> *multiline-quoted-text* → *multiline-quoted-text-item* *multiline-quoted-text*_?_ \ -> *multiline-quoted-text-item* → *escaped-character* \ -> *multiline-quoted-text-item* → Any Unicode scalar value except **`\`** \ -> *multiline-quoted-text-item* → *escaped-newline* -> -> *interpolated-string-literal* → *string-literal-opening-delimiter* *interpolated-text*_?_ *string-literal-closing-delimiter* \ -> *interpolated-string-literal* → *multiline-string-literal-opening-delimiter* *multiline-interpolated-text*_?_ *multiline-string-literal-closing-delimiter* -> -> *interpolated-text* → *interpolated-text-item* *interpolated-text*_?_ \ -> *interpolated-text-item* → **`\(`** *expression* **`)`** | *quoted-text-item* -> -> *multiline-interpolated-text* → *multiline-interpolated-text-item* *multiline-interpolated-text*_?_ \ -> *multiline-interpolated-text-item* → **`\(`** *expression* **`)`** | *multiline-quoted-text-item* -> -> *escape-sequence* → **`\`** *extended-string-literal-delimiter* \ -> *escaped-character* → *escape-sequence* **`0`** | *escape-sequence* **`\`** | *escape-sequence* **`t`** | *escape-sequence* **`n`** | *escape-sequence* **`r`** | *escape-sequence* **`"`** | *escape-sequence* **`'`** \ -> *escaped-character* → *escape-sequence* **`u`** **`{`** *unicode-scalar-digits* **`}`** \ -> *unicode-scalar-digits* → Between one and eight hexadecimal digits -> -> *escaped-newline* → *escape-sequence* *inline-spaces*_?_ *line-break* - - - - - -### Regular Expression Literals - -A regular expression literal is a sequence of characters -surrounded by slashes (`/`) with the following form: - -```swift -/<#regular expression#>/ -``` - -Regular expression literals -must not begin with an unescaped tab or space, -and they can't contain -an unescaped slash (`/`), -a carriage return, or a line feed. - -Within a regular expression literal, -a backslash is understood as a part of that regular expression, -not just as an escape character like in string literals. -It indicates that the following special character -should be interpreted literally, -or that the following nonspecial character -should be interpreted in a special way. -For example, -`/\(/` matches a single left parenthesis -and `/\d/` matches a single digit. - - - -A regular expression literal delimited by extended delimiters -is a sequence of characters surrounded by slashes (`/`) -and a balanced set of one or more number signs (`#`). -A regular expression literal -delimited by extended delimiters has the following forms: - -```swift -#/<#regular expression#>/# - -#/ -<#regular expression#> -/# -``` - -A regular expression literal that uses extended delimiters -can begin with an unescaped space or tab, -contain unescaped slashes (`/`), -and span across multiple lines. -For a multiline regular expression literal, -the opening delimiter must be at the end of a line, -and the closing delimiter must be on its own line. -Inside a multiline regular expression literal, -the extended regular expression syntax is enabled by default --- -specifically, whitespace is ignored and comments are allowed. - - - -If you use more than one number sign to form -a regular expression literal delimited by extended delimiters, -don't place whitespace in between the number signs: - -```swift -let regex1 = ##/abc/## // OK -let regex2 = # #/abc/# # // Error -``` - - - -If you need to make an empty regular expression literal, -you must use the extended delimiter syntax. - -> Grammar of a regular expression literal: -> -> *regular-expression-literal* → *regular-expression-literal-opening-delimiter* *regular-expression* *regular-expression-literal-closing-delimiter* \ -> *regular-expression* → Any regular expression -> -> *regular-expression-literal-opening-delimiter* → *extended-regular-expression-literal-delimiter*_?_ **`/`** \ -> *regular-expression-literal-closing-delimiter* → **`/`** *extended-regular-expression-literal-delimiter*_?_ -> -> *extended-regular-expression-literal-delimiter* → **`#`** *extended-regular-expression-literal-delimiter*_?_ - -## Operators - -The Swift standard library defines a number of operators for your use, -many of which are discussed in -and . -The present section describes which characters can be used to define custom operators. - -Custom operators can begin with one of the ASCII characters -`/`, `=`, `-`, `+`, `!`, `*`, `%`, `<`, `>`, -`&`, `|`, `^`, `?`, or `~`, or one of the Unicode characters -defined in the grammar below -(which include characters from the -*Mathematical Operators*, *Miscellaneous Symbols*, and *Dingbats* -Unicode blocks, among others). -After the first character, -combining Unicode characters are also allowed. - -You can also define custom operators -that begin with a dot (`.`). -These operators can contain additional dots. -For example, `.+.` is treated as a single operator. -If an operator doesn't begin with a dot, -it can't contain a dot elsewhere. -For example, `+.+` is treated as -the `+` operator followed by the `.+` operator. - - - -Although you can define custom operators that contain a question mark (`?`), -they can't consist of a single question mark character only. -Additionally, although operators can contain an exclamation point (`!`), -postfix operators can't begin with either a question mark or an exclamation point. - - - - - -> Note: The tokens `=`, `->`, `//`, `/*`, `*/`, `.`, -> the prefix operators `<`, `&`, and `?`, -> the infix operator `?`, -> and the postfix operators `>`, `!`, and `?` are reserved. -> These tokens can't be overloaded, nor can they be used as custom operators. - -The whitespace around an operator is used to determine -whether an operator is used as a prefix operator, a postfix operator, -or an infix operator. This behavior has the following rules: - -- If an operator has whitespace around both sides or around neither side, - it's treated as an infix operator. - As an example, the `+++` operator in `a+++b` and `a +++ b` is treated as an infix operator. -- If an operator has whitespace on the left side only, - it's treated as a prefix unary operator. - As an example, the `+++` operator in `a +++b` is treated as a prefix unary operator. -- If an operator has whitespace on the right side only, - it's treated as a postfix unary operator. - As an example, the `+++` operator in `a+++ b` is treated as a postfix unary operator. -- If an operator has no whitespace on the left but is followed immediately by a dot (`.`), - it's treated as a postfix unary operator. - As an example, the `+++` operator in `a+++.b` is treated as a postfix unary operator - (`a+++ .b` rather than `a +++ .b`). - -For the purposes of these rules, -the characters `(`, `[`, and `{` before an operator, -the characters `)`, `]`, and `}` after an operator, -and the characters `,`, `;`, and `:` -are also considered whitespace. - -If the `!` or `?` predefined operator has no whitespace on the left, -it's treated as a postfix operator, -regardless of whether it has whitespace on the right. -To use the `?` as the optional-chaining operator, -it must not have whitespace on the left. -To use it in the ternary conditional (`?` `:`) operator, -it must have whitespace around both sides. - -If one of the arguments to an infix operator is a regular expression literal, -then the operator must have whitespace around both sides. - -In certain constructs, operators with a leading `<` or `>` -may be split into two or more tokens. The remainder is treated the same way -and may be split again. -As a result, you don't need to add whitespace -to disambiguate between the closing `>` characters in constructs like -`Dictionary>`. -In this example, the closing `>` characters aren't treated as a single token -that may then be misinterpreted as a bit shift `>>` operator. - - - -To learn how to define new, custom operators, -see and . -To learn how to overload existing operators, -see . - - - -> Grammar of operators: -> -> *operator* → *operator-head* *operator-characters*_?_ \ -> *operator* → *dot-operator-head* *dot-operator-characters* -> -> *operator-head* → **`/`** | **`=`** | **`-`** | **`+`** | **`!`** | **`*`** | **`%`** | **`<`** | **`>`** | **`&`** | **`|`** | **`^`** | **`~`** | **`?`** \ -> *operator-head* → U+00A1–U+00A7 \ -> *operator-head* → U+00A9 or U+00AB \ -> *operator-head* → U+00AC or U+00AE \ -> *operator-head* → U+00B0–U+00B1 \ -> *operator-head* → U+00B6, U+00BB, U+00BF, U+00D7, or U+00F7 \ -> *operator-head* → U+2016–U+2017 \ -> *operator-head* → U+2020–U+2027 \ -> *operator-head* → U+2030–U+203E \ -> *operator-head* → U+2041–U+2053 \ -> *operator-head* → U+2055–U+205E \ -> *operator-head* → U+2190–U+23FF \ -> *operator-head* → U+2500–U+2775 \ -> *operator-head* → U+2794–U+2BFF \ -> *operator-head* → U+2E00–U+2E7F \ -> *operator-head* → U+3001–U+3003 \ -> *operator-head* → U+3008–U+3020 \ -> *operator-head* → U+3030 -> -> *operator-character* → *operator-head* \ -> *operator-character* → U+0300–U+036F \ -> *operator-character* → U+1DC0–U+1DFF \ -> *operator-character* → U+20D0–U+20FF \ -> *operator-character* → U+FE00–U+FE0F \ -> *operator-character* → U+FE20–U+FE2F \ -> *operator-character* → U+E0100–U+E01EF \ -> *operator-characters* → *operator-character* *operator-characters*_?_ -> -> *dot-operator-head* → **`.`** \ -> *dot-operator-character* → **`.`** | *operator-character* \ -> *dot-operator-characters* → *dot-operator-character* *dot-operator-characters*_?_ -> -> *infix-operator* → *operator* \ -> *prefix-operator* → *operator* \ -> *postfix-operator* → *operator* - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/ReferenceManual/Patterns.md b/swift-6-beta.docc/ReferenceManual/Patterns.md deleted file mode 100644 index 07dced067..000000000 --- a/swift-6-beta.docc/ReferenceManual/Patterns.md +++ /dev/null @@ -1,506 +0,0 @@ -# Patterns - -Match and destructure values. - -A *pattern* represents the structure of a single value -or a composite value. -For example, the structure of a tuple `(1, 2)` is a comma-separated list of two -elements. Because patterns represent the structure of a value rather than any -one particular value, you can match them with a variety of values. -For instance, the pattern `(x, y)` matches the tuple `(1, 2)` and any other -two-element tuple. In addition to matching a pattern with a value, -you can extract part or all of a composite value and bind each part -to a constant or variable name. - -In Swift, there are two basic kinds of patterns: -those that successfully match any kind of value, -and those that may fail to match a specified value at runtime. - -The first kind of pattern is used for destructuring values -in simple variable, constant, and optional bindings. -These include wildcard patterns, identifier patterns, -and any value binding or tuple patterns containing -them. You can specify a type annotation for these patterns -to constrain them to match only values of a certain type. - -The second kind of pattern is used for full pattern matching, -where the values you're trying to match against may not be there at runtime. -These include enumeration case patterns, optional patterns, expression patterns, -and type-casting patterns. You use these patterns in a case label of a `switch` -statement, a `catch` clause of a `do` statement, -or in the case condition of an `if`, `while`, -`guard`, or `for`-`in` statement. - -> Grammar of a pattern: -> -> *pattern* → *wildcard-pattern* *type-annotation*_?_ \ -> *pattern* → *identifier-pattern* *type-annotation*_?_ \ -> *pattern* → *value-binding-pattern* \ -> *pattern* → *tuple-pattern* *type-annotation*_?_ \ -> *pattern* → *enum-case-pattern* \ -> *pattern* → *optional-pattern* \ -> *pattern* → *type-casting-pattern* \ -> *pattern* → *expression-pattern* - -## Wildcard Pattern - -A *wildcard pattern* matches and ignores any value and consists of an underscore -(`_`). Use a wildcard pattern when you don't care about the values being -matched against. For example, the following code iterates through the closed range `1...3`, -ignoring the current value of the range on each iteration of the loop: - -```swift -for _ in 1...3 { - // Do something three times. -} -``` - - - -> Grammar of a wildcard pattern: -> -> *wildcard-pattern* → **`_`** - -## Identifier Pattern - -An *identifier pattern* matches any value and binds the matched value to a -variable or constant name. -For example, in the following constant declaration, `someValue` is an identifier pattern -that matches the value `42` of type `Int`: - -```swift -let someValue = 42 -``` - - - -When the match succeeds, the value `42` is bound (assigned) -to the constant name `someValue`. - -When the pattern on the left-hand side of a variable or constant declaration -is an identifier pattern, -the identifier pattern is implicitly a subpattern of a value-binding pattern. - -> Grammar of an identifier pattern: -> -> *identifier-pattern* → *identifier* - -## Value-Binding Pattern - -A *value-binding pattern* binds matched values to variable or constant names. -Value-binding patterns that bind a matched value to the name of a constant -begin with the `let` keyword; those that bind to the name of variable -begin with the `var` keyword. - -Identifiers patterns within a value-binding pattern -bind new named variables or constants to their matching values. For example, -you can decompose the elements of a tuple and bind the value of each element to a -corresponding identifier pattern. - -```swift -let point = (3, 2) -switch point { -// Bind x and y to the elements of point. -case let (x, y): - print("The point is at (\(x), \(y)).") -} -// Prints "The point is at (3, 2)." -``` - - - -In the example above, `let` distributes to each identifier pattern in the -tuple pattern `(x, y)`. Because of this behavior, the `switch` cases -`case let (x, y):` and `case (let x, let y):` match the same values. - -> Grammar of a value-binding pattern: -> -> *value-binding-pattern* → **`var`** *pattern* | **`let`** *pattern* - - - -## Tuple Pattern - -A *tuple pattern* is a comma-separated list of zero or more patterns, enclosed in -parentheses. Tuple patterns match values of corresponding tuple types. - -You can constrain a tuple pattern to match certain kinds of tuple types -by using type annotations. -For example, the tuple pattern `(x, y): (Int, Int)` in the constant declaration -`let (x, y): (Int, Int) = (1, 2)` matches only tuple types in which -both elements are of type `Int`. - -When a tuple pattern is used as the pattern in a `for`-`in` statement -or in a variable or constant declaration, it can contain only wildcard patterns, -identifier patterns, optional patterns, or other tuple patterns that contain those. -For example, -the following code isn't valid because the element `0` in the tuple pattern `(x, 0)` is -an expression pattern: - -```swift -let points = [(0, 0), (1, 0), (1, 1), (2, 0), (2, 1)] -// This code isn't valid. -for (x, 0) in points { - /* ... */ -} -``` - - - -The parentheses around a tuple pattern that contains a single element have no effect. -The pattern matches values of that single element's type. For example, the following are -equivalent: - - - -```swift -let a = 2 // a: Int = 2 -let (a) = 2 // a: Int = 2 -let (a): Int = 2 // a: Int = 2 -``` - - - -> Grammar of a tuple pattern: -> -> *tuple-pattern* → **`(`** *tuple-pattern-element-list*_?_ **`)`** \ -> *tuple-pattern-element-list* → *tuple-pattern-element* | *tuple-pattern-element* **`,`** *tuple-pattern-element-list* \ -> *tuple-pattern-element* → *pattern* | *identifier* **`:`** *pattern* - -## Enumeration Case Pattern - -An *enumeration case pattern* matches a case of an existing enumeration type. -Enumeration case patterns appear in `switch` statement -case labels and in the case conditions of `if`, `while`, `guard`, and `for`-`in` -statements. - -If the enumeration case you're trying to match has any associated values, -the corresponding enumeration case pattern must specify a tuple pattern that contains -one element for each associated value. For an example that uses a `switch` statement -to match enumeration cases containing associated values, -see . - -An enumeration case pattern also matches -values of that case wrapped in an optional. -This simplified syntax lets you omit an optional pattern. -Note that, -because `Optional` is implemented as an enumeration, -`.none` and `.some` can appear -in the same switch as the cases of the enumeration type. - -```swift -enum SomeEnum { case left, right } -let x: SomeEnum? = .left -switch x { -case .left: - print("Turn left") -case .right: - print("Turn right") -case nil: - print("Keep going straight") -} -// Prints "Turn left" -``` - - - -> Grammar of an enumeration case pattern: -> -> *enum-case-pattern* → *type-identifier*_?_ **`.`** *enum-case-name* *tuple-pattern*_?_ - -## Optional Pattern - -An *optional pattern* matches values wrapped in a `some(Wrapped)` case -of an `Optional` enumeration. -Optional patterns consist of an identifier pattern followed immediately by a question mark -and appear in the same places as enumeration case patterns. - -Because optional patterns are syntactic sugar for `Optional` -enumeration case patterns, -the following are equivalent: - -```swift -let someOptional: Int? = 42 -// Match using an enumeration case pattern. -if case .some(let x) = someOptional { - print(x) -} - -// Match using an optional pattern. -if case let x? = someOptional { - print(x) -} -``` - - - -The optional pattern provides a convenient way to -iterate over an array of optional values in a `for`-`in` statement, -executing the body of the loop only for non-`nil` elements. - -```swift -let arrayOfOptionalInts: [Int?] = [nil, 2, 3, nil, 5] -// Match only non-nil values. -for case let number? in arrayOfOptionalInts { - print("Found a \(number)") -} -// Found a 2 -// Found a 3 -// Found a 5 -``` - - - -> Grammar of an optional pattern: -> -> *optional-pattern* → *identifier-pattern* **`?`** - -## Type-Casting Patterns - -There are two type-casting patterns, the `is` pattern and the `as` pattern. -The `is` pattern appears only in `switch` statement -case labels. The `is` and `as` patterns have the following form: - -```swift -is <#type#> -<#pattern#> as <#type#> -``` - -The `is` pattern matches a value if the type of that value at runtime is the same as -the type specified in the right-hand side of the `is` pattern --- or a subclass of that type. -The `is` pattern behaves like the `is` operator in that they both perform a type cast -but discard the returned type. - -The `as` pattern matches a value if the type of that value at runtime is the same as -the type specified in the right-hand side of the `as` pattern --- or a subclass of that type. -If the match succeeds, -the type of the matched value is cast to the *pattern* specified in the right-hand side -of the `as` pattern. - -For an example that uses a `switch` statement -to match values with `is` and `as` patterns, -see . - -> Grammar of a type casting pattern: -> -> *type-casting-pattern* → *is-pattern* | *as-pattern* \ -> *is-pattern* → **`is`** *type* \ -> *as-pattern* → *pattern* **`as`** *type* - -## Expression Pattern - -An *expression pattern* represents the value of an expression. -Expression patterns appear only in `switch` statement -case labels. - -The expression represented by the expression pattern -is compared with the value of an input expression -using the pattern-matching operator (`~=`) from the Swift standard library. -The matches succeeds -if the `~=` operator returns `true`. By default, the `~=` operator compares -two values of the same type using the `==` operator. -It can also match a value with a range of values, -by checking whether the value is contained within the range, -as the following example shows. - -```swift -let point = (1, 2) -switch point { -case (0, 0): - print("(0, 0) is at the origin.") -case (-2...2, -2...2): - print("(\(point.0), \(point.1)) is near the origin.") -default: - print("The point is at (\(point.0), \(point.1)).") -} -// Prints "(1, 2) is near the origin." -``` - - - -You can overload the `~=` operator to provide custom expression matching behavior. -For example, you can rewrite the above example to compare the `point` expression -with a string representations of points. - -```swift -// Overload the ~= operator to match a string with an integer. -func ~= (pattern: String, value: Int) -> Bool { - return pattern == "\(value)" -} -switch point { -case ("0", "0"): - print("(0, 0) is at the origin.") -default: - print("The point is at (\(point.0), \(point.1)).") -} -// Prints "The point is at (1, 2)." -``` - - - -> Grammar of an expression pattern: -> -> *expression-pattern* → *expression* - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/ReferenceManual/Statements.md b/swift-6-beta.docc/ReferenceManual/Statements.md deleted file mode 100644 index 50bdc3458..000000000 --- a/swift-6-beta.docc/ReferenceManual/Statements.md +++ /dev/null @@ -1,1461 +0,0 @@ -# Statements - -Group expressions and control the flow of execution. - -In Swift, there are three kinds of statements: simple statements, compiler control statements, -and control flow statements. -Simple statements are the most common and consist of either an expression or a declaration. -Compiler control statements allow the program to change aspects of the compiler's behavior -and include a conditional compilation block and a line control statement. - -Control flow statements are used to control the flow of execution in a program. -There are several types of control flow statements in Swift, including -loop statements, branch statements, and control transfer statements. -Loop statements allow a block of code to be executed repeatedly, -branch statements allow a certain block of code to be executed -only when certain conditions are met, -and control transfer statements provide a way to alter the order in which code is executed. -In addition, Swift provides a `do` statement to introduce scope, -and catch and handle errors, -and a `defer` statement for running cleanup actions just before the current scope exits. - -A semicolon (`;`) can optionally appear after any statement -and is used to separate multiple statements if they appear on the same line. - -> Grammar of a statement: -> -> *statement* → *expression* **`;`**_?_ \ -> *statement* → *declaration* **`;`**_?_ \ -> *statement* → *loop-statement* **`;`**_?_ \ -> *statement* → *branch-statement* **`;`**_?_ \ -> *statement* → *labeled-statement* **`;`**_?_ \ -> *statement* → *control-transfer-statement* **`;`**_?_ \ -> *statement* → *defer-statement* **`;`**_?_ \ -> *statement* → *do-statement* **`;`**_?_ \ -> *statement* → *compiler-control-statement* \ -> *statements* → *statement* *statements*_?_ - - - -## Loop Statements - -Loop statements allow a block of code to be executed repeatedly, -depending on the conditions specified in the loop. -Swift has three loop statements: -a `for`-`in` statement, -a `while` statement, -and a `repeat`-`while` statement. - -Control flow in a loop statement can be changed by a `break` statement -and a `continue` statement and is discussed in and - below. - -> Grammar of a loop statement: -> -> *loop-statement* → *for-in-statement* \ -> *loop-statement* → *while-statement* \ -> *loop-statement* → *repeat-while-statement* - -### For-In Statement - -A `for`-`in` statement allows a block of code to be executed -once for each item in a collection (or any type) -that conforms to the -[`Sequence`](https://developer.apple.com/documentation/swift/sequence) protocol. - -A `for`-`in` statement has the following form: - -```swift -for <#item#> in <#collection#> { - <#statements#> -} -``` - -The `makeIterator()` method is called on the *collection* expression -to obtain a value of an iterator type --- that is, -a type that conforms to the -[`IteratorProtocol`](https://developer.apple.com/documentation/swift/iteratorprotocol) protocol. -The program begins executing a loop -by calling the `next()` method on the iterator. -If the value returned isn't `nil`, -it's assigned to the *item* pattern, -the program executes the *statements*, -and then continues execution at the beginning of the loop. -Otherwise, the program doesn't perform assignment or execute the *statements*, -and it's finished executing the `for`-`in` statement. - -> Grammar of a for-in statement: -> -> *for-in-statement* → **`for`** **`case`**_?_ *pattern* **`in`** *expression* *where-clause*_?_ *code-block* - -### While Statement - -A `while` statement allows a block of code to be executed repeatedly, -as long as a condition remains true. - -A `while` statement has the following form: - -```swift -while <#condition#> { - <#statements#> -} -``` - -A `while` statement is executed as follows: - -1. The *condition* is evaluated. - - If `true`, execution continues to step 2. - If `false`, the program is finished executing the `while` statement. -2. The program executes the *statements*, and execution returns to step 1. - -Because the value of the *condition* is evaluated before the *statements* are executed, -the *statements* in a `while` statement can be executed zero or more times. - -The value of the *condition* -must be of type `Bool` or a type bridged to `Bool`. -The condition can also be an optional binding declaration, -as discussed in . - -> Grammar of a while statement: -> -> *while-statement* → **`while`** *condition-list* *code-block* -> -> *condition-list* → *condition* | *condition* **`,`** *condition-list* \ -> *condition* → *expression* | *availability-condition* | *case-condition* | *optional-binding-condition* -> -> *case-condition* → **`case`** *pattern* *initializer* \ -> *optional-binding-condition* → **`let`** *pattern* *initializer*_?_ | **`var`** *pattern* *initializer*_?_ - -### Repeat-While Statement - -A `repeat`-`while` statement allows a block of code to be executed one or more times, -as long as a condition remains true. - -A `repeat`-`while` statement has the following form: - -```swift -repeat { - <#statements#> -} while <#condition#> -``` - -A `repeat`-`while` statement is executed as follows: - -1. The program executes the *statements*, - and execution continues to step 2. -2. The *condition* is evaluated. - - If `true`, execution returns to step 1. - If `false`, the program is finished executing the `repeat`-`while` statement. - -Because the value of the *condition* is evaluated after the *statements* are executed, -the *statements* in a `repeat`-`while` statement are executed at least once. - -The value of the *condition* -must be of type `Bool` or a type bridged to `Bool`. - -> Grammar of a repeat-while statement: -> -> *repeat-while-statement* → **`repeat`** *code-block* **`while`** *expression* - -## Branch Statements - -Branch statements allow the program to execute certain parts of code -depending on the value of one or more conditions. -The values of the conditions specified in a branch statement -control how the program branches and, therefore, what block of code is executed. -Swift has three branch statements: -an `if` statement, a `guard` statement, and a `switch` statement. - -Control flow in an `if` statement or a `switch` statement can be changed by a `break` statement -and is discussed in below. - -> Grammar of a branch statement: -> -> *branch-statement* → *if-statement* \ -> *branch-statement* → *guard-statement* \ -> *branch-statement* → *switch-statement* - -### If Statement - -An `if` statement is used for executing code -based on the evaluation of one or more conditions. - -There are two basic forms of an `if` statement. -In each form, the opening and closing braces are required. - -The first form allows code to be executed only when a condition is true -and has the following form: - -```swift -if <#condition#> { - <#statements#> -} -``` - -The second form of an `if` statement provides an additional *else clause* -(introduced by the `else` keyword) -and is used for executing one part of code when the condition is true -and another part of code when the same condition is false. -When a single else clause is present, an `if` statement has the following form: - -```swift -if <#condition#> { - <#statements to execute if condition is true#> -} else { - <#statements to execute if condition is false#> -} -``` - -The else clause of an `if` statement can contain another `if` statement -to test more than one condition. -An `if` statement chained together in this way has the following form: - -```swift -if <#condition 1#> { - <#statements to execute if condition 1 is true#> -} else if <#condition 2#> { - <#statements to execute if condition 2 is true#> -} else { - <#statements to execute if both conditions are false#> -} -``` - -The value of any condition in an `if` statement -must be of type `Bool` or a type bridged to `Bool`. -The condition can also be an optional binding declaration, -as discussed in . - -> Grammar of an if statement: -> -> *if-statement* → **`if`** *condition-list* *code-block* *else-clause*_?_ \ -> *else-clause* → **`else`** *code-block* | **`else`** *if-statement* - -### Guard Statement - -A `guard` statement is used to transfer program control out of a scope -if one or more conditions aren't met. - -A `guard` statement has the following form: - -```swift -guard <#condition#> else { - <#statements#> -} -``` - -The value of any condition in a `guard` statement -must be of type `Bool` or a type bridged to `Bool`. -The condition can also be an optional binding declaration, -as discussed in . - -Any constants or variables assigned a value -from an optional binding declaration in a `guard` statement condition -can be used for the rest of the guard statement's enclosing scope. - -The `else` clause of a `guard` statement is required, -and must either call a function with the `Never` return type -or transfer program control outside the guard statement's enclosing scope -using one of the following statements: - -- `return` -- `break` -- `continue` -- `throw` - -Control transfer statements are discussed in below. -For more information on functions with the `Never` return type, -see . - -> Grammar of a guard statement: -> -> *guard-statement* → **`guard`** *condition-list* **`else`** *code-block* - -### Switch Statement - -A `switch` statement allows certain blocks of code to be executed -depending on the value of a control expression. - -A `switch` statement has the following form: - -```swift -switch <#control expression#> { -case <#pattern 1#>: - <#statements#> -case <#pattern 2#> where <#condition#>: - <#statements#> -case <#pattern 3#> where <#condition#>, - <#pattern 4#> where <#condition#>: - <#statements#> -default: - <#statements#> -} -``` - -The *control expression* of the `switch` statement is evaluated -and then compared with the patterns specified in each case. -If a match is found, -the program executes the *statements* listed within the scope of that case. -The scope of each case can't be empty. -As a result, you must include at least one statement -following the colon (`:`) of each case label. Use a single `break` statement -if you don't intend to execute any code in the body of a matched case. - -The values of expressions your code can branch on are very flexible. For example, -in addition to the values of scalar types, such as integers and characters, -your code can branch on the values of any type, including floating-point numbers, strings, -tuples, instances of custom classes, and optionals. -The value of the *control expression* can even be matched to the value of a case in an enumeration -and checked for inclusion in a specified range of values. -For examples of how to use these various types of values in `switch` statements, -see in . - -A `switch` case can optionally contain a `where` clause after each pattern. -A *where clause* is introduced by the `where` keyword followed by an expression, -and is used to provide an additional condition -before a pattern in a case is considered matched to the *control expression*. -If a `where` clause is present, the *statements* within the relevant case -are executed only if the value of the *control expression* -matches one of the patterns of the case and the expression of the `where` clause evaluates to `true`. -For example, a *control expression* matches the case in the example below -only if it's a tuple that contains two elements of the same value, such as `(1, 1)`. - -```swift -case let (x, y) where x == y: -``` - - - -As the above example shows, patterns in a case can also bind constants -using the `let` keyword (they can also bind variables using the `var` keyword). -These constants (or variables) can then be referenced in a corresponding `where` clause -and throughout the rest of the code within the scope of the case. -If the case contains multiple patterns that match the control expression, -all of the patterns must contain the same constant or variable bindings, -and each bound variable or constant must have the same type -in all of the case's patterns. - - - -A `switch` statement can also include a default case, introduced by the `default` keyword. -The code within a default case is executed only if no other cases match the control expression. -A `switch` statement can include only one default case, -which must appear at the end of the `switch` statement. - -Although the actual execution order of pattern-matching operations, -and in particular the evaluation order of patterns in cases, is unspecified, -pattern matching in a `switch` statement behaves -as if the evaluation is performed in source order --- that is, -the order in which they appear in source code. -As a result, if multiple cases contain patterns that evaluate to the same value, -and thus can match the value of the control expression, -the program executes only the code within the first matching case in source order. - - - - - -#### Switch Statements Must Be Exhaustive - -In Swift, -every possible value of the control expression’s type -must match the value of at least one pattern of a case. -When this simply isn’t feasible -(for example, when the control expression’s type is `Int`), -you can include a default case to satisfy the requirement. - -#### Switching Over Future Enumeration Cases - -A *nonfrozen enumeration* is a special kind of enumeration -that may gain new enumeration cases in the future --- -even after you compile and ship an app. -Switching over a nonfrozen enumeration requires extra consideration. -When a library's authors mark an enumeration as nonfrozen, -they reserve the right to add new enumeration cases, -and any code that interacts with that enumeration -*must* be able to handle those future cases without being recompiled. -Code that's compiled in library evolution mode, -code in the Swift standard library, -Swift overlays for Apple frameworks, -and C and Objective-C code can declare nonfrozen enumerations. -For information about frozen and nonfrozen enumerations, -see . - -When switching over a nonfrozen enumeration value, -you always need to include a default case, -even if every case of the enumeration already has a corresponding switch case. -You can apply the `@unknown` attribute to the default case, -which indicates that the default case should match only enumeration cases -that are added in the future. -Swift produces a warning -if the default case matches -any enumeration case that's known at compiler time. -This future warning informs you that the library author -added a new case to the enumeration -that doesn't have a corresponding switch case. - -The following example switches over all three existing cases of -the Swift standard library's [`Mirror.AncestorRepresentation`](https://developer.apple.com/documentation/swift/mirror/ancestorrepresentation) -enumeration. -If you add additional cases in the future, -the compiler generates a warning to indicate -that you need to update the switch statement -to take the new cases into account. - -```swift -let representation: Mirror.AncestorRepresentation = .generated -switch representation { -case .customized: - print("Use the nearest ancestor’s implementation.") -case .generated: - print("Generate a default mirror for all ancestor classes.") -case .suppressed: - print("Suppress the representation of all ancestor classes.") -@unknown default: - print("Use a representation that was unknown when this code was compiled.") -} -// Prints "Generate a default mirror for all ancestor classes." -``` - - - -#### Execution Does Not Fall Through Cases Implicitly - -After the code within a matched case has finished executing, -the program exits from the `switch` statement. -Program execution doesn't continue or "fall through" to the next case or default case. -That said, if you want execution to continue from one case to the next, -explicitly include a `fallthrough` statement, -which simply consists of the `fallthrough` keyword, -in the case from which you want execution to continue. -For more information about the `fallthrough` statement, -see below. - -> Grammar of a switch statement: -> -> *switch-statement* → **`switch`** *expression* **`{`** *switch-cases*_?_ **`}`** \ -> *switch-cases* → *switch-case* *switch-cases*_?_ \ -> *switch-case* → *case-label* *statements* \ -> *switch-case* → *default-label* *statements* \ -> *switch-case* → *conditional-switch-case* -> -> *case-label* → *attributes*_?_ **`case`** *case-item-list* **`:`** \ -> *case-item-list* → *pattern* *where-clause*_?_ | *pattern* *where-clause*_?_ **`,`** *case-item-list* \ -> *default-label* → *attributes*_?_ **`default`** **`:`** -> -> *where-clause* → **`where`** *where-expression* \ -> *where-expression* → *expression* -> -> *conditional-switch-case* → *switch-if-directive-clause* *switch-elseif-directive-clauses*_?_ *switch-else-directive-clause*_?_ *endif-directive* \ -> *switch-if-directive-clause* → *if-directive* *compilation-condition* *switch-cases*_?_ \ -> *switch-elseif-directive-clauses* → *elseif-directive-clause* *switch-elseif-directive-clauses*_?_ \ -> *switch-elseif-directive-clause* → *elseif-directive* *compilation-condition* *switch-cases*_?_ \ -> *switch-else-directive-clause* → *else-directive* *switch-cases*_?_ - - - -## Labeled Statement - -You can prefix a loop statement, an `if` statement, a `switch` statement, -or a `do` statement with a *statement label*, -which consists of the name of the label followed immediately by a colon (:). -Use statement labels with `break` and `continue` statements to be explicit -about how you want to change control flow in a loop statement or a `switch` statement, -as discussed in and - below. - -The scope of a labeled statement is the entire statement following the statement label. -You can nest labeled statements, but the name of each statement label must be unique. - -For more information and to see examples -of how to use statement labels, -see in . - - - -> Grammar of a labeled statement: -> -> *labeled-statement* → *statement-label* *loop-statement* \ -> *labeled-statement* → *statement-label* *if-statement* \ -> *labeled-statement* → *statement-label* *switch-statement* \ -> *labeled-statement* → *statement-label* *do-statement* -> -> *statement-label* → *label-name* **`:`** \ -> *label-name* → *identifier* - -## Control Transfer Statements - -Control transfer statements can change the order in which code in your program is executed -by unconditionally transferring program control from one piece of code to another. -Swift has five control transfer statements: a `break` statement, a `continue` statement, -a `fallthrough` statement, a `return` statement, and a `throw` statement. - -> Grammar of a control transfer statement: -> -> *control-transfer-statement* → *break-statement* \ -> *control-transfer-statement* → *continue-statement* \ -> *control-transfer-statement* → *fallthrough-statement* \ -> *control-transfer-statement* → *return-statement* \ -> *control-transfer-statement* → *throw-statement* - -### Break Statement - -A `break` statement ends program execution of a loop, -an `if` statement, or a `switch` statement. -A `break` statement can consist of only the `break` keyword, -or it can consist of the `break` keyword followed by the name of a statement label, -as shown below. - -```swift -break -break <#label name#> -``` - -When a `break` statement is followed by the name of a statement label, -it ends program execution of the loop, -`if` statement, or `switch` statement named by that label. - -When a `break` statement isn't followed by the name of a statement label, -it ends program execution of the `switch` statement or the innermost enclosing loop -statement in which it occurs. -You can't use an unlabeled `break` statement to break out of an `if` statement. - -In both cases, program control is then transferred to the first line -of code following the enclosing loop or `switch` statement, if any. - -For examples of how to use a `break` statement, -see and -in . - -> Grammar of a break statement: -> -> *break-statement* → **`break`** *label-name*_?_ - -### Continue Statement - -A `continue` statement ends program execution of the current iteration of a loop -statement but doesn't stop execution of the loop statement. -A `continue` statement can consist of only the `continue` keyword, -or it can consist of the `continue` keyword followed by the name of a statement label, -as shown below. - -```swift -continue -continue <#label name#> -``` - -When a `continue` statement is followed by the name of a statement label, -it ends program execution of the current iteration -of the loop statement named by that label. - -When a `continue` statement isn't followed by the name of a statement label, -it ends program execution of the current iteration -of the innermost enclosing loop statement in which it occurs. - -In both cases, program control is then transferred to the condition -of the enclosing loop statement. - -In a `for` statement, -the increment expression is still evaluated after the `continue` statement is executed, -because the increment expression is evaluated after the execution of the loop's body. - -For examples of how to use a `continue` statement, -see and -in . - -> Grammar of a continue statement: -> -> *continue-statement* → **`continue`** *label-name*_?_ - -### Fallthrough Statement - -A `fallthrough` statement consists of the `fallthrough` keyword -and occurs only in a case block of a `switch` statement. -A `fallthrough` statement causes program execution to continue -from one case in a `switch` statement to the next case. -Program execution continues to the next case -even if the patterns of the case label don't match -the value of the `switch` statement's control expression. - -A `fallthrough` statement can appear anywhere inside a `switch` statement, -not just as the last statement of a case block, -but it can't be used in the final case block. -It also can't transfer control into a case block -whose pattern contains value binding patterns. - -For an example of how to use a `fallthrough` statement in a `switch` statement, -see -in . - -> Grammar of a fallthrough statement: -> -> *fallthrough-statement* → **`fallthrough`** - -### Return Statement - -A `return` statement occurs in the body of a function or method definition -and causes program execution to return to the calling function or method. -Program execution continues at the point immediately following the function or method call. - -A `return` statement can consist of only the `return` keyword, -or it can consist of the `return` keyword followed by an expression, as shown below. - -```swift -return -return <#expression#> -``` - -When a `return` statement is followed by an expression, -the value of the expression is returned to the calling function or method. -If the value of the expression doesn't match the value of the return type -declared in the function or method declaration, -the expression's value is converted to the return type -before it's returned to the calling function or method. - -> Note: As described in , a special form of the `return` statement (`return nil`) -> can be used in a failable initializer to indicate initialization failure. - - - -When a `return` statement isn't followed by an expression, -it can be used only to return from a function or method that doesn't return a value -(that is, when the return type of the function or method is `Void` or `()`). - -> Grammar of a return statement: -> -> *return-statement* → **`return`** *expression*_?_ - -### Throw Statement - -A `throw` statement occurs in the body of a throwing function or method, -or in the body of a closure expression whose type is marked with the `throws` keyword. - -A `throw` statement causes a program to end execution of the current scope -and begin error propagation to its enclosing scope. -The error that's thrown continues to propagate until it's handled by a `catch` clause -of a `do` statement. - -A `throw` statement consists of the `throw` keyword -followed by an expression, as shown below. - -```swift -throw <#expression#> -``` - -The value of the *expression* must have a type that conforms to -the `Error` protocol. -If the `do` statement or function that contains the `throw` statement -declares the type of errors it throws, -the value of the *expression* must be an instance of that type. - -For an example of how to use a `throw` statement, -see -in . - -> Grammar of a throw statement: -> -> *throw-statement* → **`throw`** *expression* - -## Defer Statement - -A `defer` statement is used for executing code -just before transferring program control outside of the scope -that the `defer` statement appears in. - -A `defer` statement has the following form: - -```swift -defer { - <#statements#> -} -``` - -The statements within the `defer` statement are executed -no matter how program control is transferred. -This means that a `defer` statement can be used, for example, -to perform manual resource management such as closing file descriptors, -and to perform actions that need to happen even if an error is thrown. - -The *statements* in the `defer` statement -are executed at the end of the scope that encloses the `defer` statement. - -```swift -func f(x: Int) { - defer { print("First defer") } - - if x < 10 { - defer { print("Second defer") } - print("End of if") - } - - print("End of function") -} -f(x: 5) -// Prints "End of if" -// Prints "Second defer" -// Prints "End of function" -// Prints "First defer" -``` - - - -In the code above, -the `defer` in the `if` statement -executes before the `defer` declared in the function `f` -because the scope of the `if` statement ends -before the scope of the function. - -If multiple `defer` statements appear in the same scope, -the order they appear is the reverse of the order they're executed. -Executing the last `defer` statement in a given scope first -means that statements inside that last `defer` statement -can refer to resources that will be cleaned up by other `defer` statements. - -```swift -func f() { - defer { print("First defer") } - defer { print("Second defer") } - print("End of function") -} -f() -// Prints "End of function" -// Prints "Second defer" -// Prints "First defer" -``` - - - -The statements in the `defer` statement can't -transfer program control outside of the `defer` statement. - -> Grammar of a defer statement: -> -> *defer-statement* → **`defer`** *code-block* - -## Do Statement - -The `do` statement is used to introduce a new scope -and can optionally contain one or more `catch` clauses, -which contain patterns that match against defined error conditions. -Variables and constants declared in the scope of a `do` statement -can be accessed only within that scope. - -A `do` statement in Swift is similar to -curly braces (`{}`) in C used to delimit a code block, -and doesn't incur a performance cost at runtime. - -A `do` statement has the following form: - -```swift -do { - try <#expression#> - <#statements#> -} catch <#pattern 1#> { - <#statements#> -} catch <#pattern 2#> where <#condition#> { - <#statements#> -} catch <#pattern 3#>, <#pattern 4#> where <#condition#> { - <#statements#> -} catch { - <#statements#> -} -``` - -A `do` statement can optionally specify the type of error it throws, -which has the following form: - -```swift -do throws(<#type#>) { - try <#expression#> -} catch <#pattern> { - <#statements#> -} catch { - <#statements#> -} -``` - -If the `do` statement includes a `throws` clause, -the `do` block can throw errors of only the specified *type*. -The *type* must be -a concrete type that conforms to the `Error` protocol, -an opaque type that conforms to the `Error` protocol, -or the boxed protocol type `any Error`. -If the `do` statement doesn't specify the type of error it throws, -Swift infers the error type as follows: - -- If every `throws` statement and `try` expression in the `do` code block - is nested inside of an exhaustive error-handling mechanism, - then Swift infers that the `do` statement is nonthrowing. - -- If the `do` code block contains code that throws - errors of only a single type - outside of exhaustive error handling, - other than throwing `Never`, - then Swift infers that the `do` statement throws that concrete error type. - -- If the `do` code block contains code that throws - errors of more than a single type - outside of exhaustive error handling, - then Swift infers that the `do` statement throws `any Error`. - -For more information about working with errors that have explicit types, -see . - -If any statement in the `do` code block throws an error, -program control is transferred -to the first `catch` clause whose pattern matches the error. -If none of the clauses match, -the error propagates to the surrounding scope. -If an error is unhandled at the top level, -program execution stops with a runtime error. - -Like a `switch` statement, -the compiler attempts to infer whether `catch` clauses are exhaustive. -If such a determination can be made, the error is considered handled. -Otherwise, the error can propagate out of the containing scope, -which means -the error must be handled by an enclosing `catch` clause -or the containing function must be declared with `throws`. - -A `catch` clause that has multiple patterns -matches the error if any of its patterns match the error. -If a `catch` clause contains multiple patterns, -all of the patterns must contain the same constant or variable bindings, -and each bound variable or constant must have the same type -in all of the `catch` clause's patterns. - - - -To ensure that an error is handled, -use a `catch` clause with a pattern that matches all errors, -such as a wildcard pattern (`_`). -If a `catch` clause doesn't specify a pattern, -the `catch` clause matches and binds any error to a local constant named `error`. -For more information about the patterns you can use in a `catch` clause, -see . - -To see an example of how to use a `do` statement with several `catch` clauses, -see . - -> Grammar of a do statement: -> -> *do-statement* → **`do`** *throws-clause*_?_ *code-block* *catch-clauses*_?_ \ -> *catch-clauses* → *catch-clause* *catch-clauses*_?_ \ -> *catch-clause* → **`catch`** *catch-pattern-list*_?_ *code-block* \ -> *catch-pattern-list* → *catch-pattern* | *catch-pattern* **`,`** *catch-pattern-list* \ -> *catch-pattern* → *pattern* *where-clause*_?_ - -## Compiler Control Statements - -Compiler control statements allow the program to change aspects of the compiler's behavior. -Swift has three compiler control statements: -a conditional compilation block -a line control statement, -and a compile-time diagnostic statement. - -> Grammar of a compiler control statement: -> -> *compiler-control-statement* → *conditional-compilation-block* \ -> *compiler-control-statement* → *line-control-statement* \ -> *compiler-control-statement* → *diagnostic-statement* - -### Conditional Compilation Block - -A conditional compilation block allows code to be conditionally compiled -depending on the value of one or more compilation conditions. - -Every conditional compilation block begins with the `#if` compilation directive -and ends with the `#endif` compilation directive. -A simple conditional compilation block has the following form: - -```swift -#if <#compilation condition#> - <#statements#> -#endif -``` - -Unlike the condition of an `if` statement, -the *compilation condition* is evaluated at compile time. -As a result, -the *statements* are compiled and executed only if the *compilation condition* -evaluates to `true` at compile time. - -The *compilation condition* can include the `true` and `false` Boolean literals, -an identifier used with the `-D` command line flag, or any of the platform -conditions listed in the table below. - -| Platform condition | Valid arguments | -| ------------------ | --------------- | -| `os()` | `macOS`, `iOS`, `watchOS`, `tvOS`, `visionOS`, `Linux`, `Windows` | -| `arch()` | `i386`, `x86_64`, `arm`, `arm64` | -| `swift()` | `>=` or `<` followed by a version number | -| `compiler()` | `>=` or `<` followed by a version number | -| `canImport()` | A module name | -| `targetEnvironment()` | `simulator`, `macCatalyst` | - - - - - -The version number for the `swift()` and `compiler()` platform conditions -consists of a major number, optional minor number, optional patch number, and so on, -with a dot (`.`) separating each part of the version number. -There must not be whitespace between the comparison operator and the version number. -The version for `compiler()` is the compiler version, -regardless of the Swift version setting passed to the compiler. -The version for `swift()` is the language version currently being compiled. -For example, if you compile your code using the Swift 5 compiler in Swift 4.2 mode, -the compiler version is 5 and the language version is 4.2. -With those settings, -the following code prints all three messages: - -```swift -#if compiler(>=5) -print("Compiled with the Swift 5 compiler or later") -#endif -#if swift(>=4.2) -print("Compiled in Swift 4.2 mode or later") -#endif -#if compiler(>=5) && swift(<5) -print("Compiled with the Swift 5 compiler or later in a Swift mode earlier than 5") -#endif -// Prints "Compiled with the Swift 5 compiler or later" -// Prints "Compiled in Swift 4.2 mode or later" -// Prints "Compiled with the Swift 5 compiler or later in a Swift mode earlier than 5" -``` - - - - - -The argument for the `canImport()` platform condition -is the name of a module that may not be present on all platforms. -The module can include periods (`.`) in its name. -This condition tests whether it's possible to import the module, -but doesn't actually import it. -If the module is present, the platform condition returns `true`; -otherwise, it returns `false`. - - - - - - - -The `targetEnvironment()` platform condition -returns `true` when code is being compiled for the specified environment; -otherwise, it returns `false`. - -> Note: The `arch(arm)` platform condition doesn't return `true` for ARM 64 devices. -> The `arch(i386)` platform condition returns `true` -> when code is compiled for the 32–bit iOS simulator. - - - - - - - -You can combine and negate compilation conditions using the logical operators -`&&`, `||`, and `!` -and use parentheses for grouping. -These operators have the same associativity and precedence as the -logical operators that are used to combine ordinary Boolean expressions. - -Similar to an `if` statement, -you can add multiple conditional branches to test for different compilation conditions. -You can add any number of additional branches using `#elseif` clauses. -You can also add a final additional branch using an `#else` clause. -Conditional compilation blocks that contain multiple branches -have the following form: - -```swift -#if <#compilation condition 1#> - <#statements to compile if compilation condition 1 is true#> -#elseif <#compilation condition 2#> - <#statements to compile if compilation condition 2 is true#> -#else - <#statements to compile if both compilation conditions are false#> -#endif -``` - -> Note: Each statement in the body of a conditional compilation block is parsed -> even if it's not compiled. -> However, there's an exception -> if the compilation condition includes a `swift()` or `compiler()` platform condition: -> The statements are parsed -> only if the language or compiler version matches -> what is specified in the platform condition. -> This exception ensures that an older compiler doesn't attempt to parse -> syntax introduced in a newer version of Swift. - -For information about how you can wrap -explicit member expressions in conditional compilation blocks, -see . - -> Grammar of a conditional compilation block: -> -> *conditional-compilation-block* → *if-directive-clause* *elseif-directive-clauses*_?_ *else-directive-clause*_?_ *endif-directive* -> -> *if-directive-clause* → *if-directive* *compilation-condition* *statements*_?_ \ -> *elseif-directive-clauses* → *elseif-directive-clause* *elseif-directive-clauses*_?_ \ -> *elseif-directive-clause* → *elseif-directive* *compilation-condition* *statements*_?_ \ -> *else-directive-clause* → *else-directive* *statements*_?_ \ -> *if-directive* → **`#if`** \ -> *elseif-directive* → **`#elseif`** \ -> *else-directive* → **`#else`** \ -> *endif-directive* → **`#endif`** -> -> *compilation-condition* → *platform-condition* \ -> *compilation-condition* → *identifier* \ -> *compilation-condition* → *boolean-literal* \ -> *compilation-condition* → **`(`** *compilation-condition* **`)`** \ -> *compilation-condition* → **`!`** *compilation-condition* \ -> *compilation-condition* → *compilation-condition* **`&&`** *compilation-condition* \ -> *compilation-condition* → *compilation-condition* **`||`** *compilation-condition* -> -> *platform-condition* → **`os`** **`(`** *operating-system* **`)`** \ -> *platform-condition* → **`arch`** **`(`** *architecture* **`)`** \ -> *platform-condition* → **`swift`** **`(`** **`>=`** *swift-version* **`)`** | **`swift`** **`(`** **`<`** *swift-version* **`)`** \ -> *platform-condition* → **`compiler`** **`(`** **`>=`** *swift-version* **`)`** | **`compiler`** **`(`** **`<`** *swift-version* **`)`** \ -> *platform-condition* → **`canImport`** **`(`** *import-path* **`)`** \ -> *platform-condition* → **`targetEnvironment`** **`(`** *environment* **`)`** -> -> *operating-system* → **`macOS`** | **`iOS`** | **`watchOS`** | **`tvOS`** | **`visionOS`** | **`Linux`** | **`Windows`** \ -> *architecture* → **`i386`** | **`x86_64`** | **`arm`** | **`arm64`** \ -> *swift-version* → *decimal-digits* *swift-version-continuation*_?_ \ -> *swift-version-continuation* → **`.`** *decimal-digits* *swift-version-continuation*_?_ \ -> *environment* → **`simulator`** | **`macCatalyst`** - - - -### Line Control Statement - -A line control statement is used to specify a line number and filename -that can be different from the line number and filename of the source code being compiled. -Use a line control statement to change the source code location -used by Swift for diagnostic and debugging purposes. - -A line control statement has the following forms: - -```swift -#sourceLocation(file: <#file path#>, line: <#line number#>) -#sourceLocation() -``` - -The first form of a line control statement changes the values -of the `#line`, `#file`, `#fileID`, and `#filePath` -literal expressions, beginning with the line of code following the line control statement. -The *line number* changes the value of `#line`, -and is any integer literal greater than zero. -The *file path* changes the value of `#file`, `#fileID`, and `#filePath`, -and is a string literal. -The specified string becomes the value of `#filePath`, -and the last path component of the string is used by the value of `#fileID`. -For information about `#file`, `#fileID`, and `#filePath`, -see . - -The second form of a line control statement, `#sourceLocation()`, -resets the source code location back to the default line numbering and file path. - -> Grammar of a line control statement: -> -> *line-control-statement* → **`#sourceLocation`** **`(`** **`file:`** *file-path* **`,`** **`line:`** *line-number* **`)`** \ -> *line-control-statement* → **`#sourceLocation`** **`(`** **`)`** \ -> *line-number* → A decimal integer greater than zero \ -> *file-path* → *static-string-literal* - -### Compile-Time Diagnostic Statement - -Prior to Swift 5.9, -the `#warning` and `#error` statements emit a diagnostic during compilation. -This behavior is now provided by -the [`warning(_:)`][] and [`error(_:)`][] macros in the Swift standard library. - -[`warning(_:)`]: http://developer.apple.com/documentation/swift/documentation/swift/warning(_:) -[`error(_:)`]: http://developer.apple.com/documentation/swift/documentation/swift/error(_:) - -## Availability Condition - -An *availability condition* is used as a condition of an `if`, `while`, -and `guard` statement to query the availability of APIs at runtime, -based on specified platforms arguments. - -An availability condition has the following form: - -```swift -if #available(<#platform name#> <#version#>, <#...#>, *) { - <#statements to execute if the APIs are available#> -} else { - <#fallback statements to execute if the APIs are unavailable#> -} -``` - -You use an availability condition to execute a block of code, -depending on whether the APIs you want to use are available at runtime. -The compiler uses the information from the availability condition -when it verifies that the APIs in that block of code are available. - -The availability condition takes a comma-separated list of platform names and versions. -Use `iOS`, `macOS`, `watchOS`, `tvOS` and `visionOS` for the platform names, -and include the corresponding version numbers. -The `*` argument is required and specifies that, on any other platform, -the body of the code block guarded by the availability condition -executes on the minimum deployment target specified by your target. - -Unlike Boolean conditions, you can't combine availability conditions using -logical operators like `&&` and `||`. -Instead of using `!` to negate an availability condition, -use an unavailability condition, which has the following form: - -```swift -if #unavailable(<#platform name#> <#version#>, <#...#>) { - <#fallback statements to execute if the APIs are unavailable#> -} else { - <#statements to execute if the APIs are available#> -} -``` - -The `#unavailable` form is syntactic sugar that negates the condition. -In an unavailability condition, -the `*` argument is implicit and must not be included. -It has the same meaning as the `*` argument in an availability condition. - -> Grammar of an availability condition: -> -> *availability-condition* → **`#available`** **`(`** *availability-arguments* **`)`** \ -> *availability-condition* → **`#unavailable`** **`(`** *availability-arguments* **`)`** \ -> *availability-arguments* → *availability-argument* | *availability-argument* **`,`** *availability-arguments* \ -> *availability-argument* → *platform-name* *platform-version* \ -> *availability-argument* → **`*`** -> -> -> -> *platform-name* → **`iOS`** | **`iOSApplicationExtension`** \ -> *platform-name* → **`macOS`** | **`macOSApplicationExtension`** \ -> *platform-name* → **`macCatalyst`** | **`macCatalystApplicationExtension`** \ -> *platform-name* → **`watchOS`** | **`watchOSApplicationExtension`** \ -> *platform-name* → **`tvOS`** | **`tvOSApplicationExtension`** \ -> *platform-name* → **`visionOS`** | **`visionOSApplicationExtension`** \ -> *platform-version* → *decimal-digits* \ -> *platform-version* → *decimal-digits* **`.`** *decimal-digits* \ -> *platform-version* → *decimal-digits* **`.`** *decimal-digits* **`.`** *decimal-digits* - - - - - - - - - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/ReferenceManual/SummaryOfTheGrammar.md b/swift-6-beta.docc/ReferenceManual/SummaryOfTheGrammar.md deleted file mode 100644 index 3b72e5fd9..000000000 --- a/swift-6-beta.docc/ReferenceManual/SummaryOfTheGrammar.md +++ /dev/null @@ -1,1113 +0,0 @@ -# Summary of the Grammar - -Read the whole formal grammar. - - - -## Lexical Structure - -> Grammar of whitespace: -> -> *whitespace* → *whitespace-item* *whitespace*_?_ \ -> *whitespace-item* → *line-break* \ -> *whitespace-item* → *inline-space* \ -> *whitespace-item* → *comment* \ -> *whitespace-item* → *multiline-comment* \ -> *whitespace-item* → U+0000, U+000B, or U+000C -> -> *line-break* → U+000A \ -> *line-break* → U+000D \ -> *line-break* → U+000D followed by U+000A -> -> *inline-spaces* → *inline-space* *inline-spaces*_?_ \ -> *inline-space* → U+0009 or U+0020 -> -> *comment* → **`//`** *comment-text* *line-break* \ -> *multiline-comment* → **`/*`** *multiline-comment-text* **`*/`** -> -> *comment-text* → *comment-text-item* *comment-text*_?_ \ -> *comment-text-item* → Any Unicode scalar value except U+000A or U+000D -> -> *multiline-comment-text* → *multiline-comment-text-item* *multiline-comment-text*_?_ \ -> *multiline-comment-text-item* → *multiline-comment* \ -> *multiline-comment-text-item* → *comment-text-item* \ -> *multiline-comment-text-item* → Any Unicode scalar value except **`/*`** or **`*/`** - -> Grammar of an identifier: -> -> *identifier* → *identifier-head* *identifier-characters*_?_ \ -> *identifier* → **`` ` ``** *identifier-head* *identifier-characters*_?_ **`` ` ``** \ -> *identifier* → *implicit-parameter-name* \ -> *identifier* → *property-wrapper-projection* \ -> *identifier-list* → *identifier* | *identifier* **`,`** *identifier-list* -> -> *identifier-head* → Upper- or lowercase letter A through Z \ -> *identifier-head* → **`_`** \ -> *identifier-head* → U+00A8, U+00AA, U+00AD, U+00AF, U+00B2–U+00B5, or U+00B7–U+00BA \ -> *identifier-head* → U+00BC–U+00BE, U+00C0–U+00D6, U+00D8–U+00F6, or U+00F8–U+00FF \ -> *identifier-head* → U+0100–U+02FF, U+0370–U+167F, U+1681–U+180D, or U+180F–U+1DBF \ -> *identifier-head* → U+1E00–U+1FFF \ -> *identifier-head* → U+200B–U+200D, U+202A–U+202E, U+203F–U+2040, U+2054, or U+2060–U+206F \ -> *identifier-head* → U+2070–U+20CF, U+2100–U+218F, U+2460–U+24FF, or U+2776–U+2793 \ -> *identifier-head* → U+2C00–U+2DFF or U+2E80–U+2FFF \ -> *identifier-head* → U+3004–U+3007, U+3021–U+302F, U+3031–U+303F, or U+3040–U+D7FF \ -> *identifier-head* → U+F900–U+FD3D, U+FD40–U+FDCF, U+FDF0–U+FE1F, or U+FE30–U+FE44 \ -> *identifier-head* → U+FE47–U+FFFD \ -> *identifier-head* → U+10000–U+1FFFD, U+20000–U+2FFFD, U+30000–U+3FFFD, or U+40000–U+4FFFD \ -> *identifier-head* → U+50000–U+5FFFD, U+60000–U+6FFFD, U+70000–U+7FFFD, or U+80000–U+8FFFD \ -> *identifier-head* → U+90000–U+9FFFD, U+A0000–U+AFFFD, U+B0000–U+BFFFD, or U+C0000–U+CFFFD \ -> *identifier-head* → U+D0000–U+DFFFD or U+E0000–U+EFFFD -> -> *identifier-character* → Digit 0 through 9 \ -> *identifier-character* → U+0300–U+036F, U+1DC0–U+1DFF, U+20D0–U+20FF, or U+FE20–U+FE2F \ -> *identifier-character* → *identifier-head* \ -> *identifier-characters* → *identifier-character* *identifier-characters*_?_ -> -> *implicit-parameter-name* → **`$`** *decimal-digits* \ -> *property-wrapper-projection* → **`$`** *identifier-characters* - -> Grammar of a literal: -> -> *literal* → *numeric-literal* | *string-literal* | *regular-expression-literal* | *boolean-literal* | *nil-literal* -> -> *numeric-literal* → **`-`**_?_ *integer-literal* | **`-`**_?_ *floating-point-literal* \ -> *boolean-literal* → **`true`** | **`false`** \ -> *nil-literal* → **`nil`** - -> Grammar of an integer literal: -> -> *integer-literal* → *binary-literal* \ -> *integer-literal* → *octal-literal* \ -> *integer-literal* → *decimal-literal* \ -> *integer-literal* → *hexadecimal-literal* -> -> *binary-literal* → **`0b`** *binary-digit* *binary-literal-characters*_?_ \ -> *binary-digit* → Digit 0 or 1 \ -> *binary-literal-character* → *binary-digit* | **`_`** \ -> *binary-literal-characters* → *binary-literal-character* *binary-literal-characters*_?_ -> -> *octal-literal* → **`0o`** *octal-digit* *octal-literal-characters*_?_ \ -> *octal-digit* → Digit 0 through 7 \ -> *octal-literal-character* → *octal-digit* | **`_`** \ -> *octal-literal-characters* → *octal-literal-character* *octal-literal-characters*_?_ -> -> *decimal-literal* → *decimal-digit* *decimal-literal-characters*_?_ \ -> *decimal-digit* → Digit 0 through 9 \ -> *decimal-digits* → *decimal-digit* *decimal-digits*_?_ \ -> *decimal-literal-character* → *decimal-digit* | **`_`** \ -> *decimal-literal-characters* → *decimal-literal-character* *decimal-literal-characters*_?_ -> -> *hexadecimal-literal* → **`0x`** *hexadecimal-digit* *hexadecimal-literal-characters*_?_ \ -> *hexadecimal-digit* → Digit 0 through 9, a through f, or A through F \ -> *hexadecimal-literal-character* → *hexadecimal-digit* | **`_`** \ -> *hexadecimal-literal-characters* → *hexadecimal-literal-character* *hexadecimal-literal-characters*_?_ - -> Grammar of a floating-point literal: -> -> *floating-point-literal* → *decimal-literal* *decimal-fraction*_?_ *decimal-exponent*_?_ \ -> *floating-point-literal* → *hexadecimal-literal* *hexadecimal-fraction*_?_ *hexadecimal-exponent* -> -> *decimal-fraction* → **`.`** *decimal-literal* \ -> *decimal-exponent* → *floating-point-e* *sign*_?_ *decimal-literal* -> -> *hexadecimal-fraction* → **`.`** *hexadecimal-digit* *hexadecimal-literal-characters*_?_ \ -> *hexadecimal-exponent* → *floating-point-p* *sign*_?_ *decimal-literal* -> -> *floating-point-e* → **`e`** | **`E`** \ -> *floating-point-p* → **`p`** | **`P`** \ -> *sign* → **`+`** | **`-`** - -> Grammar of a string literal: -> -> *string-literal* → *static-string-literal* | *interpolated-string-literal* -> -> *string-literal-opening-delimiter* → *extended-string-literal-delimiter*_?_ **`"`** \ -> *string-literal-closing-delimiter* → **`"`** *extended-string-literal-delimiter*_?_ -> -> *static-string-literal* → *string-literal-opening-delimiter* *quoted-text*_?_ *string-literal-closing-delimiter* \ -> *static-string-literal* → *multiline-string-literal-opening-delimiter* *multiline-quoted-text*_?_ *multiline-string-literal-closing-delimiter* -> -> *multiline-string-literal-opening-delimiter* → *extended-string-literal-delimiter*_?_ **`"""`** \ -> *multiline-string-literal-closing-delimiter* → **`"""`** *extended-string-literal-delimiter*_?_ \ -> *extended-string-literal-delimiter* → **`#`** *extended-string-literal-delimiter*_?_ -> -> *quoted-text* → *quoted-text-item* *quoted-text*_?_ \ -> *quoted-text-item* → *escaped-character* \ -> *quoted-text-item* → Any Unicode scalar value except **`"`**, **`\`**, U+000A, or U+000D -> -> *multiline-quoted-text* → *multiline-quoted-text-item* *multiline-quoted-text*_?_ \ -> *multiline-quoted-text-item* → *escaped-character* \ -> *multiline-quoted-text-item* → Any Unicode scalar value except **`\`** \ -> *multiline-quoted-text-item* → *escaped-newline* -> -> *interpolated-string-literal* → *string-literal-opening-delimiter* *interpolated-text*_?_ *string-literal-closing-delimiter* \ -> *interpolated-string-literal* → *multiline-string-literal-opening-delimiter* *multiline-interpolated-text*_?_ *multiline-string-literal-closing-delimiter* -> -> *interpolated-text* → *interpolated-text-item* *interpolated-text*_?_ \ -> *interpolated-text-item* → **`\(`** *expression* **`)`** | *quoted-text-item* -> -> *multiline-interpolated-text* → *multiline-interpolated-text-item* *multiline-interpolated-text*_?_ \ -> *multiline-interpolated-text-item* → **`\(`** *expression* **`)`** | *multiline-quoted-text-item* -> -> *escape-sequence* → **`\`** *extended-string-literal-delimiter* \ -> *escaped-character* → *escape-sequence* **`0`** | *escape-sequence* **`\`** | *escape-sequence* **`t`** | *escape-sequence* **`n`** | *escape-sequence* **`r`** | *escape-sequence* **`"`** | *escape-sequence* **`'`** \ -> *escaped-character* → *escape-sequence* **`u`** **`{`** *unicode-scalar-digits* **`}`** \ -> *unicode-scalar-digits* → Between one and eight hexadecimal digits -> -> *escaped-newline* → *escape-sequence* *inline-spaces*_?_ *line-break* - -> Grammar of a regular expression literal: -> -> *regular-expression-literal* → *regular-expression-literal-opening-delimiter* *regular-expression* *regular-expression-literal-closing-delimiter* \ -> *regular-expression* → Any regular expression -> -> *regular-expression-literal-opening-delimiter* → *extended-regular-expression-literal-delimiter*_?_ **`/`** \ -> *regular-expression-literal-closing-delimiter* → **`/`** *extended-regular-expression-literal-delimiter*_?_ -> -> *extended-regular-expression-literal-delimiter* → **`#`** *extended-regular-expression-literal-delimiter*_?_ - -> Grammar of operators: -> -> *operator* → *operator-head* *operator-characters*_?_ \ -> *operator* → *dot-operator-head* *dot-operator-characters* -> -> *operator-head* → **`/`** | **`=`** | **`-`** | **`+`** | **`!`** | **`*`** | **`%`** | **`<`** | **`>`** | **`&`** | **`|`** | **`^`** | **`~`** | **`?`** \ -> *operator-head* → U+00A1–U+00A7 \ -> *operator-head* → U+00A9 or U+00AB \ -> *operator-head* → U+00AC or U+00AE \ -> *operator-head* → U+00B0–U+00B1 \ -> *operator-head* → U+00B6, U+00BB, U+00BF, U+00D7, or U+00F7 \ -> *operator-head* → U+2016–U+2017 \ -> *operator-head* → U+2020–U+2027 \ -> *operator-head* → U+2030–U+203E \ -> *operator-head* → U+2041–U+2053 \ -> *operator-head* → U+2055–U+205E \ -> *operator-head* → U+2190–U+23FF \ -> *operator-head* → U+2500–U+2775 \ -> *operator-head* → U+2794–U+2BFF \ -> *operator-head* → U+2E00–U+2E7F \ -> *operator-head* → U+3001–U+3003 \ -> *operator-head* → U+3008–U+3020 \ -> *operator-head* → U+3030 -> -> *operator-character* → *operator-head* \ -> *operator-character* → U+0300–U+036F \ -> *operator-character* → U+1DC0–U+1DFF \ -> *operator-character* → U+20D0–U+20FF \ -> *operator-character* → U+FE00–U+FE0F \ -> *operator-character* → U+FE20–U+FE2F \ -> *operator-character* → U+E0100–U+E01EF \ -> *operator-characters* → *operator-character* *operator-characters*_?_ -> -> *dot-operator-head* → **`.`** \ -> *dot-operator-character* → **`.`** | *operator-character* \ -> *dot-operator-characters* → *dot-operator-character* *dot-operator-characters*_?_ -> -> *infix-operator* → *operator* \ -> *prefix-operator* → *operator* \ -> *postfix-operator* → *operator* - -## Types - -> Grammar of a type: -> -> *type* → *function-type* \ -> *type* → *array-type* \ -> *type* → *dictionary-type* \ -> *type* → *type-identifier* \ -> *type* → *tuple-type* \ -> *type* → *optional-type* \ -> *type* → *implicitly-unwrapped-optional-type* \ -> *type* → *protocol-composition-type* \ -> *type* → *opaque-type* \ -> *type* → *metatype-type* \ -> *type* → *any-type* \ -> *type* → *self-type* \ -> *type* → **`(`** *type* **`)`** - -> Grammar of a type annotation: -> -> *type-annotation* → **`:`** *attributes*_?_ **`inout`**_?_ *type* - -> Grammar of a type identifier: -> -> *type-identifier* → *type-name* *generic-argument-clause*_?_ | *type-name* *generic-argument-clause*_?_ **`.`** *type-identifier* \ -> *type-name* → *identifier* - -> Grammar of a tuple type: -> -> *tuple-type* → **`(`** **`)`** | **`(`** *tuple-type-element* **`,`** *tuple-type-element-list* **`)`** \ -> *tuple-type-element-list* → *tuple-type-element* | *tuple-type-element* **`,`** *tuple-type-element-list* \ -> *tuple-type-element* → *element-name* *type-annotation* | *type* \ -> *element-name* → *identifier* - -> Grammar of a function type: -> -> *function-type* → *attributes*_?_ *function-type-argument-clause* **`async`**_?_ *throws-clause*_?_ **`->`** *type* -> -> *function-type-argument-clause* → **`(`** **`)`** \ -> *function-type-argument-clause* → **`(`** *function-type-argument-list* **`...`**_?_ **`)`** -> -> *function-type-argument-list* → *function-type-argument* | *function-type-argument* **`,`** *function-type-argument-list* \ -> *function-type-argument* → *attributes*_?_ **`inout`**_?_ *type* | *argument-label* *type-annotation* \ -> *argument-label* → *identifier* -> -> *throws-clause* → **`throws`** | **`throws`** **`(`** *type* **`)`** - -> Grammar of an array type: -> -> *array-type* → **`[`** *type* **`]`** - -> Grammar of a dictionary type: -> -> *dictionary-type* → **`[`** *type* **`:`** *type* **`]`** - -> Grammar of an optional type: -> -> *optional-type* → *type* **`?`** - -> Grammar of an implicitly unwrapped optional type: -> -> *implicitly-unwrapped-optional-type* → *type* **`!`** - -> Grammar of a protocol composition type: -> -> *protocol-composition-type* → *type-identifier* **`&`** *protocol-composition-continuation* \ -> *protocol-composition-continuation* → *type-identifier* | *protocol-composition-type* - -> Grammar of an opaque type: -> -> *opaque-type* → **`some`** *type* - -> Grammar of a boxed protocol type: -> -> *boxed-protocol-type* → **`any`** *type* - -> Grammar of a metatype type: -> -> *metatype-type* → *type* **`.`** **`Type`** | *type* **`.`** **`Protocol`** - -> Grammar of an Any type: -> -> *any-type* → **`Any`** - -> Grammar of a Self type: -> -> *self-type* → **`Self`** - -> Grammar of a type inheritance clause: -> -> *type-inheritance-clause* → **`:`** *type-inheritance-list* \ -> *type-inheritance-list* → *attributes*_?_ *type-identifier* | *attributes*_?_ *type-identifier* **`,`** *type-inheritance-list* - -## Expressions - -> Grammar of an expression: -> -> *expression* → *try-operator*_?_ *await-operator*_?_ *prefix-expression* *infix-expressions*_?_ \ - -> Grammar of a prefix expression: -> -> *prefix-expression* → *prefix-operator*_?_ *postfix-expression* \ -> *prefix-expression* → *in-out-expression* - -> Grammar of an in-out expression: -> -> *in-out-expression* → **`&`** *primary-expression* - -> Grammar of a try expression: -> -> *try-operator* → **`try`** | **`try`** **`?`** | **`try`** **`!`** - -> Grammar of an await expression: -> -> *await-operator* → **`await`** - -> Grammar of an infix expression: -> -> *infix-expression* → *infix-operator* *prefix-expression* \ -> *infix-expression* → *assignment-operator* *try-operator*_?_ *await-operator*_?_ *prefix-expression* \ -> *infix-expression* → *conditional-operator* *try-operator*_?_ *await-operator*_?_ *prefix-expression* \ -> *infix-expression* → *type-casting-operator* \ -> *infix-expressions* → *infix-expression* *infix-expressions*_?_ - -> Grammar of an assignment operator: -> -> *assignment-operator* → **`=`** - -> Grammar of a conditional operator: -> -> *conditional-operator* → **`?`** *expression* **`:`** - -> Grammar of a type-casting operator: -> -> *type-casting-operator* → **`is`** *type* \ -> *type-casting-operator* → **`as`** *type* \ -> *type-casting-operator* → **`as`** **`?`** *type* \ -> *type-casting-operator* → **`as`** **`!`** *type* - -> Grammar of a primary expression: -> -> *primary-expression* → *identifier* *generic-argument-clause*_?_ \ -> *primary-expression* → *literal-expression* \ -> *primary-expression* → *self-expression* \ -> *primary-expression* → *superclass-expression* \ -> *primary-expression* → *conditional-expression* \ -> *primary-expression* → *closure-expression* \ -> *primary-expression* → *parenthesized-expression* \ -> *primary-expression* → *tuple-expression* \ -> *primary-expression* → *implicit-member-expression* \ -> *primary-expression* → *wildcard-expression* \ -> *primary-expression* → *macro-expansion-expression* \ -> *primary-expression* → *key-path-expression* \ -> *primary-expression* → *selector-expression* \ -> *primary-expression* → *key-path-string-expression* - -> Grammar of a literal expression: -> -> *literal-expression* → *literal* \ -> *literal-expression* → *array-literal* | *dictionary-literal* | *playground-literal* -> -> *array-literal* → **`[`** *array-literal-items*_?_ **`]`** \ -> *array-literal-items* → *array-literal-item* **`,`**_?_ | *array-literal-item* **`,`** *array-literal-items* \ -> *array-literal-item* → *expression* -> -> *dictionary-literal* → **`[`** *dictionary-literal-items* **`]`** | **`[`** **`:`** **`]`** \ -> *dictionary-literal-items* → *dictionary-literal-item* **`,`**_?_ | *dictionary-literal-item* **`,`** *dictionary-literal-items* \ -> *dictionary-literal-item* → *expression* **`:`** *expression* -> -> *playground-literal* → **`#colorLiteral`** **`(`** **`red`** **`:`** *expression* **`,`** **`green`** **`:`** *expression* **`,`** **`blue`** **`:`** *expression* **`,`** **`alpha`** **`:`** *expression* **`)`** \ -> *playground-literal* → **`#fileLiteral`** **`(`** **`resourceName`** **`:`** *expression* **`)`** \ -> *playground-literal* → **`#imageLiteral`** **`(`** **`resourceName`** **`:`** *expression* **`)`** - -> Grammar of a self expression: -> -> *self-expression* → **`self`** | *self-method-expression* | *self-subscript-expression* | *self-initializer-expression* -> -> *self-method-expression* → **`self`** **`.`** *identifier* \ -> *self-subscript-expression* → **`self`** **`[`** *function-call-argument-list* **`]`** \ -> *self-initializer-expression* → **`self`** **`.`** **`init`** - -> Grammar of a superclass expression: -> -> *superclass-expression* → *superclass-method-expression* | *superclass-subscript-expression* | *superclass-initializer-expression* -> -> *superclass-method-expression* → **`super`** **`.`** *identifier* \ -> *superclass-subscript-expression* → **`super`** **`[`** *function-call-argument-list* **`]`** \ -> *superclass-initializer-expression* → **`super`** **`.`** **`init`** - -> Grammar of a conditional expression: -> -> *conditional-expression* → *if-expression* | *switch-expression* -> -> *if-expression* → **`if`** *condition-list* **`{`** *statement* **`}`** *if-expression-tail* \ -> *if-expression-tail* → **`else`** *if-expression* \ -> *if-expression-tail* → **`else`** **`{`** *statement* **`}`** -> -> *switch-expression* → **`switch`** *expression* **`{`** *switch-expression-cases* **`}`** \ -> *switch-expression-cases* → *switch-expression-case* *switch-expression-cases*_?_ \ -> *switch-expression-case* → *case-label* *statement* \ -> *switch-expression-case* → *default-label* *statement* - -> Grammar of a closure expression: -> -> *closure-expression* → **`{`** *attributes*_?_ *closure-signature*_?_ *statements*_?_ **`}`** -> -> *closure-signature* → *capture-list*_?_ *closure-parameter-clause* **`async`**_?_ *throws-clause*_?_ *function-result*_?_ **`in`** \ -> *closure-signature* → *capture-list* **`in`** -> -> *closure-parameter-clause* → **`(`** **`)`** | **`(`** *closure-parameter-list* **`)`** | *identifier-list* \ -> *closure-parameter-list* → *closure-parameter* | *closure-parameter* **`,`** *closure-parameter-list* \ -> *closure-parameter* → *closure-parameter-name* *type-annotation*_?_ \ -> *closure-parameter* → *closure-parameter-name* *type-annotation* **`...`** \ -> *closure-parameter-name* → *identifier* -> -> *capture-list* → **`[`** *capture-list-items* **`]`** \ -> *capture-list-items* → *capture-list-item* | *capture-list-item* **`,`** *capture-list-items* \ -> *capture-list-item* → *capture-specifier*_?_ *identifier* \ -> *capture-list-item* → *capture-specifier*_?_ *identifier* **`=`** *expression* \ -> *capture-list-item* → *capture-specifier*_?_ *self-expression* \ -> *capture-specifier* → **`weak`** | **`unowned`** | **`unowned(safe)`** | **`unowned(unsafe)`** - -> Grammar of an implicit member expression: -> -> *implicit-member-expression* → **`.`** *identifier* \ -> *implicit-member-expression* → **`.`** *identifier* **`.`** *postfix-expression* - -> Grammar of a parenthesized expression: -> -> *parenthesized-expression* → **`(`** *expression* **`)`** - -> Grammar of a tuple expression: -> -> *tuple-expression* → **`(`** **`)`** | **`(`** *tuple-element* **`,`** *tuple-element-list* **`)`** \ -> *tuple-element-list* → *tuple-element* | *tuple-element* **`,`** *tuple-element-list* \ -> *tuple-element* → *expression* | *identifier* **`:`** *expression* - -> Grammar of a wildcard expression: -> -> *wildcard-expression* → **`_`** - -> Grammar of a macro-expansion expression: -> -> *macro-expansion-expression* → **`#`** *identifier* *generic-argument-clause*_?_ *function-call-argument-clause*_?_ *trailing-closures*_?_ - -> Grammar of a key-path expression: -> -> *key-path-expression* → **`\`** *type*_?_ **`.`** *key-path-components* \ -> *key-path-components* → *key-path-component* | *key-path-component* **`.`** *key-path-components* \ -> *key-path-component* → *identifier* *key-path-postfixes*_?_ | *key-path-postfixes* -> -> *key-path-postfixes* → *key-path-postfix* *key-path-postfixes*_?_ \ -> *key-path-postfix* → **`?`** | **`!`** | **`self`** | **`[`** *function-call-argument-list* **`]`** - -> Grammar of a selector expression: -> -> *selector-expression* → **`#selector`** **`(`** *expression* **`)`** \ -> *selector-expression* → **`#selector`** **`(`** **`getter:`** *expression* **`)`** \ -> *selector-expression* → **`#selector`** **`(`** **`setter:`** *expression* **`)`** - -> Grammar of a key-path string expression: -> -> *key-path-string-expression* → **`#keyPath`** **`(`** *expression* **`)`** - -> Grammar of a postfix expression: -> -> *postfix-expression* → *primary-expression* \ -> *postfix-expression* → *postfix-expression* *postfix-operator* \ -> *postfix-expression* → *function-call-expression* \ -> *postfix-expression* → *initializer-expression* \ -> *postfix-expression* → *explicit-member-expression* \ -> *postfix-expression* → *postfix-self-expression* \ -> *postfix-expression* → *subscript-expression* \ -> *postfix-expression* → *forced-value-expression* \ -> *postfix-expression* → *optional-chaining-expression* - -> Grammar of a function call expression: -> -> *function-call-expression* → *postfix-expression* *function-call-argument-clause* \ -> *function-call-expression* → *postfix-expression* *function-call-argument-clause*_?_ *trailing-closures* -> -> *function-call-argument-clause* → **`(`** **`)`** | **`(`** *function-call-argument-list* **`)`** \ -> *function-call-argument-list* → *function-call-argument* | *function-call-argument* **`,`** *function-call-argument-list* \ -> *function-call-argument* → *expression* | *identifier* **`:`** *expression* \ -> *function-call-argument* → *operator* | *identifier* **`:`** *operator* -> -> *trailing-closures* → *closure-expression* *labeled-trailing-closures*_?_ \ -> *labeled-trailing-closures* → *labeled-trailing-closure* *labeled-trailing-closures*_?_ \ -> *labeled-trailing-closure* → *identifier* **`:`** *closure-expression* - -> Grammar of an initializer expression: -> -> *initializer-expression* → *postfix-expression* **`.`** **`init`** \ -> *initializer-expression* → *postfix-expression* **`.`** **`init`** **`(`** *argument-names* **`)`** - -> Grammar of an explicit member expression: -> -> *explicit-member-expression* → *postfix-expression* **`.`** *decimal-digits* \ -> *explicit-member-expression* → *postfix-expression* **`.`** *identifier* *generic-argument-clause*_?_ \ -> *explicit-member-expression* → *postfix-expression* **`.`** *identifier* **`(`** *argument-names* **`)`** \ -> *explicit-member-expression* → *postfix-expression* *conditional-compilation-block* -> -> *argument-names* → *argument-name* *argument-names*_?_ \ -> *argument-name* → *identifier* **`:`** - -> Grammar of a postfix self expression: -> -> *postfix-self-expression* → *postfix-expression* **`.`** **`self`** - -> Grammar of a subscript expression: -> -> *subscript-expression* → *postfix-expression* **`[`** *function-call-argument-list* **`]`** - -> Grammar of a forced-value expression: -> -> *forced-value-expression* → *postfix-expression* **`!`** - -> Grammar of an optional-chaining expression: -> -> *optional-chaining-expression* → *postfix-expression* **`?`** - -## Statements - -> Grammar of a statement: -> -> *statement* → *expression* **`;`**_?_ \ -> *statement* → *declaration* **`;`**_?_ \ -> *statement* → *loop-statement* **`;`**_?_ \ -> *statement* → *branch-statement* **`;`**_?_ \ -> *statement* → *labeled-statement* **`;`**_?_ \ -> *statement* → *control-transfer-statement* **`;`**_?_ \ -> *statement* → *defer-statement* **`;`**_?_ \ -> *statement* → *do-statement* **`;`**_?_ \ -> *statement* → *compiler-control-statement* \ -> *statements* → *statement* *statements*_?_ - -> Grammar of a loop statement: -> -> *loop-statement* → *for-in-statement* \ -> *loop-statement* → *while-statement* \ -> *loop-statement* → *repeat-while-statement* - -> Grammar of a for-in statement: -> -> *for-in-statement* → **`for`** **`case`**_?_ *pattern* **`in`** *expression* *where-clause*_?_ *code-block* - -> Grammar of a while statement: -> -> *while-statement* → **`while`** *condition-list* *code-block* -> -> *condition-list* → *condition* | *condition* **`,`** *condition-list* \ -> *condition* → *expression* | *availability-condition* | *case-condition* | *optional-binding-condition* -> -> *case-condition* → **`case`** *pattern* *initializer* \ -> *optional-binding-condition* → **`let`** *pattern* *initializer*_?_ | **`var`** *pattern* *initializer*_?_ - -> Grammar of a repeat-while statement: -> -> *repeat-while-statement* → **`repeat`** *code-block* **`while`** *expression* - -> Grammar of a branch statement: -> -> *branch-statement* → *if-statement* \ -> *branch-statement* → *guard-statement* \ -> *branch-statement* → *switch-statement* - -> Grammar of an if statement: -> -> *if-statement* → **`if`** *condition-list* *code-block* *else-clause*_?_ \ -> *else-clause* → **`else`** *code-block* | **`else`** *if-statement* - -> Grammar of a guard statement: -> -> *guard-statement* → **`guard`** *condition-list* **`else`** *code-block* - -> Grammar of a switch statement: -> -> *switch-statement* → **`switch`** *expression* **`{`** *switch-cases*_?_ **`}`** \ -> *switch-cases* → *switch-case* *switch-cases*_?_ \ -> *switch-case* → *case-label* *statements* \ -> *switch-case* → *default-label* *statements* \ -> *switch-case* → *conditional-switch-case* -> -> *case-label* → *attributes*_?_ **`case`** *case-item-list* **`:`** \ -> *case-item-list* → *pattern* *where-clause*_?_ | *pattern* *where-clause*_?_ **`,`** *case-item-list* \ -> *default-label* → *attributes*_?_ **`default`** **`:`** -> -> *where-clause* → **`where`** *where-expression* \ -> *where-expression* → *expression* -> -> *conditional-switch-case* → *switch-if-directive-clause* *switch-elseif-directive-clauses*_?_ *switch-else-directive-clause*_?_ *endif-directive* \ -> *switch-if-directive-clause* → *if-directive* *compilation-condition* *switch-cases*_?_ \ -> *switch-elseif-directive-clauses* → *elseif-directive-clause* *switch-elseif-directive-clauses*_?_ \ -> *switch-elseif-directive-clause* → *elseif-directive* *compilation-condition* *switch-cases*_?_ \ -> *switch-else-directive-clause* → *else-directive* *switch-cases*_?_ - -> Grammar of a labeled statement: -> -> *labeled-statement* → *statement-label* *loop-statement* \ -> *labeled-statement* → *statement-label* *if-statement* \ -> *labeled-statement* → *statement-label* *switch-statement* \ -> *labeled-statement* → *statement-label* *do-statement* -> -> *statement-label* → *label-name* **`:`** \ -> *label-name* → *identifier* - -> Grammar of a control transfer statement: -> -> *control-transfer-statement* → *break-statement* \ -> *control-transfer-statement* → *continue-statement* \ -> *control-transfer-statement* → *fallthrough-statement* \ -> *control-transfer-statement* → *return-statement* \ -> *control-transfer-statement* → *throw-statement* - -> Grammar of a break statement: -> -> *break-statement* → **`break`** *label-name*_?_ - -> Grammar of a continue statement: -> -> *continue-statement* → **`continue`** *label-name*_?_ - -> Grammar of a fallthrough statement: -> -> *fallthrough-statement* → **`fallthrough`** - -> Grammar of a return statement: -> -> *return-statement* → **`return`** *expression*_?_ - -> Grammar of a throw statement: -> -> *throw-statement* → **`throw`** *expression* - -> Grammar of a defer statement: -> -> *defer-statement* → **`defer`** *code-block* - -> Grammar of a do statement: -> -> *do-statement* → **`do`** *throws-clause*_?_ *code-block* *catch-clauses*_?_ \ -> *catch-clauses* → *catch-clause* *catch-clauses*_?_ \ -> *catch-clause* → **`catch`** *catch-pattern-list*_?_ *code-block* \ -> *catch-pattern-list* → *catch-pattern* | *catch-pattern* **`,`** *catch-pattern-list* \ -> *catch-pattern* → *pattern* *where-clause*_?_ - -> Grammar of a compiler control statement: -> -> *compiler-control-statement* → *conditional-compilation-block* \ -> *compiler-control-statement* → *line-control-statement* \ -> *compiler-control-statement* → *diagnostic-statement* - -> Grammar of a conditional compilation block: -> -> *conditional-compilation-block* → *if-directive-clause* *elseif-directive-clauses*_?_ *else-directive-clause*_?_ *endif-directive* -> -> *if-directive-clause* → *if-directive* *compilation-condition* *statements*_?_ \ -> *elseif-directive-clauses* → *elseif-directive-clause* *elseif-directive-clauses*_?_ \ -> *elseif-directive-clause* → *elseif-directive* *compilation-condition* *statements*_?_ \ -> *else-directive-clause* → *else-directive* *statements*_?_ \ -> *if-directive* → **`#if`** \ -> *elseif-directive* → **`#elseif`** \ -> *else-directive* → **`#else`** \ -> *endif-directive* → **`#endif`** -> -> *compilation-condition* → *platform-condition* \ -> *compilation-condition* → *identifier* \ -> *compilation-condition* → *boolean-literal* \ -> *compilation-condition* → **`(`** *compilation-condition* **`)`** \ -> *compilation-condition* → **`!`** *compilation-condition* \ -> *compilation-condition* → *compilation-condition* **`&&`** *compilation-condition* \ -> *compilation-condition* → *compilation-condition* **`||`** *compilation-condition* -> -> *platform-condition* → **`os`** **`(`** *operating-system* **`)`** \ -> *platform-condition* → **`arch`** **`(`** *architecture* **`)`** \ -> *platform-condition* → **`swift`** **`(`** **`>=`** *swift-version* **`)`** | **`swift`** **`(`** **`<`** *swift-version* **`)`** \ -> *platform-condition* → **`compiler`** **`(`** **`>=`** *swift-version* **`)`** | **`compiler`** **`(`** **`<`** *swift-version* **`)`** \ -> *platform-condition* → **`canImport`** **`(`** *import-path* **`)`** \ -> *platform-condition* → **`targetEnvironment`** **`(`** *environment* **`)`** -> -> *operating-system* → **`macOS`** | **`iOS`** | **`watchOS`** | **`tvOS`** | **`visionOS`** | **`Linux`** | **`Windows`** \ -> *architecture* → **`i386`** | **`x86_64`** | **`arm`** | **`arm64`** \ -> *swift-version* → *decimal-digits* *swift-version-continuation*_?_ \ -> *swift-version-continuation* → **`.`** *decimal-digits* *swift-version-continuation*_?_ \ -> *environment* → **`simulator`** | **`macCatalyst`** - -> Grammar of a line control statement: -> -> *line-control-statement* → **`#sourceLocation`** **`(`** **`file:`** *file-path* **`,`** **`line:`** *line-number* **`)`** \ -> *line-control-statement* → **`#sourceLocation`** **`(`** **`)`** \ -> *line-number* → A decimal integer greater than zero \ -> *file-path* → *static-string-literal* - -> Grammar of an availability condition: -> -> *availability-condition* → **`#available`** **`(`** *availability-arguments* **`)`** \ -> *availability-condition* → **`#unavailable`** **`(`** *availability-arguments* **`)`** \ -> *availability-arguments* → *availability-argument* | *availability-argument* **`,`** *availability-arguments* \ -> *availability-argument* → *platform-name* *platform-version* \ -> *availability-argument* → **`*`** -> -> *platform-name* → **`iOS`** | **`iOSApplicationExtension`** \ -> *platform-name* → **`macOS`** | **`macOSApplicationExtension`** \ -> *platform-name* → **`macCatalyst`** | **`macCatalystApplicationExtension`** \ -> *platform-name* → **`watchOS`** | **`watchOSApplicationExtension`** \ -> *platform-name* → **`tvOS`** | **`tvOSApplicationExtension`** \ -> *platform-name* → **`visionOS`** | **`visionOSApplicationExtension`** \ -> *platform-version* → *decimal-digits* \ -> *platform-version* → *decimal-digits* **`.`** *decimal-digits* \ -> *platform-version* → *decimal-digits* **`.`** *decimal-digits* **`.`** *decimal-digits* - -## Declarations - -> Grammar of a declaration: -> -> *declaration* → *import-declaration* \ -> *declaration* → *constant-declaration* \ -> *declaration* → *variable-declaration* \ -> *declaration* → *typealias-declaration* \ -> *declaration* → *function-declaration* \ -> *declaration* → *enum-declaration* \ -> *declaration* → *struct-declaration* \ -> *declaration* → *class-declaration* \ -> *declaration* → *actor-declaration* \ -> *declaration* → *protocol-declaration* \ -> *declaration* → *initializer-declaration* \ -> *declaration* → *deinitializer-declaration* \ -> *declaration* → *extension-declaration* \ -> *declaration* → *subscript-declaration* \ -> *declaration* → *operator-declaration* \ -> *declaration* → *precedence-group-declaration* \ - -> Grammar of a top-level declaration: -> -> *top-level-declaration* → *statements*_?_ - -> Grammar of a code block: -> -> *code-block* → **`{`** *statements*_?_ **`}`** - -> Grammar of an import declaration: -> -> *import-declaration* → *attributes*_?_ **`import`** *import-kind*_?_ *import-path* -> -> *import-kind* → **`typealias`** | **`struct`** | **`class`** | **`enum`** | **`protocol`** | **`let`** | **`var`** | **`func`** \ -> *import-path* → *identifier* | *identifier* **`.`** *import-path* - -> Grammar of a constant declaration: -> -> *constant-declaration* → *attributes*_?_ *declaration-modifiers*_?_ **`let`** *pattern-initializer-list* -> -> *pattern-initializer-list* → *pattern-initializer* | *pattern-initializer* **`,`** *pattern-initializer-list* \ -> *pattern-initializer* → *pattern* *initializer*_?_ \ -> *initializer* → **`=`** *expression* - -> Grammar of a variable declaration: -> -> *variable-declaration* → *variable-declaration-head* *pattern-initializer-list* \ -> *variable-declaration* → *variable-declaration-head* *variable-name* *type-annotation* *code-block* \ -> *variable-declaration* → *variable-declaration-head* *variable-name* *type-annotation* *getter-setter-block* \ -> *variable-declaration* → *variable-declaration-head* *variable-name* *type-annotation* *getter-setter-keyword-block* \ -> *variable-declaration* → *variable-declaration-head* *variable-name* *initializer* *willSet-didSet-block* \ -> *variable-declaration* → *variable-declaration-head* *variable-name* *type-annotation* *initializer*_?_ *willSet-didSet-block* -> -> *variable-declaration-head* → *attributes*_?_ *declaration-modifiers*_?_ **`var`** \ -> *variable-name* → *identifier* -> -> *getter-setter-block* → *code-block* \ -> *getter-setter-block* → **`{`** *getter-clause* *setter-clause*_?_ **`}`** \ -> *getter-setter-block* → **`{`** *setter-clause* *getter-clause* **`}`** \ -> *getter-clause* → *attributes*_?_ *mutation-modifier*_?_ **`get`** *code-block* \ -> *setter-clause* → *attributes*_?_ *mutation-modifier*_?_ **`set`** *setter-name*_?_ *code-block* \ -> *setter-name* → **`(`** *identifier* **`)`** -> -> *getter-setter-keyword-block* → **`{`** *getter-keyword-clause* *setter-keyword-clause*_?_ **`}`** \ -> *getter-setter-keyword-block* → **`{`** *setter-keyword-clause* *getter-keyword-clause* **`}`** \ -> *getter-keyword-clause* → *attributes*_?_ *mutation-modifier*_?_ **`get`** \ -> *setter-keyword-clause* → *attributes*_?_ *mutation-modifier*_?_ **`set`** -> -> *willSet-didSet-block* → **`{`** *willSet-clause* *didSet-clause*_?_ **`}`** \ -> *willSet-didSet-block* → **`{`** *didSet-clause* *willSet-clause*_?_ **`}`** \ -> *willSet-clause* → *attributes*_?_ **`willSet`** *setter-name*_?_ *code-block* \ -> *didSet-clause* → *attributes*_?_ **`didSet`** *setter-name*_?_ *code-block* - -> Grammar of a type alias declaration: -> -> *typealias-declaration* → *attributes*_?_ *access-level-modifier*_?_ **`typealias`** *typealias-name* *generic-parameter-clause*_?_ *typealias-assignment* \ -> *typealias-name* → *identifier* \ -> *typealias-assignment* → **`=`** *type* - -> Grammar of a function declaration: -> -> *function-declaration* → *function-head* *function-name* *generic-parameter-clause*_?_ *function-signature* *generic-where-clause*_?_ *function-body*_?_ -> -> *function-head* → *attributes*_?_ *declaration-modifiers*_?_ **`func`** \ -> *function-name* → *identifier* | *operator* -> -> *function-signature* → *parameter-clause* **`async`**_?_ *throws-clause*_?_ *function-result*_?_ \ -> *function-signature* → *parameter-clause* **`async`**_?_ **`rethrows`** *function-result*_?_ \ -> *function-result* → **`->`** *attributes*_?_ *type* \ -> *function-body* → *code-block* -> -> *parameter-clause* → **`(`** **`)`** | **`(`** *parameter-list* **`)`** \ -> *parameter-list* → *parameter* | *parameter* **`,`** *parameter-list* \ -> *parameter* → *external-parameter-name*_?_ *local-parameter-name* *parameter-type-annotation* *default-argument-clause*_?_ \ -> *parameter* → *external-parameter-name*_?_ *local-parameter-name* *parameter-type-annotation* \ -> *parameter* → *external-parameter-name*_?_ *local-parameter-name* *parameter-type-annotation* **`...`** -> -> *external-parameter-name* → *identifier* \ -> *local-parameter-name* → *identifier* \ -> *parameter-type-annotation* → **`:`** *attributes*_?_ *parameter-modifier*_?_ *type* \ -> *parameter-modifier* → **`inout`** | **`borrowing`** | **`consuming`** -> *default-argument-clause* → **`=`** *expression* - -> Grammar of an enumeration declaration: -> -> *enum-declaration* → *attributes*_?_ *access-level-modifier*_?_ *union-style-enum* \ -> *enum-declaration* → *attributes*_?_ *access-level-modifier*_?_ *raw-value-style-enum* -> -> *union-style-enum* → **`indirect`**_?_ **`enum`** *enum-name* *generic-parameter-clause*_?_ *type-inheritance-clause*_?_ *generic-where-clause*_?_ **`{`** *union-style-enum-members*_?_ **`}`** \ -> *union-style-enum-members* → *union-style-enum-member* *union-style-enum-members*_?_ \ -> *union-style-enum-member* → *declaration* | *union-style-enum-case-clause* | *compiler-control-statement* \ -> *union-style-enum-case-clause* → *attributes*_?_ **`indirect`**_?_ **`case`** *union-style-enum-case-list* \ -> *union-style-enum-case-list* → *union-style-enum-case* | *union-style-enum-case* **`,`** *union-style-enum-case-list* \ -> *union-style-enum-case* → *enum-case-name* *tuple-type*_?_ \ -> *enum-name* → *identifier* \ -> *enum-case-name* → *identifier* -> -> *raw-value-style-enum* → **`enum`** *enum-name* *generic-parameter-clause*_?_ *type-inheritance-clause* *generic-where-clause*_?_ **`{`** *raw-value-style-enum-members* **`}`** \ -> *raw-value-style-enum-members* → *raw-value-style-enum-member* *raw-value-style-enum-members*_?_ \ -> *raw-value-style-enum-member* → *declaration* | *raw-value-style-enum-case-clause* | *compiler-control-statement* \ -> *raw-value-style-enum-case-clause* → *attributes*_?_ **`case`** *raw-value-style-enum-case-list* \ -> *raw-value-style-enum-case-list* → *raw-value-style-enum-case* | *raw-value-style-enum-case* **`,`** *raw-value-style-enum-case-list* \ -> *raw-value-style-enum-case* → *enum-case-name* *raw-value-assignment*_?_ \ -> *raw-value-assignment* → **`=`** *raw-value-literal* \ -> *raw-value-literal* → *numeric-literal* | *static-string-literal* | *boolean-literal* - -> Grammar of a structure declaration: -> -> *struct-declaration* → *attributes*_?_ *access-level-modifier*_?_ **`struct`** *struct-name* *generic-parameter-clause*_?_ *type-inheritance-clause*_?_ *generic-where-clause*_?_ *struct-body* \ -> *struct-name* → *identifier* \ -> *struct-body* → **`{`** *struct-members*_?_ **`}`** -> -> *struct-members* → *struct-member* *struct-members*_?_ \ -> *struct-member* → *declaration* | *compiler-control-statement* - -> Grammar of a class declaration: -> -> *class-declaration* → *attributes*_?_ *access-level-modifier*_?_ **`final`**_?_ **`class`** *class-name* *generic-parameter-clause*_?_ *type-inheritance-clause*_?_ *generic-where-clause*_?_ *class-body* \ -> *class-declaration* → *attributes*_?_ **`final`** *access-level-modifier*_?_ **`class`** *class-name* *generic-parameter-clause*_?_ *type-inheritance-clause*_?_ *generic-where-clause*_?_ *class-body* \ -> *class-name* → *identifier* \ -> *class-body* → **`{`** *class-members*_?_ **`}`** -> -> *class-members* → *class-member* *class-members*_?_ \ -> *class-member* → *declaration* | *compiler-control-statement* - -> Grammar of an actor declaration: -> -> *actor-declaration* → *attributes*_?_ *access-level-modifier*_?_ **`actor`** *actor-name* *generic-parameter-clause*_?_ *type-inheritance-clause*_?_ *generic-where-clause*_?_ *actor-body* \ -> *actor-name* → *identifier* \ -> *actor-body* → **`{`** *actor-members*_?_ **`}`** -> -> *actor-members* → *actor-member* *actor-members*_?_ \ -> *actor-member* → *declaration* | *compiler-control-statement* - -> Grammar of a protocol declaration: -> -> *protocol-declaration* → *attributes*_?_ *access-level-modifier*_?_ **`protocol`** *protocol-name* *type-inheritance-clause*_?_ *generic-where-clause*_?_ *protocol-body* \ -> *protocol-name* → *identifier* \ -> *protocol-body* → **`{`** *protocol-members*_?_ **`}`** -> -> *protocol-members* → *protocol-member* *protocol-members*_?_ \ -> *protocol-member* → *protocol-member-declaration* | *compiler-control-statement* -> -> *protocol-member-declaration* → *protocol-property-declaration* \ -> *protocol-member-declaration* → *protocol-method-declaration* \ -> *protocol-member-declaration* → *protocol-initializer-declaration* \ -> *protocol-member-declaration* → *protocol-subscript-declaration* \ -> *protocol-member-declaration* → *protocol-associated-type-declaration* \ -> *protocol-member-declaration* → *typealias-declaration* - -> Grammar of a protocol property declaration: -> -> *protocol-property-declaration* → *variable-declaration-head* *variable-name* *type-annotation* *getter-setter-keyword-block* - -> Grammar of a protocol method declaration: -> -> *protocol-method-declaration* → *function-head* *function-name* *generic-parameter-clause*_?_ *function-signature* *generic-where-clause*_?_ - -> Grammar of a protocol initializer declaration: -> -> *protocol-initializer-declaration* → *initializer-head* *generic-parameter-clause*_?_ *parameter-clause* *throws-clause*_?_ *generic-where-clause*_?_ \ -> *protocol-initializer-declaration* → *initializer-head* *generic-parameter-clause*_?_ *parameter-clause* **`rethrows`** *generic-where-clause*_?_ - -> Grammar of a protocol subscript declaration: -> -> *protocol-subscript-declaration* → *subscript-head* *subscript-result* *generic-where-clause*_?_ *getter-setter-keyword-block* - -> Grammar of a protocol associated type declaration: -> -> *protocol-associated-type-declaration* → *attributes*_?_ *access-level-modifier*_?_ **`associatedtype`** *typealias-name* *type-inheritance-clause*_?_ *typealias-assignment*_?_ *generic-where-clause*_?_ - -> Grammar of an initializer declaration: -> -> *initializer-declaration* → *initializer-head* *generic-parameter-clause*_?_ *parameter-clause* **`async`**_?_ *throws-clause*_?_ *generic-where-clause*_?_ *initializer-body* \ -> *initializer-declaration* → *initializer-head* *generic-parameter-clause*_?_ *parameter-clause* **`async`**_?_ **`rethrows`** *generic-where-clause*_?_ *initializer-body* \ -> *initializer-head* → *attributes*_?_ *declaration-modifiers*_?_ **`init`** \ -> *initializer-head* → *attributes*_?_ *declaration-modifiers*_?_ **`init`** **`?`** \ -> *initializer-head* → *attributes*_?_ *declaration-modifiers*_?_ **`init`** **`!`** \ -> *initializer-body* → *code-block* - -> Grammar of a deinitializer declaration: -> -> *deinitializer-declaration* → *attributes*_?_ **`deinit`** *code-block* - -> Grammar of an extension declaration: -> -> *extension-declaration* → *attributes*_?_ *access-level-modifier*_?_ **`extension`** *type-identifier* *type-inheritance-clause*_?_ *generic-where-clause*_?_ *extension-body* \ -> *extension-body* → **`{`** *extension-members*_?_ **`}`** -> -> *extension-members* → *extension-member* *extension-members*_?_ \ -> *extension-member* → *declaration* | *compiler-control-statement* - -> Grammar of a subscript declaration: -> -> *subscript-declaration* → *subscript-head* *subscript-result* *generic-where-clause*_?_ *code-block* \ -> *subscript-declaration* → *subscript-head* *subscript-result* *generic-where-clause*_?_ *getter-setter-block* \ -> *subscript-declaration* → *subscript-head* *subscript-result* *generic-where-clause*_?_ *getter-setter-keyword-block* \ -> *subscript-head* → *attributes*_?_ *declaration-modifiers*_?_ **`subscript`** *generic-parameter-clause*_?_ *parameter-clause* \ -> *subscript-result* → **`->`** *attributes*_?_ *type* - -> Grammar of a macro declaration: -> -> *macro-declaration* → *macro-head* *identifier* *generic-parameter-clause*_?_ *macro-signature* *macro-definition*_?_ *generic-where-clause* \ -> *macro-head* → *attributes*_?_ *declaration-modifiers*_?_ **`macro`** \ -> *macro-signature* → *parameter-clause* *macro-function-signature-result*_?_ \ -> *macro-function-signature-result* → **`->`** *type* \ -> *macro-definition* → **`=`** *expression* - -> Grammar of an operator declaration: -> -> *operator-declaration* → *prefix-operator-declaration* | *postfix-operator-declaration* | *infix-operator-declaration* -> -> *prefix-operator-declaration* → **`prefix`** **`operator`** *operator* \ -> *postfix-operator-declaration* → **`postfix`** **`operator`** *operator* \ -> *infix-operator-declaration* → **`infix`** **`operator`** *operator* *infix-operator-group*_?_ -> -> *infix-operator-group* → **`:`** *precedence-group-name* - -> Grammar of a precedence group declaration: -> -> *precedence-group-declaration* → **`precedencegroup`** *precedence-group-name* **`{`** *precedence-group-attributes*_?_ **`}`** -> -> *precedence-group-attributes* → *precedence-group-attribute* *precedence-group-attributes*_?_ \ -> *precedence-group-attribute* → *precedence-group-relation* \ -> *precedence-group-attribute* → *precedence-group-assignment* \ -> *precedence-group-attribute* → *precedence-group-associativity* -> -> *precedence-group-relation* → **`higherThan`** **`:`** *precedence-group-names* \ -> *precedence-group-relation* → **`lowerThan`** **`:`** *precedence-group-names* -> -> *precedence-group-assignment* → **`assignment`** **`:`** *boolean-literal* -> -> *precedence-group-associativity* → **`associativity`** **`:`** **`left`** \ -> *precedence-group-associativity* → **`associativity`** **`:`** **`right`** \ -> *precedence-group-associativity* → **`associativity`** **`:`** **`none`** -> -> *precedence-group-names* → *precedence-group-name* | *precedence-group-name* **`,`** *precedence-group-names* \ -> *precedence-group-name* → *identifier* - -> Grammar of a declaration modifier: -> -> *declaration-modifier* → **`class`** | **`convenience`** | **`dynamic`** | **`final`** | **`infix`** | **`lazy`** | **`optional`** | **`override`** | **`postfix`** | **`prefix`** | **`required`** | **`static`** | **`unowned`** | **`unowned`** **`(`** **`safe`** **`)`** | **`unowned`** **`(`** **`unsafe`** **`)`** | **`weak`** \ -> *declaration-modifier* → *access-level-modifier* \ -> *declaration-modifier* → *mutation-modifier* \ -> *declaration-modifier* → *actor-isolation-modifier* \ -> *declaration-modifiers* → *declaration-modifier* *declaration-modifiers*_?_ -> -> *access-level-modifier* → **`private`** | **`private`** **`(`** **`set`** **`)`** \ -> *access-level-modifier* → **`fileprivate`** | **`fileprivate`** **`(`** **`set`** **`)`** \ -> *access-level-modifier* → **`internal`** | **`internal`** **`(`** **`set`** **`)`** \ -> *access-level-modifier* → **`package`** | **`package`** **`(`** **`set`** **`)`** \ -> *access-level-modifier* → **`public`** | **`public`** **`(`** **`set`** **`)`** \ -> *access-level-modifier* → **`open`** | **`open`** **`(`** **`set`** **`)`** -> -> *mutation-modifier* → **`mutating`** | **`nonmutating`** -> -> *actor-isolation-modifier* → **`nonisolated`** - -## Attributes - -> Grammar of an attribute: -> -> *attribute* → **`@`** *attribute-name* *attribute-argument-clause*_?_ \ -> *attribute-name* → *identifier* \ -> *attribute-argument-clause* → **`(`** *balanced-tokens*_?_ **`)`** \ -> *attributes* → *attribute* *attributes*_?_ -> -> *balanced-tokens* → *balanced-token* *balanced-tokens*_?_ \ -> *balanced-token* → **`(`** *balanced-tokens*_?_ **`)`** \ -> *balanced-token* → **`[`** *balanced-tokens*_?_ **`]`** \ -> *balanced-token* → **`{`** *balanced-tokens*_?_ **`}`** \ -> *balanced-token* → Any identifier, keyword, literal, or operator \ -> *balanced-token* → Any punctuation except **`(`**, **`)`**, **`[`**, **`]`**, **`{`**, or **`}`** - -## Patterns - -> Grammar of a pattern: -> -> *pattern* → *wildcard-pattern* *type-annotation*_?_ \ -> *pattern* → *identifier-pattern* *type-annotation*_?_ \ -> *pattern* → *value-binding-pattern* \ -> *pattern* → *tuple-pattern* *type-annotation*_?_ \ -> *pattern* → *enum-case-pattern* \ -> *pattern* → *optional-pattern* \ -> *pattern* → *type-casting-pattern* \ -> *pattern* → *expression-pattern* - -> Grammar of a wildcard pattern: -> -> *wildcard-pattern* → **`_`** - -> Grammar of an identifier pattern: -> -> *identifier-pattern* → *identifier* - -> Grammar of a value-binding pattern: -> -> *value-binding-pattern* → **`var`** *pattern* | **`let`** *pattern* - -> Grammar of a tuple pattern: -> -> *tuple-pattern* → **`(`** *tuple-pattern-element-list*_?_ **`)`** \ -> *tuple-pattern-element-list* → *tuple-pattern-element* | *tuple-pattern-element* **`,`** *tuple-pattern-element-list* \ -> *tuple-pattern-element* → *pattern* | *identifier* **`:`** *pattern* - -> Grammar of an enumeration case pattern: -> -> *enum-case-pattern* → *type-identifier*_?_ **`.`** *enum-case-name* *tuple-pattern*_?_ - -> Grammar of an optional pattern: -> -> *optional-pattern* → *identifier-pattern* **`?`** - -> Grammar of a type casting pattern: -> -> *type-casting-pattern* → *is-pattern* | *as-pattern* \ -> *is-pattern* → **`is`** *type* \ -> *as-pattern* → *pattern* **`as`** *type* - -> Grammar of an expression pattern: -> -> *expression-pattern* → *expression* - -## Generic Parameters and Arguments - -> Grammar of a generic parameter clause: -> -> *generic-parameter-clause* → **`<`** *generic-parameter-list* **`>`** \ -> *generic-parameter-list* → *generic-parameter* | *generic-parameter* **`,`** *generic-parameter-list* \ -> *generic-parameter* → *type-name* \ -> *generic-parameter* → *type-name* **`:`** *type-identifier* \ -> *generic-parameter* → *type-name* **`:`** *protocol-composition-type* -> -> *generic-where-clause* → **`where`** *requirement-list* \ -> *requirement-list* → *requirement* | *requirement* **`,`** *requirement-list* \ -> *requirement* → *conformance-requirement* | *same-type-requirement* -> -> *conformance-requirement* → *type-identifier* **`:`** *type-identifier* \ -> *conformance-requirement* → *type-identifier* **`:`** *protocol-composition-type* \ -> *same-type-requirement* → *type-identifier* **`==`** *type* - -> Grammar of a generic argument clause: -> -> *generic-argument-clause* → **`<`** *generic-argument-list* **`>`** \ -> *generic-argument-list* → *generic-argument* | *generic-argument* **`,`** *generic-argument-list* \ -> *generic-argument* → *type* - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/ReferenceManual/Types.md b/swift-6-beta.docc/ReferenceManual/Types.md deleted file mode 100644 index c22299b07..000000000 --- a/swift-6-beta.docc/ReferenceManual/Types.md +++ /dev/null @@ -1,1376 +0,0 @@ -# Types - -Use built-in named and compound types. - -In Swift, there are two kinds of types: named types and compound types. -A *named type* is a type that can be given a particular name when it's defined. -Named types include classes, structures, enumerations, and protocols. -For example, -instances of a user-defined class named `MyClass` have the type `MyClass`. -In addition to user-defined named types, -the Swift standard library defines many commonly used named types, -including those that represent arrays, dictionaries, and optional values. - -Data types that are normally considered basic or primitive in other languages --- -such as types that represent numbers, characters, and strings --- -are actually named types, -defined and implemented in the Swift standard library using structures. -Because they're named types, -you can extend their behavior to suit the needs of your program, -using an extension declaration, -discussed in and . - -A *compound type* is a type without a name, defined in the Swift language itself. -There are two compound types: function types and tuple types. -A compound type may contain named types and other compound types. -For example, the tuple type `(Int, (Int, Int))` contains two elements: -The first is the named type `Int`, -and the second is another compound type `(Int, Int)`. - -You can put parentheses around a named type or a compound type. -However, adding parentheses around a type doesn't have any effect. -For example, `(Int)` is equivalent to `Int`. - -This chapter discusses the types defined in the Swift language itself -and describes the type inference behavior of Swift. - -> Grammar of a type: -> -> *type* → *function-type* \ -> *type* → *array-type* \ -> *type* → *dictionary-type* \ -> *type* → *type-identifier* \ -> *type* → *tuple-type* \ -> *type* → *optional-type* \ -> *type* → *implicitly-unwrapped-optional-type* \ -> *type* → *protocol-composition-type* \ -> *type* → *opaque-type* \ -> *type* → *boxed-protocol-type* \ -> *type* → *metatype-type* \ -> *type* → *any-type* \ -> *type* → *self-type* \ -> *type* → **`(`** *type* **`)`** - -## Type Annotation - -A *type annotation* explicitly specifies the type of a variable or expression. -Type annotations begin with a colon (`:`) and end with a type, -as the following examples show: - -```swift -let someTuple: (Double, Double) = (3.14159, 2.71828) -func someFunction(a: Int) { /* ... */ } -``` - - - -In the first example, -the expression `someTuple` is specified to have the tuple type `(Double, Double)`. -In the second example, -the parameter `a` to the function `someFunction` is specified to have the type `Int`. - -Type annotations can contain an optional list of type attributes before the type. - -> Grammar of a type annotation: -> -> *type-annotation* → **`:`** *attributes*_?_ *type* - -## Type Identifier - -A *type identifier* refers to either a named type -or a type alias of a named or compound type. - -Most of the time, a type identifier directly refers to a named type -with the same name as the identifier. -For example, `Int` is a type identifier that directly refers to the named type `Int`, -and the type identifier `Dictionary` directly refers -to the named type `Dictionary`. - -There are two cases in which a type identifier doesn't refer to a type with the same name. -In the first case, a type identifier refers to a type alias of a named or compound type. -For instance, in the example below, -the use of `Point` in the type annotation refers to the tuple type `(Int, Int)`. - -```swift -typealias Point = (Int, Int) -let origin: Point = (0, 0) -``` - - - -In the second case, a type identifier uses dot (`.`) syntax to refer to named types -declared in other modules or nested within other types. -For example, the type identifier in the following code references the named type `MyType` -that's declared in the `ExampleModule` module. - -```swift -var someValue: ExampleModule.MyType -``` - - - -> Grammar of a type identifier: -> -> *type-identifier* → *type-name* *generic-argument-clause*_?_ | *type-name* *generic-argument-clause*_?_ **`.`** *type-identifier* \ -> *type-name* → *identifier* - -## Tuple Type - -A *tuple type* is a comma-separated list of types, enclosed in parentheses. - -You can use a tuple type as the return type of a function -to enable the function to return a single tuple containing multiple values. -You can also name the elements of a tuple type and use those names to refer to -the values of the individual elements. An element name consists of an identifier -followed immediately by a colon (:). For an example that demonstrates both of -these features, see . - -When an element of a tuple type has a name, -that name is part of the type. - -```swift -var someTuple = (top: 10, bottom: 12) // someTuple is of type (top: Int, bottom: Int) -someTuple = (top: 4, bottom: 42) // OK: names match -someTuple = (9, 99) // OK: names are inferred -someTuple = (left: 5, right: 5) // Error: names don't match -``` - - - -All tuple types contain two or more types, -except for `Void` which is a type alias for the empty tuple type, `()`. - -> Grammar of a tuple type: -> -> *tuple-type* → **`(`** **`)`** | **`(`** *tuple-type-element* **`,`** *tuple-type-element-list* **`)`** \ -> *tuple-type-element-list* → *tuple-type-element* | *tuple-type-element* **`,`** *tuple-type-element-list* \ -> *tuple-type-element* → *element-name* *type-annotation* | *type* \ -> *element-name* → *identifier* - -## Function Type - -A *function type* represents the type of a function, method, or closure -and consists of a parameter and return type separated by an arrow (`->`): - -```swift -(<#parameter type#>) -> <#return type#> -``` - -The *parameter type* is comma-separated list of types. -Because the *return type* can be a tuple type, -function types support functions and methods -that return multiple values. - -A parameter of the function type `() -> T` -(where `T` is any type) -can apply the `autoclosure` attribute -to implicitly create a closure at its call sites. -This provides a syntactically convenient way -to defer the evaluation of an expression -without needing to write an explicit closure -when you call the function. -For an example of an autoclosure function type parameter, -see . - -A function type can have variadic parameters in its *parameter type*. -Syntactically, -a variadic parameter consists of a base type name followed immediately by three dots (`...`), -as in `Int...`. A variadic parameter is treated as an array that contains elements -of the base type name. For instance, the variadic parameter `Int...` is treated -as `[Int]`. For an example that uses a variadic parameter, -see . - -To specify an in-out parameter, prefix the parameter type with the `inout` keyword. -You can't mark a variadic parameter or a return type with the `inout` keyword. -In-out parameters are discussed in . - -If a function type has only one parameter -and that parameter's type is a tuple type, -then the tuple type must be parenthesized when writing the function's type. -For example, -`((Int, Int)) -> Void` -is the type of a function that takes a single parameter -of the tuple type `(Int, Int)` -and doesn't return any value. -In contrast, without parentheses, -`(Int, Int) -> Void` is the type -of a function that takes two `Int` parameters -and doesn't return any value. -Likewise, because `Void` is a type alias for `()`, -the function type `(Void) -> Void` -is the same as `(()) -> ()` --- -a function that takes a single argument that's an empty tuple. -These types aren't the same as `() -> ()` --- -a function that takes no arguments. - -Argument names in functions and methods -aren't part of the corresponding function type. -For example: - - - -```swift -func someFunction(left: Int, right: Int) {} -func anotherFunction(left: Int, right: Int) {} -func functionWithDifferentLabels(top: Int, bottom: Int) {} - -var f = someFunction // The type of f is (Int, Int) -> Void, not (left: Int, right: Int) -> Void. -f = anotherFunction // OK -f = functionWithDifferentLabels // OK - -func functionWithDifferentArgumentTypes(left: Int, right: String) {} -f = functionWithDifferentArgumentTypes // Error - -func functionWithDifferentNumberOfArguments(left: Int, right: Int, top: Int) {} -f = functionWithDifferentNumberOfArguments // Error -``` - - - -Because argument labels aren't part of a function's type, -you omit them when writing a function type. - -```swift -var operation: (lhs: Int, rhs: Int) -> Int // Error -var operation: (_ lhs: Int, _ rhs: Int) -> Int // OK -var operation: (Int, Int) -> Int // OK -``` - - - -If a function type includes more than a single arrow (`->`), -the function types are grouped from right to left. -For example, -the function type `(Int) -> (Int) -> Int` is understood as `(Int) -> ((Int) -> Int)` --- -that is, a function that takes an `Int` and returns -another function that takes and returns an `Int`. - -Function types for functions -that can throw or rethrow an error must include the `throws` keyword. -You can include a type after `throws` in parentheses -to specify the type of error that the function throws. -The throw error type must conform to the `Error` protocol. -Writing `throws` without specifying a type -is the same as writing `throws(any Error)`. -Omitting `throws` is the same as writing `throws(Never)`. -The error type that a function throws -can be any type that conforms to `Error`, -including generic types, boxed protocol types, and opaque types. - -The type of error that a function throws is part of that function's type, -and a subtype relationship between error types -means the corresponding function types are also subtypes. -For example, if you declare a custom `MyError` type, -the relationship between some function types is as follows, -from supertype to subtype: - -1. Functions that throw any error, marked `throws(any Error)` -1. Functions that throw a specific error, marked `throws(MyError)` -1. Functions that don't throw, marked `throws(Never)` - -As a result of these subtype relationships: - -- You can use a nonthrowing function - in the same places as a throwing function. -- You can use a function that throws a concrete error type - in the same places as a throwing function. -- You can use a function that throws a more specific error type - in the same places as a function that throws a more general error type. - -If you use an associated type or a generic type parameter -as the thrown error type in a function type, -then that associated type or generic type parameter -is implicitly required to conform to the `Error` protocol. - -Throwing and rethrowing functions are described in - -and . - -Function types for asynchronous functions -must be marked with the `async` keyword. -The `async` keyword is part of a function's type, -and synchronous functions are subtypes of asynchronous functions. -As a result, you can use a synchronous function -in the same places as an asynchronous one. -For information about asynchronous functions, -see . - - - -### Restrictions for Nonescaping Closures - -A parameter that's a nonescaping function -can't be stored in a property, variable, or constant of type `Any`, -because that might allow the value to escape. - - - -A parameter that's a nonescaping function -can't be passed as an argument to another nonescaping function parameter. -This restriction helps Swift perform -more of its checks for conflicting access to memory -at compile time instead of at runtime. -For example: - -```swift -let external: (() -> Void) -> Void = { _ in () } -func takesTwoFunctions(first: (() -> Void) -> Void, second: (() -> Void) -> Void) { - first { first {} } // Error - second { second {} } // Error - - first { second {} } // Error - second { first {} } // Error - - first { external {} } // OK - external { first {} } // OK -} -``` - - - -In the code above, -both of the parameters to `takesTwoFunctions(first:second:)` are functions. -Neither parameter is marked `@escaping`, -so they're both nonescaping as a result. - -The four function calls marked "Error" in the example above -cause compiler errors. -Because the `first` and `second` parameters -are nonescaping functions, -they can't be passed as arguments to another nonescaping function parameter. -In contrast, -the two function calls marked "OK" don't cause a compiler error. -These function calls don't violate the restriction -because `external` isn't one of the parameters of `takesTwoFunctions(first:second:)`. - -If you need to avoid this restriction, mark one of the parameters as escaping, -or temporarily convert one of the nonescaping function parameters to an escaping function -by using the `withoutActuallyEscaping(_:do:)` function. -For information about avoiding conflicting access to memory, -see . - -> Grammar of a function type: -> -> *function-type* → *attributes*_?_ *function-type-argument-clause* **`async`**_?_ *throws-clause*_?_ **`->`** *type* -> -> *function-type-argument-clause* → **`(`** **`)`** \ -> *function-type-argument-clause* → **`(`** *function-type-argument-list* **`...`**_?_ **`)`** -> -> *function-type-argument-list* → *function-type-argument* | *function-type-argument* **`,`** *function-type-argument-list* \ -> *function-type-argument* → *attributes*_?_ *parameter-modifier*_?_ *type* | *argument-label* *type-annotation* \ -> *argument-label* → *identifier* -> -> *throws-clause* → **`throws`** | **`throws`** **`(`** *type* **`)`** - - - -## Array Type - -The Swift language provides the following syntactic sugar for the Swift standard library -`Array` type: - -```swift -[<#type#>] -``` - -In other words, the following two declarations are equivalent: - -```swift -let someArray: Array = ["Alex", "Brian", "Dave"] -let someArray: [String] = ["Alex", "Brian", "Dave"] -``` - - - -In both cases, the constant `someArray` -is declared as an array of strings. The elements of an array can be accessed -through subscripting by specifying a valid index value in square brackets: -`someArray[0]` refers to the element at index 0, `"Alex"`. - -You can create multidimensional arrays by nesting pairs of square brackets, -where the name of the base type of the elements is contained in the innermost -pair of square brackets. -For example, you can create -a three-dimensional array of integers using three sets of square brackets: - -```swift -var array3D: [[[Int]]] = [[[1, 2], [3, 4]], [[5, 6], [7, 8]]] -``` - - - -When accessing the elements in a multidimensional array, -the left-most subscript index refers to the element at that index in the outermost -array. The next subscript index to the right refers to the element -at that index in the array that's nested one level in. And so on. This means that in -the example above, `array3D[0]` refers to `[[1, 2], [3, 4]]`, -`array3D[0][1]` refers to `[3, 4]`, and `array3D[0][1][1]` refers to the value 4. - -For a detailed discussion of the Swift standard library `Array` type, -see . - -> Grammar of an array type: -> -> *array-type* → **`[`** *type* **`]`** - -## Dictionary Type - -The Swift language provides the following syntactic sugar for the Swift standard library -`Dictionary` type: - -```swift -[<#key type#>: <#value type#>] -``` - -In other words, the following two declarations are equivalent: - -```swift -let someDictionary: [String: Int] = ["Alex": 31, "Paul": 39] -let someDictionary: Dictionary = ["Alex": 31, "Paul": 39] -``` - - - -In both cases, the constant `someDictionary` -is declared as a dictionary with strings as keys and integers as values. - -The values of a dictionary can be accessed through subscripting -by specifying the corresponding key in -square brackets: `someDictionary["Alex"]` refers to the value associated -with the key `"Alex"`. -The subscript returns an optional value of the dictionary's value type. -If the specified key isn't contained in the dictionary, -the subscript returns `nil`. - -The key type of a dictionary must conform to the Swift standard library `Hashable` protocol. - - - -For a detailed discussion of the Swift standard library `Dictionary` type, -see . - -> Grammar of a dictionary type: -> -> *dictionary-type* → **`[`** *type* **`:`** *type* **`]`** - -## Optional Type - -The Swift language defines the postfix `?` as syntactic sugar for -the named type `Optional`, which is defined in the Swift standard library. -In other words, the following two declarations are equivalent: - -```swift -var optionalInteger: Int? -var optionalInteger: Optional -``` - - - - - -In both cases, the variable `optionalInteger` -is declared to have the type of an optional integer. -Note that no whitespace may appear between the type and the `?`. - -The type `Optional` is an enumeration with two cases, `none` and `some(Wrapped)`, -which are used to represent values that may or may not be present. -Any type can be explicitly declared to be (or implicitly converted to) an optional type. -If you don't provide an initial value when you declare an -optional variable or property, its value automatically defaults to `nil`. - - - -If an instance of an optional type contains a value, -you can access that value using the postfix operator `!`, as shown below: - -```swift -optionalInteger = 42 -optionalInteger! // 42 -``` - - - - - -Using the `!` operator to unwrap an optional -that has a value of `nil` results in a runtime error. - -You can also use optional chaining and optional binding to conditionally perform an -operation on an optional expression. If the value is `nil`, -no operation is performed and therefore no runtime error is produced. - -For more information and to see examples that show how to use optional types, -see . - -> Grammar of an optional type: -> -> *optional-type* → *type* **`?`** - -## Implicitly Unwrapped Optional Type - -The Swift language defines the postfix `!` as syntactic sugar for -the named type `Optional`, which is defined in the Swift standard library, -with the additional behavior that -it's automatically unwrapped when it's accessed. -If you try to use an implicitly unwrapped optional that has a value of `nil`, -you'll get a runtime error. -With the exception of the implicit unwrapping behavior, -the following two declarations are equivalent: - -```swift -var implicitlyUnwrappedString: String! -var explicitlyUnwrappedString: Optional -``` - -Note that no whitespace may appear between the type and the `!`. - -Because implicit unwrapping -changes the meaning of the declaration that contains that type, -optional types that are nested inside a tuple type or a generic type ---- such as the element types of a dictionary or array --- -can't be marked as implicitly unwrapped. -For example: - -```swift -let tupleOfImplicitlyUnwrappedElements: (Int!, Int!) // Error -let implicitlyUnwrappedTuple: (Int, Int)! // OK - -let arrayOfImplicitlyUnwrappedElements: [Int!] // Error -let implicitlyUnwrappedArray: [Int]! // OK -``` - -Because implicitly unwrapped optionals -have the same `Optional` type as optional values, -you can use implicitly unwrapped optionals -in all the same places in your code -that you can use optionals. -For example, you can assign values of implicitly unwrapped -optionals to variables, constants, and properties of optionals, and vice versa. - -As with optionals, if you don't provide an initial value when you declare an -implicitly unwrapped optional variable or property, -its value automatically defaults to `nil`. - -Use optional chaining to conditionally perform an -operation on an implicitly unwrapped optional expression. -If the value is `nil`, -no operation is performed and therefore no runtime error is produced. - -For more information about implicitly unwrapped optional types, -see . - -> Grammar of an implicitly unwrapped optional type: -> -> *implicitly-unwrapped-optional-type* → *type* **`!`** - -## Protocol Composition Type - -A *protocol composition type* defines a type that conforms to each protocol -in a list of specified protocols, -or a type that's a subclass of a given class -and conforms to each protocol in a list of specified protocols. -Protocol composition types may be used only when specifying a type -in type annotations, -in generic parameter clauses, -and in generic `where` clauses. - - - -Protocol composition types have the following form: - -```swift -<#Protocol 1#> & <#Protocol 2#> -``` - -A protocol composition type allows you to specify a value whose type conforms to the requirements -of multiple protocols without explicitly defining a new, named protocol -that inherits from each protocol you want the type to conform to. -For example, -you can use the protocol composition type `ProtocolA & ProtocolB & ProtocolC` -instead of declaring a new protocol -that inherits from `ProtocolA`, `ProtocolB`, and `ProtocolC`. -Likewise, you can use `SuperClass & ProtocolA` -instead of declaring a new protocol -that's a subclass of `SuperClass` and conforms to `ProtocolA`. - -Each item in a protocol composition list is one of the following; -the list can contain at most one class: - -- The name of a class -- The name of a protocol -- A type alias whose underlying type - is a protocol composition type, a protocol, or a class. - -When a protocol composition type contains type aliases, -it's possible for the same protocol to appear -more than once in the definitions --- -duplicates are ignored. -For example, -the definition of `PQR` in the code below -is equivalent to `P & Q & R`. - -```swift -typealias PQ = P & Q -typealias PQR = PQ & Q & R -``` - - - -> Grammar of a protocol composition type: -> -> *protocol-composition-type* → *type-identifier* **`&`** *protocol-composition-continuation* \ -> *protocol-composition-continuation* → *type-identifier* | *protocol-composition-type* - -## Opaque Type - -An *opaque type* defines a type -that conforms to a protocol or protocol composition, -without specifying the underlying concrete type. - -Opaque types appear as the return type of a function or subscript, -or the type of a property. -Opaque types can't appear as part of a tuple type or a generic type, -such as the element type of an array or the wrapped type of an optional. - -Opaque types have the following form: - -```swift -some <#constraint#> -``` - -The *constraint* is a class type, -protocol type, -protocol composition type, -or `Any`. -A value can be used as an instance of the opaque type -only if it's an instance of a type -that conforms to the listed protocol or protocol composition, -or inherits from the listed class. -Code that interacts with an opaque value -can use the value only in ways -that are part of the interface defined by the *constraint*. - - - -At compile time, -a value whose type is opaque has a specific concrete type, -and Swift can use that underlying type for optimizations. -However, -the opaque type forms a boundary -that information about that underlying type can't cross. - -Protocol declarations can't include opaque types. -Classes can't use an opaque type as the return type of a nonfinal method. - -A function that uses an opaque type as its return type -must return values that share a single underlying type. -The return type can include types -that are part of the function's generic type parameters. -For example, a function `someFunction()` -could return a value of type `T` or `Dictionary`. - -> Grammar of an opaque type: -> -> *opaque-type* → **`some`** *type* - -## Boxed Protocol Type - -A *boxed protocol type* defines a type -that conforms to a protocol or protocol composition, -with the ability for that conforming type -to vary while the program is running. - -Boxed protocol types have the following form: - -```swift -any <#constraint#> -``` - -The *constraint* is a protocol type, -protocol composition type, -a metatype of a protocol type, -or a metatype of a protocol composition type. - -At runtime, -an instance of a boxed protocol type can contain a value -of any type that satisfies the *constraint*. -This behavior contrasts with how an opaque types work, -where there is some specific conforming type known at compile time. -The additional level of indirection that's used -when working with a boxed protocol type is called :newTerm:`boxing`. -Boxing typically requires a separate memory allocation for storage -and an additional level of indirection for access, -which incurs a performance cost at runtime. - -Applying `any` to the `Any` or `AnyObject` types -has no effect, -because those types are already boxed protocol types. - - - - - - - -> Grammar of a boxed protocol type: -> -> *boxed-protocol-type* → **`any`** *type* - -## Metatype Type - -A *metatype type* refers to the type of any type, -including class types, structure types, enumeration types, and protocol types. - -The metatype of a class, structure, or enumeration type is -the name of that type followed by `.Type`. -The metatype of a protocol type --- not the concrete type that -conforms to the protocol at runtime --- -is the name of that protocol followed by `.Protocol`. -For example, the metatype of the class type `SomeClass` is `SomeClass.Type` -and the metatype of the protocol `SomeProtocol` is `SomeProtocol.Protocol`. - -You can use the postfix `self` expression to access a type as a value. -For example, `SomeClass.self` returns `SomeClass` itself, -not an instance of `SomeClass`. -And `SomeProtocol.self` returns `SomeProtocol` itself, -not an instance of a type that conforms to `SomeProtocol` at runtime. -You can call the `type(of:)` function with an instance of a type -to access that instance's dynamic, runtime type as a value, -as the following example shows: - -```swift -class SomeBaseClass { - class func printClassName() { - print("SomeBaseClass") - } -} -class SomeSubClass: SomeBaseClass { - override class func printClassName() { - print("SomeSubClass") - } -} -let someInstance: SomeBaseClass = SomeSubClass() -// The compile-time type of someInstance is SomeBaseClass, -// and the runtime type of someInstance is SomeSubClass -type(of: someInstance).printClassName() -// Prints "SomeSubClass" -``` - - - -For more information, -see [`type(of:)`](https://developer.apple.com/documentation/swift/2885064-type) -in the Swift standard library. - -Use an initializer expression to construct an instance of a type -from that type's metatype value. -For class instances, -the initializer that's called must be marked with the `required` keyword -or the entire class marked with the `final` keyword. - -```swift -class AnotherSubClass: SomeBaseClass { - let string: String - required init(string: String) { - self.string = string - } - override class func printClassName() { - print("AnotherSubClass") - } -} -let metatype: AnotherSubClass.Type = AnotherSubClass.self -let anotherInstance = metatype.init(string: "some string") -``` - - - -> Grammar of a metatype type: -> -> *metatype-type* → *type* **`.`** **`Type`** | *type* **`.`** **`Protocol`** - -## Any Type - -The `Any` type can contain values from all other types. -`Any` can be used as the concrete type -for an instance of any of the following types: - -- A class, structure, or enumeration -- A metatype, such as `Int.self` -- A tuple with any types of components -- A closure or function type - -```swift -let mixed: [Any] = ["one", 2, true, (4, 5.3), { () -> Int in return 6 }] -``` - - - -When you use `Any` as a concrete type for an instance, -you need to cast the instance to a known type -before you can access its properties or methods. -Instances with a concrete type of `Any` -maintain their original dynamic type -and can be cast to that type using one of the type-cast operators --- -`as`, `as?`, or `as!`. -For example, -use `as?` to conditionally downcast the first object in a heterogeneous array -to a `String` as follows: - -```swift -if let first = mixed.first as? String { - print("The first item, '\(first)', is a string.") -} -// Prints "The first item, 'one', is a string." -``` - - - -For more information about casting, see . - -The `AnyObject` protocol is similar to the `Any` type. -All classes implicitly conform to `AnyObject`. -Unlike `Any`, -which is defined by the language, -`AnyObject` is defined by the Swift standard library. -For more information, see - -and [`AnyObject`](https://developer.apple.com/documentation/swift/anyobject). - -> Grammar of an Any type: -> -> *any-type* → **`Any`** - -## Self Type - -The `Self` type isn't a specific type, -but rather lets you conveniently refer to the current type -without repeating or knowing that type's name. - -In a protocol declaration or a protocol member declaration, -the `Self` type refers to the eventual type that conforms to the protocol. - -In a structure, class, or enumeration declaration, -the `Self` type refers to the type introduced by the declaration. -Inside the declaration for a member of a type, -the `Self` type refers to that type. -In the members of a class declaration, -`Self` can appear only as follows: - -- As the return type of a method -- As the return type of a read-only subscript -- As the type of a read-only computed property -- In the body of a method - -For example, -the code below shows an instance method `f` -whose return type is `Self`. - - - - - - - -```swift -class Superclass { - func f() -> Self { return self } -} -let x = Superclass() -print(type(of: x.f())) -// Prints "Superclass" - -class Subclass: Superclass { } -let y = Subclass() -print(type(of: y.f())) -// Prints "Subclass" - -let z: Superclass = Subclass() -print(type(of: z.f())) -// Prints "Subclass" -``` - - - -The last part of the example above shows that -`Self` refers to the runtime type `Subclass` of the value of `z`, -not the compile-time type `Superclass` of the variable itself. - - - -Inside a nested type declaration, -the `Self` type refers to the type -introduced by the innermost type declaration. - -The `Self` type refers to the same type -as the [`type(of:)`](https://developer.apple.com/documentation/swift/2885064-type) -function in the Swift standard library. -Writing `Self.someStaticMember` to access a member of the current type -is the same as writing `type(of: self).someStaticMember`. - -> Grammar of a Self type: -> -> *self-type* → **`Self`** - -## Type Inheritance Clause - -A *type inheritance clause* is used to specify which class a named type inherits from -and which protocols a named type conforms to. -A type inheritance clause begins with a colon (`:`), -followed by a list of type identifiers. - -Class types can inherit from a single superclass and conform to any number of protocols. -When defining a class, -the name of the superclass must appear first in the list of type identifiers, -followed by any number of protocols the class must conform to. -If the class doesn't inherit from another class, -the list can begin with a protocol instead. -For an extended discussion and several examples of class inheritance, -see . - -Other named types can only inherit from or conform to a list of protocols. -Protocol types can inherit from any number of other protocols. -When a protocol type inherits from other protocols, -the set of requirements from those other protocols are aggregated together, -and any type that inherits from the current protocol must conform to all of those requirements. - -A type inheritance clause in an enumeration definition can be either a list of protocols, -or in the case of an enumeration that assigns raw values to its cases, -a single, named type that specifies the type of those raw values. -For an example of an enumeration definition that uses a type inheritance clause -to specify the type of its raw values, see . - -> Grammar of a type inheritance clause: -> -> *type-inheritance-clause* → **`:`** *type-inheritance-list* \ -> *type-inheritance-list* → *attributes*_?_ *type-identifier* | *attributes*_?_ *type-identifier* **`,`** *type-inheritance-list* - -## Type Inference - -Swift uses *type inference* extensively, -allowing you to omit the type or part of the type of many variables and expressions in your code. -For example, -instead of writing `var x: Int = 0`, you can write `var x = 0`, -omitting the type completely --- -the compiler correctly infers that `x` names a value of type `Int`. -Similarly, you can omit part of a type when the full type can be inferred from context. -For example, if you write `let dict: Dictionary = ["A": 1]`, -the compiler infers that `dict` has the type `Dictionary`. - -In both of the examples above, -the type information is passed up from the leaves of the expression tree to its root. -That is, -the type of `x` in `var x: Int = 0` is inferred by first checking the type of `0` -and then passing this type information up to the root (the variable `x`). - -In Swift, type information can also flow in the opposite direction --- from the root down to the leaves. -In the following example, for instance, -the explicit type annotation (`: Float`) on the constant `eFloat` -causes the numeric literal `2.71828` to have an inferred type of `Float` instead of `Double`. - -```swift -let e = 2.71828 // The type of e is inferred to be Double. -let eFloat: Float = 2.71828 // The type of eFloat is Float. -``` - - - -Type inference in Swift operates at the level of a single expression or statement. -This means that all of the information needed to infer an omitted type or part of a type -in an expression must be accessible from type-checking -the expression or one of its subexpressions. - - - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - - diff --git a/swift-6-beta.docc/RevisionHistory/RevisionHistory.md b/swift-6-beta.docc/RevisionHistory/RevisionHistory.md deleted file mode 100644 index a7f089edd..000000000 --- a/swift-6-beta.docc/RevisionHistory/RevisionHistory.md +++ /dev/null @@ -1,869 +0,0 @@ -# Document Revision History - -Review the recent changes to this book. - -**XXX release date XXX** - -- Minor corrections throughout. - -**2024-06-10** - -- Updated for Swift 6. -- Added the section - with information about migrating to strict concurrency checking. -- Added the section - with information about throwing errors of a specific type. -- Added information about package-level access - to the chapter. - -**2024-03-05** - -- Updated for Swift 5.10. -- Added information about nested protocols - to the section. -- Added deprecation information - in the - and sections. - -**2023-12-11** - -- Updated for Swift 5.9.2. -- Added information about the `borrowing` and `consuming` modifiers - to the section. -- Added information in - about setting a constant's value after its declaration. -- Added more information about tasks, task groups, and task cancellation - to the chapter. -- Added information in the chapter - about implementing macros in an existing Swift package. -- Updated the section, - now that extension macros have replaced conformance macros. -- Added the section - with information about back deployment. - -**2023-09-18** - -- Updated for Swift 5.9. -- Added information about `if` and `switch` expressions - to the chapter - and the section. -- Added the chapter, - with information about generating code at compile time. -- Expanded the discussion of optionals in . -- Added an example of concurrency to . -- Added information about boxed protocol types - to the chapter. -- Added information about the `buildPartialBlock(first:)` - and `buildPartialBlock(accumulated:next:)` methods - to the section. -- Added visionOS to the lists of platforms in - - and . -- Formatted the formal grammar to use blank lines for grouping. - -**2023-03-30** - -- Updated for Swift 5.8. -- Added the section, - showing `defer` outside of error handling. -- Adopted Swift-DocC for publication. -- Minor corrections and additions throughout. - -**2022-09-12** - -- Updated for Swift 5.7. -- Added the section, - with information about sending data between actors and tasks, - and added information about the `@Sendable` and `@unchecked` attributes - to the and sections. -- Added the section - with information about creating a regular expression. -- Added information about the short form of `if`-`let` - to the section. -- Added information about `#unavailable` - to the section. - -**2022-03-14** - -- Updated for Swift 5.6. -- Updated the section - with information about using `#if` - around chained method calls and other postfix expressions. -- Updated the visual styling of figures throughout. - -**2021-09-20** - -- Updated for Swift 5.5. -- Added information about asynchronous functions, tasks, and actors - to the chapter, - and to the , - , - and sections. -- Updated the section - with information about identifiers that start with an underscore. - -**2021-04-26** - -- Updated for Swift 5.4. -- Added the - and sections - with information about result builders. -- Added the section - with information about how in-out parameters - can be implicitly converted to unsafe pointers in a function call. -- Updated the - and sections, - now that a function can have multiple variadic parameters. -- Updated the section, - now that implicit member expressions can be chained together. - -**2020-09-16** - -- Updated for Swift 5.3. -- Added information about multiple trailing closures - to the section, - and added information about how trailing closures are matched to parameters - to the section. -- Added information about synthesized implementations - of `Comparable` for enumerations - to the section. -- Added the section - now that you can write a generic `where` clause in more places. -- Added the section - with information about using unowned references with optional values. -- Added information about the `@main` attribute - to the section. -- Added `#filePath` to the section, - and updated the discussion of `#file`. -- Updated the section, - now that closures can refer to `self` implicitly in more scenarios. -- Updated the - and sections, - now that a `catch` clause can match against multiple errors. -- Added more information about `Any` - and moved it into the new section. -- Updated the section, - now that lazy properties can have observers. -- Updated the section, - now that members of an enumeration can satisfy protocol requirements. -- Updated the section - to describe when the getter is called before the observer. -- Updated the chapter - to mention atomic operations. - -**2020-03-24** - -- Updated for Swift 5.2. -- Added information about passing a key path instead of a closure - to the section. -- Added the section - with information about syntactic sugar the lets instances of - classes, structures, and enumerations be used with function call syntax. -- Updated the section, - now that subscripts support parameters with default values. -- Updated the section, - now that the `Self` can be used in more contexts. -- Updated the section - to make it clearer that an implicitly unwrapped optional value - can be used as either an optional or non-optional value. - -**2019-09-10** - -- Updated for Swift 5.1. -- Added information about functions - that specify a protocol that their return value conforms to, - instead of providing a specific named return type, - to the chapter. -- Added information about property wrappers - to the section. -- Added information about enumerations and structures - that are frozen for library evolution - to the section. -- Added the - and sections - with information about functions that omit `return`. -- Added information about using subscripts on types - to the section. -- Updated the section, - now that an enumeration case pattern can match an optional value. -- Updated the section, - now that memberwise initializers support - omitting parameters for properties that have a default value. -- Added information about dynamic members - that are looked up by key path at runtime - to the section. -- Added `macCatalyst` to the list of target environments - in . -- Updated the section, - now that `Self` can be used to refer to the type - introduced by the current class, structure, or enumeration declaration. - -**2019-03-25** - -- Updated for Swift 5.0. -- Added the section - and updated the section - with information about extended string delimiters. -- Added the section - with information about dynamically calling instances as functions - using the `dynamicCallable` attribute. -- Added the and sections - with information about handling future enumeration cases - in switch statements using - the `unknown` switch case attribute. -- Added information about the identity key path (`\.self`) - to the section. -- Added information about using the less than (`<`) operator - in platform conditions to the section. - -**2018-09-17** - -- Updated for Swift 4.2. -- Added information about accessing all of an enumeration's cases - to the section. -- Added information about `#error` and `#warning` - to the section. -- Added information about inlining - to the section - under the `inlinable` and `usableFromInline` attributes. -- Added information about members that are looked up by name at runtime - to the section - under the `dynamicMemberLookup` attribute. -- Added information about the `requires_stored_property_inits` and `warn_unqualified_access` attributes - to the section. -- Added information about how to conditionally compile code - depending on the Swift compiler version being used - to the section. -- Added information about `#dsohandle` - to the section. - -**2018-03-29** - -- Updated for Swift 4.1. -- Added information about synthesized implementations of equivalence operators - to the section. -- Added information about conditional protocol conformance - to the section - of the chapter, - and to the section - of the chapter. -- Added information about recursive protocol constraints - to the section. -- Added information about - the `canImport()` and `targetEnvironment()` platform conditions - to . - -**2017-12-04** - -- Updated for Swift 4.0.3. -- Updated the section, - now that key paths support subscript components. - -**2017-09-19** - -- Updated for Swift 4.0. -- Added information about exclusive access to memory - to the chapter. -- Added the section, - now that you can use generic `where` clauses - to constrain associated types. -- Added information about multiline string literals - to the section - of the chapter, - and to the section - of the chapter. -- Updated the discussion of the `objc` attribute - in , - now that this attribute is inferred in fewer places. -- Added the section, - now that subscripts can be generic. -- Updated the discussion - in the section - of the chapter, - and in the section - of the chapter, - now that protocol composition types can contain a superclass requirement. -- Updated the discussion of protocol extensions - in - now that `final` isn't allowed in them. -- Added information about preconditions and fatal errors - to the section. - -**2017-03-27** - -- Updated for Swift 3.1. -- Added the section - with information about extensions that include requirements. -- Added examples of iterating over a range - to the section. -- Added an example of failable numeric conversions - to the section. -- Added information to the section - about using the `available` attribute with a Swift language version. -- Updated the discussion in the section - to note that argument labels aren't allowed when writing a function type. -- Updated the discussion of Swift language version numbers - in the section, - now that an optional patch number is allowed. -- Updated the discussion - in the section, - now that Swift distinguishes between functions that take multiple parameters - and functions that take a single parameter of a tuple type. -- Removed the Dynamic Type Expression section - from the chapter, - now that `type(of:)` is a Swift standard library function. - -**2016-10-27** - -- Updated for Swift 3.0.1. -- Updated the discussion of weak and unowned references - in the chapter. -- Added information about the `unowned`, `unowned(safe)`, and `unowned(unsafe)` - declaration modifiers - in the section. -- Added a note to the section - about using an optional value when a value of type `Any` is expected. -- Updated the chapter - to separate the discussion of parenthesized expressions and tuple expressions. - -**2016-09-13** - -- Updated for Swift 3.0. -- Updated the discussion of functions in the chapter - and the section to note that - all parameters get an argument label by default. -- Updated the discussion of operators - in the chapter, - now that you implement them as type methods instead of as global functions. -- Added information about the `open` and `fileprivate` access-level modifiers - to the chapter. -- Updated the discussion of `inout` in the section - to note that it appears in front of a parameter's type - instead of in front of a parameter's name. -- Updated the discussion of the `@noescape` and `@autoclosure` attributes - in the and sections - and the chapter - now that they're type attributes, rather than declaration attributes. -- Added information about operator precedence groups - to the section - of the chapter, - and to the section - of the chapter. -- Updated discussion throughout - to use macOS instead of OS X, - `Error` instead of `ErrorProtocol`, - and protocol names such as `ExpressibleByStringLiteral` - instead of `StringLiteralConvertible`. -- Updated the discussion - in the section - of the chapter - and in the chapter, - now that generic `where` clauses are written at the end of a declaration. -- Updated the discussion in the section, - now that closures are nonescaping by default. -- Updated the discussion - in the section - of the chapter - and the section - of the chapter, - now that `if`, `while`, and `guard` statements - use a comma-separated list of conditions without `where` clauses. -- Added information about switch cases that have multiple patterns - to the section - of the chapter - and the section - of the chapter. -- Updated the discussion of function types - in the section - now that function argument labels are no longer part of a function's type. -- Updated the discussion of protocol composition types - in the section - of the chapter - and in the section - of the chapter - to use the new `Protocol1 & Protocol2` syntax. -- Updated the discussion in the Dynamic Type Expression section - to use the new `type(of:)` syntax for dynamic type expressions. -- Updated the discussion of line control statements - to use the `#sourceLocation(file:line:)` syntax - in the section. -- Updated the discussion in - to use the new `Never` type. -- Added information about playground literals - to the section. -- Updated the discussion in the section - to note that only nonescaping closures can capture in-out parameters. -- Updated the discussion about default parameters - in the section, - now that they can't be reordered in function calls. -- Updated attribute arguments to use a colon - in the chapter. -- Added information about throwing an error - inside the catch block of a rethrowing function - to the section. -- Added information about accessing the selector - of an Objective-C property's getter or setter - to the section. -- Added information to the section - about generic type aliases and using type aliases inside of protocols. -- Updated the discussion of function types in the section - to note that parentheses around the parameter types are required. -- Updated the chapter - to note that the `@IBAction`, `@IBOutlet`, and `@NSManaged` attributes - imply the `@objc` attribute. -- Added information about the `@GKInspectable` attribute - to the section. -- Updated the discussion of optional protocol requirements - in the section - to clarify that they're used only in code that interoperates with Objective-C. -- Removed the discussion of explicitly using `let` in function parameters - from the section. -- Removed the discussion of the `Boolean` protocol - from the chapter, - now that the protocol has been removed from the Swift standard library. -- Corrected the discussion of the `@NSApplicationMain` attribute - in the section. - -**2016-03-21** - -- Updated for Swift 2.2. -- Added information about how to conditionally compile code - depending on the version of Swift being used - to the section. -- Added information about how to distinguish - between methods or initializers whose names differ - only by the names of their arguments - to the section. -- Added information about the `#selector` syntax - for Objective-C selectors - to the section. -- Updated the discussion of associated types - to use the `associatedtype` keyword - in the - and sections. -- Updated information about initializers that return `nil` - before the instance is fully initialized - in the section. -- Added information about comparing tuples - to the section. -- Added information about using keywords as external parameter names - to the section. -- Updated the discussion of the `@objc` attribute - in the section to note that - enumerations and enumeration cases can use this attribute. -- Updated the section - with discussion of custom operators that contain a dot. -- Added a note - to the section - that rethrowing functions can't directly throw errors. -- Added a note to the section - about property observers being called - when you pass a property as an in-out parameter. -- Added a section about error handling - to the chapter. -- Updated figures in the - - section to show the deallocation process more clearly. -- Removed discussion of C-style `for` loops, - the `++` prefix and postfix operators, - and the `--` prefix and postfix operators. -- Removed discussion of variable function arguments - and the special syntax for curried functions. - -**2015-10-20** - -- Updated for Swift 2.1. -- Updated the - and sections - now that string interpolations can contain string literals. -- Added the section - with information about the `@noescape` attribute. -- Updated the - and sections - with information about tvOS. -- Added information about the behavior of in-out parameters - to the section. -- Added information to the section - about how values specified in closure capture lists are captured. -- Updated the - - section to clarify how assignment through optional chaining - behaves. -- Improved the discussion of autoclosures - in the section. -- Added an example that uses the `??` operator - to the chapter. - -**2015-09-16** - -- Updated for Swift 2.0. -- Added information about error handling - to the chapter, - the section, - the section, - the section, - and the section. -- Updated the section, - now that all types can conform to the `ErrorType` protocol. -- Added information about the new `try?` keyword - to the section. -- Added information about recursive enumerations - to the section - of the chapter - and the section - of the chapter. -- Added information about API availability checking - to the section - of the chapter - and the section - of the chapter. -- Added information about the new `guard` statement - to the section - of the chapter - and the section - of the chapter. -- Added information about protocol extensions - to the section - of the chapter. -- Added information about access control for unit testing - to the section - of the chapter. -- Added information about the new optional pattern - to the section - of the chapter. -- Updated the section - with information about the `repeat`-`while` loop. -- Updated the chapter, - now that `String` no longer conforms - to the `CollectionType` protocol from the Swift standard library. -- Added information about the new Swift standard library - `print(_:separator:terminator)` function - to the section. -- Added information about the behavior - of enumeration cases with `String` raw values - to the section - of the chapter - and the section - of the chapter. -- Added information about the `@autoclosure` attribute --- - including its `@autoclosure(escaping)` form --- - to the section. -- Updated the section - with information about the `@available` - and `@warn_unused_result` attributes. -- Updated the section - with information about the `@convention` attribute. -- Added an example of using multiple optional bindings - with a `where` clause - to the section. -- Added information to the section - about how concatenating string literals using the `+` operator - happens at compile time. -- Added information to the section - about comparing metatype values and using them - to construct instances with initializer expressions. -- Added a note to the section - about when user-defined assertions are disabled. -- Updated the discussion of the `@NSManaged` attribute - in the section, - now that the attribute can be applied to certain instance methods. -- Updated the section, - now that variadic parameters can be declared in any position - in a function's parameter list. -- Added information - to the section - about how a nonfailable initializer can delegate - up to a failable initializer - by force-unwrapping the result of the superclass's initializer. -- Added information about using enumeration cases as functions - to the section. -- Added information about explicitly referencing an initializer - to the section. -- Added information about build configuration - and line control statements - to the section. -- Added a note to the section - about constructing class instances from metatype values. -- Added a note to the - - section about weak references being unsuitable for caching. -- Updated a note in the section - to mention that stored type properties are lazily initialized. -- Updated the section - to clarify how variables and constants are captured in closures. -- Updated the section - to describe when you can apply the `@objc` attribute to classes. -- Added a note to the section - about the performance of executing a `throw` statement. - Added similar information about the `do` statement - in the section. -- Updated the section - with information about stored and computed type properties - for classes, structures, and enumerations. -- Updated the section - with information about labeled break statements. -- Updated a note in the section - to clarify the behavior of `willSet` and `didSet` observers. -- Added a note to the section - with information about the scope of `private` access. -- Added a note to the - - section about the differences in weak references - between garbage collected systems and ARC. -- Updated the - section - with a more precise definition of Unicode scalars. - -**2015-04-08** - -- Updated for Swift 1.2. -- Swift now has a native `Set` collection type. - For more information, see . -- `@autoclosure` is now an attribute of the parameter declaration, - not its type. - There's also a new `@noescape` parameter declaration attribute. - For more information, see . -- Type methods and properties now use the `static` keyword - as a declaration modifier. - For more information see . -- Swift now includes the `as?` and `as!` failable downcast operators. - For more information, - see . -- Added a new guide section about - . -- Removed the overflow division (`&/`) and - overflow remainder (`&%`) operators - from . -- Updated the rules for constant and - constant property declaration and initialization. - For more information, see . -- Updated the definition of Unicode scalars in string literals. - See . -- Updated to note that - a half-open range with the same start and end index will be empty. -- Updated to clarify - the capturing rules for variables. -- Updated to clarify - the overflow behavior of signed and unsigned integers -- Updated to clarify - protocol declaration scope and members. -- Updated - to clarify the syntax for - weak and unowned references in closure capture lists. -- Updated to explicitly mention - examples of supported characters for custom operators, - such as those in the Mathematical Operators, Miscellaneous Symbols, - and Dingbats Unicode blocks. -- Constants can now be declared without being initialized - in local function scope. - They must have a set value before first use. - For more information, see . -- In an initializer, constant properties can now only assign a value once. - For more information, - see . -- Multiple optional bindings can now appear in a single `if` statement - as a comma-separated list of assignment expressions. - For more information, see . -- An - must appear within a postfix expression. -- Protocol casts are no longer limited to `@objc` protocols. -- Type casts that can fail at runtime - now use the `as?` or `as!` operator, - and type casts that are guaranteed not to fail use the `as` operator. - For more information, see . - -**2014-10-16** - -- Updated for Swift 1.1. -- Added a full guide to . -- Added a description of - for protocols. -- Constants and variables of type `Any` can now contain - function instances. Updated the example in - to show how to check for and cast to a function type - within a `switch` statement. -- Enumerations with raw values - now have a `rawValue` property rather than a `toRaw()` method - and a failable initializer with a `rawValue` parameter - rather than a `fromRaw()` method. - For more information, see - and . -- Added a new reference section about - , - which can trigger initialization failure. -- Custom operators can now contain the `?` character. - Updated the reference to describe - the revised rules. - Removed a duplicate description of the valid set of operator characters - from . - -**2014-08-18** - -- New document that describes Swift 1.0, - Apple’s new programming language for building iOS and OS X apps. -- Added a new section about - in protocols. -- Added a new section about . -- can now use string interpolation. - Removed a note to the contrary. -- Updated the - section - to reflect the fact that `String` and `Character` values - can no longer be combined with the addition operator (`+`) - or addition assignment operator (`+=`). - These operators are now used only with `String` values. - Use the `String` type's `append(_:)` method - to append a single `Character` value onto the end of a string. -- Added information about the `availability` attribute to - the section. -- no longer implicitly evaluate to - `true` when they have a value and `false` when they do not, - to avoid confusion when working with optional `Bool` values. - Instead, make an explicit check against `nil` - with the `==` or `!=` operators - to find out if an optional contains a value. -- Swift now has a - (`a ?? b`), which unwraps an optional's value if it exists, - or returns a default value if the optional is `nil`. -- Updated and expanded - the section - to reflect and demonstrate that string and character comparison - and prefix / suffix comparison are now based on - Unicode canonical equivalence of extended grapheme clusters. -- You can now try to set a property's value, assign to a subscript, - or call a mutating method or operator through - . - The information about - - has been updated accordingly, - and the examples of checking for method call success in - - have been expanded to show how to check for property setting success. -- Added a new section about - - through optional chaining. -- Updated the section - to note that you can no longer append a single item to an array - with the `+=` operator. - Instead, use the `append(_:)` method, - or append a single-item array with the `+=` operator. -- Added a note that the start value `a` - for the `a...b` and `a.. chapter - to remove its introductory coverage of initializer overrides. - This chapter now focuses more on the addition of - new functionality in a subclass, - and the modification of existing functionality with overrides. - The chapter's example of - - has been rewritten to show how to override a `description` property. - (The examples of modifying an inherited property's default value - in a subclass initializer have been moved to - the chapter.) -- Updated the - section - to note that overrides of a designated initializer - must now be marked with the `override` modifier. -- Updated the section - to note that the `required` modifier is now written before - every subclass implementation of a required initializer, - and that the requirements for required initializers - can now be satisfied by automatically inherited initializers. -- Infix no longer require - the `@infix` attribute. -- The `@prefix` and `@postfix` attributes - for - have been replaced by `prefix` and `postfix` declaration modifiers. -- Added a note about the order in which - are applied - when both a prefix and a postfix operator are applied to - the same operand. -- Operator functions for - no longer use - the `@assignment` attribute when defining the function. -- The order in which modifiers are specified when defining - has changed. - You now write `prefix operator` rather than `operator prefix`, - for example. -- Added information about the `dynamic` declaration modifier - in . -- Added information about how type inference works - with . -- Added more information about curried functions. -- Added a new chapter about . -- Updated the chapter - to reflect the fact that Swift's `Character` type now represents - a single Unicode extended grapheme cluster. - Includes a new section on - - and more information about - - and . -- Updated the section - to note that Unicode scalars inside string literals - are now written as `\u{n}`, - where `n` is a hexadecimal number between 0 and 10FFFF, - the range of Unicode's codespace. -- The `NSString` `length` property is now mapped onto - Swift's native `String` type as `utf16Count`, not `utf16count`. -- Swift's native `String` type no longer has - an `uppercaseString` or `lowercaseString` property. - The corresponding section in - - has been removed, and various code examples have been updated. -- Added a new section about - . -- Added a new section about - . -- Added a new section about . -- Updated the section to note that - multiple related variables can be defined on a single line - with one type annotation. -- The `@optional`, `@lazy`, `@final`, and `@required` attributes - are now the `optional`, `lazy`, `final`, and `required` - . -- Updated the entire book to refer to `..<` as - the - (rather than the “half-closed range operator”). -- Updated the - section to note that `Dictionary` now has - a Boolean `isEmpty` property. -- Clarified the full list of characters that can be used - when defining . -- `nil` and the Booleans `true` and `false` are now . -- Swift's `Array` type now has full value semantics. - Updated the information about - and to reflect the new approach. - Also clarified the assignment and copy behavior for strings arrays and dictionaries. -- is now written as - `[SomeType]` rather than `SomeType[]`. -- Added a new section about , - which is written as `[KeyType: ValueType]`. -- Added a new section about . -- Examples of now use - the global `sorted(_:_:)` function - rather than the global `sort(_:_:)` function, - to reflect the new array value semantics. -- Updated the information about - to clarify that the memberwise structure initializer is made available - even if a structure's stored properties don't have default values. -- Updated to `..<` rather than `..` - for the . -- Added an example of . - -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. 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index 100% rename from swift-6-beta.docc/Assets/weakReference03@2x.png rename to swift-6.docc/Assets/weakReference03@2x.png diff --git a/swift-6-beta.docc/Assets/weakReference03~dark@2x.png b/swift-6.docc/Assets/weakReference03~dark@2x.png similarity index 100% rename from swift-6-beta.docc/Assets/weakReference03~dark@2x.png rename to swift-6.docc/Assets/weakReference03~dark@2x.png diff --git a/swift-6.docc/GuidedTour/AboutSwift.md b/swift-6.docc/GuidedTour/AboutSwift.md new file mode 100644 index 000000000..3f84d2bd3 --- /dev/null +++ b/swift-6.docc/GuidedTour/AboutSwift.md @@ -0,0 +1,30 @@ +# 关于 Swift + +理解这门语言的高阶目标。 + +Swift 是为手机、平板电脑、台式机、服务器或任何其他运行代码的设备编写软件的绝佳方式。它是一种安全且快速的编程语言，结合了现代语言思想的精华，并汲取了来自多元化的开源社区的智慧。 + +Swift 对于初学者来说很友好，同时也没有牺牲经验丰富的程序员所需要的强大和灵活性。它是一种企业级的编程语言，但又有着脚本语言般的表达力和可玩性。编译器经过精心优化，可以提供卓越的性能表现。同时语言本身也经过精心设计，以便于开发使用。两者都没有做出任何妥协，鱼与熊掌兼得。 + +Swift 通过采用现代编程模式来避免大量常见编程错误： + +- 变量始终在使用前被初始化。 +- 数组索引会进行越界检查。 +- 整数运算会进行溢出检查。 +- 可选类型确保 `nil` 值能够得到明确处理。 +- 内存被自动管理。 +- 错误处理机制使程序能够以可控方式从意外异常中恢复。 + +Swift 代码经过编译和优化，以充分发挥现代硬件的性能。其语法和标准库的设计遵循这样的指导思想：代码的最直观写法也应该具有最佳性能。Swift 兼具安全性和速度，使其成为从编写 "Hello, world!" 到开发整个操作系统的绝佳选择。 + +Swift 采用了现代化、轻量级的语法，这些语法对来自其他流行语言的开发者来说很熟悉。它还具有类型推断和模式匹配等强大功能，能够以清晰简洁的方式表达复杂的概念。这使得代码更易于阅读、编写和维护。 + +Swift 仍在不断发展，不断推出经过深思熟虑的新特性和强大的功能。Swift 的目标非常远大。我们非常期待看到你使用它创造出的作品。 + + diff --git a/swift-6.docc/GuidedTour/Compatibility.md b/swift-6.docc/GuidedTour/Compatibility.md new file mode 100644 index 000000000..18330285f --- /dev/null +++ b/swift-6.docc/GuidedTour/Compatibility.md @@ -0,0 +1,27 @@ +# 版本兼容性 + +了解在旧版本中可用的功能。 + +本书介绍了 Swift 6，这是 Xcode 16 中包含的默认 Swift 版本。你可以使用 Swift 6 编译器来构建 Swift 6、Swift 5、Swift 4.2 或 Swift 4 编写的代码。 + +当你使用 Swift 6 编译器构建 Swift 5 的代码时，你可以使用 Swift 6 的新功能 —— 这些功能要么默认启用，要么通过即将推出的功能标志启用。然而，要启用严格的并发检查，你需要升级到 Swift 6。 + +此外，当你使用 Xcode 15.3 构建 Swift 4 和 Swift 4.2 的代码时，大部分 Swift 5 的功能仍然可用。不过，以下更改仅适用于使用 Swift 5 的代码： + +- 返回不透明类型的函数需要 Swift 5.1 的运行时支持。 +- `try?` 表达式不会为已经返回可选值的表达式引入额外的可选性的层级。 +- 大型整数字面量初始化表达式会被推断为正确的整数类型。例如，`UInt64(0xffff_ffff_ffff_ffff)` 会计算出正确的值，而不是发生溢出。 + +并发功能需要 Swift 5 版本以及提供相应并发类型的 Swift 标准库版本。在 Apple 平台上，部署的目标版本需设置为 iOS 13、macOS 10.15、tvOS 13、watchOS 6 或 visionOS 1 以上。 + +用 Swift 6 编写的项目可以依赖 Swift 5、Swift 4.2 或 Swift 4 编写的项目，反之亦然。这意味着，如果你有一个大型项目，并将其分为多个框架，你可以逐个框架地将代码迁移到新版本。 + + diff --git a/swift-6-beta.docc/GuidedTour/GuidedTour.md b/swift-6.docc/GuidedTour/GuidedTour.md similarity index 99% rename from swift-6-beta.docc/GuidedTour/GuidedTour.md rename to swift-6.docc/GuidedTour/GuidedTour.md index c37e0eb40..f4804e2f1 100644 --- a/swift-6-beta.docc/GuidedTour/GuidedTour.md +++ b/swift-6.docc/GuidedTour/GuidedTour.md @@ -1,3 +1,9 @@ + + # A Swift Tour Explore the features and syntax of Swift. @@ -2449,12 +2455,6 @@ anyCommonElements([1, 2, 3], [3]) Writing `` is the same as writing ` ... where T: Equatable`. -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - -Unless otherwise specified, the default access level is internal, -as described in . -This means that `SomeInternalClass` and `someInternalConstant` can be written -without an explicit access-level modifier, -and will still have an access level of internal: +除非专门指定，否则默认的访问级别是 `internal`，如在 中所述。这意味着在不使用修饰符显式指定访问级别的情况下，`SomeInternalClass` 和 `someInternalConstant` 的访问级别仍然是 `internal`： ```swift -class SomeInternalClass {} // implicitly internal -let someInternalConstant = 0 // implicitly internal +class SomeInternalClass {} // 隐式指定为 internal +let someInternalConstant = 0 // 隐式指定为 internal ``` -## Custom Types - -If you want to specify an explicit access level for a custom type, -do so at the point that you define the type. -The new type can then be used wherever its access level permits. -For example, if you define a file-private class, -that class can only be used as the type of a property, -or as a function parameter or return type, -in the source file in which the file-private class is defined. - -The access control level of a type also affects -the default access level of that type's *members* -(its properties, methods, initializers, and subscripts). -If you define a type's access level as private or file private, -the default access level of its members will also be private or file private. -If you define a type's access level as internal or public -(or use the default access level of internal -without specifying an access level explicitly), -the default access level of the type's members will be internal. - -> Important: A public type defaults to having internal members, not public members. -> If you want a type member to be public, you must explicitly mark it as such. -> This requirement ensures that the public-facing API for a type is -> something you opt in to publishing, -> and avoids presenting the internal workings of a type as public API by mistake. +## 自定义类型 + +如果想为一个自定义类型指定访问级别，在定义类型时进行指定即可。这个新类型就可以在其访问级别允许的地方使用。例如，你定义了一个 fileprivate 访问级别的类，那么该类只能在定义它的源文件中使用——可以作为属性的类型、函数的参数类型或函数的返回类型。 + +一个类型的访问级别也会影响该类型的成员（其属性、方法、构造器和下标）的默认访问级别。如果你将一个类型的访问级别定义为 `private` 或 `fileprivate`，那么该类型的成员的默认访问级别也会是 `private` 或 `fileprivate`。如果你将一个类型的访问级别定义为 `internal` 或 `public`（或者使用 `internal` 的默认访问级别，而不显式指定访问级别），那么该类型的成员的默认访问级别将是 `internal`。 + +> 重要提示: `public` 类型的成员默认具有 `internal` 访问级别，而不是 `public`。如果你想让某个成员是 `public`，必须显式地将其指定为 `public`。这样可以确保公共 API 都是你经过选择才发布的，避免错误地将内部使用的接口公开。 ```swift -public class SomePublicClass { // explicitly public class - public var somePublicProperty = 0 // explicitly public class member - var someInternalProperty = 0 // implicitly internal class member - fileprivate func someFilePrivateMethod() {} // explicitly file-private class member - private func somePrivateMethod() {} // explicitly private class member +public class SomePublicClass { // 显式指定为 public 类 + public var somePublicProperty = 0 // 显式指定为 public 类成员 + var someInternalProperty = 0 // 隐式指定为 internal 类成员 + fileprivate func someFilePrivateMethod() {} // 显式指定为 fileprivate 类成员 + private func somePrivateMethod() {} // 显式指定为 private 类成员 } -class SomeInternalClass { // implicitly internal class - var someInternalProperty = 0 // implicitly internal class member - fileprivate func someFilePrivateMethod() {} // explicitly file-private class member - private func somePrivateMethod() {} // explicitly private class member +class SomeInternalClass { // 隐式指定为 internal 类 + var someInternalProperty = 0 // 隐式指定为 internal 类成员 + fileprivate func someFilePrivateMethod() {} // 显式指定为 fileprivate 类成员 + private func somePrivateMethod() {} // 显式指定为 private 类成员 } -fileprivate class SomeFilePrivateClass { // explicitly file-private class - func someFilePrivateMethod() {} // implicitly file-private class member - private func somePrivateMethod() {} // explicitly private class member +fileprivate class SomeFilePrivateClass { // 显式指定为 fileprivate 类 + func someFilePrivateMethod() {} // 隐式指定为 fileprivate 类成员 + private func somePrivateMethod() {} // 显式指定为 private 类成员 } -private class SomePrivateClass { // explicitly private class - func somePrivateMethod() {} // implicitly private class member +private class SomePrivateClass { // 显式指定为 private 类 + func somePrivateMethod() {} // 隐式指定为 private 类成员 } ``` @@ -302,13 +179,9 @@ private class SomePrivateClass { // explicitly private class ``` --> -### Tuple Types +### 元组类型 -The access level for a tuple type is -the most restrictive access level of all types used in that tuple. -For example, if you compose a tuple from two different types, -one with internal access and one with private access, -the access level for that compound tuple type will be private. +元组类型的访问级别是由元组中访问级别最严格的类型决定的。例如，你构建了一个包含两种不同类型的元组，其中一个是 `internal` 访问级别，另一个是 `private` 访问级别，那么这个元组的访问级别将是 `private`。 -> Note: Tuple types don't have a standalone definition in the way that -> classes, structures, enumerations, and functions do. -> A tuple type's access level is determined automatically -> from the types that make up the tuple type, -> and can't be specified explicitly. +> 注意: 元组类型不像类、结构体、枚举和函数那样有单独的定义。元组类型的访问级别是根据构成该元组类型的各个类型的访问级别自动确定的，不能显式指定。 -### Function Types +### 函数类型 -The access level for a function type is calculated as -the most restrictive access level of the function's parameter types and return type. -You must specify the access level explicitly as part of the function's definition -if the function's calculated access level doesn't match the contextual default. +函数类型的访问级别是根据函数的参数类型和返回类型中最严格的访问级别计算得出的。如果函数计算出的访问级别与上下文默认值不匹配，则必须在函数定义中显式指定访问级别。 -The example below defines a global function called `someFunction()`, -without providing a specific access-level modifier for the function itself. -You might expect this function to have the default access level of “internal”, -but this isn't the case. -In fact, `someFunction()` won't compile as written below: +下面的例子定义了一个名为 `someFunction()` 的全局函数，并且没有明确地指定其访问级别。你可能会认为这个函数会具有 `internal` 的默认访问级别，但事实并非如此。实际上，`someFunction()` 按照下面这种写法将无法编译： ```swift func someFunction() -> (SomeInternalClass, SomePrivateClass) { - // function implementation goes here + // 此处是函数实现部分 } ``` @@ -417,20 +279,13 @@ func someFunction() -> (SomeInternalClass, SomePrivateClass) { ``` --> -The function's return type is -a tuple type composed from two of the custom classes defined above in . -One of these classes is defined as internal, -and the other is defined as private. -Therefore, the overall access level of the compound tuple type is private -(the minimum access level of the tuple's constituent types). +该函数的返回类型是一个元组类型，由上面 中定义的两个自定义类组成。其中一个类被定义为 `internal`，另一个类被定义为 `private`。因此，这个元组类型的访问级别是 `private`（组成元组的类型中最严格的访问级别）。 -Because the function's return type is private, -you must mark the function's overall access level with the `private` modifier -for the function declaration to be valid: +因为函数的返回类型是 `private`，所以你必须在函数声明中使用 `private` 修饰符指定函数的访问级别，这样才能使函数声明有效： ```swift private func someFunction() -> (SomeInternalClass, SomePrivateClass) { - // function implementation goes here + // 此处是函数实现部分 } ``` @@ -445,22 +300,13 @@ private func someFunction() -> (SomeInternalClass, SomePrivateClass) { ``` --> -It's not valid to mark the definition of `someFunction()` -with the `public` or `internal` modifiers, -or to use the default setting of internal, -because public or internal users of the function might not have appropriate access -to the private class used in the function's return type. +将 `someFunction()` 函数指定为 `public` 或 `internal`，或者使用默认的 `internal` 访问级别都是非法的，因为函数的 `public` 或 `internal` 使用者可能无法访问函数返回类型中的 `private` 类。 -### Enumeration Types +### 枚举类型 -The individual cases of an enumeration automatically receive the same access level as -the enumeration they belong to. -You can't specify a different access level for individual enumeration cases. +枚举成员的访问级别和其所属的枚举类型相同。你不能为单个枚举成员指定不同的访问级别。 -In the example below, -the `CompassPoint` enumeration has an explicit access level of public. -The enumeration cases `north`, `south`, `east`, and `west` -therefore also have an access level of public: +在下面的例子中，`CompassPoint` 枚举被显式指定为 `public` 访问级别。因此，枚举成员 `north`、`south`、`east` 和 `west` 也具有 `public` 访问级别： ```swift public enum CompassPoint { @@ -506,22 +352,13 @@ public enum CompassPoint { ``` --> -#### Raw Values and Associated Values +#### 原始值和关联值 -The types used for any raw values or associated values in an enumeration definition -must have an access level at least as high as the enumeration's access level. -For example, -you can't use a private type as the raw-value type of -an enumeration with an internal access level. +枚举定义中的原始值或关联值的类型，其访问级别至少不能低于该枚举的访问级别。例如，你不能在访问级别为 `internal` 的枚举中使用 `private` 类型作为原始值类型。 -### Nested Types +### 嵌套类型 -The access level of a nested type is the same as its containing type, -unless the containing type is public. -Nested types defined within a public type -have an automatic access level of internal. -If you want a nested type within a public type to be publicly available, -you must explicitly declare the nested type as public. +嵌套类型的访问级别和包含它的类型的访问级别相同，除非包含它的类型是 `public`。定义在 `public` 类型中的嵌套类型，其访问级别默认是 `internal`。如果你想让这个嵌套类型拥有 `public` 访问级别，那么必须显式将其声明为 `public`。 -## Subclassing - -You can subclass any class -that can be accessed in the current access context -and that's defined in the same module as the subclass. -You can also subclass any open class -that's defined in a different module. -A subclass can't have a higher access level than its superclass --- -for example, you can't write a public subclass of an internal superclass. - -In addition, -for classes that are defined in the same module, -you can override any class member -(method, property, initializer, or subscript) -that's visible in a certain access context. -For classes that are defined in another module, -you can override any open class member. - -An override can make an inherited class member more accessible than its superclass version. -In the example below, class `A` is a public class with a file-private method called `someMethod()`. -Class `B` is a subclass of `A`, with a reduced access level of “internal”. -Nonetheless, class `B` provides an override of `someMethod()` -with an access level of “internal”, which is *higher* than -the original implementation of `someMethod()`: +## 子类 + +你可以继承同一模块中的所有有访问权限的类，也可以继承不同模块中被 `open` 修饰的类。子类的访问级别不得高于父类的访问级别。例如，你不能写一个 `public` 的子类来继承 `internal` 的父类。 + +此外，对于同一模块中定义的类，你可以重写在上下文中可访问的任意类成员（方法、属性、构造器或下标）。对于在其他模块中定义的类，你可以重写访问级别为 `open` 的任意类成员。 + +通过重写可以给子类的成员提供更高的访问级别。下面的例子中，类 `A` 是一个 `public` 类，它有一个 `fileprivate` 的方法 `someMethod()`。类 `B` 是 `A` 的子类，其访问级别降低为 `internal`。但是，类 `B` 将 `someMethod()` 的访问级别重写为 `internal`，其访问级别高于原来的访问级别： ```swift public class A { @@ -708,12 +528,7 @@ internal class B: A { ``` --> -It's even valid for a subclass member to call -a superclass member that has lower access permissions than the subclass member, -as long as the call to the superclass's member takes place within -an allowed access level context -(that is, within the same source file as the superclass for a file-private member call, -or within the same module as the superclass for an internal member call): +即使子类成员的访问级别高于父类成员，只要调用父类成员的操作发生在允许的访问级别上下文中（例如，在同一源文件中调用父类 `fileprivate` 成员，在同一模块内调用父类 `internal` 成员），那么子类成员调用访问权限较低的父类成员也是合法的： ```swift public class A { @@ -743,18 +558,13 @@ internal class B: A { ``` --> -Because superclass `A` and subclass `B` are defined in the same source file, -it's valid for the `B` implementation of `someMethod()` to call -`super.someMethod()`. +因为父类 `A` 和子类 `B` 定义在同一个源文件中，所以子类 `B` 可以在重写的 `someMethod()` 方法中调用 `super.someMethod()`。 -## Constants, Variables, Properties, and Subscripts +## 常量、变量、属性、下标 -A constant, variable, or property can't be more public than its type. -It's not valid to write a public property with a private type, for example. -Similarly, a subscript can't be more public than either its index type or return type. +常量、变量或属性的访问级别不能高于其类型的访问级别。例如，如果一个属性的类型的访问级别是 `private`，那么不能将这个属性的访问级别指定为 `public`。同样，下标的访问级别不能高于其索引类型或返回类型的访问级别。 -If a constant, variable, property, or subscript makes use of a private type, -the constant, variable, property, or subscript must also be marked as `private`: +如果常量、变量、属性或下标的类型的访问级别是 `private`，那么也必须将它们的访问级别指定为 `private`： ```swift private var privateInstance = SomePrivateClass() @@ -801,29 +611,15 @@ private var privateInstance = SomePrivateClass() ``` --> -### Getters and Setters +### Getters 和 Setters -Getters and setters for constants, variables, properties, and subscripts -automatically receive the same access level as -the constant, variable, property, or subscript they belong to. +常量、变量、属性和下标的 `getter` 和 `setter` 会自动获得与它们所属的常量、变量、属性或下标相同的访问级别。 -You can give a setter a *lower* access level than its corresponding getter, -to restrict the read-write scope of that variable, property, or subscript. -You assign a lower access level by writing -`fileprivate(set)`, `private(set)`, `internal(set)`, or `package(set)` -before the `var` or `subscript` introducer. +你可以为 `setter` 指定一个比对应 `getter` 更低的访问级别，以限制该变量、属性或下标的读写范围。你可以通过在 `var` 或 `subscript` 关键字之前写上 `fileprivate(set)`、`private(set)`、`internal(set)` 或 `package(set)` 来指定较低的访问级别。 -> Note: This rule applies to stored properties as well as computed properties. -> Even though you don't write an explicit getter and setter for a stored property, -> Swift still synthesizes an implicit getter and setter for you -> to provide access to the stored property's backing storage. -> Use `fileprivate(set)`, `private(set)`, `internal(set)`, and `package(set)` -> to change the access level -> of this synthesized setter in exactly the same way as for an explicit setter -> in a computed property. +> 注意: 这个规则同时适用于存储属性和计算属性。即使你没有为存储属性显式编写 `getter` 和 `setter`，Swift 仍会为你合成一个隐式的 `getter` 和 `setter`，用于访问该属性的存储内容。无论是隐式合成的 `setter`，还是像计算属性中显式编写的 `setter`，使用 `fileprivate(set)`、`private(set)`、`internal(set)` 和 `package(set)` 都可以改变 `setter` 的访问级别。 -The example below defines a structure called `TrackedString`, -which keeps track of the number of times a string property is modified: +下面的例子定义了一个名为 `TrackedString` 的结构体，它记录了一个字符串属性被修改的次数： ```swift struct TrackedString { @@ -851,26 +647,9 @@ struct TrackedString { ``` --> -The `TrackedString` structure defines a stored string property called `value`, -with an initial value of `""` (an empty string). -The structure also defines a stored integer property called `numberOfEdits`, -which is used to track the number of times that `value` is modified. -This modification tracking is implemented with -a `didSet` property observer on the `value` property, -which increments `numberOfEdits` every time the `value` property is set to a new value. - -The `TrackedString` structure and the `value` property -don't provide an explicit access-level modifier, -and so they both receive the default access level of internal. -However, the access level for the `numberOfEdits` property -is marked with a `private(set)` modifier -to indicate that -the property's getter still has the default access level of internal, -but the property is settable only from within -code that's part of the `TrackedString` structure. -This enables `TrackedString` to modify the `numberOfEdits` property internally, -but to present the property as a read-only property -when it's used outside the structure's definition. +`TrackedString` 结构体定义了一个用于存储 `String` 的属性 `value`，并将初始值设为 `""`（空字符串）。该结构体还定义了一个用于存储 `Int` 的属性 `numberOfEdits`，它用于记录属性 `value` 被修改的次数。这个功能是通过 `value` 属性上的 `didSet` 属性观察器实现的，每当给 `value` 赋新值时，`numberOfEdits` 都会递增。 + +结构体 `TrackedString` 和它的属性 `value` 都没有显式指定访问级别，所以它们都具有默认的访问级别 `internal`。然而，`numberOfEdits` 属性的访问级别被指定为 `private(set)`，这意味该属性的 `getter` 仍然具有 `internal` 的默认访问级别，但只能在 `TrackedString` 结构体内部进行赋值。这使得该属性只能在结构体内部修改，而在结构体的外部呈现为一个只读属性。 -If you create a `TrackedString` instance and modify its string value a few times, -you can see the `numberOfEdits` property value update to match the number of modifications: +如果你创建一个 `TrackedString` 实例并多次修改它的字符串值，你就会看到 `numberOfEdits` 属性的值和修改次数一致： ```swift var stringToEdit = TrackedString() @@ -902,7 +680,7 @@ stringToEdit.value = "This string will be tracked." stringToEdit.value += " This edit will increment numberOfEdits." stringToEdit.value += " So will this one." print("The number of edits is \(stringToEdit.numberOfEdits)") -// Prints "The number of edits is 3" +// 打印“The number of edits is 3” ``` -Although you can query the current value of the `numberOfEdits` property -from within another source file, -you can't *modify* the property from another source file. -This restriction protects the implementation details of -the `TrackedString` edit-tracking functionality, -while still providing convenient access to an aspect of that functionality. - -Note that you can assign an explicit access level for both -a getter and a setter if required. -The example below shows a version of the `TrackedString` structure -in which the structure is defined with an explicit access level of public. -The structure's members (including the `numberOfEdits` property) -therefore have an internal access level by default. -You can make the structure's `numberOfEdits` property getter public, -and its property setter private, -by combining the `public` and `private(set)` access-level modifiers: +虽然你可以从其他源文件中查询 `numberOfEdits` 属性的当前值，但不能从其他源文件中**修改**该属性。这个限制保护了 `TrackedString` 的编辑跟踪功能的实现细节，同时还提供了该功能方便的访问方式。 + +需要注意的是，你可以在必要时为 `getter` 和 `setter` 分别指定显式的访问级别。下面的例子将 `TrackedString` 结构体显式指定为了 `public` 访问级别。结构体的成员（包括 `numberOfEdits` 属性）拥有默认的访问级别 `internal`。你可以组合 `public` 和 `private(set)` 修饰符把结构体中的 `numberOfEdits` 属性的 `getter` 的访问级别设置为 `public`，而 `setter` 的访问级别设置为 `private`： ```swift public struct TrackedString { @@ -1018,61 +783,29 @@ public struct TrackedString { ``` --> -## Initializers - -Custom initializers can be assigned an access level less than or equal to -the type that they initialize. -The only exception is for required initializers -(as defined in ). -A required initializer must have the same access level as the class it belongs to. - -As with function and method parameters, -the types of an initializer's parameters can't be more private than -the initializer's own access level. - -### Default Initializers - -As described in , -Swift automatically provides a *default initializer* without any arguments -for any structure or base class -that provides default values for all of its properties -and doesn't provide at least one initializer itself. - -A default initializer has the same access level as the type it initializes, -unless that type is defined as `public`. -For a type that's defined as `public`, -the default initializer is considered internal. -If you want a public type to be initializable with a no-argument initializer -when used in another module, -you must explicitly provide a public no-argument initializer yourself -as part of the type's definition. - -### Default Memberwise Initializers for Structure Types - -The default memberwise initializer for a structure type is considered private -if any of the structure's stored properties are private. -Likewise, if any of the structure's stored properties are file private, -the initializer is file private. -Otherwise, the initializer has an access level of internal. - -As with the default initializer above, -if you want a public structure type to be initializable with a memberwise initializer -when used in another module, -you must provide a public memberwise initializer yourself as part of the type's definition. - -## Protocols - -If you want to assign an explicit access level to a protocol type, -do so at the point that you define the protocol. -This enables you to create protocols that can only be adopted within -a certain access context. - -The access level of each requirement within a protocol definition -is automatically set to the same access level as the protocol. -You can't set a protocol requirement to a different access level than -the protocol it supports. -This ensures that all of the protocol's requirements will be visible -on any type that adopts the protocol. +## 构造器 + +自定义构造器的访问级别可以低于或等于它所初始化的类型。唯一的例外是必要构造器（如 中定义的）。必要构造器必须具有与其所属类相同的访问级别。 + +与函数和方法的参数一样，构造器的参数类型的访问级别不能比构造器自身的访问级别更严格。 + +### 默认构造器 + +如 中所述，Swift 会为结构体和类自动生成一个不带参数的**默认构造器**，只要它们为所有存储型属性设置了默认初始值，并且未提供自定义的构造器。 + +默认构造器的访问级别与它所初始化的类型相同，除非该类型被定义为 `public`。对于 `public` 类型，默认构造器的访问级别将为 `internal`。如果你想让 `public` 类型在另一个模块中可以通过无参数构造器进行初始化，则必须在类型定义中显式提供一个 `public` 访问级别的无参数构造器。 + +### 结构体默认的成员逐一构造器 + +对于结构体类型，如果结构体中的任何一个存储属性是 `private`，则默认的成员逐一构造器的为 `private`。同样，如果任何存储属性是 `fileprivate`，则默认的成员逐一构造器为 `fileprivate`。否则，默认的成员逐一构造器为 `internal`。 + +与前面提到的默认构造器一样，如果你想让 `public` 结构体类型在其他模块中可以通过成员逐一构造器进行初始化，则必须在类型定义中显式提供一个 `public` 的成员逐一构造器。 + +## 协议 + +如果你想为协议类型显式指定访问级别，需要在定义协议时进行指定。这将限制该协议只能在特定的访问级别范围内被遵循。 + +协议定义中的每个要求都必须具有和该协议相同的访问级别。你不能将协议要求的访问级别设置为其他访问级别。这样才能确保遵循该协议的任何类型都能访问协议中的所有要求。 -> Note: If you define a public protocol, -> the protocol's requirements require a public access level -> for those requirements when they're implemented. -> This behavior is different from other types, -> where a public type definition implies -> an access level of internal for the type's members. +> 注意: 如果你定义了一个 `public` 协议，那么在实现该协议时，协议的所有要求也需要具有 `public` 访问级别。这点与其他类型不同，在其他类型中，如果类型的访问级别是 `public`，通常意味着该类型的成员具有 `internal` 访问级别。 -### Protocol Inheritance - -If you define a new protocol that inherits from an existing protocol, -the new protocol can have at most the same access level as the protocol it inherits from. -For example, -you can't write a public protocol that inherits from an internal protocol. - -### Protocol Conformance - -A type can conform to a protocol with a lower access level than the type itself. -For example, you can define a public type that can be used in other modules, -but whose conformance to an internal protocol can only be used -within the internal protocol's defining module. - -The context in which a type conforms to a particular protocol -is the minimum of the type's access level and the protocol's access level. -For example, if a type is public, but a protocol it conforms to is internal, -the type's conformance to that protocol is also internal. - -When you write or extend a type to conform to a protocol, -you must ensure that the type's implementation of each protocol requirement -has at least the same access level as the type's conformance to that protocol. -For example, if a public type conforms to an internal protocol, -the type's implementation of each protocol requirement must be at least internal. - -> Note: In Swift, as in Objective-C, protocol conformance is global --- -> it isn't possible for a type to conform to a protocol in two different ways -> within the same program. - -## Extensions - -You can extend a class, structure, or enumeration in any access context -in which the class, structure, or enumeration is available. -Any type members added in an extension have the same default access level as -type members declared in the original type being extended. -If you extend a public or internal type, any new type members you add -have a default access level of internal. -If you extend a file-private type, any new type members you add -have a default access level of file private. -If you extend a private type, any new type members you add -have a default access level of private. - -Alternatively, you can mark an extension with an explicit access-level modifier -(for example, `private`) -to set a new default access level for all members defined within the extension. -This new default can still be overridden within the extension -for individual type members. - -You can't provide an explicit access-level modifier for an extension -if you're using that extension to add protocol conformance. -Instead, the protocol's own access level is used to provide -the default access level for each protocol requirement implementation within the extension. +### 协议继承 + +如果你定义了一个继承自其他协议的新协议，那么新协议的访问级别最高也只能与其继承的协议相同。例如，你不能定义一个继承自 `internal` 协议的 `public` 协议。 + +### 协议遵循 + +一个类型可以遵循比其自身访问级别更低的协议。例如，你可以定义一个 `public` 类型，使其可以在其他模块中使用，但该类型对 `internal` 协议的遵循只能在定义该 `internal` 协议的模块中使用。 + +遵循协议时的上下文访问级别是类型和协议中访问级别最低的那个。例如，如果一个类型是 `public` 的，但它遵循 `internal` 协议，那么这个类型对该协议遵循的上下文访问级别也是 `internal` 的。 + +当你编写或扩展一个类型让它遵循一个协议时，你必须确保该类型对协议每一个要求的实现至少与协议的访问级别一致。例如，如果一个 `public` 类型遵循一个 `internal` 协议，那么该类型对协议每一个要求的实现必须至少是 `internal`。 + +> 注意: Swift 和 Objective-C 一样，协议遵循是全局的。在同一程序中，一个类型不可能用两种不同的方式遵循同一个协议。 + +## 扩展 + +可以在访问级别允许的情况下对类、结构体或枚举进行扩展。在扩展中添加的类型成员具有与原始类型中声明的类型成员相同的默认访问级别。如果你扩展的是 `public` 或 `internal` 类型，那么任何新增的类型成员默认的访问级别是 `internal`。如果你扩展的是 `fileprivate` 类型，那么新增的类型成员默认的访问级别是 `fileprivate`。如果你扩展的是 `private` 类型，那么新增的类型成员默认的访问级别是 `private`。 + +或者，你可以使用显式的访问级别修饰符（例如 `private`）标记一个扩展，从而为扩展内定义的所有成员指定一个新的默认访问级别。在此扩展内，这个新的默认级别仍然可以被单个类型成员显式指定的访问级别所覆盖。 + +如果你使用扩展来遵循协议的话，就不能为扩展提供显式的访问级别修饰符。在这种情况下，协议自身的访问级别将被用作扩展中每个协议要求的实现的默认访问级别。 -### Private Members in Extensions +### 扩展的私有成员 -Extensions that are in the same file as -the class, structure, or enumeration that they extend -behave as if the code in the extension -had been written as part of the original type's declaration. -As a result, you can: +扩展同一文件内的类，结构体或者枚举，扩展里的代码会表现得跟声明在原始类型里的一模一样。因此，你可以： -- Declare a private member in the original declaration, - and access that member from extensions in the same file. -- Declare a private member in one extension, - and access that member from another extension in the same file. -- Declare a private member in an extension, - and access that member from the original declaration in the same file. +- 在原始声明中声明一个 `private` 成员，并在同一文件中的扩展中访问该成员。 +- 在一个扩展中声明一个 `private` 成员，并在同一文件中的另一个扩展中访问该成员。 +- 在扩展中声明一个 `private` 成员，并在同一文件中的原始声明中访问该成员。 -This behavior means you can use extensions in the same way -to organize your code, -whether or not your types have private entities. -For example, given the following simple protocol: +这意味着你可以使用扩展来组织你的代码，无论你的类型是否包含 `private` 成员。例如，给定下面这样一个简单的协议： ```swift protocol SomeProtocol { @@ -1419,7 +1106,7 @@ protocol SomeProtocol { ``` --> -You can use an extension to add protocol conformance, like this: +你可以使用扩展来添加协议遵循，就像这样： ```swift struct SomeStruct { @@ -1452,20 +1139,15 @@ extension SomeStruct: SomeProtocol { ``` --> -## Generics +## 泛型 -The access level for a generic type or generic function is -the minimum of the access level of the generic type or function itself -and the access level of any type constraints on its type parameters. +泛型类型或泛型函数的访问级别取决于它本身的访问级别和其类型参数的类型约束的访问级别，最终由这些访问级别中的最低者决定。 -## Type Aliases +## 类型别名 -Any type aliases you define are treated as distinct types for the purposes of access control. -A type alias can have an access level less than or equal to the access level of the type it aliases. -For example, a private type alias can alias a private, file-private, internal, public, or open type, -but a public type alias can't alias an internal, file-private, or private type. +在访问控制层面，你定义的任何类型别名都被视为独立的类型。类型别名的访问级别不可以高于其表示的类型的访问级别。例如，一个 `private` 类型别名可以作为 `private`、`fileprivate`、`internal`、`public` 或 `open` 类型的别名，但一个 `public` 类型别名不能作为 `internal`、`fileprivate` 或 `private` 类型的别名。 -> Note: This rule also applies to type aliases for associated types used to satisfy protocol conformances. +> 注意: 这条规则也适用于为满足协议遵循而将类型别名用于关联类型的情况。 -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - + +`UInt8` 整数有八位，可以存储 `0` 到 `255` 之间的任何值。这个例子用二进制值 `00001111` 初始化一个 `UInt8` 整数，其前四位设置为 `0` ，后四位设置为 `1` 。这相当于十进制值 `15` 。 + + + +然后使用按位取反运算符创建一个名为 `invertedBits` 的新常量，它等于 `initialBits` ，但所有位都被取反。零变成一，一变成零。`invertedBits` 的值是 `11110000` ，等于无符号十进制值 `240` 。 + +### 按位与运算符 + +**按位与运算符** (`&` ）组合两个数字的位。它返回一个新数字，只有在两个输入数字中相应的位都等于 1 时，新数字的相应位才设置为 1: + +![](bitwiseAND) + +在下面的例子中，`firstSixBits` 和 `lastSixBits` 的值在中间四位都等于 `1` 。按位与运算符将它们组合成数字 `00111100` ，等于无符号十进制值 `60` ： + +```swift +let firstSixBits: UInt8 = 0b11111100 +let lastSixBits: UInt8 = 0b00111111 +let middleFourBits = firstSixBits & lastSixBits // equals 00111100 +``` + + + +### 按位或运算符 + +**按位或运算符** （ `|` ）比较两个数字的位。如果 **任一** 输入数字中的位等于 `1` ，则运算符返回一个新数字，其相应位设置为 `1` ： + +![](bitwiseOR) + + + +在下面的例子中， `someBits` 和 `moreBits` 的值在不同的位上设置为 `1` 。按位或运算符将它们组合成数字 `11111110` ，等于无符号十进制值 `254` ： + +```swift +let someBits: UInt8 = 0b10110010 +let moreBits: UInt8 = 0b01011110 +let combinedbits = someBits | moreBits // equals 11111110 +``` + + + +### 按位异或运算符 + +**按位异或运算符** ，或 "异或运算符" （ `^` ），比较两个数字的位。运算符返回一个新数字，在输入位不同的地方，新数字的相应位设置为 `1` ，在输入位相同的地方，新数字的相应位设置为 `0` : + +![](bitwiseXOR) + +在下面的例子中， `firstBits` 和 `otherBits` 的值在对方没有的位置各有一位设置为 `1` 。按位异或运算符在其输出值中将这两位都设置为 `1` 。`firstBits` 和 `otherBits` 中的所有其他位都匹配，在输出值中设置为 `0` ： + +```swift +let firstBits: UInt8 = 0b00010100 +let otherBits: UInt8 = 0b00000101 +let outputBits = firstBits ^ otherBits // equals 00010001 +``` + + + +### 按位左移和右移运算符 + +**按位左移运算符** (`<<` ）和 **按位右移运算符** (`>>` ）根据下面定义的规则，将数字中的所有位向左或向右移动特定数量的位置。 + +按位左移和右移相当于将整数乘以或除以 2 的幂。将整数的位向左移动一位会使其值翻倍，而向右移动一位会将其值减半。 + + + +#### 无符号整数的移位行为 + +无符号整数的位移行为如下： + +1. 现有的位按请求的位数向左或向右移动。 +2. 任何移动超出整数存储边界的位都会被丢弃。 +3. 在原始位向左或向右移动后留下的空位中插入零。 + +这种方法被称为 **逻辑移位**。 + +下图显示了 `11111111 << 1` (即 `11111111` 向左移动 1 位）和 `11111111 >> 1` (即 `11111111` 向右移动 1 位）的结果。绿色数字是移位的，灰色数字被丢弃，插入的粉色零： + +![](bitshiftUnsigned) + +以下是 Swift 代码中位移的样子： + +```swift +let shiftBits: UInt8 = 4 // 00000100 in binary +shiftBits << 1 // 00001000 +shiftBits << 2 // 00010000 +shiftBits << 5 // 10000000 +shiftBits << 6 // 00000000 +shiftBits >> 2 // 00000001 +``` + + + + + +您可以使用位移来编码和解码其他数据类型中的值： + +```swift +let pink: UInt32 = 0xCC6699 +let redComponent = (pink & 0xFF0000) >> 16 // redComponent is 0xCC, or 204 +let greenComponent = (pink & 0x00FF00) >> 8 // greenComponent is 0x66, or 102 +let blueComponent = pink & 0x0000FF // blueComponent is 0x99, or 153 +``` + + + +这个例子使用一个名为 `pink` 的 `UInt32` 常量来存储粉红色的级联样式表（CSS）颜色值。CSS 颜色值 `#CC6699` 在 Swift 的十六进制数字表示中写作 `0xCC6699` 。然后，通过按位与运算符（ `&` ）和按位右移运算符（ `>>` ）将这个颜色分解为红（ `CC` ）、绿（ `66` ）和蓝（ `99` ）分量。 + +红色分量是通过对数字 `0xCC6699` 和 `0xFF0000` 执行按位与运算得到的。`0xFF0000` 中的零有效地"掩蔽"了 `0xCC6699` 的第二和第三个字节，导致 `6699` 被忽略，留下 `0xCC0000` 作为结果。 + +然后，这个数字向右移动 16 位（ `>> 16`)。十六进制数中的每对字符使用 8 位，所以向右移动 16 位会将 `0xCC0000` 转换为 `0x0000CC` 。这与 `0xCC` 相同，十进制值为 204。 + +类似地，绿色分量是通过对数字 `0xCC6699` 和 `0x00FF00` 执行按位与运算得到的，得到输出值 `0x006600` 。然后将这个输出值向右移动八位，得到值 `0x66` ，十进制值为 102。 + +最后，蓝色分量是通过对数字 `0xCC6699` 和 `0x0000FF` 执行按位与运算得到的，得到输出值 `0x000099` 。因为 `0x000099` 已经等于 `0x99` ，十进制值为 153,所以这个值不需要向右移位就可以使用。 + +#### 有符号整数的移位行为 + +有符号整数的移位行为比无符号整数更复杂，因为有符号整数在二进制中的表示方式。(为简单起见，下面的例子基于 8 位有符号整数，但相同的原理适用于任何大小的有符号整数。) + +有符号整数使用它们的第一位（称为 **符号位** ）来表示整数是正数还是负数。符号位为 0 表示正数，符号位为 1 表示负数。 + +剩余的位（称为 **值位** ）存储实际值。正数的存储方式与无符号整数完全相同，从 0 开始向上计数。以下是 `Int8` 中数字 4 的位的样子： + +![](bitshiftSignedFour) + +符号位是 0(表示"正"),七个值位只是数字 4 的二进制表示。 + +然而，负数的存储方式不同。它们通过从 2 的 n 次方中减去它们的绝对值来存储，其中 n 是值位的数量。8 位数字有 7 个值位，所以这意味着 2 的 7 次方，即 128。 + +以下是数字 -4 在 Int8 类型中的二进制表示形式： + +![](bitshiftSignedMinusFour) + +这次，符号位是 `1`(表示"负数"),七个值位的二进制值是 `124`(即 `128 - 4`): + +![](bitshiftSignedMinusFourValue) + +这种编码负数的方法被称为 **二进制补码** 表示。虽然这看起来是一种不寻常的表示负数的方式，但它有几个优点。 + +首先，你可以将 `-1` 加到 `-4` 上，只需对所有八个位（包括符号位）执行标准的二进制加法，并在完成后丢弃任何不适合八位的内容： + +![](bitshiftSignedAddition) + +其次，二进制补码表示还允许你像正数一样将负数的位向左和向右移动，并且每向左移动一位仍然会使其加倍，每向右移动一位则会将其减半。为了实现这一点，在对有符号整数进行右移时使用了一个额外的规则：当你将有符号整数向右移动时，应用与无符号整数相同的规则，但用 **符号位** 填充左侧的任何空位，而不是用零填充。 + +![](bitshiftSigned) + +这个操作确保了有符号整数在向右移动后保持相同的符号，这被称为 **算术移位**。 + +由于正数和负数的存储方式特殊，将它们中的任何一个向右移动都会使它们更接近零。在这个移位过程中保持符号位不变意味着负整数在其值接近零时仍然保持为负数。 + +## 溢出运算符 + +如果你试图将一个数字插入到无法容纳该值的整数常量或变量中，默认情况下 Swift 会报告一个错误，而不是允许创建一个无效的值。当你处理太大或太小的数字时，这种行为提供了额外的安全性。 + +例如，`Int16` 整数类型可以保存介于 `-32768` 和 `32767` 之间的任何有符号整数。试图将 `Int16` 常量或变量设置为超出此范围的数字会导致错误： + +```swift +var potentialOverflow = Int16.max +// potentialOverflow 等于 32767,这是 Int16 可以容纳的最大值 +potentialOverflow += 1 +// this causes an error +``` + + + +当值变得过大或过小时提供错误处理，可以让你在编写边界值条件时拥有更大的灵活性。 + +然而，当你特别希望溢出条件截断可用位数时，你可以选择这种行为而不会触发错误。Swift 提供了三个算术 **溢出运算符**,可以选择整数计算的溢出行为。这些运算符都以 & 符号开头： + +- 溢出加法（ `&+`) +- 溢出减法（ `&-`) +- 溢出乘法（ `&*`) + +### 值溢出 + +数字可以在正方向和负方向上溢出。 + +这里有一个例子，展示了当允许无符号整数在正方向上溢出时会发生什么，使用溢出加法运算符（ `&+`): + +```swift +var unsignedOverflow = UInt8.max +// unsignedOverflow 等于 255,这是 UInt8 可以容纳的最大值 +unsignedOverflow = unsignedOverflow &+ 1 +// unsignedOverflow 现在等于 0 +``` + + + +变量 `unsignedOverflow` 被初始化为 `UInt8` 可以容纳的最大值(`255` ，或二进制的 `11111111`)。然后使用溢出加法运算符（ `&+` ）将其加 1。这将其二进制表示推到了 `UInt8` 可以容纳的大小之外，导致它溢出超出其边界，如下图所示。溢出加法后留在 `UInt8` 边界内的值是 `00000000` ，或零。 + +![](overflowAddition) + +当允许无符号整数在负方向上溢出时，也会发生类似的情况。这里有一个使用溢出减法运算符（ `&-` ）的例子： + +```swift +var unsignedOverflow = UInt8.min +// unsignedOverflow 等于 0,这是 UInt8 可以容纳的最小值 +unsignedOverflow = unsignedOverflow &- 1 +// unsignedOverflow 现在等于 255 +``` + + + +`UInt8` 可以容纳的最小值是零，或二进制的 `00000000` 。如果你使用溢出减法运算符（ `&-` ）从 `00000000` 中减去 1,这个数字将会溢出并绕回到 `11111111` ，或十进制的 `255` 。 + +![](overflowUnsignedSubtraction) + +有符号整数也会发生溢出。所有有符号整数的加法和减法都以按位方式执行，符号位作为被加或被减数字的一部分包含在内，如 中所述。 + +```swift +var signedOverflow = Int8.min +// signedOverflow 等于 -128,这是 Int8 可以容纳的最小值 +signedOverflow = signedOverflow &- 1 +// signedOverflow 现在等于 127 +``` + + + +`Int8` 可以容纳的最小值是 `-128` ，或二进制的 `10000000` 。使用溢出运算符从这个二进制数中减去 1 得到二进制值 `01111111` ，这会切换符号位并得到正 `127` ，这是 `Int8` 可以容纳的最大正值。 + +![](overflowSignedSubtraction) + +对于有符号和无符号整数，正方向的溢出会从最大有效整数值绕回到最小值，而负方向的溢出会从最小值绕回到最大值。 + +## 优先级和结合性 + +运算符 **优先级** 给予某些运算符比其他运算符更高的优先级;这些运算符会首先被应用。 + +运算符 **结合性** 定义了具有相同优先级的运算符如何组合在一起 --- 要么从左边组合，要么从右边组合。可以将其理解为"它们与左边的表达式相关联",或"它们与右边的表达式相关联"。 + +在计算复合表达式的顺序时，考虑每个运算符的优先级和结合性很重要。例如，运算符优先级解释了为什么下面的表达式等于 `17` 。 + +```swift +2 + 3 % 4 * 5 +// 这等于 17 +``` + + + + + +如果你严格从左到右读，你可能会期望表达式按如下方式计算： + +- `2` 加 `3` 等于 `5` +- `5` 除以 `4` 余 `1` +- `1` 乘以 `5` 等于 `5` + +然而，实际答案是 `17` ，而不是 `5` 。优先级更高的运算符在优先级较低的运算符之前被计算。在 Swift 中，就像在 C 语言中一样，余数运算符（ `%` ）和乘法运算符（ `*` ）的优先级高于加法运算符（ `+`)。因此，它们都在考虑加法之前被计算。 + +然而，余数和乘法具有 **相同** 的优先级。要确定使用的确切计算顺序，你还需要考虑它们的结合性。余数和乘法都与它们左边的表达式相关联。可以将其理解为在表达式的这些部分周围添加隐式括号，从左边开始： + + +```swift +2 + ((3 % 4) * 5) +``` + + + + + +`(3 % 4)` is `3`, so this is equivalent to: + +```swift +2 + (3 * 5) +``` + + + + + +`(3 * 5)` is `15`, so this is equivalent to: + +```swift +2 + 15 +``` + + + + + +这个计算得出的最终答案是 `17` 。 + +有关 Swift 标准库提供的运算符的信息，包括运算符优先级组和结合性设置的完整列表，请参阅[运算符声明](https://developer.apple.com/documentation/swift/operator_declarations)。 + +> 注意：Swift 的运算符优先级和结合性规则比 C 和 Objective-C 中的更简单和可预测。 然而，这意味着它们与基于 C 的语言并不完全相同。 在将现有代码移植到 Swift 时，请注意确保运算符交互仍然按照您预期的方式运行。 + +## 运算符方法 + +类和结构体可以为现有运算符提供自定义的实现。这被称为 **重载** 现有运算符。 + +下面的示例展示了如何为自定义结构体实现算术加法运算符（ `+` ）。算术加法运算符是一个二元运算符，因为它操作两个目标，并且它是一个中缀运算符，因为它出现在这两个目标之间。 + +该示例定义了一个 `Vector2D` 结构体，用于表示二维位置向量 `(x, y)` ，然后定义了一个 **运算符方法** 来将 `Vector2D` 结构体的实例相加： + +```swift +struct Vector2D { + var x = 0.0, y = 0.0 +} + +extension Vector2D { + static func + (left: Vector2D, right: Vector2D) -> Vector2D { + return Vector2D(x: left.x + right.x, y: left.y + right.y) + } +} +``` + + + +运算符方法被定义为 `Vector2D` 的类型方法，方法名称与要重载的运算符（ `+` ）相匹配。 由于加法不是向量的基本行为，所以类型方法在 `Vector2D` 的扩展中定义，而不是在 `Vector2D` 的主结构声明中定义。 因为算术加法运算符是一个二元运算符，所以这个运算符方法接受两个类型为 `Vector2D` 的输入参数，并返回一个单一的输出值，也是 `Vector2D` 类型。 + +在这个实现中，输入参数被命名为 `left` 和 `right` ，代表 `+` 运算符左侧和右侧的 `Vector2D` 实例。 该方法返回一个新的 `Vector2D` 实例，其 `x` 和 `y` 属性初始化为相加的两个 `Vector2D` 实例的 `x` 和 `y` 属性之和。 + +这个类型方法可以作为现有 `Vector2D` 实例之间的中缀运算符使用： + +```swift +let vector = Vector2D(x: 3.0, y: 1.0) +let anotherVector = Vector2D(x: 2.0, y: 4.0) +let combinedVector = vector + anotherVector +// combinedVector 是一个 Vector2D 实例，值为 (5.0, 5.0) +``` + + + +这个例子将向量 `(3.0, 1.0)` 和 `(2.0, 4.0)` 相加，得到向量 `(5.0, 5.0)` ，如下图所示。 + +![](vectorAddition) + +### 前缀和后缀运算符 + +上面的例子展示了自定义实现二元中缀运算符。 类和结构体还可以提供标准 **一元运算符** 的实现。 一元运算符操作单个目标。 如果它们在目标之前，则是 **前缀** 运算符（如 `-a` ），如果在目标之后，则是 **后缀** 运算符（如 `b!` ）。 + +通过在声明运算符方法时在 `func` 关键字之前写入 `prefix` 或 `postfix` 修饰符来实现前缀或后缀一元运算符： + +```swift +extension Vector2D { + static prefix func - (vector: Vector2D) -> Vector2D { + return Vector2D(x: -vector.x, y: -vector.y) + } +} +``` + + + +上面的例子为 `Vector2D` 实例实现了一元减运算符（ `-a` ）。 一元减运算符是一个前缀运算符，因此这个方法必须用 `prefix` 修饰符限定。 + +对于简单的数值，一元减运算符将正数转换为其负数等价物，反之亦然。 `Vector2D` 实例的相应实现对 `x` 和 `y` 属性执行此操作： + +```swift +let positive = Vector2D(x: 3.0, y: 4.0) +let negative = -positive +// negative 是一个 Vector2D 实例，值为 (-3.0, -4.0) +let alsoPositive = -negative +// alsoPositive 是一个 Vector2D 实例，值为 (3.0, 4.0) +``` + + + +### 复合赋值运算符 + +**复合赋值运算符** 将赋值（ `=` ）与另一个操作结合在一起。 例如，加法赋值运算符（ `+=` ）将加法和赋值组合成一个操作。 你将复合赋值运算符的左输入参数类型标记为 `inout` ，因为参数的值将直接从运算符方法内部修改。 + +下面的例子为 `Vector2D` 实例实现了一个加法赋值运算符方法： + +```swift +extension Vector2D { + static func += (left: inout Vector2D, right: Vector2D) { + left = left + right + } +} +``` + + + +因为之前定义了加法运算符，所以这里不需要重新实现加法过程。 相反，加法赋值运算符方法利用现有的加法运算符方法，并使用它将左值设置为左值加右值： + +```swift +var original = Vector2D(x: 1.0, y: 2.0) +let vectorToAdd = Vector2D(x: 3.0, y: 4.0) +original += vectorToAdd +// original 现在的值为 (4.0, 6.0) +``` + + + +> 注意：不能重载默认赋值运算符（ `=` ）。 只有复合赋值运算符可以被重载。 同样，三元条件运算符（ `a ? b : c` ）也不能被重载。 + + + +### 等价运算符 + +默认情况下，自定义类和结构体没有 **等价运算符** 的实现，这些运算符包括 **相等** 运算符（ `==` ）和 **不相等** 运算符（ `!=` ）。通常你需要实现 `==` 运算符，并使用 Swift 标准库的默认 `!=` 运算符实现，它会对 `==` 运算符的结果进行取反。 有两种方法可以实现 `==` 运算符：你可以自己实现它，或者对于许多类型，你可以要求 Swift 为你合成一个实现。 在这两种情况下，你都需要添加对 Swift 标准库的 `Equatable` 协议的遵循。 + +你可以像实现其他中缀运算符一样提供 `==` 运算符的实现： + +```swift +extension Vector2D: Equatable { + static func == (left: Vector2D, right: Vector2D) -> Bool { + return (left.x == right.x) && (left.y == right.y) + } +} +``` + + + +上面的例子实现了一个 `==` 运算符来检查两个 `Vector2D` 实例是否具有相等的值。 在 `Vector2D` 的上下文中，将"相等"理解为"两个实例具有相同的 `x` 值和 `y` 值"是有意义的，因此这就是运算符实现所使用的逻辑。 + +现在你可以使用这个运算符来检查两个 `Vector2D` 实例是否相等： + +```swift +let twoThree = Vector2D(x: 2.0, y: 3.0) +let anotherTwoThree = Vector2D(x: 2.0, y: 3.0) +if twoThree == anotherTwoThree { + print("These two vectors are equivalent.") +} +// Prints "These two vectors are equivalent." +``` + + + +在许多简单的情况下，你可以要求 Swift 为你提供等价运算符的合成实现，如 中所述。 + +## 自定义运算符 + +除了 Swift 提供的标准运算符外，你还可以声明和实现自己的 **自定义运算符**。有关可用于定义自定义运算符的字符列表，请参见 。 + +新运算符使用 `operator` 关键字在全局级别声明，并用 `prefix`、`infix` 或 `postfix` 修饰符标记： + +```swift +prefix operator +++ +``` + + + +上面的例子定义了一个名为 `+++` 的新前缀运算符。这个运算符在 Swift 中没有现有的含义，因此在下面特定的 `Vector2D` 实例工作环境中给予了它自己的自定义含义。出于本例的目的，`+++` 被视为一个新的"前缀加倍"运算符。它通过使用先前定义的加法赋值运算符将向量加到自身上，从而将 `Vector2D` 实例的 `x` 和 `y` 值加倍。要实现 `+++` 运算符，你需要向 `Vector2D` 添加一个名为 `+++` 的类型方法，如下所示： + +```swift +extension Vector2D { + static prefix func +++ (vector: inout Vector2D) -> Vector2D { + vector += vector + return vector + } +} + +var toBeDoubled = Vector2D(x: 1.0, y: 4.0) +let afterDoubling = +++toBeDoubled +// toBeDoubled 现在的值为 (2.0, 8.0) +// afterDoubling 值也为 (2.0, 8.0) +``` + + + +### 自定义中缀运算符的优先级 + +每个自定义中缀运算符都属于一个优先级组。优先级组指定了一个运算符相对于其他中缀运算符的优先级，以及运算符的结合性。有关这些特性如何影响中缀运算符与其他中缀运算符交互的解释，请参见 。 + +没有被明确放入优先级组的自定义中缀运算符会被赋予一个默认优先级组，其优先级紧高于三元条件运算符的优先级。 + +下面的例子定义了一个名为 `+-` 的新自定义中缀运算符，它属于 `AdditionPrecedence` 优先级组： + +```swift +infix operator +-: AdditionPrecedence +extension Vector2D { + static func +- (left: Vector2D, right: Vector2D) -> Vector2D { + return Vector2D(x: left.x + right.x, y: left.y - right.y) + } +} +let firstVector = Vector2D(x: 1.0, y: 2.0) +let secondVector = Vector2D(x: 3.0, y: 4.0) +let plusMinusVector = firstVector +- secondVector +// plusMinusVector 是一个 Vector2D 实例，其值为 (4.0, -2.0) +``` + + + +这个运算符将两个向量的 `x` 值相加，并从第一个向量的 `y` 值中减去第二个向量的 `y` 值。由于它本质上是一个"加法"运算符，因此它被赋予了与加法中缀运算符（如 `+` 和 `-`)相同的优先级组。有关 Swift 标准库提供的运算符的信息，包括运算符优先级组和结合性设置的完整列表，请参见 [Operator Declarations](https://developer.apple.com/documentation/swift/operator_declarations)。有关优先级组的更多信息以及定义自己的运算符和优先级组的语法，请参见 。 + +> 注意： 在定义前缀或后缀运算符时，你不需要指定优先级。但是，如果你对同一个操作数同时应用前缀和后缀运算符，后缀运算符会先被应用。 + + + +## 结果构建器 + +**结果构建器** 是你定义的一种类型，它为以自然、声明式的方式创建嵌套数据（如列表或树）添加语法。使用结果构建器的代码可以包含普通的 Swift 语法，如 `if` 和 `for` ，以处理条件或重复的数据片段。 + +下面的代码定义了几种类型，用于使用星号和文本在单行上绘图。 + +```swift +protocol Drawable { + func draw() -> String +} +struct Line: Drawable { + var elements: [Drawable] + func draw() -> String { + return elements.map { $0.draw() }.joined(separator: "") + } +} +struct Text: Drawable { + var content: String + init(_ content: String) { self.content = content } + func draw() -> String { return content } +} +struct Space: Drawable { + func draw() -> String { return " " } +} +struct Stars: Drawable { + var length: Int + func draw() -> String { return String(repeating: "*", count: length) } +} +struct AllCaps: Drawable { + var content: Drawable + func draw() -> String { return content.draw().uppercased() } +} +``` + + + +`Drawable` 协议定义了可以被绘制的东西（如线条或形状）的要求：该类型必须实现一个 `draw()` 方法。`Line` 结构表示单行绘图，它作为大多数绘图的顶层容器。要绘制一个 `Line` ，该结构调用线条的每个组件的 `draw()` 方法，然后将结果字符串连接成一个单一的字符串。`Text` 结构包装一个字符串，使其成为绘图的一部分。`AllCaps` 结构包装并修改另一个绘图，将绘图中的任何文本转换为大写。 + +通过调用这些类型的初始化器，可以创建一个绘图： + +```swift +let name: String? = "Ravi Patel" +let manualDrawing = Line(elements: [ + Stars(length: 3), + Text("Hello"), + Space(), + AllCaps(content: Text((name ?? "World") + "!")), + Stars(length: 2), +]) +print(manualDrawing.draw()) +// 打印 "***Hello RAVI PATEL!**" +``` + + + +这段代码是可行的，但有点笨拙。`AllCaps` 之后的深度嵌套括号很难阅读。当 `name` 为 `nil` 时使用 "World" 的后备逻辑必须使用 `??` 运算符内联完成，如果有更复杂的内容，这将变得很困难。如果你需要包含 switch 或 `for` 循环来构建部分绘图，就无法做到这一点。结果构建器让你可以重写这样的代码，使其看起来像普通的 Swift 代码。 + +要定义一个结果构建器，你需要在类型声明上写 `@resultBuilder` 属性。例如，这段代码定义了一个名为 `DrawingBuilder` 的结果构建器，它让你可以使用声明式语法来描述一个绘图： + +```swift +@resultBuilder +struct DrawingBuilder { + static func buildBlock(_ components: Drawable...) -> Drawable { + return Line(elements: components) + } + static func buildEither(first: Drawable) -> Drawable { + return first + } + static func buildEither(second: Drawable) -> Drawable { + return second + } +} +``` + + + +`DrawingBuilder` 结构定义了三个方法来实现结果构建器语法的部分功能。`buildBlock(_:)` 方法支持在代码块中编写一系列行。它将该块中的组件组合成一个 `Line` 。`buildEither(first:)` 和 `buildEither(second:)` 方法为 `if`-`else` 提供支持。 + +你可以将 `@DrawingBuilder` 属性应用于函数的参数，这会将传递给函数的闭包转换为结果构建器从该闭包创建的值。例如： + +```swift +func draw(@DrawingBuilder content: () -> Drawable) -> Drawable { + return content() +} +func caps(@DrawingBuilder content: () -> Drawable) -> Drawable { + return AllCaps(content: content()) +} + +func makeGreeting(for name: String? = nil) -> Drawable { + let greeting = draw { + Stars(length: 3) + Text("Hello") + Space() + caps { + if let name = name { + Text(name + "!") + } else { + Text("World!") + } + } + Stars(length: 2) + } + return greeting +} +let genericGreeting = makeGreeting() +print(genericGreeting.draw()) +// 打印 "***Hello WORLD!**" + +let personalGreeting = makeGreeting(for: "Ravi Patel") +print(personalGreeting.draw()) +// 打印 "***Hello RAVI PATEL!**" +``` + + + +`makeGreeting(for:)` 函数接受一个 `name` 参数并使用它来绘制个性化的问候语。`draw(_:)` 和 `caps(_:)` 函数都接受一个标记为 `@DrawingBuilder` 属性的单个闭包作为参数。当你调用这些函数时，你使用 `DrawingBuilder` 定义的特殊语法。Swift 将绘图的声明性描述转换为对 `DrawingBuilder` 方法的一系列调用，以构建作为函数参数传递的值。例如，Swift 将示例中对 `caps(_:)` 的调用转换为类似以下的代码： + +```swift +let capsDrawing = caps { + let partialDrawing: Drawable + if let name = name { + let text = Text(name + "!") + partialDrawing = DrawingBuilder.buildEither(first: text) + } else { + let text = Text("World!") + partialDrawing = DrawingBuilder.buildEither(second: text) + } + return partialDrawing +} +``` + + + +Swift 将 `if`-`else` 块转换为对 `buildEither(first:)` 和 `buildEither(second:)` 方法的调用。尽管你在自己的代码中不会调用这些方法，但展示转换的结果可以更容易地看到 Swift 在你使用 `DrawingBuilder` 语法时如何转换你的代码。 + +要为在特殊绘图语法中编写 `for` 循环添加支持，请添加一个 `buildArray(_:)` 方法。 + +```swift +extension DrawingBuilder { + static func buildArray(_ components: [Drawable]) -> Drawable { + return Line(elements: components) + } +} +let manyStars = draw { + Text("Stars:") + for length in 1...3 { + Space() + Stars(length: length) + } +} +``` + + + +在上面的代码中，`for` 循环创建了一个绘图数组，`buildArray(_:)` 方法将该数组转换为一个 `Line` 。 + +有关 Swift 如何将构建器语法转换为对构建器类型方法的调用的完整列表，请参阅 。 + + + + + + + + + + diff --git a/swift-6.docc/LanguageGuide/AutomaticReferenceCounting.md b/swift-6.docc/LanguageGuide/AutomaticReferenceCounting.md new file mode 100644 index 000000000..79dcbe261 --- /dev/null +++ b/swift-6.docc/LanguageGuide/AutomaticReferenceCounting.md @@ -0,0 +1,1129 @@ +# 自动引用计数 + +管理对象及其关系的生命周期。 + +Swift 使用 *自动引用计数* （ARC）来跟踪和管理应用的内存使用。在大多数情况下，这意味着 Swift 中的内存管理"自动运行"，你不需要自己考虑内存管理。当类实例不再需要时，ARC 会自动释放这些实例使用的内存。 + +然而，在少数情况下，ARC 需要更多关于代码各部分之间关系的信息，以便为你管理内存。本章描述了这些情况，并展示了如何使 ARC 管理应用的所有内存。Swift 中使用 ARC 的方法与[过渡到 ARC 发布说明](https://developer.apple.com/library/content/releasenotes/ObjectiveC/RN-TransitioningToARC/Introduction/Introduction.html)中描述的在 Objective-C 中使用 ARC 的方法非常相似。 + +引用计数只适用于类的实例。结构体和枚举是值类型，不是引用类型，它们不是通过引用存储和传递的。 + +## ARC 如何工作 + +每次创建一个类的新实例时，ARC 都会分配一块内存来存储该实例的信息。这块内存保存了实例的类型信息，以及与该实例相关的所有存储属性的值。 + +此外，当一个实例不再需要时，ARC 会释放该实例使用的内存，以便内存可以用于其他目的。这确保了类实例在不再需要时不会占用内存空间。 + +然而，如果 ARC 在一个实例仍在使用时就将其释放，那么将无法再访问该实例的属性或调用该实例的方法。实际上，如果你试图访问该实例，你的应用很可能会崩溃。 + +为了确保实例在仍然需要时不会消失，ARC 会跟踪当前有多少属性、常量和变量正在引用每个类实例。只要至少还存在一个对该实例的活动引用，ARC 就不会释放该实例。 + +为了实现这一点，每当你将类实例分配给属性、常量或变量时，该属性、常量或变量就会对该实例进行 *强引用* 。之所以称之为"强"引用，是因为它牢牢地持有该实例，只要该强引用存在，就不允许释放该实例。 + +## ARC 的实际应用 + +下面是一个自动引用计数如何工作的例子。这个例子从一个简单的 `Person` 类开始，该类定义了一个名为 `name` 的存储常量属性: + +```swift +class Person { + let name: String + init(name: String) { + self.name = name + print("\(name) is being initialized") + } + deinit { + print("\(name) is being deinitialized") + } +} +``` + + + +`Person` 类有一个初始化器，用于设置实例的 `name` 属性并打印一条消息，表示初始化正在进行。`Person` 类还有一个析构器，当类的实例被释放时打印一条消息。 + +下面的代码片段定义了三个 `Person?` 类型的变量，这些变量在后续的代码片段中用于设置对新的 `Person` 实例的多个引用。因为这些变量是可选类型(`Person?`，而不是 `Person`)，它们会自动初始化为 `nil` 值，目前并不引用 `Person` 实例。 + +```swift +var reference1: Person? +var reference2: Person? +var reference3: Person? +``` + + + +现在你可以创建一个新的 `Person` 实例并将其分配给这三个变量之一: + +```swift +reference1 = Person(name: "John Appleseed") +// Prints "John Appleseed is being initialized" +``` + + + +注意，当你调用 `Person` 类的初始化器时，会打印消息 `"John Appleseed is being initialized"`。这确认了初始化已经发生。 + +因为新的 `Person` 实例已被分配给 `reference1` 变量，所以现在从 `reference1` 到新的 `Person` 实例有了一个强引用。因为至少存在一个强引用，ARC 会确保这个 `Person` 被保留在内存中而不被释放。 + +如果你将同一个 `Person` 实例分配给另外两个变量，就会建立两个对该实例的额外强引用: + +```swift +reference2 = reference1 +reference3 = reference1 +``` + + + +现在这个单一的 `Person` 实例有 *三个* 强引用。 + +如果你通过将 `nil` 分配给两个变量来打破这些强引用中的两个（包括原始引用），一个强引用仍然存在，`Person` 实例不会被释放: + +```swift +reference1 = nil +reference2 = nil +``` + + + +ARC 不会释放 `Person` 实例，直到第三个也是最后一个强引用被打破，这时很明显你不再使用 `Person` 实例: + +```swift +reference3 = nil +// Prints "John Appleseed is being deinitialized" +``` + + + +## 类实例之间的强引用循环 + +在上面的例子中，ARC 能够跟踪你创建的新 `Person` 实例的引用数量，并在不再需要这个 `Person` 实例时将其释放。 + +然而，可能会出现这样的情况：一个类的实例 *永远* 不会到达它有零个强引用的时刻。如果两个类实例彼此持有强引用，使得每个实例都使对方保持活跃，就可能发生这种情况。这被称为 *强引用循环* 。 + +你可以通过将类之间的某些关系定义为弱引用或无主引用，而不是强引用，来解决强引用循环。这个过程在中描述。然而，在学习如何解决强引用循环之前，了解这种循环是如何产生的是很有用的。 + +这里有一个例子，展示了如何意外创建强引用循环。这个例子定义了两个名为 `Person` 和 `Apartment` 的类，用于模拟一个公寓楼及其居民: + +```swift +class Person { + let name: String + init(name: String) { self.name = name } + var apartment: Apartment? + deinit { print("\(name) is being deinitialized") } +} + +class Apartment { + let unit: String + init(unit: String) { self.unit = unit } + var tenant: Person? + deinit { print("Apartment \(unit) is being deinitialized") } +} +``` + + + +每个 `Person` 实例都有一个 `String` 类型的 `name` 属性和一个初始为 `nil` 的可选 `apartment` 属性。`apartment` 属性是可选的，因为一个人可能并不总是有公寓。 + +同样，每个 `Apartment` 实例都有一个 `String` 类型的 `unit` 属性和一个初始为 `nil` 的可选 `tenant` 属性。租户属性是可选的，因为一个公寓可能并不总是有租户。 + +这两个类还定义了一个析构器，它打印一条消息，表明该类的一个实例正在被释放。这使你能够看到 `Person` 和 `Apartment` 实例是否如预期那样被释放。 + +下一个代码片段定义了两个可选类型的变量 `john` 和 `unit4A`，它们将在下面被设置为特定的 `Apartment` 和 `Person` 实例。由于是可选类型，这两个变量的初始值都是 `nil`: + +```swift +var john: Person? +var unit4A: Apartment? +``` + + + +现在你可以创建特定的 `Person` 实例和 `Apartment` 实例，并将这些新实例分配给 `john` 和 `unit4A` 变量: + +```swift +john = Person(name: "John Appleseed") +unit4A = Apartment(unit: "4A") +``` + + + +以下是创建并分配这两个实例后强引用的样子。`john` 变量现在对新的 `Person` 实例有一个强引用，而 `unit4A` 变量对新的 `Apartment` 实例有一个强引用: + +![](referenceCycle01) + +现在你可以将这两个实例链接在一起，使这个人有一个公寓，而这个公寓有一个租户。注意使用感叹号（ `!` ）来解包和访问存储在 `john` 和 `unit4A` 可选变量中的实例，以便可以设置这些实例的属性: + +```swift +john!.apartment = unit4A +unit4A!.tenant = john +``` + + + +以下是你将两个实例链接在一起后强引用的样子: + +![](referenceCycle02) + +不幸的是，链接这两个实例在它们之间创建了一个强引用循环。`Person` 实例现在对 `Apartment` 实例有一个强引用，而 `Apartment` 实例对 `Person` 实例有一个强引用。因此，当你打破 `john` 和 `unit4A` 变量持有的强引用时，引用计数不会降到零，实例不会被 ARC 释放: + +```swift +john = nil +unit4A = nil +``` + + + +注意，当你将这两个变量设置为 `nil` 时，两个析构器都没有被调用。强引用循环阻止了 `Person` 和 `Apartment` 实例被释放，导致你的应用出现内存泄漏。 + +以下是你将 `john` 和 `unit4A` 变量设置为 `nil` 后强引用的样子: + +![](referenceCycle03) + +`Person` 实例和 `Apartment` 实例之间的强引用仍然存在，无法被打破。 + +## 解决类实例之间的强引用循环 + +Swift 提供了两种方法来解决你使用类类型的属性时出现的强引用循环：弱引用和无主引用。 + +弱引用和无主引用使引用循环中的一个实例引用另一个实例而 *不* 对其保持强引用。这样，实例就可以相互引用而不创建强引用循环。 + +当另一个实例可能先被释放时————或者说，生命周期更短时，使用弱引用。在上面的 `Apartment` 例子中，公寓在其生命周期的某个时刻可能没有租户，所以在这种情况下，弱引用是打破引用循环的适当方式。相比之下，当另一个实例具有相同或更长的生命周期时，使用无主引用。 + + + +### 弱引用 + + *弱引用* 是一种不会对其引用的实例保持强持有的引用，因此不会阻止 ARC 处置被引用的实例。这种行为可以防止引用成为强引用循环的一部分。你可以通过在属性或变量声明前放置 `weak` 关键字来表示一个弱引用。 + +因为弱引用不会对其引用的实例保持强持有，所以有可能在弱引用仍然引用该实例时，该实例被释放。因此，当弱引用所引用的实例被释放时，ARC 会自动将弱引用设置为 `nil`。而且，由于弱引用需要允许它们的值在运行时被更改为 `nil`，它们总是被声明为可选类型的变量，而不是常量。 + +你可以像检查任何其他可选值一样检查弱引用中值的存在，并且你永远不会遇到引用已不存在的无效实例的情况。 + +> 注意：当 ARC 将弱引用设置为 `nil` 时，不会调用属性观察器。 + + + +下面的例子与上面的 `Person` 和 `Apartment` 例子相同，只有一个重要的区别。这次，`Apartment` 类型的 `tenant` 属性被声明为弱引用: + +```swift +class Person { + let name: String + init(name: String) { self.name = name } + var apartment: Apartment? + deinit { print("\(name) is being deinitialized") } +} + +class Apartment { + let unit: String + init(unit: String) { self.unit = unit } + weak var tenant: Person? + deinit { print("Apartment \(unit) is being deinitialized") } +} +``` + + + +两个变量（ `john` 和 `unit4A` ）的强引用以及两个实例之间的链接像之前一样被创建: + +```swift +var john: Person? +var unit4A: Apartment? + +john = Person(name: "John Appleseed") +unit4A = Apartment(unit: "4A") + +john!.apartment = unit4A +unit4A!.tenant = john +``` + + + +现在你已经将两个实例链接在一起，引用看起来是这样的: + +![](weakReference01) + +`Person` 实例仍然对 `Apartment` 实例有一个强引用，但 `Apartment` 实例现在对 `Person` 实例有一个 *弱* 引用。这意味着当你通过将 `john` 变量设置为 `nil` 来打破它所持有的强引用时，不再有对 `Person` 实例的强引用: + +```swift +john = nil +// Prints "John Appleseed is being deinitialized" +``` + + + +因为不再有对 `Person` 实例的强引用，它被释放，`tenant` 属性被设置为 `nil`: + +![](weakReference02) + +唯一剩下的对 `Apartment` 实例的强引用来自 `unit4A` 变量。如果你打破 *那个* 强引用，就不再有对 `Apartment` 实例的强引用: + +```swift +unit4A = nil +// Prints "Apartment 4A is being deinitialized" +``` + + + +因为不再有对 `Apartment` 实例的强引用，它也被释放: + +![](weakReference03) + +> 注意：在使用垃圾回收的系统中，弱指针有时被用来实现简单的缓存机制，因为没有强引用的对象只有在内存压力触发垃圾收集时才会被释放。然而，对于 ARC，值会在它们的最后一个强引用被移除时立即被释放，使得弱引用不适合这种目的。 + +### 无主引用 + +像弱引用一样， *无主引用* 不会对它引用的实例保持强持有。然而，与弱引用不同，无主引用是在另一个实例具有相同或更长的生命周期时使用的。你通过在属性或变量声明前放置 `unowned` 关键字来表示一个无主引用。 + +与弱引用不同，无主引用总是被期望有一个值。因此，将一个值标记为无主不会使它成为可选的，ARC 也永远不会将无主引用的值设置为 `nil`。 + + + +> 重要：只有当你确定该引用 *总是* 指向一个尚未被释放的实例时，才使用无主引用。 +> +> 如果你在该实例被释放后尝试访问无主引用的值，你会得到一个运行时错误。 + + + +下面的例子定义了两个类，`Customer` 和 `CreditCard`，它们模拟了一个银行客户和该客户可能拥有的信用卡。这两个类各自将另一个类的实例存储为属性。这种关系有可能创建一个强引用循环。 + +`Customer` 和 `CreditCard` 之间的关系与上面弱引用例子中看到的 `Apartment` 和 `Person` 之间的关系略有不同。在这个数据模型中，一个客户可能有也可能没有信用卡，但一张信用卡将 *总是* 与一个客户相关联。`CreditCard` 实例永远不会比它引用的 `Customer` 存活得更久。为了表示这一点，`Customer` 类有一个可选的 `card` 属性，但 `CreditCard` 类有一个无主的(且非可选的) `customer` 属性。 + +此外，只能通过将 `number` 值和 `customer` 实例传递给自定义 `CreditCard` 初始化器来创建新的 `CreditCard` 实例。这确保了当 `CreditCard` 实例被创建时，它总是有一个与之关联的 `customer` 实例。 + +因为信用卡总是会有一个客户，你将其 `customer` 属性定义为无主引用，以避免强引用循环: + +```swift +class Customer { + let name: String + var card: CreditCard? + init(name: String) { + self.name = name + } + deinit { print("\(name) is being deinitialized") } +} + +class CreditCard { + let number: UInt64 + unowned let customer: Customer + init(number: UInt64, customer: Customer) { + self.number = number + self.customer = customer + } + deinit { print("Card #\(number) is being deinitialized") } +} +``` + + + +> 注意:`CreditCard` 类的 `number` 属性被定义为 `UInt64` 类型而不是 `Int`，以确保 `number` 属性的容量足够大，可以在 32 位和 64 位系统上存储 16 位卡号。 + +下一个代码片段定义了一个名为 `john` 的可选 `Customer` 变量，它将用于存储对特定客户的引用。由于是可选的，这个变量的初始值为 nil: + +```swift +var john: Customer? +``` + + + +现在你可以创建一个 `Customer` 实例，并用它来初始化和分配一个新的 `CreditCard` 实例作为该客户的 `card` 属性: + +```swift +john = Customer(name: "John Appleseed") +john!.card = CreditCard(number: 1234_5678_9012_3456, customer: john!) +``` + + + +现在你已经链接了两个实例，引用看起来是这样的: + +![](unownedReference01) + +`Customer` 实例现在对 `CreditCard` 实例有一个强引用，而 `CreditCard` 实例对 `Customer` 实例有一个无主引用。 + +由于无主的 `customer` 引用，当你打破 `john` 变量持有的强引用时，不再有对 `Customer` 实例的强引用: + +![](unownedReference02) + +因为不再有对 `Customer` 实例的强引用，它被释放。在这之后，不再有对 `CreditCard` 实例的强引用，它也被释放: + +```swift +john = nil +// Prints "John Appleseed is being deinitialized" +// Prints "Card #1234567890123456 is being deinitialized" +``` + + + +上面的最后一个代码片段显示，在 `john` 变量被设置为 `nil` 后，`Customer` 实例和 `CreditCard` 实例的析构器都打印了它们的 "deinitialized" 消息。 + +> 注意：上面的例子展示了如何使用 *安全* 的无主引用。Swift 还提供了 *不安全* 的无主引用，用于你需要禁用运行时安全检查的情况 --- 例如，出于性能原因。与所有不安全操作一样，你承担了检查代码安全性的责任。 +> +> 你通过写 `unowned(unsafe)` 来表示一个不安全的无主引用。如果你在实例被释放后尝试访问不安全的无主引用，你的程序将尝试访问该实例曾经所在的内存位置，这是一个不安全的操作。 + + + +### 无主可选引用 + +你可以将对类的可选引用标记为无主的。在 ARC 所有权模型中，无主可选引用和弱引用可以在相同的上下文中使用。 + +不同之处在于，当你使用无主可选引用时，你有责任确保它始终引用一个有效的对象或被设置为 `nil`。 + +这里有一个例子，用于跟踪学校某个特定系的提供的课程: + +```swift +class Department { + var name: String + var courses: [Course] + init(name: String) { + self.name = name + self.courses = [] + } +} + +class Course { + var name: String + unowned var department: Department + unowned var nextCourse: Course? + init(name: String, in department: Department) { + self.name = name + self.department = department + self.nextCourse = nil + } +} +``` + + + +`Department` 对它提供的每门课程保持强引用。在 ARC 所有权模型中，一个系拥有它的课程。`Course` 有两个无主引用，一个指向系，另一个指向学生应该学习的下一门课程; 一门课程不拥有这两个对象中的任何一个。每门课程都是某个系的一部分，所以 `department` 属性不是可选的。然而，因为有些课程没有推荐的后续课程，`nextCourse` 属性是可选的。 + +这里是使用这些类的一个例子: + +```swift +let department = Department(name: "Horticulture") + +let intro = Course(name: "Survey of Plants", in: department) +let intermediate = Course(name: "Growing Common Herbs", in: department) +let advanced = Course(name: "Caring for Tropical Plants", in: department) + +intro.nextCourse = intermediate +intermediate.nextCourse = advanced +department.courses = [intro, intermediate, advanced] +``` + + + +上面的代码创建了一个系和它的三门课程。入门和中级课程都在它们的 `nextCourse` 属性中 +存储了一个建议的下一门课程，这个属性维护了一个无主可选引用，指向学生完成这门课程后应该学习的课程。 + +![](unownedOptionalReference) + +无主可选引用不会对它包装的类实例保持强持有，因此它不会阻止 ARC 释放该实例。它在 ARC 下的行为与无主引用相同，除了无主可选引用可以是 `nil`。 + +像非可选的无主引用一样，你有责任确保 `nextCourse` 始终引用一个尚未被释放的课程。在这个例子中，当你从 `department.courses` 中删除一门课程时，你还需要移除其他课程可能对它的任何引用。 + +> 注意：可选值的底层类型是 `Optional`，它是 Swift 标准库中的一个枚举。然而，可选类型是值类型不能被标记为 `unowned` 这个规则的一个例外。 +> +> 包装类的可选类型不使用引用计数，所以你不需要对可选类型保持强引用。 + + + +### 无主引用和隐式解包可选属性 + +上面的弱引用和无主引用的例子涵盖了两种更常见的场景，在这些场景中需要打破强引用循环。 + +`Person` 和 `Apartment` 的例子展示了一种情况，其中两个属性都允许为 `nil`，有可能造成强引用循环。这种情况最好用弱引用来解决。 + +`Customer` 和 `CreditCard` 的例子展示了一种情况，其中一个属性允许为 `nil`，而另一个属性不能为 `nil`，有可能造成强引用循环。这种情况最好用无主引用来解决。 + +然而，还有第三种情况，其中 *两个* 属性都应该始终有值，一旦初始化完成，两个属性都不应该为 `nil`。在这种情况下，将一个类上的无主属性与另一个类上的隐式解包可选属性结合使用是很有用的。 + +这使得两个属性在初始化完成后都可以直接访问（无需可选解包），同时仍然避免了引用循环。本节将向你展示如何设置这样的关系。 + +下面的例子定义了两个类，`Country` 和 `City`，每个类都将另一个类的实例存储为属性。在这个数据模型中，每个国家必须始终有一个首都，每个城市必须始终属于一个国家。为了表示这一点，`Country` 类有一个 `capitalCity` 属性，`City` 类有一个 `country` 属性: + +```swift +class Country { + let name: String + var capitalCity: City! + init(name: String, capitalName: String) { + self.name = name + self.capitalCity = City(name: capitalName, country: self) + } +} + +class City { + let name: String + unowned let country: Country + init(name: String, country: Country) { + self.name = name + self.country = country + } +} +``` + + + +为了建立两个类之间的相互依赖关系，`City` 的初始化器接受一个 `Country` 实例，并将这个实例存储在其 `country` 属性中。 + +`City` 的初始化器是在 `Country` 的初始化器内部调用的。然而，`Country` 的初始化器在新的 `Country` 实例完全初始化之前，不能将 `self` 传递给 `City` 初始化器，如中所述。 + +为了应对这个要求，你将 `Country` 的 `capitalCity` 属性声明为隐式解包可选属性，通过在其类型注解末尾加上感叹号(`City!`)来表示。这意味着 `capitalCity` 属性有一个默认值 `nil`，像任何其他可选类型一样，但可以在不需要解包其值的情况下访问，如中所述。 + +因为 `capitalCity` 有一个默认的 `nil` 值，一旦 `Country` 实例在其初始化器中设置了 `name` 属性，新的 `Country` 实例就被认为是完全初始化的。这意味着 `Country` 初始化器可以在 `name` 属性被设置后，立即开始引用和传递隐式的 `self` 属性。因此，当 `Country` 初始化器设置自己的 `capitalCity` 属性时，`Country` 初始化器可以将 `self` 作为参数之一传递给 `City` 初始化器。 + +所有这些意味着你可以在一个语句中创建 `Country` 和 `City` 实例，而不会创建强引用循环，并且可以直接访问 `capitalCity` 属性，而不需要使用感叹号来解包其可选值: + +```swift +var country = Country(name: "Canada", capitalName: "Ottawa") +print("\(country.name)'s capital city is called \(country.capitalCity.name)") +// Prints "Canada's capital city is called Ottawa" +``` + + + +在上面的例子中，使用隐式解包可选类型意味着满足了所有两阶段类初始化器的要求。一旦初始化完成，`capitalCity` 属性就可以像非可选值一样使用和访问，同时仍然避免了强引用循环。 + +## 闭包的强引用循环 + +你在上面看到了当两个类实例属性相互保持强引用时如何创建强引用循环。你还看到了如何使用弱引用和无主引用来打破这些强引用循环。 + +如果你将一个闭包分配给类实例的一个属性，并且该闭包的主体捕获了该实例，也可能发生强引用循环。这种捕获可能发生是因为闭包的主体访问了该实例的一个属性，例如 `self.someProperty`，或者因为闭包在该实例上调用了一个方法，例如 `self.someMethod()`。无论哪种情况，这些访问都导致闭包"捕获"`self`，创建了一个强引用循环。 + +这个强引用循环发生是因为闭包像类一样，都是 *引用类型* 。你将一个闭包赋值给属性时，实际上是在赋值该闭包的 *引用* 。本质上，这个问题与之前的情况相同 --- 两个强引用相互保持对方存活。不同的是，这次不是两个类实例，而是一个类实例和一个闭包在相互保持对方存活。 + +Swift 为这个问题提供了一个优雅的解决方案，称为 *闭包捕获列表* 。然而，在你学习如何用闭包捕获列表打破强引用循环之前，先了解一下如何造成这样的循环是很有用的。 + +下面的例子展示了当使用引用 `self` 的闭包时，如何创建强引用循环。这个例子定义了一个名为 `HTMLElement` 的类，它为 HTML 文档中的单个元素提供了一个简单的模型: + +```swift +class HTMLElement { + + let name: String + let text: String? + + lazy var asHTML: () -> String = { + if let text = self.text { + return "<\(self.name)>\(text)" + } else { + return "<\(self.name) />" + } + } + + init(name: String, text: String? = nil) { + self.name = name + self.text = text + } + + deinit { + print("\(name) is being deinitialized") + } + +} +``` + + + +`HTMLElement` 类定义了一个 `name` 属性，表示元素的名称，例如用于标题元素的 `"h1"`，用于段落元素的 `"p"`，或用于换行元素的 `"br"`。`HTMLElement` 还定义了一个可选的 `text` 属性，你可以将其设置为一个字符串，该字符串表示要在该 HTML 元素内渲染的文本。 + +除了这两个简单的属性，`HTMLElement` 类还定义了一个名为 `asHTML` 的延迟属性。这个属性引用了一个闭包，该闭包将 `name` 和 `text` 组合成一个 HTML 字符串片段。`asHTML` 属性的类型是 `() -> String`，或者说"一个不接受参数并返回 `String` 值的函数"。 + +默认情况下，`asHTML` 属性被分配了一个返回 HTML 标签字符串表示的闭包。如果 `text` 值存在，这个标签包含可选的 `text` 值，如果 `text` 不存在，则不包含文本内容。对于段落元素，闭包会返回 `"

some text

"` 或 `"

"`，这取决于 `text` 属性是等于 `"some text"` 还是 `nil`。 + +`asHTML` 属性的命名和使用有点像实例方法。然而，因为 `asHTML` 是一个闭包属性而不是实例方法，如果你想为特定的 HTML 元素更改 HTML 渲染，你可以用自定义闭包替换 `asHTML` 属性的默认值。 + +例如，可以将 `asHTML` 属性设置为一个闭包，如果 `text` 属性为 `nil`，则默认返回一些文本，以防止表示返回一个空的 HTML 标签: + +```swift +let heading = HTMLElement(name: "h1") +let defaultText = "some default text" +heading.asHTML = { + return "<\(heading.name)>\(heading.text ?? defaultText)" +} +print(heading.asHTML()) +// Prints "

some default text

" +``` + + + +> 注意:`asHTML` 属性被声明为惰性属性，因为只有在元素实际需要被渲染为某个 HTML 输出目标的字符串值时才需要它。`asHTML` 是惰性属性这一事实意味着你可以在默认闭包中引用 `self`，因为直到初始化完成且确知 `self` 存在之前，惰性属性都不会被访问。 + +`HTMLElement` 类提供了一个单一的初始化器，它接受一个 `name` 参数和一个可选的 `text` 参数来初始化一个新元素。该类还定义了一个析构器，用于在 `HTMLElement` 实例被释放时打印一条消息。 + +下面是如何使用 `HTMLElement` 类来创建和打印一个新实例: + +```swift +var paragraph: HTMLElement? = HTMLElement(name: "p", text: "hello, world") +print(paragraph!.asHTML()) +// Prints "

hello, world

" +``` + + + +> 注意：上面的 `paragraph` 变量被定义为一个可选的 `HTMLElement`，这样它就可以在下面被设置为 `nil` 以演示存在强引用循环。 + +不幸的是，上面编写的 `HTMLElement` 类在 `HTMLElement` 实例和用于其默认 `asHTML` 值的闭包之间创建了一个强引用循环。循环看起来是这样的: + +![](closureReferenceCycle01) + +实例的 `asHTML` 属性持有对其闭包的强引用。然而，因为闭包在其主体内引用了 `self`(作为引用 `self.name` 和 `self.text` 的方式)，闭包 *捕获* 了 self，这意味着它持有对 `HTMLElement` 实例的强引用。两者之间创建了一个强引用循环。(关于在闭包中捕获值的更多信息，请参见。) + +> 注意：即使闭包多次引用 `self`，它也只捕获对 `HTMLElement` 实例的一个强引用。 + +如果你将 `paragraph` 变量设置为 `nil` 并打破其对 `HTMLElement` 实例的强引用，强引用循环会阻止 `HTMLElement` 实例及其闭包的释放: + +```swift +paragraph = nil +``` + + + +注意 `HTMLElement` 析构器中的消息没有被打印，这表明 `HTMLElement` 实例没有被释放。 + +## 解决闭包的强引用循环 + +你可以通过在闭包的定义中定义一个 *捕获列表* 来解决闭包和类实例之间的强引用循环。捕获列表定义了在闭包体内捕获一个或多个引用类型时要使用的规则。就像两个类实例之间的强引用循环一样，你声明每个被捕获的引用为弱引用或无主引用，而不是强引用。选择弱引用还是无主引用取决于代码不同部分之间的关系。 + +> 注意:Swift 要求你在闭包内引用 `self` 的成员时写成 `self.someProperty` 或 `self.someMethod()`(而不仅仅是 `someProperty` 或 `someMethod()`)。这有助于提醒你可能会意外捕获 `self`。 + +### 定义捕获列表 + +捕获列表中的每个项目都是 `weak` 或 `unowned` 关键字与对类实例的引用（如 `self` ）或初始化为某个值的变量（如 `delegate = self.delegate` ）的配对。这些配对写在一对方括号内，用逗号分隔。 + +如果提供了闭包的参数列表和返回类型，则将捕获列表放在它们之前: + +```swift +lazy var someClosure = { + [unowned self, weak delegate = self.delegate] + (index: Int, stringToProcess: String) -> String in + // closure body goes here +} +``` + + + +如果闭包没有指定参数列表或返回类型，因为它们可以从上下文推断出来，则将捕获列表放在闭包的最开始，后面跟着 `in` 关键字: + +```swift +lazy var someClosure = { + [unowned self, weak delegate = self.delegate] in + // closure body goes here +} +``` + + + +### 弱引用和无主引用 + +当闭包和它捕获的实例总是相互引用，并且总是同时被释放时，将闭包中的捕获定义为无主引用。 + +相反，当被捕获的引用在将来的某个时刻可能变为 `nil` 时，将捕获定义为弱引用。弱引用总是可选类型，并且当它们引用的实例被释放时自动变为 `nil`。这使你能够在闭包体内检查它们是否存在。 + + + +> 注意：如果被捕获的引用永远不会变为 `nil`，它应该始终被捕获为无主引用，而不是弱引用。 + +无主引用是解决 中 `HTMLElement` 示例强引用循环的适当捕获方法。以下是如何编写 `HTMLElement` 类以避免循环: + +```swift +class HTMLElement { + + let name: String + let text: String? + + lazy var asHTML: () -> String = { + [unowned self] in + if let text = self.text { + return "<\(self.name)>\(text)" + } else { + return "<\(self.name) />" + } + } + + init(name: String, text: String? = nil) { + self.name = name + self.text = text + } + + deinit { + print("\(name) is being deinitialized") + } + +} +``` + + + +这个 `HTMLElement` 的实现与之前的实现完全相同，除了在 `asHTML` 闭包内添加了一个捕获列表。在这种情况下，捕获列表是 `[unowned self]`，这意味着“将 self 捕获为无主引用而不是强引用”。 + +你可以像之前一样创建和打印 `HTMLElement` 实例: + +```swift +var paragraph: HTMLElement? = HTMLElement(name: "p", text: "hello, world") +print(paragraph!.asHTML()) +// Prints "

hello, world

" +``` + + + +这是有捕获列表时引用的样子: + +![](closureReferenceCycle02) + +这次，闭包对 `self` 的捕获是一个无主引用，并且不会对它捕获的 `HTMLElement` 实例保持强引用。如果你将 `paragraph` 变量的强引用设置为 `nil`，`HTMLElement` 实例就会被释放，从下面示例中打印的析构器消息可以看出这一点: + +```swift +paragraph = nil +// Prints "p is being deinitialized" +``` + + + +关于捕获列表的更多信息，请参见。 + + diff --git a/swift-6.docc/LanguageGuide/BasicOperators.md b/swift-6.docc/LanguageGuide/BasicOperators.md new file mode 100644 index 000000000..672835e1b --- /dev/null +++ b/swift-6.docc/LanguageGuide/BasicOperators.md @@ -0,0 +1,1026 @@ +# 基本运算符 + +执行赋值、算术和比较等操作。 + +*运算符*是一种特殊的符号或短语，用于检查、更改或组合值。例如，加法运算符（`+`）将两个数字相加，如 `let i = 1 + 2`，逻辑与运算符（`&&`）组合两个布尔值，如 `if enteredDoorCode && passedRetinaScan`。 + +Swift 支持类似 C 等你已所熟知的语言中的运算符，并改进了几个功能，以消除常见的编码错误。赋值运算符（`=`）不返回值，以防止它被误用时等于运算符（`==`）被意外使用。算术运算符（`+`、`-`、`*`、`/`、`%` 等）检测并禁止值溢出，以避免在处理超出存储它们的类型允许值范围的较大或较小数字时出现意外结果。你可以选择使用 Swift 的溢出运算符来处理值溢出行为，如 中所述。 + +Swift 还提供了 C 语言中没有的区间运算符，如 `a.. 涵盖了 Swift 的高级运算符，并描述了如何定义自定义运算符以及为自定义类型实现标准运算符。 + +## 术语 + +运算符可以是一元、二元或三元： + +- *一元*运算符作用于单个目标（如 `-a`）。一元*前置*运算符紧跟在其目标之前（如 `!b`），一元*后置*运算符紧跟在其目标之后（如 `c!`）。 +- *二元*运算符作用于两个目标（如 `2 + 3`），是*中置*的，因为它们出现在两个目标之间。 +- *三元*运算符作用于三个目标。与 C 一样，Swift 只有一个三元运算符，即三元条件运算符（`a ? b : c`）。 + +运算符影响的值称为*操作数*。在表达式 `1 + 2` 中，`+` 符号是一个中置运算符，它的两个操作数是值 `1` 和 `2`。 + +## 赋值运算符 + +*赋值运算符*（`a = b`）用 `b` 的值初始化或更新 `a` 的值： + +```swift +let b = 10 +var a = 5 +a = b +// a 现在等于 10 +``` + + + +如果赋值运算符的右侧是一个包含多个值的元组，可以将其元素同时分解为多个常量或变量： + +```swift +let (x, y) = (1, 2) // x 等于 1, y 等于 2 +``` + + + + + + + +与 C 和 Objective-C 中的赋值运算符不同，Swift 中的赋值运算符本身不返回值。以下语句无效： + +```swift +if x = y { // 这是无效的，因为 x = y 不返回值。 +} +``` + + + +因为 Swift 语言规定 `if x = y` 这种写法是无效的，这个特性可以防止不小心使用赋值运算符（=） 而非等于运算符（==）。Swift 帮助你避免代码中出现这种错误。 + + + +## 算术运算符 + +Swift 为所有数值类型支持四种标准*算术运算符*： + +- 加法（`+`） +- 减法（`-`） +- 乘法（`*`） +- 除法（`/`） + +```swift +1 + 2 // 等于 3 +5 - 3 // 等于 2 +2 * 3 // 等于 6 +10.0 / 2.5 // 等于 4.0 +``` + + + +Swift 的算术运算符与 C 和 Objective-C 中的不同，默认情况下不允许值溢出。您可以选择使用 Swift 的溢出运算符（如 `a &+ b`）来启用值溢出行为。请参阅 。 + +加法运算符也支持 `String` 拼接： + +```swift +"hello, " + "world" // 等于 "hello, world" +``` + + + +### 余数运算符 + +*余数运算符*（`a % b`）计算出 `b` 在 `a` 中能容纳多少个倍数，并返回剩余的值（称为*余数*）。 + +> 注意: 需要注意的是，尽管余数运算符在其他语言中也被称为模运算符， +> 但在 Swift 中对负数的处理与模运算符有所不同。 + + + +让我们来看看余数运算符是如何工作的。 +要计算 `9 % 4`，首先要确定 `9` 中可以包含多少个 `4`： + +![](remainderInteger) + +我们可以在 `9` 中容纳两个 `4`，剩余的是 `1`（用橙色表示）。 + +在 Swift 中，这可以写作： + +```swift +9 % 4 // 等于 1 +``` + + + +为了确定 `a % b` 的答案，`%` 运算符计算以下等式并返回 `余数` 作为输出： + +`a` = (`b` x `某个乘数`) + `余数` + +其中 `某个乘数` 是 `b` 在 `a` 中能容纳的最大倍数。 + +将 `9` 和 `4` 代入此等式，得： + +`9` = (`4` x `2`) + `1` + +当计算 `a` 为负值时，采用相同的方法： + +```swift +-9 % 4 // 等于 -1 +``` + + + +将 `-9` 和 `4` 代入等式，得： + +`-9` = (`4` x `-2`) + `-1` + +因此余数值为 `-1`。 + +对于 b 为负值的情况，其符号将被忽略。这意味着 `a % b` 和 `a % -b` 总是给出相同的答案。 + +### 一元负号运算符 + +数值的正负号可以使用前缀 `-` 切换，称为*一元负号运算符*： + +```swift +let three = 3 +let minusThree = -three // minusThree 等于 -3 +let plusThree = -minusThree // plusThree 等于 3，或 "负负三" +``` + + + +一元负号运算符（`-`）直接加在它所作用的值前面，中间没有任何空格。 + +### 一元正号运算符 + +*一元正号运算符*（`+`）只是返回它所作用的值，不做任何改变： + +```swift +let minusSix = -6 +let alsoMinusSix = +minusSix // alsoMinusSix 等于 -6 +``` + + + +虽然一元加运算符实际上什么也没做，但是当你使用一元减运算符表示负数时，你可以使用它来使你的代码对正数也保持对称性。 + +## 复合赋值运算符 + +与 C 语言一样，Swift 提供了*复合赋值运算符*，它将赋值（`=`）与另一个操作结合起来。 +一个例子是*加法赋值运算符*（`+=`）： + +```swift +var a = 1 +a += 2 +// a 现在等于 3 +``` + + + +表达式 `a += 2` 是 `a = a + 2` 的简写。 +实际上，加法和赋值被合并成一个同时执行这两个任务的运算符。 + +> 注意: 复合赋值运算符不会返回值。 +> 例如，你不能写 `let b = a += 2`。 + +有关 Swift 标准库提供的运算符的信息，请参阅 [Operator Declarations](https://developer.apple.com/documentation/swift/operator_declarations)。 + +## 比较运算符 + +Swift 支持以下比较运算符： + +- 等于（`a == b`） +- 不等于（`a != b`） +- 大于（`a > b`） +- 小于（`a < b`） +- 大于等于（`a >= b`） +- 小于等于（`a <= b`） + +> 注意: Swift 还提供了两个*标识运算符*（`===` 和 `!==`）， +> 你可以用它们来测试两个对象引用是否指向同一个对象实例。 +> 更多信息请参阅 。 + +每个比较运算符都返回一个 `Bool` 值来指示语句是否为真： + +```swift +1 == 1 // true 因为 1 等于 1 +2 != 1 // true 因为 2 不等于 1 +2 > 1 // true 因为 2 大于 1 +1 < 2 // true 因为 1 小于 2 +1 >= 1 // true 因为 1 大于等于 1 +2 <= 1 // false 因为 2 不小于等于 1 +``` + + + +比较运算符通常用于条件语句中，例如 `if` 语句： + +```swift +let name = "world" +if name == "world" { + print("hello, world") +} else { + print("I'm sorry \(name), but I don't recognize you") +} // 打印 "hello, world", 因为 name 确实等于 "world"。 +``` + + + +关于 `if` 语句的更多信息，请参阅 。 + +如果两个元组具有相同的类型和相同数量的值，则可以比较它们。 +元组是从左到右逐个值进行比较的，直到比较发现两个不相等的值为止。 +这两个值将进行比较，并且该比较的结果决定了整个元组比较的结果。 +如果所有元素都相等，那么这两个元组本身就相等。 +例如： + +```swift +(1, "zebra") < (2, "apple") // 为 true，因为 1 小于 2; "zebra" 和 "apple" 未比较 +(3, "apple") < (3, "bird") // 为 true，因为 3 等于 3，而 "apple" 小于 "bird" +(4, "dog") == (4, "dog") // 为 true，因为 4 等于 4，而 "dog" 等于 "dog" +``` + + + +在上面的示例中，您可以看到第一行的从左到右比较行为。 +因为 `1` 小于 `2`，所以 `(1, "zebra")` 被认为小于 `(2, "apple")`，而不管元组中的任何其他值如何。 +即使 `"zebra"` 不小于 `"apple"`，也无关紧要，因为比较已经由元组的第一个元素决定了。 +但是，当元组的第一个元素相同时，它们的第二个元素*会*进行比较 —— 这就是第二行和第三行发生的情况。 + +只有当给定的运算符可以应用于各自元组中的每个值时，元组才能与该运算符进行比较。例如，如下面的代码所示，您可以比较两个类型为 `(String, Int)` 的元组，因为 `String` 和 `Int` 值都可以使用 `<` 运算符进行比较。相反，类型为 `(String, Bool)` 的两个元组不能使用 `<` 运算符进行比较，因为 `<` 运算符不能应用于 `Bool` 值。 + +```swift +("blue", -1) < ("purple", 1) // 可以，计算结果为 true +("blue", false) < ("purple", true) // 错误，因为 < 不能比较布尔值 +``` + + + + + +> 注意: Swift 标准库包含用于具有少于七个元素的元组的比较运算符。 +> 要比较具有七个或更多元素的元组， +> 您必须自己实现比较运算符。 + + + +## 三元条件运算符 + +三元条件运算符是一种特殊的运算符，用于根据给定条件选择两个值中的一个。它由三个部分组成，语法格式为`问题 ? 答案1 : 答案2`。它根据问题的真假值来选择计算哪个表达式，并返回该表达式的值。如果`问题`为真，它会计算`答案1`并返回其值;否则，它会计算`答案2`并返回其值。 + +三元条件运算符是以下代码的简写形式： + +```swift +if question { + answer1 +} else { + answer2 +} +``` + + + + + +下面是一个例子，用于计算表格行的高度。如果该行有标题，则行高应比内容高度高 50 点；如果该行没有标题，则行高应比内容高度高 20 点： + +```swift +let contentHeight = 40 +let hasHeader = true +let rowHeight = contentHeight + (hasHeader ? 50 : 20) +// rowHeight 等于 90 +``` + + + +上面的例子是下面代码的简写形式： + +```swift +let contentHeight = 40 +let hasHeader = true +let rowHeight: Int +if hasHeader { + rowHeight = contentHeight + 50 +} else { + rowHeight = contentHeight + 20 +} +// rowHeight 等于 90 +``` + + + +第一个例子使用三元条件运算符意味着`rowHeight`可以在一行代码中设置为正确的值，这比第二个例子中使用的代码更加简洁。 + +三元条件运算符提供了一种有效的简写方式来决定考虑两个表达式中的哪一个。不过，要谨慎使用三元条件运算符。如果过度使用，代码的可读性会下降。避免将多个三元条件运算符实例组合成一个复合语句。 + +## 空合并运算符 + +*空合并运算符*（`a ?? b`）如果可选项`a`包含一个值，则会解包该值，否则会返回默认值`b`（如果`a`为`nil`）。表达式`a`始终是一个可选类型。表达式`b`必须与存储在`a`中的类型相匹配。 + +空合并运算符是以下代码的简写形式： + +```swift +a != nil ? a! : b +``` + + + +上面的代码使用三元条件运算符和强制解包（`a!`）来访问 `a` 中包装的值（当 `a` 不是 `nil` 时），否则返回 `b`。空合并运算符提供了一种更优雅的方式，以简洁和可读的形式封装这种条件检查和解包。 + +> 注意: 如果 `a` 的值是非 `nil` 的，则不会计算 `b` 的值。这被称为*短路求值*。 + +下面的示例使用空合并运算符在默认颜色名称和可选用户定义的颜色名称之间进行选择： + +```swift +let defaultColorName = "red" +var userDefinedColorName: String? // 默认为 nil + +var colorNameToUse = userDefinedColorName ?? defaultColorName +// userDefinedColorName 为空，所以 colorNameToUse 为默认值 "red" +``` + + + +变量 `userDefinedColorName` 被定义为一个可选的 `String` 类型，默认值为 `nil`。由于 `userDefinedColorName` 是一个可选类型，你可以使用空合并运算符来考虑它的值。在上面的例子中，该运算符被用于确定一个名为 `colorNameToUse` 的 `String` 变量的初始值。因为 `userDefinedColorName` 是 `nil`，所以表达式 `userDefinedColorName ?? defaultColorName` 返回 `defaultColorName` 的值，即 `"red"`。 + +如果你给 `userDefinedColorName` 赋予一个非 `nil` 的值，并再次执行 nil 合并运算符检查，那么 `userDefinedColorName` 包裹的值将被使用，而不是默认值： + +```swift +userDefinedColorName = "green" +colorNameToUse = userDefinedColorName ?? defaultColorName +// userDefinedColorName 不是 nil，所以 colorNameToUse 被设置为 "green" +``` + + + +## 区间运算符 + +Swift 包含几个*区间运算符*，这些是表达一个值范围的快捷方式。 + +### 闭区间运算符 + +*闭区间运算符*（`a...b`）定义了一个从 `a` 到 `b` 的范围，包括 `a` 和 `b` 的值。`a` 的值不能大于 `b`。 + + + + + + + +闭区间运算符在需要使用所有值的情况下很有用，例如在 `for-in` 循环中： + +```swift +for index in 1...5 { + print("\(index) 乘以 5 等于 \(index * 5)") +} +// 1 乘以 5 等于 5 +// 2 乘以 5 等于 10 +// 3 乘以 5 等于 15 +// 4 乘以 5 等于 20 +// 5 乘以 5 等于 25 +``` + + + +更多关于 `for-in` 循环的内容，请参阅 。 + +### 半开区间运算符 + +*半开区间运算符*（`a.. let range = 1..<2 + >> print(type(of: range)) + << Range + ``` +--> + + + + + +半开区间对于处理从基数 0 开始的列表（如数组）时特别有用，因为它可以计数到列表长度（但不包括列表长度）： + +```swift +let names = ["Anna", "Alex", "Brian", "Jack"] +let count = names.count +for i in 0.. let names = ["Anna", "Alex", "Brian", "Jack"] + -> let count = names.count + >> assert(count == 4) + -> for i in 0.. + +注意数组包含四个元素，但 `0..。 + +### 单侧区间 + +闭区间运算符有一种替代形式，用于一直延伸到尽可能远的区间 —— 例如，一个包含从索引 2 到数组末尾所有元素的区间。 +在这些情况下，你可以省略区间运算符的一侧值。 +这种区间被称为*单侧区间*，因为运算符只有一侧有值。 +例如： + +```swift +for name in names[2...] { print(name) } +// Brian +// Jack + +for name in names[...2] { print(name) } +// Anna +// Alex +// Brian +``` + + + +半开区间运算符也有一种只写最后一个值的单侧形式。 +就像在两侧都包含值时一样，最后一个值不包含在区间内。 +例如： + +```swift +for name in names[..<2] { print(name) } +// Anna +// Alex +``` + + + +单侧区间不仅可以用于下标，还可以用于其他上下文。 +对于省略了第一个值的单侧区间，你不能遍历它，因为不清楚他从哪里开始迭代。 +你*可以*遍历省略了最后一个值的单侧区间；但是，由于该区间无限延伸，请确保为循环添加一个显式的结束条件。 +你还可以检查单侧区间是否包含特定值，如下面的代码所示。 + +```swift +let range = ...5 +range.contains(7) // false +range.contains(4) // true +range.contains(-1) // true +``` + + + +## 逻辑运算符 + +*逻辑运算符*修改或组合布尔逻辑值 `true` 和 `false`。 +Swift 支持 C 语言中的三个标准逻辑运算符： + +- 逻辑非（`!a`） +- 逻辑与（`a && b`） +- 逻辑或（`a || b`） + +### 逻辑非运算符 + +*逻辑非运算符*（`!a`）反转布尔值，使 `true` 变为 `false`，`false` 变为 `true`。 + +逻辑非运算符是一个前置运算符，紧跟在它所操作的值之前，中间没有空格。 +它可以读作"非 `a`"，如下例所示： + +让我们来看一个简单的例子： + +```swift +let allowedEntry = false +if !allowedEntry { + print("ACCESS DENIED") +} // 打印 "ACCESS DENIED" +``` + + + +短语 `if !allowedEntry` 可以理解为 "如果不允许进入"。只有当 "不允许进入" 为真时，才会执行后续的那一行；也就是说，如果 `allowedEntry` 为 `false`。 + +正如这个例子所示，谨慎选择布尔常量和变量名可以帮助保持代码的可读性和简洁性，同时避免双重否定或令人困惑的逻辑语句。 + +### 逻辑与运算符 + +*逻辑与运算符*（`a && b`） 创建逻辑表达式，其中两个值都必须为 `true`，整个表达式才为 `true`。如果任一值为 `false`，整个表达式也将为 `false`。事实上，如果*第一个*值为 `false`，第二个值甚至不会被评估，因为它无论如何都不可能使整个表达式等于 `true`。这被称为*短路求值*。 + +下面的例子考虑了两个 `Bool` 值，只有在两个值都为 `true` 时才允许访问： + +```swift +let enteredDoorCode = true +let passedRetinaScan = false +if enteredDoorCode && passedRetinaScan { + print("Welcome!") +} else { + print("ACCESS DENIED") +} // 打印 "ACCESS DENIED" +``` + + + +### 逻辑或运算符 + +*逻辑或运算符*（`a || b`） 是由两个相邻的管道字符组成的中置运算符。你可以使用它来创建逻辑表达式，在这种表达式中，只要*其中一个*值为 `true`，整个表达式就为 `true`。 + +与上面的逻辑与运算符一样，逻辑或运算符也使用短路求值来考虑它的表达式。如果逻辑或表达式的左侧为 `true`，右侧就不会被评估，因为它无法改变整个表达式的结果。 + +在下面的例子中，第一个 `Bool` 值（`hasDoorKey`） 为 `false`，但第二个值（`knowsOverridePassword`） 为 `true`。由于有一个值为 `true`，整个表达式也将评估为 `true`，因此允许访问： + +```swift +let hasDoorKey = false +let knowsOverridePassword = true +if hasDoorKey || knowsOverridePassword { + print("Welcome!") +} else { + print("ACCESS DENIED") +} // 打印 "Welcome!" +``` + + + +### 组合逻辑运算符 + +你可以组合多个逻辑运算符来创建更长的复合表达式： + +```swift +if enteredDoorCode && passedRetinaScan || hasDoorKey || knowsOverridePassword { + print("Welcome!") +} else { + print("ACCESS DENIED") +} // 打印 "Welcome!" +``` + + + +这个例子使用多个 `&&` 和 `||` 运算符来创建一个更长的复合表达式。然而，`&&` 和 `||` 运算符仍然只作用于两个值，所以这实际上是三个较小的表达式链接在一起。这个例子可以理解为： + +如果我们输入了正确的门禁代码并通过了视网膜扫描，或者我们有一把有效的门钥匙，或者我们知道紧急情况下的覆盖密码，那么就允许访问。 + +根据 `enteredDoorCode`、`passedRetinaScan` 和 `hasDoorKey` 的值，前两个子表达式为 `false`。然而，由于知道紧急覆盖密码，整个复合表达式仍然评估为 `true`。 + +> 注意: Swift 逻辑运算符 `&&` 和 `||` 遵循从左到右的结合顺序，这意味着带有多个逻辑运算符的复合表达式会首先评估最左边的子表达式。 + +### 显式括号 + +有时即使不严格需要，也有必要使用括号来提高复杂表达式的可读性，让表达式的意图更加清晰。 +在上面的门禁示例中，为复合表达式的第一部分添加括号是有用的，可以明确表达其意图： + +```swift +if (enteredDoorCode && passedRetinaScan) || hasDoorKey || knowsOverridePassword { + print("Welcome!") +} else { + print("ACCESS DENIED") +} // 打印 "Welcome!" +``` + + + +括号明确表示前两个条件被视为整体逻辑中的一种可能状态。 +虽然复合表达式的输出没有改变，但整体意图对读者来说更加清晰明了。 +可读性永远比简洁性更重要，因此在有助于阐明意图的地方使用括号是很有必要的。 + + diff --git a/swift-6.docc/LanguageGuide/ClassesAndStructures.md b/swift-6.docc/LanguageGuide/ClassesAndStructures.md new file mode 100644 index 000000000..0370f045d --- /dev/null +++ b/swift-6.docc/LanguageGuide/ClassesAndStructures.md @@ -0,0 +1,655 @@ +# 结构体和类 + +封装数据的自定义模型类型。 + +*结构体*和*类*是通用、灵活的构造，是程序代码的基石。 +你可以使用与定义常量、变量和函数相同的语法， +为结构体和类添加属性和方法以增加功能。 + +与其他编程语言不同，Swift 不需要为自定义结构体和类创建单独的接口和实现文件。 +在 Swift 中，当你在单个文件中定义结构体或类， +该类或结构体的外部接口会自动提供给其他代码使用。 + +> 注意: 类的实例我们通常称为*对象*。 +> 但是，与其他语言相比，Swift 中的*结构体*和*类*在功能上更为接近， +> 本章大部分介绍的功能都适用于结构体*或*类的实例， +> 因此，本章使用了更通用的术语：*实例*。 + +## 比较结构体和类 + +Swift 中的结构体和类有许多共同点。两者都可以: + +- 定义属性来存储值 +- 定义方法来提供功能 +- 定义下标来使用下标语法访问其值 +- 定义构造器来设置初始状态 +- 被扩展以增加默认实现之外的功能 +- 遵循协议以提供某种标准功能 + +更多信息，请参阅 、、、、 和 。 + +相比结构体,类有一些额外的功能: + +- 一个类可以继承另一个类的特征。 +- 类型转换允许你在运行时检查和解释类实例的类型。 +- 析构器允许一个类的实例释放其分配的任何资源。 +- 引用计数允许对类实例有多个引用。 + +更多信息，请参阅 、、 和 。 + +类支持的额外功能以增加其复杂性为代价。 +作为一般准则，我们会优先使用结构体， +因为它们更易于理解，并在适当或必要时使用类。 +在实践中，这意味着你定义的大多数自定义类型将是结构体和枚举。 +有关更详细的比较，请参阅 [选择结构体和类](https://developer.apple.com/documentation/swift/choosing_between_structures_and_classes)。 + +> 注意: 类和 actor 共享许多相同的特征和行为。 +> 有关 actor 的信息，请参阅 。 + +### 定义语法 + +结构体和类有类似的定义语法。 +你使用 `struct` 关键字引入结构体， +使用 `class` 关键字引入类。 +两者都将整个定义放在一对大括号中： + +```swift +struct SomeStructure { + // 结构体定义在这里 +} + +class SomeClass { + // 类定义在这里 +} +``` + + + +> 注意: 每当你定义新的结构体或类时， +> 你就定义了一个新的 Swift 类型。 +> 使用 `UpperCamelCase` 来命名类型 +> （如此处的 `SomeStructure` 和 `SomeClass`）， +> 以匹配标准 Swift 类型的大小写 +> （如 `String`、`Int` 和 `Bool`）。 +> 使用 `lowerCamelCase` 来命名属性和方法 +> （如 `frameRate` 和 `incrementCount`）， +> 以便将它们与类型名称区分开。 + +下面是结构体定义和类定义的示例： + +```swift +struct Resolution { + var width = 0 + var height = 0 +} +class VideoMode { + var resolution = Resolution() + var interlaced = false + var frameRate = 0.0 + var name: String? +} +``` + + + +上面的示例定义了一个名为 `Resolution` 的新结构体， +用于描述基于像素的显示分辨率。 +该结构体包含了名为 `width` 和 `height` 的两个储存属性。 +存储属性是作为结构体或类的一部分绑定存储的常量或变量。 +当这两个属性初始整数值为 `0` 时，他们会被推断为 `Int` 类型。 + +上面的示例还定义了一个名为 `VideoMode` 的类， +用于描述视频显示的特定视频模式。 +该结构体有四个变量存储属性。 +第一个 `resolution` 被初始化为一个新的 `Resolution` 结构体实例， +它的属性类型被推断为 `Resolution`。 +新的 `VideoMode` 实例还会初始化其他三个属性。 +分别是初始化为 `false` 的 `interlaced`（表示 “非隔行扫描视频”）、 +初始化为 `0.0` 的播放帧率和一个值为可选 `String` 的 `name`。 +由于 `name` 属性是可选类型，它会自动获得默认值 `nil`，又称 “空 `name` 值”。 + +### 结构体和类的实例 + +`Resolution` 结构体定义和 `VideoMode` 类定义本身 +只描述了 `Resolution` 或 `VideoMode` 将是什么样子。 +它们并不描述特定的分辨率或视频模式。 +要做到这一点，你需要创建结构体的实例。 + +创建实例的语法对于结构体和类都非常相似： + +```swift +let someResolution = Resolution() +let someVideoMode = VideoMode() +``` + + + +结构体和类都使用构造器语法来创建新实例。 +最简单的初始化语法形式是使用结构体的类型名后跟空括号， +例如 `Resolution()` 或 `VideoMode()`。 +这将创建类或结构体的新实例，并将任何属性初始化为其默认值。 +类和结构体的初始化在 中有更详细的描述。 + + + +### 访问属性 + +你可以使用*点语法*访问实例的属性。 +在点语法中，实例名后面紧跟属性名， +中间用句点（`.`）分隔，不留任何空格： + +```swift +print("The width of someResolution is \(someResolution.width)") +// 打印 "The width of someResolution is 0" +``` + + + +在这个例子中，`someResolution.width` 指的是 +`someResolution` 的 `width` 属性， +并返回其默认初始值 `0`。 + +你可以深入到子属性， +例如 `VideoMode` 中 `resolution` 属性的 `width` 属性： + +```swift +print("The width of someVideoMode is \(someVideoMode.resolution.width)") +// 打印 "The width of someVideoMode is 0" +``` + + + +你还可以使用点语法为可变属性赋值： + +```swift +someVideoMode.resolution.width = 1280 +print("The width of someVideoMode is now \(someVideoMode.resolution.width)") +// 打印 "The width of someVideoMode is now 1280" +``` + + + +### 结构体逐一成员构造器 + +所有结构体都有一个自动生成的*逐一成员构造器*， +你可以使用它来初始化新结构体实例的成员属性。 +可以通过属性成员名称将新实例的属性初始值传递给成员构造器： + +```swift +let vga = Resolution(width: 640, height: 480) +``` + + + +与结构体不同，类实例没有默认的逐一成员构造器。 +构造器在 中有更详细的描述。 + + + +## 结构体和枚举是值类型 + +*值类型*是一种在被赋值给变量或常量时， +或者在传递给函数时，其值会被*复制*的类型。 + + + +在前面的章节中，您实际上已经广泛使用了值类型。 +事实上，Swift 中所有的基本类型：整数、浮点数、布尔值、字符串、数组和字典都是值类型， +在底层它们都是以结构体的形式实现的。 + +在 Swift 中，所有的结构体和枚举都是值类型。 +这意味着它们的实例，以及实例中所包含的任何值类型的属性在代码中传递时都会被复制。 + +> 注意: Swift 标准库定义的集合类型，如数组、字典和字符串， +> 使用了一种优化技术来降低复制操作的性能消耗。它们不会立即进行复制， +> 而是共享了原始实例和任何副本之间存储元素的内存。 +> 如果集合的某个副本被修改，则在修改之前会复制元素。 +> 您在代码中看到的样子就像立即进行了复制一样。 + +考虑下面这个使用前面示例中的 `Resolution` 结构体的例子： + +```swift +let hd = Resolution(width: 1920, height: 1080) +var cinema = hd +``` + + + +这个例子声明了一个名为 `hd` 的常量， +并将其设置为一个使用全高清视频宽度和高度 +(1920 像素宽、1080 像素高) 初始化的 `Resolution` 实例。 + +然后，它声明了一个名为 `cinema` 的变量， +并将其设置为 `hd` 的当前值。因为 `Resolution` 是一个结构体， +所以会对现有实例进行*复制*，并将这个新副本赋值给 `cinema`。 +尽管 `hd` 和 `cinema` 现在具有相同的宽度和高度， +但在底层它们是两个完全不同的实例。 + +接下来，为符合数字电影放映的需求（2048 像素宽、1080 像素高），`cinema` 的 `width` 属性被修改为稍微宽一点的 2K 标准： + +```swift +cinema.width = 2048 +``` + + + +检查 `cinema` 的 `width` 属性会发现它确实已经变为 `2048`： + +```swift +print("cinema is now \(cinema.width) pixels wide") +// 打印 "cinema is now 2048 pixels wide" +``` + + + +然而,原始 `hd` 实例的 `width` 属性仍然保持旧值 `1920`: + +```swift +print("hd is still \(hd.width) pixels wide") +// 打印 "hd is still 1920 pixels wide" +``` + + + +当 `hd` 赋值给 `cinema` 时， +存储在 `hd` 中的*值*被复制到了新的 `cinema` 实例中。 +最终结果是拥有完全相同值的两个独立的实例。 +然而，由于它们是独立的实例，将 `cinema` 的宽度设置为 `2048` +并不会影响存储在 `hd` 中的宽度，如下图所示： + +![](sharedStateStruct) + +枚举的行为也是一样的： + +```swift +enum CompassPoint { + case north, south, east, west + mutating func turnNorth() { + self = .north + } +} +var currentDirection = CompassPoint.west +let rememberedDirection = currentDirection +currentDirection.turnNorth() + +print("The current direction is \(currentDirection)") +// 打印 "The current direction is north" +print("The remembered direction is \(rememberedDirection)") +// 打印 "The current direction is west" +``` + + + +当 `currentDirection` 赋值给 `rememberedDirection`， +它实际上是拷贝了这个值。 +赋值过程结束后再修改 `currentDirection` 的值并不影响 +`rememberedDirection`所储存的原始值的拷贝。 + + + +## 类是引用类型 + +不同于值类型每次都会创建一个新的副本， +引用类型在被赋值给变量或常量， +或者在被传递给函数时不会被复制。 +而是指向同一个现有实例的引用。 + +下面通过一个例子来说明引用类型的工作原理： + +```swift +let tenEighty = VideoMode() +tenEighty.resolution = hd +tenEighty.interlaced = true +tenEighty.name = "1080i" +tenEighty.frameRate = 25.0 +``` + + + +这个例子声明了一个名为 `tenEighty` 的新常量， +并将其设置为指向 `VideoMode` 类的一个新实例。 +视频模式的分辨率被设置为之前的 HD 分辨率 `1920` x `1080`， +它被设置为逐行扫描模式，名称设置为 `"1080i"`， +帧率设置为每秒 25.0 帧。 + +接下来，将 `tenEighty` 赋值给一个名为 `alsoTenEighty` 的新常量，并修改 `alsoTenEighty` 的帧率： + +```swift +let alsoTenEighty = tenEighty +alsoTenEighty.frameRate = 30.0 +``` + + + +由于类是引用类型，tenEighty 和 alsoTenEighty 实际上 +指向的是同一个 `VideoMode` 实例。 +换句话说，它们只是同一个实例的两个不同名称，如下图所示： + +![](sharedStateClass) + +通过检查 `tenEighty` 的 `frameRate`， +可以看到它正确地显示了 `VideoMode` 实例的新帧率 `30.0`。 + +```swift +print("The frameRate property of tenEighty is now \(tenEighty.frameRate)") +// 打印 "The frameRate property of tenEighty is now 30.0" +``` + + + +这个例子也展示了引用类型更难理解和维护。 +如果 `tenEighty` 和 `alsoTenEighty` 在你程序的代码中相距很远， +那么很难找到所有改变视频模式的方式。 +无论你在哪里使用 `tenEighty`， +你也必须考虑使用 `alsoTenEighty` 的代码，反之亦然。 +相比之下，值类型更容易理解，因为操作同一个值的代码都集中在一起。 + +需要注意的是，`tenEighty` 和 `alsoTenEighty` 虽然被声明为常量而不是变量， +但你仍然可以修改 `tenEighty.frameRate` 和 `alsoTenEighty.frameRate`， +tenEighty 和 alsoTenEighty 常量的值本身并没有变化。 +它们并不“存储”这个 VideoMode 实例， +而仅仅是对 VideoMode 实例的引用。 +所以，改变的是底层 `VideoMode` 实例的 `frameRate` 属性， +而不是指向 `VideoMode` 的常量引用的值。 + + + + + +### 恒等运算符 + +因为类是引用类型，所以多个常量和变量在底层可能会引用同一个类实例。 +（对于结构体和枚举来说则不是这样，因为它们在被赋值给常量、 +变量或传递给函数时，总是会被复制。 + + + + + +有时需要找出两个常量或变量是否引用了同一个类的实例。 +为了实现这一点，Swift 提供了两个恒等运算符： + +- 恒等 (`===`) +- 不恒等 (`!==`) + +使用这些运算符来检查两个常量或变量是否引用了同一个实例： + +```swift +if tenEighty === alsoTenEighty { + print("tenEighty and alsoTenEighty refer to the same VideoMode instance.") +} +// 打印 "tenEighty and alsoTenEighty refer to the same VideoMode instance." +``` + + + +注意*恒等*（用三个等号 `===` 表示）与*相等*（用两个等号`==`表示）意义不同。 +*恒等*意味着两个类型的常量或变量引用了完全相同的类实例。 +*相等*意味着两个实例在值上被认为是相等或等价的， +具体的*相等*定义由类型的设计者决定。 + +当你定义自己的自定义结构体和类时， +你有责任决定什么才算作两个实例相等。 +定义自己的 `==` 和 `!=` 运算符实现的过程在 中有描述。 + + + + + + + +### 指针 + +如果你有 C、C++ 或 Objective-C 的经验， +你可能知道这些语言使用指针来引用内存中的地址。 +一个指向某个引用类型实例的 Swift 常量或变量相当于 C 中的指针， +但它不是直接指向内存地址，也不需要使用星号（`*`）来表示创建引用。 +相反，这些引用像 Swift 中任何其他常量或变量一样定义。 +Swift 标准库提供了指针和缓冲区类型， +如果你需要直接与指针交互，请参阅 [手动内存管理](https://developer.apple.com/documentation/swift/swift_standard_library/manual_memory_management)。 + + + + + + + + diff --git a/swift-6.docc/LanguageGuide/Closures.md b/swift-6.docc/LanguageGuide/Closures.md new file mode 100644 index 000000000..3f2985e0f --- /dev/null +++ b/swift-6.docc/LanguageGuide/Closures.md @@ -0,0 +1,1002 @@ +# 闭包 + +将执行的代码组合在一起，而不需要创建命名函数。 + +**闭包** 是可以在你的代码中传递和使用的独立功能块。Swift 中的闭包类似于其他编程语言中的匿名函数、lambda 表达式和代码块。 + +闭包可以捕获和存储其所在上下文中任意常量和变量的引用。这个过程可以看作是将这些常量和变量 **包含** 在闭包的作用域内。 Swift 会为你管理在捕获过程中涉及到的所有内存操作。 + +> 注意: 如果你不熟悉捕获（capturing）这个概念也不用担心。 +> 在 章节有它更详细的介绍。 + +在 章节中介绍的全局和嵌套函数实际上也是特殊的闭包，闭包采用如下三种形式之一： + +- 全局函数是一个有名字但不会捕获任何值的闭包 +- 嵌套函数是一个有名字并可以捕获其封闭函数域内值的闭包 +- 闭包表达式是使用轻量级语法编写的匿名闭包，它们能够捕获其上下文中的值。 + +Swift 中的闭包表达式风格简洁明了，通过一系列优化，使得在常见的情况下可以写出简短而清晰的代码。主要优化如下： + +- 利用上下文推断参数和返回值类型 +- 单表达式闭包的隐式返回（可以省略 return 关键字） +- 简化参数名称 +- 尾随闭包语法 + +## 闭包表达式 + +在 中介绍的嵌套函数，提供了一种便捷的方式，可以在较大的函数内部命名和定义自包含的代码块。然而，有时我们需要更简洁的函数式结构，而不必完整地声明函数名称。这在处理那些接受函数作为参数的函数或方法时特别有用。 + +**闭包表达式** 是一种以简短、集中的语法编写内联闭包的方法。在保证不丢失它语法清晰和意图的同时，闭包表达式提供了几种优化的语法简写形式。下面的闭包表达式通过对 `sorted(by:)` 这一示例的多次迭代来展示这个过程，每次迭代都使用了更加简洁的方式描述了相同功能。 + +### Sorted 方法 + +Swift 标准库提供了名为 `sorted(by:)` 的方法，它会基于你提供的排序闭包表达式的判断结果对数组中的值（类型确定）进行排序。一旦它完成排序过程，`sorted(by:)` 方法会返回一个与旧数组类型大小相同类型的新数组，该数组的元素有着正确的排序顺序。原数组不会被 `sorted(by:)` 方法修改。 + +下面的闭包表达式示例使用 `sorted(by:)` 方法对一个 `String` 类型的数组进行字母逆序排序。以下是初始数组： + +```swift +let names = ["Chris", "Alex", "Ewa", "Barry", "Daniella"] +``` + + + +`sorted(by:)` 方法接受一个闭包，该闭包函数需要传入与数组元素类型相同的两个值，并返回一个布尔类型值，来表明排序后第一个参数排在第二个参数前面还是后面。如果第一个参数值出现在第二个参数值 **前面**，排序闭包函数需要返回 `true`，反之返回 `false`。 + +该例子对一个 `String` 类型的数组进行排序，因此排序闭包函数类型需为 `(String, String) -> Bool`。 + +提供排序闭包函数的一种方式是编写一个正确类型的普通函数，并将其作为 `sorted(by:)` 方法的参数传入： + +```swift +func backward(_ s1: String, _ s2: String) -> Bool { + return s1 > s2 +} +var reversedNames = names.sorted(by: backward) +// reversedNames 等于 ["Ewa", "Daniella", "Chris", "Barry", "Alex"] +``` + + + +如果第一个字符串 （ `s1` ） 大于第二个字符串 （ `s2` ），`backward(_:_:)` 函数将返回 `true`，表示在新的数组中 `s1` 应该出现在 `s2` 前。对于字符串中的字符，“大于”表示“在字母顺序较晚出现”。这意味着字母 `"B"` “大于”字母 `"A"`，字符串 `"Tom"` 大于字符串 `"Tim"`。这给出了一个字母逆序排序，将 `"Barry"` 放在 `"Alex"` 之前，依此类推。 + +然而，这是一种相当繁琐的编写方式，本质上是一个单表达式函数 (`a > b`)。对于这个例子来说，利用闭包表达式语法可以更好地构造一个内联排序闭包。 + +### 闭包表达式语法 + +闭包表达式语法的一般形式如下： + +```swift +{ (<#parameters#>) -> <#return type#> in + <#statements#> +} +``` + +闭包表达式语法中的 **参数** 可以是 in-out 参数，但不能具有默认值。如果你命名了可变参数，也可以使用可变参数。元组也可以用作参数类型和返回类型。 + +下面的示例展示了上面的 `backward(_:_:)` 函数对应的闭包表达式版本： + +```swift +reversedNames = names.sorted(by: { (s1: String, s2: String) -> Bool in + return s1 > s2 +}) +``` + + + +需要注意的是，内联闭包的参数和返回类型的声明与 `backward(_:_:)` 函数的声明相同。在这两种情况下，它都写为 `(s1: String, s2: String) -> Bool`。但是在内联闭包表达式中，参数和返回类型都写入 **大括号** 内，而不是大括号外。 + +闭包函数主体的开始以 `in` 关键字开始。这个关键字表示闭包的参数和返回类型定义已经结束，接下来是闭包的实际内容。 + +因为闭包的主体很短，甚至可以写在一行上： + +```swift +reversedNames = names.sorted(by: { (s1: String, s2: String) -> Bool in return s1 > s2 } ) +``` + + + +这说明对 `sorted(by:)` 方法的整体调用保持不变。一对括号仍然包裹着方法的整个参数。然而，参数现在变为了内联闭包。 + +### 根据上下文推断类型 + +因为排序闭包是作为参数传递给方法的，所以 Swift 可以推断其参数的类型和返回的值的类型。`sorted(by:)` 方法被一个字符串数组调用，因此其参数必须是 `(String, String) -> Bool` 类型的函数。这意味着 `(String, String)` 和 `Bool` 类型不需要作为闭包表达式定义的一部分。由于可以推断所有类型，因此也可以省略返回箭头 （ `->` ） 和参数名称两边的括号： + + +```swift +reversedNames = names.sorted(by: { s1, s2 in return s1 > s2 } ) +``` + + + +当将闭包作为内联表达式传递给函数或方法时，Swift 通常能够推断出参数类型和返回类型。这意味着，在使用闭包作为函数或方法参数时，你不必写出完整的闭包形式。 + +不过，如果你希望让代码更加清晰，也可以显式地声明类型。特别是当这样做能避免给读者造成歧义时，我们鼓励这种做法。以 `sorted(by:)` 方法为例，从排序这个操作就可以清楚地看出闭包的用途。读者可以安全地假设这个闭包很可能在处理 `String` 类型的值，因为它是用来协助对字符串数组进行排序的。 + +### 单表达式闭包的隐式返回 + +单表达式闭包可以通过从其声明中省略 `return` 关键字来隐式返回其单表达式的结果，如上版本的例子可以改写为： + +```swift +reversedNames = names.sorted(by: { s1, s2 in s1 > s2 } ) +``` + + + +在这个例子中，`sorted(by:)` 方法的参数类型明确了闭包必须返回一个 `Bool` 值。因为闭包的主体包含了一个返回 `Bool` 值的单个表达式 （ `s1 > s2` ），因此不存在歧义，并且可以省略 `return` 关键字。 + +### 简写参数名称 + +Swift 自动为内联闭包提供了简写参数名称功能，你可以直接通过 `$0`、`$1`、`$2` 等来引用闭包参数的值，以此类推。 + +如果在闭包表达式中使用这些简写参数名，你可以省略闭包定义中的参数列表。Swift 会根据函数的预期类型来推断出这些简写参数的类型。你使用的最大编号的简写参数决定了闭包接受的参数数量。由于此时闭包表达式仅由其函数体组成，因此 `in` 关键字也可以省略： + +```swift +reversedNames = names.sorted(by: { $0 > $1 } ) +``` + + + +在这个例子中，`$0` 和 `$1` 表示闭包的第一个和第二个 `String` 参数。由于 `$1` 是编号最大的简写参数，因此闭包可以理解为需要两个参数。因为这里的 `sorted(by:)` 函数需要一个参数都是字符串的闭包，所以简写参数 `$0` 和 `$1` 都是 `String` 类型。 + + + +### 运算符方法 + +实际上有一种 **更短** 的方法来编写上面的闭包表达式。Swift 的 `String` 类型定义了关于大于运算符 （ `>` ） 的字符串实现方法，该方法具有两个 `String` 类型的参数，并返回一个 `Bool` 类型的值。这正好符合 `sorted(by:)` 方法所需的函数类型。因此，你可以直接传入大于运算符，Swift 会推断出你想使用它的字符串特定实现： + +```swift +reversedNames = names.sorted(by: >) +``` + + + +更多关于运算符方法的内容请查看 . + +## 尾随闭包 + +如果你需要将闭包表达式作为函数的最后一个参数传递给函数，并且闭包表达式很长，则将其编写为 **尾随闭包** 的形式可能会很有用。在函数调用的括号后编写一个尾随闭包，该尾随闭包仍然会作为该函数的一个参数。使用尾随闭包语法时，不用在函数调用过程中为第一个闭包声明参数名。函数调用可以包含多个尾随闭包；但是，下面的前几个示例只使用了单个尾随闭包。 + +```swift +func someFunctionThatTakesAClosure(closure: () -> Void) { + // 函数主体在这里 +} + +// 以下是如何在不使用尾随闭包的情况下调用此函数的示例： + +someFunctionThatTakesAClosure(closure: { + // 闭包的主体在这里 +}) + +// 以下是如何使用尾随闭包调用此函数的示例： + +someFunctionThatTakesAClosure() { + // 尾随闭包的主体在这里 +} +``` + + + +上面 部分的字符串排序闭包可以写在 `sorted(by:)` 方法的括号之外作为尾随闭包： + +```swift +reversedNames = names.sorted() { $0 > $1 } +``` + + + + +如果一个闭包表达式是函数或方法的唯一参数，并且把该表达式写作尾随闭包，则在调用函数或方法时，无需在函数名或方法名后编写一对括号 `()` ： + +```swift +reversedNames = names.sorted { $0 > $1 } +``` + + + +当闭包足够长以至于无法在单行上内联写入时，尾随闭包变得非常有用。例如，Swift 的 `Array` 类型有一个 `map(_:)` 方法，该方法将闭包表达式作为其唯一参数。对于数组中的每一个元素，都将调用一次闭包，并返回该元素的映射值（可能是其他类型的）。你可以通过在传递给 `map(_:)` 的闭包中编写代码来指定映射的性质和返回值的类型。 + +将提供的闭包应用于每个数组元素后，`map(_:)` 方法返回一个新数组，其中包含了所有新的映射值，其顺序与原始数组中的相应值相同。 + +下例是如何使用带有尾随闭包的 `map(_:)` 方法将 `Int` 类型数组转换为 `String` 类型数组。数组 `[16, 58, 510]` 用于创建新的数组 `["OneSix", "FiveEight", "FiveOneZero"]` ： + +```swift +let digitNames = [ + 0: "Zero", 1: "One", 2: "Two", 3: "Three", 4: "Four", + 5: "Five", 6: "Six", 7: "Seven", 8: "Eight", 9: "Nine" +] +let numbers = [16, 58, 510] +``` + + + +上面的代码创建了一个整型数位和它们英文版本名字相映射的字典。同时还定义了一个准备转换为字符串数组的整型数组。 + +你现在可以通过将一个闭包表达式作为尾随闭包传递给数组的 `map(_:)` 方法， 来使用 `numbers` 数组创建一个 `String` 类型的数组。 + +```swift +let strings = numbers.map { (number) -> String in + var number = number + var output = "" + repeat { + output = digitNames[number % 10]! + output + number /= 10 + } while number > 0 + return output +} +// strings 的类型被推断为 [String] +// 它的值是 ["OneSix", "FiveEight", "FiveOneZero"] +``` + + + +`map(_:)` 方法为数组中的每个元素调用一次闭包表达式。你无需指定闭包的输入参数 `number` 的类型，因为可以从要映射的数组类型进行推断。 + +在这个例子中，变量 `number` 被初始化为闭包参数 `number` 的值，这样就可以在闭包内部修改这个值。（注意，函数和闭包的参数本身总是常量。）闭包表达式还指定了 `String` 作为返回类型，表明映射后的新数组将存储字符串类型的值。 + + +闭包表达式每次调用时都会生成一个名为 `output` 的字符串。它使用求余运算符 （ `number % 10` ） 计算 `number` 的最后一位数字，并使用此数字在 `digitNames` 字典中查找所映射的字符串。这个闭包可用于创建任何一个大于零的整数的字符串表示形式。 + +> 注意: 对 `digitNames` 字典的下标访问后面跟着一个感叹号（ `!` ）。这是因为字典的下标返回一个可选值，表示如果键不存在，字典查找可能会失败。在上面的例子中，我们可以保证 number % 10 总是 digitNames 字典的有效键，所以使用感叹号来强制解包下标返回的可选 `String` 值。 + +从 `digitNames` 字典中获取的字符串会被添加到 `output` 的开头，这样就巧妙地实现了数字的反向字符串构建。（例如，`number % 10` 的计算结果：对于 `16` 得到 `6`，对于 `58` 得到 `8`，对于 `510` 得到 `0`。） + +然后将 `number` 变量除以 `10`。因为它是一个整数，所以在除法过程中会向下舍入，所以 `16` 变成 `1` , `58` 变成 `5` , `510` 变成 `51` 。 + +重复该过程，直到 `number` 等于 `0`，此时闭包返回 `output` 字符串，并通过 `map(_:)` 方法添加到输出到映射数组中。 + +在上面的示例中使用尾随闭包语法，巧妙地在函数后封装了闭包的具体功能，而无需将整个闭包包裹在 `map(_:)` 方法的外括号中。 + +如果一个函数采用多个闭包，则省略第一个尾随闭包的参数名，并声明其余的尾随闭包。例如，以下函数将为照片库加载图片： + + +```swift +func loadPicture(from server: Server, completion: (Picture) -> Void, onFailure: () -> Void) { + if let picture = download("photo.jpg", from: server) { + completion(picture) + } else { + onFailure() + } +} +``` + + + +当您调用此函数来加载图片时，将提供两个闭包。第一个闭包是一个完成处理程序，在成功下载后显示图片。第二个闭包是一个错误处理程序，用于向用户显示错误。 + +```swift +loadPicture(from: someServer) { picture in + someView.currentPicture = picture +} onFailure: { + print("Couldn't download the next picture.") +} +``` + + + +在此示例中，`loadPicture(from:completion:onFailure:)` 函数将其网络任务分配到后台，并在网络任务完成时调用两个完成处理程序之一。通过这种方法编写函数，可以清楚地将负责处理网络故障的代码与在成功下载后更新用户界面的代码分开，而不是只使用一个闭包处理两种情况。 + +> 注意: （完成处理）Completion handlers 可能会变得难以阅读，特别是当你需要嵌套多个处理时。一种替代方法是使用异步代码，详情请参阅 章节。 + +## 值捕获 + +闭包可以从定义它的环境上下文中 **捕获** 常量和变量。即使定义这些常量和变量的原作用域已经不存在，闭包仍然可以在闭包函数体内引用和修改这些值。 + +在 Swift 中，可以捕获值的最简单的闭包形式是嵌套函数，它编写在另一个函数体中。嵌套函数可以捕获其外部函数的任何参数，也可以捕获在外部函数中定义的任何常量和变量。 + +`makeIncrementer(forIncrement:)` 函数有一个 `Int` 类型的参数，其参数标签是 `forIncrement`，参数名是 `amount`。传递给这个参数的值指定了每次调用返回的增量器函数时，`runningTotal` 应该增加的数量。`makeIncrementer` 函数内部定义了一个名为 `incrementer` 的嵌套函数，它执行实际的增量操作。这个函数只是将 `amount` 加到 `runningTotal` 上，并返回结果 + +```swift +func makeIncrementer(forIncrement amount: Int) -> () -> Int { + var runningTotal = 0 + func incrementer() -> Int { + runningTotal += amount + return runningTotal + } + return incrementer +} +``` + + + +`makeIncrementer` 的返回类型为 `() -> Int`。这意味着它返回一个 **函数**，而不是一个简单的值。它返回的函数没有参数，每次调用它时都返回一个 `Int` 值。要了解函数如何返回其他函数，请参阅 。 + +`makeIncrementer(forIncrement:)` 函数定义了一个名为 `runningTotal` 的整数变量，用于存储即将返回的增量器的当前累计总和。这个变量初始化为 `0`。 + +`makeIncrementer(forIncrement:)` 函数有一个 `Int` 类型的参数，其外部参数名为 `forIncrement`，内部参数名称为 `amount`。该参数值指定每次调用 incrementer 时将要增加的值 `runningTotal` 是多少。`makeIncrementer` 函数定义了一个名为 `incrementer` 的嵌套函数，该函数执行实际的增加操作。此函数只是将 `amount` 增加到 `runningTotal` 上，并返回结果。 + +如果单独看这个嵌套的 `incrementer()` 函数，可能会觉得有些奇怪： + +```swift +func incrementer() -> Int { + runningTotal += amount + return runningTotal +} +``` + + + +`incrementer()` 函数没有任何参数，但它在函数体内引用了 `runningTotal` 和 `amount`。这是通过捕获外围函数中 `runningTotal` 和 `amoun`t 的 引用 来实现的，并在自己的函数体中使用这些引用。通过引用捕获可以确保当 `makeIncrementer` 函数调用结束时，`runningTotal` 和 `amount` 不会消失。同时，这也保证了在下次调用 `incrementer` 函数时，`runningTotal` 仍然可用。 + +> 注意: 作为一项优化，如果值不会被闭包改变，并且该值在创建闭包后不会改变，那么 Swift 可能会捕获并存储该值的 **副本**。 +> +> Swift 也会负责被捕获变量涉及的所有内存管理工作，包括释放不再需要的变量。 + +下面是一个使用 `makeIncrementer` 的例子： + +```swift +let incrementByTen = makeIncrementer(forIncrement: 10) +``` + + + +这个例子定义了一个名为 `incrementByTen` 的常量，它引用了一个增量器函数。这个函数每次被调用时都会将其 `runningTotal` 变量增加 `10`。多次调用这个函数可以看到这个行为的实际效果： + +```swift +incrementByTen() +// 返回值为 10 +incrementByTen() +// 返回值为 20 +incrementByTen() +// 返回值为 30 +``` + + + + + +如果你创建第二个增量器，它会有自己的独立的 runningTotal 变量引用： + +```swift +let incrementBySeven = makeIncrementer(forIncrement: 7) +incrementBySeven() +// 返回值为 7 +``` + + + +再次调用原来的增量器（ `incrementByTen` ）会继续增加它自己的 `runningTotal` 变量，而不会影响 `incrementBySeven` 捕获的变量： + +```swift +incrementByTen() +// 返回值为 40 +``` +-------- + + +> 注意: 如果将闭包赋值给类实例的属性，并且闭包通过引用实例或其成员来捕获该实例，则将在闭包和实例之间创建一个强循环引用。Swift 使用 **捕获列表** 来打破这些强循环引用。有关更多信息，请参阅 . + +## 闭包是引用类型 + +在上面的示例中，`incrementBySeven` 和 `incrementByTen` 是常量，但这些常量引用的闭包仍然能够递增它们捕获的 `runningTotal` 变量。这是因为函数和闭包是 **引用类型**。 + +每当你将函数或闭包赋值给一个常量或变量时，你实际上是在将该常量或变量设置为对函数或闭包的 **引用**。在上面的示例中，`incrementByTen` **引用** 的闭包选择是常量，而不是闭包本身的内容。 + +这也意味着，如果将闭包分配给两个不同的常量或变量，则这两个常量或变量都引用同一闭包。 + +```swift +let alsoIncrementByTen = incrementByTen +alsoIncrementByTen() +// 返回值为 50 + +incrementByTen() +// 返回值为 60 +``` + + + +上面的示例表明，调用 `alsoIncrementByTen` 与调用 `incrementByTen` 相同。由于它们都引用相同的闭包，因此它们都会递增并返回相同的 runningTotal 值。 + +## 逃逸闭包 + +当闭包作为参数传递给函数，但是这个闭包在函数返回之后才被执行，该闭包被称为 **逃逸** 函数。当你声明一个将闭包作为其参数之一的函数时，你可以在参数的类型之前写入 `@escaping`，以表示这个闭包是允许逃逸的。 + +当一个闭包作为参数传递给一个函数，但在函数返回后才被调用时，我们称这个闭包从函数中 逃逸。当你声明一个接受闭包作为参数的函数时，你可以在参数类型前标注 `@escaping` ，以表明这个闭包允许逃逸。 + +闭包逃逸的一种常见方式是将其存储在函数外部定义的变量中。例如，许多启动异步操作的函数会接受一个闭包作为完成处理器（completion handler）。这种函数在启动操作后就会返回，但闭包要等到操作完成后才会被调用——这就需要闭包逃逸，以便稍后调用。示例如下： + +```swift +var completionHandlers: [() -> Void] = [] +func someFunctionWithEscapingClosure(completionHandler: @escaping () -> Void) { + completionHandlers.append(completionHandler) +} +``` + + + +`someFunctionWithEscapingClosure(_:)` 函数将闭包作为其参数，并将其添加到函数外部声明的数组中。如果不用 `@escaping` 标记此函数的参数，则会收到编译错误。 + +在逃逸闭包中引用 `self` 时，如果 `self` 指向一个类的实例，就需要特别注意。在逃逸闭包中捕获 `self` 很容易意外创建强引用循环。关于引用循环的更多信息，请参见 。 + +通常情况下，闭包会通过在其体内使用变量来隐式捕获这些变量，但在这种情况下，你需要显式地进行捕获。如果你想捕获 `self`，在使用时要明确写出 self，或者将 self 包含在闭包的捕获列表中。明确地写出 `self` 可以表达你的意图，并提醒你检查是否存在引用循环。例如，在下面的代码中，传递给 `someFunctionWithEscapingClosure(_:)` 的闭包明确地引用了 `self`。相比之下，传递给 `someFunctionWithNonescapingClosure(_:)` 的闭包是一个非逃逸闭包，这意味着它可以隐式地引用 `self`。 + +```swift +func someFunctionWithNonescapingClosure(closure: () -> Void) { + closure() +} + +class SomeClass { + var x = 10 + func doSomething() { + someFunctionWithEscapingClosure { self.x = 100 } + someFunctionWithNonescapingClosure { x = 200 } + } +} + +let instance = SomeClass() +instance.doSomething() +print(instance.x) +// 打印 “200” + +completionHandlers.first?() +print(instance.x) +// 打印 “100” +``` + + + +这是 `doSomething()` 的一个版本，它通过将 `self` 包含在闭包的捕获列表中来捕获 `self`，然后隐式引用 `self`： + +```swift +class SomeOtherClass { + var x = 10 + func doSomething() { + someFunctionWithEscapingClosure { [self] in x = 100 } + someFunctionWithNonescapingClosure { x = 200 } + } +} +``` + + + +如果 `self` 是结构体或枚举的实例，则始终可以隐式引用 `self`。但是转义闭包无法捕获其对 `self` 的可变引用。结构体和枚举不允许共享可变性，如 中所述。 + +```swift +struct SomeStruct { + var x = 10 + mutating func doSomething() { + someFunctionWithNonescapingClosure { x = 200 } // Ok + someFunctionWithEscapingClosure { x = 100 } // Error + } +} +``` + + + +在上面的示例中，对 `someFunctionWithEscapingClosure` 函数的调用是一个错误，因为它位于一个可变函数中，因此 `self` 是可变的。这违反了规则，即转义闭包不能捕获对结构的 `self` 的可变引用。 + + + + + +## 自动闭包 + +**自动闭包** 是一种自动创建的闭包，用于包装作为参数传递给函数的表达式。它不接受任何参数，当它被调用时，它返回包裹在其内部的表达式的值。这种便利语法让你能够省略闭包的大括号，用一个普通的表达式来代替显式的闭包。 + +我们经常会 **调用** 采用自动闭包的函数，但是很少去 **实现** 这样的函数。例如，`assert(condition:message:file:line:)` 函数接受自动闭包作为它的 `condition` 和 `message` 参数; 其 `condition` 参数仅在 Debug 版本中计算，而其 `message` 参数仅在 `condition` 为 `false` 时计算。 + +自动闭包允许您延迟计算，因为在你调用这个闭包之前，内部代码不会运行。延迟计算对于有副作用或高计算成本的代码非常有用，因为它使得你能控制代码的执行时机。下面的代码展示了闭包如何延时计算。 + +```swift +var customersInLine = ["Chris", "Alex", "Ewa", "Barry", "Daniella"] +print(customersInLine.count) +// 打印 “5” + +let customerProvider = { customersInLine.remove(at: 0) } +print(customersInLine.count) +// 打印 ”5“ + +print("Now serving \(customerProvider())!") +// 打印 “Now serving Chris!” +print(customersInLine.count) +// 打印 “4” +``` + + + + + + + +即使 `customersInLine` 数组的第一个元素被闭包内的代码删除，但在实际调用闭包之前，不会删除数组中的元素。如果从不调用闭包，则永远不会计算闭包内的表达式，这意味着永远不会删除数组元素。请注意，`customerProvider` 的类型不是 `String`，而是 `() -> String` --- 一个返回字符串的没有参数的函数。 + +将闭包作为参数传递给函数时，你能获得同样的延时计算行为。 + +```swift +// customersInLine 是 ["Alex", "Ewa", "Barry", "Daniella"] +func serve(customer customerProvider: () -> String) { + print("Now serving \(customerProvider())!") +} +serve(customer: { customersInLine.remove(at: 0) } ) +// 打印 “Now serving Alex!” +``` + + + +上面列表中的 `serve(customer:)` 函数接受一个显式的闭包，该闭包返回顾客的名字。下面的 `serve(customer:)` 版本执行相同的操作，但它不接受显式的闭包，而是通过将其参数类型标记为 `@autoclosure` 特性来接受一个自动闭包。现在你可以调用这个函数，就像它接受一个 `String` 参数而不是闭包一样。因为 customerProvider 参数的类型被标记为 `@autoclosure` 特性，所以参数会自动转换为闭包。 + +```swift +// customersInLine 是 ["Ewa", "Barry", "Daniella"] +func serve(customer customerProvider: @autoclosure () -> String) { + print("Now serving \(customerProvider())!") +} +serve(customer: customersInLine.remove(at: 0)) +// 打印 “Now serving Ewa!” +``` + + + +> 注意: 过度使用自动闭包可能会使您的代码难以理解。上下文和函数名称应明确表示计算正在被推迟。 + +如果您想要允许一个自动闭包可以逃逸，请同时使用 `@autoclosure` 和 `@escaping` 属性。`@escaping` 属性在上面的 中进行了描述。 + +```swift +// customersInLine 是 ["Barry", "Daniella"] +var customerProviders: [() -> String] = [] +func collectCustomerProviders(_ customerProvider: @autoclosure @escaping () -> String) { + customerProviders.append(customerProvider) +} +collectCustomerProviders(customersInLine.remove(at: 0)) +collectCustomerProviders(customersInLine.remove(at: 0)) + +print("Collected \(customerProviders.count) closures.") +// 打印 “Collected 2 closures.” +for customerProvider in customerProviders { + print("Now serving \(customerProvider())!") +} +// 打印 “Now serving Barry!” +// 打印 ”Now serving Daniella!“ +``` + + + +在上面的代码中，`collectCustomerProviders(_:)` 函数将闭包追加到 `customerProviders` 数组中，而不是调用作为其 `customerProvider` 参数传递给它的闭包。数组是在函数的作用域之外声明的，这意味着数组中的闭包可以在函数返回后执行。因此 `customerProvider` 参数必须允许逃逸出函数的作用域。 + + diff --git a/swift-6.docc/LanguageGuide/CollectionTypes.md b/swift-6.docc/LanguageGuide/CollectionTypes.md new file mode 100644 index 000000000..1d98d612a --- /dev/null +++ b/swift-6.docc/LanguageGuide/CollectionTypes.md @@ -0,0 +1,1239 @@ +# 集合类型 + +使用数组、集合和字典组织数据。 + +Swift 提供了三种主要的 **集合类型**，分别是数组、集合和字典，用于存储值集合。数组是有序的值集合。集合是无序的唯一值集合。字典是无序的键值对关联集合。 + +![](CollectionTypes_intro) + +Swift 中的数组、集合和字典对于它们可以存储的值和键的类型始终是明确的。这意味着你不能错误地将一个类型不匹配的值插入到集合中。同时，这也意味着你可以放心地知道从集合中取出的值的类型。 + +> 注意: Swift 的数组、集合和字典类型是作为 **泛型集合** 实现的。 +> 有关泛型类型和集合的更多信息，请参阅 . + + + + + + + +## 集合的可变性 + +如果您创建一个数组、集合或字典，并将其赋值给一个变量，则创建的集合将是 **可变的**。这意味着，在创建集合后，您可以通过添加、删除或更改集合中的元素来改变（或称为 **变异**）集合。如果您将数组、集合或字典分配给常量，则该集合是 **不可变的**，并且其大小和内容无法更改。 + +> 注意: 在所有不需要更改的情况下，创建不可变集合是一种良好的实践。这样做可以使你更容易理解代码，并使 Swift 编译器能够优化你创建的集合的性能。 + +## 数组 + +**数组** 将相同类型的值存储在一个有序列表中。相同的值可以在数组中以不同位置多次出现。 + +> 注意: Swift 的 `Array` 类型与 Foundation 的 `NSArray` 类进行了桥接。 +> 有关如何在 Foundation 和 Cocoa 中使用 `Array` 的更多信息，请参阅相关文档 +> [Bridging Between Array and NSArray](https://developer.apple.com/documentation/swift/array#2846730). + +### 数组类型简写语法 + +Swift 数组的类型完整写作 `Array`，其中 `Element` 是数组允许存储的值的类型。你也可以以简写形式 `[Element]` 来表示数组的类型。虽然这两种形式在功能上是相同的，但简写形式更受欢迎，并且在本指南中提到数组类型时将优先使用这种形式。 + +### 创建空数组 + +您可以使用构造器语法创建某种类型的空数组： + +```swift +var someInts: [Int] = [] +print("someInts is of type [Int] with \(someInts.count) items.") +// 打印 “someInts is of type [Int] with 0 items.“ +``` + + + +请注意，`someInts` 变量的类型根据初始化器的类型推断为 `[Int]`。 + +或者，如果上下文已经提供了类型信息，例如函数参数或已经定义类型的变量或常量，你可以使用空数组字面量 `[]`（一对空的方括号）来创建一个空数组： + +```swift +someInts.append(3) +// someInts 现在包含 1 个类型为 Int 的值 +someInts = [] +// someInts 现在是一个空数组, 但它仍是 [Int] 类型的 +``` + + + +### 使用默认值创建数组 + +Swift 的 `Array` 类型还提供了一个构造器，用于创建特定大小的数组，其所有值都设置为相同的默认值。您向此构造器传递适当类型的默认值（称为 `repeating`）：以及该值在新数组中重复的次数（称为 `count`）： + +```swift +var threeDoubles = Array(repeating: 0.0, count: 3) +// threeDoubles 的类型是 [Double]，并且等于 [0.0, 0.0, 0.0] +``` + + + +### 通过合并两个数组创建一个新数组 + +您可以通过使用加法运算符 `（+）` 将两个具有兼容类型的现有数组相加来创建新数组。新数组的类型是从您相加的两个数组的类型推断出来的： + +```swift +var anotherThreeDoubles = Array(repeating: 2.5, count: 3) +// anotherThreeDoubles 的类型是 [Double]，并且等于 [2.5, 2.5, 2.5] + +var sixDoubles = threeDoubles + anotherThreeDoubles +// sixDoubles 被推断为 [Double] 类型，并且等于 [0.0, 0.0, 0.0, 2.5, 2.5, 2.5] +``` + + + + + + + +### 使用数组字面量创建数组 + +您还可以使用 **数组字面量** 来初始化数组，这是将一个或多个值写入数组集合的简写方法。数组字面量以值列表的形式写入，用逗号分隔，用一对方括号括起来： + +```swift +[<#value 1#>, <#value 2#>, <#value 3#>] +``` + +下面的示例创建了一个名为 `shoppingList` 的数组来存储 `String` 值： + +```swift +var shoppingList: [String] = ["Eggs", "Milk"] +// shoppingList 已经用两个初始项进行了初始化 +``` + + + +`shoppingList` 变量被声明为“字符串值数组”，写作 `[String]`。由于该数组指定了值类型为 `String`，因此它只允许存储 `String` 类型的值。在这里，`shoppingList` 数组通过数组字面量初始化了两个 `String` 值（`"Eggs"` 和 `"Milk"`）。 + +> 注意: `shoppingList` 数组被声明为变量（使用 `var` 关键字）而不是常量（使用 `let` 关键字），因为在下面的示例中，更多的商品要被添加到购物清单中。 + +在这个例子中，数组字面量只包含两个 `String` 值，且没有其他内容。这与 `shoppingList` 变量的声明类型（一个只能包含 `String` 值的数组）相匹配，因此允许使用这个数组字面量来初始化 `shoppingList`，并包含两个初始项目。 + +得益于 Swift 的类型推断功能，如果您使用包含相同类型值的数组字面量进行初始化，则无需显式地写出数组的类型。`shoppingList` 的初始化可以改为以更简短的形式编写： + +```swift +var shoppingList = ["Eggs", "Milk"] +``` + + + +由于数组字面量中的所有值都是相同类型，Swift 可以推断出 `[String]` 是 `shoppingList` 变量的正确类型。 + +### 访问和修改数组 + +您可以通过数组的方法和属性或使用下标语法来访问和修改数组。 + +要找出数组中的项数，可以检查其只读属性 `count`： + +```swift +print("The shopping list contains \(shoppingList.count) items.") +// 打印 “The shopping list contains 2 items.“ +``` + + + +使用布尔值 `isEmpty` 属性作为检查 `count` 属性是否等于 `0` 的快捷方式： + +```swift +if shoppingList.isEmpty { + print("The shopping list is empty.") +} else { + print("The shopping list isn't empty.") +} +// 打印 “The shopping list isn't empty.“ +``` + + + +您可以通过调用数组的 `append(_:)` 方法将新元素添加到数组的末尾： + +```swift +shoppingList.append("Flour") +// shoppingList 现在包含 3 项，而有人正在做煎饼 +``` + + + +或者，可以使用加法赋值运算符（`+=`）将一个或多个兼容项的数组追加到现有数组中： + +```swift +shoppingList += ["Baking Powder"] +// shoppingList 现在包含 4 项 +shoppingList += ["Chocolate Spread", "Cheese", "Butter"] +// shoppingList 现在包含 7 项 +``` + + + +使用 **下标语法** 从数组中检索值，在数组名称后面的方括号内传递要检索的值的索引： + +```swift +var firstItem = shoppingList[0] +// firstItem 的值为 “Eggs” +``` + + + +> 注意: 数组中的第一项的索引为 `0`，而不是 `1`。Swift 中的数组始终是零索引的。 + +您可以使用下标语法来更改给定索引处的现有值： + +```swift +shoppingList[0] = "Six eggs" +// 列表中的第一个项现在是 “Six eggs” 而不是 “Eggs” +``` + + + +当您使用下标语法时，您指定的索引需要有效。例如，编写 `shoppingList[shoppingList.count] = "Salt"` 以尝试将项目追加到数组末尾会导致运行时错误。 + + + +您还可以使用下标语法一次更改一个范围的值，即使替换值集的长度与要替换的范围不同。以下示例将 `"Chocolate Spread"`, `"Cheese"` 和 `"Butter"` 替换为 `"Bananas"` 和 `"Apples"`： + +```swift +shoppingList[4...6] = ["Bananas", "Apples"] +// shoppingList 现在包含 6 项 +``` + + + +要将项目插入数组中指定索引处，请调用数组的 `insert(_:at:)` 方法： + +```swift +shoppingList.insert("Maple Syrup", at: 0) +// shoppingList 现在包含 7 项 +// ”Maple Syrup“ 现在是列表中的第一项 +``` + + + +对 `insert(_:at:)` 方法的调用会在购物清单的最开头插入一个值为 `"Maple Syrup"` 的新项目，由索引 `0` 表示。 + +同样，使用 `remove(at:)` 方法从数组中删除项目。此方法删除指定索引处的项目并返回已删除的项目（如果您不需要，可以忽略返回的值）： + +```swift +let mapleSyrup = shoppingList.remove(at: 0) +// 索引 0 处的项刚刚被移除了 +// shoppingList 现在包含 6 项，且不包含 Maple Syrup +// mapleSyrup 常量现在等于已移除的 “Maple Syrup” 字符串 +``` + + + +> 注意: 如果您尝试访问或修改超出数组现有边界的索引的值，将触发运行时错误。您可以在使用索引之前通过将其与数组的 `count` 属性进行比较来检查索引是否有效。数组中最大的有效索引是 `count - 1`，因为数组是从零开始编制索引的，---但是，当 `count` 为 `0`（意味着数组为空）时，没有有效的索引。 + +当删除一个项目时，数组中的任何间隙都会被关闭，因此索引 `0` 处的值再次等于 `"Six eggs"`： + +```swift +firstItem = shoppingList[0] +// firstItem 现在等于 “Six eggs” +``` + + + +如果要从数组中删除最后一项，请使用 `removeLast()` 方法而不是 `remove(at:)` 方法，以避免查询数组的 `count` 属性。与 `remove(at:)` 方法一样，`removeLast()` 返回已删除的项目： + +```swift +let apples = shoppingList.removeLast() +// 数组中的最后一项刚刚被移除了 +// shoppingList 现在包含 5 项，且不包含 apples +// apples 常量现在等于已移除的 “Apples” 字符串 +``` + + + +### 遍历一个数组 + +您可以使用 `for`-`in` 循环遍历数组中整个的值的集合： + +```swift +for item in shoppingList { + print(item) +} +// Six eggs +// Milk +// Flour +// Baking Powder +// Bananas +``` + + + +如果你需要每个项目的整数索引及其值，请使用 `enumerated()` 方法遍历数组。对于数组中的每个元素，`enumerated()` 方法返回一个由整数和项组成的元组。整数从 0 开始，每个项目按 1 计数; 如果枚举整个数组，则这些整数将与这元素的索引匹配。您可以将这些元组分解为临时常量或变量，作为遍历的一部分： + +```swift +for (index, value) in shoppingList.enumerated() { + print("Item \(index + 1): \(value)") +} +// Item 1: Six eggs +// Item 2: Milk +// Item 3: Flour +// Item 4: Baking Powder +// Item 5: Bananas +``` + + + +有关 `for`-`in` 循环的更多信息，请参阅 . + +## 集合 + +**集合** 将相同类型的不同值存储在没有定义序列化的集合中。当项的顺序不重要，或者需要确保项只出现一次时，可以使用集合而不是数组。 + +> 注意: Swift 的 `Set` 类型桥接到 Foundation 的 `NSSet` 类。 +> +> 有关将 `Set` 与 Foundation 和 Cocoa 一起使用的更多信息，请参阅 [Bridging Between Set and NSSet](https://developer.apple.com/documentation/swift/set#2845530). + + + +### 集合类型的哈希值 + +一个类型必须是 **可哈希的** 才能存储在集合中---也就是说，该类型必须提供一种为自身计算 **哈希值** 的方法。哈希值是一个 `Int` 类型的值，对于所有相等的对象，它们的哈希值相同。也就是说，如果 `a == b`，那么 `a` 的哈希值必须等于 `b` 的哈希值。 + +Swift 的所有基本类型（如 `String`、`Int`、`Double` 和 `Bool`）默认都是可哈希的，可以用作集合的值类型或字典的键类型。没有关联值的枚举 case 值（如 中描述的那样）默认也是可哈希的。 + +> 注意: 你可以通过让自定义类型遵循 Swift 标准库中的 `Hashable` 协议，将它们用作集合的值类型或字典的键类型。有关实现所需 `hash(into:)` 方法的信息，请参阅 [`Hashable`](https://developer.apple.com/documentation/swift/hashable)。有关遵守协议的信息，请参阅 。 + +### 集类型语法 + +Swift 集合的类型写作 `Set`，其中 `Element` 是集合允许存储的类型。与数组不同，集合没有相应的简写形式。 + +### 创建和初始化一个空集 + +你可以使用构造器语法创建一个特定类型的空集： + +```swift +var letters = Set() +print("letters is of type Set with \(letters.count) items.") +// 打印 “letters is of type Set with 0 items.“ +``` + + + +> 注意: `letters` 变量的类型根据构造器的类型推断为 `Set`。 + +或者如果上下文已经提供了类型信息，例如函数参数或已经定义类型的变量或常量，您可以使用空数组字面量创建一个空集合： + +```swift +letters.insert("a") +// letters 现在包含 1 个类型为 Character 的值 +letters = [] +// letters 现在是一个空集合，但仍然是 Set 类型 +``` + + + +### 使用数组字面量创建集合 + +你也可以使用数组字面量初始化一个集合，这是一种将一个或多个值写入集合的简写方式。 + +下面的示例创建了一个名为 `favoriteGenres` 的集合，用于存储 `String` 值： + +```swift +var favoriteGenres: Set = ["Rock", "Classical", "Hip hop"] +// favoriteGenres 已经用三个初始元素进行了初始化 +``` + + + +`favoriteGenres` 变量被声明为“`String` 值的集合”，写作 `Set`。由于该集合指定了值类型为 `String`，因此它 **只能** 存储 `String` 值。在这里，`favoriteGenres` 集合通过数组字面量初始化了三个 `String` 值（`"Rock"`、`"Classical"` 和 `"Hip hop"`）。 + +> 注意: `favoriteGenres` 集合被声明为变量（使用 `var` 关键字），而不是常量（使用 `let` 关键字），因为在后续的示例中会添加和移除元素。 + +集合类型无法仅通过数组字面量推断，因此必须显式声明类型 `Set`。但是，由于 Swift 的类型推断功能，如果你使用一个只包含单一类型值的数组字面量进行初始化，就无需写出集合元素的类型。`favoriteGenres` 的初始化可以用更简短的形式编写为： + +```swift +var favoriteGenres: Set = ["Rock", "Classical", "Hip hop"] +``` + + + +因为数组字面量中的所有值都是相同的类型，Swift 可以推断出 `Set` 是 `favoriteGenres` 变量的正确类型。 + +### 访问和修改集合 + +您可以通过集合的方法和属性来访问和修改集合。 + +要查找集合中的元素数量，可以检查其只读属性 `count`： + +```swift +print("I have \(favoriteGenres.count) favorite music genres.") +// 打印 “I have 3 favorite music genres.“ +``` + + + +使用布尔类型的 `isEmpty` 属性作为检查 `count` 属性是否等于 `0` 的快捷方式： + +```swift +if favoriteGenres.isEmpty { + print("As far as music goes, I'm not picky.") +} else { + print("I have particular music preferences.") +} +// 打印 “I have particular music preferences.“ +``` + + + +您可以通过调用集合的 `insert(_:)` 方法将新元素添加到集合中： + +```swift +favoriteGenres.insert("Jazz") +// favoriteGenres 现在包含 4 项 +``` + + + +你可以通过调用集合的 `remove(_:)` 方法从集合中移除一个元素，该方法会在元素是集合的成员时将其移除，并返回被移除的值；如果集合中不包含该元素，则返回 `nil`。另外，可以使用 `removeAll()` 方法移除集合中的所有元素。 + +```swift +if let removedGenre = favoriteGenres.remove("Rock") { + print("\(removedGenre)? I'm over it.") +} else { + print("I never much cared for that.") +} +// 打印 “Rock? I'm over it.“ +``` + + + +要检查集合是否包含特定元素，可以使用 `contains(_:)` 方法。 + +```swift +if favoriteGenres.contains("Funk") { + print("I get up on the good foot.") +} else { + print("It's too funky in here.") +} +// 打印 “It's too funky in here.“ +``` + + + +### 遍历集合 + +您可以使用 `for`-`in` 循环遍历集合中的值。 + +```swift +for genre in favoriteGenres { + print("\(genre)") +} +// Classical +// Jazz +// Hip hop +``` + + + +有关 `for`-`in` 循环的更多信息，请参阅 . + +Swift 的 `Set` 类型没有定义的顺序。要按特定顺序遍历集合中的值，可以使用 `sorted()` 方法，该方法返回集合的元素作为一个数组，并按 `<` 运算符排序。 + +```swift +for genre in favoriteGenres.sorted() { + print("\(genre)") +} +// Classical +// Hip hop +// Jazz +``` + + + +## 执行集合操作 + +你可以高效地执行基本的集合操作，例如将两个集合组合在一起、确定两个集合之间的共同值，或判断两个集合是否包含相同的所有值、部分值或没有相同的值。 + +### 基本集合操作 + +下图描绘了 --- `a` 和 `b` --- 两个集合，其中各种集合操作的结果由阴影区域表示。 + +![](setVennDiagram) + +- 使用 `intersection(_:)` 方法创建一个只包含两个集合共有值的新集合。 +- 使用 `symmetricDifference(_:)` 方法创建一个包含两个集合中存在但不同时存在的值的新集合。 +- 使用 `union(_:)` 方法创建一个包含两个集合中所有值的新集合。 +- 使用 `subtracting(_:)` 方法创建一个不包含指定集合中值的新集合。 + +```swift +let oddDigits: Set = [1, 3, 5, 7, 9] +let evenDigits: Set = [0, 2, 4, 6, 8] +let singleDigitPrimeNumbers: Set = [2, 3, 5, 7] + +oddDigits.union(evenDigits).sorted() +// [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] +oddDigits.intersection(evenDigits).sorted() +// [] +oddDigits.subtracting(singleDigitPrimeNumbers).sorted() +// [1, 9] +oddDigits.symmetricDifference(singleDigitPrimeNumbers).sorted() +// [1, 2, 9] +``` + + + + + +### 集合成员关系与相等 + +下图描述了三个集合 --- `a`、`b` 和 `c`，其中重叠区域表示集合间共享的元素。集合 `a` 是集合 `b` 的 **超集**，因为 `a` 包含了 `b` 中的所有元素。相反，集合 `b` 是集合 `a` 的 **子集**，因为 `b` 中的所有元素都包含在 `a` 中。集合 `b` 和集合 `c` 是 **不相交的**，因为它们没有任何共同的元素。 + +![](setEulerDiagram) + +- 使用 “等于” 运算符 （`==`）判断两个集合是否包含相同的所有值。 +- 使用 `isSubset(of:)` 方法判断一个集合的所有值是否包含在指定集合中。 +- 使用 `isSuperset(of:)` 方法判断一个集合是否包含指定集合中的所有值。 +- 使用 `isStrictSubset(of:)` 或 `isStrictSuperset(of:)` 方法判断一个集合是否是指定集合的子集或超集（但不相等）。 +- 使用 `isDisjoint(with:)` 方法判断两个集合是否没有共同的值。 + +```swift +let houseAnimals: Set = ["🐶", "🐱"] +let farmAnimals: Set = ["🐮", "🐔", "🐑", "🐶", "🐱"] +let cityAnimals: Set = ["🐦", "🐭"] + +houseAnimals.isSubset(of: farmAnimals) +// true +farmAnimals.isSuperset(of: houseAnimals) +// true +farmAnimals.isDisjoint(with: cityAnimals) +// true +``` + + + + + +## 字典 + +**字典** 将相同类型的键与集合中相同类型的值之间的关联存储在集合中，没有定义顺序。每个值都与一个唯一键相关联，该 **键** 充当字典中该值的标识符。与数组中的元素不同，字典中的元素没有指定的顺序。当您需要根据值的标识符查找值时可以使用字典，其方式与使用实际字典查找特定单词的定义的方式大致相同。 + +> 注意: Swift 的 `Dictionary` 类型与 Foundation 的 `NSDictionary` 类相互桥接。 +> +> 有关将 `Dictionary` 与 Foundation 和 Cocoa 一起使用的更多信息，请查阅 [Bridging Between Dictionary and NSDictionary](https://developer.apple.com/documentation/swift/dictionary#2846239). + + +### 字典类型简写语法 + +Swift 字典的完整类型写作 `Dictionary`，其中 `Key` 是可以用作字典键的值类型，而 `Value` 是字典为这些键存储的值类型。 + +> 注意: 字典 `Key` 类型必须遵循 `Hashable` 协议，这与集合的值类型相同。 + +您也可以将字典的类型以简写形式写成 `[Key: Value]`。虽然这两种形式在功能上是相同的，但简写形式更受欢迎，并且在本指南中提到字典类型时将使用这种形式。 + +### 创建空字典 + +与数组一样，您可以使用构造器语法创建特定类型的空 `Dictionary`： + +```swift +var namesOfIntegers: [Int: String] = [:] +// namesOfIntegers 是一个空的 [Int: String] 字典 +``` + + + +此示例创建了一个类型为 `[Int: String]` 的空字典，以存储整数值的可读名称。它的键的类型为 `Int`，值的类型为 `String`。 + +如果上下文已经提供了类型信息，您可以使用空字典字面量创建一个空字典，写作 `[:]`（在一对方括号内的冒号）。 + +```swift +namesOfIntegers[16] = "sixteen" +// namesOfIntegers 现在包含 1 个键值对 +namesOfIntegers = [:] +// namesOfIntegers 再次成为一个类型为 [Int: String] 的空字典 +``` + + + +### 使用字典字面量创建字典 + +您还可以使用 **字典字面量** 初始化字典，其语法与之前看到的数组字面量类似。字典字面量是一种简便方式，可以将一个或多个键值对写成 `Dictionary` 集合。 + +**键值对** 是键与值的组合。在字典字面量中，每个键值对中的键和值由冒号分隔。键值对以列表形式书写，用逗号分隔，并用一对方括号括起来： + +```swift +[<#key 1#>: <#value 1#>, <#key 2#>: <#value 2#>, <#key 3#>: <#value 3#>] +``` + +下面的示例创建了一个字典，用于存储国际机场的名称。在这个字典中，键是三个字母的国际航空运输协会代码，而值是机场名称： + +```swift +var airports: [String: String] = ["YYZ": "Toronto Pearson", "DUB": "Dublin"] +``` + + + +`airports` 字典被声明为类型 `[String: String]`，这意味着“这是一个键的类型为 `String`，值的类型也为 `String` 的字典”。 + +> 注意: `airports` 字典被声明为一个变量（使用 `var` 关键字），而不是常量（使用 `let` 关键字），因为在下面的示例中会向字典中添加更多机场。 + +`airports` 字典通过一个包含两个键值对的字典字面量进行初始化。第一个键值对的键是 `"YYZ"`，值是 `"Toronto Pearson"`。第二个键值对的键是 `"DUB"`，值是 `"Dublin"`。 + +这个字典字面量包含两个 `String: String` 键值对。这个键值类型与 `airports` 变量声明的类型匹配（即只能 `String` 键和只能 `String` 值的字典），因此将字典字面量赋值给 `airports` 字典是允许的，从而用两个初始项初始化该字典。 + +与数组一样，如果使用的字典字面量的键和值具有一致的类型，则不必指定字典的类型。`airports` 的初始化可以用更简短的形式来编写，如下所示： + +```swift +var airports = ["YYZ": "Toronto Pearson", "DUB": "Dublin"] +``` + + + +因为字面量中的所有键都是同一类型，所有值也是同一类型，Swift 能够推断出 `[String: String]` 是用于 `airports` 字典的正确类型。 + +### 访问和修改字典 + +您可以通过字典的方法和属性，或者使用下标语法来访问和修改字典。 + +与数组一样，您可以通过检查字典的只读属性 `count` 来获取 `Dictionary` 中元素的数量。 + +```swift +print("The airports dictionary contains \(airports.count) items.") +// 打印 “The airports dictionary contains 2 items.“ +``` + + + +使用布尔 `isEmpty` 属性可以快速检查 `count` 属性是否等于 `0`。 + +```swift +if airports.isEmpty { + print("The airports dictionary is empty.") +} else { + print("The airports dictionary isn't empty.") +} +// 打印 “The airports dictionary isn't empty.“ +``` + + + +您可以使用下标语法向字典添加新元素。使用适当类型的新键作为下标索引，并分配一个适当类型的新值： + +```swift +airports["LHR"] = "London" +// airports 字典现在包含 3 项 +``` + + + +您还可以使用下标语法来更改与特定键关联的值： + +```swift +airports["LHR"] = "London Heathrow" +// “LHR”的值已更改为“London Heathrow” +``` + + + +作为下标的替代方法，请使用字典的 `updateValue(_:forKey:)` 方法来设置或更新特定键的值。与上面的下标示例一样，`updateValue(_:forKey:)` 方法会在键不存在时为该键设置一个值，或者在该键已存在时更新其值。但是，与下标不同的是，`updateValue(_:forKey:)` 方法在执行更新后返回 **旧** 值。这使您能够检查是否进行了更新。 + +`updateValue(_:forKey:)` 方法返回字典值类型的可选值。例如，对于存储 `String` 值的字典，该方法返回类型为 `String?`，即“可选的 `String`”。这个可选值在更新前如果该键存在，则包含该键的旧值；如果该键之前没有值，则返回 `nil`。 + +```swift +if let oldValue = airports.updateValue("Dublin Airport", forKey: "DUB") { + print("The old value for DUB was \(oldValue).") +} +// 打印 “The old value for DUB was Dublin.“ +``` + + + +您还可以使用下标语法从字典中为特定键检索值。由于可能请求不存在值的键，字典的下标会返回该字典值类型的可选值。如果字典中包含所请求键的值，下标将返回一个包含该键现有值的可选值。否则，下标将返回 `nil`。 + +```swift +if let airportName = airports["DUB"] { + print("The name of the airport is \(airportName).") +} else { + print("That airport isn't in the airports dictionary.") +} +// 打印 “The name of the airport is Dublin Airport.“ +``` + + + +你可以使用下标语法通过为某个键赋值为 `nil` 来从字典中删除一个键值对。 + +```swift +airports["APL"] = "Apple International" +// “Apple International“ 不是APL的真实机场，所以删除它 +airports["APL"] = nil +// APL 已经从字典中删除 +``` + + + +您还可以使用 `removeValue(forKey:)` 方法从字典中删除键值对。该方法在键值对存在时会将其移除并返回被移除的值，如果不存在该值，则返回 `nil`。 + +```swift +if let removedValue = airports.removeValue(forKey: "DUB") { + print("The removed airport's name is \(removedValue).") +} else { + print("The airports dictionary doesn't contain a value for DUB.") +} +// 打印 “The removed airport's name is Dublin Airport.“ +``` + + + +### 遍历字典 + +您可以使用 `for`-`in` 循环遍历字典中的键值对。字典中的每个元素会作为一个 `(key, value)` 元组返回，您可以在迭代过程中将元组的成员分解为临时常量或变量。 + +```swift +for (airportCode, airportName) in airports { + print("\(airportCode): \(airportName)") +} +// LHR: London Heathrow +// YYZ: Toronto Pearson +``` + + + +有关 `for`-`in` 循环的更多信息，请参阅 . + +您还可以通过访问字典的 `keys` 和 `values` 属性，获取字典的键或值的可遍历集合。 + +```swift +for airportCode in airports.keys { + print("Airport code: \(airportCode)") +} +// Airport code: LHR +// Airport code: YYZ + +for airportName in airports.values { + print("Airport name: \(airportName)") +} +// Airport name: London Heathrow +// Airport name: Toronto Pearson +``` + + + +如果您需要将字典的键或值与采用 `Array` 实例的 API 一起使用，可以使用 `keys` 或 `values` 属性初始化一个新的数组。 + +```swift +let airportCodes = [String](airports.keys) +// airportCodes 赋值为 ["LHR", "YYZ"] + +let airportNames = [String](airports.values) +// airportNames 赋值为 ["London Heathrow", "Toronto Pearson"] +``` + + + +Swift 的 `Dictionary` 类型没有定义的顺序。要以特定顺序迭代字典的键或值，可以对其 `keys` 或 `values` 属性使用 `sorted()` 方法。 + + diff --git a/swift-6.docc/LanguageGuide/Concurrency.md b/swift-6.docc/LanguageGuide/Concurrency.md new file mode 100644 index 000000000..263806f1c --- /dev/null +++ b/swift-6.docc/LanguageGuide/Concurrency.md @@ -0,0 +1,1005 @@ +# 并发 + +执行异步操作。 + +Swift 原生支持结构化的异步和并行代码。 + +*异步代码*是能够被暂时挂起并在稍后继续执行的代码，不过在同一时刻中只有一段程序代码执行。通过挂起和恢复代码，你的程序就可以在执行耗时很长的任务时抽空执行一些快速的操作，比如在下载文件、解析文件的过程中更新 UI。*并行代码*则意味着多段代码可以在同一时刻执行；例如，一台拥有四核处理器的电脑可以同时运行四段代码（每个核心执行一项任务）。一款运用并行和异步代码编写的程序可以同时运行多个任务，且可以在等待外部系统处理时，暂时挂起一些任务。 + +并发和异步代码在增加调度灵活性的同时也会增加复杂度。Swift 能够在你在编写异步代码时，提供一些编译时检查——例如，你可以使用 actor 来安全地访问可变状态。然而，为一段运行缓慢或是有错误的代码添加并发能力，并不一定就能使它变得更快速或者更正确地运行。事实上，简单地为代码增加并发能力甚至可能导致代码问题更难排查。不过，对于的确有必要并发执行的代码来说，Swift 语言级别的并发支持能帮助你在编译时就捕捉到错误。 + +本章剩余的部分将使用*并发*一词指代异步和并行代码这一常见的组合。 + +> 如果你曾经编写过并发代码的话，那你可能习惯于使用线程。Swift 中的并发模型基于线程，但你不会直接与线程打交道。在 Swift 中，一个异步函数可以交出它在某个线程上的运行权 —— 这样，另一个异步函数在这个函数被阻塞时就能获得此在此线程上的运行权。但是，Swift 并不保证异步函数恢复运行时其将在哪条线程上运行。 + +你当然也可以不用 Swift 原生支持去写并发的代码，但这样代码的可读性会下降。比如，下面的这段代码会拉取一系列图片名称的列表，下载列表中的第一张图片然后展示给用户： + +```swift +listPhotos(inGallery: "Summer Vacation") { photoNames in + let sortedNames = photoNames.sorted() + let name = sortedNames[0] + downloadPhoto(named: name) { photo in + show(photo) + } +} +``` + + + +即便是编写这样一个简单的案例，代码中都不可避免地需要使用一系列的完成回调，这导致出现了多层嵌套的闭包。可想而知，使用这种方式编写更复杂的代码会产生更深的嵌套，从而使得代码迅速变得臃肿、难以阅读。 + +## 定义和调用异步函数 + +*异步函数* 或 *异步方法* 是一种能在运行中被挂起的特殊函数或方法。相比之下，普通的同步函数和方法只能持续运行到完成、抛出错误，或是永远不返回。异步函数或方法也能做到这三件事，但多出了等待其他资源时暂停执行的能力。在异步函数或者方法的代码块中，你需要明确标注这些可以暂停执行的位置。 + +要将一个函数或方法标记为异步，你需要在函数 / 方法签名的参数列表后边加上 `async` 关键字 —— 这和使用`throws` 关键字来标记可抛出错误的函数十分相似。如果你的函数或方法有返回值，你需要将 `async` 添加在返回箭头 (`->`) 的前面。比如，下面这段代码会从图库中提取照片名： + +```swift +func listPhotos(inGallery name: String) async -> [String] { + let result = // ... some asynchronous networking code ... + return result +} +``` + + + +对于既是异步又可抛出错误的方法或函数，请将 `async` 写在 `throws` 的前面。 + + + +在调用异步方法时，当前方法的执行会被暂时挂起，直到被调用异步方法返回。你需要在调用前增加 await 关键字来标记此处为可能的挂起点（suspension point）。这就和在调用会抛出错误的方法前需要添加 `try` 一样 —— 为了标记在发生错误时，程序的执行流程可能发生变化。在一个异步方法中，方法*只会*在调用另一个异步方法时被挂起 —— 挂起永远不会是隐式或抢占式的，所有可能的挂起点都会用 `await` 明确地标注出来。这样一来，并发代码的可读性就获得了提升。 + +举个例子，下面这段代码会读取图库中所有图片的名称，然后展示第一张图片： + +```swift +let photoNames = await listPhotos(inGallery: "Summer Vacation") +let sortedNames = photoNames.sorted() +let name = sortedNames[0] +let photo = await downloadPhoto(named: name) +show(photo) +``` + + + +由于 `listPhotos(inGallery:)` 和 `downloadPhoto(named:)` 这两个方法都需要发起网络请求，他们都可能耗时较长。为这两个函数在返回箭头前加上 `async` 可以将它们定义为异步函数，从而使得这部分代码在等待图片下载时，程序中的其他部分可以继续运行。 + +为了更好理解上面这段代码的并发本质，下面列举出这段程序其中一种可能的执行顺序： + +1. 代码从第一行开始执行到第一个 `await`，调用 `listPhotos(inGallery:)` 函数并且挂起这段代码的执行，等待这个函数的返回。 + +2. 当这段代码的执行被挂起时，程序的其他并行代码便获得了继续执行的机会。比如，也许此时后台有一个耗时长的任务会更新其他一些图库。这个新任务会一直执行到下一个被 `await` 的标记的挂起点，或者一直执行到完成。 + +3. 在 `listPhotos(inGallery:)` 函数返回之后，上面这段代码会从上次的挂起点处恢复并继续执行。函数的返回返回值会被赋给 photoNames 变量。 + +4. 定义 `sortedNames` 和 `name` 的两行代码是普通的同步代码。因为并没有被 `await` 标记，这里并不会有任何潜在的挂起点。 + +5. 接下来的 `await` 标记出现在调用 `downloadPhoto(named:)` 的地方。这里会再次暂停这段代码的执行直到函数返回，从而给了其他并行代码执行的机会。 + +6. 在 `downloadPhoto(named:)` 返回后，它的返回值会被赋值到 photo 变量中，然后被作为参数传递给 `show(_:)`。 + +代码中被 `await` 标记的挂起点表明当前这段代码可能会暂停等待异步方法或函数的返回。这也被称为*让出线程*，因为在幕后 Swift 会挂起这段代码在当前线程的执行，转而让其他代码在当前线程执行。因为有 `await` 标记的代码可以被挂起，所以在程序中只有特定的地方才能调用异步方法或函数： + +- 异步函数，方法或变量的内部的代码 + +- 静态函数 `main()` 中被打上 `@main` 标记的结构体、类或者枚举中的代码 + +- 非结构化的子任务中的代码，之后会在 中说明 + + + +你还可以通过调用 `Task.yield()` 来显式地插入挂起点。 + +[`Task.yield()`]: https://developer.apple.com/documentation/swift/task/3814840-yield + +```swift +func generateSlideshow(forGallery gallery: String) async { + let photos = await listPhotos(inGallery: gallery) + for photo in photos { + // ... 为这张照片渲染一段几秒钟的视频 ... + await Task.yield() + } +} +``` + +假设在上面这段代码中，渲染视频的部份是同步执行的，它其中不会包含任何挂起点。但是，渲染视频的任务又可能耗时很长。这种情况下，你就可以通过调用 `Task.yield()` 来手动添加挂起点。你可以通过以这种结构编写长时间运行的代码，来协助 Swift 在任务执行上取得平衡 —— 在长耗时的任务上取得进展的同时，也给程序中的其他任务提供了执行的机会。 + +在学习并发编程时， [`Task.sleep(for:tolerance:clock:)`][] 这个方法非常有用。这个方法会将当前任务挂起至少指定的时长。以下是 `listPhotos(inGallery:)` 这个函数的另一个版本，它使用 `sleep(for:tolerance:clock:)` 来模拟等待网络请求: + +[`Task.sleep(for:tolerance:clock:)`]: https://developer.apple.com/documentation/swift/task/sleep(for:tolerance:clock:) + +```swift +func listPhotos(inGallery name: String) async throws -> [String] { + try await Task.sleep(for: .seconds(2)) + return ["IMG001", "IMG99", "IMG0404"] +} +``` + + + +这个版本的 `listPhotos(inGallery:)` 既是异步的，又可能抛出错误，因为 `Task.sleep(until:tolerance:clock:)` 可能抛出错误。当你调用这个版本的 `listPhotos(inGallery:)` 时，你需要同时添加 `try` 和 `await`： + +```swift +let photos = try await listPhotos(inGallery: "A Rainy Weekend") +``` + +异步函数和可抛出错误的函数有一些相似点：当你定义一个异步或是可抛出错误的函数时，你不仅需要添加 `async` 或者 `throws`，还需要在调用这些方法的代码处添加 `await` 或 `try`。一个异步函数可以调用另一个异步函数，就像一个可抛出错误的函数也可以调用另一个可抛出错误的函数。 + +但是，这里有一个非常重要的区别。你可以通过用 `do-catch` 包裹可抛错误的代码来处理错误；也可以使用 `Result` 来存储这个错误，以便将错误交由其他地方的代码来处理。这个方法可以让你在不抛错误的函数里调用可抛错误的函数。例如： + +```swift +func availableRainyWeekendPhotos() -> Result<[String], Error> { + return Result { + try listDownloadedPhotos(inGallery: "A Rainy Weekend") + } +} +``` + +相比之下，并没有什么安全的方式可以让你在同步代码里执行异步代码并等待结果。因此，Swift 的标准库特意舍弃了这样不安全的功能 —— 如果你执意要自己编写这样的逻辑，最终很有可能导致难以排查的数据竞争、线程问题，或是死锁。当你向现有项目添加并发代码时，请自顶向下进行。特别地，请先将最顶层的代码转换为异步的，然后再开始转换被其调用的其他代码，以此类推一层一层地向下转换。自底向上改造代码的方法是不可能的，因为同步代码无法调用异步代码。 + + + +## 异步序列 + +前文的 `listPhotos(inGallery:)` 方法会在异步地准备好整个数组的所有元素后，一次性返回整个数组；另一种方式是使用*异步序列 (asynchronous sequence)* 在每个元素就绪的当下都将其返回。下面这段代码展示了如何遍历一个异步序列： + +```swift +import Foundation + +let handle = FileHandle.standardInput +for try await line in handle.bytes.lines { + print(line) +} +``` + + + +在这个事例中，我们使用 `for` 和 `await` 来代替了普通的 `for`-`in` 循环。和你调用异步函数或方法一样，在这里 `await` 也标注了潜在的挂起点。一个 `for`-`await`-`in` 循环在每一轮迭代的开头都有可能挂起，以便等待序列中下一个元素的就绪。 + + + +正如同你可以在 `for`-`in` 循环中通过遵从 [`Sequence`][] 协议来使用自定义类型一样，你也可以在 `for`-`await`-`in` 循环中通过遵循 [`AsyncSequence`] 来使用自定义类型。 + +[`Sequence`]: https://developer.apple.com/documentation/swift/sequence +[`AsyncSequence`]: https://developer.apple.com/documentation/swift/asyncsequence + + + +## 并行调用异步函数 + +使用 `await` 来调用异步方法时，在同一时刻只会有一段代码运行。在一段异步代码运行的过程中，调用方会先等待其返回，然后才会执行下一行代码。举个例子，如果你想要读取图库中的前三张照片，你可以像下方这样轮流等待三次对 `downloadPhoto(named:)` 调用的返回： + +```swift +let firstPhoto = await downloadPhoto(named: photoNames[0]) +let secondPhoto = await downloadPhoto(named: photoNames[1]) +let thirdPhoto = await downloadPhoto(named: photoNames[2]) + +let photos = [firstPhoto, secondPhoto, thirdPhoto] +show(photos) +``` + + + +这种方式有一个显著的缺点：尽管下载过程是异步的，并且其他的任务也可以在下载过程中继续执行，但每次只有一个 `downloadPhoto(named:)` 的调用会运行 —— 每张照片都只有在上一张照片完成下载后才会开始下载。然而，这些任务其实没有必要相互等待：每张照片都可以独立下载，甚至是在同一时间下载。 + +要想在调用异步函数时，允许其与周围的代码并行执行，你可以在使用 `let` 定义一个常量时，在前方添加 `async` 标注。然后，你需要在使用此常量时，添加 `await` 标记。 + +```swift +async let firstPhoto = downloadPhoto(named: photoNames[0]) +async let secondPhoto = downloadPhoto(named: photoNames[1]) +async let thirdPhoto = downloadPhoto(named: photoNames[2]) + +let photos = await [firstPhoto, secondPhoto, thirdPhoto] +show(photos) +``` + + + +在这个例子中，所有三次对 `downloadPhoto(named:)` 的调用都会立即开始，而不等待前一个调用返回。如果系统此时有足够的可用资源，三个下载任务会并行执行。注意到在这三次函数调用中，我们都没有使用 `await` 标注，因为这些调用并不会导致代码执行挂起。代码持续执行到 `photos` 被定义的那一行时，你才需要 `await` 来暂停程序执行，因为程序需要前面的调用结果才能为 `photos` 赋值。 + +你可以这样理解这两种方式的区别： + +- 当紧接下来的代码就需要依赖当前函数的返回值时，使用 `await` 来调用函数。这样一来，任务就可以按顺序执行。 +- 短时间内不需要异步函数的返回结果时，使用 `async-let` 来调用函数。这样任务就可以并行执行。 +- `await` 和 `async-let` 都允许其他任务在他们被挂起的时候执行。 +- 在两种情况下，都需要用 `await` 标记可能的挂起点，以表明代码在这些点在需要的情况下会被暂停，直到被调用的异步函数返回。 + +你也可以在同一段代码中混合使用两种方法。 + +## 任务和任务组 + +一项*任务 (task)*是一个单元的工作，且可以作为程序的一部分异步执行。所有的异步代码都在某一项任务中执行。一项任务本身只能完成一件事情，但当你创建多个任务时，Swift 可以让他们同时运行。 + +上一节中的 `async`-`let` 会隐式地创建一项子任务 —— 如果你已经知道程序需要执行什么任务，这种语法十分便捷。你也可以创建一个任务组（`TaskGroup` 的实例）然后显式地向其中添加子任务。这可以让你更好地控制优先级和任务取消，也可以让你动态决定要创建多少任务。 + +[`TaskGroup`]: https://developer.apple.com/documentation/swift/taskgroup + +任务的排列具有层级结构。同一个任务组中的所有任务都具有相同的父任务， 且他们也可以拥有自己的子任务。考虑到任务和任务组之间具有这种显式的关系，我们将这种范式称为*结构化并发 (structured concurrency)*。这种任务间的显式父 - 子关系有几种优势： + +- 杜绝了在父任务中忘记等待子任务完成的可能性。 +- 当子任务被赋予更高的优先级时，父任务的优先级也会随之自动提高。 +- 当父任务被取消时，其所有的子任务都会被自动取消。 +- 一项任务的本地值会自动而高效地扩散到子任务中。 + +在下面这个代码示例中，我们可以处理任意数量的照片下载任务： + +```swift +await withTaskGroup(of: Data.self) { group in + let photoNames = await listPhotos(inGallery: "Summer Vacation") + for name in photoNames { + group.addTask { + return await downloadPhoto(named: name) + } + } + + for await photo in group { + show(photo) + } +} +``` + +上面的代码创建了一个新的任务组，并创建了一些子任务，每个任务会下载一张照片。Swift 会在条件许可的情况下，尽可能多地并行执行这些任务。在某一项子任务完成下载后，其对应的照片就会立即被显示出来。不过，这些子任务会以任意顺序完成执行，所以这些照片的最终展示顺序也会是随机的。 + +> 如果下载照片的代码可能抛出错误，你需要调用 `withThrowingTaskGroup(of:returning:body:)`。 + +在上面的代码中，每张照片都会被下载然后展示出来，所以任务组没有返回值。对于需要返回结果的任务组，你可以在传递给 `withTaskGroup(of:returning:body:)` 的闭包中，编写聚合任务结果的逻辑。 + +``` +let photos = await withTaskGroup(of: Data.self) { group in + let photoNames = await listPhotos(inGallery: "Summer Vacation") + for name in photoNames { + group.addTask { + return await downloadPhoto(named: name) + } + } + + var results: [Data] = [] + for await photo in group { + results.append(photo) + } + + return results +} +``` + +如同之前的范例，这个示例中为每张照片创建了一个下载子任务。与之前不同的是，这里的 `for`-`await`-`in` 循环会等待下一个子任务结束，将其的结果插入到结果数组中，然后继续等待所有子任务完成。最后，这个任务组会将所有下载完毕的照片数组作为一个整体返回。 + + + +### 任务取消 + +Swift 中的并发使用的是「协作取消」模型：每个任务都应当在合适的位置检查其是否已被取消，然后对取消指令做出合理的响应。取决于任务所执行工作的性质，通常有这几种方式来响应取消指令： + +- 抛出类似 `CancellationError` 的错误 +- 返回 `nil` 或是一个空的集合 +- 返回部分完成的工作 + +如果图片较大或网络较慢，图片下载可能耗时非常长。要允许用户取消这项事务，而不必等待所有任务完成，这些任务必须要检查取消指令，并在收到指令时停止运行。有两种方式可以做到这件事：调用 `Task.checkCancellation()` 方法，或是检查 `Task.isCancelled` 属性。如果任务已被取消，调用 `checkCancellation()` 会抛出错误；一项可抛错误的任务可以将错误向外扩散，并停止任务中的所有工作。这样做的好处是代码编写简单、理解成本更低。要获得更多的灵活度，你可使用 `isCancelled` 属性，这样你就可以在停止任务的过程中，做一些例如关闭网络连接、清理临时文件的清理工作。 + +[`Task.checkCancellation()`]: https://developer.apple.com/documentation/swift/task/3814826-checkcancellation +[`Task.isCancelled` type]: https://developer.apple.com/documentation/swift/task/iscancelled-swift.type.property + +``` +let photos = await withTaskGroup(of: Optional.self) { group in + let photoNames = await listPhotos(inGallery: "Summer Vacation") + for name in photoNames { + let added = group.addTaskUnlessCancelled { + guard !Task.isCancelled else { return nil } + return await downloadPhoto(named: name) + } + guard added else { break } + } + + var results: [Data] = [] + for await photo in group { + if let photo { results.append(photo) } + } + return results +} +``` + +上面这段代码相比前一个版本有几个变动： + +- 每项任务都使用 [`TaskGroup.addTaskUnlessCancelled(priority:operation:)`][] 方法来添加, + 以避免在任务取消之后产生新的任务。 +- 在每次调用 `addTaskUnlessCancelled(priority:operation:)` 之后, + 这段代码都会确认子任务的确已经添加成功了。如果任务组已被取消，那么 `added` 的值就会为 `false` —— 这种情况下，代码会不再尝试下载更多的照片。 +- 每项任务都会在开始下载照片前，检查取消指令。如果发现任务已被取消，那么返回 `nil`。 +- 最后，任务组在收集结果时会跳过所有的 `nil`。通过返回 `nil` 来响应取消指令意味着任务组可以返回部分结果（也就是在取消的那一刻之前已经下载完成的照片），而不是将已完成的工作也一并丢弃。 + +[`TaskGroup.addTaskUnlessCancelled(priority:operation:)`]: https://developer.apple.com/documentation/swift/taskgroup/addtaskunlesscancelled(priority:operation:) + +> 若要在一项任务之外检查这项任务是否已被取消，请使用 `Task.isCancelled` 实例属性，而不是类属性。 + +[`Task.isCancelled` instance]: https://developer.apple.com/documentation/swift/task/iscancelled-swift.property + +如果你的事务需要在被取消时立即收到提醒，可以使用 [`Task.withTaskCancellationHandler(operation:onCancel:isolation:)`][] 方法。例如： + +[`Task.withTaskCancellationHandler(operation:onCancel:isolation:)`]: https://developer.apple.com/documentation/swift/withtaskcancellationhandler(operation:oncancel:isolation:) + +```swift +let task = await Task.withTaskCancellationHandler { + // ... +} onCancel: { + print("Canceled!") +} + +// ... 一段时间之后 ... +task.cancel() // 输出 "Canceled!" +``` + +在你使用取消回调时，任务取消依然是协作的：任务要么一直执行到完成，要么主动检查取消指令并提早停止。因为在取消回调开始执行时，任务本身依然还在运行，请注意避免在任务和取消回调之间共享状态 —— 这可能导致数据竞争。 + + + + + + + +### 非结构化并发 + +除了像前文所述那样以结构化的方式编写并发逻辑，Swift 也支持非结构化并发。不像从属于某个任务组的任务，一项*非结构化*的任务没有父任务。管理非结构化任务时，你将拥有最大的灵活性，可以按任意方式组织他们。但是，你也将需要对他们的正确性承担全部责任。要在当前 actor 上创建一项非结构化的任务，你可调用 [`Task.init(priority:operation:)`](https://developer.apple.com/documentation/swift/task/3856790-init) 这个构造器。要创建一项不属于当前 actor 的非结构化任务 —— 也被称作为*分离 (detached)* 任务 —— ，请调用 [`Task.detached(priority:operation:)`](https://developer.apple.com/documentation/swift/task/3856786-detached) 这个类方法。这两项操作都会返回 task 实例，便于你管理他们。比如，你可以等待他们的返回结果，也可以取消他们： + +```swift +let newPhoto = // ... some photo data ... +let handle = Task { + return await add(newPhoto, toGalleryNamed: "Spring Adventures") +} +let result = await handle.value +``` + +要了解更多有关如何管理分离任务的信息，请查看 [`Task`](https://developer.apple.com/documentation/swift/task). + + + +## Actors + +你可以使用任务来将自己的程序分割为相互独立、并行的片段。任务之间时相互隔离的，这样他们才能安全地同时运行。但有时候，你需要在任务之前共享信息。此时，你就可以使用 actors 来安全地在并行代码之间共享这些信息。 + +就和类一样，actor 也是应用类型，所以在 一文中有关引用类型和值类型的对比，同时适用于类和 actor。与类不同的是，actor 在同一时刻只允许一项任务访问其可变状态，这样多个任务同时与 actor 交互时才不会产生安全性问题。举个例子，下面是一个用于记录温度的 actor： + +```swift +actor TemperatureLogger { + let label: String + var measurements: [Int] + private(set) var max: Int + + init(label: String, measurement: Int) { + self.label = label + self.measurements = [measurement] + self.max = measurement + } +} +``` + + + +你可通过 `actor` 关键字和紧随其后的括弧来定义一个 actor。`TemperatureLogger` actor 具有可供外部代码访问的属性，也有 `max` 这个只有 actor 内部代码才能修改的属性。 + +你可通过使用与结构体和类相同的构造体语法来创建一个 `actor` 实例。在你访问一个 `actor` 的属性或方法时，需要使用 `await` 来表明这是一个潜在的挂起点。比如： + + +```swift +let logger = TemperatureLogger(label: "Outdoors", measurement: 25) +print(await logger.max) +// Prints "25" +``` + +在这个例子中，对 `logger.max` 的访问是一个可能的挂起点。这是因为其 actor 同一时刻只允许一项任务访问其可变状态。如果另一项任务正与 logger 进行交互，这段代码就需要先挂起，直到轮到他来访问这个属性。 + +相比之下，actor 内部的代码不需要使用 `await` 来访问 actor 的属性。比如，这是一个用于更新 `TemperatureLogger` 所记录温度的方法： + +```swift +extension TemperatureLogger { + func update(with measurement: Int) { + measurements.append(measurement) + if measurement > max { + max = measurement + } + } +} +``` + +`update(with:)` 这个方法已经是在 actor 上运行的了，所以它在访问 `max` 这样的属性时，不需要使用 `await` 来进行标注。这个方法也揭示了 actor 同一时刻仅允许一项任务与其可变状态交互的其中一项原因：有些对于 actor 状态的更新会暂时打破不变式。`TemperatureLogger` 这个 actor 记录了一系列温度数据以及一项最高温度，并且它应当会在记录一项新的测量数据时，更新最高温度纪录。在一次更新操作插入新的测量数据之后、更新最高温度纪录之前的那一刻，我们的温度记录器 actor 暂时处于一个非法的状态中。禁止多个任务同时与某个实例交互能够在下列事件序列中，防止问题的出现： + +1. 你的代码调用 `update(with:)` 方法，该方法先更新了 `measurements` 数组。 +2. 在你的代码能够更新 `max` 之前，其他地方的代码读取了最大温度值和温度数据列表。 +3. 然后，你的代码才更新了 `max` 数据，完成了整个更新流程。 + +在这种情况下，其它地方的代码会读取到错误的信息，因为他们对 actor 的访问被插入在了 `update(with:)` 执行过程的中间，从而读取到了不合法的数据。使用 Swift actor 能让你避免这种情况，因为它同一时刻只允许一项对其状态的操作，且代码执行只能在有 `await` 标注的挂起点处被打断；又因为 `update(with:)` 并不包含任何挂起点，其执行过程中没有任何其它代码可以访问数据。 + +如果 actor 之外的代码尝试直接访问这些属性，编译器会报错。比如： + +```swift +print(logger.max) // Error +``` + +不加 `await` 访问 `logger.max` 会失败，因为一个 actor 的属性是这个 actor 的本地受隔离状态的一部分。想要访问该属性的代码必须在此 actor 上执行，这是一个异步操作，所以必须用 `await` 标注。Swift 保证了只有在 actor 上运行的代码才能访问这个 actor 的本地状态。这种保证称为 *actor 隔离*。 + +Swift 并发模型的以下几个特点共同降低了使用者对共享可变属性的理解成本： + +- 挂起点之间的代码总是按顺序执行，且不可能被任何其它并发代码打断。 + +- 与一个 actor 本地状态交互的代码只会运行在这个 actor 之上。 + +- 一个 actor 一次只运行一段代码。 + +基于这些保证，处于一个 actor 之内、且不包含 `await` 的方法可以安全地对 actor 状态进行更新，而不用担心程序中的其它部分意外读取到不合法状态。 + +例如，下面这段代码会将测量到的温度从华氏度转换到摄氏度： + +```swift +extension TemperatureLogger { + func convertFahrenheitToCelsius() { + measurements = measurements.map { measurement in + (measurement - 32) * 5 / 9 + } + } +} +``` + +上面这段代码会逐个转换数组内的测量数据。在 map 操作的执行过程中，有些温度还是华氏度，有些则已经转换为了摄氏度。但是，因为其中没有任何代码包含 `await`，这里不会出现潜在的挂起点。这个方法所修改的状态从属于 actor，actor 为这个状态提供了保护，使其免受不在 actor 上运行的代码的读取或修改。这意味着其它代码不可能有办法读取到单位转换过程中，只被转换了一半的测量数据。 + +除了将代码编写在一个 actor 中，并在其中舍弃掉所有的挂起点之外，还有更进一步的方式来避免出现临时性的不合法状态：将这些代码移动到一个同步方法中。上方的 `convertFahrenheitToCelsius()` 方法就是一个同步方法，保证了它*永远*不可能包含挂起点。这个方法封装了会暂时造成不合法数据状态的代码，从而使得阅读代码的人更容易意识到：在这个方法完成自己的任务、并将合法的数据状态被恢复之前，没有其它代码可以运行。在未来，如果你试图向这个方法中添加并发代码，或是一个可能的挂起点，编译器会及时报错，以防引入新 bug。 + + + +## 可发送类型 + +任务和 actor 能让你将一个程序分成多个小段病安全地并行运行。一个任务或是一个 actor 的实例内部所包含的可变状态（例如变量或属性），被称为*并发域*。有些数据无法在不同的并发域之间共享，因为这些数据包含可变状态，但其又无法对重叠访问提供保护。 + +对于可以被从一个并发域共享到另一个并发域的类型，被称作*可发送类型*。例如，它可以在调用一个 actor 时被作为参数传递，或是作为一项任务的返回值返回。本章前述的几个例子没有讨论可发送性，因为这些例子使用的都是简单的值类型，而这些类型永远是可以被安全地在并发域之前传递的。相比之下，有些类型无法被安全地在并发域之间传递。比如，一个包含了可变属性、但又没有添加串行访问保护的类，如果在不同任务之间传递，可能会产生无法预测或是错误的结果。 + +你可以通过使一个类型遵循 `Sendable` 协议来将其标注为可发送的。这个协议并不包含任何代码要求，但是其包含了由 Swift 强制实施的语义要求。总的来说，有三种方式能让一个类变得可发送： + +- 这个类型是一个值类型，并且它的可变状态仅由其它的可发送数据构成 —— 比如，一个只包含可发送属性的结构体，或是一个只包含可发送关联值的枚举。 +- 这个类型不包含任何可变状态，并且它的不可变状态只由其它的可发送数据构成 —— 比如，一个仅包含只读属性的结构体或类。 +- 这个类型包含能够保证其可变属性访问安全性的代码，比如一个被标注了 `@MainActor` 的类，或是一个通过队列 / 线程来对其属性增加了串行访问保护的类。 + + + +若想了解所有的语义要求，请参见 [`Sendable`](https://developer.apple.com/documentation/swift/sendable) 协议指南。 + +有些类型总是可发送的，比如只包含可发送属性的结构体，或是只包含可发送关联值的枚举。比如： + +```swift +struct TemperatureReading: Sendable { + var measurement: Int +} + +extension TemperatureLogger { + func addReading(from reading: TemperatureReading) { + measurements.append(reading.measurement) + } +} + +let logger = TemperatureLogger(label: "Tea kettle", measurement: 85) +let reading = TemperatureReading(measurement: 45) +await logger.addReading(from: reading) +``` + + + +由于 `TemperatureReading` 是个结构体，并且只包含可发送属性，又因为这个结构体没有被标注为 `public` 或 `@usableFromInline`，它是隐式可发送的。下面这个版本的结构体隐式地遵从了 `Sendable` 协议： + +```swift +struct TemperatureReading { + var measurement: Int +} +``` + + + +要显式地标注一个类型为不可发送，你可以通过扩展来覆写对 `Sendable` 的隐式遵从： + +```swift +struct FileDescriptor { + let rawValue: CInt +} + +@available(*, unavailable) +extension FileDescriptor: Sendable { } +``` + + + +上面这段代码是 POSIX 文件描述符包装器的其中一部分。尽管文件描述符的接口使用整数来识别和处理打开的文件，且整数值是可发送的，文件描述符并不能够被安全地在并发域之间传递。 + + + +在上面的代码中，`FileDescriptor` 这个结构体符合隐式可发送的条件。但是，我们使用扩展来将其对于 `Sendable` 的遵从标注为不可用，从而防止其称为一个可发送类型。 + + + + + + diff --git a/swift-6.docc/LanguageGuide/ControlFlow.md b/swift-6.docc/LanguageGuide/ControlFlow.md new file mode 100644 index 000000000..31efbad80 --- /dev/null +++ b/swift-6.docc/LanguageGuide/ControlFlow.md @@ -0,0 +1,1729 @@ +# 控制流 + +使用分支、循环和提前退出来搭建代码的流程结构。 + + +Swift 提供了多种流程控制结构，包括可以多次执行任务的 `while` 循环，基于特定条件选择执行不同代码分支的 +`if`, `guard`, 和 `switch` 语句，还有控制流程跳转到其他代码位置的 `break` 和 `continue` +语句。 +Swift 提供了 `for`-`in` 循环，用来更简单地遍历数组（Array），字典（Dictionary），区间（Range），字符串（String）和其他序列类型。 +Swift 还提供了 `defer` 语句，用来包含离开当前代码块时要执行的代码。 + +Swift 的 `switch` 语句比许多类 C 语言要更加强大。 +它的 `case` 可以匹配多种不同的模式，包括区间匹配、元组以及类型转换。 +`switch` 语句的 `case` 中匹配的值可以声明为临时常量或变量以便在 `case` 作用域内使用，也可以配合 `where` 来描述更复杂的匹配条件。 + +## For-In 循环 + +你可以使用 `for`-`in` 循环来遍历一个集合中的所有元素，例如数组中的元素、范围内的数字或者字符串中的字符。 + +以下例子使用 `for`-`in` 遍历一个数组所有元素： + +```swift +let names = ["Anna", "Alex", "Brian", "Jack"] +for name in names { + print("Hello, \(name)!") +} +// Hello, Anna! +// Hello, Alex! +// Hello, Brian! +// Hello, Jack! +``` + + + +你也可以通过遍历一个字典来访问它的键值对。遍历字典时，字典的每项元素会以 `(key, value)` 元组的形式返回，你可以在 `for`-`in` 循环中使用显式命名后的常量来解构 `(key, value)` 元组。下面的例子中，字典的键会声明为 `animalName`, +常量，字典的值会声明为 `legCount` 常量： + +```swift +let numberOfLegs = ["spider": 8, "ant": 6, "cat": 4] +for (animalName, legCount) in numberOfLegs { + print("\(animalName)s have \(legCount) legs") +} +// cats have 4 legs +// ants have 6 legs +// spiders have 8 legs +``` + + + +字典的内容本质上是无序的，遍历元素时的顺序是无法确定的。将元素插入字典的顺序并不会决定它们被遍历的顺序。关于数组和字典的细节，参见 。 + + + +`for`-`in` 循环还可以使用数字范围。下面的例子用来输出乘法表的一部分内容： +```swift +for index in 1...5 { + print("\(index) times 5 is \(index * 5)") +} +// 1 times 5 is 5 +// 2 times 5 is 10 +// 3 times 5 is 15 +// 4 times 5 is 20 +// 5 times 5 is 25 +``` + + + +例子中用来进行遍历的元素是由闭区间操作符（`...`）表示的从 `1` 到 `5` 的数字区间（包括 `1` 和 `5` 在内）。`index` 被赋值为闭区间中的第一个数字（`1`），然后循环中的语句被执行一次。在本例中，这个循环只包含一个语句，用来输出当前 `index` 值所对应的乘 5 乘法表的结果。该语句执行后，`index` 的值被更新为闭区间中的第二个数字（`2`），之后 `print(_:separator:terminator:)` 函数会再执行一次。整个过程会进行到闭区间结尾为止。 + +上面的例子中，`index` 是一个每次循环遍历开始时被自动赋值的常量。这种情况下，`index` 在使用前不需要声明，只需要将它包含在循环的声明中，就可以对其进行隐式声明，而无需使用 `let` 关键字声明。 + +如果你不需要区间序列内每一项的值，你可以使用下划线（`_`）替代变量名来忽略这个值： + +```swift +let base = 3 +let power = 10 +var answer = 1 +for _ in 1...power { + answer *= base +} +print("\(base) to the power of \(power) is \(answer)") +// 输出 "3 to the power of 10 is 59049" +``` + + + +这个例子计算 `base` 这个数的 `power` 次幂（本例中，计算 `3` 的 `10` 次幂），从 `1`（也就是 `3` 的 `0` 次幂）开始做 `3` 的乘法，使用 `1` 到 `10` 的闭区间循环来进行 10 次。这个计算并不需要知道每一次循环中计数器具体的值，只需要执行了正确的循环次数即可。下划线符号 `_` 来替代循环中的变量能够忽略当前值，并且不提供循环遍历时对值的访问。 + +在某些情况下，你可能不想使用包括两个端点的闭区间。想象一下，你在一个手表上绘制分钟的刻度线。总共 `60` 个刻度，从 `0` 分开始。使用半开区间运算符（`..<`）来表示一个左闭右开的区间。有关区间的更多信息，请参阅 。 + +```swift +let minutes = 60 +for tickMark in 0.. let minutes = 60 + >> var result: [Int] = [] + -> for tickMark in 0..> result.append(tickMark) + } + >> print(result.first!, result.last!, result.count) + << 0 59 60 + ``` +--> + +一些用户在其 UI 中可能需要较少的刻度。他们可能更喜欢每 `5` 分钟标记一个刻度。使用 `stride(from:to:by:)` 函数跳过不需要的标记。 + +```swift +let minuteInterval = 5 +for tickMark in stride(from: 0, to: minutes, by: minuteInterval) { + // 每5分钟渲染一个刻度线（0, 5, 10, 15 ... 45, 50, 55） +} +``` + + + +可以在闭区间使用 `stride(from:through:by:)` 起到同样作用： + +```swift +let hours = 12 +let hourInterval = 3 +for tickMark in stride(from: 3, through: hours, by: hourInterval) { + // 每3小时渲染一个刻度线（3, 6, 9, 12） +} +``` + + + +以上示例使用 `for`-`in` 循环来遍历范围、数组、字典和字符串。你可以用它来遍历任何的集合，包括实现了 [`Sequence`](https://developer.apple.com/documentation/swift/sequence) 协议的自定义类或集合类型。 + + + +## While 循环 + +`while` 循环会一直运行一段语句直到条件变成 `false`。这类循环最适合用在迭代次数在第一次迭代开始前无法确定的情况下。Swift 提供两种 `while` 循环形式： + +- `while` 循环，每次在循环开始时评估条件是否符合； +- `repeat`-`while` 循环，每次在循环结束时评估条件是否符合。 + +### While + +`while` 循环从计算一个条件开始。如果条件为 `true`，会重复运行一段语句，直到条件变为 `false`。 + +下面是 `while` 循环的一般格式： + +```swift +while <#condition#> { + <#statements#> +} +``` + +下面的例子来玩一个叫做蛇和梯子（也叫做滑道和梯子）的小游戏： + + + +![](snakesAndLadders) + +游戏的规则如下： + +- 游戏盘面包括 25 个方格，游戏目标是达到或者超过第 25 个方格； +- 玩家的起始方块是“方块零”，就在棋盘的左下角。 +- 每一轮，你通过掷一个六面体骰子来确定你移动方块的步数，移动的路线由上图中横向的虚线所示； +- 如果在某轮结束，你移动到了梯子的底部，可以顺着梯子爬上去； +- 如果在某轮结束，你移动到了蛇的头部，你会顺着蛇的身体滑下去。 + +游戏盘面可以使用一个 `Int` 数组来表达。 +数组的长度由一个 `finalSquare` 常量储存， +用来初始化数组和检测最终胜利条件。 +游戏盘面由 26 个 `Int` 0 值初始化，而不是 25 个（由 0 到 25，一共 26 个）： + +```swift +let finalSquare = 25 +var board = [Int](repeating: 0, count: finalSquare + 1) +``` + + + +一些方格被设置成特定的值来表示有蛇或者梯子。梯子底部的方格是一个正值，使你可以向上移动，蛇头处的方格是一个负值，会让你向下移动： + +```swift +board[03] = +08; board[06] = +11; board[09] = +09; board[10] = +02 +board[14] = -10; board[19] = -11; board[22] = -02; board[24] = -08 +``` + + + + + +3 号方格是梯子的底部，会让你向上移动到 11 号方格，我们使用 `board[03]` 等于 `+08`（来表示 `11` 和 `3` 之间的差值）。为了对齐语句，这里使用了一元正运算符（`+i`）和一元负运算符（`-i`），并且小于 `10` 的数字都使用 0 补齐（这些语法的技巧不是必要的，只是为了让代码看起来更加整洁）。 + +```swift +var square = 0 +var diceRoll = 0 +while square < finalSquare { + // 掷骰子 + diceRoll += 1 + if diceRoll == 7 { diceRoll = 1 } + // 根据点数移动 + square += diceRoll + if square < board.count { + // 如果玩家还在棋盘上，顺着梯子爬上去或者顺着蛇滑下去 + square += board[square] + } +} +print("Game over!") +``` + + + +本例中使用了最简单的方法来模拟掷骰子。`diceRoll` 的值并不是一个随机数，而是以 `0` 为初始值，之后每一次 `while` 循环，`diceRoll` 的值增加 1 ，然后检测是否超出了最大值。当 diceRoll 的值等于 `7` 时，就超过了骰子的最大值，会被重置为 `1`。所以 diceRoll 的取值顺序会一直是 `1`，`2`，`3`，`4`，`5`，`6`，`1`，`2` 等。 + +掷完骰子后，玩家向前移动 `diceRoll` 个方格，如果玩家移动超过了第 25 个方格，这个时候游戏将会结束，为了应对这种情况，代码会首先判断 `square` 的值是否小于 `board` 的 `count` 属性，只有小于才会在 `board[square]` 上增加 `square`，来向前或向后移动（遇到了梯子或者蛇）。 + +> 注意: +> 如果没有这个检测（`square < board.count`），`board[square]` 可能会越界访问 `board` 数组，导致运行时错误。 + +当本轮 `while` 循环运行完毕，会再检测循环条件是否需要再运行一次循环。如果玩家移动到或者超过第 `25` 个方格，循环条件结果为 `false`，此时游戏结束。 + +`while` 循环比较适合本例中的这种情况，因为在 `while` 循环开始时，我们并不知道游戏要跑多久，只有在达成指定条件时循环才会结束。 + +### Repeat-While + +`while` 循环的另外一种形式是 `repeat`-`while`，它和 `while` 的区别是在判断循环条件*之前*，先执行一次循环的代码块。然后重复循环直到条件为 `false`。 + +> 注意: +> Swift 语言的 `repeat`-`while` 循环和其他语言中的 `do`-`while` 循环是类似的。 + +下面是 `repeat`-`while` 循环的一般格式： + +```swift +repeat { + <#statements#> +} while <#condition#> +``` + +还是*蛇和梯子*的游戏，使用 `repeat`-`while` 循环来替代 `while` 循环。`finalSquare`、`board`、`square` 和 `diceRoll` 的值初始化同 `while` 循环时一样： + +```swift +let finalSquare = 25 +var board = [Int](repeating: 0, count: finalSquare + 1) +board[03] = +08; board[06] = +11; board[09] = +09; board[10] = +02 +board[14] = -10; board[19] = -11; board[22] = -02; board[24] = -08 +var square = 0 +var diceRoll = 0 +``` + + + +在 `repeat`-`while` 循环版本中，循环中的*第一个*操作就是前进或后退方块上的步数。因为没有梯子会让玩家直接上到第 25 个方格，所以玩家不会通过梯子直接赢得游戏。因此在循环开始时就执行方块上的步数是安全的。 + +游戏开始时，玩家在第 `0` 号方格上，`board[0]` 一直等于 0， 不会有什么影响： + +```swift +repeat { + // 顺着梯子爬上去或者顺着蛇滑下去 + square += board[square] + // 掷骰子 + diceRoll += 1 + if diceRoll == 7 { diceRoll = 1 } + // 根据点数移动 + square += diceRoll +} while square < finalSquare +print("Game over!") +``` + + + +执行方块上的步数之后，开始掷骰子，然后玩家向前移动 `diceRoll` 个方格，本轮循环结束。 + +循环条件（`while square < finalSquare`）和 `while` 方式相同，但是只会在循环结束后进行计算。在前面的例子中，`repeat`-`while` 循环比 `while` 循环更适合这个游戏。`repeat`-`while` 方式会在条件判断 `square` *没有超出后直接运行* `square += board[square]`，比起前面 `while` 循环的版本，这种方式可以省去数组越界的检查。 + +## 条件语句 + +根据特定的条件执行特定的代码通常是十分有用的。当错误发生时，你可能想运行额外的代码；或者，当值太大或太小时，向用户显示一条消息。要实现这些功能，你就需要使用*条件语句*。 + +Swift 提供两种类型的条件语句：`if` 语句和 `switch` 语句。通常，使用 `if` 语句来评估只有少数几种可能性的简单条件。而 `switch` 语句更适用于有多种排列可能的更复杂的条件。并且 `switch` 在需要用到模式匹配（pattern-matching）的情况下会更有用。 + +### If + +`if` 语句最简单的形式就是只包含一个条件，只有该条件为 `true` 时，才执行相关代码： + +```swift +var temperatureInFahrenheit = 30 +if temperatureInFahrenheit <= 32 { + print("It's very cold. Consider wearing a scarf.") +} +// 输出 "It's very cold. Consider wearing a scarf." +``` + + + +上面的例子会判断温度是否小于等于 32 华氏度（水的冰点）。如果是，则打印一条消息；否则，不打印任何消息，继续执行 `if` 块后面的代码。 + +当然，`if` 语句允许二选一执行，叫做 *else 从句*。也就是当条件为 `false` 时，执行 `else` 语句： + +```swift +temperatureInFahrenheit = 40 +if temperatureInFahrenheit <= 32 { + print("It's very cold. Consider wearing a scarf.") +} else { + print("It's not that cold. Wear a T-shirt.") +} +// 输出 "It's not that cold. Wear a T-shirt." +``` + + + +显然，这两条分支中总有一条会被执行。由于温度已升至 `40` 华氏度，不算太冷，没必要再围围巾。因此，`else` 分支就被触发了。 + +你可以把多个 `if` 语句链接在一起，来实现更多分支： + +```swift +temperatureInFahrenheit = 90 +if temperatureInFahrenheit <= 32 { + print("It's very cold. Consider wearing a scarf.") +} else if temperatureInFahrenheit >= 86 { + print("It's really warm. Don't forget to wear sunscreen.") +} else { + print("It's not that cold. Wear a T-shirt.") +} +// 输出 "It's really warm. Don't forget to wear sunscreen." +``` + + + +在上面的例子中，额外的 `if` 语句用于判断是不是特别热。而最后的 `else` 语句被保留了下来，用于打印既不冷也不热时的消息。 + +实际上，当不需要完整判断情况的时候，最后的 `else` 语句是可选的： + +```swift +temperatureInFahrenheit = 72 +if temperatureInFahrenheit <= 32 { + print("It's very cold. Consider wearing a scarf.") +} else if temperatureInFahrenheit >= 86 { + print("It's really warm. Don't forget to wear sunscreen.") +} +``` + + + +在这个例子中，由于既不冷也不热，所以不会触发 `if` 或 `else if` 分支，也就不会打印任何消息。 + +Swift 用 `if` 提供了一种在赋值时可用的简便写法。例如，请考虑以下代码： + +```swift +let temperatureInCelsius = 25 +let weatherAdvice: String + +if temperatureInCelsius <= 0 { + weatherAdvice = "It's very cold. Consider wearing a scarf." +} else if temperatureInCelsius >= 30 { + weatherAdvice = "It's really warm. Don't forget to wear sunscreen." +} else { + weatherAdvice = "It's not that cold. Wear a T-shirt." +} + +print(weatherAdvice) +// 输出 "It's not that cold. Wear a T-shirt." +``` +上面的代码中，每个分支都为 `weatherAdvice` 设置一个值，该值打印在 `if` 语句之后。 + +使用下面的语法（称为 `if` 表达式），你可以更简洁地编写这段代码： + +```swift +let weatherAdvice = if temperatureInCelsius <= 0 { + "It's very cold. Consider wearing a scarf." +} else if temperatureInCelsius >= 30 { + "It's really warm. Don't forget to wear sunscreen." +} else { + "It's not that cold. Wear a T-shirt." +} + +print(weatherAdvice) +// 输出 "It's not that cold. Wear a T-shirt." +``` + +在此 `if` 表达式版本中，每个分支都包含一个值。如果分支的条件为 true，则该分支的值将用作 `weatherAdvice` 赋值中整个 `if` 表达式的值。每个 `if` 分支都有对应的 `else if` 分支或 `else` 分支，确保其中一个分支始终匹配，并且无论哪些条件为真， `if` 表达式始终生成一个值。 + +由于赋值的语法从 `if` 表达式外部开始，因此无需在每个分支内重复 `weatherAdvice =`。取而代之的是，每个 `if` 表达式分支会产出 `weatherAdvice` 的三个可能值之一，并使用该值为其赋值。 + +`if` 表达式的所有分支都需要包含相同类型的值。由于 Swift 会单独检查每个分支返回值的类型，所以像 `nil` 这样可以被用于多个类型的值阻碍了 Swift 自动推断 `if` 表达式的类型。因此，在这种情况下你需要明确指定类型 —— 例如： +```swift +let freezeWarning: String? = if temperatureInCelsius <= 0 { + "It's below freezing. Watch for ice!" +} else { + nil +} +``` + +在上面的代码中，`if` 表达式的一个分支有一个字符串值，另一个分支有一个 `nil` 值。`nil` 值可以用作任何可选类型的值，因此你必须明确声明 `freezeWarning` 是一个可选字符串，如中所述。 + +提供类型信息的另一种方法是为 `nil` 提供显式类型，而不是为 `freezeWarning` 提供显式类型： + +```swift +let freezeWarning = if temperatureInCelsius <= 0 { + "It's below freezing. Watch for ice!" +} else { + nil as String? +} +``` + +`if` 表达式可以通过抛出错误或调用如 `fatalError（_：file：line：）` 来响应没有返回值的意外失败。例如： + +```swift +let weatherAdvice = if temperatureInCelsius > 100 { + throw TemperatureError.boiling +} else { + "It's a reasonable temperature." +} +``` + +在此示例中，`if` 表达式会检查预报温度是否高于 100° C —— 水的沸点。高于沸点的温度会导致 `if` 表达式抛出 `.boiling` 错误，而不是返回文本摘要。即使这个 `if` 表达式可能会抛出错误，你也不要在它之前使用 `try`。有关处理错误的信息，请参阅 。 + +除了在赋值的右侧使用 `if` 表达式（如上面的示例所示）之外，你还可以将它们用作函数或闭包的返回值。 + +### Switch + +`switch` 语句会尝试把某个值与若干个模式（pattern）进行匹配。根据第一个匹配成功的模式，`switch` 语句会执行对应的代码。当有可能的情况较多时，通常用 `switch` 语句替换 `if` 语句。 + +`switch` 语句最简单的形式就是把某个值与一个或若干个相同类型的值作比较： + +```swift +switch <#some value to consider#> { +case <#value 1#>: + <#respond to value 1#> +case <#value 2#>, + <#value 3#>: + <#respond to value 2 or 3#> +default: + <#otherwise, do something else#> +} +``` + +`switch` 语句由多个 *情况* 构成，每个情况由 `case` 关键字开始。除了与特定值进行比较之外，Swift 还提供了几种方法来进行更复杂的匹配模式。本章稍后将介绍这些方法。 + +与 `if` 语句类似，每一个 `case` 都是代码执行的一条分支。`switch` 语句会决定哪一条分支应该被执行，这个流程被称作根据给定的值*路由（switching）*。 + +`switch` 语句必须是*完备的*。这就是说，每一个可能的值都必须至少有一个 `case` 分支与之对应。在某些不可能涵盖所有可能值的情况下，你可以定义一个默认分支来覆盖所有其他未明确列出的值。这个默认分支由 `default` 关键字来标明，并且必须始终出现在最后。 + +下面的例子使用 `switch` 语句来匹配一个名为 `someCharacter` 的小写字符： + +```swift +let someCharacter: Character = "z" +switch someCharacter { +case "a": + print("The first letter of the Latin alphabet") +case "z": + print("The last letter of the Latin alphabet") +default: + print("Some other character") +} +// 输出 "The last letter of the Latin alphabet" +``` + + + +在这个例子中，第一个 `case` 分支用于匹配第一个英文字母 `a`，第二个 `case` 分支用于匹配最后一个字母 `z`。因为 `switch` 语句必须有一个 `case` 分支用于覆盖所有可能的字符，而不仅仅是所有的英文字母，所以 `switch` 语句使用 `default` 分支来匹配除了 `a` 和 `z` 外的所有值，这个分支保证了 `switch` 语句的完备性。 + + +和 `if` 语句一样，`switch` 语句也具有表达式形式: + +```swift +let anotherCharacter: Character = "a" +let message = switch anotherCharacter { +case "a": + "The first letter of the Latin alphabet" +case "z": + "The last letter of the Latin alphabet" +default: + "Some other character" +} + +print(message) +// 输出 "The first letter of the Latin alphabet" +``` + +在此示例中，`switch` 表达式中的每个 `case` 都包含与 `anotherCharacter` 匹配成功时 `message` 的值。由于 `switch` 始终是完备的，因此始终会分配一个值。 + +与 `if` 表达式一样，你可以抛出错误或调用类似 `fatalError（_：file：line：）` 这样不返回值的函数，从而不给特定的 `case` 提供返回值。如上例所示，你可以在赋值的右侧使用 `switch` 表达式，也可以用作函数或闭包的返回值。 + +#### 没有隐式贯穿 + +与 C 和 Objective-C 中的 `switch` 语句不同，在 Swift 中，当匹配的 `case` 分支中的代码执行完毕后，程序会终止 `switch` 语句，而不会继续执行下一个 `case` 分支。这也就是说，不需要在 `case` 分支中显式地使用 `break` 语句。这使得 `switch` 语句更安全、更易用，也避免了漏写 `break` 语句导致多个 `case` 被执行的错误。 + +> 注意: +> 虽然在 Swift 中 `break` 不是必须的，但你依然可以在 `case` 分支中的代码执行完毕前使用 break 跳出，详情请参见。 + +每一个 `case` 分支都*必须*包含至少一条语句。像下面这样书写代码是不合法的，因为第一个 `case` 分支是空的： + +```swift +let anotherCharacter: Character = "a" +switch anotherCharacter { +case "a": // 不合法，这个分支下面没有语句 +case "A": + print("The letter A") +default: + print("Not the letter A") +} +// 这段代码会报编译错误 +``` + + + +和 C 语言里的 `switch` 语句不同，在 Swift 中，`switch` 语句不会一起匹配 `"a"` 和 `"A"`。相反的，上面的代码会引起编译期错误：`case "a":` 不包含任何可执行语句 ——这就避免了意外地从一个 case 分支贯穿到另外一个，使得代码更安全、也更直观。 + +为了让单个 `case` 同时匹配 `"a"` 和 `"A"`，可以将这个两个值组合成一个复合匹配，并且用逗号分开： + +```swift +let anotherCharacter: Character = "a" +switch anotherCharacter { +case "a", "A": + print("The letter A") +default: + print("Not the letter A") +} +// 输出 "The letter A" +``` + + + +为了可读性，复合匹配可以写成多行形式，详情请参考 。 + +> 注意: +> 如果想要显式贯穿 case 分支，请使用 `fallthrough` 关键字，详情请参考 。 + +#### 区间匹配 + +`switch` 中 `case` 分支的模式也可以是一个值的区间。下面的例子展示了如何使用区间匹配来输出任意数量所对应的自然语言计数表达： + + + +```swift +let approximateCount = 62 +let countedThings = "moons orbiting Saturn" +let naturalCount: String +switch approximateCount { +case 0: + naturalCount = "no" +case 1..<5: + naturalCount = "a few" +case 5..<12: + naturalCount = "several" +case 12..<100: + naturalCount = "dozens of" +case 100..<1000: + naturalCount = "hundreds of" +default: + naturalCount = "many" +} +print("There are \(naturalCount) \(countedThings).") +// 输出 "There are dozens of moons orbiting Saturn." +``` + + + +在上面的例子中，`approximateCount` 在一个 `switch` 语句中被评估。每一个 `case` 都把这个值与一个数字或区间进行比较。因为 `approximateCount` 落在了 12 到 100 的区间，所以 `naturalCount` 被赋值为 `"dozens of"` 值，然后执行流程就被转移到了 `switch` 语句之外。 + +#### 元组 + +我们可以使用元组在同一个 `switch` 语句中测试多个值。可以针对不同的值或值区间来测试元组的每个元素。另外，使用下划线（`_`）通配符来匹配所有可能的值。 + +下面的例子展示了如何使用一个 `(Int, Int)` 类型的元组来给下图中的点 (x, y) 进行分类： + +```swift +let somePoint = (1, 1) +switch somePoint { +case (0, 0): + print("\(somePoint) is at the origin") +case (_, 0): + print("\(somePoint) is on the x-axis") +case (0, _): + print("\(somePoint) is on the y-axis") +case (-2...2, -2...2): + print("\(somePoint) is inside the box") +default: + print("\(somePoint) is outside of the box") +} +// 输出 "(1, 1) is inside the box" +``` + + + +![](coordinateGraphSimple) + +在上面的例子中，`switch` 语句会判断某个点是否是原点 (0, 0)，是否在红色的 x 轴上，是否在绿色的 y 轴上，是否在一个以原点为中心的4x4的蓝色矩形里，或者在这个矩形外面。 + +和 C 语言不同，Swift 允许多个 `case` 匹配同一个值。实际上，在这个例子中，四个 `case` 都可以匹配点 (0, 0) 。但是，如果存在多个匹配，那么只会执行第一个被匹配到的 case 分支。由于点 (0, 0) 会首先匹配 `case (0, 0)`，因此剩下的能够匹配的分支都会被忽视掉。 + +#### 值绑定（Value Bindings） + +`switch` 的 `case` 分支允许将匹配的值声明为临时常量或变量，并且在 case 分支体内使用。因为值被绑定在 case 分支体作用域内的临时常量或变量上，所以这种行为被称为*值绑定*（value binding）。 + +下面的例子展示了如何使用一个 `(Int, Int)` 类型的元组来给下图中的点 (x, y) 进行分类： + +```swift +let anotherPoint = (2, 0) +switch anotherPoint { +case (let x, 0): + print("on the x-axis with an x value of \(x)") +case (0, let y): + print("on the y-axis with a y value of \(y)") +case let (x, y): + print("somewhere else at (\(x), \(y))") +} +// 输出 "on the x-axis with an x value of 2" +``` + + + +![](coordinateGraphMedium) + +在上面的例子中，`switch` 语句会判断某个点是否在红色的 x 轴上，是否在绿色的 y 轴上，或者不在坐标轴上。 + +这三个 `case` 都声明了占位符常量 `x` 和 `y` ，用于临时获取元组 `anotherPoint` 的一个或两个值。第一个 case ——`case (let x, 0)` 将匹配一个纵坐标为 0 的点，并把这个点的横坐标赋给临时的常量 `x`。类似的，第二个 case ——`case (0, let y)` 将匹配一个横坐标为 0 的点，并把这个点的纵坐标赋给临时的常量 `y`。 + +一旦声明了这些临时的常量，它们就可以在其对应的 case 分支里使用。在这个例子中，它们用于打印给定点的分类。 + +请注意，这个 `switch` 语句不包含 `default` 分支。这是因为最后一个 case ——`case let(x, y)` 声明了一个可以匹配余下所有值的元组。这使得 switch 语句已经完备了，因此不需要再书写默认分支。 + +#### Where + +`case` 分支可以使用 `where` 从句来判断附加的条件。 + +下面的例子把下图中的点 (x, y) 进行了分类： + +```swift +let yetAnotherPoint = (1, -1) +switch yetAnotherPoint { +case let (x, y) where x == y: + print("(\(x), \(y)) is on the line x == y") +case let (x, y) where x == -y: + print("(\(x), \(y)) is on the line x == -y") +case let (x, y): + print("(\(x), \(y)) is just some arbitrary point") +} +// 输出 "(1, -1) is on the line x == -y" +``` + + + +![](coordinateGraphComplex) + +在上面的例子中，`switch` 语句会判断某个点是否在绿色的对角线 `x == y` 上，是否在紫色的对角线 `x == -y` 上，或者不在对角线上。 + +这三个 `case` 声明了占位符常量 `x` 和 `y`，用于临时获取元组 `yetAnotherPoint` 的两个值。这两个常量被用作 `where` 语句的一部分，从而创建一个动态的过滤器（filter）。当且仅当 `where` 语句的条件为 `true` 时，匹配到的 `case` 分支才会被执行。 + +和值绑定中的例子一样，由于最后一个 case 分支匹配了余下所有可能的值，`switch` 语句就已经完备了，因此不需要再书写 `default` 分支。 + +#### 复合型 Cases（Compound Cases） + +共享同一主体的多个 switch case 可以通过在 `case` 后编写多个模式来组合，每个模式之间用逗号隔开。当 `case` 后面的任意一种模式匹配的时候，这条分支就会被匹配。并且，如果匹配列表过长，还可以分行书写： + +```swift +let someCharacter: Character = "e" +switch someCharacter { +case "a", "e", "i", "o", "u": + print("\(someCharacter) is a vowel") +case "b", "c", "d", "f", "g", "h", "j", "k", "l", "m", + "n", "p", "q", "r", "s", "t", "v", "w", "x", "y", "z": + print("\(someCharacter) is a consonant") +default: + print("\(someCharacter) isn't a vowel or a consonant") +} +// 输出 "e is a vowel" +``` + + + +这个 `switch` 语句中的第一个 case 匹配了英语中的五个小写元音字母。类似的，第二个 case 匹配了英语中所有的小写辅音字母。最终，`default` 分支匹配了其它所有字符。 + +复合匹配同样可以包含值绑定。复合匹配里所有的匹配模式，都必须包含相同的值绑定。并且每一个绑定都必须获取到相同类型的值。这保证了，无论复合匹配中的哪个模式发生了匹配，分支体内的代码，都能获取到绑定的值，并且绑定的值都有一样的类型。 + +```swift +let stillAnotherPoint = (9, 0) +switch stillAnotherPoint { +case (let distance, 0), (0, let distance): + print("On an axis, \(distance) from the origin") +default: + print("Not on an axis") +} +// 输出 "On an axis, 9 from the origin" +``` + + + +上面的 `case` 有两个模式：`(let distance, 0)` 匹配了在 x 轴上的值，`(0, let distance)` 匹配了在 y 轴上的值。两个模式都绑定了 `distance`，并且 `distance` 在两种模式下，都是整型——这意味着分支体内的代码，只要 `case` 匹配，都可以获取到 `distance` 值。 + +## 控制转移语句 + +*控制转移语句*改变你代码的执行顺序，通过它可以实现代码的跳转。Swift 有五种控制转移语句： + +- `continue` +- `break` +- `fallthrough` +- `return` +- `throw` + +我们将会在下面讨论 `continue` ， `break` 和 `fallthrough` 语句。 +`return` 语句将会在 章节讨论， +`throw` 语句会在 章节讨论。 + +### Continue + +`continue` 语句告诉一个循环体立刻停止本次循环，重新开始下次循环。就好像在说“本次循环我已经执行完了”，但是并不会离开整个循环体。 + +下面的例子把一个小写字符串中的元音字母和空格字符移除，生成了一个含义模糊的短句： + +```swift +let puzzleInput = "great minds think alike" +var puzzleOutput = "" +let charactersToRemove: [Character] = ["a", "e", "i", "o", "u", " "] +for character in puzzleInput { + if charactersToRemove.contains(character) { + continue + } + puzzleOutput.append(character) +} +print(puzzleOutput) +// 输出 "grtmndsthnklk" +``` + + + +在上面的代码中，只要匹配到元音字母或者空格字符，就调用 `continue` 语句，使本次循环结束，重新开始下次循环。这种行为使 `switch` 匹配到元音字母和空格字符时不做处理，而不是让每一个匹配到的字符都被打印。 + +### Break + +`break` 语句会立刻结束整个控制流的执行。`break` 可以在 `switch` 或循环语句中使用，用来提前结束 `switch` 或循环语句。 + +#### 循环语句中的 break + +当在一个循环体中使用 `break` 时，会立刻中断该循环体的执行，然后跳转到表示循环体结束的大括号（`}`）后的第一行代码。不会再有本次循环的代码被执行，也不会再有下次的循环产生。 + + + +#### Switch 语句中的 break（Break in a Switch Statement） + +当在一个 `switch` 语句中使用 `break` 时，会立即中断该 `switch` 语句的执行，并且跳转到表示 `switch` 语句结束的大括号（`}`）后的第一行代码。 + +这种特性可以被用来匹配和忽略一个或多个分支。因为 Swift 的 `switch` 语句必须是完备的而且不允许有为空的分支，有时为了使你的意图更明显，需要特意匹配或者忽略某个分支。那么当你想忽略某个分支时，可以在该分支内写上 `break` 语句。当那个分支被匹配到时，分支内的 `break` 语句立即结束 `switch` 代码块。 + +> 注意: +> 当一个 `case` 分支仅仅包含注释时，会被报编译时错误。注释不是代码语句而且也不能让 `switch` 分支达到被忽略的效果。你应该使用 `break` 来忽略某个分支。 + +下面的例子通过 `switch` 来判断一个 `Character` 是否表示下面四种语言之一的数字符号。为了简洁，多个值被包含在了同一个分支中。 + +```swift +let numberSymbol: Character = "三" // Chinese symbol for the number 3 +var possibleIntegerValue: Int? +switch numberSymbol { +case "1", "١", "一", "๑": + possibleIntegerValue = 1 +case "2", "٢", "二", "๒": + possibleIntegerValue = 2 +case "3", "٣", "三", "๓": + possibleIntegerValue = 3 +case "4", "٤", "四", "๔": + possibleIntegerValue = 4 +default: + break +} +if let integerValue = possibleIntegerValue { + print("The integer value of \(numberSymbol) is \(integerValue).") +} else { + print("An integer value couldn't be found for \(numberSymbol).") +} +// 输出 "The integer value of 三 is 3." +``` + + + +这个例子检查 `numberSymbol` 是否是拉丁语，阿拉伯语，中文或者泰语中的 `1` 到 `4` 之一。如果被匹配到，该 `switch` 语句的 `case` 分支给 `Int?` 类型变量 `possibleIntegerValue` 设置为对应的整数值。 + +当 `switch` 代码块执行完后，接下来的代码通过使用可选绑定来判断 `possibleIntegerValue` 是否曾经被设置过值。因为是可选类型的缘故，`possibleIntegerValue` 有一个隐式的初始值 `nil`，所以仅仅当 `possibleIntegerValue` 曾被 `switch` 语句的前四个分支中的某个设置过一个值时，可选的绑定才会被判定为成功。 + +在上面的例子中，想要把 `Character` 所有的的可能性都枚举出来是不现实的，所以使用 `default` 分支来包含所有上面没有匹配到字符的情况。由于这个 `default` 分支不需要执行任何动作，所以它只写了一条 `break` 语句。一旦落入到 `default` 分支中后，`break` 语句就完成了该分支的所有代码操作，代码继续向下，开始执行 `if let` 语句。 + +### 贯穿（Fallthrough） + +在 Swift 中，`switch` 语句不会从上一个 `case` 分支跳转到下一个 `case` 分支中。这意味着只要第一个匹配到的 `case` 分支完成了它需要执行的语句，整个 `switch` 代码块完成了它的执行。相比之下，C 语言要求你显式地插入 `break` 语句到每个 `case` 分支的末尾来阻止自动落入到下一个 `case` 分支中。Swift 的这种避免默认落入到下一个分支中的特性意味着它的 `switch` 功能要比 C 语言的更加清晰和可预测，可以避免无意识地执行多个 `case` 分支从而引发的错误。 + +如果你确实需要 C 风格的贯穿的特性，你可以在每个需要该特性的 `case` 分支中使用 `fallthrough` 关键字。下面的例子使用 `fallthrough` 来创建一个数字的描述语句。 + +```swift +let integerToDescribe = 5 +var description = "The number \(integerToDescribe) is" +switch integerToDescribe { +case 2, 3, 5, 7, 11, 13, 17, 19: + description += " a prime number, and also" + fallthrough +default: + description += " an integer." +} +print(description) +// 输出 "The number 5 is a prime number, and also an integer." +``` + + + +这个例子定义了一个 `String` 类型的变量 `description` 并且给它设置了一个初始值。函数使用 `switch` 语句来判断 `integerToDescribe` 变量的值。当 `integerToDescribe` 的值属于列表中的质数之一时，该函数在 `description` 后添加一段文字，来表明这个数字是一个质数。然后它使用 `fallthrough` 关键字来“贯穿”到 `default` 分支中。`default` 分支在 `description` 的最后添加一段额外的文字，至此 `switch` 代码块执行完了。 + +如果 `integerToDescribe` 的值不属于列表中的任何质数，那么它不会匹配到第一个 `case` 分支。而这里没有其他特别的分支情况，所以 `integerToDescribe` 匹配到 `default` 分支中。 + +当 `switch` 语句执行完后，使用 `print(_:separator:terminator:)` 函数打印该数字的描述。在这个例子中，数字 `5` 被准确的识别为了一个质数。 + +> 注意: The `fallthrough` +> `fallthrough` 关键字不会检查它下一个将会落入执行的 `case` 中的匹配条件。`fallthrough` 简单地使代码继续连接到下一个 `case` （或 `default`）中的代码，这和 C 语言标准中的 `switch` 语句特性是一样的。 + +### 带标签的语句 + +在 Swift 中，你可以在循环体和条件语句中嵌套循环体和条件语句来创造复杂的控制流结构。并且，循环体和条件语句都可以使用 `break` 语句来提前结束整个代码块。因此，显式地指明 `break` 语句想要终止的是哪个循环体或者条件语句，会很有用。类似地，如果你有许多嵌套的循环体，显式指明 `continue` 语句想要影响哪一个循环体也会非常有用。 + +为了实现这个目的，你可以使用*语句标签（statement label）*来标记一个循环体或者条件语句，对于一个条件语句，你可以使用 `break` 加标签的方式，来结束这个被标记的语句。对于一个循环语句，你可以使用 `break` 或者 `continue` 加标签，来结束或者继续这条被标记语句的执行。 + +声明一个带标签的语句是通过在该语句的关键字的同一行前面放置一个标签，作为这个语句的前导关键字（introducer keyword），并且该标签后面跟随一个冒号。下面是一个针对 `while` 循环体的标签语法，同样的规则适用于所有的循环体和 `switch` 语句。 + +```swift +<#label name#>: while <#condition#> { + <#statements#> +} +``` + +下面的例子是前面章节中*蛇和梯子*游戏的适配版本，在此版本中，我们在一个带有标签的 `while` 循环体中使用 `break` 和 `continue` 语句。这次，游戏增加了一条额外的规则： + +- 为了获胜，你必须*刚好*落在第 25 个方块中。 + +如果某次掷骰子使你的移动超出第 25 个方块，你必须重新掷骰子，直到你掷出的骰子数刚好使你能落在第 25 个方块中。 + +游戏的棋盘和之前一样： + +![](snakesAndLadders) + +`finalSquare`、`board`、`square` 和 `diceRoll` 值使用和之前一样的方式初始化： + +```swift +let finalSquare = 25 +var board = [Int](repeating: 0, count: finalSquare + 1) +board[03] = +08; board[06] = +11; board[09] = +09; board[10] = +02 +board[14] = -10; board[19] = -11; board[22] = -02; board[24] = -08 +var square = 0 +var diceRoll = 0 +``` + + + +这个版本的游戏使用 `while` 循环和 `switch` 语句来实现游戏的逻辑。`while` 循环有一个标签名 `gameLoop`，来表明它是游戏的主循环。 + +该 `while` 循环体的条件判断语句是 `while square != finalSquare`，这表明你必须刚好落在方格25中。 + +```swift +gameLoop: while square != finalSquare { + diceRoll += 1 + if diceRoll == 7 { diceRoll = 1 } + switch square + diceRoll { + case finalSquare: + // 骰子数刚好使玩家移动到最终的方格里，游戏结束。 + break gameLoop + case let newSquare where newSquare > finalSquare: + // 骰子数将会使玩家的移动超出最后的方格，那么这种移动是不合法的，玩家需要重新掷骰子 + continue gameLoop + default: + // 有效移动，做正常的处理 + square += diceRoll + square += board[square] + } +} +print("Game over!") +``` + + + +每次循环迭代开始时掷骰子。与之前玩家掷完骰子就立即移动不同，这里使用了 `switch` 语句来考虑每次移动可能产生的结果，从而决定玩家本次是否能够移动。 + +- 如果骰子数刚好使玩家移动到最终的方格里，游戏结束。`break gameLoop` 语句跳转控制去执行 `while` 循环体后的第一行代码。 +- 如果骰子数将会使玩家的移动超出最后的方格，那么这种移动是不合法的，玩家需要重新掷骰子。`continue gameLoop` 语句结束本次 `while` 循环，开始下一次循环。 +- 在剩余的所有情况中，骰子数产生的都是有效的移动。玩家向前移动 `diceRoll` 个方格，然后游戏逻辑再处理玩家当前是否处于蛇头或者梯子的底部。接着本次循环结束，控制跳转到 `while` 循环体的条件判断语句处，再决定是否需要继续执行下次循环。 + + +> 注意: +> 如果上述的 `break` 语句没有使用 `gameLoop` 标签，那么它将会中断 `switch` 语句而不是 `while` 循环。使用 `gameLoop` 标签清晰的表明了 `break` 想要中断的是哪个控制语句。 +> 同时请注意，当调用 `continue gameLoop` 去跳转到下一次循环迭代时，这里使用 `gameLoop` 标签并不是严格必须的。因为在这个游戏中，只有一个循环体，所以 `continue` 语句会影响到哪个循环体是没有歧义的。然而，`continue` 语句使用 `gameLoop` 标签也是没有危害的。这样做符合标签的使用规则，同时参照旁边的 `break gameLoop`，能够使游戏的逻辑更加清晰和易于理解。 + +## 提前退出 + +像 `if` 语句一样，`guard` 语句的执行取决于一个表达式的布尔值。`guard` 来要求条件必须为真时才会执行 `guard` 语句后的代码。不同于 `if` 语句，`guard` 语句总是有一个 `else` 从句，如果条件不为真则执行 `else` 从句中的代码。 + +```swift +func greet(person: [String: String]) { + guard let name = person["name"] else { + return + } + + print("Hello \(name)!") + + guard let location = person["location"] else { + print("I hope the weather is nice near you.") + return + } + + print("I hope the weather is nice in \(location).") +} + +greet(person: ["name": "John"]) +// 输出 "Hello John!" +// 输出 "I hope the weather is nice near you." +greet(person: ["name": "Jane", "location": "Cupertino"]) +// 输出 "Hello Jane!" +// 输出 "I hope the weather is nice in Cupertino." +``` + + + +如果 `guard` 语句的条件被满足，则继续执行 `guard` 语句大括号后的代码。使用可选绑定作为条件的一部分来进行赋值的任何变量或常量都可用于 `guard` 语句出现后余下的代码块。 + +如果条件不被满足，在 `else` 分支上的代码就会被执行。这个分支必须转移控制以退出 `guard` 语句所在的代码段。它可以用控制转移语句如 `return`、`break`、`continue` 或者 `throw` 做这件事，或者调用一个不返回的方法或函数，例如 `fatalError()`。 + +相比于可以实现同样功能的 `if` 语句，按需使用 `guard` 语句会提升我们代码的可读性。它可以让你不用把判断条件不成立时会执行的代码包裹在 `else` 块中，并且它允许你将处理违反条件的代码保留在条件旁边。 + +## 延迟执行的操作（Deferred Actions） + +与 `if` 和 `while` 等控制流结构不同，`if` 和 `while` 允许你控制是否执行部分代码或执行代码的次数，而 `defer` 控制*何时*执行一段代码。你可以使用 `defer` 块编写代码，这些代码将在以后程序到达当前代码块的末尾时执行。例如： + +```swift +var score = 1 +if score < 10 { + defer { + print(score) + } + score += 5 +} +// 输出 "6" +``` + + + +在上面的示例中，`defer` 代码块内的代码在退出 `if` 语句作用域之前执行。首先，运行 `if` 语句中的代码，该语句将分数增加 5。然后，在退出 `if` 语句的作用域之前，将运行被延迟的代码，该代码会打印 `score`。 + +无论程序如何退出该范围，`defer` 内的代码始终运行。这包括提前退出函数、中断 `for` 循环或抛出错误等代码。此特性使得 `defer` 对于需要保证成对的操作非常有用（例如手动分配和释放内存、打开和关闭底层文件描述符以及在数据库中开始和结束事务），因为你可以在代码中将这两个操作写在一起。例如，下面的代码通过在代码块中加减 100 来临时奖励分数： + +```swift +var score = 3 +if score < 100 { + score += 100 + defer { + score -= 100 + } + // 这里可以写其他使用分数和奖励的代码。 + print(score) +} +// 输出 "103" +``` + + + +如果在同一作用域中编写多个 `defer` 代码块，则先写的 `defer` 代码块中的代码后执行。 + +```swift +if score < 10 { + defer { + print(score) + } + defer { + print("The score is:") + } + score += 5 +} +// 输出 "The score is:" +// 输出 "6" +``` + + + +如果你的应用程序停止运行（例如发生了运行时错误或崩溃），则延迟的代码不会执行。但是，延迟代码会在抛出错误之后执行。有关将 `defer` 与错误处理结合使用的信息，请参阅 。 + +## 检测 API 可用性 + +Swift 内置了对检查 API 可用性的支持，这可以确保你不会在给定的部署目标上不小心使用不可用的 API。 + +编译器使用 SDK 中的可用信息来验证我们的代码中使用的所有 API 在项目指定的部署目标上是否可用。如果我们尝试使用一个不可用的 API，Swift 会在编译时报错。 + +我们在 `if` 或 `guard` 语句中使用 *可用性条件 (availability condition)* 去有条件的执行一段代码，具体取决于你要使用的 API 在运行时是否可用。编译器使用从可用性条件语句中获取的信息去验证，在这个代码块中调用的 API 是否可用。 + +```swift +if #available(iOS 10, macOS 10.12, *) { + // 在 iOS 使用 iOS 10 的 API, 在 macOS 使用 macOS 10.12 的 API +} else { + // 使用先前版本的 iOS 和 macOS 的 API +} +``` + + + +以上可用性条件指定，`if` 语句的代码块仅仅在 iOS 10 或 macOS 10.12 及更高版本才运行。最后一个参数，`*`，是必须的，用于指定在所有其它平台中，如果版本号高于你的设备指定的最低版本，`if` 语句的代码块将会运行。 + +在它一般的形式中，可用性条件使用了一个平台名字和版本的列表。平台名字可以是 `iOS`，`macOS`，`watchOS`，`tvOS`，和 `visionOS` ---请访问 来获取完整列表。除了指定像 iOS 8 或 macOS 10.10 的大版本号，也可以指定像 iOS 11.2.6 以及 macOS 10.13.3 的小版本号。 + +```swift +if #available(<#platform name#> <#version#>, <#...#>, *) { + <#statements to execute if the APIs are available#> +} else { + <#fallback statements to execute if the APIs are unavailable#> +} +``` + +当你在 `guard` 语句中使用可用性条件时，它将细化用于该代码块中其余代码的可用性信息。 + +```swift +@available(macOS 10.12, *) +struct ColorPreference { + var bestColor = "blue" +} + +func chooseBestColor() -> String { + guard #available(macOS 10.12, *) else { + return "gray" + } + let colors = ColorPreference() + return colors.bestColor +} +``` + + + +在上面的例子中，结构体 `ColorPreference` 需要 macOS 10.12 或更高的版本。函数 `ChooseBestColor()` 先以一个可用性防护开头，若平台版本过低无法运行 `ColorPreference` 时，将回退到总是可用的行为上。而在 `guard` 语句后，你将能够使用 macOS 10.12 或更高版本的API。 + +除了 `#available` 以外， Swift 还支持通过不可用性条件来进行不可用性检查。举例如下，两种检查都能实现同样的效果： + +```swift +if #available(iOS 10, *) { +} else { + // 回退代码 +} + +if #unavailable(iOS 10) { + // 回退代码 +} +``` + + + +若可用性检查只提供了回退代码，改用 `#unavailable` 能提升程序整体的可读性。 + + + + diff --git a/swift-6.docc/LanguageGuide/Deinitialization.md b/swift-6.docc/LanguageGuide/Deinitialization.md new file mode 100644 index 000000000..647d327fc --- /dev/null +++ b/swift-6.docc/LanguageGuide/Deinitialization.md @@ -0,0 +1,193 @@ +# 析构过程 + +释放需要自定义清理的资源。 + +析构器仅适用于类类型，**析构器**会在类实例被释放之前立即调用。使用 `deinit` 关键字来编写析构器，类似于使用 `init` 关键字编写构造器。 + +## 析构过程原理 + +Swift 会在实例不再需要时自动释放它们，以释放资源。Swift 通过**自动引用计数**（*ARC*）来管理实例的内存，如 中所述。通常，在实例被释放时不需要执行手动清理。然而，当你处理自己的资源时，可能需要进行一些额外的清理。例如，如果创建一个自定义类来打开文件并向其中写入数据，可能需要在类实例被释放之前关闭文件。 + +类定义中每个类最多只能有一个析构器。析构器不接受任何参数，并且是没有括号的： + +```swift +deinit { + // 执行析构过程 +} +``` + + + +析构器会在实例释放之前自动调用。不能自行调用析构器。父类的析构器会被子类继承，并且在子类析构器实现的末尾被自动调用。即使子类没有提供自己的析构器，父类的析构器也总是会被调用。 + +由于实例在析构器调用完成后才会被释放，因此析构器可以访问该实例的所有属性，并可以根据这些属性修改其行为（例如查找需要关闭的文件的名称）。 + +## 析构器实践 + +下面是一个析构器实践的例子。这个例子定义了两个新类型，`Bank` 和 `Player`，用于一个简单的游戏。`Bank` 类管理一种虚构的货币，这种货币的流通量永远不会超过 10,000 个金币。游戏中只能有一个 `Bank`，因此 `Bank` 被实现为一个类，使用类型属性和类型方法来存储和管理其当前状态： + +```swift +class Bank { + static var coinsInBank = 10_000 + static func distribute(coins numberOfCoinsRequested: Int) -> Int { + let numberOfCoinsToVend = min(numberOfCoinsRequested, coinsInBank) + coinsInBank -= numberOfCoinsToVend + return numberOfCoinsToVend + } + static func receive(coins: Int) { + coinsInBank += coins + } +} +``` + + + +`Bank` 通过其 `coinsInBank` 属性跟踪当前持有的金币数量。它还提供了两个方法——`distribute(coins:)` 和 `receive(coins:)`——来处理金币的分发和接收。 + +`distribute(coins:)` 方法在分发金币之前检查银行中是否有足够的金币。如果金币不足，`Bank` 会返回一个比请求的数量更小的数字（如果银行中没有金币，则返回零）。该方法返回一个整型值，表示实际提供的金币数量。 + +`receive(coins:)` 方法只是将接收到的金币数量重新添加到银行的金币库中。 + +`Player` 类描述了游戏中的一个玩家。每个玩家的钱包中随时都会存储一定数量的金币，由 `coinsInPurse` 属性表示： + +```swift +class Player { + var coinsInPurse: Int + init(coins: Int) { + coinsInPurse = Bank.distribute(coins: coins) + } + func win(coins: Int) { + coinsInPurse += Bank.distribute(coins: coins) + } + deinit { + Bank.receive(coins: coinsInPurse) + } +} +``` + + + +每个 `Player` 实例在初始化时都会从银行获得指定的金币作为初始津贴，但是如果可用金币不足，`Player` 实例获得的金币数量可能会少于指定数量。 + +`Player` 类定义了一个 `win(coins:)` 方法，该方法从银行获取一定数量的金币并将其添加到玩家的钱包中。`Player` 类还实现了一个析构器，该析构器在 `Player` 实例被释放之前调用。在这里，析构器只是将玩家的所有金币返还给银行： + +```swift +var playerOne: Player? = Player(coins: 100) +print("A new player has joined the game with \(playerOne!.coinsInPurse) coins") +// 打印 "A new player has joined the game with 100 coins" +print("There are now \(Bank.coinsInBank) coins left in the bank") +// 打印 "There are now 9900 coins left in the bank" +``` + + + +创建了一个新的 `Player` 实例的时候，会向 `Bank` 请求获取 100 个金币（如果有的话）。这个 `Player` 实例被存储在一个名为 `playerOne` 的可选类型变量中。这里使用可选变量是因为玩家可以在任何时候退出游戏。通过可选类型可以跟踪当前是否有玩家在游戏中。 + +由于 `playerOne` 是一个可选类型，因此在访问其 `coinsInPurse` 属性以打印默认金币数量时，以及每次调用其 `win(coins:)` 方法时，都需要使用感叹号（`!`）来强制解包： + +```swift +playerOne!.win(coins: 2_000) +print("PlayerOne won 2000 coins & now has \(playerOne!.coinsInPurse) coins") +// 打印 "PlayerOne won 2000 coins & now has 2100 coins" +print("The bank now only has \(Bank.coinsInBank) coins left") +// 打印 "The bank now only has 7900 coins left" +``` + + + +在这里，玩家赢得了 2,000 个金币。玩家的钱包现在有 2,100 个金币，而银行只剩下 7,900 个金币。 + +```swift +playerOne = nil +print("PlayerOne has left the game") +// 打印 "PlayerOne has left the game" +print("The bank now has \(Bank.coinsInBank) coins") +// 打印 "The bank now has 10000 coins" +``` + + + +玩家现在已经退出游戏。这是通过将可选类型变量 `playerOne` 设置为 `nil` 来表示的，意味着“没有 `Player` 实例”。在这一刻，`playerOne` 变量对 `Player` 实例的引用被断开。没有其他属性或变量仍然引用该 `Player` 实例，因此它会被释放以回收其内存。在此之前，它的析构器会被自动调用，并将其金币返还给银行。 + + diff --git a/swift-6.docc/LanguageGuide/Enumerations.md b/swift-6.docc/LanguageGuide/Enumerations.md new file mode 100644 index 000000000..8d924b932 --- /dev/null +++ b/swift-6.docc/LanguageGuide/Enumerations.md @@ -0,0 +1,664 @@ +# 枚举 + +定义一个包含可能值的列表的自定义类型。 + +*枚举*为一组相关的值定义了一个共同的类型，并使你能够以类型安全的方式在代码中使用这些值。 + +如果你对 C 语言很熟悉，你会知道 C 的枚举将相关的名称分配给一组整数值。Swift 中的枚举更加灵活，不必为每一个枚举成员提供一个值。如果给枚举成员提供一个值（称为*原始*值），则该值的类型可以是字符串、字符，或是一个整数或浮点数。 + +此外，枚举成员可以指定*任意*类型的关联值存储到枚举成员中，就像其他语言中的联合体（unions）或变体（variants）。每一个枚举成员都可以有适当类型的关联值。 + +在 Swift 中，枚举类型是一等（first-class）类型。它们采用了很多在传统上只被类所支持的特性，例如计算属性（computed properties），用于提供枚举值的附加信息，实例方法（instance methods），用于提供和枚举值相关联的功能。枚举也可以定义构造函数（initializers）来提供一个初始值；可以在原始实现的基础上扩展它们的功能；还可以遵循协议（protocols）来提供标准的功能。 + +想了解更多相关信息，请参阅，，，，和。 + + + +## 枚举语法 + +使用 `enum` 关键词来创建枚举并且把它们的整个定义放在一对大括号内： + +```swift +enum SomeEnumeration { + // 枚举定义放在这里 +} +``` + + + +下面是用枚举表示指南针四个方向的例子： + +```swift +enum CompassPoint { + case north + case south + case east + case west +} +``` + + + +枚举中定义的值（如 `north`，`south`，`east` 和 `west`）是这个枚举的*成员值*（或成员）。你可以使用 `case` 关键字来定义新的枚举成员值。 + +>注意: +>与 C 和 Objective-C 语言不同，Swift 的枚举成员在被创建时不会被赋予一个默认的整型值。在上面的 `CompassPoint` 例子中，`north`，`south`，`east` 和 `west` 不会被隐式地赋值为 `0`，`1`，`2` 和 `3`。相反，这些枚举成员本身就是完备的值，这些值的类型是已经明确定义好的 `CompassPoint` 类型。 + +多个成员值可以出现在同一行上，用逗号隔开： + +```swift +enum Planet { + case mercury, venus, earth, mars, jupiter, saturn, uranus, neptune +} +``` + + + +每个枚举都定义了一个全新的类型。像 Swift 中其他类型一样，它们的名字（如 `CompassPoint` 和 `Planet`）以一个大写字母开头。给枚举类型使用单数名称而非复数名称，以便它们能够自然而然地表达其含义： + +```swift +var directionToHead = CompassPoint.west +``` + + + +`directionToHead` 的类型可以在它被 `CompassPoint` 的某个值初始化时推断出来。一旦 `directionToHead` 被声明为 `CompassPoint` 类型，你可以使用更简短的点语法将其设置为不同的 `CompassPoint` 值： + +```swift +directionToHead = .east +``` + + + +当 `directionToHead` 的类型已知时，再次为其赋值可以省略枚举类型名。在使用具有显式类型的枚举值时，这种写法让代码具有更好的可读性。 + +## 使用 Switch 语句匹配枚举值 + +你可以使用 `switch` 语句匹配单个枚举值： + +```swift +directionToHead = .south +switch directionToHead { +case .north: + print("Lots of planets have a north") +case .south: + print("Watch out for penguins") +case .east: + print("Where the sun rises") +case .west: + print("Where the skies are blue") +} +// 打印 "Watch out for penguins" +``` + + + +你可以这样理解这段代码： + +“判断 `directionToHead` 的值。当它等于 `.north`时，打印 `“Lots of planets have a north”`。当它等于 `.south`时，打印 `"Watch out for penguins"`。” + +……以此类推。 + +正如在中介绍的那样，在判断一个枚举类型的值时，`switch` 语句必须穷举所有情况。如果遗漏了 `.west` 这种情况，上面那段代码将无法通过编译，因为它没有考虑到 `CompassPoint` 的全部成员。强制穷举确保了枚举成员不会被意外遗漏。 + +当不需要匹配每个枚举成员的时候，你可以提供一个 `default` 分支来涵盖所有未明确列出的枚举成员： + +```swift +let somePlanet = Planet.earth +switch somePlanet { +case .earth: + print("Mostly harmless") +default: + print("Not a safe place for humans") +} +// 打印 "Mostly harmless" +``` + + + +## 枚举成员的遍历 + +在一些情况下，你会需要得到一个包含枚举所有成员的集合。你可以这样实现：在枚举的名称后编写 `: CaseIterable` 来启用此功能，令枚举遵循 `CaseIterable` 协议。Swift 会生成一个 `allCases` 属性，用于表示一个包含枚举所有成员的集合。下面是一个例子： + +```swift +enum Beverage: CaseIterable { + case coffee, tea, juice +} +let numberOfChoices = Beverage.allCases.count +print("\(numberOfChoices) beverages available") +// 打印 "3 beverages available" +``` + + + +在上面的例子中，通过 `Beverage.allCases` 可以访问到一个包含 `Beverage` 枚举所有成员的集合。`allCases` 的使用方法和其它一般集合一样——集合中的元素是枚举类型的实例，因此在本例中这些元素是 `Beverage` 的值。在上面的例子中，统计了总共有多少个枚举成员。在下面的例子中我们来演示如何使用 `for`-`in` 循环来遍历所有枚举成员。 + +```swift +for beverage in Beverage.allCases { + print(beverage) +} +// coffee +// tea +// juice +``` + + + +在前面的例子中，使用的语法表明这个枚举遵循 [`CaseIterable`](https://developer.apple.com/documentation/swift/caseiterable) 协议。想了解协议的相关信息，请参阅。 + +## 关联值 + +在枚举语法那一小节的例子中演示了如何为枚举的成员定义值和类型，你可以将常量或变量设置为 `Planet.earth`，并在赋值之后检查这个值。然而，有时候把其他类型的值与枚举的成员值一起存储起来会很有用。这额外的信息称为*关联值*，并且你每次在代码中使用该枚举成员时，还可以修改这个关联值。 + +你可以定义 Swift 枚举来存储任意类型的关联值，如果需要的话，每个枚举成员的关联值类型可以各不相同。枚举的这种特性跟其他语言中的*可识别联合*（discriminated unions），*标签联合*（tagged unions），或者*变体*（variants）相似。 + +例如，假设一个库存跟踪系统需要利用两种不同类型的条形码来跟踪商品。有些商品上标有使用 `0` 到 `9` 数字的 UPC 格式的一维条形码。每一个条形码都有一个代表数字系统的数字，该数字后接五位代表厂商代码的数字，接下来是五位代表产品代码的数字。最后一个数字是检查位，用来验证代码是否被正确扫描： + +![](barcode_UPC) + +其他商品上标有 QR 码格式的二维码，它可以使用任何 ISO 8859-1 字符，并且可以编码一个最多拥有 2,953 个字符的字符串： + +![](barcode_QR) + +这便于库存跟踪系统用包含四个整型值的元组存储 UPC 码，以及用任意长度的字符串储存 QR 码。 + +在 Swift 中，使用如下方式定义一个表示两种商品条形码的枚举： + +```swift +enum Barcode { + case upc(Int, Int, Int, Int) + case qrCode(String) +} +``` + + + +这段代码可以这么理解： + +“定义一个名为 `Barcode` 的枚举类型，它的一个成员值是具有 (`Int`，`Int`，`Int`，`Int`) 类型关联值的 `upc`，另一个成员值是具有 `String` 类型关联值的 `qrCode`。” + +这个定义不提供任何 `Int` 或 `String` 类型的关联值，它只是定义了当 `Barcode` 常量和变量等于 `Barcode.upc` 或 `Barcode.qrCode` 时可以存储的关联值的*类型*。 + +然后你可以使用任意一种条形码类型创建新的条形码，例如： + +```swift +var productBarcode = Barcode.upc(8, 85909, 51226, 3) +``` + + + +上面的例子创建了一个名为 `productBarcode` 的变量，并将 `Barcode.upc` 赋值给它，关联的元组值为 `(8, 85909, 51226, 3)`。 + +同一个商品可以分配不同类型的条形码，例如： + +```swift +productBarcode = .qrCode("ABCDEFGHIJKLMNOP") +``` + + + +这时，原始的 `Barcode.upc` 和其整数关联值被新的 `Barcode.qrCode` 和其字符串关联值所替代。`Barcode` 类型的常量和变量可以存储一个 `.upc` 或者一个 `.qrCode`（连同它们的关联值），但是在同一时间只能存储这两个值中的一个。 + +你可以使用一个 switch 语句来检查不同的条形码类型，和中的例子一样。然而，这一次，关联值可以被提取出来作为 switch 语句的一部分。你可以在 `switch` 的 `case` 分支代码中提取每个关联值作为一个常量（用 `let` 前缀）或者作为一个变量（用 `var` 前缀）来使用： + +```swift +switch productBarcode { +case .upc(let numberSystem, let manufacturer, let product, let check): + print("UPC: \(numberSystem), \(manufacturer), \(product), \(check).") +case .qrCode(let productCode): + print("QR code: \(productCode).") +} +// 打印 "QR code: ABCDEFGHIJKLMNOP." +``` + + + +如果一个枚举成员的所有关联值都被提取为常量，或者都被提取为变量，为了简洁，你可以只在成员名称前标注一个 `let` 或者 `var`： + +```swift +switch productBarcode { +case let .upc(numberSystem, manufacturer, product, check): + print("UPC : \(numberSystem), \(manufacturer), \(product), \(check).") +case let .qrCode(productCode): + print("QR code: \(productCode).") +} +// 打印 "QR code: ABCDEFGHIJKLMNOP." +``` + + + +## 原始值 + +在小节的条形码例子中，演示了如何声明存储不同类型关联值的枚举成员。作为关联值的替代选择，枚举成员可以被默认值（称为*原始值*）预填充，这些原始值的类型必须相同。 + +这是一个使用 ASCII 码作为原始值的枚举： + +```swift +enum ASCIIControlCharacter: Character { + case tab = "\t" + case lineFeed = "\n" + case carriageReturn = "\r" +} +``` + + + +上面的例子中，枚举类型 `ASCIIControlCharacter` 的原始值类型被定义为 `Character`，并设置了一些比较常见的 ASCII 控制字符。`Character` 的描述详见部分。 + +原始值可以是字符串、字符，或者任意整型值或浮点型值。每个原始值在枚举声明中必须是唯一的。 + +>注意: +>原始值和关联值是*不同*的。原始值是在定义枚举时被预先填充的值，像上述三个 ASCII 码。对于一个特定的枚举成员，它的原始值始终不变。关联值是创建一个基于枚举成员的常量或变量时才设置的值，枚举成员的关联值可以变化。 + +### 原始值的隐式赋值 + +在使用原始值为整数或者字符串类型的枚举时，不需要显式地为每一个枚举成员设置原始值，当你没有手动赋值时，Swift 将会自动为你赋值。 + +例如，当使用整数作为原始值时，隐式赋值的值依次递增 1。如果第一个枚举成员没有设置原始值，其原始值将为 `0`。 + +下面的枚举是对之前 `Planet` 这个枚举的一个细化，利用整型的原始值来表示每个行星在太阳系中的顺序： + +```swift +enum Planet: Int { + case mercury = 1, venus, earth, mars, jupiter, saturn, uranus, neptune +} +``` + + + +在上面的例子中，`Plant.mercury` 的显式原始值为 `1`，`Planet.venus` 的隐式原始值为 `2`，依此类推。 + +当使用字符串作为枚举类型的原始值时，每个枚举成员的隐式原始值为该枚举成员的名称。 + +下面的例子是前面 `CompassPoint` 枚举的细化，使用字符串类型的原始值来表示各个方向的名称： + +```swift +enum CompassPoint: String { + case north, south, east, west +} +``` + + + +上面例子中，`CompassPoint.south` 拥有隐式原始值 `"south"`，依此类推。 + +使用枚举成员的 `rawValue` 属性可以访问该枚举成员的原始值： + +```swift +let earthsOrder = Planet.earth.rawValue +// earthsOrder 值为 3 + +let sunsetDirection = CompassPoint.west.rawValue +// sunsetDirection 值为 "west" +``` + + + +### 使用原始值初始化枚举实例 + +如果在定义枚举类型的时候使用了原始值，那么将会自动获得一个构造器，这个构造器接收一个叫做 `rawValue` 的参数，参数类型即为原始值的类型，返回值则是枚举成员或 `nil`。你可以使用这个构造器来创建一个新的枚举实例。 + +这个例子利用原始值 `7` 创建了枚举成员 `Uranus`： + +```swift +let possiblePlanet = Planet(rawValue: 7) +// possiblePlanet 类型为 Planet? 值为 Planet.uranus +``` + + + +然而，并非所有 `Int` 值都可以找到一个匹配的行星。因此，原始值构造器总是返回一个*可选的*枚举成员。在上面的例子中，`possiblePlanet` 是 `Planet?` 类型，或者说“可选的 `Planet`”。 + +>注意: +>原始值构造器是一个可失败构造器，因为并不是每一个原始值都有与之对应的枚举成员。更多信息请参阅。 + +如果你试图寻找一个位置为 `11` 的行星，通过原始值构造器返回的可选 `Planet` 值将是 `nil`： + +```swift +let positionToFind = 11 +if let somePlanet = Planet(rawValue: positionToFind) { + switch somePlanet { + case .earth: + print("Mostly harmless") + default: + print("Not a safe place for humans") + } +} else { + print("There isn't a planet at position \(positionToFind)") +} +// 打印 "There isn't a planet at position 11" +``` + + + +这个例子使用了可选绑定（optional binding），试图通过原始值 `11` 来访问一个行星。`if let somePlanet = Planet(rawValue: 11)` 语句创建了一个可选 `Planet`，如果可选 `Planet` 的值存在，就会赋值给 `somePlanet`。在这个例子中，无法检索到位置为 `11` 的行星，所以 `else` 分支被执行。 + + + +## 递归枚举 + +*递归枚举*是一种枚举类型，其中一个或多个枚举成员的关联值是同一种枚举的另一个实例。你可以在枚举成员前加上 `indirect` 来表示该成员可递归。使用递归枚举时，编译器会插入一个间接层。 + +例如，下面的例子中，枚举类型存储了简单的算术表达式： + +```swift +enum ArithmeticExpression { + case number(Int) + indirect case addition(ArithmeticExpression, ArithmeticExpression) + indirect case multiplication(ArithmeticExpression, ArithmeticExpression) +} +``` + + + +你也可以在枚举类型前面加上 `indirect` 关键字来表明它的所有成员都是可递归的： + +```swift +indirect enum ArithmeticExpression { + case number(Int) + case addition(ArithmeticExpression, ArithmeticExpression) + case multiplication(ArithmeticExpression, ArithmeticExpression) +} +``` + + + +上面定义的枚举类型可以存储三种算术表达式：纯数字、两个表达式相加、两个表达式相乘。枚举成员 `addition` 和 `multiplication` 的关联值也是算术表达式——这些关联值可以嵌套表达式。例如，表达式 `(5 + 4) * 2`，乘号右边是一个数字，左边则是另一个表达式。因为数据是嵌套的，因而用来存储数据的枚举类型也需要支持这种嵌套——这意味着枚举类型需要支持递归。下面的代码展示了使用 `ArithmeticExpression` 这个递归枚举创建表达式 `(5 + 4) * 2`： + +```swift +let five = ArithmeticExpression.number(5) +let four = ArithmeticExpression.number(4) +let sum = ArithmeticExpression.addition(five, four) +let product = ArithmeticExpression.multiplication(sum, ArithmeticExpression.number(2)) +``` + + + +要操作具有递归性质的数据结构，使用递归函数是一种直截了当的方式。例如，下面是一个对算术表达式求值的函数： + +```swift +func evaluate(_ expression: ArithmeticExpression) -> Int { + switch expression { + case let .number(value): + return value + case let .addition(left, right): + return evaluate(left) + evaluate(right) + case let .multiplication(left, right): + return evaluate(left) * evaluate(right) + } +} + +print(evaluate(product)) +// 打印 "18" +``` + + + +该函数如果遇到纯数字，就直接返回该数字的值。如果遇到的是加法或乘法运算，则分别计算左边表达式和右边表达式的值，然后相加或相乘。 + + diff --git a/swift-6.docc/LanguageGuide/ErrorHandling.md b/swift-6.docc/LanguageGuide/ErrorHandling.md new file mode 100644 index 000000000..d8da59ad6 --- /dev/null +++ b/swift-6.docc/LanguageGuide/ErrorHandling.md @@ -0,0 +1,715 @@ +# 错误处理 + +响应错误并从错误中恢复。 + +**错误处理(Error handling)** 是对程序中的错误条件做出响应并从中恢复的过程。Swift 为在运行时抛出、捕获、传递和处理可恢复错误提供了一等支持。 + +有些操作并不能保证总是能执行完成或生成有用的结果。可选类型用于表示值缺失，但当操作失败时，了解造成失败的原因有助于你的代码作出相应的应对。 + +以从磁盘文件读取和处理数据为例。该任务失败的原因有很多，包括指定路径下的文件不存在、文件没有读取权限或文件编码格式不兼容。通过区分这些不同的失败情况来让程序处理和解决一些错误，并将无法解决的错误告知用户。 + +> 注意: +> Swift 中的错误处理与 Cocoa 和 Objective-C 中使用 `NSError` 类的错误处理模式互操作。 +> 有关该类的更多信息，请参阅 [在 Swift 中处理 Cocoa 错误](https://developer.apple.com/documentation/swift/cocoa_design_patterns/handling_cocoa_errors_in_swift). + +## 表示与抛出错误 + +在 Swift 中，错误由遵循 Error 协议的类型值表示。这个空协议表示一种类型可用于错误处理。 + +Swift 枚举特别适用于一组相关的错误条件，枚举的关联值还可以提供错误状态的额外信息。例如，您可以用以下方式表示在游戏中操作自动售货机的错误条件： + +```swift +enum VendingMachineError: Error { + case invalidSelection // 不可选择 + case insufficientFunds(coinsNeeded: Int) // 金额不足 + case outOfStock // 缺货 +} +``` + + + +通过抛出错误可以让你表明发生了意外情况，导致正常的流程无法继续执行。您可以使用 throw 语句来抛出错误。例如，下面的代码抛出了一个错误，表示自动售货机还需要 5 枚硬币： + +```swift +throw VendingMachineError.insufficientFunds(coinsNeeded: 5) +``` + + + +## 处理错误 + +当错误被抛出时，周围的部分代码必须负责处理该错误，例如，纠正这个问题、尝试其他方法或将错误通知用户。 + +在 Swift 中有四种处理错误的方法。您可以把函数抛出的错误传递给调用此函数的代码、使用 do-catch 语句处理错误、将错误作为可选类型处理、或者断言此错误根本不会发生。下文将对每种方法分段说明。 + +当函数抛出错误时，程序的流程会发生改变，因此快速识别代码中可能抛出错误的地方非常重要。要识别代码中的这些地方，请在调用可能抛出错误的函数、方法或构造器的代码之前加上 `try` 关键字或 `try?` 或 `try!` 变体。下文将对这些关键字进行说明。 + +> 注意: +> Swift 中的错误处理与其他语言中的错误处理类似，使用 `try`, `catch` and `throw` 关键字。 +> 区别于其他语言（包括 Objective-C ）的是，Swift 中的错误处理并不涉及解除调用栈,而解除调用栈的过程可能会耗费大量计算资源。 +> 因此，`throw` 语句的性能特征与 `return` 语句的性能特征相当。 + +### 用throwing函数传递错误 + +为了表示函数、方法或构造器可以抛出错误，您可以在函数声明中的参数后写入 `throws` 关键字。标有 `throws` 的函数称为 **throwing函数**。如果该函数指定了返回类型，则应在返回箭头（`->`）之前写入 `throws` 关键字。 + + + +```swift +func canThrowErrors() throws -> String + +func cannotThrowErrors() -> String +``` + + + + + + + + + + + +throwing会将内部抛出的错误传递到调用它的作用域。 + +> 注意: +> 只有throwing函数可以传递错误。任何在非throwing函数中抛出的错误都必须在函数内部处理。 + +在下面的示例中，`VendingMachine` 类有一个 `vend(itemNamed:)` 方法。该方法会在请求物品不存在、缺货或者投入金额小于物品价格时，抛出相应的 `VendingMachineError` 错误： + +```swift +struct Item { + var price: Int + var count: Int +} + +class VendingMachine { + var inventory = [ + "Candy Bar": Item(price: 12, count: 7), + "Chips": Item(price: 10, count: 4), + "Pretzels": Item(price: 7, count: 11) + ] + var coinsDeposited = 0 + + func vend(itemNamed name: String) throws { + guard let item = inventory[name] else { + throw VendingMachineError.invalidSelection + } + + guard item.count > 0 else { + throw VendingMachineError.outOfStock + } + + guard item.price <= coinsDeposited else { + throw VendingMachineError.insufficientFunds(coinsNeeded: item.price - coinsDeposited) + } + + coinsDeposited -= item.price + + var newItem = item + newItem.count -= 1 + inventory[name] = newItem + + print("Dispensing \(name)") + } +} +``` + + + +在 `vend(itemNamed:)` 方法的实现中使用 `guard` 语句来确保在物品成功购买所需条件中不满足任一一个条件时提前退出方法并抛出相应的错误。由于 `throw` 语句会立刻退出方法，所以物品只有在所有条件都满足时才会被售出。 + +由于 `vend(itemNamed:)` 方法会传递出它抛出的任何错误，因此调用此方法的代码必须直接处理错误 ---使用 `do`-`catch` 语句, `try?`, 或 `try!` ---或者把这些错误继续传递下去。例如,下面例子中的 `buyFavoriteSnack(person:vendingMachine:)` 也是一个throwing函数，所以 `vend(itemNamed:)` 方法抛出的任何错误将传递到调用 `buyFavoriteSnack(person:vendingMachine:)` 函数的位置。 + +```swift +let favoriteSnacks = [ + "Alice": "Chips", + "Bob": "Licorice", + "Eve": "Pretzels", +] +func buyFavoriteSnack(person: String, vendingMachine: VendingMachine) throws { + let snackName = favoriteSnacks[person] ?? "Candy Bar" + try vendingMachine.vend(itemNamed: snackName) +} +``` + + + +在这个示例中， `buyFavoriteSnack(person: vendingMachine:)` 函数会查找某人最喜欢的零食并尝试通过调用 `vend(itemNamed:)` 方法为其购买。由于 `vend(itemNamed:)` 方法可能会出错，因此在调用该方法时会在前面加上 `try` 关键字。 + +throwing 构造器能像 throwing 函数一样传递错误。例如，下表中的 `PurchasedSnack` 结构体的构造器在初始化过程中调用了一个throwing函数，并将抛出的错误传递给这个构造器的调用者来处理这些错误。 + +```swift +struct PurchasedSnack { + let name: String + init(name: String, vendingMachine: VendingMachine) throws { + try vendingMachine.vend(itemNamed: name) + self.name = name + } +} +``` + + + +### 使用 Do-Catch 处理错误 + +您可以使用 `do`-`catch` 语句通过运行闭包来处理错误。如果 `do` 子句中的代码抛出错误，就会与 `catch` 子句进行匹配，以确定哪个子句可以处理该错误。 + +下面是 `do`-`catch` 语句的一般形式： + +```swift +do { + try <#expression#> + <#statements#> +} catch <#pattern 1#> { + <#statements#> +} catch <#pattern 2#> where <#condition#> { + <#statements#> +} catch <#pattern 3#>, <#pattern 4#> where <#condition#> { + <#statements#> +} catch { + <#statements#> +} +``` + +您可以在 `catch` 后编写一个模式，以指示该子句可以处理哪些错误。如果 `catch` 子句没有模式，该子句将匹配任何错误，并将错误绑定到名为 `error` 的局部常量。有关模式匹配的更多信息，请参阅 。 + + + +例如，以下代码匹配了 `VendingMachineError` 枚举的全部三种情况。 + +```swift +var vendingMachine = VendingMachine() +vendingMachine.coinsDeposited = 8 +do { + try buyFavoriteSnack(person: "Alice", vendingMachine: vendingMachine) + print("Success! Yum.") +} catch VendingMachineError.invalidSelection { + print("Invalid Selection.") +} catch VendingMachineError.outOfStock { + print("Out of Stock.") +} catch VendingMachineError.insufficientFunds(let coinsNeeded) { + print("Insufficient funds. Please insert an additional \(coinsNeeded) coins.") +} catch { + print("Unexpected error: \(error).") +} +// 打印 "Insufficient funds. Please insert an additional 2 coins." +``` + + + +在上面的示例中，`buyFavoriteSnack(person:vendingMachine:)` 函数在 `try` 表达式中被调用，因为它可能会抛出错误。如果抛出错误，执行将立即转移到 `catch` 子句，由其决定是否继续传递。如果错误没有被匹配，它将被最后的 `catch` 子句捕获，并绑定到本地 `error` 常量。如果没有错误抛出，则执行 `do` 语句中的其余语句。 + +`catch` 子句不必处理 `do` 子句中的代码可能抛出的所有错误。如果没有 `catch` 子句处理错误，则错误会传播到周围的作用域。但是，传播的错误必须由 **某个** 周围作用域处理。在非throwing函数中， `do`-`catch` 语句必须处理错误。在throwing函数中，必须由 `do`-`catch` 语句或调用者处理错误。如果错误传递到了顶层作用域却依然没有被处理，则会出现运行时错误。 + +例如，在编写上述示例时，只要不是 `VendingMachineError` 中声明的错误，都会被调用函数捕获： + +```swift +func nourish(with item: String) throws { + do { + try vendingMachine.vend(itemNamed: item) + } catch is VendingMachineError { + print("Couldn't buy that from the vending machine.") + } +} + +do { + try nourish(with: "Beet-Flavored Chips") +} catch { + print("Unexpected non-vending-machine-related error: \(error)") +} +// 打印 "Couldn't buy that from the vending machine." +``` + + + +在 `nourish(with:)` 函数中，如果 `vend(itemNamed:)` 抛出的错误属于 `VendingMachineError` 枚举的情况之一，`nourish(with:)` 会通过打印信息来处理该错误。否则，`nourish(with:)` 会将错误传递到它的调用方。然后，该错误将被通用的 `catch` 子句捕获。 + +另一种捕获多个相关错误的方式是将它们放在 `catch` 后，并用逗号分隔。例如： + +```swift +func eat(item: String) throws { + do { + try vendingMachine.vend(itemNamed: item) + } catch VendingMachineError.invalidSelection, VendingMachineError.insufficientFunds, VendingMachineError.outOfStock { + print("Invalid selection, out of stock, or not enough money.") + } +} +``` + + + + + +`eat(item:)` 函数列出了要捕获的自动售货机错误，其错误文本与它列出的相对应。如果列出来的三个错误中任意一个被抛出，该 `catch` 子句将通过打印一条消息来处理这些错误。任何其他错误都会传播到周围的作用域，包括以后可能添加的任何自动售货机错误(`VendingMachineError`)。 + +### 将错误转换成可选值 + +您可以使用 `try?` 将错误转换为可选值来处理错误。如果在计算 `try?` 表达式时抛出错误，则表达式的值为 `nil` 。例如，在以下代码中，`x` 和 `y` 有着相同的数值和等价的含义： + +```swift +func someThrowingFunction() throws -> Int { + // ... +} + +let x = try? someThrowingFunction() + +let y: Int? +do { + y = try someThrowingFunction() +} catch { + y = nil +} +``` + + + +如果 `someThrowingFunction()` 抛出一个错误，泽 `x` 和 `y` 的值是 `nil`。否则 `x` 和 `y` 的值就是该函数的返回值。注意，无论 `someThrowingFunction()` 的返回值类型是什么类型，`x` 和 `y` 都是这个类型的可选类型。例子中此函数返回一个整型，所以 `x` 和 `y` 是可选整型。 + +使用 `try?` 可让您在希望以相同方式处理所有错误时编写简洁的错误处理代码。例如，以下代码使用多种方法获取数据，如果所有方法都失败，则返回 `nil` 。 + +```swift +func fetchData() -> Data? { + if let data = try? fetchDataFromDisk() { return data } + if let data = try? fetchDataFromServer() { return data } + return nil +} +``` + + + +### 禁用错误传递 + +有时，您知道一个throwing函数或方法实际上不会在运行时抛出错误。在这种情况下，您可以在表达式之前写入 `try!` 以禁用错误传递，并在运行时断言不会抛出错误的情况下封装调用。如果实际抛出了错误，将会发生运行时错误。 + +例如，下面的代码使用了 `loadImage(atPath:)` 函数，该函数从给定的路径加载图片资源，如果无法加载图像则抛出一个错误。在这种情况下，因为图片是和应用绑定的，运行时不会有错误抛出，所以适合禁用错误传递。 + +```swift +let photo = try! loadImage(atPath: "./Resources/John Appleseed.jpg") +``` + + + +## 指定错误类型 + +上述所有示例都使用了最常见的错误处理方式，即代码抛出的错误可以是符合 `Error` 协议的任何类型的值。这种方法符合实际情况，即您不可能提前知道代码运行时可能发生的所有错误，尤其是在传递其他地方抛出的错误时。它还反映了一个事实，即错误会随着时间的推移而改变。新版本的库，包括你的依赖库使用的库，可能会产生新的错误，而现实世界中用户配置的复杂性可能会暴露出开发或测试过程中不可见的故障模式。上述示例中的错误处理代码始终包含一个默认情况，用于处理没有特定 `catch` 子句的错误。 + +大多数 Swift 代码都不会指定所抛出错误的类型。不过，在以下特殊情况下，您可能会限制代码只抛出一种特定类型的错误： + +- 在不支持动态分配内存的嵌入式系统上运行代码时。抛出 `any Error` 或其他封装的协议类型(boxed protocol type)的实例需要在运行时分配内存来存储错误。相比之下，抛出特定类型的错误可以让 Swift 避免为错误进行堆分配。 + +- 当错误是代码单元（如库）的实现细节，而不是代码接口的一部分时。由于错误只来自于库，而不是来自于其他依赖库或库的客户端，因此您可以列出所有可能出现的错误的详尽列表。而且，由于这些错误是库的实现细节，因此可以全部在库内部处理。 + +- 在只传递由通用参数描述的错误的代码中，例如函数接收一个闭包参数并传递来自该闭包的任何错误。有关传播特定错误类型与使用 rethrows 的比较，请参阅 。 + +例如，思考一下如何实现汇总评分并使用以下错误类型的代码： + +```swift +enum StatisticsError: Error { + case noRatings + case invalidRating(Int) +} +``` + +要指定函数只抛出 `StatisticsError` 值作为其错误，您可以在声明函数时写入 `throws(StatisticsError)` 而不是 `throws` 。这种语法也被称为 **指定类型抛错(typed throws)**，因为您在声明中的 `throws` 后面指定了错误类型。例如，下面的函数抛出 `StatisticsError` 值作为其错误。 + +```swift +func summarize(_ ratings: [Int]) throws(StatisticsError) { + guard !ratings.isEmpty else { throw .noRatings } + + var counts = [1: 0, 2: 0, 3: 0] + for rating in ratings { + guard rating > 0 && rating <= 3 else { throw .invalidRating(rating) } + counts[rating]! += 1 + } + + print("*", counts[1]!, "-- **", counts[2]!, "-- ***", counts[3]!) +} +``` + +上面的代码中，`summarize(_:)` 函数汇总了评分从 1 到 3 数量的列表。如果输入无效，该函数将抛出 `StatisticsError` 实例。上面代码中抛出错误的两个地方都省略了错误类型，因为函数已经指定了错误类型。在这样的函数中抛出错误时，您可以使用省略形式 `throw .noRatings` 代替 `throw StatisticsError.noRatings` 。 + +当您在函数开头指定错误类型时，Swift 会检查您是否抛出了其他错误。例如，如果您尝试在 `VendingMachineError` 函数中使用本章前面示例中的 `summarize(_:)` 函数，该代码将在编译时产生错误。 + +您可以在普通的throwing函数中调用使用指定类型抛错的函数： + +```swift +func someThrowingFunction() -> throws { + let ratings = [1, 2, 3, 2, 2, 1] + try summarize(ratings) +} +``` + +上面的代码没有为 `someThrowingFunction()` 指定错误类型，因此它抛出了 `any Error`。您也可以将错误类型写为 `throws(any Error)`; 下面的代码等同于上面的代码： + +```swift +func someThrowingFunction() -> throws(any Error) { + let ratings = [1, 2, 3, 2, 2, 1] + try summarize(ratings) +} +``` + +在此代码中，`someThrowingFunction()` 会传递 `summarize(_:)` 抛出的任何错误。`summarize(_：)` 抛出的错误总是 `StatisticsError` 值，这也是 `someThrowingFunction()` 可抛出的有效错误。 + +就像您可以使用 `Never` 的返回类型编写一个永不返回的函数一样，您也可以使用 `throws(Never)` 编写一个永不抛出错误的函数： + +```swift +func nonThrowingFunction() throws(Never) { + // ... +} +``` +这个函数不能抛出错误，因为不可能创建一个 `Never` 类型的值来抛出。 + +除了指定函数的错误类型外，您还可以为 `do`-`catch` 语句编写特定的错误类型子句。例如： + +```swift +let ratings = [] +do throws(StatisticsError) { + try summarize(ratings) +} catch { + switch error { + case .noRatings: + print("No ratings available") + case .invalidRating(let rating): + print("Invalid rating: \(rating)") + } +} +// 打印 "No ratings available" +``` + +在此代码中，写入 `do throws(StatisticsError)` 表明 `do`-`catch` 语句抛出 `StatisticsError` 值作为其错误。与其他 `do`-`catch` 语句一样，`catch` 子句可以处理所有可能的错误，也可以传递未处理的错误让周围的作用域处理。此代码使用 `switch` 语句处理所有错误，每个枚举值有一个分支(case)。与其他没有模式的 `catch` 子句一样，该子句匹配任何错误并将错误绑定到名为 `error` 的局部常量。`do`-`catch` 语句会抛出 `StatisticsError` 值，所以 `error` 是 `StatisticsError` 类型的值。 + +上述 `catch` 子句使用 `switch` 语句来匹配和处理每个可能的错误。如果您尝试在不更新错误处理代码的情况下为 `StatisticsError` 添加新的分支(case)，Swift 将提示您错误，因为 `switch` 语句不再穷尽所有分支。对于捕获自身所有错误的库，您可以使用这种方法来确保任何新错误都有相应的新代码来处理。 + +如果函数或 `do` 闭包只抛出单一类型的错误，Swift 会推断该代码使用了指定类型抛错。使用这种更短的语法，您可以将上述 `do`-`catch` 示例编写如下： + +```swift +let ratings = [] +do { + try summarize(ratings) +} catch { + switch error { + case .noRatings: + print("No ratings available") + case .invalidRating(let rating): + print("Invalid rating: \(rating)") + } +} +// 打印 "No ratings available" +``` + +尽管上面的 `do`-`catch` 块没有指定它抛出的错误类型，Swift 仍会推断它抛出 `StatisticsError` 。您可以显式地编写 `throws(any Error)` 以避免让 Swift 推断出指定类型抛错。 + +## 指定清理操作 + +您可以使用 `defer` 语句在代码执行离开当前代码块之前执行一组语句。无论是以何种方式离开当前代码块--是由于抛出错误，还是由于 return 或 break 等语句，该语句都可让您执行任何必要的清理。例如，您可以使用 `defer` 语句来确保关闭文件描述符，以及释放手动分配的内存。 + +`defer` 语句将代码的执行延迟到当前的作用域退出之前。该语句由 `defer` 关键字和要被延迟执行的语句组成。延迟执行的语句不能包含任何控制转移语句，例如 `break`、`return` 语句，或是抛出一个错误。延迟执行的操作会按照它们声明的顺序从后往前执行——也就是说，第一条 `defer` 语句中的代码最后才执行，第二条 `defer` 语句中的代码倒数第二个执行，以此类推。最后一条语句会第一个执行。 + +```swift +func processFile(filename: String) throws { + if exists(filename) { + let file = open(filename) + defer { + close(file) + } + while let line = try file.readline() { + // 处理文件。 + } + // close(file) 会在这里被调用，即作用域的最后。 + } +} +``` + + + +上面的示例使用 `defer` 语句确保 `open(_:)` 函数有相应的 `close(_:)` 调用。 + +即使不涉及错误处理代码，您也可以使用 `defer` 语句。有关详细信息，请参阅 。 + + diff --git a/swift-6-beta.docc/LanguageGuide/Extensions.md b/swift-6.docc/LanguageGuide/Extensions.md similarity index 51% rename from swift-6-beta.docc/LanguageGuide/Extensions.md rename to swift-6.docc/LanguageGuide/Extensions.md index 38832b0a1..d268dc299 100644 --- a/swift-6-beta.docc/LanguageGuide/Extensions.md +++ b/swift-6.docc/LanguageGuide/Extensions.md @@ -1,31 +1,22 @@ -# Extensions +# 扩展 -Add functionality to an existing type. +为现有类型添加功能。 -*Extensions* add new functionality to an existing -class, structure, enumeration, or protocol type. -This includes the ability to extend types -for which you don't have access to the original source code -(known as *retroactive modeling*). -Extensions are similar to categories in Objective-C. -(Unlike Objective-C categories, Swift extensions don't have names.) +**扩展（Extensions）** 用于为现有的类、结构体、枚举或协议类型添加新功能。这包括了扩展那些您无法访问原始源代码的类型的能力（即*追溯建模*）。扩展和 Objective-C 的分类很相似。（与 Objective-C 分类不同的是，Swift 扩展是没有名字的。） -Extensions in Swift can: +Swift 中的扩展可以： -- Add computed instance properties and computed type properties -- Define instance methods and type methods -- Provide new initializers -- Define subscripts -- Define and use new nested types -- Make an existing type conform to a protocol + - 添加计算实例属性和计算类属性 + - 定义实例方法和类方法 + - 提供新的构造器 + - 定义下标 + - 定义和使用新的嵌套类型 + - 使已经存在的类型遵循一个协议 -In Swift, -you can even extend a protocol to provide implementations of its requirements -or add additional functionality that conforming types can take advantage of. -For more details, see . +在 Swift 中，你甚至可以扩展协议以提供其需要的实现，或者添加额外功能给遵循的类型所使用。你可以从 获取更多细节。 -> Note: Extensions can add new functionality to a type, -> but they can't override existing functionality. +> 注意: +> 扩展可以给一个类型添加新的功能，但是不能重写已经存在的功能。 -## Extension Syntax +## 扩展的语法 -Declare extensions with the `extension` keyword: +使用 `extension` 关键字声明扩展： ```swift extension SomeType { - // new functionality to add to SomeType goes here + // 在这里给 SomeType 添加新的功能 } ``` @@ -96,14 +87,11 @@ extension SomeType { ``` --> -An extension can extend an existing type to make it adopt one or more protocols. -To add protocol conformance, -you write the protocol names -the same way as you write them for a class or structure: +扩展可以扩充一个现有的类型，给它添加一个或多个协议。在添加协议的遵循声明时，协议名称的写法和类或者结构体一样： ```swift extension SomeType: SomeProtocol, AnotherProtocol { - // implementation of protocol requirements goes here + // 协议所需要的实现写在这里 } ``` @@ -119,23 +107,15 @@ extension SomeType: SomeProtocol, AnotherProtocol { ``` --> -Adding protocol conformance in this way is described in -. +这种遵循协议的方式在 中有描述。 -An extension can be used to extend an existing generic type, -as described in . -You can also extend a generic type to conditionally add functionality, -as described in . +扩展可以使用在现有泛型类型上，就像 中描述的一样。你还可以使用扩展给泛型类型有条件地添加功能，就像 中描述的一样。 -> Note: If you define an extension to add new functionality to an existing type, -> the new functionality will be available on all existing instances of that type, -> even if they were created before the extension was defined. +> 注意: 对一个现有的类型，如果你定义了一个扩展来添加新的功能，那么这个类型的所有实例都可以使用这个新功能，包括那些在扩展定义之前就存在的实例。 -## Computed Properties +## 计算属性 -Extensions can add computed instance properties and computed type properties to existing types. -This example adds five computed instance properties to Swift's built-in `Double` type, -to provide basic support for working with distance units: +扩展可以给现有类型添加计算实例属性和计算类属性。这个例子给 Swift 内建的 `Double` 类型添加了五个计算型实例属性，以提供基本的距离单位处理功能： ```swift extension Double { @@ -147,10 +127,10 @@ extension Double { } let oneInch = 25.4.mm print("One inch is \(oneInch) meters") -// Prints "One inch is 0.0254 meters" +// 打印“One inch is 0.0254 meters” let threeFeet = 3.ft print("Three feet is \(threeFeet) meters") -// Prints "Three feet is 0.914399970739201 meters" +// 打印“Three feet is 0.914399970739201 meters” ``` -These computed properties express that a `Double` value -should be considered as a certain unit of length. -Although they're implemented as computed properties, -the names of these properties can be appended to -a floating-point literal value with dot syntax, -as a way to use that literal value to perform distance conversions. - -In this example, a `Double` value of `1.0` is considered to represent “one meter”. -This is why the `m` computed property returns `self` --- -the expression `1.m` is considered to calculate a `Double` value of `1.0`. - -Other units require some conversion to be expressed as a value measured in meters. -One kilometer is the same as 1,000 meters, -so the `km` computed property multiplies the value by `1_000.00` -to convert into a number expressed in meters. -Similarly, there are 3.28084 feet in a meter, -and so the `ft` computed property divides the underlying `Double` value -by `3.28084`, to convert it from feet to meters. - -These properties are read-only computed properties, -and so they're expressed without the `get` keyword, for brevity. -Their return value is of type `Double`, -and can be used within mathematical calculations wherever a `Double` is accepted: +这些计算属性表示一个 `Double` 值应该被视为某种长度单位。尽管它们是作为计算属性实现的，但是这些属性的名称可以使用点语法附加到浮点字面量值之后，作为一种使用该字面量值执行距离转换的方式。 + +在这个例子中，`Double` 类型的 `1.0` 代表的是“一米”。这就是为什么计算属性 `m` 返回的是 `self` ——表达式 `1.m` 被认为是计算一个 `Double` 类型的 `1.0`。 + +其他单位需要进行一些转换，才能表示为以米为单位的值。一千米等于 1000 米，所以计算属性 `km` 将该值乘以 `1_000.00`来将其转换为以米为单位的数字。类似地，一米等于 3.28084 英尺，因此计算属性 `ft` 将底层的 `Double` 值除以 `3.28084` 来将其从英尺转换为米。 + +这些属性是只读的计算属性，因此为了简便，它们的表达方式省略了 `get` 关键字。它们的返回值是 `Double` 类型，可以在任何接受 `Double` 的数学计算中使用: ```swift let aMarathon = 42.km + 195.m print("A marathon is \(aMarathon) meters long") -// Prints "A marathon is 42195.0 meters long" +// 打印“A marathon is 42195.0 meters long” ``` -> Note: Extensions can add new computed properties, but they can't add stored properties, -> or add property observers to existing properties. +> 注意: 扩展可以添加新的计算属性，但是它们不能添加存储属性，也不能为现有属性添加属性观察器。 -## Initializers - -Extensions can add new initializers to existing types. -This enables you to extend other types to accept -your own custom types as initializer parameters, -or to provide additional initialization options -that were not included as part of the type's original implementation. - -Extensions can add new convenience initializers to a class, -but they can't add new designated initializers or deinitializers to a class. -Designated initializers and deinitializers -must always be provided by the original class implementation. - -If you use an extension to add an initializer to a value type that provides -default values for all of its stored properties -and doesn't define any custom initializers, -you can call the default initializer and memberwise initializer for that value type -from within your extension's initializer. -This wouldn't be the case if you had written the initializer -as part of the value type's original implementation, -as described in . - -If you use an extension to add an initializer to a structure -that was declared in another module, -the new initializer can't access `self` until it calls -an initializer from the defining module. - -The example below defines a custom `Rect` structure to represent a geometric rectangle. -The example also defines two supporting structures called `Size` and `Point`, -both of which provide default values of `0.0` for all of their properties: +## 构造器 + +扩展可以为现有类型添加新的构造器。这使你可以扩展其他类型以接受你自己的自定义类型作为构造器参数，或提供类型的原始实现中未包含的其他构造选项。 + +扩展可以为一个类添加新的便利构造器（convenience initializer），但是它们不能为一个类添加新的指定构造器（designated initializer）或析构器（deinitializer）。指定构造器和析构器必须始终由类的原始实现提供。 + +如果你使用扩展为一个值类型添加构造器，并且该值类型提供了所有存储属性的默认值，且没有定义任何自定义构造器，你就可以在扩展的构造器中调用该值类型的默认构造器和成员构造器。如果你已经将构造器写在该值类型的原始实现中，则不适用于这种情况，正如 中所描述的那样。 + +如果你使用扩展为另一个模块中声明的结构体添加构造器，那么在调用定义模块中的构造器之前，新的构造器是不能访问 `self` 的。 + +以下示例定义了一个自定义的 `Rect` 结构体来表示几何矩形。该示例还定义了两个辅助结构体 `Size` 和 `Point`，它们都为所有属性提供了默认值 `0.0`： ```swift struct Size { @@ -293,10 +237,7 @@ struct Rect { ``` --> -Because the `Rect` structure provides default values for all of its properties, -it receives a default initializer and a memberwise initializer automatically, -as described in . -These initializers can be used to create new `Rect` instances: +因为 `Rect` 结构体为所有属性都提供了默认值，它会自动获得默认构造器和成员构造器，如 中所述。这些构造器可用于创建新的 `Rect` 实例: ```swift let defaultRect = Rect() @@ -314,8 +255,7 @@ let memberwiseRect = Rect(origin: Point(x: 2.0, y: 2.0), ``` --> -You can extend the `Rect` structure to provide an additional initializer -that takes a specific center point and size: +你可以通过扩展 `Rect` 结构体来额外提供一个允许指定 center 和 size 的构造器： ```swift extension Rect { @@ -341,16 +281,12 @@ extension Rect { ``` --> -This new initializer starts by calculating an appropriate origin point based on -the provided `center` point and `size` value. -The initializer then calls the structure's automatic memberwise initializer -`init(origin:size:)`, which stores the new origin and size values -in the appropriate properties: +这个新的构造器首先根据提供的 `center` 和 `size` 计算一个适当的原点。然后这个构造器调用结构体自带的成员构造器 `init(origin:size:)`，它会将新的 origin 和 size 值储存在相应的属性中： ```swift let centerRect = Rect(center: Point(x: 4.0, y: 4.0), size: Size(width: 3.0, height: 3.0)) -// centerRect's origin is (2.5, 2.5) and its size is (3.0, 3.0) +// centerRect 的 origin 是 (2.5, 2.5) 并且它的 size 是 (3.0, 3.0) ``` -> Note: If you provide a new initializer with an extension, -> you are still responsible for making sure that each instance is fully initialized -> once the initializer completes. +> 注意: 如果你使用扩展提供了一个新的构造器，你仍然需要确保在构造器完成时每个实例都被完全初始化。 -## Methods +## 方法 -Extensions can add new instance methods and type methods to existing types. -The following example adds a new instance method called `repetitions` to the `Int` type: +扩展可以为现有类型添加新的实例方法和类方法。以下示例为 `Int` 类型添加了一个名为 `repetitions` 的新实例方法： ```swift extension Int { @@ -397,12 +330,9 @@ extension Int { ``` --> -The `repetitions(task:)` method takes a single argument of type `() -> Void`, -which indicates a function that has no parameters and doesn't return a value. +`repetitions(task:)` 方法仅接受一个类型为 `() -> Void` 的参数，它表示一个没有参数没有返回值的函数。 -After defining this extension, -you can call the `repetitions(task:)` method on any integer -to perform a task that many number of times: +定义了这个扩展之后，你可以对任意整形数值调用 `repetitions(task:)` 方法，以执行指定次数的任务: ```swift 3.repetitions { @@ -426,15 +356,11 @@ to perform a task that many number of times: ``` --> -### Mutating Instance Methods +### 变值实例方法 -Instance methods added with an extension can also modify (or *mutate*) the instance itself. -Structure and enumeration methods that modify `self` or its properties -must mark the instance method as `mutating`, -just like mutating methods from an original implementation. +通过扩展添加的实例方法同样也可以修改（modify）（或 *改变（mutating）*）实例本身。修改 `self` 或其属性的结构体和枚举方法，必须将实例方法标记为 `mutating`，就像原始实现中的变值方法（mutating methods）一样。 -The example below adds a new mutating method called `square` to Swift's `Int` type, -which squares the original value: +下面的示例为 Swift 的 `Int` 类型添加了一个新的变值方法 `square`，它可以计算原始值的平方： ```swift extension Int { @@ -444,7 +370,7 @@ extension Int { } var someInt = 3 someInt.square() -// someInt is now 9 +// someInt 现在是 9 ``` -## Subscripts +## 下标 -Extensions can add new subscripts to an existing type. -This example adds an integer subscript to Swift's built-in `Int` type. -This subscript `[n]` returns the decimal digit `n` places in -from the right of the number: +扩展可以为现有类型添加新的下标。这个示例为 Swift 内置的 `Int` 类型添加了一个整数下标。下标 `[n]` 返回数字从右边数第 `n` 位的十进制数字: -- `123456789[0]` returns `9` -- `123456789[1]` returns `8` +- `123456789[0]` 返回 `9` +- `123456789[1]` 返回 `8` -…and so on: +……以此类推： ```swift extension Int { @@ -486,13 +409,13 @@ extension Int { } } 746381295[0] -// returns 5 +// 返回 5 746381295[1] -// returns 9 +// 返回 9 746381295[2] -// returns 2 +// 返回 2 746381295[8] -// returns 7 +// 返回 7 ``` -If the `Int` value doesn't have enough digits for the requested index, -the subscript implementation returns `0`, -as if the number had been padded with zeros to the left: +如果 `Int` 值的数字位数不足以满足所请求的索引，那么下标实现会返回 `0`，就好像在数字左边补上了 0： ```swift 746381295[9] -// returns 0, as if you had requested: +// 返回 0，就好像你进行了这个请求： 0746381295[9] ``` @@ -570,9 +491,9 @@ as if the number had been padded with zeros to the left: Tracking bug is --> -## Nested Types +## 嵌套类型 -Extensions can add new nested types to existing classes, structures, and enumerations: +扩展可以给现有的类，结构体，和枚举添加新的嵌套类型： ```swift extension Int { @@ -614,17 +535,11 @@ extension Int { ``` --> -This example adds a new nested enumeration to `Int`. -This enumeration, called `Kind`, -expresses the kind of number that a particular integer represents. -Specifically, it expresses whether the number is -negative, zero, or positive. +这个例子给 `Int` 添加了一个新的嵌套枚举。这个枚举叫做 `Kind`，表示特定整数所代表的数字类型。具体来说，它表示数字是负数、零还是正数。 -This example also adds a new computed instance property to `Int`, -called `kind`, -which returns the appropriate `Kind` enumeration case for that integer. +这个例子也给 `Int` 添加了一个新的计算实例属性，叫做 `kind`，它返回被操作整数所对应的 `Kind` 枚举值。 -The nested enumeration can now be used with any `Int` value: +现在，这个嵌套枚举可以用于任何 `Int` 值： ```swift func printIntegerKinds(_ numbers: [Int]) { @@ -641,7 +556,7 @@ func printIntegerKinds(_ numbers: [Int]) { print("") } printIntegerKinds([3, 19, -27, 0, -6, 0, 7]) -// Prints "+ + - 0 - 0 + " +// 打印“+ + - 0 - 0 + ” ``` -This function, `printIntegerKinds(_:)`, -takes an input array of `Int` values and iterates over those values in turn. -For each integer in the array, -the function considers the `kind` computed property for that integer, -and prints an appropriate description. - -> Note: `number.kind` is already known to be of type `Int.Kind`. -> Because of this, all of the `Int.Kind` case values -> can be written in shorthand form inside the `switch` statement, -> such as `.negative` rather than `Int.Kind.negative`. +这个函数 `printIntegerKinds(_:)` 接受一个 `Int` 值的数组作为输入，并逐个遍历这些值。对于数组中的每个整数，该函数都会检查它的 `kind` 计算属性，然后打印适当的描述。 -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). +> 注意: `number.kind` 已经被认为是 `Int.Kind` 类型。所以，在 `switch` 语句中所有的 `Int.Kind` 枚举值可以被缩写，例如使用 `.negative` 替代 `Int.Kind.negative`。 diff --git a/swift-6.docc/LanguageGuide/Functions.md b/swift-6.docc/LanguageGuide/Functions.md new file mode 100644 index 000000000..4cff7ce09 --- /dev/null +++ b/swift-6.docc/LanguageGuide/Functions.md @@ -0,0 +1,1054 @@ +# 函数 + +定义和调用函数，标注参数并使用其返回值。 + +**函数(Functions)** 是执行特定任务的独立代码块。你可以通过给函数命名来标识某个函数的功能，这个名字可以被用来在需要的时候“调用”这个函数来完成它的任务。 + +Swift 的统一函数语法非常灵活，从没有形参名称的简单 C 风格函数到每个形参都有局部和外部参数名的复杂 Objective-C 风格方法，它都能表达。形参可以提供默认值，以简化函数调用，也可以作为 in-out 参数传递，在函数执行完毕后，传入的变量值将被修改。 + +Swift 中的每个函数都有一个类型，由函数的形参值类型和返回值类型组成。您可以像使用 Swift 中的其他类型一样使用该类型，这样就可以简单地将函数作为形参传递给其他函数，也可以从其他函数返回函数。或者还可以在其他函数中编写函数，以便在嵌套函数作用域中封装功能。 + +## 定义和调用函数 +定义函数时，您可以定义一个或多个有名字和类型的值，作为函数的输入，即 **形参**。您还可以选择性地定义某种类型的值作为函数执行结束时的输出，即函数的 **返回类型**。 + +每个函数都有一个 **函数名**，它描述了函数执行的任务。要使用函数，您需要使用函数名“调用”该函数，并向其传递与函数形参类型相匹配的输入值（称为 **实参**）。函数的实参必须与函数形参表里的顺序一致。 + +下面示例中的函数名为 'greet(person:)'，因为它要做的就是接收一个人的姓名作为输入，并返回对这个人的问候语句。为此，您需要定义一个名为 `person` 的 `String` 输入参数值，以及一个包含对该人的问候语的 `String` 类型返回值： + +```swift +func greet(person: String) -> String { + let greeting = "Hello, " + person + "!" + return greeting +} +``` + + + +所有这些信息都汇总起来成为函数的 **定义(definition)** ，并以 **func** 作为前缀。使用 **返回箭头** `->`（连字符后跟一个直角括号），后面跟要返回的类型名称，来表示函数的返回类型。 + +定义描述了函数的作用、期望接收的内容以及完成后的返回内容。有了定义，就可以在代码的其他地方明确地调用函数： + +```swift +print(greet(person: "Anna")) +// 打印 "Hello, Anna!" +print(greet(person: "Brian")) +// 打印 "Hello, Brian!" +``` + + + +调用 `greet(person:)` 函数时，请在 `person` 参数标签后传递 `String` 值，例如 `greet(person: "Anna")` 。由于函数返回的是 `String` 类型的值，因此 `greet(person:)` 可以被包含在 `print(_:separator:terminator:)` 的调用中，用来输出这个函数的返回值，如上所示。 + +> 注意: `print(_:separator:terminator:)` 函数的第一个参数并没有设置一个标签，而其他的参数是可选的，因为他们已经有了默认值。 +> 关于这些函数语法上的变化详见下方 和 。 + +在 `greet(person:)` 的函数体中，先定义了一个新的名为 `greeting` 的 `String` 常量，并将其设置为简单的问候信息。然后用 `return` 关键字把这个问候返回出去。一旦 `return greeting` 被调用，该函数执行完毕并返回 `greeting` 的当前值。 + +您可以使用不同的输入值多次调用 `greet(person:)` 函数。上面的示例显示了在输入值为 `"Anna"` 和 `"Brian"` 时调用该函数的结果。在每种情况下，函数都会返回一个定制的问候语。 + +为了简化这个函数的定义，可以将消息的创建和返回语句合并为一行： + +```swift +func greetAgain(person: String) -> String { + return "Hello again, " + person + "!" +} +print(greetAgain(person: "Anna")) +// 打印 "Hello again, Anna!" +``` + + + +## 函数参数与返回值 + +Swift 中的函数参数和返回值非常灵活。您可以定义任何函数，从只带有单个未命名参数的简单实用程序函数，到带有丰富参数名和不同参数选项的复杂函数。 + +### 无参数函数 + +函数可以没有参数。下面这个函数就是一个无参数函数，每次它被调用时，总是返回固定的 `String` 消息： + +```swift +func sayHelloWorld() -> String { + return "hello, world" +} +print(sayHelloWorld()) +// 打印 "hello, world" +``` + + + +即使函数不需要任何参数，在函数定义的名称后仍需要加上括号。在调用函数时，函数名后面也要加上一对空括号。 + +### 多参数函数 + +函数可以有多个输入参数，这些参数写在函数的括号内，用逗号隔开。 + +下面的函数将一个人的名字和他是否被问候过作为输入，并返回适合该人的问候语： + +```swift +func greet(person: String, alreadyGreeted: Bool) -> String { + if alreadyGreeted { + return greetAgain(person: person) + } else { + return greet(person: person) + } +} +print(greet(person: "Tim", alreadyGreeted: true)) +// 打印 "Hello again, Tim!" +``` + + + +您通过将标有 `person` 的 `String` 参数值和标有 `alreadyGreeted` 的 `Bool` 参数值同时传递给 `greet(person:alreadyGreeted:)` 函数，并用逗号隔开来调用它。请注意，该函数不同于前面章节中的 `greet(person:)` 函数。虽然这两个函数的名称都以 `greet` 开头，但 `greet(person:alreadyGreeted:)` 函数需要两个参数，而 `greet(person:)` 函数只需要一个参数。 + +### 无返回值函数 + +函数无需定义返回类型。下面是 `greet(person:)` 函数的另一个版本，它直接打印了一个 `String` 值，而不是返回它： + +```swift +func greet(person: String) { + print("Hello, \(person)!") +} +greet(person: "Dave") +// 打印 "Hello, Dave!" +``` + + + +因为这个函数不需要返回值，所以这个函数的定义中没有返回箭头（`->`）和返回类型。 + +> 注意: +> 严格来说，这个版本的 `greet(person:)` 函数 **确实** 仍然返回一个值，即使没有定义返回值。 +> 没有定义返回类型的函数会返回 `Void` 类型的特殊值。 +> 这是一个空元组，写成 `()`。 + +函数被调用时，其返回值可被忽略： + +```swift +func printAndCount(string: String) -> Int { + print(string) + return string.count +} +func printWithoutCounting(string: String) { + let _ = printAndCount(string: string) +} +printAndCount(string: "hello, world") +// 打印 "hello, world" 并返回 12 +printWithoutCounting(string: "hello, world") +// 打印 "hello, world" 但没有返回值 +``` + + + + + +第一个函数 `printAndCount(string:)` 打印一个字符串并返回 `Int` 类型的字符数。第二个函数 `printWithoutCounting(string:)` 调用了第一个函数，但是忽略了它的返回值。当第二个函数被调用时，消息依然会被第一个函数打印，但是返回值不会被用到。 + +> 注意: +> 返回值可以被忽略，但定义了有返回值的函数必须返回一个值，如果在函数定义底部没有返回任何值，将导致编译时错误。 + + + +### 多重返回值函数 + +你可以用元组（tuple）类型让多个值作为一个复合值从函数中返回。 + +下面的示例定义了一个名为 `minMax(array:)` 的函数，用于查找 `Int` 数组中最小值和最大值： + +```swift +func minMax(array: [Int]) -> (min: Int, max: Int) { + var currentMin = array[0] + var currentMax = array[0] + for value in array[1.. currentMax { + currentMax = value + } + } + return (currentMin, currentMax) +} +``` + + + +`minMax(array:)` 函数返回一个包含两个 `Int` 值的元组。这些值被标记为 `min` 和 `max` 以便在查询函数返回值时可以通过名称访问它们。 + +在 `minMax(array:)` 的函数体中，在开始的时候设置两个工作变量 `currentMin` 和 `currentMax` 的值为数组中的第一个数。然后函数会遍历数组中剩余的值并检查该值是否分别小于或大于 `currentMin` 和 `currentMax`。最后数组中的最小值与最大值作为一个包含两个 `Int` 值的元组返回。 + +由于元组的成员值已经被命名，因此可以使用点语法访问它们，以获取最小值和最大值： + +```swift +let bounds = minMax(array: [8, -6, 2, 109, 3, 71]) +print("min is \(bounds.min) and max is \(bounds.max)") +// 打印 "min is -6 and max is 109" +``` + + + +请注意，元组的成员无需在元组从函数中返回时命名，因为它们的名字已经在函数返回类型中指定了。 + +#### 可选元组返回类型 + +如果函数返回的元组类型有可能整个元组都“没有值”，你可以使用 **可选** 元组返回类型反映整个元组可以是 `nil` 的事实。您可以通过在元组类型的结尾括号后放置问号来定义可选的元组返回类型，例如 `(Int, Int)?` 或 `(String, Int, Bool)?`。 + +> 注意: 可选元组类型如 `(Int, Int)?` 与元组包含可选类型如 `(Int?, Int?)` 是不同的。 +> 可选的元组类型，整个元组是可选的，而不只是元组中的每个元素值。 + +上面的 `minMax(array:)` 函数返回一个包含两个 `Int` 值的元组。但是，该函数没有对传入的数组执行任何安全检查。如果 `array` 参数为一个空数组，则上面定义的 `minMax(array:)` 函数在尝试访问 `array[0]` 时将触发运行时错误。 + +为了安全地处理空数组问题，将 `minMax(array:)` 函数改写为使用可选元组返回类型，并且当数组为空时返回 `nil`： + +```swift +func minMax(array: [Int]) -> (min: Int, max: Int)? { + if array.isEmpty { return nil } + var currentMin = array[0] + var currentMax = array[0] + for value in array[1.. currentMax { + currentMax = value + } + } + return (currentMin, currentMax) +} +``` + + + +您可以使用可选绑定来检查此版本的 `minMax(array:)` 函数是返回实际元组值还是 `nil` 值： + +```swift +if let bounds = minMax(array: [8, -6, 2, 109, 3, 71]) { + print("min is \(bounds.min) and max is \(bounds.max)") +} +// 打印 "min is -6 and max is 109" +``` + + + +### 隐式返回的函数 + +如果函数的整个主体是一个表达式，则函数隐式返回该表达式。例如，下面的两个函数具有相同的行为： + +```swift +func greeting(for person: String) -> String { + "Hello, " + person + "!" +} +print(greeting(for: "Dave")) +// 打印 "Hello, Dave!" + +func anotherGreeting(for person: String) -> String { + return "Hello, " + person + "!" +} +print(anotherGreeting(for: "Dave")) +// 打印 "Hello, Dave!" +``` + + + +`greeting(for:)` 函数的完整定义就是它返回的问候信息，这意味着它可以使用这种较短的形式。`anotherGreeting(for:)` 函数使用 `return` 关键字返回相同的问候信息但显得函数更长。如果您编写的函数只有一行 `return` ，则可以省略 `return` 。 + +正如你将会在 里看到的，属性的 getter 也可以使用隐式返回。 + +> 注意: +> 作为隐式返回值编写的代码必须返回某个值。 +> 例如，你不能使用 `print(13)` 作为隐式返回值。 +> 然而，你可以使用不返回值的函数（如 `fatalError("Oh no!")`）作为隐式返回值，因为 Swift 知道它们并不会产生任何隐式返回。 + + + +## 函数参数标签和参数名称 + +每个函数参数都有一个 **参数标签（argument label）** 和 **参数名称（parameter name）**。参数标签在调用函数时使用；调用的时候需要将函数的参数标签写在对应的参数前面。参数名称用于函数的实现；默认情况下，函数参数使用参数名称来作为它们的参数标签。 + +```swift +func someFunction(firstParameterName: Int, secondParameterName: Int) { + // 在函数体内，firstParameterName 和 secondParameterName 代表参数中的第一个和第二个参数值 +} +someFunction(firstParameterName: 1, secondParameterName: 2) +``` + + + +所有参数都必须有唯一的名称。虽然多个参数有可能具有相同的参数标签，但唯一的参数标签有助于提高代码的可读性。 + + + +### 指定参数标签 + +在参数名之前写入参数标签，用空格隔开： + +```swift +func someFunction(argumentLabel parameterName: Int) { + // In the function body, parameterName refers to the argument value + // for that parameter. +} +``` + + + +下面是 `greet(person:)` 函数的一个变体，它接收一个人的名字和家乡，并返回一个问候语： + +```swift +func greet(person: String, from hometown: String) -> String { + return "Hello \(person)! Glad you could visit from \(hometown)." +} +print(greet(person: "Bill", from: "Cupertino")) +// 打印 "Hello Bill! Glad you could visit from Cupertino." +``` + + + +参数标签的使用能够让一个函数在调用时更有表达力，更类似自然语言，并且仍保持了函数内部的可读性以及清晰的意图。 + +### 省略参数标签 + +如果不需要参数的参数标签，请写一个下划线（`_`）来代替该参数的显式参数标签。 + +```swift +func someFunction(_ firstParameterName: Int, secondParameterName: Int) { + // 在函数体内，firstParameterName 和 secondParameterName 代表参数中的第一个和第二个参数值 +} +someFunction(1, secondParameterName: 2) +``` + + + +如果参数有参数标签，则在调用函数时 **必须** 使用标签来标示参数。 + +### 默认参数值 + +你可以在函数体中通过给参数赋值来为任意一个 **参数定义默认值（Deafult Value）**。如果定义了默认值，您就可以在调用函数时省略该参数。 + +```swift +func someFunction(parameterWithoutDefault: Int, parameterWithDefault: Int = 12) { + // 如果你在调用时候不传第二个参数，parameterWithDefault 会值为 12 传入到函数体中。 +} +someFunction(parameterWithoutDefault: 3, parameterWithDefault: 6) // parameterWithDefault is 6 +someFunction(parameterWithoutDefault: 4) // parameterWithDefault is 12 +``` + + + +将不带有默认值的参数放在函数参数列表的最前。没有缺省值的参数通常对函数的意义更为重要--将它们写在前面，无论是否省略了任何缺省参数，都能更容易地识别出调用的是同一个函数。 + +### 可变参数 + +**可变参数（variadic parameter）** 接受零个或多个指定类型的值。使用可变参数可以指定在调用函数时，参数可以传入不同数量的输入值。通过在参数类型名称后插入三个句点字符（`...`）来编写可变参数。 + +可变参数的值将在函数体中变为相应类型的数组。例如，名称为 `numbers` 和类型为 `Double...` 的可变参数，在函数体中以名为 `numbers` 类型为 `[Double]` 的数组常量形式提供。 + +下面的示例计算任意长度数字列表的 **算术平均数（arithmetic mean）**（也称为 **平均值（average）**）： + +```swift +func arithmeticMean(_ numbers: Double...) -> Double { + var total: Double = 0 + for number in numbers { + total += number + } + return total / Double(numbers.count) +} +arithmeticMean(1, 2, 3, 4, 5) +// 返回 3.0，是这 5 个数的平均数 +arithmeticMean(3, 8.25, 18.75) +// 返回 10.0，是这 3 个数的平均数 +``` + + + + + +一个函数可以有多个可变参数。可变参数之后的第一个形参必须有参数标签。通过参数标签，可以清楚地知道哪些参数传递给可变参数，哪些参数传递给可变参数之后的形参。 + + + + + +### 输入输出参数 + +函数参数默认为常量。在函数体中更改函数参数的值会导致编译时错误。这意味着您不能错误地更改参数的值。如果您希望函数修改参数的值，并希望这些更改在函数调用结束后仍然有效，请将该参数定义为 **in-out parameter（输入输出参数）**。 + +您可以通过在参数类型前添加 `inout` 关键字来编写输入输出参数。输入输出参数有一个值，该值 **传入** 给函数，被函数修改后，然后被 **传出** 函数，以替换原来的值。有关传入传出参数行为及相关编译器优化的详细讨论，请参阅 。 + +你只能传递变量给输入输出参数。不能传入常量或者字面量，因为这些是不能被修改的。当传入的参数作为输入输出参数时，需要在参数名前加 （`&`） 符，表示这个值可以被函数修改。 + +> 注意: +> 输入输出参数不能有默认值，而且可变参数不能标记为 `inout`。 + +下例中，`swapTwoInts(_:_:)` 函数有两个分别叫做 `a` 和 `b` 的输入输出参数： + +```swift +func swapTwoInts(_ a: inout Int, _ b: inout Int) { + let temporaryA = a + a = b + b = temporaryA +} +``` + + + +`swapTwoInts(_:_:)` 函数简单地交换 `a` 与 `b` 的值。该函数先将 `a` 的值存到一个临时常量 `temporaryA` 中，然后将 `b` 的值赋给 `a`，最后将 `temporaryA` 赋值给 `b`。 + +您可以使用两个类型为 `Int` 的变量调用 `swapTwoInts(_:_:)` 函数来交换它们的值。请注意，`someInt` 和 `anotherInt` 在传递给 `swapTwoInts(_：_:)` 函数前，都加了 & 前缀： + +```swift +var someInt = 3 +var anotherInt = 107 +swapTwoInts(&someInt, &anotherInt) +print("someInt is now \(someInt), and anotherInt is now \(anotherInt)") +// 打印 "someInt is now 107, and anotherInt is now 3" +``` + + + +上面的示例展示了 `someInt` 和 `anotherInt` 的原始值被 `swapTwoInt(_：_:)` 函数修改，即使它们是在函数体外定义的。 + +> 注意: +> 输入输出参数和函数的返回值是不一样的。 +> 上面的 `swapTwoInts` 函数并没有定义任何返回类型或返回值，但仍然修改了 `someInt` 和 `anotherInt` 的值。 +> 输入输出参数是函数在其函数体范围之外产生影响的另一种方式。 + + + +## 函数类型 + +每个函数都有一个特定的 **函数类型**，由函数的参数类型和返回类型组成。 + +例如： + +```swift +func addTwoInts(_ a: Int, _ b: Int) -> Int { + return a + b +} +func multiplyTwoInts(_ a: Int, _ b: Int) -> Int { + return a * b +} +``` + + + +此示例定义了两个简单的数学函数，分别称为 `addTwoInt` 和 `multiplyTwoInt`。这些函数分别接收两个 `Int` 值，并返回一个 `Int` 值，执行有关的数学运算的结果。 + +这两个函数的类型都是 `(Int, Int) -> Int`。这可以理解为： + +“这个函数类型有两个 `Int` 型的参数并返回一个 `Int` 型的值”。 + +下面是另一个例子，一个没有参数或返回值的函数： + +```swift +func printHelloWorld() { + print("hello, world") +} +``` + + + +该函数的类型是 `() -> Void`，或“没有参数，并返回 `Void` 类型的函数”。 + +### 使用函数类型 + +在 Swift 中，您可以像使用其他类型一样使用函数类型。例如，你可以定义一个类型为函数的常量或变量，并将适当的函数赋值给它： + +```swift +var mathFunction: (Int, Int) -> Int = addTwoInts +``` + + + +这可以理解为： + +”定义一个叫做 `mathFunction` 的变量，类型是‘接收两个 `Int` 值并返回一个 `Int` 值的函数’。将此新变量设置为引用名为 `addTwoInts` 的函数"。 + +`addTwoInts(_:_:)` 函数与 `mathFunction` 变量具有相同的类型，所以这个赋值过程在 Swift 类型检查（type-check）中是允许的。 + +现在，你可以用 `mathFunction` 来调用被赋值的函数了： + +```swift +print("Result: \(mathFunction(2, 3))") +// Prints "Result: 5" +``` + + + +与非函数类型相同，具有相同匹配类型的不同函数也可以分配给同一个变量： + +```swift +mathFunction = multiplyTwoInts +print("Result: \(mathFunction(2, 3))") +// 打印 "Result: 6" +``` + + + +与其他类型一样，当您将函数赋值给常量或变量时，可以让 Swift 来推断函数类型： + +```swift +let anotherMathFunction = addTwoInts +// anotherMathFunction 被推断为 (Int, Int) -> Int 类型 +``` + + + + + +### 函数类型作为参数类型 + +您可以使用函数类型例如 `(Int, Int) -> Int` 作为另一个函数的参数类型。这样你可以将函数的一部分实现留给函数的调用者来提供。 + +下面是一个打印上述数学函数结果的示例： + +```swift +func printMathResult(_ mathFunction: (Int, Int) -> Int, _ a: Int, _ b: Int) { + print("Result: \(mathFunction(a, b))") +} +printMathResult(addTwoInts, 3, 5) +// 打印 "Result: 8" +``` + + + +此示例定义了 `printMathResult(_:_:_:)` 函数，它有三个参数。第一个参数名为 `mathFunction`, 其类型为 `(Int, Int) -> Int`。您可以传入任何这种类型的函数作为第一个参数。第二个和第三个参数分别为 `a` 和 `b`，它们都是 `Int` 类型。这些值将用作所提供数学函数的两个输入值。 + +当 `printMathResult(_:_:_:)` 被调用时，它被传入 `addTwoInts(_:_:)` 函数以及整数值 `3` 和 `5`。它用传入的 `3` 和 `5` 调用传入的函数，并输出结果：`8`。 + +`printMathResult(_:_:_:)` 的作用是打印调用适当类型数学函数的结果。它不关心传入函数是如何实现的，重要的是函数的类型是不是正确的。这使得 `printMathResult(_:_:_:)` 能够以类型安全的方式将其部分功能移交给函数的调用者。 + +### 函数类型作为返回类型 + +你可以将函数类型作为另一个函数的返回类型。方法是在返回函数的返回箭头（`->`）后紧接着写入一个完整的函数类型。 + +下一个示例定义了两个简单函数，分别称为 `stepForward(_:)` 和 `stepBackward(_:)`。`stepForward(_:)` 函数返回的值比输入值大 1，而 `stepBackward(_:)` 函数返回的值比输入值小 1。这两个函数的类型都是 `(Int) -> Int`： + +```swift +func stepForward(_ input: Int) -> Int { + return input + 1 +} +func stepBackward(_ input: Int) -> Int { + return input - 1 +} +``` + + + +如下名为 `chooseStepFunction(backward:)` 的函数，它的返回类型是 `(Int) -> Int`。`chooseStepFunction(backward:)` 根据布尔值 `backward` 来返回 `stepForward(_:)` 函数或 `stepBackward(_:)` 函数： + +```swift +func chooseStepFunction(backward: Bool) -> (Int) -> Int { + return backward ? stepBackward : stepForward +} +``` + + + +您现在可以用 `chooseStepFunction(backward:)` 来获得一个前进或后退方向的函数： + +```swift +var currentValue = 3 +let moveNearerToZero = chooseStepFunction(backward: currentValue > 0) +// moveNearerToZero 现在指向 stepBackward() 函数。 +``` + + + +上面的示例确定是否需要一个正步长或负步长来使 `currentValue` 的变量逐渐趋近于零。`currentValue` 的初始值为 3，这意味着 `currentValue > 0` 返回 `true`，这使得 `chooseStepFunction(backward:)` 返回 `stepBackward(_:)` 函数。返回函数的引用存储在了 `moveNearerToZero` 的常量中。 + +现在， `moveNearerToZero` 指向了正确的函数，它可以用来计数归零： + +```swift +print("Counting to zero:") +// Counting to zero: +while currentValue != 0 { + print("\(currentValue)... ") + currentValue = moveNearerToZero(currentValue) +} +print("zero!") +// 3... +// 2... +// 1... +// zero! +``` + + + +## 嵌套函数 + +到目前为止，您在本章中遇到的所有函数都是 **全局函数（global functions）**，它们定义在全局域中。您还可以在其他函数的主体中定义函数，即 **嵌套函数（nested functions）**。 + +默认情况下，嵌套函数是对外界不可见的，但仍可被其外围函数（enclosing function）调用。外围函数也可以返回它的某个嵌套函数，以便在另一个作用域中使用这个函数。 + +您可以重写上面的 `chooseStepFunction(backward:)` 示例，以使用并返回嵌套函数： + +```swift +func chooseStepFunction(backward: Bool) -> (Int) -> Int { + func stepForward(input: Int) -> Int { return input + 1 } + func stepBackward(input: Int) -> Int { return input - 1 } + return backward ? stepBackward : stepForward +} +var currentValue = -4 +let moveNearerToZero = chooseStepFunction(backward: currentValue > 0) +// moveNearerToZero 现在引用了嵌套的 stepForward() 函数 +while currentValue != 0 { + print("\(currentValue)... ") + currentValue = moveNearerToZero(currentValue) +} +print("zero!") +// -4... +// -3... +// -2... +// -1... +// zero! +``` + + + + diff --git a/swift-6.docc/LanguageGuide/Generics.md b/swift-6.docc/LanguageGuide/Generics.md new file mode 100644 index 000000000..731762914 --- /dev/null +++ b/swift-6.docc/LanguageGuide/Generics.md @@ -0,0 +1,1533 @@ +# 泛型 + +编写适用于多种类型的代码，并指定对这些类型的要求。 + +泛型代码让你能根据自定义的需求，编写出适用于任意类型的、灵活可复用的函数及类型。你可避免编写重复的代码，而是用一种清晰抽象的方式来表达代码的意图。 + +泛型是 Swift 最强大的特性之一，很多 Swift 标准库是基于泛型代码构建的。实际上，即使你没有意识到，在 *语言指南* 中也是一直使用泛型。例如，Swift 的 `Array` 和 `Dictionary` 都是泛型集合。你可以创建一个 `Int` 类型数组，也可创建一个 `String` 类型数组，甚至可以是任意其他 Swift 类型的数组。同样，你也可以创建一个存储任意指定类型的字典，并对该类型没有限制。 + +## 泛型解决的问题 + +下面是一个标准的非泛型函数 `swapTwoInts(_:_:)`，它用于交换两个 `Int` 类型的值： + +```swift +func swapTwoInts(_ a: inout Int, _ b: inout Int) { + let temporaryA = a + a = b + b = temporaryA +} +``` + + + +这个函数使用输入输出参数（inout）来交换 `a` 和 `b` 的值，具体请参考 . + +`swapTwoInts(_:_:)` 函数将 `b` 的原始值换给了 `a`，将 `a` 的原始值换给了 `b`，你可以调用这个函数来交换两个 `Int` 类型变量： + +```swift +var someInt = 3 +var anotherInt = 107 +swapTwoInts(&someInt, &anotherInt) +print("someInt is now \(someInt), and anotherInt is now \(anotherInt)") +// 打印 "someInt is now 107, and anotherInt is now 3" +``` + + + +`swapTwoInts(_:_:)` 函数很实用，但它只能作用于 `Int` 类型。如果你想交换两个 `String` 类型值，或者 `Double` 类型值，你必须编写对应的函数，类似下面 `swapTwoStrings(_:_:)` 和 `swapTwoDoubles(_:_:)` 函数： + +```swift +func swapTwoStrings(_ a: inout String, _ b: inout String) { + let temporaryA = a + a = b + b = temporaryA +} + +func swapTwoDoubles(_ a: inout Double, _ b: inout Double) { + let temporaryA = a + a = b + b = temporaryA +} +``` + + + +你可能注意到了，`swapTwoInts(_:_:)`、`swapTwoStrings(_:_:)` 和 `swapTwoDoubles(_:_:)` 函数体是一样的，唯一的区别是它们接受的参数类型`（`Int`、`String` 和 `Double`）`。 + +在实际应用中，通常需要一个更实用更灵活的函数来交换两个*任意*类型的值，幸运的是，泛型代码帮你解决了这种问题。（这些函数的泛型版本已经在下面定义好了。） + +> 注意: 在上面三个函数中, +> `a` 和 `b` 类型必须相同。如果 `a` 和 `b` 类型不同，那它们俩就不能互换值。Swift 是类型安全的语言，所以它不允许一个 `String` 类型的变量和一个 `Double` 类型的变量互换值。试图这样做将导致编译错误。 + +## 泛型函数 + +*泛型函数*可适用于任意类型，下面是函数 `swapTwoInts(_:_:)` 的泛型版本，命名为 `swapTwoValues(_:_:)`： + +```swift +func swapTwoValues(_ a: inout T, _ b: inout T) { + let temporaryA = a + a = b + b = temporaryA +} +``` + + + + + +`swapTwoValues(_:_:)` 和 `swapTwoInts(_:_:)` 函数体内容相同，它们只在第一行稍有不同，如下所示： + +```swift +func swapTwoInts(_ a: inout Int, _ b: inout Int) +func swapTwoValues(_ a: inout T, _ b: inout T) +``` + + + +泛型版本的函数使用了一个*占位符*类型名称（这里叫做`T`），而不是一个*实际*的类型名称（例如`Int`、`String`或`Double`）。`占位符类型`名称并不关心 `T` 必须是什么类型，但它要求 `a` 和 `b` 必须是相同类型的 `T`，无论 `T` 代表什么。每次调用 `swapTwoValues(_:_:)` 函数时，都会确定`T`的实际类型。` + +泛型函数和非泛型函数的另外一个不同之处在于这个泛型函数名`swapTwoValues(_:_:)`后面跟着占位类型名（`T`），并用尖括号括起来（``）。这个尖括号告诉 Swift 那个 `T` 是 `swapTwoValues(_:_:)` 函数定义内的一个占位类型名，因此 Swift 不会去查找名为 `T` 的实际类型。 + +`swapTwoValues(_:_:)` 函数现在可以像 `swapTwoInts(_:_:)` 那样调用，不同的是它能接受两个*任意*类型的值，条件是这两个值有着相同的类型。`swapTwoValues(_:_:)` 函数被调用时，`T` 所代表的类型都会由传入的值的类型推断出来。 + +在下面的两个例子中，`T` 分别代表 `Int` 和 `String`： + +```swift +var someInt = 3 +var anotherInt = 107 +swapTwoValues(&someInt, &anotherInt) +// someInt 现在是 107, 而 anotherInt 现在是 3 + +var someString = "hello" +var anotherString = "world" +swapTwoValues(&someString, &anotherString) +// someString 现在是 "world", 而 anotherString 现在是 "hello" +``` + + + +> 注意: 上面定义的 `swapTwoValues(_:_:)` 函数是受 `swap(_:_:)` 函数启发而实现的。后者存在于 Swift 标准库，你可以在你的应用程序中使用它。如果你在代码中需要类似 `swapTwoValues(_:_:)` 函数的功能，你可以使用已存在的 `swap(_:_:)` 函数。 + +## 类型参数 + +上面 `swapTwoValues(_:_:)` 例子中，占位类型 `T` 是一个*类型参数*的例子，类型参数指定并命名一个占位类型，并且紧随在函数名后面，使用一对尖括号括起来（例如 ``）。 + +一旦一个类型参数被指定，你可以用它来定义一个函数的参数类型（例如 `swapTwoValues(_:_:)` 函数中的参数 `a` 和 `b`），或者作为函数的返回类型，还可以用作函数主体中的类型注解。在这些情况下，类型参数会在函数调用时被实际类型所替换。（在上面的 `swapTwoValues(_:_:)` 例子中，当函数第一次被调用时，`T` 被 `Int` 替换，第二次调用时，被 `String` 替换。） + +你可以通过在尖括号内写多个类型参数名称，并用逗号分隔，来提供多个类型参数。 + +## 命名类型参数 + +大多情况下，类型参数具有描述性的名称，例如字典 `Dictionary` 中的 `Key` 和 `Value` 及数组 `Array` 中的 `Element`，这能告诉阅读代码的人这些类型参数与泛型类型或函数之间的关系。然而，当它们之间没有有意义的关系时，通常使用单个字符来表示，例如 `T`、`U`、`V`，例如上面演示函数 `swapTwoValues(_:_:)` 中的 `T`。 + +> 注意: 请始终使用大写字母开头的驼峰命名法（例如 `T` 和 `MyTypeParameter`）来为类型参数命名，以表明它们是占位*类型*，而不是一个值。 + +## 泛型类型 + +除了泛型函数，Swift 还允许自定义*泛型类型*。这些自定义类、结构体和枚举可以适用于*任意*类型，类似于 `Array` 和 `Dictionary`。 + +本节将向你展示如何编写一个名为 `Stack`（栈）的泛型集合类型。栈是值的有序集合，和数组类似，但比数组有更严格的操作限制。数组允许在其中任意位置插入或是删除元素。而栈只允许在集合的末端添加新的元素（称之为*入栈*）。类似的，栈也只能从末端移除元素（称之为*出栈*）。 + +> 注意: 栈的概念已被 `UINavigationController` 类用来构造视图控制器的导航结构。你通过调用 `UINavigationController` 的 `pushViewController(_:animated:)` 方法来添加新的视图控制器到导航栈，通过 `popViewControllerAnimated(_:)` 方法来从导航栈中移除视图控制器。每当你需要一个严格的“后进先出”方式来管理集合，栈都是最实用的模型。 + +下图展示了入栈（push）和出栈（pop）的行为： + +![](stackPushPop) + +1. 现在有三个值在栈中。 +2. 第四个值被压入到栈的顶部。 +3. 现在栈中有四个值，最近入栈的那个值在顶部。 +4. 栈中最顶部的那个值被移除出栈。 +5. 一个值移除出栈后，现在栈又只有三个值了。 + +下面展示如何编写一个非泛型版本的栈，以 `Int` 型的栈为例： + +```swift +struct IntStack { + var items: [Int] = [] + mutating func push(_ item: Int) { + items.append(item) + } + mutating func pop() -> Int { + return items.removeLast() + } +} +``` + + + +这个结构体在栈中使用一个名为 `items` 的`Array`属性来存储值。栈提供了两个方法：`push(_:)` 和 `pop()`，用来向栈中压入值以及从 `Stack` 中移除值。这些方法被标记为 `mutating`，因为它们需要*修改*结构体的 `items` 数组。 + +上面的 `IntStack` 结构体只能用于 `Int` 类型。可以定义一个泛型 `Stack` 结构体，从而能够处理任意类型的值。 + +下面是 `Stack` 的泛型版本： + +```swift +struct Stack { + var items: [Element] = [] + mutating func push(_ item: Element) { + items.append(item) + } + mutating func pop() -> Element { + return items.removeLast() + } +} +``` + + +> 注意：`Stack` 基本上和 `IntStack` 相同，只是用占位类型参数 `Element` 代替了实际的 `Int` 类型。这个类型参数包裹在紧随结构体名的一对尖括号里（``）。 + +`Element` 为待提供的类型定义了一个占位名。这种待提供的类型可以在结构体的定义中通过 `Element` 来引用。在这个例子中，`Element` 在如下三个地方被用作占位符： + +- 创建 `items` 属性，使用 `Element` 类型的空数组对其进行初始化。 +- 指定 `push(_:)` 方法的唯一参数 `item` 的类型必须是 `Element` 类型。 +- 指定 `pop()` 方法的返回值类型必须是 `Element` 类型。 + +由于 `Stack` 是泛型类型，因此可以用来创建适用于 Swift 中*任意*有效类型的栈，就像 `Array` 和 `Dictionary` 那样。 + +你可以通过在尖括号中写出栈中需要存储的数据类型来创建并初始化一个 `Stack` 实例。例如，要创建一个 `String` 类型的栈，可以写成 `Stack()`： + +```swift +var stackOfStrings = Stack() +stackOfStrings.push("uno") +stackOfStrings.push("dos") +stackOfStrings.push("tres") +stackOfStrings.push("cuatro") +// 栈中现在有 4 个字符串 +``` + + + +下图展示了 `stackOfStrings` 如何将这四个值压栈： + +![](stackPushedFourStrings) + +从栈中移除并返回栈顶部的值，例如 `"cuatro"`： + +```swift +let fromTheTop = stackOfStrings.pop() +// fromTheTop 的值为 “cuatro”，现在栈中还有 3 个字符串 +``` + + + +下图展示了栈顶部的 `"cuatro"` 出栈的过程： + +![](stackPoppedOneString) + +## 泛型扩展 + +当对泛型类型进行扩展时，你并不需要提供类型参数列表作为定义的一部分。 +相反，可以在扩展中直接使用*原始*类型定义中的类型参数列表，并且这些来自原始类型中的参数名称会被用作原始定义中类型参数的引用。 + +下面的例子扩展了泛型类型 `Stack`，为其添加了一个名为 `topItem` 的只读计算属性，它将会返回当前栈顶元素且不会将其从栈中移除： + +```swift +extension Stack { + var topItem: Element? { + return items.isEmpty ? nil : items[items.count - 1] + } +} +``` + + + +`topItem` 属性会返回 `Element` 类型的可选值。 +- 当栈为空的时候，`topItem` 会返回 `nil`； +- 当栈不为空的时候，`topItem` 会返回 `items` 数组中的最后一个元素。 + +> 注意: 这个扩展并没有定义类型参数列表。相反的，`Stack` 类型已有的类型参数名称 `Element`，被用在扩展中来表示计算属性 `topItem` 的可选类型。 + +计算属性 `topItem` 现在可以直接用来访问和查询 `Stack` 的顶部元素，而不会将这个顶部元素移除。 + +```swift +if let topItem = stackOfStrings.topItem { + print("The top item on the stack is \(topItem).") +} +// 打印 "The top item on the stack is tres." +``` + + + +泛型类型的扩展也可以包括对扩展类型实例的要求，以便这些实例可以获得新的功能，这一部分将在 中进行讨论. + +## 类型约束 + +`swapTwoValues(_:_:)` 函数和 `Stack` 类可以与任何类型一起使用。 +然而，有时对可以与泛型函数和泛型类型一起使用的类型强制执行某些*类型约束*是有用的。 +类型约束指定类型参数必须继承自特定的类，或者遵循特定协议或协议组合。 + +例如, Swift 的 `Dictionary` 类型对字典的键的类型做了些限制。在 中，字典键的类型必须是*可哈希的（hashable）*。也就是说，必须有一种方法能够唯一地表示它。字典键之所以要是可哈希的，是为了便于检查字典中是否已经包含某个特定键的值。若没有这个要求，字典将无法判断是否可以插入或替换某个指定键的值，也不能查找到已经存储在字典中的指定键的值。 + +这个要求通过对字典键类型的类型约束来强制执行，该约束指定键类型必须遵循Swift标准库中定义的 `Hashable` 协议。Swift的所有基本类型（如 `String`、`Int`、`Double` 和`Bool`）默认都是可哈希的。 +如何让自定义类型遵循 `Hashable` 协议，可以查看文档 [遵循 Hashable 协议](https://developer.apple.com/documentation/swift/hashable#2849490). + +你可以在创建自定义泛型类型时定义自己的类型约束，这些约束为泛型编程提供了强大的功能。像 `Hashable` 这样的抽象概念根据类型的概念特征而不是其具体类型来描述类型。 + +### 类型约束语法 + +你可以通过在类型参数的名称后添加一个类或协议约束，并用冒号分隔，来编写类型约束。下面将展示泛型函数中类型约束的基本语法（与泛型类型的语法相同）： + + +```swift +func someFunction(someT: T, someU: U) { + // function body goes here +} +``` + + + +上面这个函数有两个类型参数。第一个类型参数 `T` 必须是 `SomeClass` 子类；第二个类型参数 `U` 必须遵循 `SomeProtocol` 协议。 + +### 类型约束实践 + +这是一个 `findIndex(ofString:in:)` 的非泛型函数，该函数的功能是在一个 `String` 数组中查找输入 `String` 值的索引。若查找到匹配的字符串，`findIndex(ofString:in:)` 函数返回该字符串在数组中的索引值，否则返回 `nil` ： + +```swift +func findIndex(ofString valueToFind: String, in array: [String]) -> Int? { + for (index, value) in array.enumerated() { + if value == valueToFind { + return index + } + } + return nil +} +``` + + + +`findIndex(ofString:in:)` 函数可以用于查找字符串数组中的某个字符串的第一个索引值： + +```swift +let strings = ["cat", "dog", "llama", "parakeet", "terrapin"] +if let foundIndex = findIndex(ofString: "llama", in: strings) { + print("The index of llama is \(foundIndex)") +} +// Prints "The index of llama is 2" +``` + + + +如果只能查找字符串在数组中的索引，用处不是很大。不过，你可以用占位类型 `T` 替换 `String` 类型来写出具有相同功能的泛型函数 `findIndex(_:_:)`。 + +下面展示了 `findIndex(ofString:in:)` 函数的泛型版本 `findIndex(of:in:)`。请注意这个函数返回值的类型仍然是 `Int?`，这是因为函数返回的是一个可选的索引值，而不是从数组中得到的一个可选值。需要提醒的是，这个函数无法通过编译，原因将在函数后说明： + +```swift +func findIndex(of valueToFind: T, in array:[T]) -> Int? { + for (index, value) in array.enumerated() { + if value == valueToFind { + return index + } + } + return nil +} +``` + + + +上面所写的函数无法通过编译。问题出在相等性判定上，即 "`if value == valueToFind`"。不是所有的 Swift 类型都可以用等式符（`==`）进行比较。例如，如果你自定义类或结构体来描述复杂的数据模型，对于这个类或结构体而言，Swift 无法明确知道“相等”意味着什么。正因如此，无法保证这段代码适用于*每一个*可能的类型 `T`，当你试图编译这部分代码时就会出现相应的错误。 + +不过，Swift 并不会让我们对所有这类问题无从下手。Swift 标准库中定义了一个 `Equatable` 协议，该协议要求任何遵循该协议的类型必须实现等式符（`==`）及不等符（`!=`），从而能对该类型的任意两个值进行比较。所有的 Swift 标准类型自动支持 `Equatable` 协议。 + + + +任何遵循 `Equatable` 的类型都可以安全地与 `findIndex(of:in:)` 函数一起使用，因为它们保证支持等于操作符。为了表明这一点，你需要在定义函数时将 `Equatable` 作为类型参数的约束来写入： + +```swift +func findIndex(of valueToFind: T, in array:[T]) -> Int? { + for (index, value) in array.enumerated() { + if value == valueToFind { + return index + } + } + return nil +} +``` + + + +`findIndex(of:in:)` 类型参数写做 `T: Equatable`，表示“任何遵循 `Equatable` 协议的类型 `T`”。 + +`findIndex(of:in:)` 函数现在可以成功编译了，并且适用于任何遵循 `Equatable` 的类型，如 `Double` 或 `String`： + +```swift +let doubleIndex = findIndex(of: 9.3, in: [3.14159, 0.1, 0.25]) +// doubleIndex is an optional Int with no value, because 9.3 isn't in the array +let stringIndex = findIndex(of: "Andrea", in: ["Mike", "Malcolm", "Andrea"]) +// stringIndex is an optional Int containing a value of 2 +``` + + + + + + + +## 关联类型 + +定义一个协议时，声明一个或多个关联类型作为协议定义的一部分将会非常有用。*关联类型*为协议中的某个类型提供了一个占位符名称，其代表的实际类型在协议被遵循时才会被指定。关联类型通过 `associatedtype` 关键字来指定。 + +### 关联类型实践 + +下面例子定义了一个 `Container` 协议，该协议定义了一个关联类型 `Item`： + +```swift +protocol Container { + associatedtype Item + mutating func append(_ item: Item) + var count: Int { get } + subscript(i: Int) -> Item { get } +} +``` + + + +`Container` 协议定义了三个任何遵循该协议的类型（即容器）必须提供的功能: + +- 必须可以通过 `append(_:)` 方法添加一个新元素到容器里。 +- 必须可以通过 `count` 属性获取容器中元素的数量，并返回一个 `Int` 值。 +- 必须可以通过索引值类型为 `Int` 的下标检索到容器中的每一个元素。 + +该协议没有指定容器中的元素的类型以及如何存储。该协议只指定了任何遵循 `Container` 协议的类型必现提供上述三个功能。在遵循该协议的前提下，容器也可以提供其他额外的功能。 + +任何遵循 `Container` 协议的类型必须能够指定它存储的值的类型。具体来说，它必须确保添加到容器内的元素以及下标返回的元素类型都是正确的。 + +为了定义这些条件，`Container` 协议需要在不知道容器中元素的具体类型的情况下引用这种类型。`Container` 协议需要指定任何通过 `append(_:)` 方法添加到容器中的元素和容器内的元素是相同类型，并且通过容器下标返回的元素的类型也是这种类型。 + +为此，`Container` 协议声明了一个关联类型 `Item`，写作 `associatedtype Item`。协议没有定义 `Item` 是什么，这个信息留给遵循协议的类型来提供。尽管如此，`Item` 别名提供了一种方式来引用 `Container` 中元素的类型，并将之用于 `append(_:)` 方法和下标，从而保证任何 `Container` 的行为都能如预期。 + +以下是上文中非泛型的 IntStack，通过遵循 Container 协议，修改后的版本： + +```swift +struct IntStack: Container { + // IntStack 原始实现 + var items: [Int] = [] + mutating func push(_ item: Int) { + items.append(item) + } + mutating func pop() -> Int { + return items.removeLast() + } + // 遵循Container 协议的实现部分 + typealias Item = Int + mutating func append(_ item: Int) { + self.push(item) + } + var count: Int { + return items.count + } + subscript(i: Int) -> Int { + return items[i] + } +} +``` + + + +`IntStack` 类型实现了 `Container` 协议的三项要求，，并且在每种情况下都封装了 `IntStack` 类型的现有功能的一部分，以满足这些要求。 + +此外，`IntStack` 在实现 `Container` 协议的要求时，指定 `Item` 为 `Int` 类型，即 `typealias Item = Int`，从而将 `Container` 协议中抽象的 `Item` 类型转换为具体的 `Int` 类型。 + +得益于 Swift 的类型推断机制， +实际上在 `IntStack` 的定义中不需要声明 `Item` 为 `Int`。因为 `IntStack` 遵循 `Container` 协议的所有要求，`Swift` 只需通过 `append(_:)` 方法的 `item` 参数类型和下标返回值的类型，就可以推断出 `Item` 的具体类型。事实上，如果你在上面的代码中删除了 `typealias Item = Int` 这一行，一切也可正常工作，因为 Swift 清楚地知道 `Item` 应该是哪种类型。 + +你也可以让泛型 `Stack` 结构体遵循 `Container` 协议: + +```swift +struct Stack: Container { + // Stack 的原始实现部分 + var items: [Element] = [] + mutating func push(_ item: Element) { + items.append(item) + } + mutating func pop() -> Element { + return items.removeLast() + } + // Container 协议的实现部分 + mutating func append(_ item: Element) { + self.push(item) + } + var count: Int { + return items.count + } + subscript(i: Int) -> Element { + return items[i] + } +} +``` + + + +这一次，类型参数 `Element` 被用作 `append(_:)` 方法的 `item` 参数类型和下标的返回类型。因此，Swift 可以推断出 `Element` 即是 `item` 的类型。 + +### 扩展现有类型来指定关联类型 + +在中描述了如何利用扩展让一个已存在的类型遵循一个协议，这包括使用了关联类型协议 + +Swift 的 `Array` 类型已经提供 `append(_:)` 方法，`count` 属性，以及带有 `Int` 索引的下标来检索其元素。这三个功能都遵循 `Container` 协议的要求，也就意味着你只需声明 `Array` 遵循 `Container` 协议，就可以扩展 `Array`，使其遵循 `Container` 协议。你可以通过一个空扩展来实现这点，正如中的描述 + +```swift +extension Array: Container {} +``` + + + +`Array` 已有的 `append(_:)` 方法和下标使 Swift 能够推断出 `Item` 的具体类型，就像上面提到的泛型 `Stack` 类型一样。在定义此扩展后，你可以将任何 Array 作为 `Container` 使用。 + + +### 给关联类型添加约束 + +你可以在协议中为关联类型添加类型约束，以要求遵循该协议的类型满足这些约束。。例如，下面的代码定义了 `Container` 协议， 其要求关联类型 `Item` 必须遵循 `Equatable` 协议： + +```swift +protocol Container { + associatedtype Item: Equatable + mutating func append(_ item: Item) + var count: Int { get } + subscript(i: Int) -> Item { get } +} +``` + + + +为了遵守 `Container` 协议，`Item` 类型也必须遵守 `Equatable` 协议。 + +### 在关联类型约束里使用协议 + +协议可以作为它自身的要求出现。例如，有一个协议细化了 `Container` 协议，添加了一个 `suffix(_:)` 方法。`suffix(_:)` 方法返回容器中从后往前给定数量的元素，并把它们存储在一个 `Suffix` 类型的实例里。 + +```swift +protocol SuffixableContainer: Container { + associatedtype Suffix: SuffixableContainer where Suffix.Item == Item + func suffix(_ size: Int) -> Suffix +} +``` + + + +在这个协议里，`Suffix` 是一个关联类型，就像上边例子中 `Container` 的 `Item` 类型一样。`Suffix` 拥有两个约束：它必须遵循 `SuffixableContainer` 协议（就是当前定义的协议），以及它的 `Item` 类型必须是和容器里的 `Item` 类型相同。`Item` 的约束是一个 `where` 分句，它在下面 中有讨论。 + +这是上面 中 `Stack` 类型的拓展，它遵循了 `SuffixableContainer` 协议： + +```swift +extension Stack: SuffixableContainer { + func suffix(_ size: Int) -> Stack { + var result = Stack() + for index in (count-size)..() +stackOfInts.append(10) +stackOfInts.append(20) +stackOfInts.append(30) +let suffix = stackOfInts.suffix(2) +// suffix 包含 20 和 30 +``` + + + +在上面的例子中，`Suffix` 是 `Stack` 的关联类型，也是 `Stack` ，所以 `Stack` 的后缀运算返回另一个 `Stack` 。另外，遵循 `SuffixableContainer` 的类型可以拥有一个与它自己不同的 `Suffix` 类型——也就是说后缀运算可以返回不同的类型。比如说，这里有一个非泛型 `IntStack` 类型的扩展，它遵循了 `SuffixableContainer` 协议，使用 `Stack` 作为它的后缀类型而不是 `IntStack`： + +```swift +extension IntStack: SuffixableContainer { + func suffix(_ size: Int) -> Stack { + var result = Stack() + for index in (count-size)..。 +} +``` + + + +## 泛型 Where 语句 + +让你能够为泛型函数、下标、类型的类型参数定义一些强制要求。 + +对关联类型添加约束通常是非常有用的。你可以通过定义一个泛型 `where` 子句来实现。通过泛型 `where` 子句让关联类型遵循某个特定的协议，以及某个特定的类型参数和关联类型必须类型相同。你可以通过将 `where` 关键字紧跟在类型参数列表后面来定义 `where` 子句，`where` 子句后跟一个或者多个针对关联类型的约束，以及一个或多个类型参数和关联类型间的相等关系。你可以在函数体或者类型的大括号之前添加 `where` 子句。 + +下面的例子定义了一个名为 `allItemsMatch` 的泛型函数，用来检查两个 `Container` 实例是否包含相同顺序的相同元素。如果所有的元素能够匹配，那么返回 `true`，否则返回 `false`。 + +被检查的两个 `Container` 可以不是相同类型的容器（虽然它们可以相同），但它们必须拥有相同类型的元素。这个要求通过一个类型约束以及一个 `where` 子句来表示： + +```swift +func allItemsMatch + (_ someContainer: C1, _ anotherContainer: C2) -> Bool + where C1.Item == C2.Item, C1.Item: Equatable { + + // 检查两个容器含有相同数量的元素 + if someContainer.count != anotherContainer.count { + return false + } + + // 检查每一对元素是否相等 + for i in 0.. func allItemsMatch + (_ someContainer: C1, _ anotherContainer: C2) -> Bool + where C1.Item == C2.Item, C1.Item: Equatable { + --- + // Check that both containers contain the same number of items. + if someContainer.count != anotherContainer.count { + return false + } + --- + // Check each pair of items to see if they're equivalent. + for i in 0.. + +这个函数接受 `someContainer` 和 `anotherContainer` 两个参数。参数 `someContainer` 的类型为 `C1`，参数 `anotherContainer` 的类型为 `C2`。`C1` 和 `C2` 是容器的两个占位类型参数，函数被调用时才能确定它们的具体类型。 + +这个函数的类型参数列表还定义了对两个类型参数的要求： + +- `C1` 必须遵循 `Container` 协议（写作 `C1: Container`）。 +- `C2` 必须遵循 `Container` 协议（写作 `C2: Container`）。 +- `C1` 的 `Item` 必须和 `C2` 的 `Item` 类型相同（写作 `C1.Item == C2.Item`）。 +- `C1` 的 `Item` 必须遵循 `Equatable` 协议（写作 `C1.Item: Equatable`）。 + +前两个要求定义在函数的类型形式参数列表里，后两个要求定义在函数的泛型 where 分句中。 + +这些要求意味着： + +- `someContainer` 是一个 `C1` 类型的容器。 +- `anotherContainer` 是一个 `C2` 类型的容器。 +- `someContainer` 和 `anotherContainer` 包含相同类型的元素。 +- `someContainer` 中的元素可以通过不等于操作符（!=）来检查它们是否相同。 + +第三个和第四个要求结合起来意味着 `anotherContainer` 中的项也可以使用 `!=` 操作符进行检查，因为它们与 `someContainer` 中的项类型完全相同。 + +这些要求使得 `allItemsMatch(_:_:)` 函数能够比较两个容器，即使它们是不同类型的容器。 + +`allItemsMatch(_:_:)` 函数首先检查两个容器是否包含相同数量的项。如果它们元素个数不同，则不可能匹配，函数会返回 `false`。 + +在进行此检查之后，函数使用 `for`-`in` 循环和半开区间操作符 (`..<`) 遍历 `someContainer` 中的所有项。对于每一项，函数检查 `someContainer` 中的元素是否不等于 `anotherContainer` 中的对应元素。如果两项不相等，则两个容器不匹配，函数返回 `false`。 + +如果循环结束时没有发现不匹配的情况，则两个容器匹配，函数返回 `true`。 + +以下是 `allItemsMatch(_:_:)` 函数的示例： + +```swift +var stackOfStrings = Stack() +stackOfStrings.push("uno") +stackOfStrings.push("dos") +stackOfStrings.push("tres") + +var arrayOfStrings = ["uno", "dos", "tres"] + +if allItemsMatch(stackOfStrings, arrayOfStrings) { + print("All items match.") +} else { + print("Not all items match.") +} +// 打印 "All items match." +``` + + + +上述示例创建了一个 `Stack` 实例来存储 `String` 值，并将三个字符串压入栈中。该示例还使用包含与栈中相同的三个字符串的数组字面量创建了一个 `Array` 实例。即使栈和数组类型不同，但它们都遵循 `Container` 协议，并且都包含相同类型的值。因此，你可以将这两个容器作为参数来调用 `allItemsMatch(_:_:)` 函数。在上述示例中，`allItemsMatch(_:_:)` 函数正确地报告了两个容器中的所有元素都是相互匹配的。 + +## 具有泛型 Where 子句的扩展 + +你也可以使用泛型 `where` 子句作为扩展的一部分。下面的示例扩展了前面的例子中的泛型 `Stack` 结构，添加了一个 `isTop(_:)` 方法。 + +```swift +extension Stack where Element: Equatable { + func isTop(_ item: Element) -> Bool { + guard let topItem = items.last else { + return false + } + return topItem == item + } +} +``` + + + +这个新的 `isTop(_:)` 方法首先检查栈是否为空，然后将给定的元素与栈顶的元素进行比较。如果你尝试在没有泛型 `where` 子句的情况下这样做，你会遇到一个问题：`isTop(_:)` 的实现使用了 `==` 操作符，但 `Stack` 的定义并不要求其元素遵循 `Equatable` 协议的，因此使用 `==` 操作符会导致编译时错误。使用泛型 `where` 子句可以为扩展添加新的条件，这样扩展只在栈中的元素遵循 `Equatable` 协议时才添加 `isTop(_:)` 方法。 + +以下是 `isTop(_:)` 方法的实际运行方式： + +```swift +if stackOfStrings.isTop("tres") { + print("Top element is tres.") +} else { + print("Top element is something else.") +} +// 打印 "Top element is tres." +``` + + + +如果尝试在其元素不遵循 `Equatable` 协议的栈上调用 `isTop(_:)` 方法，则会收到编译时错误。 + +```swift +struct NotEquatable { } +var notEquatableStack = Stack() +let notEquatableValue = NotEquatable() +notEquatableStack.push(notEquatableValue) +notEquatableStack.isTop(notEquatableValue) // 报错 +``` + + + +你可以使用泛型 `where` 子句去扩展一个协议。基于以前的示例，下面的示例扩展了 `Container` 协议，添加一个 `startsWith(_:)` 方法。 + +```swift +extension Container where Item: Equatable { + func startsWith(_ item: Item) -> Bool { + return count >= 1 && self[0] == item + } +} +``` + + + + + +`startsWith(_:)` 方法首先确保容器中至少有一个元素，然后检查容器中的第一个元素是否与给定的元素相匹配。这个新的 `startsWith(_:)` 方法可以用于任何遵循 `Container` 协议的类型，包括上面使用的栈和数组，只要容器中的元素是遵循 `Equatable` 的。 + +```swift +if [9, 9, 9].startsWith(42) { + print("Starts with 42.") +} else { + print("Starts with something else.") +} +// Prints "Starts with something else." +``` + + + +泛型 `where` 子句在上面的例子中要求 `Item` 遵循一个协议，但你也可以编写一个泛型 `where` 子句去要求 `Item` 为特定类型。例如： + +```swift +extension Container where Item == Double { + func average() -> Double { + var sum = 0.0 + for index in 0.. extension Container where Item == Double { + func average() -> Double { + var sum = 0.0 + for index in 0.. print([1260.0, 1200.0, 98.6, 37.0].average()) + <- 648.9 + ``` +--> + +此示例为 `Item` 类型是 `Double` 的容器中添加了一个 `average()` 方法。它遍历容器中的所有元素，将它们相加，然后除以容器的元素数量来计算平均值。为了进行浮点数除法，它将元素数量从 `Int` 类型显式转换为 `Double` 类型。 + +你可以在扩展中使用泛型 `where` 子句包含多个条件，就像在其他地方编写泛型 `where` 子句一样。用逗号分隔列表中的每个条件。 + + + +## 包含上下文关系的 where 子句 + + 当你使用泛型时，可以为没有独立类型约束的声明添加 `where` 子句。例如，你可以在泛型类型的下标或泛型类型扩展中的方法上编写泛型 `where` 子句。`Container` 结构是泛型的，下面示例通过 `where` 子句让新的方法声明其调用所需要满足的类型约束。 + +```swift +extension Container { + func average() -> Double where Item == Int { + var sum = 0.0 + for index in 0.. Bool where Item: Equatable { + return count >= 1 && self[count-1] == item + } +} +let numbers = [1260, 1200, 98, 37] +print(numbers.average()) +// 打印 "648.75" +print(numbers.endsWith(37)) +// 打印 "true" +``` + + + +这个示例在 `Container` 中添加了一个当元素是整数时可使用的 `average()` 方法；还添加了一个当元素遵循 `equatable` 协议时使用 `endsWith(_:)` 方法。这两个函数都包含一个泛型 `where` 子句，该子句为 `Container` 原始声明中的泛型 `Item` 类型参数添加了类型约束。 + +如果你想在不使用上下文 `where` 子句的情况下编写这段代码，你需要为每个泛型 `where` 子句编写两个扩展。上面的示例和下面的示例具有相同的行为。 + +```swift +extension Container where Item == Int { + func average() -> Double { + var sum = 0.0 + for index in 0.. Bool { + return count >= 1 && self[count-1] == item + } +} +``` + + + +在使用上下文 `where` 子句的示例中，由于每个方法的泛型 `where` 子句都声明了需要满足的要求，因此`average()` 和 `endsWith(_:)` 的实现都可以放在在同一个扩展中。将这些要求移动到扩展的泛型 `where` 子句进行声明也能起到同样的效果，但每一个扩展只能有一个必备条件。 + +## 具有泛型 Where 子句的关联类型 + +你可以在关联类型上后面加上一个泛型 `where` 子句。例如，假设你想创建一个包含迭代器（`Iterator`）的 `Container`，类似于 Swift 标准库中的 `Sequence` 协议。你可以这样编写： + +```swift +protocol Container { + associatedtype Item + mutating func append(_ item: Item) + var count: Int { get } + subscript(i: Int) -> Item { get } + + associatedtype Iterator: IteratorProtocol where Iterator.Element == Item + func makeIterator() -> Iterator +} +``` + + + + + +在 `Iterator` 上的泛型 `where` 子句要求无论迭代器的元素类型如何，迭代器中的元素类型必须和容器的元素类型保持一致。`makeIterator()` 函数提供对容器迭代器的访问。 + + + +对于继承自另一个协议的协议，你可以通过在协议声明中添加泛型 `where` 子句来为继承的关联类型添加约束。例如，以下代码声明了一个 `ComparableContainer` 协议，该协议要求 `Item` 遵循 `Comparable`协议： + +```swift +protocol ComparableContainer: Container where Item: Comparable { } +``` + + + + + + + +## 泛型下标 + +下标可以是泛型，并且可以添加泛型 `where` 子句。你可以在 `subscript` 之后的尖括号内写入占位符类型，并在下标主体的开括号（`{`）之前写入泛型 `where` 子句。例如： + + + + +```swift +extension Container { + subscript(indices: Indices) -> [Item] + where Indices.Iterator.Element == Int { + var result: [Item] = [] + for index in indices { + result.append(self[index]) + } + return result + } +} +``` + + + + + +这个对 `Container` 协议的扩展添加了一个下标，该下标接受一个索引序列并返回一个包含每个给定索引所在的值的数组。这个泛型下标的约束如下： + +- 尖括号中的泛型参数 `Indices` 必须是遵循 Swift 标准库中 `Sequence` 协议的类型。 +- 下标接受一个单一参数 `indices`，它是该 `Indices` 类型的一个实例。 +- 泛型 `where` 子句要求序列的迭代器必须遍历 `Int` 类型的元素。这确保了序列中的索引与用于容器的索引类型相同。 + +综合起来，这些约束意味着传入给 `indices` 参数的值是一个整型序列。 + + + + + + diff --git a/swift-6.docc/LanguageGuide/Inheritance.md b/swift-6.docc/LanguageGuide/Inheritance.md new file mode 100644 index 000000000..137824c17 --- /dev/null +++ b/swift-6.docc/LanguageGuide/Inheritance.md @@ -0,0 +1,486 @@ +# 继承 + +通过子类化来添加或重写功能。 + +一个类可以从另一个类*继承*方法、属性和其他特性。当一个类从另一个类继承时，继承的类被称为*子类*，而被它继承的类被称为*父类*。继承是 Swift 中区别类与其他类型的基本特性。 + +Swift 中的类可以调用和访问属于其父类的方法、属性和下标，并且可以通过重写这些方法、属性和下标来优化或修改他们的行为。Swift 通过检查重写定义是否与父类定义相匹配来帮助确保您的覆盖是正确的。 + +类还可以为继承的属性添加属性观察器，以便在属性值发生变化时得到通知。无论最初是定义为存储属性还是计算属性，都可以为任何属性添加属性观察器。 + +## 定义基类 + +如果一个类没有继承其他类，那他就是一个基类。 + +> 注意: Swift 中没有统一的基类，所有类的起源都是平等的。您不指定父类的类会自动成为基类。 + +下面的示例定义了一个名为 `Vehicle` 的基类。这个基类定义了一个名为 `currentSpeed` 的存储属性，默认值为 `0.0`（推断出属性类型为 `Double`）。`currentSpeed` 属性的值被一个只读计算 `String` 属性 `description` 用于创建车辆的描述。 + +`Vehicle` 基类还定义了一个名为 `makeNoise()` 的方法。这个方法对于基类 Vehicle 的实例不会做任何事情，但稍后会被 `Vehicle` 的子类自定义： + +```swift +class Vehicle { + var currentSpeed = 0.0 + var description: String { + return "traveling at \(currentSpeed) miles per hour" + } + func makeNoise() { + // 不做任何事情 - 不是任何一辆车都会发出噪音。 + } +} +``` + + + +您可以使用*初始化语法*创建一个新的 `Vehicle` 实例，写作类型名后跟一对空括号： + +```swift +let someVehicle = Vehicle() +``` + + + +创建了一个新的 `Vehicle` 实例后，您可以访问它的 `description` 属性来打印车辆当前速度的描述: + +```swift +print("Vehicle: \(someVehicle.description)") +// Vehicle: traveling at 0.0 miles per hour +``` + + + +`Vehicle` 类定义了任意车辆的通用特性，但本身并不太有用。要使其更有用，您需要对其进行完善以描述更具体的车辆类型。 + +## 子类化 + +*子类化*是基于现有类创建新类的行为。子类继承现有类的特性，然后您可以对其进行完善。您还可以向子类添加新的特性。 + +要指示子类有一个父类，请在父类名前写子类名，中间用冒号分隔： + +```swift +class SomeSubclass: SomeSuperclass { + // 子类定义在这里 +} +``` + + + +下面的示例定义了一个名为 `Bicycle` 的子类，它的父类是 `Vehicle`： + +```swift +class Bicycle: Vehicle { + var hasBasket = false +} +``` + + + +新的 `Bicycle` 类自动获得了 `Vehicle` 的所有特性，例如它的 `currentSpeed` 和 `description` 属性以及 `makeNoise()` 方法。 + +除了继承的特性之外，`Bicycle` 类还定义了一个新的存储属性 `hasBasket`，默认值为 `false` （推断该属性的类型为 `Bool`）。 + +默认情况下，您创建的任何新 `Bicycle` 实例都默认没有篮子。在创建实例后，您可以将特定 `Bicycle` 实例的 `hasBasket` 属性设置为 `true`： + +```swift +let bicycle = Bicycle() +bicycle.hasBasket = true +``` + + + +您还可以修改 `Bicycle` 实例继承的 `currentSpeed` 属性，并查询实例继承的 `description` 属性： + +```swift +bicycle.currentSpeed = 15.0 +print("Bicycle: \(bicycle.description)") +// Bicycle: traveling at 15.0 miles per hour +``` + + + +子类本身也可以被子类化。下一个示例创建了一个 `Bicycle` 的子类，用于双人自行车，称为 “tandem”： + +```swift +class Tandem: Bicycle { + var currentNumberOfPassengers = 0 +} +``` + + + +`Tandem` 继承了 `Bicycle` 的所有属性和方法，而 `Bicycle` 又继承了 `Vehicle` 的所有属性和方法。`Tandem` 子类还添加了一个名为 `currentNumberOfPassengers` 的新存储属性，默认值为 `0`。 + +如果您创建一个 `Tandem` 实例，您可以使用它的任何新属性和继承的属性，并查询它从 `Vehicle` 继承的只读属性：`description` + +```swift +let tandem = Tandem() +tandem.hasBasket = true +tandem.currentNumberOfPassengers = 2 +tandem.currentSpeed = 22.0 +print("Tandem: \(tandem.description)") +// Tandem: traveling at 22.0 miles per hour +``` + + + +## 重写 + +子类可以提供自己的自定义实现来覆盖它将从父类继承的实例方法、类型方法、实例属性、类型属性或下标。这被称为*重写*。 + +要重写将被继承的特性，您需要在重写定义前加上 `override` 关键字。这样做可以明确您打算提供重写，而不是由于疏忽而提供了相同的定义。无意间的重写可能会导致意外行为，任何没有 `override` 关键字的重写在编译代码时都会被诊断为错误。 + +`override` 关键字还会提示 Swift 编译器检查您的重写类的父类（或其父类之一）是否有与您提供的重写定义相匹配的声明。这个检查可以确保您的重写定义是正确的。 + +### 访问父类方法、属性和下标 + +当您为子类提供方法、属性或下标重写时，有时使用现有父类实现作为重写的一部分是很有用的。例如，您可以改进现有实现的行为，或在现有继承的变量中存储修改后的值。 + +在适当的情况下，您可以使用 `super` 前缀来访问父类的方法、属性或下标： + +- 一个被重写的名为 `someMethod()` 的方法可以在重写方法实现中通过调用 `super.someMethod()` 来调用父类版本的 `someMethod()`。 +- 一个被重写的名为 `someProperty` 的属性可以在重写的 getter 或 setter 实现中通过 `super.someProperty` 来访问父类版本的 `someProperty`。 +- 一个被重写的针对 `someIndex` 的下标可以在重写的下标实现中通过 `super[someIndex]` 来访问同一下标的父类版本。 + +### 重写方法 + +您可以重写继承的实例或类型方法，以在子类中提供该方法的定制或替代实现。 + +下面的示例定义了一个名为 `Train` 的新 `Vehicle` 子类，它重写了从 `Vehicle` 继承的 `makeNoise()` 方法: + +```swift +class Train: Vehicle { + override func makeNoise() { + print("Choo Choo") + } +} +``` + + + +如果您创建一个新的 `Train` 实例并调用它的 `makeNoise()` 方法，您可以看到调用了 `Train` 子类版本的方法: + +```swift +let train = Train() +train.makeNoise() +// 打印 "Choo Choo" +``` + + + +### 重写属性 + +您可以重写继承的实例或类型属性，为该属性提供自己的自定义 getter 和 setter，或添加属性观察器以使重写的属性能够观察底层属性值的变化。 + +#### 重写属性 Getter 和 Setter + +您可以为任何继承的属性提供自定义 getter（如果需要的话还有 setter），无论继承的属性在源码中是作为存储属性还是计算属性实现的。子类不知道继承属性的存储或计算性质，它只知道继承的属性有一个特定的名称和类型。您必须始终声明要重写的属性的名称和类型，以便编译器检查您的重写与父类中具有相同名称和类型的属性相匹配。 + +您可以通过在子类属性重写中提供 getter 和 setter 来将继承的只读属性表示为可读写属性。但是您不能将继承的可读写属性声明为只读属性。 + +> 如果您在属性重写中提供了 setter，您也必须为该重写提供 getter。如果您不想在重写的 getter 中修改继承属性的值。您可以简单地通过从 getter 返回 `super.someProperty` 来传递继承的值。其中 `someProperty` 是您正在重写的属性的名称。 + +下面的示例定义了一个名为 `Car` 的新类，它是 `Vehicle` 的子类。`Car` 类引入了一个名为 `gear` 的新存储属性，默认整数值为 `1`。`Car` 类还重写了它从 `Vehicle` 继承的 `description` 属性，以提供包含当前档位的自定义描述: + +```swift +class Car: Vehicle { + var gear = 1 + override var description: String { + return super.description + " in gear \(gear)" + } +} +``` + + + +`description` 属性的重写首先调用 `super.description`，它返回 `Vehicle` 类的 `description` 属性。然后,`Car` 类的 `description` 版本在此描述的末尾添加了一些额外文本，以提供有关当前档位的信息。 + +如果您创建 `Car` 类的实例并设置其 `gear` 和 `currentSpeed` 属性，您可以看到它的 `description` 属性返回在 `Car` 类中定义的定制描述： + +```swift +let car = Car() +car.currentSpeed = 25.0 +car.gear = 3 +print("Car: \(car.description)") +// 打印 "Car: traveling at 25.0 miles per hour in gear 3" +``` + + + +#### 重写属性观察器 + +您可以使用属性重写的方式为继承的属性添加属性观察器。这样无论该属性最初是如何实现的，您都能够在继承属性的值发生变化时得到通知。有关属性观察器的更多信息，请参阅 。 + +> 注意: 你无法为继承的常量存储属性或继承的只读计算属性添加属性观察器。这些属性的值无法被修改，所以在重写时提供 `willSet` 或 `didSet` 实现是不合适的。 +> +> 另请注意，你不能为同一属性提供重写的 setter 和重写的属性观察器。如果你想观察属性值的变化，并且你已经为该属性提供了自定义 setter，你可以简单地在自定义 setter 中观察任何值的变化。 + +以下示例创建了一个名为 `AutomaticCar` 的新类，它继承自 `Car` 类。`AutomaticCar` 类表示一辆带有自动变速箱的汽车，根据当前速度自动选择合适的挡位： + +```swift +class AutomaticCar: Car { + override var currentSpeed: Double { + didSet { + gear = Int(currentSpeed / 10.0) + 1 + } + } +} +``` + + + +每当你设置 `AutomaticCar` 实例的 `currentSpeed` 属性时，该属性的 `didSet` 观察器会根据新速度为实例的 `gear` 属性设置一个合适的挡位。具体来说，属性观察器选择一个挡位，该挡位是新 `currentSpeed` 值除以 10 向下取整后加 1。速度为 `35.0` 时会挂 `4` 挡: + +```swift +let automatic = AutomaticCar() +automatic.currentSpeed = 35.0 +print("AutomaticCar: \(automatic.description)") +// AutomaticCar: traveling at 35.0 miles per hour in gear 4 +``` + + + +## 防止重写 + +你可以通过将其标记为 *final* 来防止方法、属性或下标被重写。在方法、属性或下标的引入关键字前写 `final` 修饰符。（如 `final var`、`final func`、`final class func` 和 `final subscript`） + + +任何尝试在子类中重写 final 方法、属性或下标的行为都会在编译时报错。你在扩展中添加到类的方法、属性或下标也可以在扩展的定义中标记为 final。有关更多信息，请参阅 。 + + + +你可以通过在类定义（`final class`）中在 `class` 关键字前写 `final` 修饰符来将整个类标记为 final。任何尝试子类化 final 类的行为都会在编译时报错。 + + + + + + + + + + diff --git a/swift-6.docc/LanguageGuide/Initialization.md b/swift-6.docc/LanguageGuide/Initialization.md new file mode 100644 index 000000000..d10ffd776 --- /dev/null +++ b/swift-6.docc/LanguageGuide/Initialization.md @@ -0,0 +1,2399 @@ +# 构造过程 +设置类型中存储属性的初始值并执行一次性构造过程。 + +*构造过程*是使用类、结构体或枚举等实例之前的准备过程。这个过程包括为该实例的每个存储属性设置初始值，并执行任何其他必要的设置或构造过程，以确保新实例在使用前已经完成正确的构造。 + +你可以通过定义*构造器*来实现这个构造过程，它就像是用来创建特定类型新实例的特殊方法。与Objective-C构造器不同，Swift构造器没有返回值，它们的主要作用是确保类型的新实例在首次使用前被正确构造。 + +类的实例可以通过实现*析构器*来执行它释放之前自定义的清理工作。想了解析构器的更多内容，请参见 . + +## 存储属性的初始赋值 + +创建类和结构体的实例时，*必须*为它们所有的存储属性设置一个适当的初始值。存储属性不能处于不确定的状态。 + +你可以在构造器中为存储属性设置初始值，或者在定义属性时赋予默认值。以下部分将会详细介绍这两种方法。 + +> 注意：当你为存储属性赋予默认值，或者在构造器中设置其初始值时，该属性的值会被直接设置，而不会触发任何属性观察器。 + +### 构造器 + +*构造器* 被调用来创建某个特定类型的新实例。构造器在最简单的形式中就像一个没有参数的实例方法，以关键字`init` 来命名： + +```swift +init() { + // 在此处执行构造过程 +} +``` + + + +下面例子中定义了一个用来保存华氏温度的结构体 `Fahrenheit`，它拥有一个 `Double` 类型的存储型属性 `temperature`： + +```swift +struct Fahrenheit { + var temperature: Double + init() { + temperature = 32.0 + } +} +var f = Fahrenheit() +print("The default temperature is \(f.temperature)° Fahrenheit") +// 打印 "The default temperature is 32.0° Fahrenheit" +``` + + + +这个结构体只定义了一个不带形参的构造器 `init`，并在里面将存储型属性 `temperature` 的值初始化为 `32.0`（华氏温度下水的冰点）。 + +### 默认属性值 + +如上所示，你可以在构造器中设置存储属性的初始值。同样，你也可以在属性声明时为其设置默认值。 + +> 注意：如果一个属性总是使用相同的初始值，建议直接提供一个默认值，而不是每次都在构造器中设置值。两种方法的最终结果是一样的，但默认值使属性的初始化与其声明结合的更加紧密。它能让你的构造器更简洁、更清晰，且能够通过默认值推断属性的类型。默认值还能使你更容易利用默认构造器和构造器继承等特性，如本章后面所述。 + +你可以通过在声明 `temperature` 属性时提供一个默认值，将上面的 `Fahrenheit` 结构体写成如下更简单的形式： + +```swift +struct Fahrenheit { + var temperature = 32.0 +} +``` + + + +## 自定义构造过程 + +你可以自定义构造过程，比如提供输入的形参、可选属性类型或者给常量属性赋值，这些都将在后面章节中提到。 + +### 形参的构造过程 + +你可以在自定义构造过程的定义中提供*构造形参*，指定其值的类型和名字。构造形参的功能和语法与函数和方法的形参相同。 + +下面例子中定义了一个用来保存摄氏温度的结构体 `Celsius`。它定义了两个不同的构造器：`init(fromFahrenheit:)` 和 `init(fromKelvin:)`，二者分别通过接受不同温标下的温度值来创建新的实例： + +```swift +struct Celsius { + var temperatureInCelsius: Double + init(fromFahrenheit fahrenheit: Double) { + temperatureInCelsius = (fahrenheit - 32.0) / 1.8 + } + init(fromKelvin kelvin: Double) { + temperatureInCelsius = kelvin - 273.15 + } +} +let boilingPointOfWater = Celsius(fromFahrenheit: 212.0) +// boilingPointOfWater.temperatureInCelsius 是 100.0 +let freezingPointOfWater = Celsius(fromKelvin: 273.15) +// freezingPointOfWater.temperatureInCelsius 是 0.0 +``` + + + +第一个构造器只拥有一个构造形参，其实参标签为 `fromFahrenheit`，形参命名为 `fahrenheit`；第二个构造器也拥有一个构造形参，其实参标签为 `fromKelvin`，形参命名为 `kelvin`。这两个构造器都将单一的实参转换成摄氏温度值，并保存在属性 `temperatureInCelsius` 中。 + + + +### 形参命名和实参标签 + +与函数和方法形参相同，构造形参可以同时具有在构造器内部使用的形参名称和在调用构造器时使用的实参标签。 + +然而，构造器在括号前没有像函数和方法那样的可辨别的方法名。因此，构造器的形参名称和类型在确定应调用哪个构造器时起着至关重要的作用。正因为如此，如果你没有提供实参标签，Swift会为构造器的每个形参自动提供一个实参标签。 + + +下面例子中的 `Color` 结构体包含了三个 `Double` 类型的常量（ `red` 、 `green`、 `blue` ）,表明颜色中红、绿、蓝成分的含量。 +`Color` 提供了一个构造器，为红蓝绿提供三个合适 `Double` 类型的形参命名。`Color` 也提供了第二个构造器，它只包含名为 `white` 的 `Double` 类型的形参，它为三个颜色的属性提供相同的值。 + +```swift +struct Color { + let red, green, blue: Double + init(red: Double, green: Double, blue: Double) { + self.red = red + self.green = green + self.blue = blue + } + init(white: Double) { + red = white + green = white + blue = white + } +} +``` + + + +两种构造器都能通过为每一个构造器形参提供命名值来创建一个新的 Color 实例： + +```swift +let magenta = Color(red: 1.0, green: 0.0, blue: 1.0) +let halfGray = Color(white: 0.5) +``` + + + +> 注意: 如果不通过实参标签传值，这个构造器是没法调用的。如果构造器定义了某个实参标签，就必须使用它，忽略它将导致编译期错误： + +```swift +let veryGreen = Color(0.0, 1.0, 0.0) +// 报编译期错误-需要实参标签 +``` + + + +### 不带实参标签的构造器形参 + +如果你不希望构造器的某个形参使用实参标签，可以使用下划线 `(_)` 来代替显式的实参标签来重写默认行为。 + +接下来是 带来的扩展版本的 `Celsius`, 增加了一个构造器用于从已经是摄氏温标的 `Double` 值创建一个新的 `Celsius` 实例: + +```swift +struct Celsius { + var temperatureInCelsius: Double + init(fromFahrenheit fahrenheit: Double) { + temperatureInCelsius = (fahrenheit - 32.0) / 1.8 + } + init(fromKelvin kelvin: Double) { + temperatureInCelsius = kelvin - 273.15 + } + init(_ celsius: Double) { + temperatureInCelsius = celsius + } +} +let bodyTemperature = Celsius(37.0) +// bodyTemperature.temperatureInCelsius 是 37.0 +``` + + + +构造器调用 `Celsius(37.0)` 的意图非常明确，不需要实参标签。因此，适合使用构造器 `init(_ celsius: Double)`来通过未命名的 `Double` 值来构造 `Celsius` 。 + +### 可选属性类型 + +如果你自定义的类型有一个逻辑上允许值为空的存储型属性——无论是因为它无法在初始化时赋值，还是因为它在之后某个时机可以赋值为空——都需要将它声明为 `可选类型`。可选类型的属性将自动初始化为 `nil`，表示这个属性是特意在构造过程设置为空。 + +下面例子中定义了类 `SurveyQuestion`，它包含一个可选 `String` 属性 `response`： + +```swift +class SurveyQuestion { + var text: String + var response: String? + init(text: String) { + self.text = text + } + func ask() { + print(text) + } +} +let cheeseQuestion = SurveyQuestion(text: "Do you like cheese?") +cheeseQuestion.ask() +// 打印 "Do you like cheese?" +cheeseQuestion.response = "Yes, I do like cheese." +``` + + + +对调查问卷问题的答案，唯有提问后方能揭晓，因此 `response` 属性被声明为 `String?` 类型，即“可选的 `String` ”。当一个新的 `SurveyQuestion` 实例被初始化时，它会自动被赋予默认值 `nil`，表示“尚无字符串”。 + +### 构造过程中常量属性的赋值 + +你可以在构造过程中的任意时间点给常量属性赋值，只要在构造过程结束时将它设置成确定的值。一旦常量属性被赋值，它将永远不可更改。 + + + + + +> 注意: 对于类的实例来说，它的常量属性只能在类的构造过程中修改，不能在子类中修改。 + +你可以修改上面的 `SurveyQuestion` 示例，将问题的 `text` 属性从变量属性改为常量属性，以表明一旦 `SurveyQuestion` 实例被创建，`text` 是不会改变的。即使 `text` 属性现在是一个常量，它仍然可以在类的构造器中被设置： + +```swift +class SurveyQuestion { + let text: String + var response: String? + init(text: String) { + self.text = text + } + func ask() { + print(text) + } +} +let beetsQuestion = SurveyQuestion(text: "How about beets?") +beetsQuestion.ask() +// 打印 "How about beets?" +beetsQuestion.response = "I also like beets. (But not with cheese.)" +``` + + + +## 默认构造器 + +如果结构体或类为所有属性提供了默认值，又没有提供任何自定义的构造器，那么 Swift 会给这些结构体或类提供一个默认构造器。这个默认构造器将简单地创建一个所有属性值都设置为它们默认值的实例。 + + + +下面例子中定义了一个类 `ShoppingListItem`，它封装了购物清单中的某一物品的名字（`name`）、数量（`quantity`）和购买状态 `purchase state`： + +```swift +class ShoppingListItem { + var name: String? + var quantity = 1 + var purchased = false +} +var item = ShoppingListItem() +``` + + + +由于 `ShoppingListItem` 类的所有属性都有默认值，并且它是一个没有超类的基类。因此，`ShoppingListItem` 会自动获得一个默认构造器实现，该实现会创建一个新实例，并将其所有属性设置为默认值。（`name` 属性是一个可选的 `String` 属性，因此它会自动接收一个默认值 `nil`，即使这个值没有在代码中写出。）上面的示例使用 `ShoppingListItem` 类的默认构造器来创建一个类的新实例（ `ShoppingListItem()`形式的构造语法），并将其赋值给变量 `item` 。 + +![](initializersExample03) + +### 结构体类型的成员逐一构造器 + +如果结构体类型没有定义任何自定义构造器，它们会自动获得*逐一成员构造器*。与默认构造器不同，即使存储属性没有默认值，结构体也会获得逐一成员构造器。 + + + +成员逐一构造器是一种用于初始化新结构体实例里成员属性的快捷方法。新实例的属性的初始值可以通过名称传递给逐一成员构造器。 + +下面的示例定义了一个名为 `Size` 的结构体，该结构体有两个属性，分别是 `width` 和 `height`。通过赋予默认值 `0.0`，这两个属性都被推断为 `Double` 类型。 + +`Size` 结构体会自动获得一个 `init(width:height:)` 逐一成员构造器，你可以用它来构造一个新的 `Size` 实例： + +```swift +struct Size { + var width = 0.0, height = 0.0 +} +let twoByTwo = Size(width: 2.0, height: 2.0) +``` + + + +When you call a memberwise initializer, +you can omit values for any properties +that have default values. +In the example above, +the `Size` structure has a default value +for both its `height` and `width` properties. +You can omit either property or both properties, +and the initializer uses the default value for anything you omit. +For example: +当你调用逐一成员构造器时，你可以省略任何具有默认值的属性。在上面的示例中，`Size` 结构体的 `height` 和 `width` 属性都有默认值。你可以省略其中一个或两个属性，构造器会使用默认值。例如： + +```swift +let zeroByTwo = Size(height: 2.0) +print(zeroByTwo.width, zeroByTwo.height) +// 输出 "0.0 2.0" + +let zeroByZero = Size() +print(zeroByZero.width, zeroByZero.height) +// Prints "0.0 0.0" +``` + + + +## 值类型的构造器代理 + +构造器可以调用其他构造器来完成实例的部分构造过程。这个过程被称为*构造器代理*，它避免了在多个构造器中重复代码。 + +构造器代理的实现规则和形式在值类型和类类型中有所不同。值类型（结构体和枚举类型）不支持继承，所以构造器代理的过程相对简单，因为它们只能代理给自己的其它构造器。 +然而，类不一样，它可以继承自其他类（请参考）。这意味着类有责任保证其所有继承的存储型属性在构造时也能正确的初始化。这些责任将在后续章节中介绍。 + +对于值类型，你可以使用 `self.init` 在自定义的构造器中引用相同值类型的构造器。并且，你只能在构造器内部调用 `self.init` 。 + +请注意，如果你为某个值类型定义了一个自定义构造器，你将无法访问到默认构造器（如果是结构体，还将无法访问逐一成员构造器）。这种限制避免了在一个更复杂的构造器中做了额外的重要设置，但有人不小心使用自动生成的构造器而导致错误的情况。 + +> 注意: 如果你希望默认构造器、逐一成员构造器以及你自己的自定义构造器都能用来创建实例，可以将自定义的构造器写到扩展（extension）中，而不是写在值类型的原始定义中。想查看更多内容，请查看. + +下面的示例中定义了一个自定义结构体 `Rect` ，用来代表几何矩形。这个示例需要两个辅助的结构体 `Size` 和 `Point` ，它们各自为其所有的属性提供了默认值 `0.0` . + +```swift +struct Size { + var width = 0.0, height = 0.0 +} +struct Point { + var x = 0.0, y = 0.0 +} +``` + + + +你可以通过三种方式为 `Rect` 创建实例 (1)使用含有默认值 `origin` 和 `size` 属性来初始化；(2)提供指定的 `origin` 和 `size` 实例来初始化；(3)提供指定的 `center` 和 `size` 来初始化。在下面 `Rect` 结构体定义中，我们为这三种方法提供了三个自定义的构造器: + +```swift +struct Rect { + var origin = Point() + var size = Size() + init() {} + init(origin: Point, size: Size) { + self.origin = origin + self.size = size + } + init(center: Point, size: Size) { + let originX = center.x - (size.width / 2) + let originY = center.y - (size.height / 2) + self.init(origin: Point(x: originX, y: originY), size: size) + } +} +``` + + + +第一个 `Rect` 构造器 `init()` ，在功能上跟没有自定义构造器时自动获得的默认构造器是一样的。这个构造器的函数是空的，使用一对大括号 `{}` 来表示。 +调用这个构造器将返回一个 `Rect` 实例，它的`origin` 和 `size` 属性都使用定义时的默认值 `Point(x: 0.0, y: 0.0)` 和 `Size(width: 0.0, height: 0.0)` : + +```swift +let basicRect = Rect() +// basicRect 的 origin 是 (0.0, 0.0) ， size 是 (0.0, 0.0) +``` + + + +第二个 `Rect` 构造器 `init(origin:size:)` ，在功能上跟结构体在没有自定义构造器时获得的逐一成员构造器是一样的。这个构造器只是简单地将 `origin` 和 `size` 的实参值赋给对应的存储类型: + +```swift +let originRect = Rect(origin: Point(x: 2.0, y: 2.0), + size: Size(width: 5.0, height: 5.0)) +// originRect 的 origin 是 (2.0, 2.0)，size 是 (5.0, 5.0) +``` + + + +第三个 `Rect` 构造器 `init(center:size:)` 要稍微更复杂一点。它先通过 `center` 和 `size` 的值计算出 `origin` 的坐标，然后再调用（或者说 *代理* 给） `init(origin:size:)` 构造器来将新的 `origin` 和 `size` 值赋值到对应的属性中： + +```swift +let centerRect = Rect(center: Point(x: 4.0, y: 4.0), + size: Size(width: 3.0, height: 3.0)) +// centerRect 的 origin 是 (2.5, 2.5)，size 是 (3.0, 3.0) +``` + + + +构造器 init(center:size:)` 可以直接将 `origin` 和 `size` 的新值赋值到对应的属性中。然而，构造器 `init(center:size:)` 通过使用提供了相关功能的现有构造器将会更加便捷（而且意图更清晰）。 + +> 注意: 如果你想使用另一种不需要自己定义的 `init()` and `init(origin:size:)` 构造器的方式来实现这个例子, 请参考 . + +## 类的继承和构造过程 + +类里面的所有存储型属性———包括所有继承自父类的属性——都必须在构造过程中设置初始值。 + +Swift 为类类型提供了两种构造器来确保实例中所有存储型属性都能获得初始值，它们被称为指定构造器和便利构造器。 + +### 指定构造器和便利构造器 + +*指定构造器* 是类中最主要的构造器。一个指定构造器将初始化类中提供的所有属性，并调用合适的父类构造器让构造过程沿着父类链继续往上进行。 + +类倾向于拥有极少的指定构造器，普遍的是一个类只拥有一个指定构造器。指定构造器像一个个“漏斗”放在构造过程发生的地方，让构造过程沿着父类链继续往上进行。 + +每一个类都必须至少拥有一个指定构造器。在某些情况下，许多类通过继承了父类中的指定构造器而满足了这个条件。具体内容请参考后续章节。 + +*便利构造器* 是类中比较次要的、辅助型的构造器。你可以定义便利构造器来调用同一个类中的指定构造器，并为部分形参提供默认值。你也可以定义便利构造器来创建一个特殊用途或特定输入值的实例。 + +你应当只在必要的时候为类提供便利构造器，比方说某种情况下通过使用便利构造器来快捷调用某个指定构造器，能够节省更多开发时间并让类的构造过程更清晰明了。 + +### 指定构造器和便利构造器的语法 + +类的指定构造器的写法跟值类型简单构造器一样: + +```swift +init(<#parameters#>) { + <#statements#> +} +``` + +便利构造器也采用相同样式的写法，但需要在 `init` 关键字之前放置 `convenience` 关键字，并使用空格将其分开: + +```swift +convenience init(<#parameters#>) { + <#statements#> +} +``` + +### 类类型的构造器代理 + +为了简化指定构造器和便利构造器之间的调用关系，Swift 构造器之间的代理调用遵循以下三条规则: + +- term **规则 1**: + 指定构造器必须调用其直接父类的的指定构造器。 + +- term **规则 2**: + 便利构造器必须调用 *相同* 类中定义的其它构造器. + +- term **规则 3**: + 便利构造器最后必须调用指定构造器。 + +一个更方便记忆的方法是: + +- 指定构造器必须总是 *向上* 代理 +- 便利构造器必须总是 *横向* 代理 + +这些规则可以通过下面图例来说明: + +![](initializerDelegation01) + +如图所示，父类中包含一个指定构造器和两个便利构造器。其中一个便利构造器调用了另外一个便利构造器，而后者又调用了唯一的指定构造器。这满足了上面提到的规则 2 和 3。这个父类没有自己的父类，所以规则 1 没有用到。 + +子类中包含两个指定构造器和一个便利构造器。便利构造器必须调用两个指定构造器中的任意一个，因为它只能调用同一个类里的其他构造器。这满足了上面提到的规则 2 和 3。而两个指定构造器必须调用父类中唯一的指定构造器，这满足了规则 1。 + +> 注意: 这些规则不会影响类的实例如何 *创建* 。任何上图中展示的构造器都可以用来创建完全初始化的实例。这些规则只影响类的构造器如何实现。 + +下面图例中展示了一种涉及四个类的更复杂的类层级结构。它演示了指定构造器是如何在类层级中充当“漏斗”的作用，在类的构造器链上简化了类之间的相互关系。 + +![](initializerDelegation02) + +### 两段式构造过程 + +Swift 中类的构造过程包含两个阶段。第一个阶段，类中的每个存储型属性赋一个初始值。当每个存储型属性的初始值被赋值后，第二阶段开始，它给每个类一次机会，在新实例准备使用之前进一步自定义它们的存储型属性。 + +两段式构造过程的使用让构造过程更安全，同时在整个类层级结构中给予了每个类完全的灵活性。两段式构造过程可以防止属性值在初始化之前被访问，也可以防止属性被另外一个构造器意外地赋予不同的值。 + +> 注意: Swift 的两段式构造过程跟 Objective-C 中的构造过程类似。最主要的区别在于阶段 1，Objective-C 给每一个属性赋值零或空值(比如说 `0` 或 `nil`)。 +> Swift 的构造流程则更加灵活，它允许你设置定制的初始值，并自如应对某些属性不能以 `0` 或 `nil` 作为合法默认值的情况。 + +Swift 编译器将执行 4 种有效的安全检查，以确保两段式构造过程不出错地完成： + +- term **安全检查 1**: + 指定构造器必须保证它所在类的所有属性都初始化完成，之后才能将其它构造任务向上代理给父类中的构造器。 + +如上所述，一个对象的内存只有在其所有存储型属性确定之后才能完全初始化。为了满足这一规则，指定构造器必须保证它所在类的属性在它往上代理之前先完成初始化。 + +- term **安全检查 2**: + 指定构造器必须在为继承的属性设置新值之前向上代理调用父类构造器。如果没这么做，指定构造器赋予的新值将被父类中的构造器所覆盖。 + +- term **安全检查 3**: + 便利构造器必须为任意属性（包括所有同类中定义的）赋新值之前代理调用其它构造器。如果没这么做，便利构造器赋予的新值将被该类的指定构造器所覆盖。 + +- term **安全检查 4**: + 构造器在第一阶段构造完成之前，不能调用任何实例方法，不能读取任何实例属性的值，不能引用 `self` 作为一个值。 + +类的实例在第一阶段结束以前并不是完全有效的。只有第一阶段完成后，类的实例才是有效的，才能访问属性和调用方法。 + +以下是基于上述安全检查的两段式构造过程展示： + +**阶段 1** + +- 类的某个指定构造器或便利构造器被调用. +- 完成类的新实例内存的分配，但此时内存还没有被初始化. +- 指定构造器确保其所在类引入的所有存储型属性都已赋初值。存储型属性所属的内存完成初始化。 +- 指定构造器切换到父类的构造器，对其存储属性完成相同的任务。 +- 这个过程沿着类的继承链一直往上执行，直到到达继承链的最顶部。 +- 当到达了继承链最顶部，而且继承链的最后一个类已确保所有的存储型属性都已经赋值，这个实例的内存被认为已经完全初始化。此时阶段 1 完成。 + +**阶段 2** + +- 从继承链顶部往下，继承链中每个类的指定构造器都有机会进一步自定义实例。构造器此时可以访问 `self` 、修改它的属性并调用实例方法等等。 +- 最终，继承链中任意的便利构造器有机会自定义实例和使用 `self`. + +下图展示了在假定的子类和父类之间的构造阶段 1: + +![](twoPhaseInitialization01) + +在这个示例中，构造过程从对子类中一个便利构造器的调用开始。这个便利构造器此时还不能修改任何属性，它会代理到该类中的指定构造器。 + +如安全检查 1 所示，指定构造器将确保所有子类的属性都有值。然后它将调用父类的指定构造器，并沿着继承链一直往上完成父类的构造过程。 + +父类中的指定构造器确保所有父类的属性都有值。由于没有更多的父类需要初始化，也就无需继续向上代理。 + +一旦父类中所有属性都有了初始值，实例的内存被认为是完全初始化，阶段 1 完成。 + +以下展示了相同构造过程的阶段 2： + +![](twoPhaseInitialization02) + +父类中的指定构造器现在有机会进一步自定义实例（尽管这不是必须的）。 + +一旦父类中的指定构造器完成调用，子类中的指定构造器可以执行更多的自定义操作（这也不是必须的）。 + +最终，一旦子类的指定构造器完成调用，最开始被调用的便利构造器可以执行更多的自定义操作。 + +### 构造器的继承和重写 + +跟 Objective-C 中的子类不同，Swift 中的子类默认情况下不会继承父类的构造器。Swift 的这种机制可以防止一个父类的简单构造器被一个更精细的子类继承，而在用来创建子类的新实例时没有完全或错误被初始化。 + +> 注意: 父类的构造器仅会在安全和适当的某些情况下 *被* 继承。具体内容请参考后续章节 . + +假如你希望自定义的子类中能提供一个或多个跟父类相同的构造器，你可以在子类中提供这些构造器的自定义实现。 + +当你在编写一个和父类中指定构造器相匹配的子类构造器时，你实际上是在重写父类的这个指定构造器。因此，你必须在定义子类构造器时带上 `override` 修饰符。即使你重写的是系统自动提供的默认构造器，也需要带上 `override` 修饰符，具体内容请参考. + +正如重写属性，方法或者是下标，`override` 修饰符会让编译器去检查父类中是否有相匹配的指定构造器，并验证构造器参数是否按预想中被指定。 + +> 注意: 当你重写一个父类的指定构造器时，你总是需要写 `override` 修饰符，即使是为了实现子类的便利构造器。 + + + + + +相反，如果你编写了一个和父类便利构造器相匹配的子类构造器，由于子类不能直接调用父类的便利构造器（每个规则都在上文有所描述）。 +因此，严格意义上来讲，你的子类并未对一个父类构造器提供重写。最后的结果就是，你在子类中“重写”一个父类便利构造器时，不需要加 `override` 修饰符。 + + + +在下面的例子中定义了一个叫 `Vehicle` 的基类。基类中声明了一个存储型属性 `numberOfWheels` ，它是默认值为 `Int` 类型的 `0` 。 `numberOfWheels` 属性用在一个描述车辆特征 `String` 类型为 `description` 的计算型属性中: + +```swift +class Vehicle { + var numberOfWheels = 0 + var description: String { + return "\(numberOfWheels) wheel(s)" + } +} +``` + + + +`Vehicle` 类只为存储型属性提供默认值，也没有提供自定义构造器。因此，它会自动获得一个默认构造器，具体内容请参考。默认构造器（如果有的话）总是类中的指定构造器，可以用于创建 `numberOfWheels` 为 `0` 的 `Vehicle` 实例: + +```swift +let vehicle = Vehicle() +print("Vehicle: \(vehicle.description)") +// Vehicle: 0 wheel(s) +``` + + + +下面例子中定义了一个 `Vehicle` 的子类 `Bicycle`: + +```swift +class Bicycle: Vehicle { + override init() { + super.init() + numberOfWheels = 2 + } +} +``` + + + +子类 `Bicycle` 定义了一个自定义指定构造器 `init()`。这个指定构造器和父类的指定构造器相匹配，所以 `Bicycle` 中这个版本的构造器需要带上 `override` 修饰符。 + +`Bicycle` 的构造器 `init()` 以调用 `super.init()` 方法开始，这个方法的作用是调用 `Bicycle` 的父类 `Vehicle` 的默认构造器。这样可以确保 `Bicycle` 在修改属性之前，它所继承的属性 `numberOfWheels` 能被 `Vehicle` 类初始化。在调用 `super.init()` 之后，属性 `numberOfWheels` 的原值被新值 `2` 替换。 + + +如果你创建一个 `Bicycle` 实例, 你可以调用继承的 `description` 计算型属性去查看属性 `numberOfWheels` 是否有改变: + +```swift +let bicycle = Bicycle() +print("Bicycle: \(bicycle.description)") +// Bicycle: 2 wheel(s) +``` + + + +如果子类的构造器没有在阶段 2 过程中做自定义操作，并且父类有一个同步、无参数的指定构造器，你可以在所有子类的存储属性赋值之后省略 `super.init()` 的调用。若父类的构造器是异步的，你就需要明确地写入 `await super.init()` 。 + +这个例子定义了另一个 `Vehicle` 的子类 `Hoverboard` ，只设置它的 `color` 属性。这个构造器依赖隐式调用父类的构造器来完成，而不是显式调用 `super.init()` 。 + +```swift +class Hoverboard: Vehicle { + var color: String + init(color: String) { + self.color = color + // super.init() 在这里被隐式调用 + } + override var description: String { + return "\(super.description) in a beautiful \(color)" + } +} +``` + + + +`Hoverboard` 的实例用 `Vehicle` 构造器里默认的轮子数量。 + +```swift +let hoverboard = Hoverboard(color: "silver") +print("Hoverboard: \(hoverboard.description)") +// Hoverboard: 0 wheel(s) 以美丽的银色 +``` + + + +> 注意: 子类可以在构造过程修改继承来的变量属性，但是不能修改继承来的常量属性。 + + + +### 构造器的自动继承 + +如上所述，子类在默认情况下不会继承父类的构造器。但是如果满足特定条件，父类构造器是可以 *被* 自动继承的。事实上，这意味着对于许多常见场景你不必重写父类的构造器，并且可以在安全的情况下以最小的代价继承父类的构造器。 + +假设你为子类中引入的所有新属性都提供了默认值，以下 2 个规则将适用: + +- term *规则 1**: + 如果子类没有定义任何指定构造器，它将自动继承父类所有的指定构造器。 + +- term **规则 2**: + 如果子类提供了 *所有* 父类指定构造器的实现——无论是通过规则 1 继承过来的，还是提供了自定义实现——它将自动继承父类所有的便利构造器。 + +即使你在子类中添加了更多的便利构造器，这两条规则仍然适用。 + +> 注意: 子类可以将父类的指定构造器实现为便利构造器来满足规则 2。 + + + + + +### 指定构造器和便利构造器实践 + +接下来的例子将在实践中展示指定构造器、便利构造器以及构造器的自动继承。这个例子定义了包含三个类 `Food`, `RecipeIngredient`, 以及 `ShoppingListItem` 的层级结构，并将演示它们的构造器是如何相互作用的。 + +类层次中的基类是 `Food` ，它是一个简单的用来封装食物名字的类。 `Food` 类引入了一个叫做 `name` 的 `String` 类型的属性，并且提供了两个构造器来创建 `Food` 实例: + +```swift +class Food { + var name: String + init(name: String) { + self.name = name + } + convenience init() { + self.init(name: "[Unnamed]") + } +} +``` + + + +下图展示了 `Food` 的构造链路: + +![](initializersExample01) + +类类型没有默认的逐一成员构造器，所以 `Food` 类提供了一个接受单一参数 `name` 的指定构造器。这个构造器可以使用一个特定的名字来创建新的 `Food` 实例: + +```swift +let namedMeat = Food(name: "Bacon") +// namedMeat 的名字是 "Bacon" +``` + + + +`Food` 类中的构造器 `init(name: String)` 被定义为一个 *指定* 构造器，因为它能确保 `Food` 实例的所有存储型属性都被初始化。 `Food` 类没有父类，所以 `init(name: String)` 构造器不需要调用 `super.init()` 来完成构造过程。 + +`Food` 类样提供了一个没有参数的便利构造器 `init()` 。这个 `init()` 构造器为新食物提供了一个默认的占位名字，通过横向代理到指定构造器 `init(name: String)` 并给参数 `name` 赋值为 `[Unnamed]` 来实现: + +```swift +let mysteryMeat = Food() +// mysteryMeat 的名字是 "[Unnamed]" +``` + + + +层级中的第二个类是 `Food` 的子类 `RecipeIngredient` 。 `RecipeIngredient` 类用来表示食谱中的一项原料。它引入了 `Int` 类型的属性 `quantity` (以及从 `Food` 继承过来的 `name` 属性)，并且定义了两个构造器来创建 `RecipeIngredient` 实例: + +```swift +class RecipeIngredient: Food { + var quantity: Int + init(name: String, quantity: Int) { + self.quantity = quantity + super.init(name: name) + } + override convenience init(name: String) { + self.init(name: name, quantity: 1) + } +} +``` + + + +下图中展示了 `RecipeIngredient` 类的构造器链: + +![](initializersExample02) + +`RecipeIngredient` 类拥有一个指定构造器 `init(name: String, quantity: Int)`, 它可以用来填充 `RecipeIngredient` 的所有属性值。 这个构造器一开始先将传入的 `quantity` 实参赋值给 `quantity` 属性，这个属性也是唯一在 `RecipeIngredient` 中新引入的属性。随后，构造器向上代理到父类 `Food` 的 `init(name: String)` 。这个过程满足 中的安全检查 1。 + +`RecipeIngredient` 也定义了一个便利构造器 `init(name: String)` ，它只通过 `name` 来创建 `RecipeIngredient` 的实例。这个便利构造器假设任意 `RecipeIngredient` 实例的 `quantity` 为 `1` ，所以不需要显式的质量即可创建出实例。这个便利构造器的定义可以更加方便和快捷地创建实例，并且避免了创建多个 `quantity` 为 `1` 的 `RecipeIngredient` 实例时的代码重复。这个便利构造器只是简单地横向代理到类中的指定构造器，并为 `quantity` 参数传递 `1` 。 + +`RecipeIngredient` 的便利构造器 `init(name: String)` 使用了跟 `Food` 中指定构造器 `init(name: String)` 相同的形参。由于这个便利构造器重写了父类的指定构造器 `init(name: String)` ， 因此必须在前面使用 `override` 修饰符(参见) + +尽管 `RecipeIngredient` 将父类的指定构造器重写为了便利构造器，但是它依然提供了父类的所有指定构造器的实现。因此，`RecipeIngredient` 会自动继承父类的所有便利构造器。 + +在这个示例中， `RecipeIngredient` 的父类是 `Food`,它有一个便利构造器 `init()`。这个便利构造器会被 `RecipeIngredient` 继承。这个继承版本的 `init()` 在功能上跟 `Food` 提供的版本是一样的,只是它会代理到 `RecipeIngredient` 版本的 `init(name: String)` 而不是 `Food` 提供的版本. + +所有的这三种构造器都可以用来创建新的 `RecipeIngredient` 实例: + +```swift +let oneMysteryItem = RecipeIngredient() +let oneBacon = RecipeIngredient(name: "Bacon") +let sixEggs = RecipeIngredient(name: "Eggs", quantity: 6) +``` + + + +类层级中第三个也是最后一个类 `ShoppingListItem` 是 `RecipeIngredient` 的子类。它构建了购物单中出现的某一种食谱原料。 + +购物单中的每一项总是从未购买状态开始的。为了呈现这一事实， `ShoppingListItem` 引入了一个 Boolean（布尔类型）的属性 `purchased` ，它的默认值是 `false` 。 `ShoppingListItem` 还添加了一个计算型属性 `description` ， 它提供了关于 `ShoppingListItem` 实例的一些文字描述: + +```swift +class ShoppingListItem: RecipeIngredient { + var purchased = false + var description: String { + var output = "\(quantity) x \(name)" + output += purchased ? " ✔" : " ✘" + return output + } +} +``` + + + +> 注意: `ShoppingListItem` 没有定义构造器来为 `purchased` 提供初始值，因为添加到购物单的物品的初始状态总是未购买。 + + +因为它为自己引入的所有属性都提供了默认值，并且自己没有定义任何构造器， `ShoppingListItem` 将自动继承 *所有* 父类中的指定构造器和便利构造器。 + +下图展示了这三个类的构造器链: + +![](initializersExample03) + +你可以使用三个继承来的构造器来创建 `ShoppingListItem` 的新实例: + +```swift +var breakfastList = [ + ShoppingListItem(), + ShoppingListItem(name: "Bacon"), + ShoppingListItem(name: "Eggs", quantity: 6), +] +breakfastList[0].name = "Orange juice" +breakfastList[0].purchased = true +for item in breakfastList { + print(item.description) +} +// 1 x Orange juice ✔ +// 1 x Bacon ✘ +// 6 x Eggs ✘ +``` + + + +如上所述，例子中通过字面量方式创建了一个数组 `breakfastList` ，它包含了三个 `ShoppingListItem` 实例，因此数组的类型也能被自动推导为 `[ShoppingListItem]` 。 在数组创建完之后，数组中第一个 `ShoppingListItem` 实例的名字从 `"[Unnamed]"` 更改为 `"Orange juice"` ，并标记状态为已购买。打印数组中每个元素的描述显示了它们都已按照预期被赋值。 + + + + + + + +## 可失败构造器 + +有时，定义一个构造器可失败的类，结构体或者枚举是很有用的。这里的“失败” 指的是，如给构造器传入无效的形参，或缺少某种所需的外部资源，又或是不满足某种必要的条件等。 + +为了妥善处理这种构造过程中可能会失败的情况。你可以在一个类，结构体或是枚举类型的定义中，添加一个或多个可失败构造器。其语法为在 `init` 关键字后面添加问号 (`init?`) 。 + +> 注意: 可失败构造器的参数名和参数类型，不能与其它非可失败构造器的参数名，及其参数类型相同。 + + + +可失败构造器会创建一个类型为自身类型的 *可选* 类型的对象。你通过 `return nil` 语句来表明可失败构造器在何种情况下应该 “失败”。 + +> 注意: 严格来说，构造器都不支持返回值。因为构造器本身的作用，只是为了确保对象能被正确构造。因此你只是用 `return nil` 表明可失败构造器构造失败，而不要用关键字 `return` 来表明构造成功。 + +例如，实现针对数字类型转换的可失败构造器。确保数字类型之间的转换能保持精确的值，使用这个 `init(exactly:)` 构造器。如果类型转换不能保持值不变，则这个构造器构造失败。 + +```swift +let wholeNumber: Double = 12345.0 +let pi = 3.14159 + +if let valueMaintained = Int(exactly: wholeNumber) { + print("\(wholeNumber) conversion to Int maintains value of \(valueMaintained)") +} +// 打印 "12345.0 conversion to Int maintains value of 12345" + +let valueChanged = Int(exactly: pi) +// valueChanged 是 Int? 类型，不是 Int 类型 + +if valueChanged == nil { + print("\(pi) conversion to Int doesn't maintain value") +} +// 打印 "3.14159 conversion to Int doesn't maintain value" +``` + + + +下面示例中，定义一个名为 `Animal` 的结构体，其中有一个名为 `species` 的 `String` 类型的常量属性。同时该结构体还定义了一个接受一个名为 `species` 的 `String` 类型形参的可失败构造器。这个可失败构造器检查传入的 `species` 值是否为一个空字符串。如果为空字符串，则构造失败。否则， `species` 属性被赋值，构造成功。 + + +```swift +struct Animal { + let species: String + init?(species: String) { + if species.isEmpty { return nil } + self.species = species + } +} +``` + + + +你可以通过该可失败构造器来尝试构建一个 `Animal` 的实例，并检查构造过程是否成功: + +```swift +let someCreature = Animal(species: "Giraffe") +// someCreature 的类型是 Animal?, 而不是 Animal + +if let giraffe = someCreature { + print("An animal was initialized with a species of \(giraffe.species)") +} +// 打印 "An animal was initialized with a species of Giraffe" +``` + + + +如果你给该可失败构造器传入一个空字符串到形参 `species` ，则会导致构造失败: + +```swift +let anonymousCreature = Animal(species: "") +// anonymousCreature 的类型是 Animal?, 而不是 Animal + +if anonymousCreature == nil { + print("The anonymous creature couldn't be initialized") +} +// 打印 "The anonymous creature couldn't be initialized" +``` + + + +> 注意: 检查空字符串的值 (如 `""` 而不是 `"Giraffe"`) 不等同于检查 `nil` 以表示 *可选* `String` 值的缺失。 +> 上述示例中的空字符串 (`""`) 是一个合法且非可选的 `String`. +> 然而, 对于 `Animal` 这个类来说，其 `species` 属性的值为空字符串是不合适的。 +> 为了体现这个限制，如果发现使用空字符串，构造器就会触发初始化失败的错误。 + +### 枚举类型的可失败构造器 + +你可以通过一个基于一个或多个形参的可失败构造器来获取枚举类型中特定的枚举成员。如果提供的形参无法匹配任何枚举成员，则构造失败。 + +下面的示例中，定义了一个 `TemperatureUnit` 的枚举类型。其中包含了三个可能的枚举状态 (`kelvin`, `celsius`, and `fahrenheit`)，以及一个根据表示温度单位的 `Character` 值找出合适的枚举成员的可失败构造器: + +```swift +enum TemperatureUnit { + case kelvin, celsius, fahrenheit + init?(symbol: Character) { + switch symbol { + case "K": + self = .kelvin + case "C": + self = .celsius + case "F": + self = .fahrenheit + default: + return nil + } + } +} +``` + + + +你可以利用该可失败构造器在三个枚举成员中选择合适的枚举成员，当形参不能和任何枚举成员相匹配时，则构造失败： + +```swift +let fahrenheitUnit = TemperatureUnit(symbol: "F") +if fahrenheitUnit != nil { + print("This is a defined temperature unit, so initialization succeeded.") +} +// 打印 "This is a defined temperature unit, so initialization succeeded." + +let unknownUnit = TemperatureUnit(symbol: "X") +if unknownUnit == nil { + print("This isn't a defined temperature unit, so initialization failed.") +} +// 打印 "This isn't a defined temperature unit, so initialization failed." +``` + + + +### 带原始值的枚举类型的可失败构造器 + +带原始值的枚举类型会自带一个可失败构造器 `init?(rawValue:)` ，该可失败构造器有一个合适的原始值类型的 `rawValue` 形参，选择找到的相匹配的枚举成员，找不到则构造失败。 + +因此 `TemperatureUnit` 的例子可以用原始值类型的 `Character` 和进阶的 `init?(rawValue:)` 构造器重写为: + +```swift +enum TemperatureUnit: Character { + case kelvin = "K", celsius = "C", fahrenheit = "F" +} + +let fahrenheitUnit = TemperatureUnit(rawValue: "F") +if fahrenheitUnit != nil { + print("This is a defined temperature unit, so initialization succeeded.") +} +// 打印 "This is a defined temperature unit, so initialization succeeded." + +let unknownUnit = TemperatureUnit(rawValue: "X") +if unknownUnit == nil { + print("This isn't a defined temperature unit, so initialization failed.") +} +// 打印 "This isn't a defined temperature unit, so initialization failed." +``` + + + +### 构造失败的传递 + +类、结构体、枚举的可失败构造器可以横向代理到它们自己其他的可失败构造器。类似的，子类的可失败构造器也能向上代理到父类的可失败构造器。 + +无论是向上代理还是横向代理，如果你代理到的其他可失败构造器触发构造失败，整个构造过程将立即终止，接下来的任何构造代码不会再被执行。 + + + + + + + +> 注意: 可失败构造器也可以代理到其它的不可失败构造器。通过这种方式，你可以增加一个可能的失败状态到现有的构造过程中。 + +下面这个示例中定义了一个 `Product` 的子类 `CartItem` 。 `CartItem` 类建立了一个在线购物车中的物品的模型，它有一个名为 `quantity` 的常量存储型属性，并确保给属性的值至少为 `1` : + + +```swift +class Product { + let name: String + init?(name: String) { + if name.isEmpty { return nil } + self.name = name + } +} + +class CartItem: Product { + let quantity: Int + init?(name: String, quantity: Int) { + if quantity < 1 { return nil } + self.quantity = quantity + super.init(name: name) + } +} +``` + + + +`CartItem` 可失败构造器首先验证接收的 `quantity` 值是否大于等于1。倘若 `quantity` 值无效，则立即终止整个构造过程，返回失败结果，且不再执行余下代码。同样地， `Product` 的可失败构造器首先检查 `name` 值， 如果 `name` 值为空字符串，则构造器立即执行失败。 + +如果你通过传入一个非空的 `name` 以及 大于等于 `1` 的 `quantity` 来创建一个 `CartItem` 实例，构建方法将成功执行: + +```swift +if let twoSocks = CartItem(name: "sock", quantity: 2) { + print("Item: \(twoSocks.name), quantity: \(twoSocks.quantity)") +} +// 打印 "Item: sock, quantity: 2" +``` + + + +如果你用值为 `0` 的 `quantity` 来创建一个 `CartItem` 实例，那么将导致 `CartItem` 的构造器失败: + +```swift +if let zeroShirts = CartItem(name: "shirt", quantity: 0) { + print("Item: \(zeroShirts.name), quantity: \(zeroShirts.quantity)") +} else { + print("Unable to initialize zero shirts") +} +// 打印 "Unable to initialize zero shirts" +``` + + + +同样地，如果你尝试传入一个值为空字符串的 `name` 来创建一个 `CartItem` 实例，那么将导致父类 `Product` 的构造过程失败: + +```swift +if let oneUnnamed = CartItem(name: "", quantity: 1) { + print("Item: \(oneUnnamed.name), quantity: \(oneUnnamed.quantity)") +} else { + print("Unable to initialize one unnamed product") +} +// 打印 "Unable to initialize one unnamed product" +``` + + + +### 重写一个可失败构造器 + +如同其它的构造器，你可以在子类中重写父类的可失败构造器。或者你也可以用子类的非可失败构造器重写一个父类的可失败构造器。这使你可以定义一个不会构造失败的子类，即使父类的构造器允许构造失败。 + +注意，当你用子类的非可失败构造器重写父类的可失败构造器时，向上代理到父类的可失败构造器的唯一方式是对父类的可失败构造器的返回值进行强制解包。 + +> 注意: 你可以用非可失败构造器重写可失败构造器，但反过来却不行。 + + + +下面的示例定义了 `Document` 类。这个类模拟一个文档并可以用 `name` 属性来构造，属性的值必须为一个非空的字符串或 `nil` ，但不能是一个空字符串: + +```swift +class Document { + var name: String? + // 该构造器创建了一个 name 属性的值为 nil 的 document 实例 + init() {} + // 该构造器创建了一个 name 属性的值为非空字符串的 document 实例 + init?(name: String) { + if name.isEmpty { return nil } + self.name = name + } +} +``` + + + +下一个示例中定义了一个 `Document` 的子类 `AutomaticallyNamedDocument` 。 `AutomaticallyNamedDocument` 子类重写了所有父类引入的指定构造器。这些重写确保了 `AutomaticallyNamedDocument` 在没有 `name` 值时传递空字符串给 `init(name:)` 构造器，具有初始 `name` 值 `"[Untitled]"` 。 + +```swift +class AutomaticallyNamedDocument: Document { + override init() { + super.init() + self.name = "[Untitled]" + } + override init(name: String) { + super.init() + if name.isEmpty { + self.name = "[Untitled]" + } else { + self.name = name + } + } +} +``` + + + +`AutomaticallyNamedDocument` 用一个不可失败构造器 `init?(name:)` 重写了父类的可失败构造器。因为子类用另一种方式处理了空字符串的情况，所以不再需要一个可失败构造器，因此子类用一个不可失败构造器代替了父类的可失败构造器。 + +你可以在子类的不可失败构造器中使用强制解包来调用父类的可失败构造器。比如，下面的 `UntitledDocument` 子类总是被命名为 `"[Untitled]"` 并且在初始化阶段使用父类的可失败构造器 `init(name:)` 。 + +```swift +class UntitledDocument: Document { + override init() { + super.init(name: "[Untitled]")! + } +} +``` + + + +在这个例子中，如果在调用父类的可失败构造器 `init(name:)` 时传入的是字符串，那么强制解包操作会引发运行时错误。不过，因为这里是通过字符串常量来调用它，构造器不会失败，所以在这个例子中并不会发生运行时错误。 + +### init! 可失败构造器 + +通常来说我们通过在 `init` 关键字后添加问号的方式 (`init?`) 来定义一个可失败的构造器。但你也可以通过在 `init` 后面添加感叹号的方式来定义一个可失败构造器 (`init!`) ，该可失败构造器将会构建一个对应类型的隐式解包可选类型的对象。 + +你可以在 `init?` 中代理到 `init!` ，反之亦然。你也可以用 `init?` 重写 `init!` ，反之亦然。你还可以用 `init` 代理到 `init!` ，不过，一旦 `init!` 构造失败，则会触发断言。 + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +## 必要构造器 + +在类的构造器前添加 `required` 修饰符表明所有该类的子类都必须实现该构造器: + +```swift +class SomeClass { + required init() { + // 构造器的实现代码 + } +} +``` + + + + + + + +在子类重写父类的必要构造器时，必须在子类的构造器前也添加 `required` 修饰符，表明该构造器要求也应用于继承链后面的子类。在重写父类中必要的指定构造器时，不需要添加 `override` 修饰符: + + +```swift +class SomeSubclass: SomeClass { + required init() { + // 子类的必要构造器实现 + } +} +``` + + + + + +> 注意: 如果子类继承的构造器能满足必要构造器的要求，则无须在子类中显式提供必要构造器的实现。 + + + + + + + +## 通过闭包或函数设置属性的默认值 + +如果某个存储型属性的默认值需要一些自定义或设置，你可以使用闭包或全局函数为其提供定制的默认值。每当某个属性所在类型的新实例被构造时，对应的闭包或函数会被调用，而它们的返回值会当做默认值赋值给这个属性。 + +这种类型的闭包或函数通常会创建一个跟属性类型相同的临时变量，然后修改它的值以满足预期的初始状态，最后返回这个临时变量，作为属性的默认值。 + +下面模板介绍了如何用闭包为属性提供默认值： + +```swift +class SomeClass { + let someProperty: SomeType = { + // 在这个闭包中给 someProperty 创建一个默认值 + // someValue 必须和 SomeType 类型相同 + return someValue + }() +} +``` + + + +注意闭包结尾的花括号后面接了一对空的小括号。这用来告诉 Swift 立即执行此闭包。如果你忽略了这对括号，相当于将闭包本身作为值赋值给了属性，而不是将闭包的返回值赋值给属性。 + + + +> 注意: 如果你使用闭包来初始化属性，请记住在闭包执行时，实例的其它部分都还没有初始化。这意味着你不能在闭包里访问其它属性，即使这些属性有默认值。同样，你也不能使用隐式的 `self` 属性，或者调用任何实例方法。 + +下面的示例中定义了一个 `Chessboard` 结构体，它构建了西洋跳棋游戏的棋盘。西洋跳棋游戏在一副黑白格交替的 8 x 8 的棋盘中进行的： + +![](chessBoard) + +为了呈现这副游戏棋盘， `Chessboard` 结构体定义了一个属性 `boardColors` ，它是一个包含64个 `Bool` 值的数组。数组中值为 `true` 表示为一个黑格，值为 `false` 表示为一个白格。数组中第一个元素代表棋盘上左上角的格子，最后一个元素代表棋盘上右下角的格子。 . + +`boardColors` 数组是同一个闭包初始化并设置颜色值的： array is initialized with a closure to set up its color values: + +```swift +struct Chessboard { + let boardColors: [Bool] = { + var temporaryBoard: [Bool] = [] + var isBlack = false + for i in 1...8 { + for j in 1...8 { + temporaryBoard.append(isBlack) + isBlack = !isBlack + } + isBlack = !isBlack + } + return temporaryBoard + }() + func squareIsBlackAt(row: Int, column: Int) -> Bool { + return boardColors[(row * 8) + column] + } +} +``` + + + +每当一个新的 `Chessboard` 实例被创建时，闭包就会被执行， `boardColors` 的默认值会被计算出来并返回。上面的示例中的闭包会计算出棋盘中每个格子对应的颜色，并将这些值保存到一个临时数组 `temporaryBoard` 中，最后在构建完成时将此数组作为闭包返回值返回。这个返回的数组会保存到 `boardColors` 中，并可以通过 `squareIsBlackAt(row:column:)` 工具函数来查询: + +```swift +let board = Chessboard() +print(board.squareIsBlackAt(row: 0, column: 1)) +// 打印 "true" +print(board.squareIsBlackAt(row: 7, column: 7)) +// 打印 "false" +``` + + + + diff --git a/swift-6.docc/LanguageGuide/Macros.md b/swift-6.docc/LanguageGuide/Macros.md new file mode 100644 index 000000000..96431d2ae --- /dev/null +++ b/swift-6.docc/LanguageGuide/Macros.md @@ -0,0 +1,510 @@ +# 宏 + +在编译时使用宏来生成代码。 + +宏会在编译你的源代码时对其进行转换，从而让你避免手动编写重复的代码。在编译过程中，Swift 会先展开代码中的所有宏，然后再像往常一样构建代码。 + +![一个宏展开示意图。左侧是 Swift 代码的风格化表示。右侧是由宏添加了几行的相同的代码。](macro-expansion) + +宏展开始终是一种加法操作：宏会添加新代码，但绝不会删除或修改现有代码。 + +每个宏的输入和宏展开的输出都会被检查，以确保它们是语法上有效的 Swift 代码。同样，你传给宏的值以及宏生成的代码中的值也会被检查，以确保它们具有正确的类型。此外，如果宏的实现在展开宏时遇到错误，编译器会将其视为编译错误。这些保证让使用了宏的代码更容易被推导，也让人更容易发现诸如宏使用不当或宏实现有错误这样的问题。 + +Swift 有两种宏: + +- *独立宏（Freestanding macros）* 可单独出现，无需被附加到任何声明中。 + +- *附加宏（Attached macros）* 会修改它被附加到的声明。 + +附加宏和独立宏的调用方式略有不同，但它们都遵循相同的宏展开模型，并都使用相同的方法来实现。下面的章节将更详细地描述这两种宏。 + +## 独立宏 + +要调用独立宏，需要在其名称前写入井号 (`#`)，并在名称后的括号中写入宏的参数。例如： + +```swift +func myFunction() { + print("Currently running \(#function)") + #warning("Something's wrong") +} +``` + +在函数内的第一行中，`#function` 调用了 Swift 标准库中的 [`function()`][] 宏。当你编译此代码时，Swift 会调用该宏的实现，将 `#function` 替换为当前函数的名称。当你运行这段代码并调用 `myFunction()` 时，它会打印 “Currently running myFunction()”。在第二行中，`#warning` 调用了 Swift 标准库中的 [`warning(_:)`][] 宏，来生成一个自定义的编译时警告。 + +[`function()`]: https://developer.apple.com/documentation/swift/function() +[`warning(_:)`]: https://developer.apple.com/documentation/swift/warning(_:) + +独立宏可以像 `#function` 所做的那样产出一个值，也可以像 `#warning` 所做的那样在编译时执行一个操作。 + + +## 附加宏 + +要调用附加宏，需要在其名称前写入 at 符号 (`@`) ，并在名称后的括号中写入宏的参数。 + +附加宏会修改它们所附加到的声明。它们为被附加到的声明添加代码，比如定义一个新的方法或者增加对某个协议的遵循。 + +例如，请看下面这段不使用宏的代码： + +```swift +struct SundaeToppings: OptionSet { + let rawValue: Int + static let nuts = SundaeToppings(rawValue: 1 << 0) + static let cherry = SundaeToppings(rawValue: 1 << 1) + static let fudge = SundaeToppings(rawValue: 1 << 2) +} +``` + +在这段代码中，`SundaeToppings` 选项集中的每个选项都包括对构造器的调用，这是重复的手动操作。这样的实现方式在添加新选项时容易出错，比如在行尾键入错误的数字。 + +下面是该代码使用宏后的替代版本： + +```swift +@OptionSet +struct SundaeToppings { + private enum Options: Int { + case nuts + case cherry + case fudge + } +} +``` + +此版本的 `SundaeToppings` 调用了 `@OptionSet` 宏。这个宏会读取私有枚举类中的枚举值列表，并为其中的每个值生成常量列表，同时也会为结构体增加对 [`OptionSet`][] 协议的遵循。 + +[`OptionSet`]: https://developer.apple.com/documentation/swift/optionset + + + + +作为对比，`@OptionSet` 宏展开后是下面这样。这段代码不是由你自己编写的，只有当你特别要求 Swift 展示宏的展开时，你才会看到它。 + +```swift +struct SundaeToppings { + private enum Options: Int { + case nuts + case cherry + case fudge + } + + typealias RawValue = Int + var rawValue: RawValue + init() { self.rawValue = 0 } + init(rawValue: RawValue) { self.rawValue = rawValue } + static let nuts: Self = Self(rawValue: 1 << Options.nuts.rawValue) + static let cherry: Self = Self(rawValue: 1 << Options.cherry.rawValue) + static let fudge: Self = Self(rawValue: 1 << Options.fudge.rawValue) +} +extension SundaeToppings: OptionSet { } +``` + +这个结构体中的私有枚举类之后的所有代码都来自于 `@OptionSet` 宏。使用宏生成所有静态变量的 `SundaeToppings` 版本比前面手动编码的版本更易于阅读和维护。 + +## 宏的声明 + +在大多数 Swift 代码中，当你实现某个符号（如函数或类型）时，不需要单独额外的声明。但是，宏的声明和实现是分开的。宏的声明包含其名称、所需的参数、可以被使用的位置以及它可以生成怎样的代码。宏的实现则包含通过生成 Swift 代码来展开这个宏所需的代码。 + +你可以使用 `macro` 关键字引入一个宏的声明。例如，下面是前面例子中使用到的 `@OptionSet` 宏的声明的一部分： + +```swift +public macro OptionSet() = + #externalMacro(module: "SwiftMacros", type: "OptionSetMacro") +``` + +第一行指定了宏的名称和它的参数 —— 名称是 `OptionSet`，并且不带任何参数。第二行使用 Swift 标准库中的 [`externalMacro(module:type:)`][] 宏来告诉 Swift 这个宏的实现在哪里。在这个例子中，`SwiftMacros` 模块包含一个名为 `OptionSetMacro` 并实现了 `@OptionSet` 宏的类型。 + +[`externalMacro(module:type:)`]: https://developer.apple.com/documentation/swift/externalmacro(module:type:) + +因为 `OptionSet` 是一个附加宏，它的名称使用大驼峰式命名法，就像结构体和类的名称那样。独立宏的名称使用小驼峰式命名法，就像变量和函数的名称那样。 + +> 注意: +> 宏的可访问性总是被声明为 `public` 的。 +> 由于声明宏的代码与使用宏的代码位于不同的模块中，因此没有任何地方可以应用一个非公开可访问的宏。 + +宏的声明定义了宏的*角色* —— 包括宏在源代码中可以被调用的位置以及宏可以生成的代码种类。每个宏都有一个或多个角色，作为属性的一部分写在宏声明的开头。下面是 `@OptionSet` 的更完整的声明，包括了指定它的角色的属性： + +```swift +@attached(member) +@attached(extension, conformances: OptionSet) +public macro OptionSet() = + #externalMacro(module: "SwiftMacros", type: "OptionSetMacro") +``` + +`@attached` 属性在此声明中出现了两次，每个宏角色各用了一次。第一次使用时，`@attached(member)` 表示这个宏会向被作用到的类型添加新的成员。按 `OptionSet` 协议以及一些附加成员的要求，`@OptionSet` 宏添加了一个 `init(rawValue:)` 构造器。第二次使用时，`@attached(extension, conformances: OptionSet)` 声明了 `@OptionSet` 会添加对 `OptionSet` 协议的遵循。`@OptionSet` 宏会扩展被作用到的类型，使其遵循 `OptionSet` 协议。 + +对于独立宏，你可以编写 `@freestanding` 属性来指定其角色： + +``` +@freestanding(expression) +public macro line() -> T = + /* ... 宏实现的位置 ... */ +``` + + + +上面的 `#line` 宏具有 `expression`（表达式）的角色。表达式宏可以产生一个值，或者执行一个编译时操作，比如生成一个警告。 + +除了宏的角色外，宏的声明还提供了有关这个宏所生成的符号名称的信息。当宏的声明提供了一个名称列表时，它保证只生成使用这些名称的声明，这有助于理解和调试生成的代码。下面是 `@OptionSet` 的完整声明： + +```swift +@attached(member, names: named(RawValue), named(rawValue), + named(`init`), arbitrary) +@attached(extension, conformances: OptionSet) +public macro OptionSet() = + #externalMacro(module: "SwiftMacros", type: "OptionSetMacro") +``` + +在上面的声明中，`@attached(member)` 宏在 `names:` 标签后为 `@OptionSet` 宏所生成的每个符号添加了参数。这个宏声明了名为 `RawValue`, `rawValue` 和 `init` 的符号 —— 因为这些名称是预先知道的，宏的声明明确列出了它们。 + +这个宏声明还在名称列表后添加了 `arbitrary`，这将允许宏生成一些在使用该宏之前未知名称的声明。例如，当 `@OptionSet` 宏被应用于上述的 `SundaeToppings` 结构体时，它将生成与枚举类成员 `nuts`, `cherry` 和 `fudge` 相对应的类型属性。 + +要了解更多信息，包括宏角色的完整列表，请参阅 中的 和 。 + +## 宏的展开 + +在构建使用了宏的 Swift 代码时，编译器会调用宏的实现来展开它们。 + +![显示宏展开的四个步骤的图表。输入是 Swift 源代码。源代码变成了一棵树，代表代码的结构。宏的实现向这棵树添加了新的分支。结果是带有添加过代码的 Swift 源代码。](macro-expansion-full) + +具体来说，Swift 会以以下方式展开宏： + +1. 编译器读取代码，创建语法的内存表示。 + +2. 编译器将部分内存表示发送给宏的实现，宏将在此基础上展开。 + +3. 编译器将宏的调用替换为它的展开形式。 + +4. 编译器使用展开后的源代码继续进行编译。 + +为了阐述具体的步骤，用以下代码来举例： + +``` +let magicNumber = #fourCharacterCode("ABCD") +``` + +`#fourCharacterCode` 宏接受一个长度为四个字符的字符串作为输入，并返回一个无符号的 32 位整数，该整数对应于组成字符串的字符的 ASCII 码值的组合。一些文件格式使用这样的整数来标识数据，因为它们紧凑且在调试器中仍然可读。下面的 的部分展示了如何实现这个宏。 + +为了展开上述代码中的宏，编译器读取 Swift 文件并创建该代码的内存表示，也就是*抽象语法树*（AST）。AST 使得代码的结构变得清晰，也使得编写与该结构进行交互的代码变得更容易 —— 例如编写编译器或宏的实现，都需要与 AST 进行交互。以下是上述代码的 AST 表示，略微简化，省略了一些额外的细节： + +![一个树状图，以常量作为根节点。该常量有一个名为 magicNumber 的名称和一个值。该常量的值是一个宏调用。这个宏调用有一个名为 fourCharacterCode 的名称和它的参数。参数是一个值为 ABCD 的字符串字面量。](macro-ast-original) + +上面的图展示了该代码的结构是如何在内存中表示的。AST 中的每个节点对应源代码的一部分。AST 的 “Constant declaration（常量声明）”节点下有两个子节点，分别表示常量声明的两个部分：它的名称和它的值。“Macro call（宏调用）”节点则有表示宏的名称和传递给宏的参数列表的子节点。 + +作为构建这个 AST 的一部分，编译器会检查源代码是否是有效的 Swift 代码。例如，`#fourCharacterCode` 只接受一个参数，且该参数必须是一个字符串。如果你尝试传递一个整数参数，或者在字符串字面量的末尾忘记了引号 (`"`)，你会在这个过程中的这个点上获得一个错误。 + +编译器会找到代码中调用宏的地方，并加载实现这些宏的外部二进制文件。对于每个宏调用，编译器将抽象语法树（AST）的一部分传递给该宏的实现。以下是这个部分 AST 的表示： + +![一个树状图，以一个宏调用（Macro call）作为根节点。这个宏调用有一个名为 fourCharacterCode 的名称和参数。这个参数是一个值为 ABCD 的字符串字面量。](macro-ast-input) + +`#fourCharacterCode` 宏的实现会在展开这个宏时读取这个部分 AST 作为输入。宏的实现仅对其接收到的部分 AST 进行操作，这意味着无论这个宏的前后代码是什么，它的展开方式始终不变。这一限制有助于使宏展开更易于理解，并帮助你的代码能更快得到构建，因为 Swift 可以不必展开那些未变更过的宏。 + +Swift 能通过限制实现宏的代码，帮助宏的作者避免意外读取其他输入： + +- 传递给宏实现的抽象语法树（AST）仅包含表示该宏的 AST 节点，而不包括其前后的任何代码。 + +- 宏的实现运行在一个沙盒环境中，这可以防止其访问文件系统或网络。 + +除了这些保护措施，宏的作者有责任不读取或修改宏输入以外的任何内容。例如，宏的展开不得依赖于当前的时间。 + +`#fourCharacterCode` 的实现会生成了一个包含展开后代码的新 AST。以下是上述代码会返回给编译器的内容： + +![一个具有 UInt32 类型的整型字面量 1145258561 的树形图。](macro-ast-output) + +当编译器接收到这个展开结果时，它用包含了这个宏展开结果的 AST 节点替换掉包含了宏调用的 AST 节点。在宏展开后，编译器会再次检查以确保程序仍然是语法上有效的 Swift 代码，并且所有的类型都是正确的。这会生成一个可以像往常一样编译的最终 AST： + +![一个树状图，以常量作为根节点。该常量有一个名为 magicNumber 的名称和一个值。该常量的值是 UInt32 类型的整型字面量 1145258561。](macro-ast-result) + +这个 AST 对应于如下的 Swift 代码： + +``` +let magicNumber = 1145258561 as UInt32 +``` + +在这个例子中，作为输入的源代码只有一个宏，但一个真实的程序可能有某个相同宏的多个实例以及对不同宏的多个调用。编译器会一次展开一个宏。 + +如果一个宏出现在另一个宏的内部，则先展开外部宏 —— 这使得外部宏可以在自己被展开之前修改它的内部宏。 + + + +## 实现一个宏 + +要实现一个宏，你需要两个组件：一个是执行这个宏展开的类型，另一个是用来声明这个宏并将其暴露为 API 的库。这些部分与使用这个宏的代码分开构建，即使这个宏和它的使用端是一起开发的也是如此，因为这个宏的实现是作为构建这个宏的使用端的一部分而运行的。 + +要使用 Swift 包管理器来创建新的宏，请运行 `swift package init --type macro` —— 这会创建几个文件，包括一个宏的实现和声明的模板。 + +要在现有项目中添加宏，请按如下方式编辑 `Package.swift` 文件的开头： + +- 在 `swift-tools-version` 注释中设置 Swift 工具版本为 5.9 或更高版本。 +- 导入 `CompilerPluginSupport` 模块。 +- 在 `platforms` 列表中将 macOS 10.15 作为最低部署目标。 + +下面的代码展示了作为示例的 `Package.swift` 文件的开头。 + +```swift +// swift-tools-version: 5.9 + +import PackageDescription +import CompilerPluginSupport + +let package = Package( + name: "MyPackage", + platforms: [ .iOS(.v17), .macOS(.v13)], + // ... +) +``` + +接下来，在现有的 `Package.swift` 文件中为宏的实现和宏的声明所在的库分别添加一个构建目标。例如，你可以添加类似于下面这样的内容，注意更改名称以匹配你的项目： + +```swift +targets: [ + // 执行源代码转换的宏的实现。 + .macro( + name: "MyProjectMacros", + dependencies: [ + .product(name: "SwiftSyntaxMacros", package: "swift-syntax"), + .product(name: "SwiftCompilerPlugin", package: "swift-syntax") + ] + ), + + // 暴露宏作为它的 API 的一部分的库。 + .target(name: "MyProject", dependencies: ["MyProjectMacros"]), +] +``` + +上面的代码定义了两个构建目标：`MyProjectMacros` 包含宏的实现，而 `MyProject` 则让这些宏变得可被使用。 + +宏的实现使用 [SwiftSyntax][] 模块，通过 AST 以结构化的方式与 Swift 代码进行交互。如果你使用 Swift 包管理器创建了一个新的宏包，生成的 `Package.swift` 文件将自动包含对 SwiftSyntax 的依赖关系。如果你要在现有项目中添加宏，请自行在 `Package.swift` 文件中添加对 SwiftSyntax 的依赖： + +[SwiftSyntax]: http://github.com/apple/swift-syntax/ + +```swift +dependencies: [ + .package(url: "https://github.com/apple/swift-syntax", from: "509.0.0") +], +``` + +根据宏的角色，宏的实现需要遵守 SwiftSyntax 中的相应协议。例如，对于上一节中的 `#fourCharacterCode`，下面是一个实现该宏的结构： + +```swift +import SwiftSyntax +import SwiftSyntaxMacros + +public struct FourCharacterCode: ExpressionMacro { + public static func expansion( + of node: some FreestandingMacroExpansionSyntax, + in context: some MacroExpansionContext + ) throws -> ExprSyntax { + guard let argument = node.argumentList.first?.expression, + let segments = argument.as(StringLiteralExprSyntax.self)?.segments, + segments.count == 1, + case .stringSegment(let literalSegment)? = segments.first + else { + throw CustomError.message("Need a static string") + } + + let string = literalSegment.content.text + guard let result = fourCharacterCode(for: string) else { + throw CustomError.message("Invalid four-character code") + } + + return "\(raw: result) as UInt32" + } +} + +private func fourCharacterCode(for characters: String) -> UInt32? { + guard characters.count == 4 else { return nil } + + var result: UInt32 = 0 + for character in characters { + result = result << 8 + guard let asciiValue = character.asciiValue else { return nil } + result += UInt32(asciiValue) + } + return result +} +enum CustomError: Error { case message(String) } +``` + +如果要将此宏添加到现有的使用 Swift 包管理的项目中，请添加一个类型作为宏的构建目标的入口点，并列出构建目标定义的宏： + +```swift +import SwiftCompilerPlugin + +@main +struct MyProjectMacros: CompilerPlugin { + var providingMacros: [Macro.Type] = [FourCharacterCode.self] +} +``` + +`#fourCharacterCode` 宏是一个产出一个表达式的独立宏，因此，实现它的 `FourCharacterCode` 类型需遵循 `ExpressionMacro` 协议。`ExpressionMacro` 协议有一个要求，即有一个 `expansion(of:in:)` 方法来展开 AST。有关宏的角色列表及其相应的 SwiftSyntax 协议，请参阅 中的 和 。 + +要展开 `#fourCharacterCode` 宏，Swift 会将使用了此宏的代码的 AST 发送给包含该宏的实现的库。在这个库的内部，Swift 会调用 `FourCharacterCode.expansion(of:in:)` 方法，并将 AST 和上下文作为参数传递给该方法。`expansion(of:in:)` 的实现会找到作为参数传递给 `#fourCharacterCode` 的字符串，并计算出相对应的 32 位无符号整型字面量的值。 + +在上面的示例中，第一个 `guard` 块从 AST 中提取出字符串字面量，并将该 AST 节点赋值给 `literalSegment`。第二个 `guard` 块调用私有 `fourCharacterCode(for:)` 函数。如果宏使用不当，这两个代码块都可能会抛出错误 —— 错误信息会在被不当调用的位置作为编译器错误抛出。例如，如果你尝试以 `#fourCharacterCode("AB" + "CD")` 的方式来调用该宏，编译器会显示错误信息 "Need a static string"（“需要一个静态字符串”）。 + +`expansion(of:in:)` 方法返回了一个 `ExprSyntax` 的实例，`ExprSyntax` 是 SwiftSyntax 中的一种用于表示 AST 中的表达式的类型。由于此类型遵循 `StringLiteralConvertible` 协议，作为一种轻量级的语法，这个宏的实现就使用了一个简单字符串字面量来创建其结果。所有从宏实现中返回的 SwiftSyntax 类型都遵循 `StringLiteralConvertible` 协议，因此你也可以在实现任何宏时使用这种方法。 + + + + + + + + +## 开发和调试宏 + +宏非常适合使用测试驱动的方式进行开发：宏可以将一个 AST 转换成另一个 AST，而无需依赖任何外部状态，也无需更改任何外部状态。此外，你还可以用字符串字面量创建语法节点，从而简化了测试输入的设置。你还可以读取 AST 的 `description` 属性来获取一个用来与预期值进行比较的字符串。例如，下面是对前面章节中的 `#fourCharacterCode` 宏的一个测试： + +```swift +let source: SourceFileSyntax = + """ + let abcd = #fourCharacterCode("ABCD") + """ + +let file = BasicMacroExpansionContext.KnownSourceFile( + moduleName: "MyModule", + fullFilePath: "test.swift" +) + +let context = BasicMacroExpansionContext(sourceFiles: [source: file]) + +let transformedSF = source.expand( + macros:["fourCharacterCode": FourCharacterCode.self], + in: context +) + +let expectedDescription = + """ + let abcd = 1145258561 as UInt32 + """ + +precondition(transformedSF.description == expectedDescription) +``` + +上面的示例使用了一个 precondition 来测试宏，但你也可以使用测试框架来代替它。 + + + + diff --git a/swift-6.docc/LanguageGuide/MemorySafety.md b/swift-6.docc/LanguageGuide/MemorySafety.md new file mode 100644 index 000000000..15a7dd71d --- /dev/null +++ b/swift-6.docc/LanguageGuide/MemorySafety.md @@ -0,0 +1,535 @@ +# 内存安全 + +通过合理地组织代码来避免内存访问冲突。 + +默认情况下，Swift 会阻止代码中不安全的行为。例如，Swift 会确保所有变量都在使用前被初始化、内存不可在被释放后访问、还会对数组的下标做越界检查。 + +Swift 会要求修改内存的代码拥有对被修改区域的独占访问权，以确保对同一块内存区域的多次访问不会产生冲突。由于 Swift 会自动管理内存，大多数情况下你不需要考虑内存访问相关的问题。但是，为了避免编写出会产生内存访问冲突的代码，你还是很有必要了解什么情况下会出现这种问题。如果你的代码有内存访问冲突的问题，系统会在编译时或运行时报错。 + + + +## 理解内存访问冲突 + +在进行变量赋值、函数传参这样的操作时，你的代码会访问内存。举个例子，下面的代码包含了一次读操作和一次写操作： + +```swift +// 向 one 所在的内存区域发起一次写操作 +var one = 1 + +// 向 one 所在的内存区域发起一次读操作 +print("We're number \(one)!") +``` + + + + + +当多个不同地方的代码试图访问同一块内存区域时，访问冲突就有可能出现。对一块内存区域的同时访问可能导致程序出现无法预测或不稳定的行为。Swift 中有许多修改数值的方式，其中一些会横跨多行代码，这意味着修改某个数值的过程本身也有可能产生对此数值的访问。 + +要理解这个问题，你可以尝试想象一下在纸上更新一个预算表的流程。更新预算表分为两步：第一步你需要先添加每个预算项目的名字和数额，第二步才是更新预算总额。在整个更新流程的之前及之后，你可以从预算中读取任何信息，而这些信息都是正确的，就像下图所示一样。 + +![](memory_shopping) + +但是，在你向预算表添加项目的过程中，它会短暂地处于一个临时、不合法的状态，因为预算总额此时还没有被更新以反映这些新添加的项目。在项目添加的过程中读取到的总额是不正确的。 + +这个例子还展示了一个在你修复内存访问冲突问题中会遇到的挑战：有时可能存在多种修复冲突的方式，但它们最终产生的结果并不相同，且很难确定到底哪种结果是符合预期的。在这个例子中，$5 或者 $320 都可以是正确答案 —— 这取决于你的「预期」是读取到更新前的总额还是更新后的总额。在你修复访问冲突之前，你需要先明确内存访问的目的和预期结果是什么。 + +> 注意：如果你编写过并发或多线程代码，访问冲突问题可能对你来说并不陌生。但是，这里讨论的访问冲突即便在单线程环境中也有可能产生，而此时并没有并发或多线程的代码存在。 +> +> 如果你的程序在单线程上运行时产生了内存访问冲突问题，Swift 保证会抛出编译时或运行时错误。对于多线程代码，你可以使用 [Thread Sanitizer](https://developer.apple.com/documentation/xcode/diagnosing_memory_thread_and_crash_issues_early) 来检测不同线程间的访问冲突。 + + + +### 内存访问的特点 + +在访问冲突的语境下，我们需要考虑内存访问的三个特点：此次访问是读还是写、访问的时长、被访问内存区域的位置。特别地，两次内存访问会在满足以下所有条件时产生冲突： + +- 它们不是都是读操作，也不是都是原子化操作。 +- 它们访问的是同一块内存区域。 +- 它们的时间窗口出现了重叠。 + +读操作和写操作之间的区别是显而易见的：写操作会改变内存区域，而读操作不会。「内存区域」指的是被访问的内容（例如变量、常量、属性）。内存访问的要么瞬间完成，要么持续较长时间。 + +一次访问如果满足以下条件之一则为*原子*访问: +- 是对 [`Atomic`] 或 [`AtomicLazyReference`] 的原子操作调用 +- 仅使用 C 原子操作 +否则就是非原子访问。 +关于 C 原子函数的列表,请参阅 `stdatomic(3)` 手册页。 + +[`Atomic`]: https://developer.apple.com/documentation/synchronization/atomic +[`AtomicLazyReference`]: https://developer.apple.com/documentation/synchronization/atomiclazyreference + + + +如果在一次内存访问的过程中没有任何其他代码可以在其开始后、结束前运行，则这次访问是**瞬时**完成的。其性质决定了两次瞬时访问不可能同时发生。大多数内存访问都是瞬时完成的。比如，下面这段代码中的所有读写操作都是瞬时完成的： + +```swift +func oneMore(than number: Int) -> Int { + return number + 1 +} + +var myNumber = 1 +myNumber = oneMore(than: myNumber) +print(myNumber) +// Prints "2" +``` + + + +然而，也有其他被称作**长时访问**的内存访问 —— 它们的执行过程会「横跨」其它代码的执行。长时访问和瞬时访问的区别在于：前者执行开始后、结束前的这段时间内，其它的代码有可能会执行，我们称之为**重叠**。一次长时访问可以与其它的长时或瞬时访问重叠。 + +重叠访问通常出现在函数和方法的 in-out 参数以及结构体的变值方法中。下文中会讨论 Swift 中具体哪些类型的代码会使用长时访问。 + + + +## 对 In-Out 参数的访问冲突 + +一个函数会对它所有的 in-out 参数保持长时写访问。in-out 参数的写访问会在所有非 in-out 参数处理完之后开始，直到函数执行完毕为止。如果存在多个 in-out 参数，则写访问的开始顺序和参数的排列顺序一致。 + +这种长时保持的写访问带来的问题是：即便作用域和访问权限规则允许，你也不能再访问以 in-out 形式传入的原始变量。这是因为任何访问原始变量的行为都会造成冲突，例如： + +```swift +var stepSize = 1 + +func increment(_ number: inout Int) { + number += stepSize +} + +increment(&stepSize) +// 错误：stepSize 访问冲突 +``` + + + +在上面的代码里，`stepSize` 是一个全局变量，并且它可以通常可以在 `increment(_:)` 里被访问。然而，对于 `stepSize` 的读访问与 `number` 的写访问重叠了。就像下面展示的那样，`number` 和 `stepSize` 都指向了同一个内存区域。针对同一块内存区域的读和写访问重叠了，因此产生了冲突。 + +![](memory_increment) + +其中一个解决冲突的方式是显式地复制一份 `stepSize`： + +```swift +// 显式复制 +var copyOfStepSize = stepSize +increment(©OfStepSize) + +// 更新原来的值 +stepSize = copyOfStepSize +// stepSize 现在的值是 2 +``` + + + +由于你在调用 `increment(_:)` 前复制了 `stepSize`，显然 `copyOfStepSize` 会以当前 `stepSize` 的值增加。读访问在写访问开始前就结束了，所以不会产生冲突。 + +对于 in-out 参数保持长时写访问的另一个后果是，往同一个函数的多个 in-out 参数里传入同一个变量也会产生冲突。例如： + +```swift +func balance(_ x: inout Int, _ y: inout Int) { + let sum = x + y + x = sum / 2 + y = sum - x +} +var playerOneScore = 42 +var playerTwoScore = 30 +balance(&playerOneScore, &playerTwoScore) // OK +balance(&playerOneScore, &playerOneScore) +// 错误：playerOneScore 访问冲突 +``` + + + +上面的 `balance(_:_:)` 函数会将传入的两个参数平均化。将 `playerOneScore` 和 `playerTwoScore` 作为参数传入不会产生错误 —— 虽然这两个写访问在时间上重叠了，但它们访问的是不同的内存位置；相反，将 `playerOneScore` 同时传入两个参数则会冲突，因为这样会发起两次在时间上重叠、针对同一内存位置的写访问。 + +> 因为操作符也是函数，它们也会对 in-out 参数保持长时访问。例如，假设 `balance(_:_:)` 是一个名为 `<^>` 的操作符函数，那么 `playerOneScore <^> playerOneScore` 也会造成像 `balance(&playerOneScore, &playerOneScore)` 一样的冲突。 + + + +## 方法中的 self 访问冲突 + + + + + +一个结构体的变值（mutating）方法会在其被调用期间保持对于 `self` 的长时写访问。想象这样一个游戏：其中每个玩家都有一定的生命值，受到伤害时会减少；玩家还会有能量值，会在玩家使用特殊技能时减少。 + +```swift +struct Player { + var name: String + var health: Int + var energy: Int + + static let maxHealth = 10 + mutating func restoreHealth() { + health = Player.maxHealth + } +} +``` + + + +在上面的 `restoreHealth()` 方法中，对于 `self` 的写访问开始于方法的开头，并持续到方法返回为止。在这个例子中，`restoreHealth()` 中没有任何其他的代码会对 `Player` 实例的属性产生重叠访问。但是，下面的 `shareHealth(with:)` 方法会将另一个 `Player` 实例作为 in-out 参数接受，这样则会产生重叠访问的可能性。 + +```swift +extension Player { + mutating func shareHealth(with teammate: inout Player) { + balance(&teammate.health, &health) + } +} + +var oscar = Player(name: "Oscar", health: 10, energy: 10) +var maria = Player(name: "Maria", health: 5, energy: 10) +oscar.shareHealth(with: &maria) // OK +``` + + + +在上面的例子中，通过调用 `shareHealth(with:)` 方法来将 Oscar 的生命值分享给 Maria 并不会造成冲突。因为 `oscar` 是变值方法中 `self` 所对应的值，所以方法执行过程存在对于 `oscar` 的写访问；在相同的时间窗口内，方法对于 `maria` 也会有写访问，因为 `maria` 是作为 in-out 参数传入的。尽管这两次写访问在时间上发生了重叠，它们并不冲突。 + +![](memory_share_health_maria) + +然而，如果你将 `oscar` 所谓参数传入 `shareHealth(with:)`，就会产生冲突了： + +```swift +oscar.shareHealth(with: &oscar) +// 错误：对 oscar 的访问出现冲突 +``` + + + +在整个变值方法执行期间，方法不仅需要对保持对 `self` 的写访问，其 in-out 参数还需要对 `teammate` 保持相同时长的写访问。在方法内，`self` 和 `teammate` 都指向了内存中的同一位置（如下图所示）。因为两次写访问不仅在时间上重叠，访问的内存区域也重叠了，所以产生了冲突。 + +![](memory_share_health_oscar) + +## 属性访问冲突 + +结构体、元组、枚举这样的类型是由多个独立的值构成的（例如结构体的属性、元组的元素）。它们都是值类型，所以修改值的任何一部分都是对于整个值的修改。这意味着对于其中任何一个属性的读或写访问，都需要对于整个值的访问。例如，对于元组元素的重叠写访问会造成冲突： + +```swift +var playerInformation = (health: 10, energy: 20) +balance(&playerInformation.health, &playerInformation.energy) +// 错误：对 playerInformation 的属性访问有冲突 +``` + + + +在上面的示例中，因为出现了对于 `playerInformation` 的重叠写访问，对元组中的元素调用 `balance(_:_:)` 就会产生冲突。`playerInformation.health` 和 `playerInformation.energy` 都被作为 in-out 参数被传入了，这意味着 `balance(_:_:)` 在执行期间需要保持对它们的写访问，而这又意味着 `balance(_:_:)` 需要保持两次对 `playerInformation` 整体的重叠写访问，这样一来就发生了访问冲突。 + +下方的代码展示了对一个存储在全局变量中的结构体的重叠写访问，这会导致相同的错误。 + +```swift +var holly = Player(name: "Holly", health: 10, energy: 10) +balance(&holly.health, &holly.energy) // 错误 +``` + + + +在实践中，大多数时候对于结构体属性的重叠访问是安全的。举个例子，如果上方例子中的变量 `holly` 是一个本地变量而非全局变量，编译器可以证明对于该结构体的属性的重叠访问是安全的： + +```swift +func someFunction() { + var oscar = Player(name: "Oscar", health: 10, energy: 10) + balance(&oscar.health, &oscar.energy) // OK +} +``` + + + +在上方的例子中，Oscar 的生命值和能量值被作为两个 in-out 参数传递给了 `balance(_:_:)`。此时编译器能够证明内存是安全的，因为这两个属性并不会以任何方式交互。 + +要保证内存安全，限制结构体属性的重叠访问并不总是必要的。内存安全性是我们想获得的一种保证，但是「独占访问」是相比「内存安全」更加严格的一种要求 —— 这意味着即使有些代码违反了「独占访问」的原则，它也可以是内存安全的。只要编译器可以「证明」这种非专属的访问是内存安全的，Swift 就会允许这样的代码存在。特别地，在以下条件满足时，编译器就可以证明对结构体属性的重叠访问是安全的： + +- 代码只访问了实例的存储属性，而没有访问计算属性或类属性 +- 结构体是本地变量，而非全局变量的值 +- 结构体要么没有被闭包捕获，要么只被非逃逸闭包捕获了 + +如果编译器无法证明访问安全性，它就会拒绝访问。 + + + + + + + + + + + + +[`Atomic`]: https://developer.apple.com/documentation/synchronization/atomic +[`AtomicLazyReference`]: https://developer.apple.com/documentation/synchronization/atomiclazyreference diff --git a/swift-6.docc/LanguageGuide/Methods.md b/swift-6.docc/LanguageGuide/Methods.md new file mode 100644 index 000000000..af4a225fa --- /dev/null +++ b/swift-6.docc/LanguageGuide/Methods.md @@ -0,0 +1,581 @@ + +# 方法 + +定义并调用属于某个实例或者类型的函数。 + +*方法*是与特定类型关联的函数。 +类、结构体和枚举都可以定义实例方法，这些方法封装了特定的任务和功能，用于处理给定类型的实例。 +类、结构体和枚举还可以定义类型方法，这些方法与类型本身相关联。 +类型方法类似于 Objective-C 中的类方法。 + +结构体和枚举在 Swift 中能够定义方法，这是与 C 和 Objective-C 的一个重大区别。 +在 Objective-C 中，只有类可以定义方法。 +而在 Swift 中，无论你选择定义类、结构体或枚举，你都可以灵活地为你创建的类型定义方法。 + +## 实例方法 + +*实例方法*是属于某个类、结构体或枚举实例的函数。 +它们通过提供访问和修改实例属性的方式，或者提供与实例功能相关的操作，来支持实例的整体功能。 +实例方法有着和函数完全一样的语法，具体描述可以参见 。 + +你在所属类型的开闭大括号内编写实例方法。 +实例方法可以隐式访问该类型的所有其他实例方法和属性。 +实例方法只能在该类型的特定实例上调用，而不能在没有实例的情况下独立调用。 + +这是一个定义简单“Counter”类的示例，可用于计算某个动作发生的次数： + +```swift +class Counter { + var count = 0 + func increment() { + count += 1 + } + func increment(by amount: Int) { + count += amount + } + func reset() { + count = 0 + } +} +``` + + + +`Counter` 类定义了三个实例方法： + +- `increment()` 将计数器增加 `1`。 +- `increment(by: Int)` 将计数器增加指定的整数值。 +- `reset()` 将计数器重置为零。 + +`Counter` 类还声明了一个变量属性 `count`，用于跟踪当前的计数器值。 + +你可以使用与属性相同的点语法来调用实例方法： + +```swift +let counter = Counter() +// the initial counter value is 0 +counter.increment() +// the counter's value is now 1 +counter.increment(by: 5) +// the counter's value is now 6 +counter.reset() +// the counter's value is now 0 +``` + + + +函数参数可以同时拥有一个参数名称（在函数体内使用）和一个参数标签（在调用函数时使用），参见。 +方法参数也是如此，因为方法只是与某种类型关联的函数。 + +### self 属性 + +每个类型的实例都有一个隐式属性，称为 `self`，它与实例本身完全等同。 +你可以在实例方法中使用 `self` 属性来引用当前实例。 + +上面示例中的 `increment()` 方法可以这样编写： + +```swift +func increment() { + self.count += 1 +} +``` + + + + + +实际上，在代码中你不需要经常写 `self`。 +如果你没有显式地写出 `self`，当你在方法中使用已知的属性或方法名称时，Swift 会假设你是在引用当前实例的属性或方法。 +这种假设在 `Counter` 类的三个实例方法中通过直接使用 `count`（而非 `self.count`）得到了体现。 + +此规则的主要例外情况发生在实例方法的形参名称与该实例的属性名称相同时。 +在这种情况下，形参名称优先，此时就需要以更明确的方式引用属性。你可以使用 `self` 属性来区分形参名称和属性名称。 + +在如下情况中，`self` 用于区分一个名为 x 的方法形参和一个同样名为 x 的实例属性： + +```swift +struct Point { + var x = 0.0, y = 0.0 + func isToTheRightOf(x: Double) -> Bool { + return self.x > x + } +} +let somePoint = Point(x: 4.0, y: 5.0) +if somePoint.isToTheRightOf(x: 1.0) { + print("This point is to the right of the line where x == 1.0") +} +// Prints "This point is to the right of the line where x == 1.0" +``` + + + +如果没有 `self` 前缀，Swift 会假定 `x` 的两个使用都指的是名为 `x` 的方法形参。 + +### 从实例方法内部修改值类型 + +结构体和枚举是 *值类型*。默认情况下，值类型的属性不能在其实例方法内部被修改。 + + + +然而，如果你需要在特定方法内部修改结构体或枚举的属性，你可以为该方法启用 *mutating* 行为。 +这样方法就可以在内部改变其属性，并且所有的更改在方法结束时会写回到原始结构体中。 +该方法还可以将一个全新的实例赋值给隐式的 `self` 属性，并且这个新实例将在方法结束时替换现有实例。 + +你可以通过在该方法的 `func` 关键字前添加 `mutating` 关键字来启用这种行为： + +```swift +struct Point { + var x = 0.0, y = 0.0 + mutating func moveBy(x deltaX: Double, y deltaY: Double) { + x += deltaX + y += deltaY + } +} +var somePoint = Point(x: 1.0, y: 1.0) +somePoint.moveBy(x: 2.0, y: 3.0) +print("The point is now at (\(somePoint.x), \(somePoint.y))") +// Prints "The point is now at (3.0, 4.0)" +``` + + + +上面的 `Point` 结构体定义了一个 mutating 的 `moveBy(x:y:)` 方法，该方法可以将一个 `Point` 实例移动一定的距离。 +这个方法直接修改其调用的点，而不是返回一个新的点。为了使该方法能够修改其属性，`mutating` 关键字被添加到它的定义中。 + +请注意，你不能在结构体类型的常量上调用 mutating 方法，因为其属性不能被改变，即使它们是可变属性， +参见： + +```swift +let fixedPoint = Point(x: 3.0, y: 3.0) +fixedPoint.moveBy(x: 2.0, y: 3.0) +// this will report an error +``` + + + + + +### 在 mutating 方法中给 self 赋值 + +mutating 方法可以将一个全新的实例赋值给隐式的 `self` 属性。上面的 `Point` 示例可以改写为以下方式： + +```swift +struct Point { + var x = 0.0, y = 0.0 + mutating func moveBy(x deltaX: Double, y deltaY: Double) { + self = Point(x: x + deltaX, y: y + deltaY) + } +} +``` + + + +这个版本的 mutating `moveBy(x:y:)` 方法创建了一个新的结构体，其 `x` 和 `y` 值被设置为目标位置。 +调用这个替代版本方法与调用早期版本方法的最终结果完全相同。 + +枚举的 mutating 方法可以将隐式的 `self` 参数设置为同一枚举的不同case： + +```swift +enum TriStateSwitch { + case off, low, high + mutating func next() { + switch self { + case .off: + self = .low + case .low: + self = .high + case .high: + self = .off + } + } +} +var ovenLight = TriStateSwitch.low +ovenLight.next() +// ovenLight is now equal to .high +ovenLight.next() +// ovenLight is now equal to .off +``` + + + +这个示例定义了一个三状态开关的枚举。每次调用其 `next()` 方法时，开关将在三种不同的电源状态（`off`, `low` 和 `high`）之间循环切换。 + +## 类型方法 + +如上所述，实例方法是在特定类型的实例上调用的方法。 +你还可以定义在类型本身上调用的方法。这类方法称为 *类型方法*。 +你可以通过在方法的 `func` 关键字前添加 `static` 关键字来标识类型方法。 +类可以使用 `class` 关键字代替 `static` ，以允许子类覆盖父类对该方法的实现。 + +> 注意：在 Objective-C 中，你只能为 Objective-C 类定义类型级方法。 +> 在 Swift 中，你可以为所有类、结构体和枚举定义类型级方法。 +> 每个类型方法都明确作用于其对应的类型。 + +类型方法与实例方法一样，使用点语法调用。 +然而你是在类型上调用类型方法，而不是在该类型的实例上调用。 +下面是如何在一个名为 `SomeClass`的类上调用类型方法的示例： + +```swift +class SomeClass { + class func someTypeMethod() { + // type method implementation goes here + } +} +SomeClass.someTypeMethod() +``` + + + +在类型方法的主体内部，隐式的 `self` 属性指的是类型本身，而不是该类型的实例。 +这意味着你可以使用 `self` 来区分类型属性和类型方法形参，正如你在实例属性和实例方法形参中所做的那样。 + +更一般地说，在类型方法的主体内使用的任何未限定的方法和属性名称，都将指向其他类型级的方法和属性。 +类型方法可以直接通过其他方法的名称调用另一个类型方法，而无需在前面加上类型名称。 +类似地，结构体和枚举上的类型方法可以通过使用类型属性的名称来访问类型属性，无需加类型名称前缀。 + +下面的示例定义了一个名为 `LevelTracker` 的结构，用于跟踪玩家在游戏不同关卡或阶段的进度。 +这是一个单人游戏，但可以在单个设备上存储多个玩家的信息。 + +游戏的所有关卡（除了第一关）在首次游玩时都是锁定的。 +每当一个玩家完成一个关卡，该关卡将对设备上的所有玩家解锁。 +`LevelTracker` 结构使用类型属性和方法来跟踪游戏中哪些关卡已被解锁。 +同时，它还跟踪每个玩家的当前关卡。 + +```swift +struct LevelTracker { + static var highestUnlockedLevel = 1 + var currentLevel = 1 + + static func unlock(_ level: Int) { + if level > highestUnlockedLevel { highestUnlockedLevel = level } + } + + static func isUnlocked(_ level: Int) -> Bool { + return level <= highestUnlockedLevel + } + + @discardableResult + mutating func advance(to level: Int) -> Bool { + if LevelTracker.isUnlocked(level) { + currentLevel = level + return true + } else { + return false + } + } +} +``` + + + +`LevelTracker` 结构跟踪所有玩家已解锁的最高关卡。 +该值存储在名为 `highestUnlockedLevel` 的类型属性中。 + +`LevelTracker` 还定义了两个类型函数来处理 `highestUnlockedLevel` 属性。 +第一个是名为 `unlock(_:)` 的类型函数，它在解锁新关卡时更新 `highestUnlockedLevel` 的值。 +第二个是名为 `isUnlocked(_:)` 的便利类型函数，如果特定关卡编号已经解锁，则返回 `true`。 +（请注意，这些类型方法可以直接访问 `highestUnlockedLevel` 类型属性，而不需要写成 `LevelTracker.highestUnlockedLevel`。） + +除了类型属性和类型方法，`LevelTracker` 还跟踪每个玩家在游戏中的进度。 +它使用一个名为 `currentLevel` 的实例属性来记录玩家当前正在玩的关卡。 + +为了帮助管理 `currentLevel` 属性，`LevelTracker` 定义了一个名为 `advance(to:)` 的实例方法。 +在更新 `currentLevel` 之前，该方法会检查请求的新关卡是否已经解锁。 +`advance(to:)` 方法返回一个布尔值，以指示是否成功设置了 `currentLevel`。 +由于调用 `advance(to:)` 方法的代码不一定会关注返回值，因此该函数被标记为 `@discardableResult` 属性。 +有关此属性的更多信息，请参见 。 + +`LevelTracker` 结构与下面展示的 `Player` 类一起使用，用于跟踪和更新每个玩家的进度： + +```swift +class Player { + var tracker = LevelTracker() + let playerName: String + func complete(level: Int) { + LevelTracker.unlock(level + 1) + tracker.advance(to: level + 1) + } + init(name: String) { + playerName = name + } +} +``` + + + +`Player` 类创建了一个新的 `LevelTracker` 实例来跟踪该玩家的进度。 +它还提供了一个名为 `complete(level:)`的方法，每当玩家完成某个关卡时调用。 +此方法为所有玩家解锁下一个关卡，并更新该玩家的进度，使其进入下一个关卡。 +（ `advance(to:)` 的Boolean返回值被忽略，因为通过前一行对 `LevelTracker.unlock(_:)` 的调用，关卡已被解锁。） + +你可以为一个新玩家创建 `Player` 类的实例，并查看当该玩家完成第一关时会发生什么： + +```swift +var player = Player(name: "Argyrios") +player.complete(level: 1) +print("highest unlocked level is now \(LevelTracker.highestUnlockedLevel)") +// Prints "highest unlocked level is now 2" +``` + + + +如果你创建第二个玩家，并尝试将其移动到尚未被任何玩家解锁的关卡，那么尝试设置玩家的当前等级时会失败： + +```swift +player = Player(name: "Beto") +if player.tracker.advance(to: 6) { + print("player is now on level 6") +} else { + print("level 6 hasn't yet been unlocked") +} +// Prints "level 6 hasn't yet been unlocked" +``` + + + + + + diff --git a/swift-6.docc/LanguageGuide/NestedTypes.md b/swift-6.docc/LanguageGuide/NestedTypes.md new file mode 100644 index 000000000..29efa7a2b --- /dev/null +++ b/swift-6.docc/LanguageGuide/NestedTypes.md @@ -0,0 +1,167 @@ +# 嵌套类型 + +在另一个类型的作用域内定义类型。 + +枚举常被创建以支持特定类或结构的功能。同样的，为了在更复杂类型的上下文中使用而定义纯工具性结构，以及通常与特定类型结合使用的协议，也显得十分便利。为了实现这一点，Swift 允许你定义 *嵌套类型*，即在支持的类型定义中嵌套枚举、结构和协议等辅助类型。 + +要在一个类型中嵌套另一个类型，只需将其定义写在外部类型的大括号内。类型可以根据需要嵌套到任意层次。 + +## 使用嵌套类型 + +下面的示例定义了一个名为`BlackjackCard` 的结构，用于模拟 Blackjack 游戏中的扑克牌。`BlackjackCard`结构包含两个嵌套的枚举类型，分别是 `Suit` 和`Rank`。 + +在 Blackjack 中，Ace 牌的值可以是 1 或 11。这个特性通过嵌套在 `Rank`枚举中的一个名为 `Values` 的结构表示： + +```swift +struct BlackjackCard { + + // 嵌套的 Suit 枚举 + enum Suit: Character { + case spades = "♠", hearts = "♡", diamonds = "♢", clubs = "♣" + } + + // 嵌套的 Rank 枚举 + enum Rank: Int { + case two = 2, three, four, five, six, seven, eight, nine, ten + case jack, queen, king, ace + struct Values { + let first: Int, second: Int? + } + var values: Values { + switch self { + case .ace: + return Values(first: 1, second: 11) + case .jack, .queen, .king: + return Values(first: 10, second: nil) + default: + return Values(first: self.rawValue, second: nil) + } + } + } + + // BlackjackCard 属性和方法 + let rank: Rank, suit: Suit + var description: String { + var output = "suit is \(suit.rawValue)," + output += " value is \(rank.values.first)" + if let second = rank.values.second { + output += " or \(second)" + } + return output + } +} +``` + + + +`Suit`枚举描述了四种常见的扑克牌花色，以及用于表示其符号的原始 `Character` 值。 + +`Rank` 枚举描述了十三种可能的扑克牌等级，以及用于表示其数值的原始 `Int` 值 +（这个原始的 `Int` 值不适用于杰克 (Jack)、皇后 (Queen)、国王 (King) 和王牌 (Ace) 牌。） + +正如上面提到的，`Rank` 枚举定义了一个它自己的进一步嵌套的结构，叫做 `Values`。 +这个结构封装了大多数牌只有一个值，而王牌 (Ace) 有两个值的事实。 +`Values` 结构定义了两个属性来表示这一点： + +- `first`，类型为 `Int` +- `second`，类型为 `Int?`，或“可选的 `Int`” + +`Rank`还定义了一个计算属性`values`，它返回一个 `Values`结构的实例。 该计算属性会根据牌的等级 (rank) 生成一个新的`Values`实例，并为不同的等级分配合适的值。 对于`jack`、`queen`、`king`和`ace`，使用特殊的值。 对于数字牌，则使用等级的原始 `Int` 值。 + +`BlackjackCard` 结构本身有两个属性 —— `rank` 和 `suit`。它还定义了一个名为 `description` 的计算属性，该属性使用 `rank` 和 `suit` 中存储的值来构建牌的名称和数值的描述。`description` 属性使用可选绑定来检查是否存在第二个值，如果有，则为第二个值插入额外的描述信息。 + +由于 `BlackjackCard` 是一个没有自定义初始化器的结构体，它具有一个隐式的成员逐一初始化器，正如在 [[doc:Initialization#Memberwise-Initializers-for-Structure-Types](doc:Initialization#Memberwise-Initializers-for-Structure-Types)]中所描述的那样。你可以使用这个初始化器来初始化一个名为 `theAceOfSpades` 的新常量： + +```swift +let theAceOfSpades = BlackjackCard(rank: .ace, suit: .spades) +print("theAceOfSpades: \(theAceOfSpades.description)") +// 输出 "theAceOfSpades: suit is ♠, value 是 1 或者 11" +``` + + + +尽管 `Rank` 和 `Suit` 嵌套在 `BlackjackCard` 中，它们的类型可以从上下文中推断出来，因此在初始化这个实例时，可以仅通过它们的名称（如 `.ace` 和 `.spades`）来引用枚举的情况。 +在上面的示例中，`description` 属性正确地记录了黑桃 A 的值为 `1` 或 `11`。 + +## 访问嵌套类型 + +要在定义上下文之外使用嵌套类型，需要在其名称前加上其外部类型的名称前缀： + +```swift +let heartsSymbol = BlackjackCard.Suit.hearts.rawValue +// heartsSymbol 是 "♡" +``` + + + +在上面的例子中，这使得 `Suit`, `Rank`, 和 `Values`的名称可以故意保持简短，因为它们的名称由定义它们的上下文自然限定。 + + + + diff --git a/swift-6.docc/LanguageGuide/OpaqueTypes.md b/swift-6.docc/LanguageGuide/OpaqueTypes.md new file mode 100644 index 000000000..0bc43f44d --- /dev/null +++ b/swift-6.docc/LanguageGuide/OpaqueTypes.md @@ -0,0 +1,704 @@ +# 不透明类型和封装协议类型 + +隐藏值类型的实现细节。 + +Swift 提供了两种隐藏值类型细节的方法：不透明类型（Opaque Type）和封装协议类型（Boxed Protocol Type）。在隔离模块和调用模块的代码上，隐藏类型信息是有用的，因为这样返回值的底层类型可以保持私有。 + +返回不透明类型的函数或方法隐藏了其返回值的类型信息。函数会将返回值类型描述为一个遵循某种协议的类型，而非一个更具体的类型。不透明类型会保留类型的身份信息 —— 编译器可以访问该类型信息，但模块的调用端则无法访问。 + +封装协议类型可以存储遵循给定协议的任何类型的实例。封装协议类型不保留类型的身份信息 —— 值的具体类型在运行时才会被知道，并且随着不同的值被存储其中，它的具体类型可能会发生变化。 + +## 不透明类型所解决的问题 + +举个例子，假设你正在编写一个用 ASCII 字符绘制几何形状的程序模块。每个几何形状结构体的基本特征是有一个 `draw()` 函数，该函数返回表示那个几何形状的字符串，这样你就可以把这个基本特征作为 `Shape` 协议的要求之一： + +```swift +protocol Shape { + func draw() -> String +} + +struct Triangle: Shape { + var size: Int + func draw() -> String { + var result: [String] = [] + for length in 1...size { + result.append(String(repeating: "*", count: length)) + } + return result.joined(separator: "\n") + } +} +let smallTriangle = Triangle(size: 3) +print(smallTriangle.draw()) +// * +// ** +// *** +``` + + + +如下面的代码所示，你可以使用泛型来实现像垂直翻转某个几何形状这样的操作。然而，这种方法有一个重要的局限性：翻转后的结果会暴露用于创建该结果的确切的泛型类型。 + +```swift +struct FlippedShape: Shape { + var shape: T + func draw() -> String { + let lines = shape.draw().split(separator: "\n") + return lines.reversed().joined(separator: "\n") + } +} +let flippedTriangle = FlippedShape(shape: smallTriangle) +print(flippedTriangle.draw()) +// *** +// ** +// * +``` + + + +又比如下面这样的代码，这种方法定义了一个能将两个形状垂直连接起来的 `JoinedShape` 结构体，如果把一个三角形与一个翻转过的三角形连接在一起，就产生了像 `JoinedShape>` 这样的类型。 + +```swift +struct JoinedShape: Shape { + var top: T + var bottom: U + func draw() -> String { + return top.draw() + "\n" + bottom.draw() + } +} +let joinedTriangles = JoinedShape(top: smallTriangle, bottom: flippedTriangle) +print(joinedTriangles.draw()) +// * +// ** +// *** +// *** +// ** +// * +``` + + + +因为我们总是需要声明完整的返回类型，所以暴露关于形状创建的详细信息会导致类型泄露，这些泄漏的类型本不应成为绘制几何形状程序模块公开接口的一部分。模块内部的代码可以以多种不同的方式构建相同的形状，而其他使用该形状的模块外部代码不应需要考虑关于变换几何形状的具体实现细节。 +像 `JoinedShape` 和 `FlippedShape` 这样的包装类型（wrapper types）对模块的用户来说并不重要，它们不应该被暴露出去。该模块的公开接口包括了连接和翻转形状等操作，这些操作会返回另一个经过操作后的 `Shape` 值。 + +## 返回一个不透明类型 + +你可以把不透明类型看作是泛型类型的反面。泛型类型允许函数的调用端选择参数和返回值的类型，而这些类型与函数的实现是分离的。例如，以下代码中的函数返回一个调用端指定的类型： + +```swift +func max(_ x: T, _ y: T) -> T where T: Comparable { ... } +``` + + + +`max(_:_:)` 的调用端会指定 `x`和 `y` 的值，这些值的类型决定了 `T` 的具体类型。调用端可以使用任何遵循 `Comparable` 协议的类型来调用这个函数。函数内部的代码以一种通用的方式编写，因此可以处理调用端提供的任何类型。`max(_:_:)` 的实现将仅使用所有遵循 `Comparable` 协议的类型所共享的功能。 + +在这一点上，返回不透明类型函数的角色是反过来的。不透明类型将允许函数的实现来选择返回值的类型，而返回值的类型与函数的调用端是分离的。例如，以下示例中的函数返回一个梯形，却没有暴露该形状的底层类型。 + +```swift +struct Square: Shape { + var size: Int + func draw() -> String { + let line = String(repeating: "*", count: size) + let result = Array(repeating: line, count: size) + return result.joined(separator: "\n") + } +} + +func makeTrapezoid() -> some Shape { + let top = Triangle(size: 2) + let middle = Square(size: 2) + let bottom = FlippedShape(shape: top) + let trapezoid = JoinedShape( + top: top, + bottom: JoinedShape(top: middle, bottom: bottom) + ) + return trapezoid +} +let trapezoid = makeTrapezoid() +print(trapezoid.draw()) +// * +// ** +// ** +// ** +// ** +// * +``` + + + +在这个例子中，`makeTrapezoid()` 函数声明它的返回类型为 `some Shape`。因此，该函数会返回一个遵循 `Shape` 协议的某种给定类型的值，却可以不必指定任何特定的具体返回类型。以这种方式编写 `makeTrapezoid()` 使其能够只需表达其公开接口的基本特征 —— 它返回的值是一个形状 —— 而不会将构成该形状的具体类型暴露为其公开接口的一部分。这个实现使用了两个三角形和一个正方形来绘制梯形，但是你也可以用其他不同的方式来实现同样的功能，而无需改变函数的返回类型。 + +这个例子凸显了不透明类型与泛型类型的反向关系。就像泛型函数的调用端一样，`makeTrapezoid()` 中的代码可以返回它所需的任何类型，只要该类型遵循 `Shape` 协议。类似于泛型函数的实现，该函数的调用端也需要以一种通用的方式来编写，以便能够兼容由 `makeTrapezoid()` 返回的任何 `Shape` 值。 + +你还可以将不透明返回类型与泛型结合使用。以下代码中的函数都返回了遵循 `Shape` 协议的某种类型的值。 + +```swift +func flip(_ shape: T) -> some Shape { + return FlippedShape(shape: shape) +} +func join(_ top: T, _ bottom: U) -> some Shape { + JoinedShape(top: top, bottom: bottom) +} + +let opaqueJoinedTriangles = join(smallTriangle, flip(smallTriangle)) +print(opaqueJoinedTriangles.draw()) +// * +// ** +// *** +// *** +// ** +// * +``` + + + +在这个例子中，`opaqueJoinedTriangles` 的值与前面章节中所举的泛型例子里的 `joinedTriangles` 的值相同。但是，与那个例子不同的是，`flip(_:)` 和 `join(_:_:)` 将泛型形状操作所返回的底层类型包装在不透明返回类型中，使得这些类型不再可见。这两个函数是泛型函数，因为它们依赖的类型是泛型类型，函数的类型参数传递出了 `FlippedShape` 和 `JoinedShape` 所需的类型信息。 + +如果一个返回不透明类型的函数从多处返回值，则所有可能的返回值必须具有相同的类型。对于一个泛型函数，它可以使用函数的泛型参数作为其返回类型，但这个返回类型仍然必须是相同的某个单一类型。例如，下面是一个*不合法的*形状翻转函数版本，它包含了正方形的一个特例： + +```swift +func invalidFlip(_ shape: T) -> some Shape { + if shape is Square { + return shape // 错误：返回类型不一致 + } + return FlippedShape(shape: shape) // 错误：返回类型不一致 +} +``` + + + +如果你用一个 `Square` 调用这个函数，它会返回一个 `Square`；否则，它会返回一个 `FlippedShape`。这违反了只能返回同一种类型值的要求，使得 `invalidFlip(_:)` 成为不合法的代码。修复 `invalidFlip(_:)` 的一种方法是将正方形特例的处理移入 `FlippedShape` 的实现中，这样可以让这个函数始终返回一个 `FlippedShape` 值： + +```swift +struct FlippedShape: Shape { + var shape: T + func draw() -> String { + if shape is Square { + return shape.draw() + } + let lines = shape.draw().split(separator: "\n") + return lines.reversed().joined(separator: "\n") + } +} +``` + + + + + +始终返回同一种单一类型的要求并不妨碍你在不透明返回类型中使用泛型。以下是一个示例函数，它将它的类型参数作为其返回值的基础类型： + +```swift +func `repeat`(shape: T, count: Int) -> some Collection { + return Array(repeating: shape, count: count) +} +``` + + + +在这个例子中，返回值的基础类型取决于 `T`：无论传入的形状是什么，`repeat(shape:count:)` 都会创建并返回该形状的数组。然而，因为返回值始终具有相同的基础类型 `[T]`，所以它遵循了具有不透明返回类型的函数必须仅返回某种单一类型值的要求。 + +## 封装协议类型 + +封装协议类型有时也被称为*存在类型（existential type）*，这个术语源于这样的一个表达：“存在一个类型 *T*，使得 *T* 遵循该协议”。要创建一个封装协议类型，在协议名称前加上 `any`。下面是一个示例： + +```swift +struct VerticalShapes: Shape { + var shapes: [any Shape] + func draw() -> String { + return shapes.map { $0.draw() }.joined(separator: "\n\n") + } +} + +let largeTriangle = Triangle(size: 5) +let largeSquare = Square(size: 5) +let vertical = VerticalShapes(shapes: [largeTriangle, largeSquare]) +print(vertical.draw()) +``` + + + +在上面的例子中，`VerticalShapes` 声明了 `shapes` 的类型为 `[any Shape]` —— 一个封装 `Shape` 类型元素的数组。数组中的每个元素可以是不同的类型，但所有这些类型都必须遵循 `Shape` 协议。为了支持这种运行时的灵活性，Swift 在必要时会增加一层间接的抽象分层 —— 这种分层被称为*封装层（Box）*，并且它有性能成本。 + +在 `VerticalShapes` 类型中，代码可以使用 `Shape` 协议所要求的方法、属性和下标操作。例如，`VerticalShapes` 的 `draw()` 方法调用了数组中每个元素的 `draw()` 方法。因为 `Shape` 协议要求必须有一个 `draw()` 方法，所以这个方法是可用的。相反，如果尝试访问三角形的 `size` 属性，或任何其他不被 `Shape` 协议所要求的属性或方法，会产生错误。 + +我们来对比一下可用于 `shapes` 的三种类型： + +- 使用泛型：通过编写 `struct VerticalShapes` 和 `var shapes: [S]`，可以创建一个数组，其元素是某种特定的形状类型，并且这个特定类型的身份对任何与数组交互的代码都是可见的。 + +- 使用不透明类型：通过编写 `var shapes: [some Shape]` 来创建一个数组，其元素是某种特定形状类型，并且这个特定类型的身份是隐藏的。 + +- 使用封装协议类型：通过编写 `var shapes: [any Shape]` 能创建一个可以存储不同类型元素的数组，并且这些类型的身份是隐藏的。 + +在上面的例子中，封装协议类型是唯一允许 `VerticalShapes` 的调用者将不同种类的形状混合在一起的方法。 + +你可以在知道被封装值的基础类型时使用一个 `as` 来进行类型转换。例如： + +```swift +if let downcastTriangle = vertical.shapes[0] as? Triangle { + print(downcastTriangle.size) +} +// 打印输出 "5" +``` + +要了解更多信息请参考。 + +## 不透明类型与封装协议类型之间的区别 + +函数返回一个不透明类型与返回一个封装协议类型看起来非常相似，但这两种返回类型在是否保留类型的身份信息上有所不同。不透明类型指的是某种特定类型，尽管函数的调用者无法看到是哪种具体类型；而封装协议类型可以指任何遵循该协议的类型。一般来说，封装协议类型在存储值的底层类型上提供了更多的灵活性，而不透明类型则需要你对这些底层类型做出更严格的保证。 + +例如，以下是前文中 `flip(_:)` 的另一个版本，它使用封装协议类型而不是不透明类型作为其返回类型： + +```swift +func protoFlip(_ shape: T) -> Shape { + return FlippedShape(shape: shape) +} +``` + + + +这个版本的 `protoFlip(_:)` 与 `flip(_:)` 的主体相同，并且它始终返回相同类型的值。与 `flip(_:)` 不同的是，`protoFlip(_:)` 的返回值其实不需要总是具有相同的类型 —— 这个返回值只需遵循 `Shape` 协议即可。换句话说，`protoFlip(_:)` 与其调用者之间的 API 约束比 `flip(_:)` 更加宽松。它保留了返回多种类型值的灵活性： + +```swift +func protoFlip(_ shape: T) -> Shape { + if shape is Square { + return shape + } + + return FlippedShape(shape: shape) +} +``` + + + +修改后的 `protoFlip(_:)` 函数根据传入的形状返回一个 `Square` 实例或一个 `FlippedShape` 实例。由这个函数在两处返回的两个翻转形状可能具有完全不同的类型。其他合法版本的这个函数在翻转同一形状的多个实例时，可能会返回不同类型的值。`protoFlip(_:)` 返回值的类型信息不够具体，这意味着许多依赖于类型信息的操作无法在返回值上使用。例如，无法编写用于比较这个函数返回结果的 `==` 运算符。 + +```swift +let protoFlippedTriangle = protoFlip(smallTriangle) +let sameThing = protoFlip(smallTriangle) +protoFlippedTriangle == sameThing // 错误 +``` + + + +示例中最后一行的错误有几个原因。最直接的问题是 `Shape` 协议的要求中不包含 `==` 运算符。如果你尝试添加一个，你会遇到的下一个问题是 `==` 运算符需要知道其左右参数的类型。此类运算符通常要接受 `Self` 类型的参数，即与遵循协议的具体类型具有一致类型的参数，但如果为协议添加 `Self` 要求，在将协议当作类型使用时将不再允许进行类型抹消（Type Erasure）。 + +将封装协议类型用作函数的返回类型，给你带来了返回任何遵循该协议的类型的灵活性。然而，这种灵活性的代价是，某些操作无法在返回的值上执行。上面的示例显示了 `==` 运算符不可用的情况 —— 它依赖于特定的类型信息，而使用封装协议类型时这些信息无法保留。 + +这种方法的另一个问题是形状变换无法嵌套。翻转三角形的结果是一个类型为 `Shape` 的值，而 `protoFlip(_:)` 函数的参数是某种遵循 `Shape` 协议的类型。然而，封装协议类型的值并不遵循该协议。因此，`protoFlip(_:)` 返回的值并不遵循 `Shape` 协议。这意味着像 `protoFlip(protoFlip(smallTriangle))` 这样试图嵌套多次变换的代码是不合法的，因为翻转后的形状不是 `protoFlip(_:)` 的合法参数。（译者注：在此例中，封装协议类型的函数返回值允许该返回值是任何遵循 `Shape` 协议的类型，但这个封装本身并不保留原始类型的信息，即“存在某种遵循 `Shape` 协议的类型，但具体是什么类型你不知道”。这种类型信息在被封装后是被抹除的。因此，虽然 `any Shape` 可以持有一个遵循 `Shape` 协议的值，但 `any Shape` 本身并不遵循 `Shape` 协议。） + +相比之下，不透明类型保留了底层类型的身份信息。Swift 可以推断出关联的类型，这使得你可以在封装协议类型不能用作返回值的地方使用不透明返回值。例如，下面是一个来自的 `Container` 协议的版本： + +```swift +protocol Container { + associatedtype Item + var count: Int { get } + subscript(i: Int) -> Item { get } +} +extension Array: Container { } +``` + + + +你不能将 `Container` 用作函数的返回类型，因为该协议具有一个关联类型（Associated Type）。你也不能将其用作泛型返回类型的约束，因为在函数体外部没有足够的信息来推断泛型类型需要是什么具体类型。（译者注：因为协议中的关联类型在定义时并未具体化，而是在实际遵循协议的类型中被确定。由于关联类型在编译时无法确定具体类型，这会对类型推断和函数的返回类型造成影响。当你尝试将 `Container` 用作函数的返回类型时，编译器无法确定 `Container` 的具体实现，因为 `Container` 只定义了协议的要求，但未指定关联类型 `Item` 的具体类型。编译器需要知道 `Container` 具体的 `Item` 类型才能确定返回值的具体类型，但 `Container` 协议并没有提供这些信息。当在泛型函数中使用 `Container` 作为约束时，也会遇到类似的问题。泛型的约束需要在编译时知道具体的类型信息，以便生成正确的代码。假设泛型类型 `C` 需要遵循 `Container` 协议，但 `Container` 的关联类型 `Item` 并未在泛型约束中指定，因此编译器无法确定 `C` 的具体类型。） + +```swift +// 错误：具有关联类型的协议不能用作返回类型。 +func makeProtocolContainer(item: T) -> Container { + return [item] +} + +// 错误：没有足够的信息来推断 C 的类型。 +func makeProtocolContainer(item: T) -> C { + return [item] +} +``` + + + +使用不透明类型 `some Container` 作为返回类型，可以表达想要的 API 合约 —— 函数返回一个容器，但不指定容器的具体类型： + +```swift +func makeOpaqueContainer(item: T) -> some Container { + return [item] +} +let opaqueContainer = makeOpaqueContainer(item: 12) +let twelve = opaqueContainer[0] +print(type(of: twelve)) +// 打印输出 "Int" +``` + + + +`twelve` 的类型被推断为 `Int`，这说明了类型推断在不透明类型中是能起作用的。在 `makeOpaqueContainer(item:)` 的实现中，不透明容器的底层类型是 `[T]`。在这种情况下，`T` 是 `Int`，所以返回值是一个整数数组，并且关联类型 `Item` 被推断为 `Int`。由于 `Container` 的下标操作（`subscript` 方法）返回 `Item` 类型的值，所以 `twelve` 的类型也被推断为 `Int`。 + + + + + diff --git a/swift-6.docc/LanguageGuide/OptionalChaining.md b/swift-6.docc/LanguageGuide/OptionalChaining.md new file mode 100644 index 000000000..dd2312b3c --- /dev/null +++ b/swift-6.docc/LanguageGuide/OptionalChaining.md @@ -0,0 +1,389 @@ +# 可选链式调用（Optional Chaining） + +在不解包的情况下访问可选值的成员。 + +可选链式调用（Optional Chaining）是一种可以在当前值可能为 `nil` 的可选值上请求和调用属性、方法及下标的方法。如果可选值有值，那么调用就会成功；如果可选值是 `nil`，那么调用将返回 `nil`。多个调用可以连接在一起形成一个调用链，如果其中任何一个节点为 `nil`，整个调用链都会失败，即返回 `nil`。 + +> 注意 +> Swift 的可选链式调用和 Objective-C 中向 `nil` 发送消息有些相像，但是 Swift 的可选链式调用可以应用于任意类型，并且能检查调用是否成功。 + + + +## 使用可选链式调用代替强制展开 + +通过在想调用的属性、方法、或下标的可选值（optional value）后面放一个问号（`?`），可以定义一个可选链。这一点很像在可选值后面放一个叹号（`!`）来强制展开它的值。它们的主要区别在于当可选值为空时可选链式调用只会调用失败，然而强制展开将会触发运行时错误。 + +为了反映可选链式调用可以在空值（`nil`）上调用的事实，不论这个调用的属性、方法及下标返回的值是不是可选值，它的返回结果都是一个可选值。你可以利用这个返回值来判断你的可选链式调用是否调用成功，如果调用有返回值则说明调用成功，返回 `nil` 则说明调用失败。 + +具体来说，可选链调用的结果，除了被包装成了一个可选值外，类型与预期的返回值类型是相同的。例如，正常情况下返回 `Int` 的属性，在通过可选链访问时，返回的是 `Int?`。 + +下面几段代码将演示可选链式调用和强制展开的不同，以及如何判断可选链是否调用成功。 + +首先定义两个类 `Person` 和 `Residence`： + +```swift +class Person { + var residence: Residence? +} + +class Residence { + var numberOfRooms = 1 +} +``` + +`Residence` 有一个 `Int` 类型的属性 `numberOfRooms`，其默认值为 `1`。`Person` 具有一个可选的 `residence` 属性，其类型为 `Residence?`。 + +假如你创建了一个新的 `Person` 实例，由于 `residence` 属性是可选型，所以初始值是 `nil`。就像下面的代码中，`john` 有一个值为 `nil` 的 `residence` 属性： + +```swift +let john = Person() +``` + +如果使用叹号（`!`）对 `john` 的 `residence` 属性中的 `numberOfRooms` 值强制解包，会触发运行时错误，因为这时 `residence` 没有可以解包的值： + +```swift +let roomCount = john.residence!.numberOfRooms +// 这会引发运行时错误 +``` + +`john.residence` 为非 `nil` 值的时候，上面的调用会成功，并且把 `roomCount` 设置为 `Int` 类型的房间数量。正如上面提到的，当 `residence` 为 `nil` 的时候上面这段代码会触发运行时错误。 + +可选链式调用提供了另一种访问 `numberOfRooms` 的方式，使用问号（`?`）来替代原来的叹号（`!`）： + +```swift +if let roomCount = john.residence?.numberOfRooms { + print("John's residence has \(roomCount) room(s).") +} else { + print("Unable to retrieve the number of rooms.") +} +// 打印 “Unable to retrieve the number of rooms.” +``` + +在 `residence` 后面添加问号之后，Swift 就会在 `residence` 不为 `nil` 的情况下访问 `numberOfRooms`。 + +因为访问 `numberOfRooms` 有可能失败，可选链式调用会返回 `Int?` 类型，或称为“可选的 `Int`”。如上例所示，当 `residence` 为 `nil` 的时候，可选的 `Int` 将会为 `nil`，表明无法访问 `numberOfRooms`。访问成功时，可选的 `Int` 值会通过可选绑定展开，并赋值给非可选类型的 `roomCount` 常量。 + +要注意的是，即使 `numberOfRooms` 是非可选的 `Int` 时，这一点也成立。只要使用可选链式调用就意味着 `numberOfRooms` 会返回一个 `Int?` 而不是 `Int`。 + +可以将一个 `Residence` 的实例赋给 `john.residence`，这样它就不再是 `nil` 了： + +```swift +john.residence = Residence() +``` + +`john.residence` 现在包含一个实际的 `Residence` 实例，而不再是 `nil`。如果你试图使用先前的可选链式调用访问 `numberOfRooms`，它现在将返回值为 `1` 的 `Int?` 类型的值： + +```swift +if let roomCount = john.residence?.numberOfRooms { + print("John's residence has \(roomCount) room(s).") +} else { + print("Unable to retrieve the number of rooms.") +} +// 打印 “John's residence has 1 room(s).” +``` + + + +## 为可选链式调用定义模型类 + +通过使用可选链式调用可以调用多层属性、方法和下标。这样可以在复杂的模型中向下访问各种子属性，并且判断能否访问子属性的属性、方法或下标。 + +下面这段代码定义了四个模型类，这些例子包括多层可选链式调用。为了方便说明，在 `Person` 和 `Residence` 的基础上增加了 `Room` 类和 `Address` 类，以及相关的属性、方法以及下标。 + +`Person` 类的定义基本保持不变： + +```swift +class Person { + var residence: Residence? +} +``` + +`Residence` 类比之前复杂些，增加了一个名为 `rooms` 的变量属性，该属性被初始化为 `[Room]` 类型的空数组： + +```swift +class Residence { + var rooms: [Room] = [] + var numberOfRooms: Int { + return rooms.count + } + subscript(i: Int) -> Room { + get { + return rooms[i] + } + set { + rooms[i] = newValue + } + } + func printNumberOfRooms() { + print("The number of rooms is \(numberOfRooms)") + } + var address: Address? +} +``` + +现在 `Residence` 有了一个存储 `Room` 实例的数组，`numberOfRooms` 属性被实现为计算型属性，而不是存储型属性。`numberOfRooms` 属性简单地返回 `rooms` 数组的 `count`属 性的值。 + +`Residence` 还提供了访问 `rooms` 数组的快捷方式，即提供可读写的下标来访问 `rooms` 数组中指定位置的元素。 + +此外，`Residence` 还提供了 `printNumberOfRooms()` 方法，这个方法的作用是打印 `numberOfRooms` 的值。 + +最后，`Residence` 还定义了一个可选属性 `address`，其类型为 `Address?`。`Address` 类的定义在下面会说明。 + +`Room` 类是一个简单类，其实例被存储在 `rooms` 数组中。该类只包含一个属性 `name`，以及一个用于将该属性设置为适当的房间名的初始化函数： + +```swift +class Room { + let name: String + init(name: String) { self.name = name } +} +``` + +最后一个类是 `Address`，这个类有三个 `String?` 类型的可选属性。`buildingName` 以及 `buildingNumber` 属性分别表示某个大厦的名称和号码，第三个属性 `street` 表示大厦所在街道的名称： + +```swift +class Address { + var buildingName: String? + var buildingNumber: String? + var street: String? + func buildingIdentifier() -> String? { + if let buildingNumber = buildingNumber, let street = street { + return "\(buildingNumber) \(street)" + } else if buildingName != nil { + return buildingName + } else { + return nil + } + } +} +``` + +`Address` 类提供了 `buildingIdentifier()` 方法，返回值为 `String?`。 如果 `buildingName` 有值则返回 `buildingName`。或者，如果 `buildingNumber` 和 `street` 均有值则返回 `buildingNumber`。否则，返回 `nil`。 + + + +## 通过可选链式调用访问属性 + +正如[使用可选链式调用代替强制展开](#optional_chaining_as_an_alternative_to_forced_unwrapping)中所述，可以通过可选链式调用在一个可选值上访问它的属性，并判断访问是否成功。 + +下面的代码创建了一个 `Person` 实例，然后像之前一样，尝试访问 `numberOfRooms` 属性： + +```swift +let john = Person() +if let roomCount = john.residence?.numberOfRooms { + print("John's residence has \(roomCount) room(s).") +} else { + print("Unable to retrieve the number of rooms.") +} +// 打印 “Unable to retrieve the number of rooms.” +``` + +因为 `john.residence` 为 `nil`，所以这个可选链式调用依旧会像先前一样失败。 + +还可以通过可选链式调用来设置属性值： + +```swift +let someAddress = Address() +someAddress.buildingNumber = "29" +someAddress.street = "Acacia Road" +john.residence?.address = someAddress +``` + +在这个例子中，通过 `john.residence` 来设定 `address` 属性也会失败，因为 `john.residence` 当前为 `nil`。 + +上面代码中的赋值过程是可选链式调用的一部分，这意味着可选链式调用失败时，等号右侧的代码不会被执行。对于上面的代码来说，很难验证这一点，因为像这样赋值一个常量没有任何副作用。下面的代码完成了同样的事情，但是它使用一个函数来创建 `Address` 实例，然后将该实例返回用于赋值。该函数会在返回前打印“Function was called”，这使你能验证等号右侧的代码是否被执行。 + +```swift +func createAddress() -> Address { + print("Function was called.") + + let someAddress = Address() + someAddress.buildingNumber = "29" + someAddress.street = "Acacia Road" + + return someAddress +} +john.residence?.address = createAddress() +``` + +没有任何打印消息，可以看出 `createAddress()` 函数并未被执行。 + + + +## 通过可选链式调用调用方法 + +可以通过可选链式调用来调用方法，并判断是否调用成功，即使这个方法没有返回值。 + +`Residence` 类中的 `printNumberOfRooms()` 方法打印当前的 `numberOfRooms` 值，如下所示： + +```swift +func printNumberOfRooms() { + print("The number of rooms is \(numberOfRooms)") +} +``` + +这个方法没有返回值。然而，没有返回值的方法具有隐式的返回类型 `Void`，如[无返回值函数](./06_Functions.html#functions_without_return_values)中所述。这意味着没有返回值的方法也会返回 `()`，或者说空的元组。 + +如果在可选值上通过可选链式调用来调用这个方法，该方法的返回类型会是 `Void?`，而不是 `Void`，因为通过可选链式调用得到的返回值都是可选的。这样我们就可以使用 `if` 语句来判断能否成功调用 `printNumberOfRooms()` 方法，即使方法本身没有定义返回值。通过判断返回值是否为 `nil` 可以判断调用是否成功： + +```swift +if john.residence?.printNumberOfRooms() != nil { + print("It was possible to print the number of rooms.") +} else { + print("It was not possible to print the number of rooms.") +} +// 打印 “It was not possible to print the number of rooms.” +``` + +同样的，可以据此判断通过可选链式调用为属性赋值是否成功。在上面的[通过可选链式调用访问属性](#accessing_properties_through_optional_chaining)的例子中，我们尝试给 `john.residence `中的 `address` 属性赋值，即使 `residence` 为 `nil` 。通过可选链式调用给属性赋值会返回 `Void?`，通过判断返回值是否为 `nil` 就可以知道赋值是否成功： + +```swift +if (john.residence?.address = someAddress) != nil { + print("It was possible to set the address.") +} else { + print("It was not possible to set the address.") +} +// 打印 “It was not possible to set the address.” +``` + + + +## 通过可选链式调用访问下标 + +通过可选链式调用，我们可以在一个可选值上访问下标，并且判断下标调用是否成功。 + +> 注意 +> 通过可选链式调用访问可选值的下标时，应该将问号放在下标方括号的前面而不是后面。可选链式调用的问号一般直接跟在可选表达式的后面。 + +下面这个例子用下标访问 `john.residence` 属性存储的 `Residence` 实例的 `rooms` 数组中的第一个房间的名称，因为 `john.residence` 为 `nil`，所以下标调用失败了： + +```swift +if let firstRoomName = john.residence?[0].name { + print("The first room name is \(firstRoomName).") +} else { + print("Unable to retrieve the first room name.") +} +// 打印 “Unable to retrieve the first room name.” +``` + +在这个例子中，问号直接放在 `john.residence` 的后面，并且在方括号的前面，因为 `john.residence` 是可选值。 + +类似的，可以通过下标，用可选链式调用来赋值： + +```swift +john.residence?[0] = Room(name: "Bathroom") +``` + +这次赋值同样会失败，因为 `residence` 目前是 `nil`。 + +如果你创建一个 `Residence` 实例，并为其 `rooms` 数组添加一些 `Room` 实例，然后将 `Residence` 实例赋值给 `john.residence`，那就可以通过可选链和下标来访问数组中的元素： + +```swift +let johnsHouse = Residence() +johnsHouse.rooms.append(Room(name: "Living Room")) +johnsHouse.rooms.append(Room(name: "Kitchen")) +john.residence = johnsHouse + +if let firstRoomName = john.residence?[0].name { + print("The first room name is \(firstRoomName).") +} else { + print("Unable to retrieve the first room name.") +} +// 打印 “The first room name is Living Room.” +``` + + + +### 访问可选类型的下标 + +如果下标返回可选类型值，比如 Swift 中 `Dictionary` 类型的键的下标，可以在下标的结尾括号后面放一个问号来在其可选返回值上进行可选链式调用： + +```swift +var testScores = ["Dave": [86, 82, 84], "Bev": [79, 94, 81]] +testScores["Dave"]?[0] = 91 +testScores["Bev"]?[0] += 1 +testScores["Brian"]?[0] = 72 +// "Dave" 数组现在是 [91, 82, 84]，"Bev" 数组现在是 [80, 94, 81] +``` + +上面的例子中定义了一个 `testScores` 数组，包含了两个键值对，把 `String` 类型的键映射到一个 `Int` 值的数组。这个例子用可选链式调用把 `"Dave"`数组中第一个元素设为 `91`，把 `"Bev"` 数组的第一个元素 `+1`，然后尝试把 `"Brian"` 数组中的第一个元素设为 `72`。前两个调用成功，因为 `testScores` 字典中包含 `"Dave"` 和 `"Bev"` 这两个键。但是 `testScores` 字典中没有 `"Brian"` 这个键，所以第三个调用失败。 + + + +## 连接多层可选链式调用 + +可以通过连接多个可选链式调用在更深的模型层级中访问属性、方法以及下标。然而，多层可选链式调用不会增加返回值的可选层级。 + +也就是说： + ++ 如果你访问的值不是可选的，可选链式调用将会返回可选值。 ++ 如果你访问的值就是可选的，可选链式调用不会让可选返回值变得“更可选”。 + +因此： + ++ 通过可选链式调用访问一个 `Int` 值，将会返回 `Int?`，无论使用了多少层可选链式调用。 ++ 类似的，通过可选链式调用访问 `Int?` 值，依旧会返回 `Int?` 值，并不会返回 `Int??`。 + +下面的例子尝试访问 `john` 中的 `residence` 属性中的 `address` 属性中的 `street` 属性。这里使用了两层可选链式调用，`residence` 以及 `address` 都是可选值： + +```swift +if let johnsStreet = john.residence?.address?.street { + print("John's street name is \(johnsStreet).") +} else { + print("Unable to retrieve the address.") +} +// 打印 “Unable to retrieve the address.” +``` + +`john.residence` 现在包含一个有效的 `Residence` 实例。然而，`john.residence.address` 的值当前为 `nil`。因此，调用 `john.residence?.address?.street` 会失败。 + +需要注意的是，上面的例子中，`street` 的属性为 `String?`。`john.residence?.address?.street` 的返回值也依然是 `String?`，即使已经使用了两层可选链式调用。 + +如果为 `john.residence.address` 赋值一个 `Address` 实例，并且为 `address` 中的 `street` 属性设置一个有效值，我们就能过通过可选链式调用来访问 `street` 属性： + +```swift +let johnsAddress = Address() +johnsAddress.buildingName = "The Larches" +johnsAddress.street = "Laurel Street" +john.residence?.address = johnsAddress + +if let johnsStreet = john.residence?.address?.street { + print("John's street name is \(johnsStreet).") +} else { + print("Unable to retrieve the address.") +} +// 打印 “John's street name is Laurel Street.” +``` + +在上面的例子中，因为 `john.residence` 包含一个有效的 `Residence` 实例，所以对 `john.residence` 的 `address` 属性赋值将会成功。 + + + +## 在方法的可选返回值上进行可选链式调用 + +上面的例子展示了如何在一个可选值上通过可选链式调用来获取它的属性值。我们还可以在一个可选值上通过可选链式调用来调用方法，并且可以根据需要继续在方法的可选返回值上进行可选链式调用。 + +在下面的例子中，通过可选链式调用来调用 `Address` 的 `buildingIdentifier()` 方法。这个方法返回 `String?` 类型的值。如上所述，通过可选链式调用来调用该方法，最终的返回值依旧会是 `String?` 类型： + +```swift +if let buildingIdentifier = john.residence?.address?.buildingIdentifier() { + print("John's building identifier is \(buildingIdentifier).") +} +// 打印 “John's building identifier is The Larches.” +``` + +如果要在该方法的返回值上进行可选链式调用，在方法的圆括号后面加上问号即可： + +```swift +if let beginsWithThe = + john.residence?.address?.buildingIdentifier()?.hasPrefix("The") { + if beginsWithThe { + print("John's building identifier begins with \"The\".") + } else { + print("John's building identifier doesn't begin with \"The\".") + } +} +// 打印 “John's building identifier begins with "The".” +``` + +> 注意 +> 在上面的例子中，在方法的圆括号后面加上问号是因为你要在 `buildingIdentifier()` 方法的可选返回值上进行可选链式调用，而不是方法本身。 \ No newline at end of file diff --git a/swift-6-beta.docc/LanguageGuide/Properties.md b/swift-6.docc/LanguageGuide/Properties.md similarity index 50% rename from swift-6-beta.docc/LanguageGuide/Properties.md rename to swift-6.docc/LanguageGuide/Properties.md index 6917539a5..2a8781249 100644 --- a/swift-6-beta.docc/LanguageGuide/Properties.md +++ b/swift-6.docc/LanguageGuide/Properties.md @@ -1,12 +1,8 @@ -# Properties +# 属性 -Access stored and computed values that are part of an instance or type. +访问属于实例或类型的存储值和计算值。 -*Properties* associate values with a particular class, structure, or enumeration. -Stored properties store constant and variable values as part of an instance, -whereas computed properties calculate (rather than store) a value. -Computed properties are provided by classes, structures, and enumerations. -Stored properties are provided only by classes and structures. +**属性**将值与特定的类、结构或枚举关联。存储属性将常量和变量值作为实例的一部分进行存储，而计算属性则计算（而不是存储）一个值。计算属性由类、结构和枚举提供。存储属性仅由类和结构体提供。 -Stored and computed properties are usually associated with instances of a particular type. -However, properties can also be associated with the type itself. -Such properties are known as type properties. +存储和计算属性通常与特定类型的实例相关联。然而，属性也可以与类型本身相关联。这种属性称为类型属性。 -In addition, you can define property observers to monitor changes in a property's value, -which you can respond to with custom actions. -Property observers can be added to stored properties you define yourself, -and also to properties that a subclass inherits from its superclass. +另外，还可以定义属性观察器来监控属性值的变化，以此来触发自定义的操作。属性观察器可以添加到类本身定义的存储属性上，也可以添加到从父类继承的属性上。 - -You can also use a property wrapper -to reuse code in the getter and setter of multiple properties. +你也可以利用属性包装器在多个属性的 getter 和 setter 中复用代码。 -## Stored Properties +## 存储属性 -In its simplest form, a stored property is a constant or variable -that's stored as part of an instance of a particular class or structure. -Stored properties can be either -*variable stored properties* (introduced by the `var` keyword) -or *constant stored properties* (introduced by the `let` keyword). +简单来说，存储属性是作为特定类或结构实例的一部分所存储的常量或变量。存储属性可以是*变量存储属性*（用关键字 `var` 定义） +也可以是*常量存储属性*（用关键字 `let` 定义）。 -You can provide a default value for a stored property as part of its definition, -as described in . -You can also set and modify the initial value for a stored property during initialization. -This is true even for constant stored properties, -as described in . +可以在定义存储属性的时候指定默认值，请参考 一节。还可以在构造过程中设置和修改存储属性的初始值，甚至修改常量存储属性的值，请参考 一节。 -The example below defines a structure called `FixedLengthRange`, -which describes a range of integers -whose range length can't be changed after it's created: +下面的例子定义了一个名为 `FixedLengthRange`的结构体，该结构体用于描述整数的区间，且这个区间值在被创建后不能被修改。 ```swift struct FixedLengthRange { @@ -82,9 +63,9 @@ struct FixedLengthRange { let length: Int } var rangeOfThreeItems = FixedLengthRange(firstValue: 0, length: 3) -// the range represents integer values 0, 1, and 2 +// 该区间表示整数 0，1，2 rangeOfThreeItems.firstValue = 6 -// the range now represents integer values 6, 7, and 8 +// 该区间现在表示整数 6，7，8 ``` -Instances of `FixedLengthRange` have -a variable stored property called `firstValue` -and a constant stored property called `length`. -In the example above, `length` is initialized when the new range is created -and can't be changed thereafter, because it's a constant property. +`FixedLengthRange` 的实例包含一个名为 `firstValue`的变量存储属性和一个名为 `length`的常量存储属性。在上面的例子中， `length` 在创建实例的时候被初始化，且之后无法修改它的值，因为它是一个常量属性。 -### Stored Properties of Constant Structure Instances +### 常量结构体实例的存储属性 -If you create an instance of a structure -and assign that instance to a constant, -you can't modify the instance's properties, -even if they were declared as variable properties: +如果创建了一个结构体实例并将其赋值给一个常量，则无法修改该实例的任何属性，即使它们被声明为可变属性： ```swift let rangeOfFourItems = FixedLengthRange(firstValue: 0, length: 4) -// this range represents integer values 0, 1, 2, and 3 +// 该区间表示整数 0，1，2，3 rangeOfFourItems.firstValue = 6 -// this will report an error, even though firstValue is a variable property +// 尽管 firstValue 是个可变属性，但这里还是会报错 ``` -Because `rangeOfFourItems` is declared as a constant (with the `let` keyword), -it isn't possible to change its `firstValue` property, -even though `firstValue` is a variable property. +因为 `rangeOfFourItems` 被声明为常量（使用 `let` 关键字），所以即使 `firstValue` 是一个可变属性，也无法再修改它了。 -This behavior is due to structures being *value types*. -When an instance of a value type is marked as a constant, -so are all of its properties. +这种行为是由于结构体属于**值类型**。当值类型的实例被声明为常量的时候，它的所有属性也就成了常量。 -The same isn't true for classes, which are *reference types*. -If you assign an instance of a reference type to a constant, -you can still change that instance's variable properties. +属于*引用类型*的类别则不一样，把一个引用类型的实例赋给一个常量后，依然可以修改该实例的可变属性。 -### Lazy Stored Properties +### 延时加载存储属性 -A *lazy stored property* is a property whose initial value isn't calculated -until the first time it's used. -You indicate a lazy stored property by writing -the `lazy` modifier before its declaration. +*延时加载存储属性*是指当第一次被调用的时候才会计算其初始值的属性。在属性声明前使用 `lazy` 来标示一个延时加载存储属性。 -> Note: You must always declare a lazy property as a variable (with the `var` keyword), -> because its initial value might not be retrieved until -> after instance initialization completes. -> Constant properties must always have a value *before* initialization completes, -> and therefore can't be declared as lazy. +> 注意: +> 必须将延时加载属性声明成变量（使用 `var` 关键词），因为属性的初始值可能在实例构造完成之后才会得到。而常量属性在构造过程完成之前必须要有初始值，因此无法声明成延时加载。 -Lazy properties are useful when the initial value for a property -is dependent on outside factors whose values aren't known -until after an instance's initialization is complete. -Lazy properties are also useful when the initial value for a property requires -complex or computationally expensive setup that shouldn't be performed -unless or until it's needed. +延时加载属性在属性的初始值依赖于外部因素，且这些因素的值在实例初始化完成后才会知道时非常有用。或者当获得属性的值因为需要复杂或者大量的计算，而应该采用需要的时候再计算的方式，延时加载属性也会很有用。 -The example below uses a lazy stored property to avoid -unnecessary initialization of a complex class. -This example defines two classes called `DataImporter` and `DataManager`, -neither of which is shown in full: +下面的例子使用了延时加载存储属性来避免复杂类中不必要的初始化工作。例子中定义了 `DataImporter` 和 `DataManager` 两个类，下面是部分代码： ```swift class DataImporter { /* - DataImporter is a class to import data from an external file. - The class is assumed to take a nontrivial amount of time to initialize. + DataImporter 是一个负责将外部文件中的数据导入的类。 + 这个类的初始化会消耗不少时间。 */ var filename = "data.txt" - // the DataImporter class would provide data importing functionality here + // 这里会提供数据导入功能 } class DataManager { lazy var importer = DataImporter() var data: [String] = [] - // the DataManager class would provide data management functionality here + // 这里会提供数据管理功能 } let manager = DataManager() manager.data.append("Some data") manager.data.append("Some more data") -// the DataImporter instance for the importer property hasn't yet been created +// DataImporter 实例的 importer 属性还没有被创建 ``` -The `DataManager` class has a stored property called `data`, -which is initialized with a new, empty array of `String` values. -Although the rest of its functionality isn't shown, -the purpose of this `DataManager` class is to manage and provide access to -this array of `String` data. - -Part of the functionality of the `DataManager` class -is the ability to import data from a file. -This functionality is provided by the `DataImporter` class, -which is assumed to take a nontrivial amount of time to initialize. -This might be because a `DataImporter` instance needs to open a file -and read its contents into memory when the `DataImporter` instance is initialized. - -Because it's possible for a `DataManager` instance to manage its data -without ever importing data from a file, -`DataManager` doesn't create a new `DataImporter` instance -when the `DataManager` itself is created. -Instead, it makes more sense to create the `DataImporter` instance -if and when it's first used. - -Because it's marked with the `lazy` modifier, -the `DataImporter` instance for the `importer` property -is only created when the `importer` property is first accessed, -such as when its `filename` property is queried: +`DataManager` 类包含一个名为 `data`的存储属性，初始值是一个空的字符串数组。这里没有给出全部代码，只需知道 `DataManager` 类的目的是管理和提供对这个字符串数组的访问即可。 + +`DataManager` 的一个功能是从文件中导入数据。这个功能由 `DataImporter` 类提供，`DataImporter` 完成初始化需要消耗不少时间：因为它的实例在初始化时可能需要打开文件并读取文件中的内容到内存中。 + +`DataManager` 管理数据时也可能不从文件中导入数据。所以当 `DataManager` 的实例被创建时，不会创建一个 `DataImporter` 的实例，更明智的做法是第一次用到 `DataManager` 的时候才去创建它。 + +由于使用了 `lazy`，`DataImporter` 的实例 `importer` 属性只有在第一次被访问的时候才被创建。比如访问它的属性 `filename` 的时候： ```swift print(manager.importer.filename) -// the DataImporter instance for the importer property has now been created -// Prints "data.txt" +// DataImporter 实例的 importer 属性现在被创建了 +// 输出“data.txt” ``` -> Note: If a property marked with the `lazy` modifier -> is accessed by multiple threads simultaneously -> and the property hasn't yet been initialized, -> there's no guarantee that the property will be initialized only once. +> 注意: +> 如果一个被标记为 `lazy` 的属性在没有初始化时就同时被多个线程访问，则无法保证该属性只会被初始化一次。 -### Stored Properties and Instance Variables +### 存储属性和实例变量 -If you have experience with Objective-C, -you may know that it provides *two* ways -to store values and references as part of a class instance. -In addition to properties, -you can use instance variables as a backing store for the values stored in a property. +如果你有过使用 Objective-C 的经验，应该知道 Objective-C 为类实例存储值和引用提供了两种方法。除了属性之外，还可以使用实例变量作为一个备份存储将变量值赋值给属性。 -Swift unifies these concepts into a single property declaration. -A Swift property doesn't have a corresponding instance variable, -and the backing store for a property isn't accessed directly. -This approach avoids confusion about how the value is accessed in different contexts -and simplifies the property's declaration into a single, definitive statement. -All information about the property --- -including its name, type, and memory management characteristics --- -is defined in a single location as part of the type's definition. +Swift 编程语言中把这些理论统一用属性来实现。Swift 中的属性没有对应的实例变量，属性的备份存储也无法直接访问。这就避免了不同场景下访问方式的困扰，同时也将属性的定义简化成一个语句。属性的全部信息——包括命名、类型和内存管理特征——作为类型定义的一部分，都定义在一个地方。 -## Computed Properties +## 计算属性 -In addition to stored properties, -classes, structures, and enumerations can define *computed properties*, -which don't actually store a value. -Instead, they provide a getter and an optional setter -to retrieve and set other properties and values indirectly. +除存储属性外，类、结构体和枚举还可以定义计算属性。计算属性不直接存储值，而是提供一个 getter 和一个可选的 setter，来间接获取和设置其他属性或变量的值。 ```swift struct Point { @@ -358,10 +278,10 @@ struct Rect { var square = Rect(origin: Point(x: 0.0, y: 0.0), size: Size(width: 10.0, height: 10.0)) let initialSquareCenter = square.center -// initialSquareCenter is at (5.0, 5.0) +// initialSquareCenter 位于（5.0， 5.0） square.center = Point(x: 15.0, y: 15.0) print("square.origin is now at (\(square.origin.x), \(square.origin.y))") -// Prints "square.origin is now at (10.0, 10.0)" +// 打印“square.origin is now at (10.0, 10.0)” ``` -This example defines three structures for working with geometric shapes: - -- `Point` encapsulates the x- and y-coordinate of a point. -- `Size` encapsulates a `width` and a `height`. -- `Rect` defines a rectangle by an origin point and a size. - -The `Rect` structure also provides a computed property called `center`. -The current center position of a `Rect` can always be determined from its `origin` and `size`, -and so you don't need to store the center point as an explicit `Point` value. -Instead, `Rect` defines a custom getter and setter for a computed variable called `center`, -to enable you to work with the rectangle's `center` as if it were a real stored property. - -The example above creates a new `Rect` variable called `square`. -The `square` variable is initialized with an origin point of `(0, 0)`, -and a width and height of `10`. -This square is represented by the light green square in the diagram below. - -The `square` variable's `center` property is then accessed through dot syntax (`square.center`), -which causes the getter for `center` to be called, -to retrieve the current property value. -Rather than returning an existing value, -the getter actually calculates and returns a new `Point` to represent the center of the square. -As can be seen above, the getter correctly returns a center point of `(5, 5)`. - -The `center` property is then set to a new value of `(15, 15)`, -which moves the square up and to the right, -to the new position shown by the dark green square in the diagram below. -Setting the `center` property calls the setter for `center`, -which modifies the `x` and `y` values of the stored `origin` property, -and moves the square to its new position. +此示例定义了三个用于处理几何形状的结构： + +- `Point` 封装了一个点的 x 和 y 坐标。 +- `Size` 包含 `width` 和 `height`. +- `Rect` 通过一个原点和大小来定义一个矩形。 + +`Rect` 还提供了一个名为 `center` 的计算属性。`Rect` 的当前中心位置始终可以从其原点和大小确定，因此不需要将中心点以 `Point` 类型的值来存储。`Rect` 的计算属性 `center`提供了自定义的 getter 和 setter 来获取和设置矩形的中心点，就像它有一个存储属性一样。 + +上面的示例创建了一个名为 `square` 的 `Rect` 变量，初始值原点是 `(0, 0)`，宽度和高度均为 `10`。如下图中的浅绿色正方形所示。 + +`square` 的 `center` 属性可以通过点运算符（`square.center`）来访问，这会调用该属性的 getter 来获取它的值。跟直接返回已经存在的值不同，getter 实际上是通过计算然后返回一个新的 `Point` 来表示 `square` 的中心点，如代码所示，它正确返回了中心点 `(5, 5)`。 + +将 `center` 属性设置为新的值 `(15, 15)`，会将正方形向上和向右移动，移动到下图中深绿色正方形所示的新位置。设置 `center` 属性会调用 `center` 的 setter，修改存储的 `origin` 属性的 `x` 和 `y` 值，从而将正方形移动到新的位置。 ![](computedProperties) -### Shorthand Setter Declaration +### 简化 Setter 声明 -If a computed property's setter doesn't define a name for the new value to be set, -a default name of `newValue` is used. -Here's an alternative version of the `Rect` structure -that takes advantage of this shorthand notation: +如果计算属性的 setter 没有为要设置的新值定义名称，则默认会使用 `newValue` 作为名称。这里是利用这种简写方式的 `Rect` 结构体的另一个版本： ```swift struct AlternativeRect { @@ -484,13 +384,9 @@ struct AlternativeRect { -### Shorthand Getter Declaration +### 简化 Getter 声明 -If the entire body of a getter is a single expression, -the getter implicitly returns that expression. -Here's another version of the `Rect` structure -that takes advantage of this shorthand notation -and the shorthand notation for setters: +如果 getter 的主体是一个单一表达式，那么 getter 会隐式返回该表达式。这里是另一个利用这种 getter 和 setter 简写方式的 `Rect` 结构体版本： ```swift struct CompactRect { @@ -530,21 +426,14 @@ struct CompactRect { ``` --> -Omitting the `return` from a getter -follows the same rules as omitting `return` from a function, -as described in . +省略 getter 中的 `return` 遵循与函数省略 `return` 相同的规则，详见 。 -### Read-Only Computed Properties +### 只读计算属性 -A computed property with a getter but no setter is known as a *read-only computed property*. -A read-only computed property always returns a value, -and can be accessed through dot syntax, but can't be set to a different value. +只有 getter 而没有 setter 的计算属性被称为*只读计算属性*。只读计算属性总是返回一个值，可以通过点运算符访问，但不能设置为其他值。 -> Note: You must declare computed properties --- including read-only computed properties --- -> as variable properties with the `var` keyword, because their value isn't fixed. -> The `let` keyword is only used for constant properties, -> to indicate that their values can't be changed once they're set -> as part of instance initialization. +> 注意: +> 你必须将计算属性——包括只读计算属性——声明为使用 `var` 关键字的变量属性，因为它们的值并非固定的。`let` 关键字只用于常量属性，表示它们的值在实例初始化时设置后就无法更改。 -You can simplify the declaration of a read-only computed property -by removing the `get` keyword and its braces: +可以通过省略 `get` 关键字和它的花括号来简化只读计算属性的声明： ```swift struct Cuboid { @@ -577,7 +465,7 @@ struct Cuboid { } let fourByFiveByTwo = Cuboid(width: 4.0, height: 5.0, depth: 2.0) print("the volume of fourByFiveByTwo is \(fourByFiveByTwo.volume)") -// Prints "the volume of fourByFiveByTwo is 40.0" +// 打印 "the volume of fourByFiveByTwo is 40.0" ``` -This example defines a new structure called `Cuboid`, -which represents a 3D rectangular box with `width`, `height`, and `depth` properties. -This structure also has a read-only computed property called `volume`, -which calculates and returns the current volume of the cuboid. -It doesn't make sense for `volume` to be settable, -because it would be ambiguous as to which values of `width`, `height`, and `depth` -should be used for a particular `volume` value. -Nonetheless, it's useful for a `Cuboid` to provide a read-only computed property -to enable external users to discover its current calculated volume. +这个示例定义了一个名为 `Cuboid` 的新结构体，用于表示一个具有 `width`、`height` 和 `depth` 属性的三维立方体。该结构体还包含一个名为 `volume` 的只读计算属性，用于计算并返回立方体的当前体积。`volume` 属性不应是可设置的，因为这样会导致对于应使用哪些 `width`、`height` 和 `depth` 值来计算特定体积产生歧义。然而，`Cuboid` 提供一个只读计算属性来让外部用户了解其当前计算的体积是非常有用的。 -## Property Observers +## 属性观察器 -Property observers observe and respond to changes in a property's value. -Property observers are called every time a property's value is set, -even if the new value is the same as the property's current value. +属性观察器用于监测并响应属性值的变化。每次属性值被设置时，无论新值是否与当前值相同，属性观察器都会被调用。 -You can add property observers in the following places: +属性观察器可以添加在以下位置： -- Stored properties that you define -- Stored properties that you inherit -- Computed properties that you inherit +- 自定义的存储属性 +- 继承的存储属性 +- 继承的计算属性 -For an inherited property, -you add a property observer by overriding that property in a subclass. -For a computed property that you define, -use the property's setter to observe and respond to value changes, -instead of trying to create an observer. -Overriding properties is described in . +对于继承的属性，可以通过在子类中重写该属性来添加属性观察器。对于自定义的计算属性，应使用属性的 setter 来观察和响应值的变化，而不是试图创建一个观察器。重写属性的相关内容详见 。 -You have the option to define either or both of these observers on a property: +可以为属性添加以下一个或两个观察器： -- `willSet` is called just before the value is stored. -- `didSet` is called immediately after the new value is stored. +- `willSet` 在值存储之前被调用。 +- `didSet` 在新值存储之后立即被调用。 -If you implement a `willSet` observer, -it's passed the new property value as a constant parameter. -You can specify a name for this parameter as part of your `willSet` implementation. -If you don't write the parameter name and parentheses within your implementation, -the parameter is made available with a default parameter name of `newValue`. +如果实现了 `willSet` 观察器，新的属性值会作为一个常量参数传递。可以在 `willSet` 实现中为这个参数指定名称。如果没有在实现中写出参数名称和括号，则该参数将以默认名称 `newValue` 提供。 -Similarly, if you implement a `didSet` observer, -it's passed a constant parameter containing the old property value. -You can name the parameter or use the default parameter name of `oldValue`. -If you assign a value to a property within its own `didSet` observer, -the new value that you assign replaces the one that was just set. +同样地，如果实现了 `didSet` 观察器，旧的属性值会作为一个常量参数传递。可以命名这个参数，也可以使用默认名称 `oldValue`。如果在 `didSet` 观察器内为属性赋值，所分配的新值将覆盖刚刚设置的值。 -> Note: The `willSet` and `didSet` observers of superclass properties -> are called when a property is set in a subclass initializer, -> after the superclass initializer has been called. -> They aren't called while a class is setting its own properties, -> before the superclass initializer has been called. +> 注意: +> 父类属性的 `willSet` 和 `didSet` 观察器会在子类初始化器中设置属性时调用，此时父类的初始化器已经被调用。而在父类初始化器被调用之前，给子类的属性赋值时不会调用子类属性的观察器。 > -> For more information about initializer delegation, -> see -> and . +> 有关初始化器代理的更多信息，请参见 和 。 -Here's an example of `willSet` and `didSet` in action. -The example below defines a new class called `StepCounter`, -which tracks the total number of steps that a person takes while walking. -This class might be used with input data from a pedometer or other step counter -to keep track of a person's exercise during their daily routine. +下面是一个 `willSet` 和 `didSet` 实际应用的例子。以下示例定义了一个名为 `StepCounter` 的新类，用于跟踪一个人在行走时所走的总步数。这个类可以与计步器或其他步数计数器的输入数据配合使用，以记录一个人在日常锻炼中的活动情况。 ```swift class StepCounter { @@ -792,14 +648,14 @@ class StepCounter { } let stepCounter = StepCounter() stepCounter.totalSteps = 200 -// About to set totalSteps to 200 -// Added 200 steps +// 将 totalSteps 的值设置为 200 +// 增加了 200 步 stepCounter.totalSteps = 360 -// About to set totalSteps to 360 -// Added 160 steps +// 将 totalSteps 的值设置为 360 +// 增加了 160 步 stepCounter.totalSteps = 896 -// About to set totalSteps to 896 -// Added 536 steps +// 将 totalSteps 的值设置为 896 +// 增加了 536 步 ``` -The `StepCounter` class declares a `totalSteps` property of type `Int`. -This is a stored property with `willSet` and `didSet` observers. +`StepCounter` 类声明了一个名为 `totalSteps` 的 `Int` 类型属性。这是一个具有 `willSet` 和 `didSet` 观察器的存储属性。 -The `willSet` and `didSet` observers for `totalSteps` are called -whenever the property is assigned a new value. -This is true even if the new value is the same as the current value. +当 `totalSteps` 属性被赋予新值时，这些观察器都会被调用，即使新值与当前值相同也是如此。 -This example's `willSet` observer uses -a custom parameter name of `newTotalSteps` for the upcoming new value. -In this example, it simply prints out the value that's about to be set. +这个例子中的 `willSet` 观察器为即将设置的新值使用了自定义参数名称 `newTotalSteps`，在这里，它简单地打印出即将被设置的值。 -The `didSet` observer is called after the value of `totalSteps` is updated. -It compares the new value of `totalSteps` against the old value. -If the total number of steps has increased, -a message is printed to indicate how many new steps have been taken. -The `didSet` observer doesn't provide a custom parameter name for the old value, -and the default name of `oldValue` is used instead. +`didSet` 观察器在 `totalSteps` 的值更新后被调用。它将 `totalSteps` 的新值与旧值进行比较。如果步数总数增加了，则会打印一条消息，指示增加了多少步数。`didSet` 观察器没有为旧值提供自定义参数名称，使用的是默认名称 `oldValue`。 -> Note: If you pass a property that has observers -> to a function as an in-out parameter, -> the `willSet` and `didSet` observers are always called. -> This is because of the copy-in copy-out memory model for in-out parameters: -> The value is always written back to the property at the end of the function. -> For a detailed discussion of the behavior of in-out parameters, -> see . +> 注意: +> 如果将一个具有观察器的属性作为 in-out 参数传递给函数，那么 `willSet` 和 `didSet` 观察器总是会被调用。这是因为 in-out 参数的复制-写回内存模型：在函数结束时，值总是会被写回属性。关于 in-out 参数行为的详细讨论，请参见 。 -## Property Wrappers - -A property wrapper adds a layer of separation -between code that manages how a property is stored -and the code that defines a property. -For example, -if you have properties that -provide thread-safety checks -or store their underlying data in a database, -you have to write that code on every property. -When you use a property wrapper, -you write the management code once when you define the wrapper, -and then reuse that management code by applying it to multiple properties. - -To define a property wrapper, -you make a structure, enumeration, or class -that defines a `wrappedValue` property. -In the code below, -the `TwelveOrLess` structure ensures that -the value it wraps always contains a number less than or equal to 12. -If you ask it to store a larger number, it stores 12 instead. +## 属性包装器 + +属性包装器在管理属性存储方式的代码和定义属性的代码之间添加了一层分离。例如，如果有一些属性需要提供线程安全检查或将其底层数据存储在数据库中，那么你必须在每个属性上编写这些代码。而使用属性包装器时，只需在定义包装器时编写一次管理代码，然后通过将其应用于多个属性来重复使用这些管理代码。 + +要定义属性包装器，需要创建一个结构体、枚举或类，并定义一个 `wrappedValue` 属性。在下面的代码中，`TwelveOrLess` 结构体确保它所包装的值始终不大于 12。如果试图存储更大的数字，它会将数字存储为 12。 ```swift @propertyWrapper @@ -933,17 +758,10 @@ struct TwelveOrLess { Always initializing the wrapped value is a simpler starting point. --> -The setter ensures that new values are less than or equal to 12, -and the getter returns the stored value. +setter 确保新值不大于 12，而 getter 返回存储的值。 -> Note: The declaration for `number` in the example above -> marks the variable as `private`, -> which ensures `number` is used only -> in the implementation of `TwelveOrLess`. -> Code that's written anywhere else -> accesses the value using the getter and setter for `wrappedValue`, -> and can't use `number` directly. -> For information about `private`, see . +> 注意: +> 上例中 `number` 的声明被标记为 `private`，这确保了 `number` 只能在 `TwelveOrLess` 的实现中使用。其他地方的代码只能通过 `wrappedValue` 的 getter 和 setter 来访问这个值，而不能直接使用 `number`。关于 `private` 的更多信息，请参见 。 -You apply a wrapper to a property -by writing the wrapper's name before the property -as an attribute. -Here's a structure that stores a rectangle -that uses the `TwelveOrLess` property wrapper -to ensure its dimensions are always 12 or less: +可以通过在属性前作为特性写上包装器的名称来应用包装器。下面是一个存储矩形的结构体，使用 `TwelveOrLess` 属性包装器来确保其尺寸始终不超过 12： ```swift struct SmallRectangle { @@ -1004,15 +817,15 @@ struct SmallRectangle { var rectangle = SmallRectangle() print(rectangle.height) -// Prints "0" +// 打印 "0" rectangle.height = 10 print(rectangle.height) -// Prints "10" +// 打印 "10" rectangle.height = 24 print(rectangle.height) -// Prints "12" +// 打印 "12" ``` -The `height` and `width` properties get their initial values -from the definition of `TwelveOrLess`, -which sets `TwelveOrLess.number` to zero. -The setter in `TwelveOrLess` treats 10 as a valid value -so storing the number 10 in `rectangle.height` proceeds as written. -However, 24 is larger than `TwelveOrLess` allows, -so trying to store 24 end up setting `rectangle.height` -to 12 instead, the largest allowed value. - -When you apply a wrapper to a property, -the compiler synthesizes code that provides storage for the wrapper -and code that provides access to the property through the wrapper. -(The property wrapper is responsible for storing the wrapped value, -so there's no synthesized code for that.) -You could write code that uses the behavior of a property wrapper, -without taking advantage of the special attribute syntax. -For example, -here's a version of `SmallRectangle` -from the previous code listing -that wraps its properties in the `TwelveOrLess` structure explicitly, -instead of writing `@TwelveOrLess` as an attribute: +`height` 和 `width` 属性的初始值来自 `TwelveOrLess` 的定义，其中将 `TwelveOrLess.number` 设置为0。`TwelveOrLess` 中的 setter 将 10 视为有效值，因此将数字 10 存储在 `rectangle.height` 中的操作能成功。然而，24 超出了 `TwelveOrLess` 允许的范围，因此尝试存储 24 最终会将 `rectangle.height` 设置为 12，这是允许的最大值。 + +当为属性应用包装器时，编译器会生成代码，为包装器提供存储空间，并通过包装器提供对属性的访问。（属性包装器负责存储被包装的值，因此不会为此生成代码。）可以编写代码使用属性包装器的行为，而不必利用特殊的特性语法。例如，下面是前面代码示例中 `SmallRectangle` 的一个版本，它明确的将其属性包装在 `TwelveOrLess` 结构体中，而不是将 `@TwelveOrLess` 写作一个特性： ```swift struct SmallRectangle { @@ -1094,26 +889,11 @@ struct SmallRectangle { ``` --> -The `_height` and `_width` properties -store an instance of the property wrapper, `TwelveOrLess`. -The getter and setter for `height` and `width` -wrap access to the `wrappedValue` property. - -### Setting Initial Values for Wrapped Properties - -The code in the examples above -sets the initial value for the wrapped property -by giving `number` an initial value in the definition of `TwelveOrLess`. -Code that uses this property wrapper -can't specify a different initial value for a property -that's wrapped by `TwelveOrLess` --- -for example, -the definition of `SmallRectangle` -can't give `height` or `width` initial values. -To support setting an initial value or other customization, -the property wrapper needs to add an initializer. -Here's an expanded version of `TwelveOrLess` called `SmallNumber` -that defines initializers that set the wrapped and maximum value: +`_height` 和 `_width` 属性存储了属性包装器 `TwelveOrLess` 的一个实例。`height` 和 `width` 的 getter 和 setter 包装了对 `wrappedValue` 属性的访问。 + +### 设置被包装属性的初始值 + +上面示例中的代码通过在 `TwelveOrLess` 的定义中为 `number` 赋予初始值来设置被包装属性的初始值。使用该属性包装器的代码不能为被 `TwelveOrLess` 包装的属性指定不同的初始值——例如，`SmallRectangle` 的定义不能为 `height` 或 `width` 赋予初始值。为了支持设置初始值或其他自定义，属性包装器需要添加一个构造器。这里是 `TwelveOrLess` 的扩展版本，名为 `SmallNumber`，它定义了可以设置被包装值和最大值的构造器： ```swift @propertyWrapper @@ -1181,16 +961,9 @@ struct SmallNumber { so it's clearer to make each init stand on its own. --> -The definition of `SmallNumber` includes three initializers --- -`init()`, `init(wrappedValue:)`, and `init(wrappedValue:maximum:)` --- -which the examples below use -to set the wrapped value and the maximum value. -For information about initialization and initializer syntax, -see . +`SmallNumber` 的定义包括三个构造器——`init()`、`init(wrappedValue:)` 和 `init(wrappedValue:maximum:)`——下面的示例使用这些构造器来设置被包装值和最大值。关于初始化和构造器语法的更多信息，请参见 。 -When you apply a wrapper to a property and you don't specify an initial value, -Swift uses the `init()` initializer to set up the wrapper. -For example: +当为属性应用包装器且未指定初始值时，Swift 使用 `init()` 构造器来设置包装器。例如： ```swift struct ZeroRectangle { @@ -1200,7 +973,7 @@ struct ZeroRectangle { var zeroRectangle = ZeroRectangle() print(zeroRectangle.height, zeroRectangle.width) -// Prints "0 0" +// 打印 "0 0" ``` -The instances of `SmallNumber` that wrap `height` and `width` -are created by calling `SmallNumber()`. -The code inside that initializer -sets the initial wrapped value and the initial maximum value, -using the default values of zero and 12. -The property wrapper still provides all of the initial values, -like the earlier example that used `TwelveOrLess` in `SmallRectangle`. -Unlike that example, -`SmallNumber` also supports writing those initial values -as part of declaring the property. - -When you specify an initial value for the property, -Swift uses the `init(wrappedValue:)` initializer to set up the wrapper. -For example: +通过调用 `SmallNumber()` 创建了用于包装 `height` 和 `width` 的 `SmallNumber` 实例。该构造器中的代码使用默认值 0 和 12 设置了初始包装值和初始最大值。属性包装器仍然提供所有的初始值，就像前面在 `SmallRectangle` 中使用 `TwelveOrLess` 的示例一样。但与该示例不同，`SmallNumber` 还支持在声明属性时写入这些初始值。 + +当为属性指定初始值时，Swift 使用 `init(wrappedValue:)` 构造器来设置包装器。例如： ```swift struct UnitRectangle { @@ -1263,7 +1025,7 @@ struct UnitRectangle { var unitRectangle = UnitRectangle() print(unitRectangle.height, unitRectangle.width) -// Prints "1 1" +// 打印 "1 1" ``` -When you write `= 1` on a property with a wrapper, -that's translated into a call to the `init(wrappedValue:)` initializer. -The instances of `SmallNumber` that wrap `height` and `width` -are created by calling `SmallNumber(wrappedValue: 1)`. -The initializer uses the wrapped value that's specified here, -and it uses the default maximum value of 12. +当在具有包装器的属性上写 `= 1` 时，这会被转换为调用 `init(wrappedValue:)` 构造器。用于包装 `height` 和 `width` 的 `SmallNumber` 实例通过调用 `SmallNumber(wrappedValue: 1)` 创建。构造器使用了这里指定的包装值，并使用默认的最大值 12。 -When you write arguments in parentheses after the custom attribute, -Swift uses the initializer that accepts those arguments to set up the wrapper. -For example, if you provide an initial value and a maximum value, -Swift uses the `init(wrappedValue:maximum:)` initializer: +当在自定义特性后面的括号中写入参数时，Swift 使用接受这些参数的构造器来设置包装器。例如，如果提供了初始值和最大值，Swift 会使用 `init(wrappedValue:maximum:)` 构造器： ```swift struct NarrowRectangle { @@ -1323,12 +1077,12 @@ struct NarrowRectangle { var narrowRectangle = NarrowRectangle() print(narrowRectangle.height, narrowRectangle.width) -// Prints "2 3" +// 打印 "2 3" narrowRectangle.height = 100 narrowRectangle.width = 100 print(narrowRectangle.height, narrowRectangle.width) -// Prints "5 4" +// 打印 "5 4" ``` -The instance of `SmallNumber` that wraps `height` -is created by calling `SmallNumber(wrappedValue: 2, maximum: 5)`, -and the instance that wraps `width` -is created by calling `SmallNumber(wrappedValue: 3, maximum: 4)`. +用于包装 `height` 的 `SmallNumber` 实例是通过调用 `SmallNumber(wrappedValue: 2, maximum: 5)` 创建的，而用于包装 `width` 的实例是通过调用 `SmallNumber(wrappedValue: 3, maximum: 4)` 创建的。 -By including arguments to the property wrapper, -you can set up the initial state in the wrapper -or pass other options to the wrapper when it's created. -This syntax is the most general way to use a property wrapper. -You can provide whatever arguments you need to the attribute, -and they're passed to the initializer. +通过在属性包装器中包含参数，可以在包装器中设置初始状态或在创建时传递其他选项。这种语法是使用属性包装器的最通用方式，可以为特性提供所需的任何参数，这些参数会被传递给构造器。 -When you include property wrapper arguments, -you can also specify an initial value using assignment. -Swift treats the assignment like a `wrappedValue` argument -and uses the initializer that accepts the arguments you include. -For example: +当包含属性包装器参数时，还可以通过赋值指定初始值。Swift 会将该赋值视为 `wrappedValue` 参数，并使用接受所包含参数的构造器。例如： ```swift struct MixedRectangle { @@ -1403,11 +1145,11 @@ struct MixedRectangle { var mixedRectangle = MixedRectangle() print(mixedRectangle.height) -// Prints "1" +// 打印 "1" mixedRectangle.height = 20 print(mixedRectangle.height) -// Prints "12" +// 打印 "12" ``` -The instance of `SmallNumber` that wraps `height` -is created by calling `SmallNumber(wrappedValue: 1)`, -which uses the default maximum value of 12. -The instance that wraps `width` -is created by calling `SmallNumber(wrappedValue: 2, maximum: 9)`. - -### Projecting a Value From a Property Wrapper - -In addition to the wrapped value, -a property wrapper can expose additional functionality -by defining a *projected value* --- -for example, a property wrapper that manages access to a database -can expose a `flushDatabaseConnection()` method on its projected value. -The name of the projected value is the same as the wrapped value, -except it begins with a dollar sign (`$`). -Because your code can't define properties that start with `$` -the projected value never interferes with properties you define. - -In the `SmallNumber` example above, -if you try to set the property to a number that's too large, -the property wrapper adjusts the number before storing it. -The code below adds a `projectedValue` property to the `SmallNumber` structure -to keep track of whether the property wrapper -adjusted the new value for the property before storing that new value. +用于包装 `height` 的 `SmallNumber` 实例是通过调用 `SmallNumber(wrappedValue: 1)` 创建的，使用默认的最大值 12。用于包装 `width` 的实例是通过调用 `SmallNumber(wrappedValue: 2, maximum: 9)` 创建的。 + +### 从属性包装器中呈现一个值 + +除了被包装的值之外，属性包装器还可以通过定义*被呈现值*来提供额外的功能——例如，管理数据库访问的属性包装器可以在其被呈现值上暴露一个 `flushDatabaseConnection()` 方法。被呈现值的名称与被包装值相同，只是以美元符号 (`$`) 开头。由于代码中不能定义以 `$` 开头的属性，因此被呈现值不会与定义的属性产生冲突。 + +在上面的 `SmallNumber` 示例中，如果尝试将属性设置为一个过大的数字，属性包装器会在存储之前调整该数字。下面的代码向 `SmallNumber` 结构体添加了一个 `projectedValue` 属性，用于跟踪属性包装器在存储新值之前是否调整了该新值。 ```swift @propertyWrapper @@ -1485,11 +1210,11 @@ var someStructure = SomeStructure() someStructure.someNumber = 4 print(someStructure.$someNumber) -// Prints "false" +// 打印 "false" someStructure.someNumber = 55 print(someStructure.$someNumber) -// Prints "true" +// 打印 "true" ``` -Writing `someStructure.$someNumber` accesses the wrapper's projected value. -After storing a small number like four, -the value of `someStructure.$someNumber` is `false`. -However, -the projected value is `true` -after trying to store a number that's too large, like 55. - -A property wrapper can return a value of any type as its projected value. -In this example, -the property wrapper exposes only one piece of information --- -whether the number was adjusted --- -so it exposes that Boolean value as its projected value. -A wrapper that needs to expose more information -can return an instance of some other type, -or it can return `self` -to expose the instance of the wrapper as its projected value. - -When you access a projected value from code that's part of the type, -like a property getter or an instance method, -you can omit `self.` before the property name, -just like accessing other properties. -The code in the following example refers to the projected value -of the wrapper around `height` and `width` as `$height` and `$width`: +写作 `someStructure.$someNumber` 时会访问包装器的被呈现值。存储像 4 这样的小数字后，`someStructure.$someNumber` 的值是 `false`。然而，在尝试存储过大的数字（如 55）后，被呈现值会变为 `true`。 + +属性包装器可以将任何类型的值作为其被呈现值。在这个例子中，属性包装器只暴露了一条信息——数字是否被调整——因此它将这个布尔值作为被呈现值暴露出来。需要暴露更多信息的包装器可以返回某种其他类型的实例，或者返回 `self`，以将包装器的实例作为被呈现值暴露出来。 + +当在类型的代码中（如属性的 getter 或实例方法）访问被呈现值时，可以省略属性名称前的 `self.`，就像访问其他属性一样。以下示例中的代码将围绕 `height` 和 `width` 的包装器的被呈现值分别引用为 `$height` 和 `$width`： ```swift enum Size { @@ -1614,43 +1321,15 @@ struct SizedRectangle { ``` --> -Because property wrapper syntax is just syntactic sugar -for a property with a getter and a setter, -accessing `height` and `width` -behaves the same as accessing any other property. -For example, -the code in `resize(to:)` accesses `height` and `width` -using their property wrapper. -If you call `resize(to: .large)`, -the switch case for `.large` sets the rectangle's height and width to 100. -The wrapper prevents the value of those properties -from being larger than 12, -and it sets the projected value to `true`, -to record the fact that it adjusted their values. -At the end of `resize(to:)`, -the return statement checks `$height` and `$width` -to determine whether -the property wrapper adjusted either `height` or `width`. - -## Global and Local Variables - -The capabilities described above for computing and observing properties -are also available to *global variables* and *local variables*. -Global variables are variables that are defined outside of any -function, method, closure, or type context. -Local variables are variables that are defined within -a function, method, or closure context. - -The global and local variables you have encountered in previous chapters -have all been *stored variables*. -Stored variables, like stored properties, -provide storage for a value of a certain type and allow that value to be set and retrieved. - -However, you can also define *computed variables* -and define observers for stored variables, -in either a global or local scope. -Computed variables calculate their value, rather than storing it, -and they're written in the same way as computed properties. +由于属性包装器语法只是带有 getter 和 setter 的属性的语法糖，访问 `height` 和 `width` 的行为与访问其他属性相同。例如，`resize(to:)` 中的代码使用它们的属性包装器访问 `height` 和 `width`。如果调用 `resize(to: .large)`，那么 `.large` 的 switch 分支会将矩形的高度和宽度设置为 100。包装器会防止这些属性的值大于 12，并将被呈现值设置为 `true`，以记录它调整了这些值的事实。在 `resize(to:)` 的末尾，return 语句检查 `$height` 和 `$width` 以确定属性包装器是否调整了 `height` 或 `width`。 + +## 全局变量和局部变量 + +上面描述的用于计算和观察属性的功能同样适用于*全局变量*和*局部变量*。全局变量是定义在任何函数、方法、闭包或类型上下文之外的变量。局部变量是在函数、方法或闭包上下文中定义的变量。 + +在前面的章节中遇到的全局变量和局部变量都是*存储变量*。存储变量与存储属性类似，为某种类型的值提供存储，并允许设置和获取该值。 + +然而，也可以在全局或局部范围内定义*计算变量*并为存储变量定义观察器。计算变量计算它们的值，而不是存储它，并且它们的写法与计算属性相同。 -> Note: Global constants and variables are always computed lazily, -> in a similar manner to . -> Unlike lazy stored properties, -> global constants and variables don't need to be marked with the `lazy` modifier. +> 注意: +> 全局常量和变量总是以类似于 的方式被延迟计算。与延迟存储属性不同，全局常量和变量不需要用 `lazy` 修饰符标记。 > -> Local constants and variables are never computed lazily. +> 局部常量和变量从不延迟计算。 -You can apply a property wrapper to a local stored variable, -but not to a global variable or a computed variable. -For example, -in the code below, `myNumber` uses `SmallNumber` as a property wrapper. +可以将属性包装器应用于局部存储变量，但不能应用于全局变量或计算变量。例如，下面的代码中，`myNumber` 使用 `SmallNumber` 作为属性包装器。 ```swift func someFunction() { @SmallNumber var myNumber: Int = 0 myNumber = 10 - // now myNumber is 10 + // 这时 myNumber 是 10 myNumber = 24 - // now myNumber is 12 + // 这时 myNumber 是 12 } ``` @@ -1720,10 +1394,7 @@ func someFunction() { ``` --> -Like when you apply `SmallNumber` to a property, -setting the value of `myNumber` to 10 is valid. -Because the property wrapper doesn't allow values higher than 12, -it sets `myNumber` to 12 instead of 24. +就像将 `SmallNumber` 应用于属性一样，将 `myNumber` 的值设置为 10 是有效的。由于属性包装器不允许超过 12 的值，它会将 `myNumber` 设置为 12，而不是 24。 -## Type Properties - -Instance properties are properties that belong to an instance of a particular type. -Every time you create a new instance of that type, -it has its own set of property values, separate from any other instance. - -You can also define properties that belong to the type itself, -not to any one instance of that type. -There will only ever be one copy of these properties, -no matter how many instances of that type you create. -These kinds of properties are called *type properties*. - -Type properties are useful for defining values that are universal to -*all* instances of a particular type, -such as a constant property that all instances can use -(like a static constant in C), -or a variable property that stores a value that's global to all instances of that type -(like a static variable in C). - -Stored type properties can be variables or constants. -Computed type properties are always declared as variable properties, -in the same way as computed instance properties. - -> Note: Unlike stored instance properties, -> you must always give stored type properties a default value. -> This is because the type itself doesn't have an initializer -> that can assign a value to a stored type property at initialization time. +## 类型属性 + +实例属性是属于特定类型实例的属性。每次创建该类型的新实例时，它都有自己的一组属性值，实例之间的属性相互独立。 + +还可以定义属于类型本身的属性，而不是属于该类型的某个实例。无论创建多少个该类型的实例，这些属性都只有一份。这类属性称为*类型属性*。 + +类型属性对于定义对特定类型的*所有*实例通用的值非常有用，例如所有实例都可以使用的常量属性（类似于 C 语言中的静态常量），或存储对该类型的所有实例都全局有效的值的变量属性（类似于 C 语言中的静态变量）。 + +存储的类型属性可以是变量或常量。计算的类型属性始终像计算实例属性一样声明为变量属性。 + +> 注意: +> 与存储实例属性不同，存储类型属性必须始终指定默认值。这是因为类型本身没有构造器，无法在初始化时为存储类型属性赋值。 > -> Stored type properties are lazily initialized on their first access. -> They're guaranteed to be initialized only once, -> even when accessed by multiple threads simultaneously, -> and they don't need to be marked with the `lazy` modifier. +> 存储类型属性在第一次访问时会被延迟初始化。即使在多个线程同时访问时，也保证只会初始化一次，并且不需要用 `lazy` 修饰符标记。 -### Type Property Syntax +### 类型属性语法 -In C and Objective-C, you define static constants and variables associated with a type -as *global* static variables. -In Swift, however, type properties are written as part of the type's definition, -within the type's outer curly braces, -and each type property is explicitly scoped to the type it supports. +在 C 和 Objective-C 中，定义与类型关联的静态常量和变量时，通常作为*全局*静态变量来定义。然而，在 Swift 中，类型属性是作为类型定义的一部分编写的，在类型的外部大括号内，每个类型属性都明确地限定在它所支持的类型范围内。 -You define type properties with the `static` keyword. -For computed type properties for class types, -you can use the `class` keyword instead -to allow subclasses to override the superclass’s implementation. -The example below shows the syntax for stored and computed type properties: +使用 `static` 关键字定义类型属性。对于类类型的计算类型属性，可以使用 `class` 关键字，允许子类重写父类的实现。下面的示例展示了存储类型属性和计算类型属性的语法： ```swift struct SomeStructure { @@ -1866,26 +1511,23 @@ class SomeClass { ``` --> -> Note: The computed type property examples above are for read-only computed type properties, -> but you can also define read-write computed type properties -> with the same syntax as for computed instance properties. +> 注意: +> 上面的计算类型属性示例是针对只读计算类型属性的，但也可以使用与计算实例属性相同的语法定义读写计算类型属性。 -### Querying and Setting Type Properties +### 获取和设置类型属性的值 -Type properties are queried and set with dot syntax, just like instance properties. -However, type properties are queried and set on the *type*, not on an instance of that type. -For example: +类型属性的查询和设置使用点语法，就像实例属性一样。然而，类型属性是针对*类型*本身进行查询和设置的，而不是针对该类型的某个实例。例如： ```swift print(SomeStructure.storedTypeProperty) -// Prints "Some value." +// 打印 "Some value." SomeStructure.storedTypeProperty = "Another value." print(SomeStructure.storedTypeProperty) -// Prints "Another value." +// 打印 "Another value." print(SomeEnumeration.computedTypeProperty) -// Prints "6" +// 打印 "6" print(SomeClass.computedTypeProperty) -// Prints "27" +// 打印 "27" ``` -The examples that follow use two stored type properties as part of a structure -that models an audio level meter for a number of audio channels. -Each channel has an integer audio level between `0` and `10` inclusive. +接下来的示例使用了两个存储类型属性，作为一个结构体的一部分，用于模拟多个音频通道的音频电平仪。每个通道的音频电平都是一个介于 `0` 到 `10` 之间的整数（包括 `0` 和 `10`）。 -The figure below illustrates how two of these audio channels can be combined -to model a stereo audio level meter. -When a channel's audio level is `0`, none of the lights for that channel are lit. -When the audio level is `10`, all of the lights for that channel are lit. -In this figure, the left channel has a current level of `9`, -and the right channel has a current level of `7`: +下图展示了如何将这两个音频通道组合在一起，以模拟一个立体声音频电平仪。当某个通道的音频电平为 `0` 时，该通道的指示灯全部熄灭；当音频电平为 `10` 时，该通道的指示灯全部点亮。在这个图中，左通道的当前电平为 `9`，右通道的当前电平为 `7`： ![](staticPropertiesVUMeter) -The audio channels described above are represented by -instances of the `AudioChannel` structure: +上述音频通道由 `AudioChannel` 结构体的实例表示： ```swift struct AudioChannel { @@ -1927,11 +1561,11 @@ struct AudioChannel { var currentLevel: Int = 0 { didSet { if currentLevel > AudioChannel.thresholdLevel { - // cap the new audio level to the threshold level + // 将当前音量限制在阈值之内 currentLevel = AudioChannel.thresholdLevel } if currentLevel > AudioChannel.maxInputLevelForAllChannels { - // store this as the new overall maximum input level + // 存储当前音量作为新的最大输入音量 AudioChannel.maxInputLevelForAllChannels = currentLevel } } @@ -1962,40 +1596,21 @@ struct AudioChannel { ``` --> -The `AudioChannel` structure defines two stored type properties to support its functionality. -The first, `thresholdLevel`, defines the maximum threshold value an audio level can take. -This is a constant value of `10` for all `AudioChannel` instances. -If an audio signal comes in with a higher value than `10`, -it will be capped to this threshold value (as described below). - -The second type property is -a variable stored property called `maxInputLevelForAllChannels`. -This keeps track of the maximum input value that has been received -by *any* `AudioChannel` instance. -It starts with an initial value of `0`. - -The `AudioChannel` structure also defines -a stored instance property called `currentLevel`, -which represents the channel's current audio level on a scale of `0` to `10`. - -The `currentLevel` property has a `didSet` property observer -to check the value of `currentLevel` whenever it's set. -This observer performs two checks: - -- If the new value of `currentLevel` is greater than the allowed `thresholdLevel`, - the property observer caps `currentLevel` to `thresholdLevel`. -- If the new value of `currentLevel` (after any capping) is higher than - any value previously received by *any* `AudioChannel` instance, - the property observer stores the new `currentLevel` value in - the `maxInputLevelForAllChannels` type property. - -> Note: In the first of these two checks, -> the `didSet` observer sets `currentLevel` to a different value. -> This doesn't, however, cause the observer to be called again. - -You can use the `AudioChannel` structure to create -two new audio channels called `leftChannel` and `rightChannel`, -to represent the audio levels of a stereo sound system: +`AudioChannel` 结构体定义了两个存储类型属性来支持其功能。第一个，`thresholdLevel`，定义了音频电平的最大阈值。对于所有 `AudioChannel` 实例，这个值是一个恒定的 `10`。如果输入的音频信号值高于 `10`，它将被限制在这个阈值内（如下所述）。 + +第二个类型属性是一个名为 `maxInputLevelForAllChannels` 的变量存储属性，用于跟踪*任何* `AudioChannel` 实例所接收到的最大输入值。该属性初始值为 `0`。 + +`AudioChannel` 结构体还定义了一个存储实例属性，称为 `currentLevel`，表示通道当前的音频电平，范围从 `0` 到 `10`。 + +`currentLevel` 属性有一个 `didSet` 属性观察器，每当设置 `currentLevel` 时检查其值。这个观察器执行两个检查： + +- 如果 `currentLevel` 的新值大于允许的 `thresholdLevel`，属性观察器会将 `currentLevel` 限制在 `thresholdLevel`。 +- 如果 `currentLevel` 的新值（在任何限制之后）高于之前*任何* `AudioChannel` 实例接收到的值，属性观察器会将新的 `currentLevel` 值存储在 `maxInputLevelForAllChannels` 类型属性中。 + +> 注意: +> 在这两个检查中的第一个中，`didSet` 观察器将 `currentLevel` 设置为不同的值。但这不会导致观察器再次被调用。 + +可以使用 `AudioChannel` 结构体创建两个新的音频通道，称为 `leftChannel` 和 `rightChannel`，用于表示立体声音响系统的音频电平： ```swift var leftChannel = AudioChannel() @@ -2011,16 +1626,13 @@ var rightChannel = AudioChannel() ``` --> -If you set the `currentLevel` of the *left* channel to `7`, -you can see that the `maxInputLevelForAllChannels` type property -is updated to equal `7`: - +如果将*左*通道的 `currentLevel` 设置为 `7`，可以看到 `maxInputLevelForAllChannels` 类型属性更新为 `7`： ```swift leftChannel.currentLevel = 7 print(leftChannel.currentLevel) -// Prints "7" +// 打印 "7" print(AudioChannel.maxInputLevelForAllChannels) -// Prints "7" +// 打印 "7" ``` -If you try to set the `currentLevel` of the *right* channel to `11`, -you can see that the right channel's `currentLevel` property -is capped to the maximum value of `10`, -and the `maxInputLevelForAllChannels` type property is updated to equal `10`: +如果尝试将*右*通道的 `currentLevel` 设置为 `11`，可以看到右通道的 `currentLevel` 属性被限制在最大值 `10`，并且 `maxInputLevelForAllChannels` 类型属性更新为 `10`： ```swift rightChannel.currentLevel = 11 print(rightChannel.currentLevel) -// Prints "10" +// 打印 "10" print(AudioChannel.maxInputLevelForAllChannels) -// Prints "10" +// 打印 "10" ``` -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - + +## 协议语法 + +协议的定义方式与类、结构体和枚举的定义非常相似： + +```swift +protocol SomeProtocol { + // 这里是协议的定义部分 +} +``` + + + +要让自定义类型遵循某个协议，在定义类型时，需要在类型名称后加上协议名称，中间以冒号（`:`）分隔。遵循多个协议时，各协议之间用逗号（`,`）分隔： + +```swift +struct SomeStructure: FirstProtocol, AnotherProtocol { + // 这里是结构体的定义部分 +} +``` + + + +如果一个类拥有父类，应该将父类名放在任何遵循的协议名之前，以逗号分隔： + +```swift +class SomeClass: SomeSuperclass, FirstProtocol, AnotherProtocol { + // 这里是类的定义部分 +} +``` + + + +> 注意: 由于协议是类型，它们的名称应以大写字母开头（如 `FullyNamed` 和 `RandomNumberGenerator`），以与 Swift 中其他类型的命名规范（如 `Int`、`String` 和 `Double`）保持一致。 + +## 属性要求 + +协议可以要求遵循协议的类型提供特定名称和类型的实例属性或类型属性。协议不指定属性是存储属性还是计算属性，它只指定属性的名称和类型。此外，协议还指定属性是可读的还是可**读写**的。 + +如果协议要求属性可读写，那么该属性不能是常量属性或只读的计算属性。如果协议只要求属性是可读的，那么该属性不仅可以是可读的，如果你自己的代码需要的话，还可以是可写的。 + +协议总是用 `var` 关键字来声明变量属性，在类型声明后加上 `{ set get }`来表示属性是可读可写的，可读属性则用 `{ get }` 来表示： + +```swift +protocol SomeProtocol { + var mustBeSettable: Int { get set } + var doesNotNeedToBeSettable: Int { get } +} +``` + + + +在协议中定义类型属性时，总是使用 `static` 关键字作为前缀。当一个类遵循协议时，除了 `static` 关键字，还可以使用 `class` 关键字来声明类型属性： + +```swift +protocol AnotherProtocol { + static var someTypeProperty: Int { get set } +} +``` + + + +如下所示，这是一个只含有一个实例属性要求的协议： + +```swift +protocol FullyNamed { + var fullName: String { get } +} +``` + + + +`FullyNamed` 协议除了要求遵循协议的类型提供 `fullName` 属性外，并没有其他特别的要求。这个协议表示，任何遵循 `FullyNamed` 的类型，都必须有一个可读的 `String` 类型的实例属性 `fullName`。 + +下面是一个遵循 `FullyNamed` 协议的简单结构体： + +```swift +struct Person: FullyNamed { + var fullName: String +} +let john = Person(fullName: "John Appleseed") +// john.fullName 为 "John Appleseed" +``` + + + +这个例子中定义了一个叫做 `Person` 的结构体，用来表示一个具有名字的人。它在定义的第一行声明了它遵循 `FullyNamed` 协议。 + +每个 `Person` 实例都有一个 `String` 类型的存储属性 `fullName`。这正好满足了 `FullyNamed` 协议的要求，也就意味着 `Person` 结构体正确地遵循了协议。（如果协议要求未被完全满足，Swift 在编译时会报错。） + +下面是一个更为复杂的类，它采用并遵循了 `FullyNamed` 协议： + +```swift +class Starship: FullyNamed { + var prefix: String? + var name: String + init(name: String, prefix: String? = nil) { + self.name = name + self.prefix = prefix + } + var fullName: String { + return (prefix != nil ? prefix! + " " : "") + name + } +} +var ncc1701 = Starship(name: "Enterprise", prefix: "USS") +// ncc1701.fullName 为 "USS Enterprise" +``` + + + +`Starship` 类把 `fullName` 作为只读的计算属性来实现。每一个 `Starship` 类的实例都有一个名为 `name` 的非可选属性和一个名为 `prefix` 的可选属性。当 `prefix` 存在时，计算属性 `fullName` 会将 `prefix` 插入到 `name` 之前，从而得到一个带有 `prefix` 的 `fullName`。 + + + +## 方法要求 + +协议可以要求遵循协议的类型实现某些指定的实例方法和类方法。这些方法的编写方式与普通实例方法和类型方法完全相同，都写在协议定义的一部分中，但没有大括号或方法主体。协议允许使用可变参数，和普通方法的定义方式相同。但是，不能在协议定义中为方法参数指定默认值。 + +正如属性要求中所述，在协议中定义类方法的时候，总是使用 `static` 关键字作为前缀。即使在类中实现时，类方法要求使用 `class` 或 `static` 作为关键字前缀，这条规则仍然适用： + +```swift +protocol SomeProtocol { + static func someTypeMethod() +} +``` + + + +下面的例子定义了一个只含有一个实例方法的协议： + +```swift +protocol RandomNumberGenerator { + func random() -> Double +} +``` + + + +`RandomNumberGenerator` 协议要求遵循协议的类型必须拥有一个名为 `random`，返回值类型为 `Double` 的实例方法。尽管这里并未指明，但是我们假设返回值是从 `0.0` 到（但不包括）`1.0`。 + +`RandomNumberGenerator` 协议并不关心每一个随机数是怎样生成的 —— 它只要求生成器提供一种标准的方式来生成新的随机数。 + +这里有一个采用并遵循 `RandomNumberGenerator` 协议的类。该类实现了一个叫做 **线性同余生成器（linear congruential generator）** 的伪随机数算法。 + +```swift +class LinearCongruentialGenerator: RandomNumberGenerator { + var lastRandom = 42.0 + let m = 139968.0 + let a = 3877.0 + let c = 29573.0 + func random() -> Double { + lastRandom = ((lastRandom * a + c) + .truncatingRemainder(dividingBy:m)) + return lastRandom / m + } +} +let generator = LinearCongruentialGenerator() +print("Here's a random number: \(generator.random())") +// 打印 “Here's a random number: 0.37464991998171” +print("And another one: \(generator.random())") +// 打印 “And another one: 0.729023776863283” +``` + + + +## 变值方法要求 + +有时需要在方法中改变（或 **变值（mutate）** ）方法所属的实例。例如，在值类型（即结构体和枚举）的实例方法中，将 `mutating` 关键字作为方法的前缀，写在 `func` 关键字之前，表示可以在该方法中修改它所属的实例以及实例的任意属性的值。这一过程在 章节中有详细描述。 + +如果你在协议中定义了一个实例方法，该方法会改变遵循该协议的类型的实例，那么在定义协议时需要在方法前加 `mutating` 关键字。这使得结构体和枚举能够遵循此协议并满足此方法要求。 + +> 注意: 实现协议中的 `mutating` 方法时，若是类类型，则不用写 `mutating` 关键字。`mutating` 关键字只用于结构体和枚举。 + +如下所示，`Togglable` 协议只定义了一个名为 `toggle` 的实例方法。顾名思义，`toggle()` 方法将改变实例属性，从而切换遵循该协议类型的实例的状态。 + +`toggle()` 方法在定义的时候，使用 `mutating` 关键字标记，这表明当它被调用时，该方法将会改变遵循协议的类型的实例： + +```swift +protocol Togglable { + mutating func toggle() +} +``` + + + +当使用枚举或结构体来实现 `Togglable` 协议时，需要提供一个被标记为 `mutating` 的 `toggle()` 方法。 + +下面定义了一个名为 `OnOffSwitch` 的枚举。这个枚举在两种状态之间进行切换，用枚举成员 `On` 和 `Off` 表示。枚举的 `toggle()` 方法被标记为 `mutating`，以满足 `Togglable` 协议的要求： + +```swift +enum OnOffSwitch: Togglable { + case off, on + mutating func toggle() { + switch self { + case .off: + self = .on + case .on: + self = .off + } + } +} +var lightSwitch = OnOffSwitch.off +lightSwitch.toggle() +// lightSwitch 现在的值为 .on +``` + + + +## 构造器要求 + +协议可以要求遵循协议的类型实现指定的构造器。你可以像编写普通构造器那样，在协议的定义里写下构造器的声明，但不需要写花括号和构造器的实体： + +```swift +protocol SomeProtocol { + init(someParameter: Int) +} +``` + + + +### 协议构造器要求的类实现 + +你可以在遵循协议的类中实现构造器，无论是作为指定构造器，还是作为便利构造器。无论哪种情况，你都必须为构造器实现标上 `required` 修饰符： + +```swift +class SomeClass: SomeProtocol { + required init(someParameter: Int) { + // 这里是构造器的实现部分 + } +} +``` + + + + + +使用 `required` 修饰符可以确保所有子类也必须提供此构造器实现，从而也能遵循协议。 + +关于 `required` 构造器的更多内容，参考 。 + + + + + +> 注意: 如果类已经被标记为 `final`，那么不需要在协议构造器的实现中使用 `required` 修饰符，因为 `final` 类不能有子类。关于 `final` 修饰符的更多内容，参见 。 + + + +如果一个子类重写了父类的指定构造器，并且该构造器满足了某个协议的构造器要求，那么该构造器的实现需要同时标注 `required` 和 `override` 修饰符： + +```swift +protocol SomeProtocol { + init() +} + +class SomeSuperClass { + init() { + // 这里是构造器的实现部分 + } +} + +class SomeSubClass: SomeSuperClass, SomeProtocol { + // 因为遵循协议，需要加上 required；因为继承自父类，需要加上 override + required override init() { + // 这里是构造器的实现部分 + } +} +``` + + + +### 可失败构造器要求 + +协议还可以为遵循协议的类型定义可失败构造器要求，详见 。 + +遵循协议的类型可以通过可失败构造器（`init?`）或非可失败构造器（`init`）来满足协议中定义的可失败构造器要求。协议中定义的非可失败构造器要求可以通过非可失败构造器（`init`）或隐式解包可失败构造器（`init!`）来满足。 + + + + + + + + + + + + + + + + + +## 协议作为类型 + +协议本身并不实现任何功能。尽管如此，你仍然可以在代码中将协议用作类型。 + +最常见的将协议用作类型的方式是将其用作泛型约束。具有泛型约束的代码可以与任何符合该协议的类型一起工作，具体的类型由使用该 API 的代码选择。例如，当你调用一个函数并传入一个参数，而该参数的类型是泛型时，调用者会选择具体的类型。 + +在代码中使用不透明类型时，可以与某个符合该协议的类型一起工作。底层类型在编译时是已知的，API 实现会选择该类型，但该类型的身份对 API 的使用方是隐藏的。使用不透明类型可以防止 API 的实现细节泄露到抽象层之外 —— 例如，通过隐藏函数的具体返回类型，并仅保证该值符合给定的协议。 + +代码使用装箱（boxed）的协议类型时，可以与任何在运行时选择的、符合该协议的类型一起工作。为了支持这种运行时的灵活性，Swift 在必要时会添加一个间接层 —— 称为 **箱子（box）**，这会带来性能开销。由于这种灵活性，Swift 在编译时无法知道底层类型，这意味着你只能访问协议所要求的成员。要访问底层类型的任何其他 API，都需要在运行时进行类型转换。 + +关于使用协议作为泛型约束的信息，参考 。关于不透明类型和装箱协议类型的信息，参考 。 + + +## 代理 + +**代理（Delegate）** 是一种设计模式，它允许类或结构体将一些需要它们负责的功能代理给其他类型的实例。代理模式的实现很简单：定义协议来封装那些需要被代理的功能，这样就能确保遵循协议的类型能提供这些功能。代理模式可以用来响应特定的动作，或者接收外部数据源提供的数据，而无需关心外部数据源的类型。 + +下面的例子定义了两个基于骰子游戏的协议： +以下示例定义了一个骰子游戏，以及一个用于跟踪游戏进度的嵌套协议： + +```swift +class DiceGame { + let sides: Int + let generator = LinearCongruentialGenerator() + weak var delegate: Delegate? + + init(sides: Int) { + self.sides = sides + } + + func roll() -> Int { + return Int(generator.random() * Double(sides)) + 1 + } + + func play(rounds: Int) { + delegate?.gameDidStart(self) + for round in 1...rounds { + let player1 = roll() + let player2 = roll() + if player1 == player2 { + delegate?.game(self, didEndRound: round, winner: nil) + } else if player1 > player2 { + delegate?.game(self, didEndRound: round, winner: 1) + } else { + delegate?.game(self, didEndRound: round, winner: 2) + } + } + delegate?.gameDidEnd(self) + } + + protocol Delegate: AnyObject { + func gameDidStart(_ game: DiceGame) + func game(_ game: DiceGame, didEndRound round: Int, winner: Int?) + func gameDidEnd(_ game: DiceGame) + } +} +``` + +`DiceGame` 类实现了一个游戏，每个玩家轮流掷骰子，掷出最高点数的玩家赢得该轮。它使用前文中示例中的线性同余生成器来生成骰子掷出的随机数。 + +`DiceGame.Delegate` 协议可用于跟踪骰子游戏的进度。由于 `DiceGame.Delegate` 协议总是在骰子游戏的上下文中使用，因此它被嵌套在 `DiceGame` 类内部。协议可以嵌套在类型声明（如结构体和类）内部，只要外部声明不是泛型。关于嵌套类型的更多信息，参见 。 + +为了防止强引用循环，代理被声明为弱引用。关于弱引用的更多信息，参见 。将协议标记为 class-only 允许 `DiceGame` 类声明其代理必须使用弱引用。一个 class-only 协议通过继承自 `AnyObject` 来标记，如 中所述。 + +`DiceGame.Delegate` 提供了三个方法来跟踪游戏的进度。这三个方法被整合到上面的 `play(rounds:)` 方法的游戏逻辑中。当新游戏开始、新回合开始或游戏结束时，`DiceGame` 类会调用它的代理方法。 + +因为 `delegate` 属性是 **可选的** `DiceGame.Delegate`，所以 `play(rounds:)` 方法在每次调用代理方法时都使用可选链，如 中所述。如果 `delegate` 属性为 nil，这些代理调用将被忽略。如果 `delegate` 属性不为 nil，则会调用代理方法，并将 `DiceGame` 实例作为参数传递。 + +下一个示例展示了一个名为 `DiceGameTracker` 的类，它遵循了 `DiceGame.Delegate` 协议： + +```swift +class DiceGameTracker: DiceGame.Delegate { + var playerScore1 = 0 + var playerScore2 = 0 + func gameDidStart(_ game: DiceGame) { + print("Started a new game") + playerScore1 = 0 + playerScore2 = 0 + } + func game(_ game: DiceGame, didEndRound round: Int, winner: Int?) { + switch winner { + case 1: + playerScore1 += 1 + print("Player 1 won round \(round)") + case 2: playerScore2 += 1 + print("Player 2 won round \(round)") + default: + print("The round was a draw") + } + } + func gameDidEnd(_ game: DiceGame) { + if playerScore1 == playerScore2 { + print("The game ended in a draw.") + } else if playerScore1 > playerScore2 { + print("Player 1 won!") + } else { + print("Player 2 won!") + } + } +} +``` + +`DiceGameTracker` 类实现了 `DiceGame.Delegate` 协议要求的所有三个方法。它通过这些方法在新游戏开始时将两个玩家的分数清零，在每轮结束时更新他们的分数，以及在游戏结束时宣布获胜者。 + +以下是 `DiceGame` 和 `DiceGameTracker` 的实际运行情况: + +```swift +let tracker = DiceGameTracker() +let game = DiceGame(sides: 6) +game.delegate = tracker +game.play(rounds: 3) +// 开始新游戏 +// Player 2 won round 1 +// Player 2 won round 2 +// Player 1 won round 3 +// Player 2 won! +``` + +## 在扩展里添加协议遵循 + +即便无法修改源代码，你依然可以通过扩展令已有类型采用并遵循协议。扩展可以为已有类型添加属性、方法、下标以及构造器，因此可以符合协议中可能需要的任意要求。关于扩展的更多详情，参见 。 + +> 注意: 当一个协议的遵循被添加到实例类型的扩展中时，现有的实例会自动采用并遵循该协议。 + +例如下面这个 `TextRepresentable` 协议，任何想要通过文本表示一些内容的类型都可以实现该协议。这些想要表示的内容可以是实例本身的描述，也可以是实例当前状态的文本描述： + +```swift +protocol TextRepresentable { + var textualDescription: String { get } +} +``` + + + +上面提到的 `Dice` 类可以被扩展以采用并遵循 `TextRepresentable` 协议： + + + +```swift +extension Dice: TextRepresentable { + var textualDescription: String { + return "A \(sides)-sided dice" + } +} +``` + + + +通过扩展遵循并适配协议，和在原始定义中采用并遵循协议的效果完全相同。协议名称写在类型名之后，以冒号隔开，然后在扩展的大括号内实现协议要求的内容。 + +现在所有 `Dice` 的实例都可以被看做 `TextRepresentable` 类型： + +```swift +let d12 = Dice(sides: 12, generator: LinearCongruentialGenerator()) +print(d12.textualDescription) +// 打印 “A 12-sided dice” +``` + + + +同样，`SnakesAndLadders` 类也可以通过扩展来适配和遵循 `TextRepresentable` 协议： + +```swift +extension SnakesAndLadders: TextRepresentable { + var textualDescription: String { + return "A game of Snakes and Ladders with \(finalSquare) squares" + } +} +print(game.textualDescription) +// 打印 “A game of Snakes and Ladders with 25 squares” +``` + + + +### 有条件地遵循协议 + +泛型类型可能只在某些情况下满足一个协议的要求，比如当类型的泛型形式参数遵循对应协议时。你可以通过在扩展类型时列出限制让泛型类型有条件地遵循某协议。在你采用协议的名字后面写泛型 `where` 分句。更多关于泛型 `where` 分句，参见 。 + +下面的扩展让 `Array` 类型只要在存储遵循 `TextRepresentable` 协议的元素时，就遵循 `TextRepresentable` 协议。 + +```swift +extension Array: TextRepresentable where Element: TextRepresentable { + var textualDescription: String { + let itemsAsText = self.map { $0.textualDescription } + return "[" + itemsAsText.joined(separator: ", ") + "]" + } +} +let myDice = [d6, d12] +print(myDice.textualDescription) +// 打印 "[A 6-sided dice, A 12-sided dice]" +``` + + + +### 在扩展里声明协议遵循 + +当一个类型已经遵循了某个协议中的所有要求，却还没有声明遵循该协议时，可以通过空的扩展来让它遵循该协议： + +```swift +struct Hamster { + var name: String + var textualDescription: String { + return "A hamster named \(name)" + } +} +extension Hamster: TextRepresentable {} +``` + + + +从现在起，`Hamster` 的实例可以作为 `TextRepresentable` 类型使用： + +```swift +let simonTheHamster = Hamster(name: "Simon") +let somethingTextRepresentable: TextRepresentable = simonTheHamster +print(somethingTextRepresentable.textualDescription) +// 打印 “A hamster named Simon” +``` + + + +> 注意: 即使满足了协议的所有要求，类型也不会自动遵循协议，必须显式地遵循协议。 + +## 使用合成实现来遵循协议 + +Swift 可以在很多简单场景下自动提供遵循 `Equatable`、`Hashable` 和 `Comparable` 协议的实现。在使用这些合成实现之后，无需再编写重复的样板代码来实现这些协议所要求的方法。 + + + +Swift 为以下几种自定义类型提供了 `Equatable` 协议的合成实现： + +- 只包含遵循 `Equatable` 协议的存储属性的结构体 +- 只包含遵循 `Equatable` 协议的关联类型的枚举 +- 没有任何关联类型的枚举 + +在包含类型原始声明的文件中声明对 `Equatable` 协议的遵循，可以得到 `==` 操作符的合成实现，且无需自己编写任何关于 `==` 的实现代码。`Equatable` 协议同时包含 `!=` 操作符的默认实现。 + +下面的例子中定义了一个 `Vector3D` 结构体来表示一个类似 `Vector2D` 的三维向量 `(x, y, z)`。由于 `x`、`y` 和 `z` 都是满足 `Equatable` 的类型，`Vector3D` 可以得到等价运算符的合成实现。 + +```swift +struct Vector3D: Equatable { + var x = 0.0, y = 0.0, z = 0.0 +} + +let twoThreeFour = Vector3D(x: 2.0, y: 3.0, z: 4.0) +let anotherTwoThreeFour = Vector3D(x: 2.0, y: 3.0, z: 4.0) +if twoThreeFour == anotherTwoThreeFour { + print("These two vectors are also equivalent.") +} +// 打印 "These two vectors are also equivalent." +``` + + + + + +Swift 为以下几种自定义类型提供了 `Hashable` 协议的合成实现： + +- 只包含遵循 `Hashable` 协议的存储属性的结构体 +- 只包含遵循 `Hashable` 协议的关联类型的枚举 +- 没有任何关联类型的枚举 + +在包含类型原始声明的文件中声明对 `Hashable` 协议的遵循，可以得到 `hash(into:)` 的合成实现，且无需自己编写任何关于 `hash(into:)` 的实现代码。 + +Swift 为没有原始值的枚举类型提供了 `Comparable` 协议的合成实现。如果枚举类型包含关联类型，那这些关联类型也必须同时遵循 `Comparable` 协议。在包含原始枚举类型声明的文件中声明其对 `Comparable` 协议的遵循，可以得到 `<` 操作符的合成实现，且无需自己编写任何关于 `<` 的实现代码。`Comparable` 协议同时包含 `<=`、`>` 和 `>=` 操作符的默认实现。 + +下面的例子中定义了 `SkillLevel` 枚举类型，其中定义了 beginner（初学者）、intermediate（中级）和 expert（专家）三种类型，专家类型会由额外的 stars（星级）数量来进行排名。 + +```swift +enum SkillLevel: Comparable { + case beginner + case intermediate + case expert(stars: Int) +} +var levels = [SkillLevel.intermediate, SkillLevel.beginner, + SkillLevel.expert(stars: 5), SkillLevel.expert(stars: 3)] +for level in levels.sorted() { + print(level) +} +// 打印 “beginner” +// 打印 “intermediate” +// 打印 “expert(stars: 3)” +// 打印 “expert(stars: 5)” +``` + + + + + + + +## 协议类型的集合 + +协议类型可以在数组或者字典这样的集合中使用，在 提到了这样的用法。下面的例子创建了一个元素类型为 `TextRepresentable` 的数组： + +```swift +let things: [TextRepresentable] = [game, d12, simonTheHamster] +``` + + + +现在可以遍历 `things` 数组，并打印每个元素的文本表示： + +```swift +for thing in things { + print(thing.textualDescription) +} +// A game of Snakes and Ladders with 25 squares +// A 12-sided dice +// A hamster named Simon +``` + + + +注意 `thing` 常量是 `TextRepresentable` 类型而不是 `Dice`，`DiceGame`，`Hamster` 等类型，即使实例在幕后确实是这些类型中的一种。由于 `thing` 是 `TextRepresentable` 类型，任何 `TextRepresentable` 的实例都有一个 `textualDescription` 属性，所以在每次循环中可以安全地访问 `thing.textualDescription`。 + +## 协议的继承 + +协议能够 **继承（inherit）** 一个或多个其他协议，可以在继承的协议的基础上增加新的要求。协议的继承语法与类的继承相似，多个被继承的协议间用逗号分隔： + +```swift +protocol InheritingProtocol: SomeProtocol, AnotherProtocol { + // 这里是协议的定义部分 +} +``` + + + +如下所示，`PrettyTextRepresentable` 协议继承了上面提到的 `TextRepresentable` 协议： + +```swift +protocol PrettyTextRepresentable: TextRepresentable { + var prettyTextualDescription: String { get } +} +``` + + + +例子中定义了一个新的协议 `PrettyTextRepresentable`，它继承自 `TextRepresentable` 协议。任何遵循 `PrettyTextRepresentable` 协议的类型，除了必须满足 `TextRepresentable` 协议的要求，**还要** 额外满足 `PrettyTextRepresentable` 协议的要求。在这个例子中，`PrettyTextRepresentable` 协议额外要求遵循协议的类型提供一个返回值为 `String` 类型的 `prettyTextualDescription` 属性。 + +如下所示，扩展 `SnakesAndLadders`，使其采用并遵循 `PrettyTextRepresentable` 协议： + +```swift +extension SnakesAndLadders: PrettyTextRepresentable { + var prettyTextualDescription: String { + var output = textualDescription + ":\n" + for index in 1...finalSquare { + switch board[index] { + case let ladder where ladder > 0: + output += "▲ " + case let snake where snake < 0: + output += "▼ " + default: + output += "○ " + } + } + return output + } +} +``` + + + +上述扩展令 `SnakesAndLadders` 遵循了 `PrettyTextRepresentable` 协议，并提供了协议要求的 `prettyTextualDescription` 属性。每个 `PrettyTextRepresentable` 类型同时也是 `TextRepresentable` 类型，所以在 `prettyTextualDescription` 的实现中，可以访问 `textualDescription` 属性，然后拼接上冒号和换行符，接着遍历数组中的元素，拼接一个几何图形来表示每个棋盘方格的内容： + +- 当从数组中取出的元素的值大于 `0` 时，用 `▲` 表示。 +- 当从数组中取出的元素的值小于 `0` 时，用 `▼` 表示。 +- 当从数组中取出的元素的值等于 `0` 时，用 `○` 表示。 + +任意 `SankesAndLadders` 的实例都可以使用 `prettyTextualDescription` 属性来打印一个漂亮的文本描述： + +```swift +print(game.prettyTextualDescription) +// 一个有 25 个方格的蛇梯棋（Snakes and Ladders）游戏: +// ○ ○ ▲ ○ ○ ▲ ○ ○ ▲ ▲ ○ ○ ○ ▼ ○ ○ ○ ○ ▼ ○ ○ ▼ ○ ▼ ○ +``` + + + +## 类专属的协议 + +你通过添加 `AnyObject` 关键字到协议的继承列表，就可以限制协议只能被类类型遵循（而不能是结构体类型或者枚举类型）。 + +```swift +protocol SomeClassOnlyProtocol: AnyObject, SomeInheritedProtocol { + // 这里是类专属协议的定义部分 +} +``` + + + +在以上例子中，协议 `SomeClassOnlyProtocol` 只能被类类型遵循。如果尝试让结构体或枚举类型遵循 `SomeClassOnlyProtocol`，则会导致编译时错误。 + +> 注意: 当协议定义的要求需要遵循协议的类型必须是引用语义而非值语义时，应该使用类类型专属协议。关于引用语义和值语义的更多内容，参见 和 。 + + + + + +## 协议组合 + +要求一个类型同时遵循多个协议是很有用的。你可以使用 **协议组合** 来组合多个协议到一个要求里。协议组合的行为就和你定义的临时局部协议一样，拥有组合中所有协议的需求。协议组合不定义任何新的协议类型。 + +协议组合使用 `SomeProtocol & AnotherProtocol` 的形式。你可以列举任意数量的协议，用和符号（`&`）分开。除了协议列表，协议组合也能包含类类型，这允许你标明一个需要的父类。 + +下面的例子中，将 `Named` 和 `Aged` 两个协议按照上述语法组合成一个协议，作为函数参数的类型： + +```swift +protocol Named { + var name: String { get } +} +protocol Aged { + var age: Int { get } +} +struct Person: Named, Aged { + var name: String + var age: Int +} +func wishHappyBirthday(to celebrator: Named & Aged) { + print("Happy birthday, \(celebrator.name), you're \(celebrator.age)!") +} +let birthdayPerson = Person(name: "Malcolm", age: 21) +wishHappyBirthday(to: birthdayPerson) +// 打印 “Happy birthday Malcolm - you're 21!” +``` + + + +在这个例子中，`Named` 协议包含 `String` 类型的 `name` 属性。`Aged` 协议包含 `Int` 类型的 `age` 属性。`Person` 结构体遵循了这两个协议。 + +这个例子中也定义了一个 `wishHappyBirthday(to:)` 函数，其参数 `celebrator` 的类型为 `Named & Aged`，这意味着“任何同时遵循 Named 和 Aged 协议的类型”。它不关心参数的具体类型，只要参数遵循这两个协议即可。 + +这个例子随后创建了一个名为 `birthdayPerson` 的 `Person` 的实例，作为参数传递给了 `wishHappyBirthday(to:)` 函数。因为 `Person` 同时遵循这两个协议，所以这个参数合法，`wishHappyBirthday(to:)` 函数就能打印生日问候语。 + +这里有一个例子：将 `Location` 类和前面的 `Named` 协议进行组合： + +```swift +class Location { + var latitude: Double + var longitude: Double + init(latitude: Double, longitude: Double) { + self.latitude = latitude + self.longitude = longitude + } +} +class City: Location, Named { + var name: String + init(name: String, latitude: Double, longitude: Double) { + self.name = name + super.init(latitude: latitude, longitude: longitude) + } +} +func beginConcert(in location: Location & Named) { + print("Hello, \(location.name)!") +} + +let seattle = City(name: "Seattle", latitude: 47.6, longitude: -122.3) +beginConcert(in: seattle) +// 打印 “Hello, Seattle!” +``` + + + +`beginConcert(in:)` 函数接受一个类型为 `Location & Named` 的参数，这意味着“任何 `Location` 的子类，并且遵循 `Named` 协议”。在这个例子中，`City` 就满足这样的条件。 + +将 `birthdayPerson` 传入 `beginConcert(in:)` 函数是不合法的，因为 `Person` 不是 `Location` 的子类。同理，如果你新建一个类继承于 `Location`，但是没有遵循 `Named` 协议，用这个类的实例去调用 `beginConcert(in:)` 函数也是不合法的。 + +## 检查是否遵循协议 + +你可以使用 中描述的 `is` 和 `as` 操作符来检查是否遵循某协议，并且可以类型转换到指定的协议。检查和转换协议的语法与检查和转换类型是完全一样的： + +- `is` 用来检查实例是否遵循某个协议，若遵循则返回 `true`，否则返回 `false`。 +- `as?` 返回一个可选值，当实例遵循某个协议时，返回类型为协议类型的可选值，否则返回 `nil`。 +- `as!` 将实例强制向下转换到某个协议类型，如果强转失败，将触发运行时错误。 + +下面的例子定义了一个 `HasArea` 协议，该协议定义了一个 `Double` 类型的可读属性 `area`： + +```swift +protocol HasArea { + var area: Double { get } +} +``` + + + +如下所示，`Circle` 类和 `Country` 类都遵循了 `HasArea` 协议： + +```swift +class Circle: HasArea { + let pi = 3.1415927 + var radius: Double + var area: Double { return pi * radius * radius } + init(radius: Double) { self.radius = radius } +} +class Country: HasArea { + var area: Double + init(area: Double) { self.area = area } +} +``` + + + +`Circle` 类把 `area` 属性实现为基于存储型属性 `radius` 的计算型属性。`Country` 类则把 `area` 属性实现为存储型属性。这两个类都正确地遵循了 `HasArea` 协议。 + +如下所示，`Animal` 是一个未遵循 `HasArea` 协议的类： + +```swift +class Animal { + var legs: Int + init(legs: Int) { self.legs = legs } +} +``` + + + +`Circle`，`Country`，`Animal` 并没有一个共同的基类，尽管如此，它们都是类，它们的实例都可以作为 `AnyObject` 类型的值，存储在同一个数组中： + +```swift +let objects: [AnyObject] = [ + Circle(radius: 2.0), + Country(area: 243_610), + Animal(legs: 4) +] +``` + + + +`objects` 数组使用字面量初始化，数组包含一个 `radius` 为 `2` 的 `Circle` 的实例，一个保存了英国国土面积的 `Country` 实例和一个 `legs` 为 `4` 的 `Animal` 实例。 + +如下所示，`objects` 数组可以被迭代，并对迭代出的每一个元素进行检查，看它是否遵循 `HasArea` 协议： + +```swift +for object in objects { + if let objectWithArea = object as? HasArea { + print("Area is \(objectWithArea.area)") + } else { + print("Something that doesn't have an area") + } +} +// Area is 12.5663708 +// Area is 243610.0 +// Something that doesn't have an area +``` + + + +当迭代出的元素遵循 `HasArea` 协议时，将 `as?` 操作符返回的可选值通过可选绑定，绑定到 `objectWithArea` 常量上。`objectWithArea` 是 `HasArea` 协议类型的常量，因此 `area` 属性可以类型安全地被访问和打印。 + +`objects` 数组中实际的元素的类型并不会因为强转而丢失类型信息，它们仍然是 `Circle`，`Country`，`Animal` 类型。然而，当它们被赋值给 `objectWithArea` 常量时，只被视为 `HasArea` 类型，因此只有 `area` 属性能够被访问。 + + + + + +## 可选协议要求 + + + + + +协议可以定义 **可选要求（optional requirements）**，遵循协议的类型可以选择是否实现这些要求。在协议中使用 `optional` 关键字作为前缀来定义可选要求。可选要求用在你需要和 Objective-C 打交道的代码中。协议和可选要求都必须带上 `@objc` 属性。注意被标记为 `@objc` 的协议只能被类遵循，不能被结构体和枚举遵循。 + +使用可选要求中的方法或者属性时，它们的类型会自动变成可选的。比如，一个类型为 `(Int) -> String` 的方法会变成 `((Int) -> String)?`。需要注意的是整个函数类型是可选的，而不是函数的返回值。 + +协议中的可选要求可通过可选链式调用来使用，因为遵循协议的类型可能没有实现这些可选要求。类似 `someOptionalMethod?(someArgument)` 这样，你可以在可选方法名称后加上 `?` 来调用可选方法。和可选链式有关的详细内容，可参见 。 + +下面的例子定义了一个名为 `Counter` 的用于整数计数的类，它使用外部的数据源来提供每次的增量。数据源由 `CounterDataSource` 协议定义，它包含两个可选要求： + +```swift +@objc protocol CounterDataSource { + @objc optional func increment(forCount count: Int) -> Int + @objc optional var fixedIncrement: Int { get } +} +``` + + + +`CounterDataSource` 协议定义了一个可选方法 `increment(forCount:)` 和一个可选属性 `fiexdIncrement`，它们使用了不同的方法来从数据源中获取适当的增量值。 + +> 注意: 严格来讲，`CounterDataSource` 协议中的方法和属性都是可选的，因此遵循协议的类可以不实现这些要求，尽管技术上允许这样做，不过作为一个数据源，最好不要这样写。 + +下面定义的 `Counter` 类含有 `CounterDataSource?` 类型的可选属性 `dataSource`，如下所示： + +```swift +class Counter { + var count = 0 + var dataSource: CounterDataSource? + func increment() { + if let amount = dataSource?.increment?(forCount: count) { + count += amount + } else if let amount = dataSource?.fixedIncrement { + count += amount + } + } +} +``` + + + +`Counter` 类使用变量属性 `count` 来存储当前值。该类还定义了一个 `increment` 方法，每次调用该方法的时候，将会增加 `count` 的值。 + +`increment()` 方法首先尝试使用其数据源的 `increment(forCount:)` 方法实现来得到每次的增量。`increment()` 方法使用可选链式调用来尝试调用 `increment(forCount:)`，并将当前的 `count` 值作为参数传入。 + +这里使用了 **两** 层可选链式调用。首先，由于 `dataSource` 可能为 `nil`，因此在 `dataSource` 后边加上了 `?`，以此表明只在 `dataSource` 非空时才去调用 `increment(forCount:)` 方法。其次，即使 `dataSource` **确实**存在 ，也无法保证其是否实现了 `increment(forCount:)` 方法，因为这个方法是可选的。在这里，`increment(forCount:)` 可能没有被实现的可能性，也通过可选链被处理了。只有当 `increment(forCount:)` 存在时——也就是说，如果它不是 `nil` ——才会调用 `increment(forCount:)`。这就是为什么 `increment(forCount:)` 也在名称后面写有一个问号。 + +调用 `increment(forCount:)` 方法在上述两种情形下都有可能失败，所以返回值为 `Int?` **可选** 类型，即使在 `CounterDataSource` 协议中，`increment(forCount:)` 的返回值类型是非可选 `Int`。另外，即使这里使用了两层可选链式调用，最后的返回结果依旧是单层的可选类型。关于使用多层可选链式调用的更多信息，参见 。 + +在调用 `increment(forCount:)` 方法后，可选 `Int` 型的返回值通过可选绑定解包并赋值给常量 `amount`。如果可选值确实包含一个数值，也就是说，数据源和方法都存在，并且数据源方法返回了一个有效值，就会将解包后的 `amount` 加到 `count` 上，增量操作就完成了。 + +如果 **没有** 从 `increment(forCount:)` 方法获取到值，可能由于 `dataSource` 为 `nil`，或者它并没有实现 `increment(forCount:)` 方法，那么 `increment()` 方法将试图从数据源的 `fixedIncrement` 属性中获取增量。`fixedIncrement` 是一个可选属性，因此属性值是一个可选 `Int` 值，即使该属性在 `CounterDataSource` 协议中的类型是非可选的 `Int`。 + +下面的例子展示了 `CounterDataSource` 的简单实现。`ThreeSource` 类遵循了 `CounterDataSource` 协议，它实现了可选属性 `fixedIncrement`，每次会返回 `3`： + +```swift +class ThreeSource: NSObject, CounterDataSource { + let fixedIncrement = 3 +} +``` + + + +你可以使用 `ThreeSource` 的实例作为新 `Counter` 实例的数据源： + +```swift +var counter = Counter() +counter.dataSource = ThreeSource() +for _ in 1...4 { + counter.increment() + print(counter.count) +} +// 3 +// 6 +// 9 +// 12 +``` + + + +上述代码新建了一个 `Counter` 实例，并将它的数据源设置为一个 `ThreeSource` 的实例，然后调用 `increment()` 方法 4 次。正如预期一样，每次调用都会将 `count` 的值增加 3. + +下面是一个更为复杂的数据源 `TowardsZeroSource`，它将使得 `Counter` 实例的 `count` 属性的值增加或减少，最终变为 0： + +```swift +class TowardsZeroSource: NSObject, CounterDataSource { + func increment(forCount count: Int) -> Int { + if count == 0 { + return 0 + } else if count < 0 { + return 1 + } else { + return -1 + } + } +} +``` + + + +`TowardsZeroSource` 实现了 `CounterDataSource` 协议中的 `increment(forCount:)` 可选方法，以 `count` 参数为依据，来确定计数的方向。如果 `count` 已经为 0，此方法将返回 `0`，以此表明之后不应再有计数操作发生。 + +你可以使用一个 `TowardsZeroSource` 实例将 `Counter` 实例来从 `-4` 数到 `0`。一旦达到 `0`，数值便不会再有变动： + +```swift +counter.count = -4 +counter.dataSource = TowardsZeroSource() +for _ in 1...5 { + counter.increment() + print(counter.count) +} +// -3 +// -2 +// -1 +// 0 +// 0 +``` + + + +## 协议扩展 + +协议可以通过扩展来为遵循协议的类型提供方法、初始化方法、下标以及计算属性的实现。通过这种方式，你可以基于协议本身来实现这些功能，而无需在每个遵循协议的类型中都重复同样的实现，也无需使用全局函数。 + +例如，可以扩展 `RandomNumberGenerator` 协议来提供 `randomBool()` 方法。该方法使用协议中定义的 `random()` 方法来返回一个随机的 `Bool` 值： + +```swift +extension RandomNumberGenerator { + func randomBool() -> Bool { + return random() > 0.5 + } +} +``` + + + +通过添加协议扩展，所有遵循协议的类型，都能自动获得这个扩展所增加的方法实现，而无需任何额外修改。 + +```swift +let generator = LinearCongruentialGenerator() +print("Here's a random number: \(generator.random())") +// 打印 “Here's a random number: 0.37464991998171” +print("And here's a random Boolean: \(generator.randomBool())") +// 打印 “And here's a random Boolean: true” +``` + + + + + +协议扩展可以为遵循协议的类型增加实现，但不能声明该协议继承自另一个协议。协议的继承只能在协议声明处进行指定。 + +### 提供默认实现 + +你可以通过协议扩展来为协议要求的方法、计算属性提供默认的实现。如果遵循协议的类型为这些要求提供了自己的实现，那么这些自定义实现将会替代扩展中的默认实现被使用。 + +> 注意: 通过协议扩展为协议要求提供的默认实现和可选的协议要求不同。虽然在这两种情况下，遵循协议的类型都无需自己实现这些要求，但是通过扩展提供的默认实现可以直接调用，而无需使用可选链式调用。 + +例如，`PrettyTextRepresentable` 协议继承自 `TextRepresentable` 协议，可以为其提供一个默认的 `prettyTextualDescription` 属性来简单地返回 `textualDescription` 属性的值： + +```swift +extension PrettyTextRepresentable { + var prettyTextualDescription: String { + return textualDescription + } +} +``` + + + + + + + + + + + + + + + +### 为协议扩展添加限制条件 + +在扩展协议的时候，可以指定一些限制条件，只有遵循协议的类型满足这些限制条件时，才能获得协议扩展提供的默认实现。这些限制条件写在协议名之后，使用 `where` 子句来描述，更多和泛型 where 子句的内容，参见 。 + +例如，你可以扩展 `Collection` 协议，适用于集合中的元素遵循了 `Equatable` 协议的情况。通过限制集合元素遵循 `Equatable` 协议（Swift 标准库的一部分）， 你可以使用 `==` 和 `!=` 操作符来检查两个元素的等价性和非等价性。 + +```swift +extension Collection where Element: Equatable { + func allEqual() -> Bool { + for element in self { + if element != self.first { + return false + } + } + return true + } +} +``` + + + +如果集合中的所有元素都一致，`allEqual()` 方法才返回 `true`。 + +例如两个整数数组，一个数组的所有元素都是一样的，另一个不一样： + +```swift +let equalNumbers = [100, 100, 100, 100, 100] +let differentNumbers = [100, 100, 200, 100, 200] +``` + + + +由于数组遵循 `Collection` 并且整数遵循 `Equatable`，`equalNumbers` 和 `differentNumbers` 都可以使用 `allEqual()` 方法： + +```swift +print(equalNumbers.allEqual()) +// 打印 “true” +print(differentNumbers.allEqual()) +// 打印 “false” +``` + + + +> 注意: 如果一个遵循类型满足了为同一方法或属性提供实现的多个有限制条件的协议扩展的要求，Swift 会使用最贴合限制的实现。 + + + + + + diff --git a/swift-6.docc/LanguageGuide/StringsAndCharacters.md b/swift-6.docc/LanguageGuide/StringsAndCharacters.md new file mode 100644 index 000000000..3ce168057 --- /dev/null +++ b/swift-6.docc/LanguageGuide/StringsAndCharacters.md @@ -0,0 +1,1453 @@ +# 字符串和字符 + +存储和操作文本 + +**字符串** 是一系列字符的集合，例如 `"hello, world"`，`"albatross"`。Swift 的字符串通过 `String` 类型来表示。而 `String` 内容的访问方式有多种，例如以 `Character` 值的集合。 + +Swift 的 `String` 和 `Character` 类型提供了一种快速且兼容 Unicode 的方式来处理代码中的文本内容。创建和操作字符串的语法与 C 语言中字符串操作相似，轻量并且易读。通过 `+` 符号就可以非常简单的实现两个字符串的拼接操作。与 Swift 中其他值一样，能否更改字符串的值，取决于其被定义为常量还是变量。你可以在已有字符串中插入常量、变量、字面量和表达式从而形成更长的字符串，这一过程也被称为字符串插值。在显示、存储和打印创建自定义字符串值时，字符串插值操作尤其有用。 + +尽管语法简易，但 Swift 中的 `String` 类型的实现却很快速和现代化。每一个字符串都是由编码无关的 Unicode 字符组成，并支持以多种 Unicode 表示形式访问字符。 + +> 注意: +> Swift 的 `String` 类型与 Foundation `NSString` 类进行了无缝桥接。 +> Foundation 还对 `String` 进行扩展使其可以访问 `NSString` 类型中定义的方法。 +> 这意味着调用那些 NSString 的方法，你无需进行任何类型转换。 +> +> 更多关于在 Foundation 和 Cocoa 中使用 `String` 的信息请查看 [Bridging Between String and NSString](https://developer.apple.com/documentation/swift/string#2919514). + +## 字符串字面量 + +你可以在代码里使用一段预定义的字符串值作为 **字符串字面量**。字符串字面量是由一对双引号(`"`)包裹着的具有固定顺序的字符集。 + +使用字符串文字作为常量或变量的初始值: + +```swift +let someString = "Some string literal value" +``` + + + +注意，Swift 之所以推断 `someString` 常量为 `String` 类型，是因为它使用了字面量方式进行初始化。 + +### 多行字符串字面量 + +如果你需要一个多行字符串，那就使用多行字符串字面量 — 由一对三个双引号包裹着的具有固定顺序的文本字符集: + + + +```swift +let quotation = """ +The White Rabbit put on his spectacles. "Where shall I begin, +please your Majesty?" he asked. + +"Begin at the beginning," the King said gravely, "and go on +till you come to the end; then stop." +""" +``` + + + +一个多行字符串字面量包含了所有的在开启和关闭引号（`"""`）中的行。这个字符从开启引号（`"""`）之后的第一行开始，到关闭引号（`"""`）之前为止。这就意味着字符串开启引号之后（`"""`）或者结束引号（`"""`）之前都没有换行符号。（译者：下面两个字符串其实是一样的，虽然第二个使用了多行字符串的形式） + +```swift +let singleLineString = "These are the same." +let multilineString = """ +These are the same. +""" +``` + + + +如果你的代码中，多行字符串字面量包含换行符的话，则多行字符串字面量中也会包含换行符。如果你想换行，以便加强代码的可读性，但是你又不想在你的多行字符串字面量中出现换行符的话，你可以用在行尾写一个反斜杠（`\`）作为续行符。 + +```swift +let softWrappedQuotation = """ +The White Rabbit put on his spectacles. "Where shall I begin, \ +please your Majesty?" he asked. + +"Begin at the beginning," the King said gravely, "and go on \ +till you come to the end; then stop." +""" +``` + + + +为了让一个多行字符串字面量开始和结束于换行符，请将换行写在第一行和最后一行，例如： + +```swift +let lineBreaks = """ + +This string starts with a line break. +It also ends with a line break. + +""" +``` + + + + + +一个多行字符串字面量能够缩进来匹配周围的代码。关闭引号（`"""`）之前的空白字符串告诉 Swift 编译器其他各行多少空白字符串需要忽略。然而，如果你在某行的前面写的空白字符串超出了关闭引号（`"""`）之前的空白字符串，则超出部分将被包含在多行字符串字面量中。 + +![](multilineStringWhitespace) + + + + + +在上面的例子中，尽管整个多行字符串字面量都是缩进的（源代码缩进），第一行和最后一行没有以空白字符串开始（实际的变量值）。中间一行的缩进用空白字符串（源代码缩进）比关闭引号（`"""`）之前的空白字符串多，所以，它的行首将有 4 个空格。 + +### 字符串字面量的特殊字符 + +字符串字面量可以包含以下特殊字符： + +- 转义字符 `\0`(空字符)、`\\`(反斜线)、`\r`(水平制表符)、`\t`(换行符)、`\n`(回车符)、`\"`(双引号)、`\'`(单引号)。 + +- 任意的 Unicode 标量，可以写成 `\u{`n`}`(u 为小写)，其中n为任意一到八位十六进制数且可用的 Unicode 位码。 + (Unicode在文档 中进行解析讨论) + + + +下面的代码为各种特殊字符的使用示例。 `wiseWords` 常量包含了两个双引号。 `dollarSign`、`blackHeart` 和 `sparklingHeart` 常量演示了三种不同格式的 Unicode 标量： + +```swift +let wiseWords = "\"Imagination is more important than knowledge\" - Einstein" +// "Imagination is more important than knowledge" - Einstein +let dollarSign = "\u{24}" // $, Unicode scalar U+0024 +let blackHeart = "\u{2665}" // ♥, Unicode scalar U+2665 +let sparklingHeart = "\u{1F496}" // 💖, Unicode scalar U+1F496 +``` + + + +由于多行字符串字面量使用了三个双引号，而不是一个，所以你可以在多行字符串字面量里直接使用双引号（`"`）而不必加上转义符（`\`）。要在多行字符串字面量中使用 `"""` 的话，就需要使用至少一个转义符（`\`）： + +```swift +let threeDoubleQuotationMarks = """ +Escaping the first quotation mark \""" +Escaping all three quotation marks \"\"\" +""" +``` + + + +### 扩展字符串分隔符 + +您可以将字符串字面值置于扩展分隔符内，以便在字符串中包含特殊字符而不触发其转义效果。您将字符串置于双引号（`"`）内，并在其周围加上井号（`#`）。例如，打印字符串字面值 `#"Line 1\nLine 2"#` 会打印出换行符转义序列（`\n`），而不是将字符串打印在两行上。 + +在字符串字面值中，如果您需要某个字符的特殊效果，需使字符串中紧跟转义字符（`\`）后的井号（`#`）数量相匹配。例如，如果您的字符串是 `#"Line 1\nLine 2"#` 并且您想要换行，可以使用 `#"Line 1\#nLine 2"#` 替代。同样，`###"Line1###nLine2"###` 也能实现换行效果。 + +扩展分隔符创建的字符串文字也可以是多行字符串文字。您可以使用扩展分隔符在多行字符串中包含文本 `"""`，覆盖原有的结束文字的默认行为。例如： + +```swift +let threeMoreDoubleQuotationMarks = #""" +Here are three more double quotes: """ +"""# +``` + + + +## 初始化空字符串 + +要创建一个空字符串 `String` 作为初始值，可以将空的字符串字面量赋值给变量，也可以初始化一个新的 `String` 实例： + +```swift +var emptyString = "" // 空字符串字面量 +var anotherEmptyString = String() // 初始化方法 +// 两个字符串均为空并等价。 +``` + + + +通过检查字符串值的布尔型 `isEmpty` 属性，来确定该字符串值是否为空。 + +```swift +if emptyString.isEmpty { + print("Nothing to see here") +} +// 打印输出 "Nothing to see here" +``` + + + + + +## 字符串可变性 + +你可以通过将一个特定字符串分配给一个变量来对其进行修改，或者分配给一个常量来保证其不会被修改： + +```swift +var variableString = "Horse" +variableString += " and carriage" +// variableString 现在为 "Horse and carriage" + +let constantString = "Highlander" +constantString += " and another Highlander" +// 这会报告一个编译错误 compile-time error - 常量字符串不可以被修改。 +``` + + + + + +> 注意: +> 在 Objective-C 和 Cocoa 中，需要通过选择两个不同的类（NSString 和 NSMutableString）来指定字符串是否可以被修改。 + +## 字符串是值类型 + +在 Swift 中 `String` 类型是值类型。如果你创建了一个新的字符串，那么当其进行常量、变量赋值操作，或在函数/方法中传递时，会进行值拷贝。在前述任一情况下，都会对已有字符串值创建新副本，并对该新副本而非原始字符串进行传递或赋值操作。值类型在中进行了详细描述。 + +Swift 默认拷贝 `String` 的行为保证了在函数/方法向你传递的 `String` 所属权属于你，无论该值来自于哪里。你可以确信传递的 `String` 不会被修改，除非你自己去修改它。 + +在实际编译时，Swift 编译器会优化字符串的使用，使实际的复制只发生在绝对必要的情况下，这意味着你将字符串作为值类型的同时可以获得极高的性能。 + +## 使用字符 + +如果您想要获取某个字符串里的每一个字符值，可以采用 `for`-`in` 循环的方式对该字符串进行遍历操作，从而逐个访问到其中的每一个字符。 + +```swift +for character in "Dog!🐶" { + print(character) +} +// D +// o +// g +// ! +// 🐶 +``` + + + +`for`-`in` 循环在 中进行了详细描述。 + +另外, 当您有一个只包含一个字符的字符串字面量时，通过明确给出 `Character` 这种类型的标注，就能够将其转化为一个独立存在的 `Character` 类型的常量或者变量。 + +```swift +let exclamationMark: Character = "!" +``` + + + +字符串可以通过传递一个值类型为 `Character` 的数组作为自变量来初始化： + +```swift +let catCharacters: [Character] = ["C", "a", "t", "!", "🐱"] +let catString = String(catCharacters) +print(catString) +// 打印 "Cat!🐱" +``` + + + +## 连接字符串和字符 + +字符串可以通过加法运算符（`+`）相加在一起（或称“连接”）创建一个新的字符串： + +```swift +let string1 = "hello" +let string2 = " there" +var welcome = string1 + string2 +// welcome 现在等于 "hello there" +``` + + + +你也可以通过加法赋值运算符（`+=`）将一个字符串添加到一个已经存在字符串变量上： + +```swift +var instruction = "look over" +instruction += string2 +// instruction 现在等于 "look over there" +``` + + + +你可以用 `append()` 方法将一个字符附加到一个字符串变量的尾部： + +```swift +let exclamationMark: Character = "!" +welcome.append(exclamationMark) +// welcome 现在等于 "hello there!" +``` + + + +> 注意: +> 你不能将一个字符串或者字符添加到一个已经存在的字符变量上，因为字符变量只能包含一个字符。 + +如果你需要使用多行字符串字面量来拼接字符串，并且你需要字符串每一行都以换行符结尾，包括最后一行, 示例如下： + +```swift +let badStart = """ + one + two + """ +let end = """ + three + """ +print(badStart + end) +// 打印两行: +// one +// twothree + +let goodStart = """ + one + two + + """ +print(goodStart + end) +// 打印三行: +// one +// two +// three +``` + + + +上面的代码，把 `badStart` 和 `end` 拼接起来的字符串非我们想要的结果。因为 `badStart` 最后一行没有换行符，它与 `end` 的第一行结合到了一起。相反的，`goodStart` 的每一行都以换行符结尾，所以它与 `end` 拼接的字符串总共有三行，正如我们期望的那样。 + +## 字符串插值 + +**字符串** 插值是一种构建新字符串的方式，可以在其中包含常量、变量、字面量和表达式。**字符串字面量** 和 **多行字符串字面量** 都可以使用字符串插值。你插入的字符串字面量的每一项都在以反斜线为前缀的圆括号中： + +```swift +let multiplier = 3 +let message = "\(multiplier) times 2.5 is \(Double(multiplier) * 2.5)" +// message is "3 times 2.5 is 7.5" +``` + + + +在上面的例子中，`multiplier` 的值以 `\(multiplier)` 的形式被插入到一个字符串常量中。当创建字符串执行插值计算时此占位符会被替换为 `multiplier` 实际的值。 + +`multiplier` 的值也作为字符串中后面表达式的一部分。该表达式计算 `Double(multiplier) * 2.5` 的值并将结果（`7.5`）插入到字符串中。在这个例子中，表达式写为 `\(Double(multiplier) * 2.5)` 并包含在字符串字面量中。 + +你可以使用扩展字符串分隔符创建字符串，来包含不想作为字符串插值处理的字符。例如： + +```swift +print(#"Write an interpolated string in Swift using \(multiplier)."#) +// 打印 "Write an interpolated string in Swift using \(multiplier)." +``` + + + +如果要在使用扩展字符串分隔符的字符串中使用字符串插值，需要在反斜杠后面添加与开头和结尾数量相同扩展字符串分隔符。例如： + +```swift +print(#"6 times 7 is \#(6 * 7)."#) +// 打印 "6 times 7 is 42." +``` + + + +> 注意: +> 插值字符串中写在括号中的表达式不能包含非转义反斜杠（`\`），并且不能包含回车或换行符。不过，插值字符串可以包含其他字面量。 + +## Unicode + +`Unicode` 是一个用于在不同书写系统中对文本进行编码、表示和处理的国际标准。它使你可以用标准格式表示来自任意语言几乎所有的字符，并能够对文本文件或网页这样的外部资源中的字符进行读写操作。Swift 的 `String` 和 `Character` 类型是完全兼容 Unicode 标准的。 + +### Unicode 标量 + +Swift 的 `String` 类型是基于 Unicode 标量建立的。Unicode 标量是对应字符或者修饰符的唯一的 21 位数字，例如 `U+0061` 表示小写的拉丁字母（`LATIN SMALL LETTER A`）（"`a`"），`U+1F425` 表示小鸡表情（`FRONT-FACING BABY CHICK`）（"`🐥`"）。 + +请注意，并非所有 21 位 Unicode 标量值都分配给字符，某些标量被保留用于将来分配或用于 UTF-16 编码。已分配的标量值通常也有一个名称，例如上面示例中的 `LATIN SMALL LETTER A` 和 `FRONT-FACING BABY CHICK`。 + +### 可扩展的字形群集 + +每一个 Swift 的 `Character` 类型代表一个 **可扩展的字形群**。而一个可扩展的字形群构成了人类可读的单个字符，它由一个或多个（当组合时） Unicode 标量的序列组成。 + +举个例子，字母 `é` 可以用单一的 Unicode 标量 `é`(`LATIN SMALL LETTER E WITH ACUTE`, 或者 `U+00E9`)来表示。然而一个标准的字母 `e`(`LATIN SMALL LETTER E` 或者 `U+0065`) 加上一个急促重音（`COMBINING ACTUE ACCENT`）的标量（`U+0301`），这样一对标量就表示了同样的字母 `é`。 这个急促重音的标量形象的将 `e` 转换成了 `é`。 + +在这两种情况中，字母 `é` 代表了一个单一的 Swift 的 `Character` 值，同时代表了一个可扩展的字形群。在第一种情况，这个字形群包含一个单一标量；而在第二种情况，它是包含两个标量的字形群： + +```swift +let eAcute: Character = "\u{E9}" // é +let combinedEAcute: Character = "\u{65}\u{301}" // e 后面加上 ́ +// eAcute 是 é, combinedEAcute 是 é +``` + + + +可扩展的字形集是一个将许多复杂的脚本字符表示为单个字符值的灵活方式。例如，来自朝鲜语字母表的韩语音节能表示为组合或分解的有序排列。在 Swift 都会表示为同一个单一的 `Character` 值： + +```swift +let precomposed: Character = "\u{D55C}" // 한 +let decomposed: Character = "\u{1112}\u{1161}\u{11AB}" // ᄒ, ᅡ, ᆫ +// precomposed 是 한, decomposed 是 한 +``` + + + +可拓展的字符群集可以使包围记号（例如 `COMBINING ENCLOSING CIRCLE` 或者 `U+20DD`）的标量包围其他 Unicode 标量，作为一个单一的 `Character` 值： + +```swift +let enclosedEAcute: Character = "\u{E9}\u{20DD}" +// enclosedEAcute 是 é⃝ +``` + + + +地域性指示符号的 Unicode 标量可以组合成一个单一的 `Character` 值，例如 `REGIONAL INDICATOR SYMBOL LETTER U`(`U+1F1FA`)和 `REGIONAL INDICATOR SYMBOL LETTER S`(`U+1F1F8`)： + +```swift +let regionalIndicatorForUS: Character = "\u{1F1FA}\u{1F1F8}" +// regionalIndicatorForUS 是 🇺🇸 +``` + + + +## 计算字符数量 + +如果想要获得一个字符串中 `Character` 值的数量，可以使用 `count` 属性： + +```swift +let unusualMenagerie = "Koala 🐨, Snail 🐌, Penguin 🐧, Dromedary 🐪" +print("unusualMenagerie has \(unusualMenagerie.count) characters") +// 打印 "unusualMenagerie has 40 characters" +``` + + + +注意在 Swift 中，使用可拓展的字符群集作为 `Character` 值来连接或改变字符串时，并不一定会更改字符串的字符数量。 + +例如，如果你用四个字符的单词 `cafe` 初始化一个新的字符串，然后添加一个 `COMBINING ACTUE ACCENT`(`U+0301`)作为字符串的结尾。最终这个字符串的字符数量仍然是 `4`，因为第四个字符是 `é`，而不是 `e`： + +```swift +var word = "cafe" +print("the number of characters in \(word) is \(word.count)") +// 打印 "the number of characters in cafe is 4" + +word += "\u{301}" // 拼接一个重音，U+0301 + +print("the number of characters in \(word) is \(word.count)") +// 打印 "the number of characters in café is 4" +``` + + + +> 注意: +> 可扩展的字形群可以由多个 Unicode 标量组成。这意味着不同的字符以及相同字符的不同表示方式可能需要不同数量的内存空间来存储。所以 Swift 中的字符在一个字符串中并不一定占用相同的内存空间数量。因此在没有获取字符串的可扩展的字符群的范围时候，就不能计算出字符串的字符数量。如果你正在处理一个长字符串，需要注意 `count` 属性必须遍历全部的 Unicode 标量，来确定字符串的字符数量。 +> +> 另外需要注意的是通过 `count` 属性返回的字符数量并不总是与包含相同字符的 `NSString` 的 `length` 属性相同。`NSString` 的 `length` 属性是利用 UTF-16 表示的十六位代码单元数字，而不是 Unicode 可扩展的字符群集。 + +## 访问和修改字符串 + +你可以通过字符串的属性和方法来访问和修改它，当然也可以用下标语法完成。 + +### 字符串索引 + +每一个 `String` 值都有一个关联的索引（**index**）类型，`String.Index`，它对应着字符串中的每一个 `Character` 的位置。 + +前面提到，不同的字符可能会占用不同数量的内存空间，所以要知道 `Character` 的确定位置，就必须从 `String` 开头遍历每一个 Unicode 标量直到结尾。因此，Swift 的字符串不能用整数（integer）做索引。 + +使用 `startIndex` 属性可以获取一个 `String` 的第一个 `Character` 的索引。使用 `endIndex` 属性可以获取最后一个 `Character` 的后一个位置的索引。因此，`endIndex` 属性不能作为一个字符串的有效下标。如果 `String` 是空串，`startIndex` 和 `endIndex` 是相等的。 + +通过调用 `String` 的 `index(before:)` 或 `index(after:)` 方法，可以立即得到前面或后面的一个索引。你还可以通过调用 `index(_:offsetBy:)` 方法来获取对应偏移量的索引，这种方式可以避免多次调用 `index(before:)` 或 `index(after:)` 方法。 + +你可以使用下标语法来访问 `String` 特定索引的 `Character`。 + +```swift +let greeting = "Guten Tag!" +greeting[greeting.startIndex] +// G +greeting[greeting.index(before: greeting.endIndex)] +// ! +greeting[greeting.index(after: greeting.startIndex)] +// u +let index = greeting.index(greeting.startIndex, offsetBy: 7) +greeting[index] +// a +``` + + + +试图获取越界索引对应的 `Character`，将引发一个运行时错误。 + +```swift +greeting[greeting.endIndex] // Error +greeting.index(after: greeting.endIndex) // Error +``` + + + + + +使用 `indices` 属性会创建一个包含全部索引的范围（`Range`），用来在一个字符串中访问单个字符。 + +```swift +for index in greeting.indices { + print("\(greeting[index]) ", terminator: "") +} +// 打印 "G u t e n T a g ! " +``` + + + + + +> 注意: +> 你可以在任意一个确认的并遵循 `Collection` 协议的类型里面使用 `startIndex` 和 `endIndex` 属性或者 `index(before:)` 、`index(after:)` 和 `index(_:offsetBy:)` 方法，如上文所示是使用在 `String` 中，你也可以使用在 `Array`、`Dictionary` 和 `Set` 中。 + +### 插入和删除 + +调用 `insert(_:at:)` 方法可以在一个字符串的指定索引插入一个字符，调用 `insert(contentsOf:at:)` 方法可以在一个字符串的指定索引插入一段字符串。 + +```swift +var welcome = "hello" +welcome.insert("!", at: welcome.endIndex) +// welcome 变量现在等于 "hello!" + +welcome.insert(contentsOf: " there", at: welcome.index(before: welcome.endIndex)) +// welcome 变量现在等于 "hello there!" +``` + + + +调用 `remove(at:)` 方法可以在一个字符串的指定索引删除一个字符，调用 `removeSubrange(_:)` 方法可以在一个字符串的指定索引删除一个子字符串。 + +```swift +welcome.remove(at: welcome.index(before: welcome.endIndex)) +// welcome 现在等于 "hello there" + +let range = welcome.index(welcome.endIndex, offsetBy: -6).. welcome.remove(at: welcome.index(before: welcome.endIndex)) + /> welcome now equals \"\(welcome)\" + let range = welcome.index(welcome.endIndex, offsetBy: -6).. welcome.removeSubrange(range) + /> welcome now equals \"\(welcome)\" + + + + +> 注意: +> 你可以在任意一个确认的并遵循 `RangeReplaceableCollection` 协议的类型里面使用 `insert(*:at:)`、`insert(contentsOf:at:)`、`remove(at:)` 和 `removeSubrange(*:)` 方法，如上文所示是使用在 `String` 中，你也可以使用在 `Array`、`Dictionary` 和 `Set` 中。 + +## 子字符串 + +当你从字符串中获取一个子字符串 —— 例如，使用下标或者 `prefix(_:)` 之类的方法 —— 就可以得到一个 [`Substring`](https://developer.apple.com/documentation/swift/substring) 的实例，而非另外一个 `String`。 +Swift 里的 `Substring` 绝大部分函数都跟 `String` 一样，意味着你可以使用同样的方式去操作 `Substring` 和 `String`。然而，跟 `String` 不同的是，你只有在短时间内需要操作字符串时，才会使用 `Substring`。当你需要长时间保存结果时，就把 `Substring` 转化为 `String`, 示例如下： + +```swift +let greeting = "Hello, world!" +let index = greeting.firstIndex(of: ",") ?? greeting.endIndex +let beginning = greeting[.. let greeting = "Hello, world!" + -> let index = greeting.firstIndex(of: ",") ?? greeting.endIndex + -> let beginning = greeting[.. beginning is \"\(beginning)\" + let newString = String(beginning) + ``` +--> + +就像 `String`，每一个 `Substring` 都会在内存里保存字符集。而 `String` 和 `Substring` 的区别在于性能优化上，`Substring` 可以重用原 `String` 的内存空间，或者另一个 `Substring` 的内存空间（`String` 也有同样的优化，但如果两个 `String` 共享内存的话，它们就会相等）。这一优化意味着你在修改 `String` 和 `Substring` 之前都不需要消耗性能去复制内存。就像前面说的那样，`Substring` 不适合长期存储 —— 因为它重用了原 `String` 的内存空间，原 `String` 的内存空间必须保留直到它的 `Substring` 不再被使用为止。 + +上面的例子，`greeting` 是一个 `String`，意味着它在内存里有一片空间保存字符集。而由于 `beginning` 是 `greeting` 的 `Substring`，它重用了 `greeting` 的内存空间。相反，`newString` 是一个 `String` —— 它是使用 `Substring` 创建的，拥有一片自己的内存空间。下面的图展示了他们之间的关系： + + + +![](stringSubstring) + +> 注意: +> `String` 和 `Substring` 都遵循 +> [`StringProtocol`](https://developer.apple.com/documentation/swift/stringprotocol)协议, 这意味着操作字符串的函数使用 `StringProtocol` 会更加方便。你可以传入 `String` 或 `Substring` 去调用函数。 + +## 比较字符串 + +Swift 提供了三种方式来比较文本值：字符串字符相等、前缀相等和后缀相等。 + +### 字符串和字符相等 + +字符串/字符可以用等于操作符（`==`）和不等于操作符（`!=`），详细描述在 : + +```swift +let quotation = "We're a lot alike, you and I." +let sameQuotation = "We're a lot alike, you and I." +if quotation == sameQuotation { + print("These two strings are considered equal") +} +// 打印 "These two strings are considered equal" +``` + + + +如果两个字符串值（或两个字符）的可扩展字形群集在规范上等效，那么它们就被认为是相等的。而扩展字形群集在规范上等效是指它们具有相同的语言意义和外观，即便它们是由不同的 Unicode 标量组成。 + + + + + +例如，`LATIN SMALL LETTER E WITH ACUTE`(`U+00E9`)就是标准相等于 `LATIN SMALL LETTER E`(`U+0065`)后面加上 `COMBINING ACUTE ACCENT`(`U+0301`)。这两个字符群集都是表示字符 `é` 的有效方式，所以它们被认为是标准相等的： + +```swift +// "Voulez-vous un café?" 使用 LATIN SMALL LETTER E WITH ACUTE +let eAcuteQuestion = "Voulez-vous un caf\u{E9}?" + +// "Voulez-vous un café?" 使用 LATIN SMALL LETTER E and COMBINING ACUTE ACCENT +let combinedEAcuteQuestion = "Voulez-vous un caf\u{65}\u{301}?" + +if eAcuteQuestion == combinedEAcuteQuestion { + print("These two strings are considered equal") +} +// 打印 "These two strings are considered equal" +``` + + + +相反，英语中的 `LATIN CAPITAL LETTER A`(`U+0041`，或者 `A`)不等于俄语中的 `CYRILLIC CAPITAL LETTER A`(`U+0410`，或者 `A`)。视觉上相似，但语言含义却不同。 + +```swift +let latinCapitalLetterA: Character = "\u{41}" + +let cyrillicCapitalLetterA: Character = "\u{0410}" + +if latinCapitalLetterA != cyrillicCapitalLetterA { + print("These two characters aren't equivalent.") +} +// 打印 "These two characters aren't equivalent." +``` + + + +> 注意: 在 Swift 中，字符串和字符并不区分地域（not locale-sensitive）。 + + + +### 前缀和后缀相等 + +通过调用字符串的 `hasPrefix(*:)`或`hasSuffix(*:)` 方法来检查字符串是否拥有特定前缀或后缀，两个方法均接收一个 `String` 类型的参数，并返回一个布尔值。 + + + + + +下面的例子以一个字符串数组表示莎士比亚话剧《罗密欧与朱丽叶》中前两场的场景位置： + +```swift +let romeoAndJuliet = [ + "Act 1 Scene 1: Verona, A public place", + "Act 1 Scene 2: Capulet's mansion", + "Act 1 Scene 3: A room in Capulet's mansion", + "Act 1 Scene 4: A street outside Capulet's mansion", + "Act 1 Scene 5: The Great Hall in Capulet's mansion", + "Act 2 Scene 1: Outside Capulet's mansion", + "Act 2 Scene 2: Capulet's orchard", + "Act 2 Scene 3: Outside Friar Lawrence's cell", + "Act 2 Scene 4: A street in Verona", + "Act 2 Scene 5: Capulet's mansion", + "Act 2 Scene 6: Friar Lawrence's cell" +] +``` + + + +你可以调用 `hasPrefix(_:)` 方法来计算话剧中第一幕的场景数： + +```swift +var act1SceneCount = 0 +for scene in romeoAndJuliet { + if scene.hasPrefix("Act 1 ") { + act1SceneCount += 1 + } +} +print("There are \(act1SceneCount) scenes in Act 1") +// 打印 "There are 5 scenes in Act 1" +``` + + + +相似地，你可以用 `hasSuffix(_:)` 方法来计算发生在不同地方的场景数： + +```swift +var mansionCount = 0 +var cellCount = 0 +for scene in romeoAndJuliet { + if scene.hasSuffix("Capulet's mansion") { + mansionCount += 1 + } else if scene.hasSuffix("Friar Lawrence's cell") { + cellCount += 1 + } +} +print("\(mansionCount) mansion scenes; \(cellCount) cell scenes") +// 打印 "6 mansion scenes; 2 cell scenes" +``` + + + +> 注意: +> `hasPrefix(*:)` 和 `hasSuffix(*:)` 方法都是在每个字符串中逐字符比较其可扩展的字符群集是否标准相等，详细描述在 . + +## 字符串的 Unicode 表示形式 + +当一个 Unicode 字符串被写进文本文件或者其他储存时，字符串中的 Unicode 标量会用 Unicode 定义的几种 `编码格式`（encoding forms）编码。每一个字符串中的小块编码都被称 `代码单元`（code units）。这些包括 UTF-8 编码格式（编码字符串为 8 位的代码单元），UTF-16 编码格式（编码字符串位 16 位的代码单元），以及 UTF-32 编码格式（编码字符串32位的代码单元）。 + +Swift 提供了几种不同的方式来访问字符串的 Unicode 表示形式。你可以利用 `for-in` 来对字符串进行遍历，从而以 Unicode 可扩展的字符群集的方式访问每一个 `Character` 值。该过程在 中进行了描述。 + +另外，能够以其他三种 Unicode 兼容的方式访问字符串的值： + +- UTF-8 代码单元集合（利用字符串的 `utf8` 属性进行访问） +- UTF-16 代码单元集合（利用字符串的 `utf16` 属性进行访问） +- 21 位的 Unicode 标量值集合，也就是字符串的 UTF-32 编码格式（利用字符串的 `unicodeScalars` 属性进行访问） + +下面由 `D`,`o`,`g`,`‼`(`DOUBLE EXCLAMATION MARK`, Unicode 标量 `U+203C`)和 🐶(`DOG FACE`，Unicode 标量为 `U+1F436`)组成的字符串中的每一个字符代表着一种不同的表示： + +```swift +let dogString = "Dog‼🐶" +``` + + + +### UTF-8 表示 + +你可以通过遍历 `String` 的 `utf8` 属性来访问它的 `UTF-8` 表示。其为 `String.UTF8View` 类型的属性，`UTF8View` 是无符号 8 位（`UInt8`）值的集合，每一个 `UInt8` 值都是一个字符的 UTF-8 表示： + +![](UTF8) + +```swift +for codeUnit in dogString.utf8 { + print("\(codeUnit) ", terminator: "") +} +print("") +// 打印 "68 111 103 226 128 188 240 159 144 182 " +``` + + + + + +上面的例子中，前三个 10 进制 `codeUnit` 值（`68`、`111`、`103`）代表了字符 `D`、`o` 和 `g`，它们的 UTF-8 表示与 ASCII 表示相同。接下来的三个 10 进制 `codeUnit` 值（`226`、`128`、`188`）是 `DOUBLE EXCLAMATION MARK` 的3字节 UTF-8 表示。最后的四个 `codeUnit` 值（`240`、`159`、`144`、`182`）是 `DOG FACE` 的4字节 UTF-8 表示。 + + + + + +### UTF-16 表示 + +你可以通过遍历 `String` 的 `utf16` 属性来访问它的 `UTF-16` 表示。其为 `String.UTF16View` 类型的属性，`UTF16View` 是无符号16位（`UInt16`）值的集合，每一个 `UInt16` 都是一个字符的 UTF-16 表示： + +![](UTF16) + +```swift +for codeUnit in dogString.utf16 { + print("\(codeUnit) ", terminator: "") +} +print("") +// 打印 "68 111 103 8252 55357 56374 " +``` + + + + + +同样，前三个 `codeUnit` 值（`68`、`111`、`103`）代表了字符 `D`、`o` 和 `g`，它们的 UTF-16 代码单元和 UTF-8 完全相同（因为这些 Unicode 标量表示 ASCII 字符）。 + +第四个 `codeUnit` 值（`8252`）是一个等于十六进制 `203C` 的的十进制值。这个代表了 `DOUBLE EXCLAMATION MARK` 字符的 Unicode 标量值 `U+203C`。这个字符在 UTF-16 中可以用一个代码单元表示。 + +第五和第六个 `codeUnit` 值（`55357` 和 `56374`）是 `DOG FACE` 字符的 UTF-16 表示。第一个值为 `U+D83D`(十进制值为 `55357`)，第二个值为 `U+DC36`(十进制值为 `56374`)。 + +### Unicode 标量表示 + +你可以通过遍历 `String` 值的 `unicodeScalars` 属性来访问它的 Unicode 标量表示。其为 `UnicodeScalarView` 类型的属性，`UnicodeScalarView` 是 `UnicodeScalar` 类型的值的集合。 + +每一个 `UnicodeScalar` 拥有一个 `value` 属性，可以返回对应的 21 位数值，用 `UInt32` 来表示： + +![](UnicodeScalar) + +```swift +for scalar in dogString.unicodeScalars { + print("\(scalar.value) ", terminator: "") +} +print("") +// 打印 "68 111 103 8252 128054 " +``` + + + + + +前三个 `UnicodeScalar` 值（`68`、`111`、`103`）的 `value` 属性仍然代表字符 `D`、`o` 和 `g`。 + +第四个 `codeUnit` 值（`8252`）仍然是一个等于十六进制 `203C` 的十进制值。这个代表了 `DOUBLE EXCLAMATION MARK` 字符的 Unicode 标量 `U+203C`。 + +第五个 `UnicodeScalar` 值的 `value` 属性，`128054`，是一个十六进制 `1F436` 的十进制表示。其等同于 `DOG FACE` 的 Unicode 标量 `U+1F436`。 + +作为查询它们的 `value` 属性的一种替代方法，每个 `UnicodeScalar` 值也可以用来构建一个新的 `String` 值，比如在字符串插值中使用： + +```swift +for scalar in dogString.unicodeScalars { + print("\(scalar) ") +} +// D +// o +// g +// ‼ +// 🐶 +``` + + + + diff --git a/swift-6.docc/LanguageGuide/Subscripts.md b/swift-6.docc/LanguageGuide/Subscripts.md new file mode 100644 index 000000000..76927a7e2 --- /dev/null +++ b/swift-6.docc/LanguageGuide/Subscripts.md @@ -0,0 +1,332 @@ + +# 下标 + +访问集合的元素 + +**下标**可以定义在类、结构体和枚举中，是访问集合、列表或序列中元素的快捷方式。可以使用下标的索引设置和获取值，而不需要再调用对应的存取方法。举例来说，用下标访问一个 `Array` 实例中的元素可以写作 `someArray[index]`，访问 `Dictionary` 实例中的元素可以写作 `someDictionary[key]`。 + +一个类型可以定义多个下标，系统会根据索引值的类型自动选择对应的下标重载。下标不限于一维，你可以定义具有多个入参的下标来满足自定义类型的需求。 + + + +## 下标语法 + +下标允许你通过在实例名称后面的方括号中传入一个或者多个索引值来对实例进行查询。它的语法类似于实例方法语法和计算型属性语法。定义下标使用 `subscript` 关键字，与定义实例方法类似，都是指定一个或多个输入参数和一个返回类型。与实例方法不同的是，下标可以设定为读写或只读。这种行为由 getter 和 setter 实现，类似计算型属性： + +```swift +subscript(index: Int) -> Int { + get { + // 返回一个适当的 Int 类型的值 + } + set(newValue) { + // 执行适当的赋值操作 + } +} +``` + + + +`newValue` 的类型与下标的返回值类型相同。如同计算型属性，可以不指定 setter 的参数（`newValue`）。如果不指定参数，setter 会提供一个名为 `newValue` 的默认参数。 + +如同只读计算型属性，对于只读下标的声明，你可以通过省略 `get` 关键字和对应的大括号组来进行简写: + +```swift +subscript(index: Int) -> Int { + // 返回一个适当的 Int 类型的值 +} +``` + + + +下面代码演示了只读下标的实现，这里定义了一个 `TimesTable` 结构体，用来表示对应整数的乘法表: + +```swift +struct TimesTable { + let multiplier: Int + subscript(index: Int) -> Int { + return multiplier * index + } +} +let threeTimesTable = TimesTable(multiplier: 3) +print("six times three is \(threeTimesTable[6])") +// 打印“six times three is 18” +``` + + + +在上例中，创建了一个 `TimesTable` 实例，用来表示整数 `3` 的乘法表。数值 `3` 被传递给结构体的构造函数，作为实例成员 `multiplier` 的值。 + +你可以通过下标访问 `threeTimesTable` 实例，例如上面演示的 `threeTimesTable[6]`。这条语句查询了乘法表中 `3` 的第六个元素，返回 `3` 的 `6` 倍即 `18`。 + +>注意: +`TimesTable` 例子基于一个固定的数学公式。将 `threeTimesTable[someIndex]` 进行赋值操作并不合适，因此下标定义为只读的。 + +## 下标用法 + +“下标”的确切含义取决于使用场景。下标通常用作访问集合、列表或序列中的成员元素的快捷方式。你可以针对自己特定的类或结构体功能来以最恰当的方式实现下标。 + +例如，Swift 的 `Dictionary` 类型实现下标用于对实例中储存的值进行存取操作。为字典设值时，在下标中使用和字典的键类型相同的键，并把一个和字典的值类型相同的值赋给这个下标： + +```swift +var numberOfLegs = ["spider": 8, "ant": 6, "cat": 4] +numberOfLegs["bird"] = 2 +``` + + + +上面的示例定义了一个名为 `numberOfLegs` 的变量，并用一个包含三对键值的字典字面量初始化它。`numberOfLegs` 字典的类型推断为 `[String: Int]`。创建字典后，此示例使用下标赋值将 `String` 类型的键 `bird` 和 `Int` 类型的值 `2` 添加到字典中。 + +更多关于 `Dictionary` 下标的信息请参考 . + +>注意: +>Swift 的 `Dictionary` 类型的下标接受并返回_可选_类型的值。对于上面的 `numberOfLegs` 字典通过下标返回的是一个 `Int?` 或者说“可选的 int”。`Dictionary` 类型之所以如此实现下标，是因为不是每个键都有对应的值，同时这也提供了一种通过键删除对应值的方式，只需将键对应的值赋值为 nil 即可。 + +## 下标选项 + +下标可以接受任意数量的入参，并且这些入参可以是任何类型。下标的返回值也可以是任意类型。 + +与函数一样，下标可以接受不同数量的参数，并且为这些参数提供默认值，如 和 中所述。但是，与函数不同的是，下标不能使用 in-out 参数。 + + + +一个类或结构体可以根据自身需要提供多个下标实现，使用下标时将通过入参的数量和类型进行区分，自动匹配合适的下标。它通常被称为 **下标重载**。 + +虽然下标采用单一入参是最常见的，但也可以根据情况定义接受多个入参的下标。下面的示例定义一个 `Matrix` 结构体，该结构体表示一个 `Double` 类型的二维矩阵。`Matrix` 结构体的下标接受两个整型参数: + +```swift +struct Matrix { + let rows: Int, columns: Int + var grid: [Double] + init(rows: Int, columns: Int) { + self.rows = rows + self.columns = columns + grid = Array(repeating: 0.0, count: rows * columns) + } + func indexIsValid(row: Int, column: Int) -> Bool { + return row >= 0 && row < rows && column >= 0 && column < columns + } + subscript(row: Int, column: Int) -> Double { + get { + assert(indexIsValid(row: row, column: column), "Index out of range") + return grid[(row * columns) + column] + } + set { + assert(indexIsValid(row: row, column: column), "Index out of range") + grid[(row * columns) + column] = newValue + } + } +} +``` + + + +`Matrix` 提供了一个接受两个入参的构造方法，入参分别是 `rows` 和 `columns`，创建了一个足够容纳 `rows * columns` 个 `Double` 类型的值的数组。通过传入数组长度和初始值 `0.0` 到数组的构造器，将矩阵中每个位置的值初始化为 `0.0`。关于数组的这种构造方法请参考。 + +你可以通过传入合适的 `row` 和 `column` 数值来构造一个新的 `Matrix` 实例: + +```swift +var matrix = Matrix(rows: 2, columns: 2) +``` + + + +上例中创建了一个两行两列的 `Matrix` 实例。该 `Matrix` 实例的 `grid` 数组按照从左上到右下的阅读顺序将矩阵扁平化存储： + +将 `row` 和 `column` 的值传入下标来为矩阵设值，下标的入参使用逗号分隔: + +```swift +matrix[0, 1] = 1.5 +matrix[1, 0] = 3.2 +``` + + + +上面两条语句分别调用下标的 setter 将矩阵右上角位置（即 `row` 为 `0`、`column` 为 `1` 的位置）的值设置为 `1.5`，将矩阵左下角位置（即 `row` 为 `1`、`column` 为 `0` 的位置）的值设置为 `3.2`: + +`Matrix` 下标的 getter 和 setter 中都含有断言，用来检查下标入参 `row` 和 `column` 的值是否有效。为了方便进行断言，`Matrix` 包含了一个名为 `indexIsValid(row:column:)` 的便利方法，用来检查入参 `row` 和 `column` 的值是否在矩阵范围内: + +```swift +func indexIsValid(row: Int, column: Int) -> Bool { + return row >= 0 && row < rows && column >= 0 && column < columns +} +``` + + + +断言在下标越界时触发: + +```swift +let someValue = matrix[2, 2] +// 断言将会触发，因为 [2, 2] 已经超过了 matrix 的范围 +``` + + + +## 类型下标 + +正如上节所述，实例下标是在特定类型的一个实例上调用的下标。你也可以定义一种在这个类型自身上调用的下标。这种下标被称作 **类型下标**。你可以通过在 `subscript` 关键字之前写下 `static` 关键字的方式来表示一个类型下标。类型可以使用 `class` 关键字来代替 `static`，它允许子类重写父类中对那个下标的实现。下面的例子展示了如何定义和调用一个类型下标: + +```swift +enum Planet: Int { + case mercury = 1, venus, earth, mars, jupiter, saturn, uranus, neptune + static subscript(n: Int) -> Planet { + return Planet(rawValue: n)! + } +} +let mars = Planet[4] +print(mars) +``` + + + + diff --git a/swift-6.docc/LanguageGuide/TheBasics.md b/swift-6.docc/LanguageGuide/TheBasics.md new file mode 100644 index 000000000..f3510c084 --- /dev/null +++ b/swift-6.docc/LanguageGuide/TheBasics.md @@ -0,0 +1,1612 @@ +# 基础知识 + +处理常见数据类型并编写基本语法。 + +Swift 提供了许多基本数据类型，包括表示整数的 `Int`、表示浮点数的 `Double`、表示布尔值的 `Bool` 和表示文本的 `String`。Swift 还提供了三种主要集合类型（`Array`（数组），`Set`（集合），和 `Dictionary`（字典））的强大版本，详见 。 + +Swift 使用变量来存储值，并通过标识名称来引用值。Swift 还广泛使用不可更改其值的变量。这些变量被称为常量，在整个 Swift 中都有使用，以便在处理无需更改的值时使代码更安全、更清晰。 + +除了熟悉的类型外，Swift 还引入了元组等高级类型。通过元组，你可以创建并传递一组值。你可以使用元组从函数返回一个包含了多个值的复合值。 + +Swift 还引入了可选类型，用于处理值缺失的情况。可选类型要么表示“变量*有*值，且等于 *x*”，要么表示“压根就*没有*值”。 + +Swift 是一种*类型安全*的语言，这意味着该语言可以帮助你明确代码可以处理的值的类型。如果你的部分代码需要字符串，类型安全可以防止你错误地将整数传递给它。同样，类型安全也能防止你不小心将可选字符串传递给需要非可选字符串的代码。类型安全可帮助你在开发过程中尽早发现并修复错误。 + +## 常量和变量 + +常量和变量将名称（如 `maximumNumberOfLoginAttempts` 或 `welcomeMessage`）与特定类型的值（如数字 `10` 或字符串 `"Hello"`）相关联。*常量*的值一旦设置就不能更改，而*变量*则可以在将来设置不同的值。 + +### 声明常量和变量 + +常量和变量在使用前必须先声明。使用 `let` 关键字声明常量，使用 `var` 关键字声明变量。下面举例说明如何使用常量和变量来追踪用户尝试登录的次数： + +```swift +let maximumNumberOfLoginAttempts = 10 +var currentLoginAttempt = 0 +``` + + + +该代码可理解为： + +“声明一个名为 `maximumNumberOfLoginAttempts` 的新常量，并赋予其 `10` 的值。然后，声明一个名为 `currentLoginAttempt` 的新变量，并赋予其 `0` 的初始值。” + +在这个示例中，允许的最大登录尝试次数被声明为一个常量，因为最大值永远不会改变。当前登录尝试计数器被声明为变量，因为每次登录尝试失败后，该值都必须递增。 + +如果代码中的某个存储值不会改变，请使用 `let` 关键字将其声明为常量。变量只用于存储会发生变化的值。 + +在声明常量或变量时，可以像上面的示例一样，在声明中为其赋值。或者你也可以稍后在程序中为变量分配初始值，只要能保证在第一次读取前它有值即可。 + +```swift +var environment = "development" +let maximumNumberOfLoginAttempts: Int +// maximumNumberOfLoginAttempts 尚无值。 + +if environment == "development" { + maximumNumberOfLoginAttempts = 100 +} else { + maximumNumberOfLoginAttempts = 10 +} +// 现在 maximumNumberOfLoginAttempts 有了值，可以读取了。 +``` + + + +在本例中，登录尝试的最大次数是常数，其值取决于环境。在开发环境中，其值为 100；在其他环境中，其值为 10。`if` 语句的两个分支都将 `maximumNumberOfLoginAttempts` 初始化为某个值，从而保证该常量一定有一个值。有关 Swift 如何在以这种方式设置初始值时检查代码的详情，请参阅 。 + +你可以在一行中声明多个常量或多个变量，中间用逗号隔开： + +```swift +var x = 0.0, y = 0.0, z = 0.0 +``` + + + +### 类型注解 + +在声明常量或变量时，可以提供*类型注解*，以明确常量或变量可以存储的值的类型。编写类型注解时，在常量或变量名后加上冒号，后跟一个空格，然后是要使用的类型名称。 + +本例为名为 `welcomeMessage` 的变量提供了一个类型注解，以指示该变量可以存储字符串值： + +```swift +var welcomeMessage: String +``` + + + +声明中的冒号表示“…类型的…”，因此上面的代码可以理解为： + +“声明一个名为 `welcomeMessage` 的字符串类型的变量。” + +短语“字符串类型”意味着“可以存储任何字符串值“。请将其理解为变量可以存储的“值的类型”。 + +现在，你可以将 `welcomeMessage` 变量设置为任何字符串值且不会出错： + +```swift +welcomeMessage = "Hello" +``` + + + +你可以在一行中定义多个相同类型的相关变量，中间用逗号隔开，并在最后一个变量名后加一个类型注解： + +```swift +var red, green, blue: Double +``` + + + +> 备注: 在实践中很少需要编写类型注解。如果你在定义常量或变量时为其提供了初始值，Swift 几乎总是可以推断出该常量或变量的类型，如 中所述。上面的 `welcomeMessage` 示例中，没有提供初始值，因此 `welcomeMessage` 变量的类型是通过类型注解指定的，而不是从初始值推断出来的。 + +### 命名常量和变量 + +常量和变量名几乎可以包含任何字符，包括 Unicode 字符： + +```swift +let π = 3.14159 +let 你好 = "你好世界" +let 🐶🐮 = "dogcow" +``` + + + +常量和变量名不能包含空格字符、数学符号、箭头、专用的 Unicode 标量值，或画线和画框字符。常量和变量名也不能以数字开头，但数字可以包含在名称的其他部分。 + +一旦声明了某个类型的常量或变量，就不能再用相同的名称声明，也不能更改它以存储不同类型的值。同时也不能将常量改为变量，或将变量改为常量。 + +> 备注: 如果要使用与 Swift 保留关键字相同的名称命名常量或变量，在使用关键字时，请用反引号 (`` ` ``) 包裹。不过，除非别无他选，请尽量避免使用关键字作为变量名称。 + +你可以将现有变量的值更改为另一个兼容类型的值。在本例中，`friendlyWelcome` 的值从 `"Hello!"` 改为了 `"Bonjour!"`： + +```swift +var friendlyWelcome = "Hello!" +friendlyWelcome = "Bonjour!" +// friendlyWelcome 的当前值是 "Bonjour!" +``` + + + +与变量不同，常量的值在设置后不能更改。在编译代码时，如果试图更改常量的值，就会报错： + +```swift +let languageName = "Swift" +languageName = "Swift++" +// 这是一个编译时错误：languageName 不能更改。 +``` + + + +### 打印常量和变量 + +使用 `print(_:separator:terminator:)` 函数可以打印常量或变量的当前值： + +```swift +print(friendlyWelcome) +// 打印 "Bonjour!" +``` + + + +`print(_:separator:terminator:)` 函数是一个全局函数，用于将一个或多个值打印到适当的输出端。例如，在 Xcode 中，`print(_:separator:termininator:)` 函数将在 Xcode 的 “控制台” 窗格中打印输出。参数 `separator`（分隔符） 和 `terminator`（结束符） 有默认值，因此在调用此函数时可以省略。默认情况下，该函数通过添加换行符来结束打印行。如果要打印一个值且不换行，可以传递一个空字符串作为结束符，例如 `print(someValue,termininator:"")`。有关带默认值参数的信息，请参阅 。 + + + + + + + +Swift 使用*字符串插值*将常量或变量的名称作为占位符包含在较长的字符串中，并提示 Swift 将其替换为该常量或变量的当前值。将名称包在括号中，并在左括号前用反斜杠进行转义： + +```swift +print("The current value of friendlyWelcome is \(friendlyWelcome)") +// 打印 "The current value of friendlyWelcome is Bonjour！" +``` + + + +> 备注: 在 中描述了所有字符串插值选项。 + +## 注释 + +使用注释在代码中包含不可执行的文本，作为给自己的注释或提醒。在编译代码时，Swift 编译器会忽略注释。 + +Swift 中的注释与 C 语言中的注释非常相似。单行注释以两个正斜杠 (`//`) 开头： + +```swift +// 这是一个注释。 +``` + + + +多行注释以正斜杠 + 星号 (`/*`) 开始，并以星号 + 正斜杠 (`*/`) 结束： + +```swift +/* 这也是一个注释 +但写了多行。*/ +``` + + + +与 C 语言中的多行注释不同，Swift 中的多行注释可以嵌套在其他多行注释中。在编写嵌套注释时，你可以先编写一个多行注释块，然后在第一个注释块中编写第二个多行注释块。然后关闭第二个注释块，接着关闭第一个注释块： + +```swift +/* 这是第一个多行注释的开始。 + /* 这是第二个嵌套的多行注释。*/ +这是第一个多行注释的结束。*/ +``` + + + +通过嵌套多行注释，即使代码中已包含多行注释，你也能快速、轻松地注释大块代码。 + +## 分号 + +与许多其他语言不同，Swift 并不要求你在代码中的每条语句后都写上分号（`;`），不过如果你愿意，也可以这样做。不过，如果你想在一行中编写多个独立语句，则*必须*使用分号： + +```swift +let cat = "🐱"; print(cat) +// 打印 "🐱" +``` + + + +## 整数 + +*整数*是没有小数成分的数字，如 `42` 和 `-23`。 整数可以是*有符号的*（正数、零或负数），也可以是*无符号的*（正数或零）。 + +Swift 提供 8、16、32 和 64 位有符号和无符号整数。这些整数遵循与 C 类似的命名规则，即 8 位无符号整数的类型是 `UInt8`，32 位有符号整数的类型是 `Int32`。与 Swift 中的所有类型一样，这些整数类型的名称也是大写的。 + +### 整数边界 + +你可以使用 `min` 和 `max` 属性访问每个整数类型的最小值和最大值： + +```swift +let minValue = UInt8.min // minValue 等于 0，类型为 UInt8 +let maxValue = UInt8.max // maxValue 等于 255，类型为 UInt8 +``` + + + +这些属性的值属于适当大小的数字类型（如上例中的 `UInt8`），因此可以在表达式中与同类型的其他值一起使用。 + +### Int（整数） + +在大多数情况下，你不需要在代码中使用特定大小的整数。Swift 提供了一种额外的整数类型 `Int`，其大小与当前平台的原生字长大小相同： + +- 在 32 位平台上，`Int` 的大小与 `Int32` 相同。 +- 在 64 位平台上，`Int` 的大小与 `Int64` 相同。 + +除非你需要使用特定大小的整数，否则在代码中始终使用 `Int` 表示整数值。这有助于代码的一致性和互操作性。即使在 32 位平台上，`Int` 也可以存储介于 `-2,147,483,648` 和 `2,147,483,647` 之间的任何值，其大小足以容纳许多整数范围。 + +### UInt（无符号整数） + +Swift 还提供了无符号整数类型 `UInt`，其大小与当前平台的原生字长大小相同： + +- 在 32 位平台上，`UInt` 的大小与 `UInt32` 相同。 +- 在 64 位平台上，`UInt` 的大小与 `UInt64` 相同。 + +> 备注: 只有在特别需要无符号整数类型且其大小与平台的原生字长大小相同时，才使用 `UInt`。如果不是这种情况，最好使用 `Int`，即使要存储的值是非负值。对整数值一致使用 `Int` 可以提高代码的互操作性，避免在不同数字类型之间进行转换，并符合 中所述的整数类型推断。 + +## 浮点数 + +*浮点数*是带有小数成分的数字，如 `3.14159`、`0.1` 和 `-273.15`。 + +与整数类型相比，浮点类型可以表示的数值范围更广，而且可以存储比 `Int` 类型大得多或小得多的数字。Swift 提供了两种带符号浮点数类型： + +- `Double` 表示 64 位浮点数。 +- `Float` 表示 32 位浮点数。 + +> 备注: `Double` 的精度至少为小数点后 15 位，而 `Float` 的精度可以少至小数点后 6 位。使用哪种浮点类型更合适，取决于代码中需要处理数值的性质和范围。在两种类型都适用的情况下，`Double` 是首选。 + + + + + +## 类型安全和类型推断 + +Swift 是一种*类型安全*的语言。类型安全语言鼓励你显式指示代码可处理的值的类型。如果代码的一部分需要 `String`，你就不能错误地将 `Int` 传递给它。 + +由于 Swift 是类型安全的语言，因此它在编译代码时会执行*类型检查*，并将任何不匹配的类型标记为错误。这样，你就能在开发过程中尽早发现并修复错误。 + +类型检查可帮助你在处理不同类型的值时避免错误。但是，这并不意味着你必须指定你声明的每个常量和变量的类型。如果你没有指定所需值的类型，Swift 会使用*类型推断*来确定适当的类型。类型推断使编译器在编译代码时，仅通过检查你提供的值，就能自动推断出特定表达式的类型。 + +由于有了类型推断，Swift 所需的类型声明比 C 或 Objective-C 等语言要少得多。常量和变量仍然是显式类型的，但为其指定类型的大部分工作都是自动完成的。 + +当你声明一个带有初始值的常量或变量时，类型推断尤其有用。通常的做法是在声明常量或变量时为其赋*字面量*。（字面量指的是直接出现在源代码中的值，如下面示例中的 `42` 和 `3.14159`）。 + +例如，如果你将字面量 `42` 赋给一个新常量，但没有说明它是什么类型，Swift 就会推断你希望该常量是一个 `Int` 常量，因为你刚使用了一个看起来像整数的数字对它进行了初始化： + +```swift +let meaningOfLife = 42 +// meaningOfLife 被推断为 Int 类型 +``` + + + +同样，如果你没有为浮点数字面量指定类型，Swift 会认为你想创建一个 `Double`： + +```swift +let pi = 3.14159 +// pi 推断为 Double 类型 +``` + + + +在推断浮点数类型时，Swift 总是选择 `Double`（而非 `Float`）。 + +如果在表达式中结合使用整数和浮点数字面量，则会根据上下文推断出 `Double` 类型： + +```swift +let anotherPi = 3 + 0.14159 +// anotherPi 也被推断为 Double 类型 +``` + + + +`3` 的字面量本身并没有显式类型，因此可以根据加法中的浮点数字面量推断出合适的输出类型 `Double`。 + +## 数值字面量 + +整数字面量可以写成： + +- 不带前缀的*十进制*数 +- 带 `0b` 前缀的*二进制*数 +- 带前缀 `0o` 的*八进制*数 +- 带前缀 `0x` 的*十六进制*数 + +以下所有整数字面量的十进制值都是 `17`： + +```swift +let decimalInteger = 17 +let binaryInteger = 0b10001 // 以二进制表示的 17 +let octalInteger = 0o21 // 以八进制表示的 17 +let hexadecimalInteger = 0x11 // 以十六进制表示的 17 +``` + + + +浮点字面量可以是十进制（不带前缀），也可以是十六进制（带 `0x` 前缀）。浮点数的小数点两边必须始终有一个数字（或十六进制数）。十进制浮点数还可以有一个可选的*指数*，用大写或小写 `e` 表示；十六进制浮点数必须有一个指数，用大写或小写 `p` 表示。 + + + + + +对于指数为 `x` 的十进制数，字面量是基数乘以 10ˣ： + +- `1.25e2` 表示 1.25 x 10² 或 `125.0`。 +- `1.25e-2` 表示 1.25 x 10⁻² 或 `0.0125`。 + +对于指数为 `x` 的十六进制数，字面量是基数乘以 2ˣ： + +- `0xFp2` 表示 15 x 2² 或 `60.0`。 +- `0xFp-2` 表示 15 x 2⁻²，或 `3.75`。 + +以下所有浮点字面量的十进制值都是 `12.1875`： + +```swift +let decimalDouble = 12.1875 +let exponentDouble = 1.21875e1 +let hexadecimalDouble = 0xC.3p0 +``` + + + +数值字面量可以包含额外的格式，以便于阅读。整数和浮点数都可以填充额外的零，也可以包含下划线，以提高可读性。这两种格式都不会影响字面量的实际值： + +```swift +let paddedDouble = 000123.456 +let oneMillion = 1_000_000 +let justOverOneMillion = 1_000_000.000_000_1 +``` + + + +## 数字类型转换 + +对代码中的所有通用整数常量和变量使用 `Int` 类型，即使它们已知是非负值。在通常情况下使用默认整数类型意味着整数常量和变量可以立即在代码中互操作，并且与整数字面量的推断类型相匹配。 + +只有当手头的任务特别需要其他整数类型时，或者因为外部数据源提供了显式大小的数据，或者为了性能、内存使用或其他必要的优化，再考虑使用其他整数类型。在这些情况下使用显式大小的类型有助于捕捉任何意外的数值溢出，并隐式地记录所使用数据的性质。 + +### 整数转换 + +每种数字类型的整数常量或变量可存储的数字范围都不同。`Int8` 常量或变量可以存储 `-128` 到 `127` 之间的数字，而 `UInt8` 常量或变量可以存储 `0` 到 `255` 之间的数字。编译代码时，如果一个数字无法放入一个规定大小的整数类型常量或变量中，就会报错： + +```swift +let cannotBeNegative: UInt8 = -1 +// UInt8 不能存储负数，因此会报错 +let tooBig: Int8 = Int8.max + 1 +// Int8 不能存储大于其最大值的数字， +// 因此也会报错 +``` + + + +由于每种数值类型可以存储不同范围的值，因此必须根据具体情况选择是否进行数值类型转换。这种选择加入的方法可以防止隐藏的转换错误，并有助于在代码中明确类型转换的意图。 + +要将一种特定的数字类型转换为另一种，需要用现有值初始化一个所需类型的新数字。在下面的示例中，常量 `twoThousand` 的类型是 `UInt16`，而常量 `one` 的类型是 `UInt8`。由于它们的类型不同，因此不能直接相加。所以，本例调用 `UInt16(one)` 创建了一个初始值为 `one` 的新 `UInt16`，并用这个值代替原来的值进行计算： + +```swift +let twoThousand: UInt16 = 2_000 +let one: UInt8 = 1 +let twoThousandAndOne = twoThousand + UInt16(one) +``` + + + +因为加法的两边现在都是 UInt16 类型，所以加法是允许的。输出常量 (`twoThousandAndOne`) 被推断为 `UInt16` 类型，因为它是两个 `UInt16` 值的和。 + +`SomeType(ofInitialValue)` 是调用 Swift 类型初始化器并赋予初始值的默认方式。在后台，`UInt16` 有一个接受 `UInt8` 值的初始化器，因此该初始化器用于从现有的 `UInt8` 生成一个新的 `UInt16`。但在这里不能使用*任意*类型，而必须是 `UInt16` 提供了初始化器的类型。扩展现有类型以提供接受新类型（包括你自己的类型定义）的初始化器，将在 中介绍。 + +### 整数和浮点数转换 + +整数和浮点数值类型之间必须进行显式转换： + +```swift +let three = 3 +let pointOneFourOneFiveNine = 0.14159 +let pi = Double(three) + pointOneFourOneFiveNine +// pi 等于 3.14159，并被推断为 Double 类型 +``` + + + +在这里，常量 `three` 的值被用来创建一个 `Double` 类型的新值，这样加法的两边都是相同的类型。如果没有这种转换，加法运算将无法进行。 + +浮点数到整数也必须进行显式转换。整数类型可以用 `Double` 或 `Float` 值进行初始化： + +```swift +let integerPi = Int(pi) +// integerPi 等于 3，并被推断为 Int 类型 +``` + + + +用这种方法初始化一个新的整数值时，浮点数总是被截断的。这意味着 `4.75` 变为 `4`，`-3.9` 变为 `-3`。 + +> 备注: 组合数值常量和变量的规则与组合数值字面量的规则不同。字面量 `3` 可以与字面量 `0.14159` 相加，因为数值字面量本身并没有显式类型。只有在编译器对其进行评估时才会推断出其类型。 + + + +## 类型别名 + +*类型别名*用于定义现有类型的替代名称。你可以使用 `typealias` 关键字定义类型别名。 + +当你想用一个更适合上下文的名称来代指现有类型时，比如在处理源自外部的特定大小的数据时，类型别名就非常有用： + +```swift +typealias AudioSample = UInt16 +``` + + + +一旦定义了类型别名，就可以在任何可以使用原名的地方使用此别名： + +```swift +var maxAmplitudeFound = AudioSample.min +// maxAmplitudeFound 现在为 0 +``` + + + +在这里，`AudioSample` 被定义为 `UInt16` 的别名。因为是别名，所以调用 `AudioSample.min` 实际上是调用 `UInt16.min`，为 `maxAmplitudeFound` 变量提供一个初始值 `0`。 + +## 布尔类型 + +Swift 有一种基本的*布尔*类型，称为 `Bool`。布尔值又被称为*逻辑值*，因为它们只能为真或假。Swift 提供了两个布尔常量值：`true` 和 `false`。 + +```swift +let orangesAreOrange = true +let turnipsAreDelicious = false +``` + + + +`orangesAreOrange` 和 `turnipsAreDelicious` 的类型已被推断为 `Bool`，因为它们使用了布尔字面量初始化。与上述的 `Int` 和 `Double` 一样，如果你在创建常量或变量时将其设置为 `true` 或 `false`，则无需将其声明为 Bool。类型推断有助于让 Swift 代码在使用类型已知的值初始化常量或变量时更简洁易读。 + +布尔值在使用条件语句（如 `if` 语句）时尤其有用： + +```swift +if turnipsAreDelicious { + print("Mmm, tasty turnips!") +} else { + print("Eww, turnips are horrible.") +} +// 打印 "Eww, turnips are horrible." +``` + + + + 将详细介绍 `if` 语句等条件语句。 + +Swift 的类型安全防止非布尔值被替换为布尔值。下面的示例代码会报告编译时错误： + +```swift +let i = 1 +if i { + // 该示例将无法编译，并报错 +} +``` + + + +不过，下面的替代示例是合法的： + +```swift +let i = 1 +if i == 1 { + // 此示例将成功编译 +} +``` + + + +`i == 1` 的比较结果是 `Bool` 类型，因此第二个示例通过了类型检查。类似 `i == 1` 的比较结果将在 中讨论。 + +与 Swift 中其他类型安全示例一样，这种方法可以避免意外错误，并确保特定代码部分的意图始终清晰明了。 + +## 元组 + +*元组*将多个值组合成一个复合值。元组内的值可以是任何类型，且不必彼此属于同一类型。 + +在下例中，`(404, "Not Found")` 是一个描述 *HTTP 状态代码*的元组。HTTP 状态代码是网络服务器在你请求网页时返回的一个特殊值。如果你请求一个不存在的网页，返回的状态代码就是 `404 Not Found`。 + +```swift +let http404Error = (404, "Not Found") +// http404Error 的类型为（Int，String），且等于（404，"Not Found"） +``` + + + +`(404，"Not Found")` 元组将一个 `Int` 和一个 `String` 组合在一起，赋予 HTTP 状态代码两个独立的值：一个数字和一个人类可读的描述。它可以描述为 “一个 `(Int, String)` 类型的元组”。 + +你可以从任何的类型排列中创建元组，而且元组中可以包含任意多的不同类型。没有什么能阻止你创建一个 `(Int, Int, Int)` 或 `(String, Bool)` 类型的元组，或者其他任何你需要的排列。 + +你可以将元组的内容*分解*成单独的常量或变量，然后像往常一样访问它们： + +```swift +let (statusCode, statusMessage) = http404Error +print("The status code is \(statusCode)") +// 打印 "The status code is 404" +print("The status message is \(statusMessage)") +// 打印 "The status message is Not Found" +``` + + + +如果只需要元组的部分值，则在分解元组时使用下划线 (`_`) 忽略不需要的部分： + +```swift +let (justTheStatusCode, _) = http404Error +print("The status code is \(justTheStatusCode)") +// 打印 "The status code is 404" +``` + + + +或者，使用从零开始的索引号访问元组中的单个元素值： + +```swift +print("The status code is \(http404Error.0)") +// 打印 "The status code is 404" +print("The status message is \(http404Error.1)") +// 打印 "The status message is Not Found" +``` + + + +你可以在定义元组时为元组中的各个元素命名： + +```swift +let http200Status = (statusCode: 200, description: "OK") +``` + + + +如果为元组中的元素命名了，就可以使用元素名访问这些元素的值： + +```swift +print("The status code is \(http200Status.statusCode)") +// 打印 "The status code is 200" +print("The status message is \(http200Status.description)") +// 打印 "The status message is OK" +``` + + + +元组作为函数的返回值特别有用。一个试图检索网页的函数可能会返回 `(Int, String)` 元组类型来描述网页检索的成功或失败。在函数中返回包含两个不同值（每个值的类型都不同）的元组，比只能返回单一类型的单个值提供了更多关于其结果的有用信息。更多信息，请参阅 。 + +> 备注: 元组适用于简单的相关值组。它们不适合创建复杂的数据结构。如果你的数据结构可能会变得复杂，请将其创建为类或结构体，而不是元组。更多信息，请参阅 。 + +## 可选 + +在可能缺失值的情况下，请使用*可选*。可选代表两种可能性：要么*存在*一个指定类型的值，并可以解包可选以访问该值；要么*根本就没有*值。 + +举一个可能缺失值的例子，Swift 的 `Int` 类型有一个初始化器，它会尝试将 `String` 值转换为 `Int` 值。但是，只有某些字符串可以转换成整数。字符串 `"123"` 可以转换成数值 `123`，但字符串 `"hello, world"` 却没有对应的数值。下面的示例使用初始化器尝试将 `String` 转换为 `Int`： + +```swift +let possibleNumber = "123" +let convertedNumber = Int(possibleNumber) +// convertedNumber 的类型是 "可选 Int"。 +``` + + + +因为上面代码中的初始化器可能会失败，所以它返回的是*可选* `Int`，而不是 `Int`。 + +要编写可选类型，需要在可选包含的类型名称后面加一个问号（`?`）。例如，可选 `Int` 的类型是 `Int?`。可选 `Int` 只能储存某个 `Int` 值或不储存任何值。它不能储存任何其他值，如 `Bool` 或 `String` 值。 + +### nil + +通过给可选变量赋特殊值 `nil`，可以将其设置为无值状态： + +```swift +var serverResponseCode: Int? = 404 +// serverResponseCode 包含一个实际 Int 值 404 +serverResponseCode = nil +// serverResponseCode 现在不包含任何值 +``` + + + +如果你定义了一个可选变量，但没有提供默认值，那么该变量将自动设置为 `nil`： + +```swift +var surveyAnswer: String? +// surveyAnswer 自动设置为 nil +``` + + + +你可以使用 `if` 语句，通过比较可选和 nil 来确定可选是否包含一个值。你可以使用“等于”操作符（`==`）或“不等于”操作符（`!=`）进行比较。 + +如果可选有一个值，它就被视为“不等于” `nil`： + +```swift +let possibleNumber = "123" +let convertedNumber = Int(possibleNumber) + +if convertedNumber != nil { + print("convertedNumber contains some integer value.") +} +// 打印 "convertedNumber contains some integer value." +``` + + + +不能在非可选常量或变量中使用 `nil`。如果代码中的常量或变量在某些条件下需要在没有值的情况下工作，请将其声明为适当类型的可选值。声明为非可选值的常量或变量保证永远不会包含 `nil` 值。如果尝试将 `nil` 赋值给一个非可选值，就会出现编译时错误。 + +通过将可选值和非可选值分开，可以显式标记哪些信息可能缺失，从而更方便编写处理缺失值的代码。你不能意外地将可选值当作非可选值来处理，因为这种错误会在编译时产生错误。在对值进行解包后，使用该值的其他代码都不需要检查 `nil`，因此不需要在代码的不同部分重复检查同一个值。 + +在访问可选值时，代码总是同时处理 `nil` 和非 `nil` 两种情况。当值缺失时，可以执行如以下各节所述的几项操作： + +- 当值为 `nil` 时，跳过对其进行操作的代码。 + +- 通过返回 `nil` 或使用 中记述的 `?.` 运算符传播 `nil` 值。 + +- 使用 `??` 运算符提供一个后备值。 + +- 使用 `!` 运算符停止程序执行。 + +> 备注: 在 Objective-C 中，`nil` 是指向不存在对象的指针。在 Swift 中，`nil` 并非指针，而是特定类型值的缺失。*任何*类型的可选都可以被设置为 `nil`，而不仅仅是对象类型。 + +### 可选绑定 + + +你可以使用可选绑定来确定可选是否包含值，如果包含，则将该值作为临时常量或变量使用。可选绑定可与 `if`、`guard` 和 `while` 语句一起使用，以检查可选中的值，并将该值提取到常量或变量中，作为单个操作的一部分。有关 `if`、`guard` 和 `while` 语句的更多信息，请参阅 。 + +使用 `if` 语句编写的可选绑定如下： + +```swift +if let <#constantName#> = <#someOptional#> { + <#statements#> +} +``` + +你可以重写 部分中的 `possibleNumber` 示例，使用可选绑定而不是强制解包： + +```swift +if let actualNumber = Int(possibleNumber) { + print("The string \"\(possibleNumber)\" has an integer value of \(actualNumber)") +} else { + print("The string \"\(possibleNumber)\" couldn't be converted to an integer") +} +// 打印 "The string "123" has an integer value of 123" +``` + + + +该代码可理解为： + +“如果 `Int(possibleNumber)` 返回的可选 `Int` 中包含一个值，则将它赋值给名为 `actualNumber` 的新常量。” + +如果转换成功，`actualNumber` 常量就可以在 `if` 语句的第一个分支中使用。这个常量已经用可选中的值进行了初始化，并具有相应的非可选类型。在本例中，`possibleNumber` 的类型是 `Int?`，因此 `actualNumber` 的类型是 `Int`。 + +如果在访问原可选常量或变量的值后不需要再引用它，则可以考虑使用相同的名称来命名新常量或变量： + +```swift +let myNumber = Int(possibleNumber) +// 这里，myNumber 是一个可选整数 +if let myNumber = myNumber { + // 这里，myNumber 是一个非可选整数 + print("My number is \(myNumber)") +} +// 打印 "My number is 123" +``` + + + +这段代码首先检查 `myNumber` 是否包含一个值，就像上一个示例中的代码一样。如果 `myNumber` 有一个值，名为 `myNumber` 的新常量的值就会被设置为该值。在 `if` 语句的正文中，`myNumber` 指的就是这个新的非可选常量。在 `if` 语句之前或之后写 `myNumber`，指的是原来的可选整数常量。 + +由于这种代码非常常见，因此可以使用更简短的语法来解包可选值：只写常量或变量的名称即可。解包后的新常量或变量隐式地使用与可选值相同的名称。 + +```swift +if let myNumber { + print("My number is \(myNumber)") +} +// 打印 "My number is 123" +``` + + + +你可以在可选绑定时使用常量或变量。如果你想在 `if` 语句的第一个分支中修改 `myNumber` 的值，你可以改写为 `if var myNumber`，这样，包含在可选中的值就可以作为变量而不是常量使用了。在 `if` 语句正文中对 `myNumber` 所做的修改仅适用于该局部变量，而*不适用于*原来的可选常量或变量。 + +你可以在一个 `if` 语句中包含任意数量的可选绑定和布尔条件，并用逗号分隔。如果可选绑定中的任何值为 `nil`，或任何布尔条件的值为 `false`，则整个 `if` 语句的条件被视为 `false`。以下 `if` 语句是等价的： + +```swift +if let firstNumber = Int("4"), let secondNumber = Int("42"), firstNumber < secondNumber && secondNumber < 100 { + print("\(firstNumber) < \(secondNumber) < 100") +} +// 打印 "4 < 42 < 100" + +if let firstNumber = Int("4") { + if let secondNumber = Int("42") { + if firstNumber < secondNumber && secondNumber < 100 { + print("\(firstNumber) < \(secondNumber) < 100") + } + } +} +// 打印 "4 < 42 < 100" +``` + + + + + +在 `if` 语句中使用可选绑定创建的常量和变量只能在 `if` 语句的正文中使用。与此相反，用 `guard` 语句创建的常量和变量仅在 `guard` 语句后的代码行中可用，如 中所述。 + +### 提供后备值 + +处理缺失值的另一种方法是使用 nil-coalescing 操作符（`??`）提供一个缺省值。如果 `??` 左边的可选值不是 `nil`，那么该值将被解包并使用。否则，将使用 `??` 右侧的值。例如，如果指定了姓名，下面的代码会用姓名问候某人，如果姓名为 `nil`，则使用通用问候语。 + +```swift +let name: String? = nil +let greeting = "Hello, " + (name ?? "friend") + "!" +print(greeting) +// 打印 "Hello, friend!" +``` + + + +关于使用 `??` 提供后备值的更多信息，请参阅 。 + +### 强制解包 + +当 `nil` 表示不可恢复的故障时（如程序员错误或状态损坏），你可以通过在可选名称的末尾添加感叹号 (`!`) 来访问底层值。这被称为*强制解包*可选的值。强制解包一个非 `nil` 值时，结果是其解包值。强制解包一个 `nil` 值则会引发运行时错误。 + +实际上，`!` 是 [`fatalError(_:file:line:)`][] 的简写。例如，下面的代码显示了两种等效的方法： + +[`fatalError(_:file:line:)`]: https://developer.apple.com/documentation/swift/fatalerror(_:file:line:) + +```swift +let possibleNumber = "123" +let convertedNumber = Int(possibleNumber) + +let number = convertedNumber! + +guard let number = convertedNumber else { + fatalError("The number was invalid") +} +``` + +上述两个版本的代码都要求于 `convertedNumber` 始终包含一个值。使用上述任一方法将该要求写入代码，可让代码在运行时检查该要求是否为真。 + +有关在运行时执行数据要求和检查假设的更多信息，请参阅 。 + +### 隐式解包可选 + +如上所述，可选表示允许常量或变量“无值”。可以用 `if` 语句检查可选值是否存在，如果可选值确实存在，则可以通过可选绑定有条件地解除对可选值的包裹。 + +有时，从程序结构中可以清楚地看出，在首次设置可选值后，该可选将*始终*有一个值。在这种情况下，无需在每次访问可选时都对其值进行检查和解包，因为你可以安全地假定它一直都有值。 + +这类可选被定义为*隐式解包可选*。在编写隐式解包可选时，需要在可选类型后面加上感叹号（`String!`）而不是问号（`String?`）。要注意不是在使用可选时在其名称后加上感叹号，而是在声明可选时在其类型后加上感叹号。 + +当首次定义可选后，可选的值立即被确认存在，并且可以确保在此后的每一个时间点都存在值时，隐式解包可选就非常有用了。在 Swift 中，隐式解包可选的主要用途是在类初始化过程中，如 中所述。 + +当变量有可能在稍后阶段变为 `nil` 时，不要使用隐式解包可选。如果需要在变量的生命周期内检查变量是否为 `nil`，请务必使用普通的可选类型。 + +隐式解包的可选在幕后是一个普通的可选值，但也可以像非可选值一样使用，而无需在每次访问时都进行解包。下面的示例显示了可选字符串和隐式解包的可选字符串在作为显式字符串访问其被包装值时的行为差异： + +```swift +let possibleString: String? = "An optional string." +let forcedString: String = possibleString! // 需要显式解包 + +let assumedString: String! = "An implicitly unwrapped optional string." +let implicitString: String = assumedString // 隐式解包 +``` + + + +你可以将隐式解包可选视为允许可选值在需要时被强制解包。在使用隐式解包的可选值时，Swift 会首先尝试将其作为普通可选值使用；如果不能将其作为可选值使用，Swift 就会强制解包该值。在上面的代码中，可选值 `assumedString` 在赋值给 `implicitString` 之前被强制解包，因为 `implicitString` 的类型是显式定义的非可选字符串。在下面的代码中，`optionalString` 没有显式类型，所以它是一个普通的可选值。 + +```swift +let optionalString = assumedString +// optionalString 的类型是 "String?"，而 assumedString 没有强制解包。 +``` + + + +如果一个隐式解包的可选值为 `nil`，而你试图访问它的被包装值，就会触发运行时错误。其结果与用感叹号来强制解包一个不包含值的普通可选完全相同。 + +你可以像检查普通可选一样，检查隐式解包的可选是否为 `nil`： + +```swift +if assumedString != nil { + print(assumedString!) +} +// 打印 "An implicitly unwrapped optional string." +``` + + + +你也可以对隐式解包的可选使用可选绑定，在单个语句中检查并解包其值： + +```swift +if let definiteString = assumedString { + print(definiteString) +} +// 打印 "An implicitly unwrapped optional string." +``` + + + +## 错误处理 + +你可以使用*错误处理*来应对程序在执行过程中可能遇到的错误情况。 + +与可以使用值的存在与否来传达函数的成功或失败的可选不同，错误处理允许你确定失败的根本原因，并在必要时将错误传播到程序的另一部分。 + +当函数遇到错误条件时，它会*抛出*一个错误。该函数的调用者可以*捕获*错误并做出适当的响应。 + +```swift +func canThrowAnError() throws { + // 此函数可能抛出错误，也可能不抛错 +} +``` + + + +函数在声明中包含 `throws` 关键字，表示它可以抛出错误。调用可以抛出错误的函数时，要在表达式前加上 `try` 关键字。 + +Swift 会自动将错误传播到当前作用域之外，直到它们被 `catch` 子句处理为止。 + +```swift +do { + try canThrowAnError() + // 无错误的情况 +} catch { + // 抛出错误的情况 +} +``` + + + +`do` 语句会创建一个新的包含作用域，允许错误传播到一个或多个 `catch` 子句。 + +下面的示例说明了如何使用错误处理来应对不同的错误条件： + +```swift +func makeASandwich() throws { + // ... +} + +do { + try makeASandwich() + eatASandwich() +} catch SandwichError.outOfCleanDishes { + washDishes() +} catch SandwichError.missingIngredients(let ingredients) { + buyGroceries(ingredients) +} +``` + + + +在本例中，如果没有干净的餐具或缺少任何配料，`makeASandwich()` 函数就会出错。由于 `makeASandwich()` 可能会出错，因此函数调用被封装在 `try` 表达式中。通过将函数调用封装在 `do` 语句中，任何抛出的错误都会传播到所提供的 `catch` 子句中。 + +如果函数没有出错，就会继续调用 `eatASandwich()` 函数。如果函数抛错且错误符合 `SandwichError.outOfCleanDishes` 的情况，则会调用 `washDishes()` 函数。如果出现与 `SandwichError.missingIngredients` 情况匹配的错误，则会调用 `buyGroceries(_:)` 函数，并使用 `catch` 模式捕获相关的 `[String]` 值。 + +在 中有对于抛出、捕获和传播错误更详细的介绍。 + +## 断言和先决条件 + +*断言*和*先决条件*是在运行时进行的检查。使用它们可以确保在执行任何进一步代码之前满足一个基本条件。如果断言或前提条件中的布尔条件为 `true`，代码将照常执行。如果条件的计算结果为 `false`，则程序的当前状态无效;代码执行结束，应用会被终止。 + +你可以使用断言和前提条件来表达你在编码时的假设和期望，因此你可以将它们作为代码的一部分。断言可以帮助你在开发过程中发现错误和不正确的假设，而前提条件可以帮助你在生产过程中发现问题。 + +除了在运行时验证你的预期，断言和前提条件也是代码中一种有用的文档形式。与上文 中讨论的错误条件不同，断言和前提条件不用于可恢复或预期的错误。因为一个失败的断言或前提条件表示程序状态无效，所以没有办法捕获一个失败的断言。从无效状态恢复是不可能的。当断言失败时，程序中至少有一个数据是无效的，但你不知道它为什么无效，也不知道是否还有其他状态也无效。 + +使用断言和先决条件并不能代替代码设计，减少无效条件出现的可能。但是，使用断言和前提条件来强制确保有效的数据和状态，会使应用在出现无效状态时以更可预测的方式终止，并使问题更容易调试。如果不对假设进行检查，可能要到很久以后，当其他地方的代码开始明显失效，以及用户数据被悄无声息地破坏后，你才会注意到这类问题。一旦检测到无效状态，立即停止执行也有助于限制无效状态造成的损害。 + +断言和前提条件的区别在于何时检查：断言只在调试构建中进行检查，而前提条件则在调试构建和生产构建中都进行检查。在生产版本中，断言中的条件不会被评估。这意味着你可以在开发过程中随意使用断言，而不会影响生产过程中的性能。 + +### 使用断言进行调试 + + + +你可以调用 Swift 标准库中的 [`assert(_:_:file:line:)`](https://developer.apple.com/documentation/swift/1541112-assert) 函数来编写断言。你可以向该函数传递一个计算结果为 `true` 或 `false` 的表达式，以及一条在条件结果为 `false` 时显示的信息。例如： + +```swift +let age = -3 +assert(age >= 0, "A person's age can't be less than zero.") +// 该断言失败的原因是 -3 并不 >= 0。 +``` + + + +在本例中，如果 `age >= 0` 的值为 `true`，即 `age` 的值为非负值，代码将继续执行。如果 `age` 的值为负数（如上面的代码），则 `age >= 0 `的值为 `false`，断言失败，应用终止。 + +你可以省略断言信息，例如，当信息只是重复解释断言条件时。 + +```swift +assert(age >= 0) +``` + + + + + +如果代码已经检查了条件，则使用 [`assertionFailure(_:file:line:)`](https://developer.apple.com/documentation/swift/1539616-assertionfailure) 函数来表示断言失败。例如： + +```swift +if age > 10 { + print("You can ride the roller-coaster or the ferris wheel.") +} else if age >= 0 { + print("You can ride the ferris wheel.") +} else { + assertionFailure("A person's age can't be less than zero.") +} +``` + + + +### 强制执行先决条件 + +当条件有可能为假，但*必须*为真才能继续执行代码时，请使用先决条件。例如，使用先决条件检查下标是否越界，或检查函数是否传递了有效值。 + +你可以通过调用 [`precondition(_:_:file:line:)`](https://developer.apple.com/documentation/swift/1540960-precondition) 函数来编写先决条件。你可以向该函数传递一个计算结果为 `true` 或 `false` 的表达式，以及一条在条件结果为 `false` 时显示的信息。例如： + +```swift +// 在下标的实现中... +precondition(index > 0, "Index must be greater than zero.") +``` + + + +你还可以调用 [`preconditionFailure(_:file:line:)`](https://developer.apple.com/documentation/swift/1539374-preconditionfailure) 函数来表示发生了故障，例如，如果执行了 switch 的默认情况，但所有有效输入数据本应由其他情况来处理。 + +> 备注: 如果以非检查模式（`-Ounchecked`）编译，则不会检查前置条件。编译器会假定前提条件总是为真，并据此优化代码。不过，无论优化设置如何，`fatalError(_:file:line:)` 函数始终会停止执行。 +> +> 在原型开发和早期开发过程中，你可以使用 `fatalError(_:file:line:)` 函数为尚未实现的功能创建存根，方法是将 `fatalError("Unimplemented")` 写成存根实现。与断言或先决条件不同，致命错误永远不会被优化掉，因此可以确保在遇到存根实现时始终停止执行。 + + + + + + diff --git a/swift-6-beta.docc/LanguageGuide/TypeCasting.md b/swift-6.docc/LanguageGuide/TypeCasting.md similarity index 51% rename from swift-6-beta.docc/LanguageGuide/TypeCasting.md rename to swift-6.docc/LanguageGuide/TypeCasting.md index eee09ac14..cbfaaeaa0 100644 --- a/swift-6-beta.docc/LanguageGuide/TypeCasting.md +++ b/swift-6.docc/LanguageGuide/TypeCasting.md @@ -1,33 +1,19 @@ -# Type Casting +# 类型转换 -Determine a value's runtime type and give it more specific type information. +确定一个值的运行时类型，并为其提供更具体的类型信息。 -*Type casting* is a way to check the type of an instance, -or to treat that instance as a different -superclass or subclass from somewhere else in its own class hierarchy. +*类型转换*是一种检查实例类型，或将该实例视为其类层次结构中不同父类或子类的方法。 -Type casting in Swift is implemented with the `is` and `as` operators. -These two operators provide a simple and expressive way -to check the type of a value or cast a value to a different type. +Swift 中的类型转换是通过 `is` 和 `as` 操作符实现的。这两个操作符为值的类型检查或类型转换提供了一种简单而富有表现力的方法。 -You can also use type casting to check whether a type conforms to a protocol, -as described in . +你还可以使用类型转换来检查类型是否遵循协议， +如 中所述。 -## Defining a Class Hierarchy for Type Casting +## 为类型转换定义类层次结构 -You can use type casting with a hierarchy of classes and subclasses -to check the type of a particular class instance -and to cast that instance to another class within the same hierarchy. -The three code snippets below define a hierarchy of classes -and an array containing instances of those classes, -for use in an example of type casting. +你可以使用类型转换在类和子类的层次结构中检查某个类实例的类型，并将该实例转换为同一层次结构中的另一个类。下面的三个代码片段定义了一个类层次结构和一个包含这些类的实例的数组，作为类型转换示例。 -The first snippet defines a new base class called `MediaItem`. -This class provides basic functionality for any kind of item that appears -in a digital media library. -Specifically, it declares a `name` property of type `String`, -and an `init(name:)` initializer. -(It's assumed that all media items, including all movies and songs, will have a name.) +第一个代码片段定义了一个新的名为 `MediaItem` 的基类。该类为数字媒体库中出现的任何类型项目提供基本功能。具体来说，它声明了一个 `String` 类型的 `name` 属性和一个 `init(name:)` 初始化器。（假设所有媒体项目，包括所有电影和歌曲，都有一个名称。） ```swift class MediaItem { @@ -51,12 +37,7 @@ class MediaItem { ``` --> -The next snippet defines two subclasses of `MediaItem`. -The first subclass, `Movie`, encapsulates additional information about a movie or film. -It adds a `director` property on top of the base `MediaItem` class, -with a corresponding initializer. -The second subclass, `Song`, adds an `artist` property and initializer -on top of the base class: +下一个代码段定义了 `MediaItem` 的两个子类。第一个子类 `Movie` 封装了电影的附加信息。它在基础 `MediaItem` 类的基础上添加了一个 `director` 属性和一个相应的初始化器。第二个子类 `Song` 在基类的基础上添加了 `artist` 属性和初始化器： ```swift class Movie: MediaItem { @@ -98,13 +79,7 @@ class Song: MediaItem { ``` --> -The final snippet creates a constant array called `library`, -which contains two `Movie` instances and three `Song` instances. -The type of the `library` array is inferred -by initializing it with the contents of an array literal. -Swift's type checker is able to deduce that `Movie` and `Song` have -a common superclass of `MediaItem`, -and so it infers a type of `[MediaItem]` for the `library` array: +最后一个代码段创建了一个名为 `library` 的常量数组，其中包含两个 `Movie` 实例和三个 `Song` 实例。`library` 数组的类型是通过使用数组字面值初始化来推断的。Swift 的类型检查程序能够推断出 `Movie` 和 `Song` 有一个共同的父类 `MediaItem`，从而推断出 library 数组的类型为 `[MediaItem]`： ```swift let library = [ @@ -114,7 +89,7 @@ let library = [ Song(name: "The One And Only", artist: "Chesney Hawkes"), Song(name: "Never Gonna Give You Up", artist: "Rick Astley") ] -// the type of "library" is inferred to be [MediaItem] +// "library" 的类型被推断为 [MediaItem] ``` -The items stored in `library` are still `Movie` and `Song` instances behind the scenes. -However, if you iterate over the contents of this array, -the items you receive back are typed as `MediaItem`, -and not as `Movie` or `Song`. -In order to work with them as their native type, -you need to *check* their type, -or *downcast* them to a different type, -as described below. +`library` 中存储的项目实际上仍然是 `Movie` 和 `Song` 实例。但是，如果遍历该数组的内容，返回的项目类型是 `MediaItem`，而不是 `Movie` 或 `Song`。为了以原始类型处理它们，你需要*检查*它们的类型，或将它们*向下转型*为不同的类型，如下文所述。 -## Checking Type +## 检查类型 -Use the *type check operator* (`is`) to check -whether an instance is of a certain subclass type. -The type check operator returns `true` if the instance is of that subclass type -and `false` if it's not. +使用*类型检查操作符* (`is`) 来检查实例是否属于某个子类类型。如果实例属于该子类类型，则类型检查操作符返回 `true`；反之则返回 `false`。 -The example below defines two variables, `movieCount` and `songCount`, -which count the number of `Movie` and `Song` instances in the `library` array: +下面的示例定义了两个变量：`movieCount` 和 `songCount`，用于计算 `library` 数组中 `Movie` 和 `Song` 实例的数量： ```swift var movieCount = 0 @@ -166,7 +130,7 @@ for item in library { } print("Media library contains \(movieCount) movies and \(songCount) songs") -// Prints "Media library contains 2 movies and 3 songs" +// 打印 "Media library contains 2 movies and 3 songs" ``` -This example iterates through all items in the `library` array. -On each pass, the `for`-`in` loop sets the `item` constant -to the next `MediaItem` in the array. - -`item is Movie` returns `true` if the current `MediaItem` -is a `Movie` instance and `false` if it's not. -Similarly, `item is Song` checks whether the item is a `Song` instance. -At the end of the `for`-`in` loop, the values of `movieCount` and `songCount` -contain a count of how many `MediaItem` instances were found of each type. - -## Downcasting - -A constant or variable of a certain class type may actually refer to -an instance of a subclass behind the scenes. -Where you believe this is the case, -you can try to *downcast* to the subclass type -with a *type cast operator* (`as?` or `as!`). - -Because downcasting can fail, -the type cast operator comes in two different forms. -The conditional form, `as?`, returns an optional value of the type you are trying to downcast to. -The forced form, `as!`, attempts the downcast and force-unwraps the result -as a single compound action. - -Use the conditional form of the type cast operator (`as?`) -when you aren't sure if the downcast will succeed. -This form of the operator will always return an optional value, -and the value will be `nil` if the downcast was not possible. -This enables you to check for a successful downcast. - -Use the forced form of the type cast operator (`as!`) -only when you are sure that the downcast will always succeed. -This form of the operator will trigger a runtime error -if you try to downcast to an incorrect class type. - -The example below iterates over each `MediaItem` in `library`, -and prints an appropriate description for each item. -To do this, it needs to access each item as a true `Movie` or `Song`, -and not just as a `MediaItem`. -This is necessary in order for it to be able to access -the `director` or `artist` property of a `Movie` or `Song` -for use in the description. - -In this example, each item in the array might be a `Movie`, -or it might be a `Song`. -You don't know in advance which actual class to use for each item, -and so it's appropriate to use the conditional form of the type cast operator (`as?`) -to check the downcast each time through the loop: +此示例遍历 `library` 数组中的所有项目。每次遍历时，`for`-`in` 循环都会将 `item` 常量设置为数组中的下一个 `MediaItem`。 + +如果当前的 `MediaItem` 是 `Movie` 实例，`item is Movie` 返回 `true`；反之则返回 `false`。同理，`item is Song` 会检查项目是否为 `Song` 实例。当 `for`-`in` 循环结束时，`movieCount` 和 `songCount` 的值包含了找到的每种类型的 `MediaItem` 实例的数量。 + +## 向下转型 + +某个类常量或变量可能实际上指向子类的实例。如果你认为情况确实如此，可以尝试使用*类型转换操作符*（`as?` 或 `as!`）来*向下转型*为子类类型。 + +由于向下转型可能失败，类型转换操作符有两种不同的形式。条件形式，即 `as?`，会返回一个与你尝试向下转型的类型相同的可选值。强制形式，即 `as!`，会执行尝试向下转型并将结果强制解包的复合操作。 + +如果不确定向下转型能否成功，请使用类型转换操作符的条件形式（`as?`）。这种形式的操作符将始终返回一个可选值，如果向下转型不成功，该值将为 `nil`。这样，你就可以检查是否成功进行了向下转型。 + +只有在确定向下转型一定会成功时，才使用类型转换操作符的强制形式（`as!`）。如果尝试向下转型到一个不正确的类类型，这种形式的操作符会触发运行时错误。 + +下面的示例遍历 `library` 中的每个 `MediaItem`，并为每个项目打印适当的描述。为此，它需要将每个项目作为真正的 `Movie` 或 `Song`来访问，而不仅仅是作为一个 `MediaItem`。因为只有这样才能访问 `Movie` 的 `director` 或 `Song` 的 `artist` 属性，并在描述中使用。 + +在本例中，数组中的每个项目可能是一部 `Movie`，也可能是一首 `Song`。我们事先并不知道哪个项目是什么类型，因此这里较为恰当的做法，是使用类型转换操作符的条件形式（`as?`）在循环中进行向下转型。 ```swift for item in library { @@ -274,36 +207,15 @@ for item in library { ``` --> -The example starts by trying to downcast the current `item` as a `Movie`. -Because `item` is a `MediaItem` instance, it's possible that it *might* be a `Movie`; -equally, it's also possible that it might be a `Song`, -or even just a base `MediaItem`. -Because of this uncertainty, the `as?` form of the type cast operator returns an *optional* value -when attempting to downcast to a subclass type. -The result of `item as? Movie` is of type `Movie?`, or “optional `Movie`”. - -Downcasting to `Movie` fails when applied to -the `Song` instances in the library array. -To cope with this, the example above uses optional binding -to check whether the optional `Movie` actually contains a value -(that is, to find out whether the downcast succeeded.) -This optional binding is written “`if let movie = item as? Movie`”, -which can be read as: - -“Try to access `item` as a `Movie`. -If this is successful, -set a new temporary constant called `movie` to -the value stored in the returned optional `Movie`.” - -If the downcasting succeeds, the properties of `movie` are then used -to print a description for that `Movie` instance, including the name of its `director`. -A similar principle is used to check for `Song` instances, -and to print an appropriate description (including `artist` name) -whenever a `Song` is found in the library. - -> Note: Casting doesn't actually modify the instance or change its values. -> The underlying instance remains the same; it's simply treated and accessed -> as an instance of the type to which it has been cast. +该示例首先尝试将当前 `item` 向下转型为 `Movie`。由于 `item` 是一个 `MediaItem` 实例，它有*可能*是一部 `Movie`；同样，它也有可能是一首 `Song`，或者只是一个基本的 `MediaItem`。由于这种不确定性，当尝试向下转型到子类类型时，类型转换运算符的 `as?` 形式会返回一个*可选*值。`item as? Movie` 的结果是 `Movie?`（“可选 `Movie`”）类型。 + +当应用到库数组中的 `Song` 实例时，向下转型为 `Movie` 会失败。为了解决这个问题，上面的示例使用了可选绑定来检查可选 `Movie` 是否实际包含一个值（即，检查向下转型是否成功。）这里的可选绑定写作“`if let movie = item as? Movie`”，可以理解为： + +“尝试将 `item` 作为 `Movie` 访问。如果成功，则将名为 `movie` 的新临时常量设置为存储在返回的可选 `Movie` 中的值。” + +如果向下转型成功，`movie` 的属性将用于打印该 `Movie` 实例的说明，包括其导演的姓名。类似的原理也用于检查 `Song` 实例，并在从资源库中找到歌曲时打印适当的描述（包括艺术家姓名）。 + +> 注意: 类型转换并不修改实例或更改其值。底层的实例保持不变，它们只是被当作转换后的类型实例来处理和访问。 -## Type Casting for Any and AnyObject +## Any 和 AnyObject 的类型转换 -Swift provides two special types for working with nonspecific types: +Swift 为处理非特定类型提供了两种通用类型： -- `Any` can represent an instance of any type at all, including function types. -- `AnyObject` can represent an instance of any class type. +- `Any` 可以表示任何类型的实例，包括函数类型。 +- `AnyObject` 可以表示任何类类型的实例。 -Use `Any` and `AnyObject` only when you explicitly need -the behavior and capabilities they provide. -It's always better to be specific about the types you expect to work with in your code. +只有在明确需要它们提供的行为和功能时，才使用 `Any` 和 `AnyObject`。在代码中最好明确指定你希望使用的类型。 -Here's an example of using `Any` to work with a mix of different types, -including function types and nonclass types. -The example creates an array called `things`, which can store values of type `Any`: +下面是一个使用 `Any` 处理不同类型（包括函数类型和非类类型）的示例。该示例创建了一个名为 `things` 的数组，可以存储 `Any` 类型的值： ```swift var things: [Any] = [] @@ -362,20 +270,10 @@ things.append({ (name: String) -> String in "Hello, \(name)" }) ``` --> -The `things` array contains -two `Int` values, two `Double` values, a `String` value, -a tuple of type `(Double, Double)`, -the movie “Ghostbusters”, -and a closure expression that takes a `String` value -and returns another `String` value. - -To discover the specific type of a constant or variable -that's known only to be of type `Any` or `AnyObject`, -you can use an `is` or `as` pattern in a `switch` statement's cases. -The example below iterates over the items in the `things` array -and queries the type of each item with a `switch` statement. -Several of the `switch` statement's cases bind their matched value to -a constant of the specified type to enable its value to be printed: +`things` 数组包含两个 `Int` 值、两个 `Double` 值、一个 `String` 值、一个类型为 `(Double, Double)` 的元组、电影 “捉鬼敢死队”，以及一个接收 `String` 值并返回另一个 `String` 值的闭包表达式。 + + +如果常量或变量的已知类型是 `Any` 或 `AnyObject`，要确定其具体类型，可以在 `switch` 语句的 case 下使用 `is` 或 `as` 模式。下面的示例遍历了 `things` 数组中的项目，并使用 `switch` 语句查询了每个项目的类型。该 `switch` 语句的一些 case 将其匹配值与指定类型的常量绑定，以便打印其值： ```swift for thing in things { @@ -453,17 +351,12 @@ for thing in things { ``` --> -> Note: The `Any` type represents values of any type, including optional types. -> Swift gives you a warning if you use an optional value -> where a value of type `Any` is expected. -> If you really do need to use an optional value as an `Any` value, -> you can use the `as` operator to explicitly cast the optional to `Any`, -> as shown below. +> 注意: `Any` 类型代表任何类型的值，包括可选类型。如果你使用的是可选值，而预期值是 `Any` 类型，Swift 会发出警告。如果你您确实需要将可选值用作 `Any` 值，可以使用 `as` 操作符显式地将可选值转换为 `Any` 值，如下所示。 > > ```swift > let optionalNumber: Int? = 3 -> things.append(optionalNumber) // Warning -> things.append(optionalNumber as Any) // No warning +> things.append(optionalNumber) // 会警告 +> things.append(optionalNumber as Any) // 不会警告 > ``` -> Beta Software: -> -> This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software. -> -> Learn more about using [Apple's beta software](https://developer.apple.com/support/beta-software/). - + +特性的第一个参数指明宏的角色： + +- Peer 宏：将 `peer` 作为此特性的第一个参数。实现该宏的类型遵循 `PeerMacro` 协议。这些宏在与宏附加的声明相同的作用域中生成新的声明。例如，将 peer 宏应用于结构体的方法可以在该结构体上定义额外的方法和属性。 + +- Member 宏：将 `member` 作为此特性的第一个参数。实现该宏的类型遵循 `MemberMacro` 协议。这些宏生成的新声明是该宏所附加的类型或扩展的成员。例如，将 member 宏应用于结构体声明可以在该结构体上定义额外的方法和属性。 + +- Member 特性：将 `memberAttribute` 作为此特性的第一个参数。实现该宏的类型遵循 `MemberAttributeMacro` 协议。这些宏将特性添加到该宏所附加的类型或扩展的成员上。 + +- Accessor 宏：将 `accessor` 作为此特性的第一个参数。实现该宏的类型遵循 `AccessorMacro` 协议。这些宏为它们附加的存储属性添加访问器，将其转换为计算属性。 + +- Extension 扩展宏：将 `extension` 作为此特性的第一个参数。实现宏的类型遵循 `ExtensionMacro` 协议。这些宏可以添加协议遵循、`where` 从句，以及宏所附加到的类型的成员的新声明。如果宏添加了协议遵循，请包含 `conformances:` 参数并指定这些协议。遵循列表包含协议名称、指向遵循列表项的类型别名，或者是遵循列表项的协议组合。嵌套类型上的扩展宏会展开为该文件顶层的扩展。你不能在扩展、类型别名或嵌套在函数内的类型上编写扩展宏，也不能使用扩展宏添加具有 peer 宏的扩展。 + +peer、member 和 accessor 宏角色需要一个 `names:` 参数，列出宏生成的符号名称。如果宏在扩展内部添加声明，扩展宏角色也需要一个 `names:` 参数。当宏声明包含 `names:` 参数时，宏实现必须仅生成与该列表匹配的名称的符号。也就是说，宏不必为每个列出的名称生成符号。该参数的值是以下一个或多个项的列表： + +- `named(<#name#>)` 其中 *name* 是那个固定的符号名称，用于一个已知的名称。 + +- `overloaded` 用于与现有符号同名的名称。 + +- `prefixed(<#prefix#>)` 其中 *prefix* 被添加到符号名称前，用于以固定字符串开头的名称。 + +- `suffixed(<#suffix#>)` 其中 *suffix* 被附加到符号名称后，用于以固定字符串结尾的名称。 + +- `arbitrary` 用于一个在宏展开之前无法确定的名称。 + +作为一个特殊情况，你可以为一个行为类似于属性包装器的宏编写 `prefixed($)`。 + + +### available + +应用此特性来表明某个声明的生命周期是相对于特定的 Swift 语言版本或特定的平台和操作系统版本的。 + +`available` 特性总是与两个或更多用逗号分隔的特性参数列表一起出现。这些参数以以下平台或语言名称之一开头： + +- `iOS` +- `iOSApplicationExtension` +- `macOS` +- `macOSApplicationExtension` +- `macCatalyst` +- `macCatalystApplicationExtension` +- `watchOS` +- `watchOSApplicationExtension` +- `tvOS` +- `tvOSApplicationExtension` +- `visionOS` +- `visionOSApplicationExtension` +- `swift` + + + + + +你还可以使用星号 (`*`) 来表示在上述所有列出的平台上声明的可用性。使用 Swift 版本号指定可用性的 `available` 特性不能再使用星号。 + +其余参数可以以任何顺序出现，并指定有关声明生命周期的附加信息，包括重要的里程碑。 + +- `unavailable` 参数表示该声明在指定平台上不可用。指定 Swift 版本可用性时无法使用此参数。 +- `introduced` 参数表示声明引入到指定平台或语言的第一个版本。它具有以下形式： + + ```swift + introduced: <#version number#> + ``` + *version number* 由一个到三个正整数组成，数字之间用句点分隔。 +- `deprecated` 参数表示声明在指定平台或语言上被弃用的的第一个版本。它具有以下形式： + + ```swift + deprecated: <#version number#> + ``` + 可选的 *version number* 由一个到三个正整数组成，数字之间用句点分隔。省略版本号表示该声明当前已被弃用，但没有提供有关何时被弃用的任何信息。如果省略版本号，也要省略冒号 (`:`)。 +- `obsoleted` 参数表示指定平台或语言中声明被废除的第一个版本。当声明被废除时，它会从指定的平台或语言中移除，并且不能再使用。它的形式如下： + + ```swift + obsoleted: <#version number#> + ``` + *version number* 由一个到三个正整数组成，数字之间用句点分隔。 +- `message` 参数提供了一个文本消息，当编译器发出关于使用已弃用或已废除声明的警告或错误时，会显示该消息。它具有以下形式： + + ```swift + message: <#message#> + ``` + *message* 由一个字符串字面量组成。 +- `renamed` 参数提供了一个文本消息，表示声明被重命名到的新名称。当发出有关使用重命名声明的错误时，编译器会显示新名称。它具有以下形式： + + ```swift + renamed: <#new name#> + ``` + *new name* 由一个字符串字面量组成。 + + 你可以将带有 `renamed` 和 `unavailable` 参数的 `available` 特性应用于类型别名声明，如下所示，以指示声明的名称在框架或库的不同版本之间发生了变化。此组合会导致编译时错误，表明声明已被重命名。 + + ```swift + // 首个版本 + protocol MyProtocol { + // 协议定义 + } + ``` + + + + + ```swift + // 后续版本将 MyProtocol 重命名为 MyRenamedProtocol + protocol MyRenamedProtocol { + // 协议定义 + } + + @available(*, unavailable, renamed: "MyRenamedProtocol") + typealias MyProtocol = MyRenamedProtocol + ``` + + + + +你可以在单个声明上应用多个 `available` 特性，以指定该声明在不同平台和不同版本的 Swift 上的可用性。如果 `available` 特性指定的平台或语言版本与当前目标不匹配，则该特性被应用到的声明将被忽略。如果你使用多个 `available` 特性，则有效的可用性是平台和 Swift 可用性的组合。 + + + +如果除了平台或语言名称参数之外，`available` 特性仅指定一个 `introduced` 参数，则可以使用以下简写语法： + +```swift +@available(<#platform name#> <#version number#>, *) +@available(swift <#version number#>) +``` + +`available` 特性的简写语法简洁地表达了多个平台的可用性。尽管这两种形式在功能上是等效的，但在可能的情况下，优先使用简写形式。 + +```swift +@available(iOS 10.0, macOS 10.12, *) +class MyClass { + // class definition +} +``` + + + +使用 Swift 版本号指定可用性的 `available` 特性不能另外指定声明的平台可用性。相反，使用单独的 `available` 特性来指定 Swift 版本的可用性和在一个或多个平台上的可用性。 + +```swift +@available(swift 3.0.2) +@available(macOS 10.12, *) +struct MyStruct { + // struct definition +} +``` + + + +### backDeployed + +将此特性应用于函数、方法、下标操作或计算属性，以便在调用或访问该符号的程序中包含符号实现的副本。你可以使用此特性来标注作为平台一部分发布的符号，例如某操作系统中所包含的 API。此特性标注可以通过在访问它们的程序中包含其实现的副本来可追溯地使用这些符号。复制实现也称为*发送到客户端*。 + +此特性接受一个 `before:` 参数，指定提供此符号的平台的第一个版本。这些平台版本与你为 `available` 特性的指定的平台版本具有相同的含义。与 `available` 特性不同，列表中不能包含星号 (`*`) 来指代所有版本。例如，考虑以下代码： + +```swift +@available(iOS 16, *) +@backDeployed(before: iOS 17) +func someFunction() { /* ... */ } +``` + +在上面的例子中，iOS SDK 从 iOS 17 开始提供 `someFunction()`。此外，SDK 通过向后兼容在 iOS 16 上提供 `someFunction()`。 + +在编译调用此函数的代码时，Swift 插入了一层间接调用，以找到该函数的实现。如果代码已包含此函数的 SDK 版本运行，则使用 SDK 的实现。否则，将使用调用者中包含的副本。在上面的示例中，当在 iOS 17 或更高版本上运行时，调用 `someFunction()` 使用 SDK 的实现，而在 iOS 16 上运行时，则使用调用者中包含的 `someFunction()` 的副本。 + +> 注意: +> 当调用者的最低部署目标与包含该符号的 SDK 的第一个版本相同或更高时，编译器可以优化掉运行时检查，直接调用 SDK 的实现。在这种情况下，如果你直接访问向后兼容的符号，编译器也可以省略客户端中符号实现的副本。 + + + +满足以下标准的函数、方法、下标操作和计算属性可以进行向后兼容： + +- 声明是 `public` 或 `@usableFromInline`。 +- 对于实例方法和类方法，该方法被标记为 `final`，并且没有标记 `@objc`。 +- 该实现满足 中描述的对内联函数的要求。 + +### discardableResult + +将此特性应用于函数或方法声明，以在调用返回值的函数或方法而不使用其结果时抑制编译器警告。 + +### dynamicCallable + +将此特性应用于类、结构体、枚举或协议，以将该类型的实例视为可调用函数。该类型必须实现 `dynamicallyCall(withArguments:)` 方法和 `dynamicallyCall(withKeywordArguments:)` 方法中的至少一个或都实现。 + +你可以像调用有任意数量参数的函数一样调用动态可调用类型的实例。 + +```swift +@dynamicCallable +struct TelephoneExchange { + func dynamicallyCall(withArguments phoneNumber: [Int]) { + if phoneNumber == [4, 1, 1] { + print("Get Swift help on forums.swift.org") + } else { + print("Unrecognized number") + } + } +} + +let dial = TelephoneExchange() + +// 使用动态方法调用 +dial(4, 1, 1) +// 打印 "Get Swift help on forums.swift.org" + +dial(8, 6, 7, 5, 3, 0, 9) +// 打印 "Unrecognized number" + +// 直接调用内部的方法 +dial.dynamicallyCall(withArguments: [4, 1, 1]) +``` + + + +`dynamicallyCall(withArguments:)` 方法的声明必须有一个遵循 [`ExpressibleByArrayLiteral`](https://developer.apple.com/documentation/swift/expressiblebyarrayliteral) 协议的单一参数——就像上面的例子中的 `[Int]`。返回类型可以是任何类型。 + +如果你实现了 `dynamicallyCall(withKeywordArguments:)` 方法，则可以在动态方法调用中包含标签。 + +```swift +@dynamicCallable +struct Repeater { + func dynamicallyCall(withKeywordArguments pairs: KeyValuePairs) -> String { + return pairs + .map { label, count in + repeatElement(label, count: count).joined(separator: " ") + } + .joined(separator: "\n") + } +} + +let repeatLabels = Repeater() +print(repeatLabels(a: 1, b: 2, c: 3, b: 2, a: 1)) +// a +// b b +// c c c +// b b +// a +``` + + + +`dynamicallyCall(withKeywordArguments:)` 方法的声明必须有一个遵循 [`ExpressibleByDictionaryLiteral`](https://developer.apple.com/documentation/swift/expressiblebydictionaryliteral) 协议的单一参数，返回类型可以是任何类型。参数的 [`Key`](https://developer.apple.com/documentation/swift/expressiblebydictionaryliteral/2294108-key) 必须遵循 [`ExpressibleByStringLiteral`](https://developer.apple.com/documentation/swift/expressiblebystringliteral)。前面的例子使用 [`KeyValuePairs`](https://developer.apple.com/documentation/swift/keyvaluepairs) 作为参数类型，以便调用者可以包含重复的参数标签——`a` 和 `b` 在对 `repeat` 的调用中出现多次。 + +如果你实现了两种不同参数类型的 `dynamicallyCall` 方法，当方法调用包含关键字参数时，将调用 `dynamicallyCall(withKeywordArguments:)`。在所有其他情况下，将调用 `dynamicallyCall(withArguments:)`。 + +你只能使用与你在某个 `dynamicallyCall` 方法实现中指定的类型匹配的参数和返回值来调用动态可调用实例。以下示例中的调用无法编译，因为没有接受 `KeyValuePairs` 的 `dynamicallyCall(withArguments:)` 的实现。 + +```swift +repeatLabels(a: "four") // 错误 +``` + + + +### dynamicMemberLookup + +将此特性应用于类、结构体、枚举或协议，以便在运行时按名称查找成员。该类型必须实现一个 `subscript(dynamicMember:)` 下标操作。 + +在显式成员表达式中，如果没有对应的命名成员声明，则该表达式被理解为对类型的 `subscript(dynamicMember:)` 下标操作的调用，并将有关成员的信息作为参数传递。下标操作可以接受一个参数，该参数可以是键路径或成员名称；如果你实现了两种下标操作，则使用接受键路径参数的下标操作。 + +`subscript(dynamicMember:)` 的实现可以接受 +[`KeyPath`](https://developer.apple.com/documentation/swift/keypath)、 +[`WritableKeyPath`](https://developer.apple.com/documentation/swift/writablekeypath) 或 [`ReferenceWritableKeyPath`](https://developer.apple.com/documentation/swift/referencewritablekeypath) 来作为键路径参数。它可以通过一个类型遵循 [`ExpressibleByStringLiteral`](https://developer.apple.com/documentation/swift/expressiblebystringliteral) 协议的参数来接受成员名称——在大多数情况下，这个类型是 `String`。下标操作的返回类型可以是任何类型。 + +通过成员名称的动态成员查找，可以创建一个包装类型，用于处理在编译时无法进行类型检查的数据，例如在将其他语言的数据桥接到 Swift 时。比如： + +```swift +@dynamicMemberLookup +struct DynamicStruct { + let dictionary = ["someDynamicMember": 325, + "someOtherMember": 787] + subscript(dynamicMember member: String) -> Int { + return dictionary[member] ?? 1054 + } +} +let s = DynamicStruct() + +// 使用动态成员查询 +let dynamic = s.someDynamicMember +print(dynamic) +// 打印 "325" + +// 直接调用底层下标 +let equivalent = s[dynamicMember: "someDynamicMember"] +print(dynamic == equivalent) +// 打印 "true" + +``` + + + +按键路径进行动态成员查找能以支持编译时类型检查的方式实现包装类型。例如： + +```swift +struct Point { var x, y: Int } + +@dynamicMemberLookup +struct PassthroughWrapper { + var value: Value + subscript(dynamicMember member: KeyPath) -> T { + get { return value[keyPath: member] } + } +} + +let point = Point(x: 381, y: 431) +let wrapper = PassthroughWrapper(value: point) +print(wrapper.x) +``` + + + +### freestanding + +将 `freestanding` 特性应用于独立宏的声明。 + + + +### frozen + +将此特性应用于结构体或枚举的声明，以限制可以对该类型所能进行的更改类型。此特性仅在以库演进模式编译时允许使用。库的未来版本不能通过添加、删除或重新排序枚举的成员或结构体的存储实例属性来更改其声明。这些更改在非冻结类型上是允许的，但会破坏冻结类型的 ABI 兼容性。 + +> 注意: +> 当编译器不处于库演进模式时，所有结构体和枚举都被隐式冻结，此特性将被忽略。 + + + + + + + +在库演进模式下，与非冻结结构体和枚举成员交互的代码会以一种特殊的方式编译，即使库的未来版本添加、移除或重新排序了该类型的一些成员，该代码也可以继续工作，而无需重新编译。编译器通过在运行时查找信息和添加间接层等技术实现了这一点。将结构体或枚举标记为冻结（frozen）会放弃这种灵活性以获得性能提升：库的未来版本只能对该类型进行有限的更改，但编译器可以在与该类型成员交互的代码中进行额外的优化。 + +冻结类型、冻结结构体的存储属性类型以及冻结枚举成员的关联值类型必须是 public，或标记为 `usableFromInline` 特性。冻结结构体的属性不能有属性观察者，并且为存储实例属性提供初始值的表达式必须遵循与可内联函数相同的限制，详见 。 + + + +要在命令行上启用库演化模式，请将 `-enable-library-evolution` 选项传递给 Swift 编译器。要在 Xcode 中启用它，请将 "Build Libraries for Distribution" 构建设置 (`BUILD_LIBRARY_FOR_DISTRIBUTION`) 设置 Yes，见：[Xcode Help](https://help.apple.com/xcode/mac/current/#/dev04b3a04ba)。 + + + +对一个冻结的枚举进行 switch 语句时，不需要包含 `default` 分支，如 中所述。在对冻结枚举进行 switch 时，如果包含 `default` 或 `@unknown default` 分支，会产生一个警告，因为这些代码永远不会被执行。 + + + + + + + +### GKInspectable + +应用此特性可将自定义 GameplayKit 组件属性公开给 SpriteKit 编辑器 UI。应用此特性还隐含着 `objc` 特性。 + + + +### inlinable + +将此特性应用于函数、方法、计算属性、下标操作、便利构造器或析构器声明，以将该声明的实现公开为模块的公共接口的一部分。编译器可以允许在调用位置用符号实现的副本替换对可内联符号的调用。 + +可内联代码可以与任何模块中声明的 `open` 和 `public` 符号进行交互，并且可以与同一模块中标记为 `usableFromInline` 特性的 `internal` 符号进行交互。可内联代码无法与 `private` 或 `fileprivate` 符号进行交互。 + +此特性不能应用于嵌套在函数内部的声明，也不能应用于 `fileprivate` 或 `private` 声明。定义在可内联函数内部的函数和闭包是隐式可内联的，即使它们不能用此特性标记。 + + + + + + + +### main + +将此特性应用于结构体、类或枚举声明，以表示它包含程序流程的顶级入口点。该类型必须提供一个不接受任何参数并返回 `Void` 的 `main` 类型函数。例如： + +```swift +@main +struct MyTopLevel { + static func main() { + // 顶层代码在此处编写 + } +} + +``` + + + +描述 `main` 特性要求的另一种方法是，特性作用于的类型必须满足与遵循以下假设协议的类型相同的要求： + +```swift +protocol ProvidesMain { + static func main() throws +} +``` + + + +编译为可执行文件的 Swift 代码最多只能包含一个顶级入口点，详见 。 + + + + + + + +### nonobjc + +将此特性应用于方法、属性、下标操作或构造器声明，以抑制隐式 `objc` 特性。`nonobjc` 特性告诉编译器此声明在 Objective-C 代码中不可用，尽管在 Objective-C 中可以表示它。 + +将此特性应用于扩展与将其应用于该扩展中未显式标记为 `objc` 特性的每个成员具有相同的效果。 + +你可以使用 `nonobjc` 特性来解决标记为 `objc` 的类中桥接方法的循环引用问题，并且将允许重载标记为 `objc` 的类中的方法和构造器。 + +标记为 `nonobjc` 特性的方法不能被标记为 `objc` 特性的方法重写。然而，标记为 `objc` 特性的方法可以被标记为 `nonobjc` 特性的方法重写。同样，标记为 `nonobjc` 特性的方法不能满足标记为 `objc` 特性的方法的协议要求。 + +### NSApplicationMain + +> 已弃用: +> 此特性已弃用；请改用 特性。在 Swift 6 中，使用此特性将会导致错误。 + +将此特性应用于一个类，以指示它是应用程序委托。使用此特性等同于调用 `NSApplicationMain(_:_:)` 函数。 + +如果你不使用此特性，请提供一个在顶层调用 `NSApplicationMain(_:_:)` 函数的 `main.swift` 文件，如下所示： + +```swift +import AppKit +NSApplicationMain(CommandLine.argc, CommandLine.unsafeArgv) +``` + + + +你编译为可执行文件的 Swift 代码最多只能包含一个顶级入口点，见 中的讨论。 + +### NSCopying + +将此特性应用于类的存储变量属性。此特性会导致属性的 setter 被合成为使用 `copyWithZone(_:)` 方法返回的属性值*副本*，而不是属性值本身。属性的类型必须遵循 `NSCopying` 协议。 + +`NSCopying` 特性的行为类似于 Objective-C 的 `copy` 属性特性。 + + + +### NSManaged + +将此特性应用于从 `NSManagedObject` 继承的类的实例方法或存储属性变量，以表明 Core Data 会在运行时根据关联的实体描述动态提供其实现。对于标有 `NSManaged` 特性的属性，Core Data 还会在运行时提供存储。应用此特性也隐含了 `objc` 特性。 + +### objc + +将此特性应用于任何可以用 Objective-C 表示的声明——例如，非嵌套类、协议、非泛型枚举（限制为整数原始值类型）、类的属性和方法（包括 getter 和 setter）、协议的可选成员、构造器和下标操作。`objc` 特性告诉编译器该声明可以在 Objective-C 代码中使用。 + +将此特性应用于扩展与将其应用于该扩展中未显式标记为 `nonobjc` 特性的每个成员具有相同的效果。 + +编译器隐式地将 `objc` 特性添加到 Objective-C 中定义的任何类的子类中。然而，子类不能是泛型的，也不能继承任何泛型类。如下所述，你可以显式地将 `objc` 特性添加到满足这些标准的子类中，以指定其 Objective-C 名称。标记为 `objc` 特性的协议不能继承未标记此特性的协议。 + +`objc` 特性在以下情况下也会被隐式添加： + +- 该声明是在子类中的重写，超类的声明具有 `objc` 特性。 +- 该声明满足具有 `objc` 特性的协议的要求。 +- 声明包含 `IBAction`、`IBSegueAction`、`IBOutlet`、`IBDesignable`、`IBInspectable`、`NSManaged` 或 `GKInspectable` 特性。 + +如果你将 `objc` 特性应用于枚举，每个枚举成员都会在 Objective-C 代码中以枚举名称和枚举成员名称的组合形式暴露出来，且枚举成员名称的首字母会大写。例如，在 Swift 中名为 `Planet` 的枚举中的一个名为 `venus` 的枚举成员在 Objective-C 代码中将会以名为 `PlanetVenus` 的成员暴露出来。 + +`objc` 特性可以选择接受一个特性参数，该参数由一个标识符组成。该标识符指定了向 Objective-C 暴露的被 `objc` 特性所应用到的实体的名称。你可以使用此参数为类、枚举、枚举成员、协议、方法、getter、setter 和构造器命名。如果你为类、协议或枚举指定 Objective-C 名称，请在名称前加上三个字母的前缀，如 [Conventions](https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/Conventions/Conventions.html#//apple_ref/doc/uid/TP40011210-CH10-SW1) 和 [Programming with Objective-C](https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/Introduction/Introduction.html#//apple_ref/doc/uid/TP40011210) 中所述。下面的示例将 `ExampleClass` 的 `enabled` 属性的 getter 暴露给 Objective-C 代码，名称为 `isEnabled`，而不仅仅是属性本身的名称。 + +```swift +class ExampleClass: NSObject { + @objc var enabled: Bool { + @objc(isEnabled) get { + // 返回适当的值 + } + } +} +``` + + + +更多信息，请参见 [Importing Swift into Objective-C](https://developer.apple.com/documentation/swift/imported_c_and_objective-c_apis/importing_swift_into_objective-c)。 + +> 注意: +> `objc` 特性的参数也可以更改该声明的运行时名称。当调用与 Objective-C 运行时交互的函数（如 [`NSClassFromString(_:)`](https://developer.apple.com/documentation/foundation/1395135-nsclassfromstring)）时，以及在应用的 Info.plist 文件中指定类名时，使用运行时名称。如果通过传递参数指定名称，则该名称将用作 Objective-C 代码中的名称和运行时名称。如果省略参数，则 Objective-C 代码中使用的名称与 Swift 代码中的名称相匹配，并且运行时名称遵循 Swift 编译器的名称重整惯例。 + +### objcMembers + +将此特性应用于类声明，以隐式地将 `objc` 特性应用于该类所有与 Objective-C 兼容的成员、其扩展、其子类及其子类的所有扩展。 + +大多数代码应使用 `objc` 特性，仅暴露需要的声明。如果你需要暴露多个声明，可以将它们分组到一个带有 `objc` 特性的扩展中。`objcMembers` 特性是一个方便的工具，适用于大量使用 Objective-C 运行时的自省功能的库。在不需要时应用 `objc` 特性可能会增加你的二进制文件大小并对性能产生不利影响。 + + + +### preconcurrency + +将此特性应用于声明，以抑制严格的并发检查。你可以将此特性应用于以下类型的声明： + +- 导入 +- 结构体, 类和 actor +- 枚举和枚举成员 +- 协议 +- 变量和常量 +- 下标操作 +- 构造器 +- 函数 + +在导入声明中，此特性降低了对使用导入模块类型的代码的并发检查的严格性。具体来说，导入模块中未显式标记为不可发送（nonsendable）的类型可以在需要可发送（sendable）类型的上下文中使用。 + +在其他声明中，此特性降低了对使用所声明符号的代码进行并发检查的严格性。当你在具有最小并发检查的范围内使用此符号时，该符号指定的与并发相关的约束，例如 `Sendable` 要求或全局 actor，将不会被检查。 + +你可以按如下方式使用此特性，以帮助将代码迁移到严格的并发检查： + +1. 启用严格检查。 +1. 对尚未启用严格检查的模块，用 `preconcurrency` 特性标注导入。 +1. 在将模块迁移到严格检查后，移除 `preconcurrency` 特性。编译器会警告你关于导入中 `preconcurrency` 特性不再有效并应被移除的任何地方。 + +对于其他声明，当你在声明中添加与并发相关的约束时，如果你仍有未迁移到严格检查的客户端，请添加 `preconcurrency` 特性。在所有客户端迁移后，删除 `preconcurrency` 特性。 + +来自 Objective-C 的声明在导入时始终被视为带有 `preconcurrency` 特性标记。 + +### propertyWrapper + +将此特性应用于类、结构体或枚举声明，以将该类型用作属性包装器。当你将此特性应用于某个类型时，你会创建一个与该类型同名的自定义特性。将该新特性应用于类、结构体或枚举的属性，以通过包装器类型的实例来包装对该属性的访问；将特性应用于局部存储变量声明，以相同方式包装对变量的访问。计算变量、全局变量和常量不能使用属性包装器。 + + + + + + + +包装器必须定义一个 `wrappedValue` 实例属性。该属性的*被包装值*是 getter 和 setter 所暴露的值。在大多数情况下，`wrappedValue` 是一个计算值，但它也可以是一个存储值。包装器定义并管理其被包装值所需的任何底层存储。编译器通过在包装属性的名称前加下划线 (`_`) 来合成包装器类型实例的存储——例如，`someProperty` 的包装器存储为 `_someProperty`。包装器的合成存储具有 `private` 的访问控制级别。 + +一个具有属性包装器的属性可以包含 `willSet` 和 `didSet` 块，但它不能覆盖编译器合成的 `get` 或 `set` 块。 + +Swift 提供了两种语法糖形式用于初始化属性包装器。你可以在被包装值的定义中使用赋值语法，将赋值右侧的表达式作为参数传递给属性包装器构造器的 `wrappedValue` 参数。你还可以在将特性应用于属性时提供参数，这些参数会传递给属性包装器的构造器。例如，在下面的代码中，`SomeStruct` 调用 `SomeWrapper` 定义的每个构造器。 + +```swift +@propertyWrapper +struct SomeWrapper { + var wrappedValue: Int + var someValue: Double + init() { + self.wrappedValue = 100 + self.someValue = 12.3 + } + init(wrappedValue: Int) { + self.wrappedValue = wrappedValue + self.someValue = 45.6 + } + init(wrappedValue value: Int, custom: Double) { + self.wrappedValue = value + self.someValue = custom + } +} + +struct SomeStruct { + // 使用 init() + @SomeWrapper var a: Int + + // 使用 init(wrappedValue:) + @SomeWrapper var b = 10 + + // 二者都使用 init(wrappedValue:custom:) + @SomeWrapper(custom: 98.7) var c = 30 + @SomeWrapper(wrappedValue: 30, custom: 98.7) var d +} +``` + + + + + + + +*被呈现值*对于一个被包装属性而言是可以让属性包装器用来暴露额外功能的第二个值。属性包装器类型的作者负责确定其被呈现值的含义，并定义被呈现值所暴露的接口。要从属性包装器中投射一个值，请在包装器类型上定义一个 `projectedValue` 实例属性。编译器通过在包装属性的名称前加上美元符号（`$`）来合成被呈现值的标识符——例如，`someProperty` 的被呈现值是 `$someProperty`。被呈现值具有与原始包装属性相同的访问控制级别。 + +```swift +@propertyWrapper +struct WrapperWithProjection { + var wrappedValue: Int + var projectedValue: SomeProjection { + return SomeProjection(wrapper: self) + } +} +struct SomeProjection { + var wrapper: WrapperWithProjection +} + +struct SomeStruct { + @WrapperWithProjection var x = 123 +} +let s = SomeStruct() +s.x // Int 类型的值 +s.$x // SomeProjection 类型的值 +s.$x.wrapper // WrapperWithProjection 类型的值 +``` + + + +### resultBuilder + +将此特性应用于类、结构体或枚举，以将该类型用作结果构造器。*结果构造器*是一种逐步构建嵌套数据结构的类型。你可以使用结果构造器来实现一种用于以自然、声明式方式创建嵌套数据结构的领域特定语言（DSL）。有关如何使用 `resultBuilder` 特性的示例，见 。 + +#### 结果构造方法 + +结果构造器实现了以下描述的静态方法。由于结果构造器的所有功能都是通过静态方法暴露的，因此你永远不需要初始化该类型的实例。结果构造器必须实现 `buildBlock(_:)` 方法，或者同时实现 `buildPartialBlock(first:)` 和 `buildPartialBlock(accumulated:next:)` 方法。其他方法（这些方法在 DSL 中启用额外功能）是可选的。结果构造器类型的声明实际上不必包含对任何协议的遵循。 + +静态方法的描述中使用了三种类型作为占位符。`Expression` 是构造器输入类型的占位符，`Component` 是部分结果类型的占位符，而 `FinalResult` 是构造器生成的结果类型的占位符。你需要用结果构造器实际使用的类型替换这些占位符。如果你的结果构建方法没有为 `Expression` 或 `FinalResult` 指定类型，那么它们默认与 `Component` 类型相同。 + +块的构造的方法如下： + +- `static func buildBlock(_ components: Component...) -> Component`：将一组部分结果组合成一个单一的部分结果。 + +- `static func buildPartialBlock(first: Component) -> Component`：从第一个组件构建一个部分结果组件。实现此方法和 `buildPartialBlock(accumulated:next:)` 以支持一次构建一个组件的块。与 `buildBlock(_:)` 相比，这种方法减少了处理不同数量参数的泛型重载的需求。 + +- `static func buildPartialBlock(accumulated: Component, next: Component) -> Component`：通过将一个累积的组件与一个新组件结合，构建一个部分结果组件。实现此方法和 `buildPartialBlock(first:)` 以支持一次构建一个组件的块。与 `buildBlock(_:)` 相比，这种方法减少了处理不同数量参数的泛型重载的需求。 + +结果构造器可以实现上述列出的所有三种块构建方法；在这种情况下，可用性决定调用哪个方法。默认情况下，Swift 会调用 `buildPartialBlock(first:)` 和 `buildPartialBlock(accumulated:next:)` 方法。要让 Swift 调用 `buildBlock(_:)` 方法，请将包围声明标记为早于 `buildPartialBlock(first:)` 和 `buildPartialBlock(accumulated:next:)` 方法的可用性。 + +其他结果构造方法如下： + +- `static func buildOptional(_ component: Component?) -> Component`：从可以为 `nil` 的部分结果构建一个部分结果。实现此方法以支持不包含 `else` 从句的 `if` 语句。 + +- `static func buildEither(first: Component) -> Component`：构建一个部分结果，其值根据某些条件而变化。实现此方法和 `buildEither(second:)` 以支持 `switch` 语句和包含 `else` 从句的 `if` 语句。 + +- `static func buildEither(second: Component) -> Component`：构建一个部分结果，其值根据某些条件而变化。实现此方法和 `buildEither(first:)` 以支持 `switch` 语句和包含 `else` 从句的 `if` 语句。 + +- `static func buildArray(_ components: [Component]) -> Component`：从部分结果的数组构造一个部分结果。实现此方法以支持 `for` 循环。 + +- `static func buildExpression(_ expression: Expression) -> Component`：从表达式构建部分结果。你可以实现此方法以执行预处理——例如，将表达式转换为内部类型——或为使用端的类型推断提供额外的信息。 + +- `static func buildFinalResult(_ component: Component) -> FinalResult`：从部分结果构建最终结果。你可以将此方法实现为结果构造器的一部分，该结果构造器对部分结果和最终结果使用不同的类型，或者在返回结果之前对结果执行其他后处理。 + +- `static func buildLimitedAvailability(_ component: Component) -> Component`：构建一个擦除类型信息的部分结果。你可以实现此方法以防止类型信息传播到执行可用性检查的编译器控制语句之外。 + +例如，下面的代码定义了一个简单的结果构造器，用于构建一个整数数组。此代码将 `Component` 和 `Expression` 定义为类型别名，以便更容易将下面的示例与上面的方法列表进行匹配。 + +```swift +@resultBuilder +struct ArrayBuilder { + typealias Component = [Int] + typealias Expression = Int + static func buildExpression(_ element: Expression) -> Component { + return [element] + } + static func buildOptional(_ component: Component?) -> Component { + guard let component = component else { return [] } + return component + } + static func buildEither(first component: Component) -> Component { + return component + } + static func buildEither(second component: Component) -> Component { + return component + } + static func buildArray(_ components: [Component]) -> Component { + return Array(components.joined()) + } + static func buildBlock(_ components: Component...) -> Component { + return Array(components.joined()) + } +} +``` + + + +#### 结果转换 + +递归应用以下语法转换，将使用结果构造器语法的代码转换为调用结果构造器类型的静态方法的代码： + +- 如果结果构造器有一个 `buildExpression(_:)` 方法，则每个表达式都变成对该方法的调用。这个转换总是首先进行。例如，以下声明是等价的： + + ```swift + @ArrayBuilder var builderNumber: [Int] { 10 } + var manualNumber = ArrayBuilder.buildExpression(10) + ``` + + + +- 赋值语句的转换方式类似于表达式，但被理解为计算 `()`。你可以定义一个 `buildExpression(_:)` 的重载，它采用 `()` 类型的参数来专门处理赋值。 +- 检查可用性条件的分支语句将成为对 `buildLimitedAvailability(_:)` 方法的调用（如果该方法已实现）。如果未实现 `buildLimitedAvailability(_:)` 方法，那么检查可用性的分支语句将使用与其他分支语句相同的转换。这种转换发生在转换为对 `buildEither(first:)`、`buildEither(second:)` 或 `buildOptional(_:)` 的调用之前。 + +你可以使用 `buildLimitedAvailability(_:)` 方法来擦除根据所采用的分支而变化的类型信息。例如，下面的 `buildEither(first:)` 和 `buildEither(second:)` 方法使用一个泛型类型，该类型捕获有关两个分支的类型信息。 + + ```swift + protocol Drawable { + func draw() -> String + } + struct Text: Drawable { + var content: String + init(_ content: String) { self.content = content } + func draw() -> String { return content } + } + struct Line: Drawable { + var elements: [D] + func draw() -> String { + return elements.map { $0.draw() }.joined(separator: "") + } + } + struct DrawEither: Drawable { + var content: Drawable + func draw() -> String { return content.draw() } + } + + @resultBuilder + struct DrawingBuilder { + static func buildBlock(_ components: D...) -> Line { + return Line(elements: components) + } + static func buildEither(first: First) + -> DrawEither { + return DrawEither(content: first) + } + static func buildEither(second: Second) + -> DrawEither { + return DrawEither(content: second) + } + } + ``` + + + + 然而，这种方法在具有可用性检查的代码中会导致问题： + + ```swift + @available(macOS 99, *) + struct FutureText: Drawable { + var content: String + init(_ content: String) { self.content = content } + func draw() -> String { return content } + } + + @DrawingBuilder var brokenDrawing: Drawable { + if #available(macOS 99, *) { + FutureText("Inside.future") // 问题所在 + } else { + Text("Inside.present") + } + } + // brokenDrawing 的类型是 Line, Line>> + ``` + + + + 在上面的代码中，`FutureText` 作为 `brokenDrawing` 类型的一部分出现，因为它是 `DrawEither` 泛型类型中的类型之一。如果在运行时 `FutureText` 不可用， 即使在该类型显式未被使用的情况下，这可能会导致你的程序崩溃。 + + 为了解决这个问题，实现一个 `buildLimitedAvailability(_:)` 方法，通过返回一个始终可用的类型来擦除类型信息。例如，下面的代码通过可用性检查构建一个 `AnyDrawable` 值。 + + ```swift + struct AnyDrawable: Drawable { + var content: Drawable + func draw() -> String { return content.draw() } + } + extension DrawingBuilder { + static func buildLimitedAvailability(_ content: some Drawable) -> AnyDrawable { + return AnyDrawable(content: content) + } + } + + @DrawingBuilder var typeErasedDrawing: Drawable { + if #available(macOS 99, *) { + FutureText("Inside.future") + } else { + Text("Inside.present") + } + } + // typeErasedDrawing 的类型是 Line>> + ``` + + + +- 一个分支语句变成了一系列对 `buildEither(first:)` 和 `buildEither(second:)` 方法的嵌套调用。语句的条件和分支情况被映射到二叉树的叶子节点上，该语句成为了对 `buildEither` 方法的嵌套调用，遵循从根节点到该叶节点的路径。 + + 例如，如果你编写一个包含三个分支的 switch 语句，编译器将使用一个具有三个叶节点的二叉树。同样，因为从根节点到第二个 case 分支的路径是“第二个子节点”，然后是“第一个子节点”，所以该 case 分支变成了像 `buildEither(first: buildEither(second: ... ))` 这样的嵌套调用。以下声明是等效的： + + ```swift + let someNumber = 19 + @ArrayBuilder var builderConditional: [Int] { + if someNumber < 12 { + 31 + } else if someNumber == 19 { + 32 + } else { + 33 + } + } + + var manualConditional: [Int] + if someNumber < 12 { + let partialResult = ArrayBuilder.buildExpression(31) + let outerPartialResult = ArrayBuilder.buildEither(first: partialResult) + manualConditional = ArrayBuilder.buildEither(first: outerPartialResult) + } else if someNumber == 19 { + let partialResult = ArrayBuilder.buildExpression(32) + let outerPartialResult = ArrayBuilder.buildEither(second: partialResult) + manualConditional = ArrayBuilder.buildEither(first: outerPartialResult) + } else { + let partialResult = ArrayBuilder.buildExpression(33) + manualConditional = ArrayBuilder.buildEither(second: partialResult) + } + ``` + + + +- 一个可能不产生值的分支语句，比如没有 `else` 从句的 `if` 语句，会变成对 `buildOptional(_:)` 的调用。如果 `if` 语句的条件满足，它的代码块会被转换并作为参数传递；否则，`buildOptional(_:)` 会以 `nil` 作为参数被调用。例如，以下声明是等价的： + + ```swift + @ArrayBuilder var builderOptional: [Int] { + if (someNumber % 2) == 1 { 20 } + } + + var partialResult: [Int]? = nil + if (someNumber % 2) == 1 { + partialResult = ArrayBuilder.buildExpression(20) + } + var manualOptional = ArrayBuilder.buildOptional(partialResult) + ``` + + + +- 如果结果构造器实现了 `buildPartialBlock(first:)` 和 `buildPartialBlock(accumulated:next:)` 方法，则代码块或 `do` 语句将变成对这些方法的调用。块内的第一条语句被转换为 `buildPartialBlock(first:)` 方法的一个参数，其余语句则变成对 `buildPartialBlock(accumulated:next:)` 方法的嵌套调用。例如，以下声明是等效的： + + ```swift + struct DrawBoth: Drawable { + var first: First + var second: Second + func draw() -> String { return first.draw() + second.draw() } + } + + @resultBuilder + struct DrawingPartialBlockBuilder { + static func buildPartialBlock(first: D) -> D { + return first + } + static func buildPartialBlock( + accumulated: Accumulated, next: Next + ) -> DrawBoth { + return DrawBoth(first: accumulated, second: next) + } + } + + @DrawingPartialBlockBuilder var builderBlock: some Drawable { + Text("First") + Line(elements: [Text("Second"), Text("Third")]) + Text("Last") + } + + let partialResult1 = DrawingPartialBlockBuilder.buildPartialBlock(first: Text("first")) + let partialResult2 = DrawingPartialBlockBuilder.buildPartialBlock( + accumulated: partialResult1, + next: Line(elements: [Text("Second"), Text("Third")]) + ) + let manualResult = DrawingPartialBlockBuilder.buildPartialBlock( + accumulated: partialResult2, + next: Text("Last") + ) + ``` + + +- 否则，代码块或 `do` 语句会变成对 `buildBlock(_:)` 方法的调用。块内的每个语句都会逐个转换，并成为 `buildBlock(_:)` 方法的参数。例如，以下声明是等价的。 + + ```swift + @ArrayBuilder var builderBlock: [Int] { + 100 + 200 + 300 + } + + var manualBlock = ArrayBuilder.buildBlock( + ArrayBuilder.buildExpression(100), + ArrayBuilder.buildExpression(200), + ArrayBuilder.buildExpression(300) + ) + ``` + + + +- 一个 `for` 循环会变成一个临时变量、一个新的 `for` 循环和对 `buildArray(_:)` 方法的调用。新的 `for` 循环遍历序列，并将每个部分结果附加到该数组中。临时数组作为参数传递给 `buildArray(_:)` 调用。例如，以下声明是等价的： + + ```swift + @ArrayBuilder var builderArray: [Int] { + for i in 5...7 { + 100 + i + } + } + + var temporary: [[Int]] = [] + for i in 5...7 { + let partialResult = ArrayBuilder.buildExpression(100 + i) + temporary.append(partialResult) + } + let manualArray = ArrayBuilder.buildArray(temporary) + ``` + + + +- 如果结果构造器有一个 `buildFinalResult(_:)` 方法，则最终结果变为对该方法的调用。此转换始终是最后进行的。 + + + + + + + +尽管转换行为是通过临时变量来描述的，但使用结果构造器实际上并不会创建任何在代码其他部分可见的新声明。 + +你不能在结果构造器转换的代码中使用 `break`、`continue`、`defer`、`guard` 或 `return` 语句、`while` 语句或 `do`-`catch` 语句。 + +转换过程不会改变代码中的声明，这使得你可以使用临时常量和变量逐步构建表达式。它也不会改变 `throw` 语句、编译时诊断语句或包含 `return` 语句的闭包。 + +只要有可能，转换就会被合并。例如，表达式 `4 + 5 * 6` 变为 `buildExpression(4 + 5 * 6)`，而不是多次调用该函数。同样，嵌套分支语句成为调用 `buildEither` 方法的单个二叉树。 + + + +#### 自定义结果构造器特性 + +创建结果构造器类型会创建一个同名的自定义特性。你可以在以下位置应用该特性： + +- 在函数声明中，结果构造器构建函数的主体。 +- 在包含 getter 的变量或下标操作声明中，结果构造器构建 getter 的主体。 +- 在函数声明中的一个参数上，结果构造器构建一个作为相应参数传递的闭包的主体。 + +应用结果构造器特性不会影响 ABI 兼容性。将结果构造器特性应用于参数会使该特性成为函数接口的一部分，这可能会影响源代码的兼容性。 + +### requires_stored_property_inits + +将此特性应用于类声明，以要求类中的所有存储属性在其定义中提供默认值。对于任何继承自 `NSManagedObject` 的类，都会推断出此特性。 + + + +### testable + +将此特性应用于 `import` 声明时，可以通过修改访问控制来简化对模块代码的测试。导入的模块中标记为 `internal` 访问级别的实体将被导入并视为 `public` 访问级别声明的实体。标记为 `internal` 或 `public` 访问级别的类和类成员将被导入并视为 `open` 访问级别声明的实体。导入的模块必须在启用了测试的情况下编译。 + +### UIApplicationMain + +> 已弃用: +> 此特性已弃用；请改用 特性。在 Swift 6 中，使用此特性将会导致错误。 + +将此特性应用于一个类，以指示它是应用程序委托。使用此特性相当于调用 `UIApplicationMain` 函数，并将此类的名称作为委托类的名称传递。 + +如果你不使用此特性，请提供一个包含顶层代码的 `main.swift` 文件，该代码调用 [`UIApplicationMain(_:_:_:_:)`](https://developer.apple.com/documentation/uikit/1622933-uiapplicationmain) 函数。例如，如果你的应用使用自定义的 `UIApplication` 子类作为其主类，请调用 `UIApplicationMain(_:_:_:_:)` 函数，而不是使用此特性。 + +编译为可执行文件的 Swift 代码最多只能包含一个顶级入口点，详见 。 + +### unchecked + +将此特性应用于协议类型，作为类型声明中采用的协议列表的一部分，以关闭对该协议要求的强制执行。 + +唯一支持的协议是 [`Sendable`](https://developer.apple.com/documentation/swift/sendable)。 + +### usableFromInline + +将此特性应用于函数、方法、计算属性、下标操作、构造器或析构器声明，以允许该符号在与声明位于同一模块中定义的内联代码中使用。声明必须具有 `internal` 访问级别修饰符。标记为 `usableFromInline` 的结构体或类只能对其属性使用公共类型或 `usableFromInline` 类型。标记为 `usableFromInline` 的枚举只能对其枚举成员的原始值和关联值使用公共类型或 `usableFromInline` 类型。 + +像 `public` 访问级别修饰符一样，这个特性将声明暴露为模块公共接口的一部分。与 `public` 不同，编译器不允许在模块外的代码中按名称引用标记为 `usableFromInline` 的声明，即使声明的符号已被导出。然而，模块外的代码仍然可以通过使用运行时行为与声明的符号进行交互。 + +标记为 `inlinable` 特性的声明可以隐式地从可内联代码中使用。虽然 `inlinable` 或 `usableFromInline` 都可以应用于 `internal` 声明，但同时应用这两个特性是错误的。 + + + +### warn_unqualified_access + +将此特性应用于顶级函数、实例方法、类方法或静态方法，以便在未使用前缀限定符（如模块名称、类型名称、实例变量或实例常量）的情况下使用该函数或方法时触发警告。使用此特性可以帮助减少在同一作用域内访问的具有相同名称的函数之间的歧义。 + +例如，Swift 标准库包括一个顶级函数 [`min(_:_:)`](https://developer.apple.com/documentation/swift/1538339-min/) 和一个用于具有可比较元素的序列的 [`min()`](https://developer.apple.com/documentation/swift/sequence/1641174-min) 方法。序列方法使用 `warn_unqualified_access` 特性声明，以帮助减少在 `Sequence` 扩展中尝试使用其中一个或另一个时的混淆。 + +### Interface Builder 使用的声明特性 + +Interface Builder 特性是声明特性，供 Interface Builder 与 Xcode 同步使用。Swift 提供了以下 Interface Builder 特性：`IBAction`、`IBSegueAction`、`IBOutlet`、`IBDesignable` 和 `IBInspectable`。这些特性在概念上与其对应的 Objective-C 特性相同。 + + + +你将 `IBOutlet` 和 `IBInspectable` 特性应用于类的属性声明。 +你将 `IBAction` 和 `IBSegueAction` 特性应用于类的方法声明， +并将 `IBDesignable` 特性应用于类声明。 + +应用 `IBAction`, `IBSegueAction`, `IBOutlet`, +`IBDesignable` 或 `IBInspectable` 特性也隐含 `objc` 特性。 + +## 类型特性 + +你只能将类型特性应用于类型。 + +### autoclosure + +使用此特性可以通过将表达式自动包装在一个无参数的闭包中来延迟对表达式的求值。你可以在函数或方法声明中将其应用于参数的类型，该参数的类型为不接受参数且返回与表达式类型相同的值的函数类型。关于如何使用 `autoclosure` 特性的示例，请参见 和 。 + +### convention + +将此特性应用于函数的类型，以指示其调用约定。 + +`convention` 特性总是与以下参数之一一起出现： + +- `swift` 参数表示一个 Swift 函数引用。这是 Swift 中函数值的标准调用约定。 +- `block` 参数表示一个与 Objective-C 兼容的块引用。函数值表示为对块对象的引用，该块对象是一个 `id` 类型兼容的 Objective-C 对象，它将其调用函数嵌入到该对象中。调用函数使用 C 调用约定。 +- `c` 参数表示一个 C 函数引用。函数值不携带上下文，并使用 C 调用约定。 + + + +除了少数例外，任何调用约定的函数都可以在需要其他调用约定的函数时使用。一个非泛型的全局函数、一个不捕获任何局部变量的局部函数，或者一个不捕获任何局部变量的闭包可以转换为 C 调用约定。其他 Swift 函数无法转换为 C 调用约定。具有 Objective-C 块调用约定的函数无法转换为 C 调用约定。 + +### escaping + +将此特性应用于函数或方法声明中的参数类型，以指示参数的值可以存储以供后续执行。这隐含着该值可以超出这次调用的生命周期。具有 `escaping` 类型特性的函数类型参数需要对属性或方法显式使用 `self.`。有关如何使用 `escaping` 特性的示例，见 。 + +### Sendable + +将此特性应用于函数的类型，以指示该函数或闭包是可发送的。 + +将此特性应用于函数类型与使非函数类型遵循 [`Sendable`](https://developer.apple.com/documentation/swift/sendable) 协议具有相同的含义。 + +如果函数或闭包在需要可发送值的上下文中使用，并且函数或闭包满足可发送的要求，则会在函数和闭包上推断有此特性。 + +可发送函数类型是相应的不可发送函数类型的子类型。 + +## Switch Case 特性 + +你只能将 switch case 特性应用于 switch case。 + +### unknown + +将此特性应用于 switch case，以指示在代码编译时不期望与任何已知的枚举 case 匹配。有关如何使用 `unknown` 特性的示例，见 。 + +> 特性的语法: +> +> *attribute* → **`@`** *attribute-name* *attribute-argument-clause*_?_ \ +> *attribute-name* → *identifier* \ +> *attribute-argument-clause* → **`(`** *balanced-tokens*_?_ **`)`** \ +> *attributes* → *attribute* *attributes*_?_ +> +> *balanced-tokens* → *balanced-token* *balanced-tokens*_?_ \ +> *balanced-token* → **`(`** *balanced-tokens*_?_ **`)`** \ +> *balanced-token* → **`[`** *balanced-tokens*_?_ **`]`** \ +> *balanced-token* → **`{`** *balanced-tokens*_?_ **`}`** \ +> *balanced-token* → 任意标识符、关键字、字面量或运算符 \ +> *balanced-token* → 除了 **`(`**、**`)`**、**`[`**、**`]`**、**`{`** 或 **`}`** 之外的任何标点符号 + + diff --git a/swift-6.docc/ReferenceManual/Declarations.md b/swift-6.docc/ReferenceManual/Declarations.md new file mode 100644 index 000000000..0548dfa42 --- /dev/null +++ b/swift-6.docc/ReferenceManual/Declarations.md @@ -0,0 +1,2848 @@ +# 声明 + +引入类型、运算符、变量以及其他名称和构造。 + +*声明*将在程序中引入一个新的名称或构造例如，你使用声明来引入函数和方法，引入变量和常量，以及定义枚举、结构体、类和协议类型。你还可以使用声明来扩展现有具名类型的行为，并将其他地方声明的符号引入到你的程序中。 + +在 Swift 中，大多数声明也是定义，因为它们在声明的同时被实现或初始化。但由于协议不实现它的成员，所以协议成员在此仅仅是声明。为了方便起见，而且这种区别在 Swift 中没那么重要，所以术语*声明*涵盖了声明和定义两种含义。 + +> 声明的语法: +> +> *declaration* → *import-declaration* \ +> *declaration* → *constant-declaration* \ +> *declaration* → *variable-declaration* \ +> *declaration* → *typealias-declaration* \ +> *declaration* → *function-declaration* \ +> *declaration* → *enum-declaration* \ +> *declaration* → *struct-declaration* \ +> *declaration* → *class-declaration* \ +> *declaration* → *actor-declaration* \ +> *declaration* → *protocol-declaration* \ +> *declaration* → *initializer-declaration* \ +> *declaration* → *deinitializer-declaration* \ +> *declaration* → *extension-declaration* \ +> *declaration* → *subscript-declaration* \ +> *declaration* → *macro-declaration* \ +> *declaration* → *operator-declaration* \ +> *declaration* → *precedence-group-declaration* + +## 顶级代码 + +Swift 源文件中的顶级代码由零个或多个语句、声明和表达式组成。默认情况下，在源文件顶层声明的变量、常量和其他具名声明可以被同一模块中每个源文件的代码访问。你可以使用访问级别修饰符来重写此默认行为，具体说明见 。 + +有两种类型的顶级代码：顶级声明和可执行的顶级代码。顶级声明仅由声明组成，允许出现在所有 Swift 源文件中。可执行的顶级代码包含语句和表达式，而不仅仅是声明，仅允许作为程序的顶级入口点。 + +编译 Swift 代码生成可执行文件时，无论文件和模块中的代码如何组织，都只能通过以下方法之一来指定顶级入口点：`main` 特性、`NSApplicationMain` 特性、`UIApplicationMain` 特性、`main.swift` 文件，或包含顶级可执行代码的文件。 + +> 顶级声明的语法: +> +> *top-level-declaration* → *statements*_?_ + +## 代码块 + +*代码块*被各种声明和控制结构用来将语句组合在一起。它具有以下形式： + +```swift +{ + <#statements#> +} +``` + +代码块中的*语句*包括声明、表达式和其他类型的语句，并按它们在源代码中出现的顺序执行。 + + + + +> 代码块的语法: +> +> *code-block* → **`{`** *statements*_?_ **`}`** + +## 导入声明 + +*导入声明*允许你访问在当前文件之外声明的符号。基本形式是导入整个模块；它由 `import` 关键字后跟模块名称组成。 + +```swift +import <#module#> +``` + +提供更多细节可以限制导入哪些符号——可以指定特定的子模块，也可以指定模块或子模块中特定的声明。使用这种限制后，在当前作用域中，只有被导入的符号是可用的，而不是整个模块中的所有声明。 + +```swift +import <#import kind#> <#module#>.<#symbol name#> +import <#module#>.<#submodule#> +``` + + + +> 导入声明的语法: +> +> *import-declaration* → *attributes*_?_ **`import`** *import-kind*_?_ *import-path* +> +> *import-kind* → **`typealias`** | **`struct`** | **`class`** | **`enum`** | **`protocol`** | **`let`** | **`var`** | **`func`** \ +> *import-path* → *identifier* | *identifier* **`.`** *import-path* + +## 常量声明 + +*常量声明*会在你的程序中引入一个具名的常量值。常量声明使用 `let` 关键字，形式如下： + +```swift +let <#constant name#>: <#type#> = <#expression#> +``` + +常量声明定义了*常量名称*与构造器*表达式*的值之间的不可变绑定；一旦常量的被赋值，就无法更改。也就是说，如果常量是用类对象初始化的，对象本身可以改变，但常量名称与它所指向的对象之间的绑定不能改变。 + +当常量声明在全局作用域时，常量必须赋值。当常量声明在函数或者方法的上下文中时，可以稍后初始化，只要保证在第一次读取其值之前已为其赋值。如果编译器能够证明常量的值从未被读取，则不要求该常量必须赋值。此分析称为*确定初始化*——编译器保证一个值在读取之前值已被赋值。 + +> 注意: +> 确定初始化无法分析包含特定领域的内容，并且对条件语句中的状态跟踪能力也有限。如果你可以确定常量始终有一个值，但编译器无法证明这一点，请尝试简化设置该值的代码路径，或改用变量声明。 + + + +当常量声明出现在类或结构体声明的上下文中时，它被视为一个*常量属性*。常量声明不是计算属性，因此没有 getter 或 setter。 + +如果*常量名称*是元组形式，元组中每一项的名称都会和初始化*表达式*中对应的值进行绑定。 + +```swift +let (firstNumber, secondNumber) = (10, 42) +``` + + + +在这个例子中，`firstNumber` 是值 `10` 的具名常量，而 `secondNumber` 是值 `42` 的具名常量。现在这两个常量可以独立使用： + +```swift +print("The first number is \(firstNumber).") +// 打印 "The first number is 10." +print("The second number is \(secondNumber).") +// 打印 "The second number is 42." +``` + + + +在常量声明中，当可以推断出*常量名称*的类型时，类型注释（`:` *type*）是可选的，详见 。 + +要声明一个常量类型属性，请使用 `static` 声明修饰符标记该声明。类的常量类型属性总是隐式为 final；你无法用 class 或 final 声明修饰符实现允许或禁止被子类重写的目的。类型属性的讨论请参见 。 + + + +有关常量的更多信息以及何时使用它们的指导，请参见 和 。 + +> 常量声明的语法 +> +> *constant-declaration* → *attributes*_?_ *declaration-modifiers*_?_ **`let`** *pattern-initializer-list* +> +> *pattern-initializer-list* → *pattern-initializer* | *pattern-initializer* **`,`** *pattern-initializer-list* \ +> *pattern-initializer* → *pattern* *initializer*_?_ \ +> *initializer* → **`=`** *expression* + +## 变量声明 + +*变量声明*会在你的程序中引入一个具名的变量值，并使用 `var` 关键字进行声明。 + +变量声明有多种形式，用于定义各种有名称的、可变的值，包括存储变量和计算变量及属性、存储变量和属性观察者，以及静态变量属性。使用哪种形式取决于变量声明的作用域以及你打算声明的变量类型。 + +> 注意: +> 你还可以在协议声明的上下文中声明属性，参见 。 + +你可以通过在子类的属性声明中标记 `override` 声明修饰符来重写属性，参见 。 + +### 存储变量和存储变量属性 + +以下形式声明了一个存储变量或存储变量属性： + +```swift +var <#variable name#>: <#type#> = <#expression#> +``` + +你可以在全局作用域、函数的局部作用域，类或结构体声明的上下文中定义这种形式的变量声明。当这种形式的变量声明在全局作用域或函数的全局作用域内声明时，它被称为*存储变量*。当它在类或结构体声明的上下文中声明时，它被称为*存储变量属性*。 + +构造器*表达式*不能出现在协议声明中，但在其他场景下，构造器*表达式*是可选的。也就是说，如果没有构造器*表达式*，变量声明必须包含显式类型注释（`:` *type*）。 + +与常量声明一样，如果变量声明省略了构造器*表达式*，则在第一次读取该变量之前必须为其设置一个值。同样，如果*变量名*是一个元组模式，则元组中每个项的名称都绑定到构造器*表达式*中的相应值。 + +如其名称所示，存储变量或存储变量属性的值存储在内存中。 + +### 计算变量和计算属性 + +以下形式声明了一个计算变量或计算属性： + +```swift +var <#variable name#>: <#type#> { + get { + <#statements#> + } + set(<#setter name#>) { + <#statements#> + } +} +``` + +你可以在全局作用域、函数的全局作用域或类、结构体、枚举或扩展声明的上下文中定义这种形式的变量声明。当这种形式的变量声明在全局作用域或函数的全局作用域内声明时，它被称为*计算变量*。当它在类、结构体或扩展声明的上下文中声明时，它被称为*计算属性*。 + +getter 用于读取值，setter 用于写入值。setter 子句是可选的，当只需要 getter 时，可以省略两个子句，直接返回请求的值，参见 。但如果提供了 setter 子句，则必须同时提供 getter 子句。 + +*setter 名称*和括号是可选的。如果你提供了 setter 名称，它将用作 setter 参数的名称。如果你不提供 setter 名称，setter 的默认参数名称是 `newValue`，参见 。 + +与存储的具名值和存储的变量属性不同，计算的具名值或计算属性的值并不会存储在内存中。 + +有关更多信息以及查看计算属性的示例，请参见 。 + +### 存储变量观察者和属性观察者 + +你还可以使用 `willSet` 和 `didSet` 观察者声明一个存储变量或属性。带有观察者的存储变量或属性具有以下形式： + +```swift +var <#variable name#>: <#type#> = <#expression#> { + willSet(<#setter name#>) { + <#statements#> + } + didSet(<#setter name#>) { + <#statements#> + } +} +``` + +你可以在全局作用域、函数的全局作用域或类或结构体声明的上下文中定义这种变量声明形式。当这种形式的变量声明在全局作用域或函数的全局作用域内声明时，观察者被称为*存储变量观察者*。当它在类或结构体声明的上下文中声明时，观察者被称为*属性观察者*。 + +你可以为任何存储属性添加属性观察者。你还可以通过在子类中重写属性，为任何继承自父类的属性（无论是存储的还是计算的）添加属性观察者，参见 。 + +构造器*表达式*在类或结构体声明的上下文中是可选的，但在其他地方是必须的。如果能通过构造器*表达式*推断出类型，则*类型*标注是可选的。通常，表达式的类型推断发生在首次读取属性时。如果在读取属性之前，初值已经被重写，则推断发生在首次写入属性时。 + + + +`willSet` 和 `didSet` 观察者提供了一种在变量或属性被赋值时的观察（和响应）方式。当变量或属性首次初始化时，观察者不会被调用。相反，它们仅在初始化上下文之外的情况下，值被设置时被调用。 + +`willSet` 观察者在变量或属性的值被设置之前被调用。新值作为常量传递给 `willSet` 观察者，因此新值在 `willSet` 子句的实现中无法更改。`didSet` 观察者在新值被设置后立即被调用。与 `willSet` 观察者不同，变量或属性的旧值会传递给 `didSet` 观察者，以防你仍然需要访问它。也就是说，如果你在其自己的 `didSet` 观察者子句中给变量或属性赋值，那么你赋的这个新值将替代刚刚设置并传递给 `willSet` 观察者的值。 + +*setter 名称*和 `willSet` 与 `didSet` 子句中的括号是可选的。如果提供了 setter 名称，它们将作为 `willSet` 和 `didSet` 观察者的参数名称。如果不提供 setter 名称，`willSet` 观察者的默认参数名称是 `newValue`，而 `didSet` 观察者的默认参数名称是 `oldValue`。 + +`didSet` 子句在提供 `willSet` 子句时是可选的。同样，在提供 `didSet` 子句时，`willSet` 子句也是可选的。 + +如果在 `didSet` 主体中引用了旧值，为了使旧值可用，在调用 `didSet` 之前，会先调用 getter。否则，新的值会被存储，而不调用超类的 getter。下面的示例显示了一个由超类定义并被其子类重写以添加观察者的计算属性。 + +```swift +class Superclass { + private var xValue = 12 + var x: Int { + get { print("Getter was called"); return xValue } + set { print("Setter was called"); xValue = newValue } + } +} + +// 这个子类在它的观察器中没有引用 oldValue， +// 因此，父类的 getter 方法中的打印语句只会执行一次 +class New: Superclass { + override var x: Int { + didSet { print("New value \(x)") } + } +} +let new = New() +new.x = 100 +// 打印 "Setter was called" +// 打印 "Getter was called" +// 打印 "New value 100" + +// 这个子类在它的观察器中引用了 oldValue， +// 因此父类的 getter 在 setter 之前会被调用一次， +// 然后再次调用以打印该值。 +class NewAndOld: Superclass { + override var x: Int { + didSet { print("Old value \(oldValue) - new value \(x)") } + } +} +let newAndOld = NewAndOld() +newAndOld.x = 200 +// 打印 "Getter was called" +// 打印 "Setter was called" +// 打印 "Getter was called" +// 打印 "Old value 12 - new value 200" +``` + + + +有关更多信息以及如何使用属性观察者的示例，请参见 。 + + +### 类型变量属性 + +要声明一个类型变量属性，请使用 `static` 声明修饰符标记声明。类可以使用 `class` 声明修饰符标记类型计算属性，以允许子类重写超类的实现。类型属性的讨论请参见 。 + +> 变量声明的语法: +> +> *variable-declaration* → *variable-declaration-head* *pattern-initializer-list* \ +> *variable-declaration* → *variable-declaration-head* *variable-name* *type-annotation* *code-block* \ +> *variable-declaration* → *variable-declaration-head* *variable-name* *type-annotation* *getter-setter-block* \ +> *variable-declaration* → *variable-declaration-head* *variable-name* *type-annotation* *getter-setter-keyword-block* \ +> *variable-declaration* → *variable-declaration-head* *variable-name* *initializer* *willSet-didSet-block* \ +> *variable-declaration* → *variable-declaration-head* *variable-name* *type-annotation* *initializer*_?_ *willSet-didSet-block* +> +> *variable-declaration-head* → *attributes*_?_ *declaration-modifiers*_?_ **`var`** \ +> *variable-name* → *identifier* +> +> *getter-setter-block* → *code-block* \ +> *getter-setter-block* → **`{`** *getter-clause* *setter-clause*_?_ **`}`** \ +> *getter-setter-block* → **`{`** *setter-clause* *getter-clause* **`}`** \ +> *getter-clause* → *attributes*_?_ *mutation-modifier*_?_ **`get`** *code-block* \ +> *setter-clause* → *attributes*_?_ *mutation-modifier*_?_ **`set`** *setter-name*_?_ *code-block* \ +> *setter-name* → **`(`** *identifier* **`)`** +> +> *getter-setter-keyword-block* → **`{`** *getter-keyword-clause* *setter-keyword-clause*_?_ **`}`** \ +> *getter-setter-keyword-block* → **`{`** *setter-keyword-clause* *getter-keyword-clause* **`}`** \ +> *getter-keyword-clause* → *attributes*_?_ *mutation-modifier*_?_ **`get`** \ +> *setter-keyword-clause* → *attributes*_?_ *mutation-modifier*_?_ **`set`** +> +> *willSet-didSet-block* → **`{`** *willSet-clause* *didSet-clause*_?_ **`}`** \ +> *willSet-didSet-block* → **`{`** *didSet-clause* *willSet-clause*_?_ **`}`** \ +> *willSet-clause* → *attributes*_?_ **`willSet`** *setter-name*_?_ *code-block* \ +> *didSet-clause* → *attributes*_?_ **`didSet`** *setter-name*_?_ *code-block* + + + +## 类型别名声明 + +*类型别名声明*将现有类型的具名别名引入到你的程序中。类型别名声明使用 `typealias` 关键字声明，具有以下形式： + +```swift +typealias <#name#> = <#existing type#> +``` + +在声明类型别名后，别名*名称*可以在程序中的任何地方替代*现有类型*使用。*现有类型*可以是具名类型或复合类型。类型别名不会创建新类型；它们只是允许一个名称引用现有类型。 + +类型别名声明可以使用泛型参数为现有的泛型类型命名。类型别名可以为现有类型的某些或所有泛型参数提供具体类型。例如： + +```swift +typealias StringDictionary = Dictionary + +// 接下来的两个字典是同一类型 +var dictionary1: StringDictionary = [:] +var dictionary2: Dictionary = [:] +``` + + + +当声明一个带有泛型参数的类型别名时，这些参数的约束必须与现有类型的泛型参数的约束完全匹配。例如： + +```swift +typealias DictionaryOfInts = Dictionary +``` + + + +因为类型别名和现有类型可以互换使用，类型别名不能引入额外的泛型约束。 + +类型别名可以通过省略声明中的所有泛型参数来转发现有类型的泛型参数。例如，这里声明的 `Diccionario` 类型别名具有与 `Dictionary` 相同的泛型参数和约束。 + +```swift +typealias Diccionario = Dictionary +``` + + + + + + +在协议声明中，类型别名可以为经常使用的类型提供一个更短和更方便的名称。例如： + +```swift +protocol Sequence { + associatedtype Iterator: IteratorProtocol + typealias Element = Iterator.Element +} + +func sum(_ sequence: T) -> Int where T.Element == Int { + // ... +} +``` + + + +没有这种类型别名，`sum` 函数必须将关联类型称为 `T.Iterator.Element`，而不是 `T.Element`。 + +另见 。 + +> 类型别名声明的语法: +> +> *typealias-declaration* → *attributes*_?_ *access-level-modifier*_?_ **`typealias`** *typealias-name* *generic-parameter-clause*_?_ *typealias-assignment* \ +> *typealias-name* → *identifier* \ +> *typealias-assignment* → **`=`** *type* + + + +## 函数声明 + +*函数声明*将一个函数或方法引入到你的程序中。在类、结构体、枚举或协议的上下文中声明的函数被称为*方法*。函数声明使用 `func` 关键字声明，具有以下形式： + +```swift +func <#function name#>(<#parameters#>) -> <#return type#> { + <#statements#> +} +``` + +如果函数的返回类型是 `Void` ，则可以省略返回类型，如下所示： + +```swift +func <#function name#>(<#parameters#>) { + <#statements#> +} +``` + +因为参数类型无法被推断出来，所以函数的每个参数必须明确指定类型。如果在参数类型前写上 `inout`，则该参数可以在函数的作用域内被修改。关于 in-out 参数的详细讨论请参见下面的 。 + +如果一个函数声明中的*语句*只包含一个表达式，则默认返回该表达式的值。只有在表达式的类型和函数的返回类型不是 `Void`，并且不是像 `Never` 那样没有任何枚举值的枚举时，才会考虑这种隐式返回语法。 + + + +函数可以使用元组类型作为返回类型来返回多个值。 + + + +在函数内部，可以定义另一个函数。这种定义在函数内部的函数，被称为*嵌套函数*。 + +如果一个嵌套函数捕获了一个保证不会逃逸的值——例如一个 in-out 参数——或者作为一个非逃逸函数参数传递，那么这个嵌套函数就是非逃逸的。否则，嵌套函数就是逃逸的。 + +有关嵌套函数的讨论，请参见 。 + +### 参数名称 + +函数的参数是一个以逗号分隔的列表，每个参数都可以是不同的类型。函数调用时的参数顺序必须和函数声明时的参数顺序一致。最简单的参数列表有着如下的形式： + +```swift +<#parameter name#>: <#parameter type#> +``` + +参数有名称和标签，名称用于在函数内访问参数，标签用于在调用函数时指定参数。默认情况下，参数的名称也可以作为标签。例如： + +```swift +func f(x: Int, y: Int) -> Int { return x + y } +f(x: 1, y: 2) // x 和 y 都带有标签 +``` + + + + + +你可以使用以下形式（选其一），重写参数标签的默认行为： + +```swift +<#argument label#> <#parameter name#>: <#parameter type#> +_ <#parameter name#>: <#parameter type#> +``` + +在参数名称前的名称会作为这个参数的显式实参标签，它可以和参数名称不同。在函数或方法调用时，相对应的参数必须使用这个实参标签。 + +参数名称前的下划线（`_`）可以去除参数的实参标签。在函数或方法调用时，相对应的参数必须去除标签。 + +```swift +func repeatGreeting(_ greeting: String, count n: Int) { /* 打招呼 n 次 */ } +repeatGreeting("Hello, world!", count: 2) // count 带有标签，greeting 没有 +``` + + + +### 参数修饰符 + +*参数修饰符*改变了参数传递给函数的方式。 + +```swift +<#argument label#> <#parameter name#>: <#parameter modifier#> <#parameter type#> +``` + +要使用参数修饰符，请在参数类型之前写 `inout`、`borrowing` 或 `consuming`。 + +```swift +func someFunction(a: inout A, b: consuming B, c: C) { ... } +``` + +#### In-Out 参数 + +默认情况下，Swift 中的函数参数是按值传递的：在函数内进行的任何更改对调用者都是不可见的。如果需要传入一个 in-out 参数，可以使用 `inout` 参数修饰符。 + +```swift +func someFunction(a: inout Int) { + a += 1 +} +``` + +当调用包含 in-out 参数的函数时，必须在 in-out 参数前加上 &（与符号），以表明函数调用可以更改该参数的值。 + +```swift +var x = 7 +someFunction(&x) +print(x) // 打印 "8" +``` + +In-out 参数的传递方式如下： + +1. 当函数被调用时，参数的值会被复制。 +2. 在函数体内，副本被修改。 +3. 当函数返回时，副本的值被赋给原始参数。 + +这种行为被称为*拷入拷出（copy-in copy-out）* 或*值结果调用（call by value result）*。例如，当一个计算属性或一个带观察者的属性作为 in-out 参数传递时，它的 getter 在函数调用中被调用，而它的 setter 在函数返回时被调用。 + +作为一种优化手段，当参数值存储在内存中的物理地址时，在函数体内部和外部均会使用同一内存位置。这种优化行为被称为*引用调用（call by reference）*; 它满足了 copy-in copy-out 模型的所有要求，同时消除了复制的开销。请使用 copy-in copy-out 给出的模型编写代码，而不依赖于引用传递优化，以便在有或没有优化的情况下都能正确运行。 + +在函数内，不要访问作为 in-out 参数传递的值，即使原始值在当前作用域中可用。访问原始值是对该值的同时访问，这违反了内存独占性。 + +```swift +var someValue: Int +func someFunction(a: inout Int) { + a += someValue +} + +// 错误：这会导致运行时排他性违规 +someFunction(&someValue) +``` + +出于同样的原因，你不能将相同的值传递给多个 in-out 参数。 + +```swift +var someValue: Int +func someFunction(a: inout Int, b: inout Int) { + a += b + b += 1 +} + +// 错误：不能将同一个值传递给多个 in-out 参数 +someFunction(&someValue, &someValue) +``` + +有关内存安全和内存独占的更多信息，请参见 。 + + + +捕获 in-out 参数的闭包或嵌套函数必须是非逃逸的。如果你需要捕获一个 in-out 参数而不对其进行修改，请使用捕获列表显式地以不可变方式捕获该参数。 + +```swift +func someFunction(a: inout Int) -> () -> Int { + return { [a] in return a + 1 } +} +``` + + + +如果你需要捕获并修改一个 in-out 参数，请使用一个显式的局部副本，例如在多线程代码中，确保所有修改在函数返回之前都已完成。 + +```swift +func multithreadedFunction(queue: DispatchQueue, x: inout Int) { + // 创建一个本地副本，并在函数结束时手动将其复制回去。 + var localX = x + defer { x = localX } + + // 异步操作 localX，然后在返回之前等待。 + queue.async { someMutatingOperation(&localX) } + queue.sync {} +} +``` + + + +有关 in-out 参数的更多讨论和示例，请参见 .。 + + + +#### 借用和消费参数 + +默认情况下，Swift 使用一套规则在函数调用之间自动管理对象生命周期，在需要时复制值。默认规则旨在在大多数情况下最小化开销——如果你想要更具体的控制，可以应用 `borrowing` 或 `consuming` 参数修饰符。在这种情况下，使用 `copy` 显式标记复制操作。 + +无论你是否使用默认规则，Swift 确保在所有情况下对象的生命周期和所有权都得到正确管理。这些参数修饰符仅影响特定使用模式的相对效率，而不影响正确性。 + + + +`borrowing` 修饰函数参数时，函数不会保留参数的值。在这种情况下，调用者保留对象的所有权，并负责对象的生命周期管理。所以当函数只是临时使用对象时，用 `borrowing` 修饰可以最大限度地减少开销。 + +```swift +// `isLessThan` 不会保留任一参数 +func isLessThan(lhs: borrowing A, rhs: borrowing A) -> Bool { + ... +} +``` + +如果函数需要保留参数的值，例如，通过将其存储在全局变量中——你可以使用 `copy` 显式地复制该值。 + +```swift +// 同样是 `isLessThan` 函数，这个 `isLessThan` 可以将最小值记录下来 +func isLessThan(lhs: borrowing A, rhs: borrowing A) -> Bool { + if lhs < storedValue { + storedValue = copy lhs + } else if rhs < storedValue { + storedValue = copy rhs + } + return lhs < rhs +} +``` + +相反，`consuming` 参数修饰符表示该函数拥有该值的所有权，负责在函数返回之前存储或销毁它。 + +```swift +// `store` 会保留它的参数，因此将其标记为 `consuming` +func store(a: consuming A) { + someGlobalVariable = a +} +``` + +使用 `consuming` 可以在调用者在函数调用后不再需要使用该对象时，最小化开销。 + +```swift +// 通常，这就是最后一次使用 value 了 +store(a: value) +``` + +如果在函数调用后继续使用可复制对象，编译器会在函数调用之前自动复制该对象。 + +```swift +// 编译器会在这里插入一个隐式副本 +store(a: someValue) // 此函数消费 someValue +print(someValue) // 这里使用的是 someValue 的副本 +``` + +与 `inout` 不同，`borrowing` 和 `consuming` 参数在调用函数时不需要任何特殊标记： + +```swift +func someFunction(a: borrowing A, b: consuming B) { ... } + +someFunction(a: someA, b: someB) +``` + +显式使用 `borrowing` 或 `consuming` 表示你希望更严格地控制运行时所有权管理的开销。因为复制可能导致意外的运行时所有权操作，所以标记为这两种修饰符的参数在没有使用显式的 `copy` 关键字的情况下不能被复制： + +```swift +func borrowingFunction1(a: borrowing A) { + // 错误：无法隐式复制 a + // 这个赋值操作需要复制，因为 `a` 只是从调用者那里借来的。 + someGlobalVariable = a +} + +func borrowingFunction2(a: borrowing A) { + // 可以：显式复制是可以的 + someGlobalVariable = copy a +} + +func consumingFunction1(a: consuming A) { + // 错误：无法隐式复制 a + // 这个赋值操作需要复制，因为后面有 `print` + someGlobalVariable = a + print(a) +} + +func consumingFunction2(a: consuming A) { + // 可以：显式复制在这种情况下有效 + someGlobalVariable = copy a + print(a) +} + +func consumingFunction3(a: consuming A) { + // 可以：不需要复制，因为这是最后一次使用 + someGlobalVariable = a +} +``` + + + + +### 特殊类型的参数 + +参数可以被忽略，数量可以不固定，还可以为其提供默认值，使用形式如下 + +```swift +_ : <#parameter type#> +<#parameter name#>: <#parameter type#>... +<#parameter name#>: <#parameter type#> = <#default argument value#> +``` + +下划线 (`_`) 参数被显式忽略，无法在函数体内访问。 + +带有基本类型名称后面紧跟三个点（`...`）的参数被理解为可变参数。紧跟在可变参数后面的参数必须有一个实参标签。一个函数可以有多个可变参数。可变参数被视为包含基本类型名称元素的数组。例如，可变参数 `Int...` 被视为 `[Int]`。有关使用可变参数的示例，请参见 。 + +带有等号（`=`）且在类型后跟随一个表达式的参数，表示该参数有一个默认值。这个给定的表达式会在函数调用时进行求值。如果在调用函数时省略了该参数，则会使用默认值。 + +```swift +func f(x: Int = 42) -> Int { return x } +f() // 有效，使用默认值 +f(x: 7) // 有效，使用提供的值 +f(7) // 无效，缺少实参标签 +``` + + + + + + + +### 特殊类型的方法 + +枚举或结构体的方法，如果修改了 `self`，必须标记为 `mutating` 声明修饰符。 + +重写超类方法的方法必须标记为 `override` 声明修饰符。没有 `override` 修饰符而重写方法，或者在不重写超类方法的情况下使用 `override` 修饰符，都是编译时错误。 + +与类型相关的方法，而不是与类型实例相关的方法，必须使用 `static` 声明修饰符来标记（枚举和结构体使用 `static`，类可以使用 `static` 或 `class` 声明修饰符）。用 `class` 声明修饰符标记的类类型方法可以被子类的实现重写；用 `class final` 或 `static` 标记的类类型方法则不能被重写。 + + + + + +### 特殊名称的方法 + +一些具有特殊名称的方法为函数调用语法提供了语法糖。如果一个类型定义了这些方法之一，该类型的实例就可以使用函数调用语法。此时的函数调用会被理解为对该实例上某个特殊命名方法的调用 + +类、结构体或枚举类型可以通过定义一个 `dynamicallyCall(withArguments:)` 方法或一个 `dynamicallyCall(withKeywordArguments:)` 方法来支持函数调用语法，参见 ，或者通过定义一个作为函数调用（call-as-function）的方法，如下所述。如果该类型同时定义了一个作为函数调用的方法和 `dynamicCallable` 特性使用的其中一个方法，则在可以使用任一方法的情况下，编译器优先选择作为函数调用的方法。 + +作为函数调用方法的名称为 `callAsFunction()`，或其他以 `callAsFunction(` 开头并带有有标签或无标签参数的名称——例如，`callAsFunction(_:_:)` 和 `callAsFunction(something:)` 也是有效的作为函数调用方法名称。 + + + +以下函数调用是等效的： + +```swift +struct CallableStruct { + var value: Int + func callAsFunction(_ number: Int, scale: Int) { + print(scale * (number + value)) + } +} +let callable = CallableStruct(value: 100) +callable(4, scale: 2) +callable.callAsFunction(4, scale: 2) +// 两个函数调用都打印 208。 +``` + + + +作为函数调用的方法和来自 `dynamicCallable` 特性的方法在将多少信息编码到类型系统与在运行时可能的动态行为之间做出了不同的权衡。当你声明一个作为函数调用的方法时，你需要指定参数的数量，以及每个参数的类型和标签。`dynamicCallable` 特性的方法仅指定用于保存参数数组的类型。 + +定义一个作为函数调用的方法，或者来自 `dynamicCallable` 特性的方法，并不允许你在函数调用表达式以外的任何上下文中将该类型的实例用作函数。例如： + +```swift +let someFunction1: (Int, Int) -> Void = callable(_:scale:) // 错误 +let someFunction2: (Int, Int) -> Void = callable.callAsFunction(_:scale:) +``` + + + +`subscript(dynamicMember:)` 下标为成员查找提供了语法糖，参见 。 + +### 抛出函数和方法 + +可以抛出错误的函数和方法必须标记 `throws` 关键字。这些函数和方法被称为*抛出函数*和*抛出方法*。它们具有以下形式： + +```swift +func <#function name#>(<#parameters#>) throws -> <#return type#> { + <#statements#> +} +``` + +抛出特定错误类型的函数具有以下形式： + +```swift +func <#function name#>(<#parameters#>) throws(<#error type#>) -> <#return type#> { + <#statements#> +} +``` + +调用抛出函数或方法的必须被包裹在一个 `try` 或 `try!` 表达式中（即，在 `try` 或 `try!` 操作符的作用域内）。 + +函数的类型包括：它是否会抛出错误，以及它抛出的错误类型。非抛出函数是抛出函数的子类型。所以，可以在使用抛出函数的地方使用非抛出函数。有关抛出错误函数类型的更多信息，请参阅 。有关处理具有显式类型的错误的示例，请参阅 。 + +你不能仅根据函数是否会抛出错误来重载一个函数。不过，你可以根据函数的*参数*是否会抛出错误来重载函数。 + +抛出方法不能重写非抛出方法，且抛出方法也不能满足协议中对非抛出方法的要求。不过，非抛出方法可以重写抛出方法，且非抛出方法也可以满足协议中对会抛出方法的要求。 + +### 再抛出函数和方法 + +函数或方法可以使用 `rethrows` 关键字声明，表示它只在其某个函数参数抛出错误时才会抛出错误。这样的函数和方法被称为*再抛出函数（rethrowing functions）*和*再抛出方法（rethrowing methods）*。再抛出函数和方法必须至少有一个会抛出错误的函数参数。 + +```swift +func someFunction(callback: () throws -> Void) rethrows { + try callback() +} +``` + + + +再抛出的函数或方法只能在 `catch` 子句中包含 `throw` 语句。这使得你可以在 `do`-`catch` 语句中调用抛出函数，并通过抛出不同的错误在 `catch` 子句中处理错误。此外，`catch` 子句必须仅处理由再抛出函数的抛出参数抛出的错误。例如，以下是无效的，因为 `catch` 子句将处理由 `alwaysThrows()` 抛出的错误。 + +```swift +func alwaysThrows() throws { + throw SomeError.error +} +func someFunction(callback: () throws -> Void) rethrows { + do { + try callback() + try alwaysThrows() // 无效，alwaysThrows() 不是一个抛出参数 + } catch { + throw AnotherError.error + } +} +``` + + + + + +抛出方法不能重写再抛出方法，抛出方法也不能满足再抛出方法的协议要求。也就是说，再抛出方法可以重写抛出方法，再抛出方法可以满足抛出方法的协议要求。 + +在泛型代码中，抛出特定错误类型是再抛出的替代方案。例如： + +```swift +func someFunction(callback: () throws(E) -> Void) throws(E) { + try callback() +} +``` + +这种传播错误的方法保留了错误的类型信息。然而，与标记一个函数 `rethrows` 不同，这种方法并不阻止该函数抛出相同类型的错误。 + + + +### 异步函数和方法 + +以异步方式运行的函数和方法必须使用 `async` 关键字标记。这类函数和方法被称为*异步函数*和*异步方法*。它们的形式如下： + +```swift +func <#function name#>(<#parameters#>) async -> <#return type#> { + <#statements#> +} +``` + +对异步函数或方法的调用必须包装在一个 `await` 表达式中——也就是说，它们必须在 `await` 操作符的作用域内。 + +`async` 关键字是函数类型的一部分，且同步函数是异步函数的子类型。因此，你可以在需要异步函数的上下文中使用同步函数。例如，你可以用同步方法重写异步方法，且同步方法可以满足对异步方法的协议要求。 + +你可以根据函数是否为异步来重载一个函数。在调用时，由上下文决定使用哪个重载：在异步上下文中，会使用异步函数，而在同步上下文中，会使用同步函数。 + +异步方法不能重写同步方法，且异步方法不能满足对同步方法的协议要求。不过，同步方法可以重写异步方法，且同步方法可以满足对异步方法的协议要求。 + + + +### 永不返回的函数 + +Swift 定义了一个 [`Never`][] 类型，表示一个函数或方法不会返回给调用者。返回类型为 `Never` 的函数和方法被称为*非返回*。非返回的函数和方法要么导致不可恢复的错误，要么开始一个无限进行的工作序列。这意味着在调用后立即运行的代码永远不会被执行。即使抛出错误的函数和再抛出错误的函数是非返回类型，它们仍然可以将程序控制权转移到相应的 `catch` 块。 + +[`Never`]: https://developer.apple.com/documentation/swift/never + +非返回的函数或方法可以在 guard 语句的 `else` 分支中调用，以结束该分支，见 。 + +你可以重写一个非返回的方法，但新方法必须保持其返回类型和非返回的行为。 + +> 函数声明的语法: +> +> *function-declaration* → *function-head* *function-name* *generic-parameter-clause*_?_ *function-signature* *generic-where-clause*_?_ *function-body*_?_ +> +> *function-head* → *attributes*_?_ *declaration-modifiers*_?_ **`func`** \ +> *function-name* → *identifier* | *operator* +> +> *function-signature* → *parameter-clause* **`async`**_?_ *throws-clause*_?_ *function-result*_?_ \ +> *function-signature* → *parameter-clause* **`async`**_?_ **`rethrows`** *function-result*_?_ \ +> *function-result* → **`->`** *attributes*_?_ *type* \ +> *function-body* → *code-block* +> +> *parameter-clause* → **`(`** **`)`** | **`(`** *parameter-list* **`)`** \ +> *parameter-list* → *parameter* | *parameter* **`,`** *parameter-list* \ +> *parameter* → *external-parameter-name*_?_ *local-parameter-name* *parameter-type-annotation* *default-argument-clause*_?_ \ +> *parameter* → *external-parameter-name*_?_ *local-parameter-name* *parameter-type-annotation* \ +> *parameter* → *external-parameter-name*_?_ *local-parameter-name* *parameter-type-annotation* **`...`** +> +> *external-parameter-name* → *identifier* \ +> *local-parameter-name* → *identifier* \ +> *parameter-type-annotation* → **`:`** *attributes*_?_ *parameter-modifier*_?_ *type* \ +> *parameter-modifier* → **`inout`** | **`borrowing`** | **`consuming`** +> *default-argument-clause* → **`=`** *expression* + + + +## 枚举声明 + +*枚举声明*将一个具名的枚举类型引入到你的程序中。 + +枚举声明有两种基本形式，使用 `enum` 关键字进行声明。使用任一形式声明的枚举的主体包含零个或多个值——称为*枚举用例*——以及任意数量的声明，包括计算属性、实例方法、类型方法、构造器、类型别名，甚至其他枚举、结构体、类和 actor 声明。枚举声明不能包含析构器或协议声明。 + +枚举类型可以采用任意数量的协议，但不能从类、结构体或其他枚举继承。 + +与类和结构体不同，枚举类型没有隐式提供的默认构造器；所有构造器必须显式声明。构造器可以委托给枚举中的其他构造器，但初始化过程只有在构造器将枚举的一个用例赋值给 `self` 后才完成。 + +像结构体但不同于类，枚举是值类型；当枚举的实例被赋值给变量或常量，或作为参数传递给函数调用时，会被复制。有关值类型的信息，请参见 。 + +你可以通过扩展声明扩展枚举类型的行为，如 中所讨论的。 + +### 任意类型的枚举成员 + +如下的形式声明了一个包含任意类型枚举用例的枚举变量： + +```swift +enum <#enumeration name#>: <#adopted protocols#> { + case <#enumeration case 1#> + case <#enumeration case 2#>(<#associated value types#>) +} +``` + +这种形式的枚举声明在其他语言中有时被叫做*可识别联合*。 + +在这种形式中，每个用例块由 `case` 关键字开始，后面跟着一个或多个枚举用例，用逗号分隔。每个用例的名称必须是唯一的。每个用例还可以指定它存储特定类型的值。这些类型在*关联值类型*元组中指定，紧接在用例名称之后。 + +存储关联值的枚举成员可以用作函数，这些函数创建具有指定关联值的枚举实例。就像函数一样，你可以获取对枚举成员的引用，并在代码中稍后应用它。 + +```swift +enum Number { + case integer(Int) + case real(Double) +} +let f = Number.integer +// f 是一个 (Int) -> Number 的函数类型 + +// 应用函数 `f` 来创建一个包含整数值的 `Number` 实例数组 +let evenInts: [Number] = [0, 2, 4, 6].map(f) +``` + + + + + +有关更多信息以及查看枚举关联值的示例，请参见 。 + +#### 间接枚举 + +枚举类型可以具有递归结构，就是说，枚举用例的关联值类型可以是枚举类型自身。然而，枚举类型的实例具有值语义，这意味着它们在内存中有固定布局。为了支持递归，编译器必须插入一个间接层。 + +要让某个枚举用例支持递归，请使用 `indirect` 声明修饰符进行标记。间接枚举成员必须具有关联值。 + + + +```swift +enum Tree { + case empty + indirect case node(value: T, left: Tree, right: Tree) +} +``` + + + +要让一个枚举类型的所有用例都支持递归，请使用 `indirect` 修饰符标记整个枚举——当枚举包含许多需要标记为 `indirect` 修饰符的用例时，这样做非常方便。 + +使用 `indirect` 修饰符标记的枚举类型可以既包含有关联值的用例，同时还可包含没有关联值的用例。但是，它不能再单独使用 `indirect` 修饰符来标记某个用例。 + + + + + + + +### 带有原始值类型的枚举 + +以下形式声明了一个枚举类型，其中包含相同基本类型的枚举成员： + +```swift +enum <#enumeration name#>: <#raw-value type#>, <#adopted protocols#> { + case <#enumeration case 1#> = <#raw value 1#> + case <#enumeration case 2#> = <#raw value 2#> +} +``` + +在这种形式中，每个用例块由 `case` 关键字开始，后面跟着一个或多个枚举用例，用逗号分隔。与第一种形式中的用例不同，每个用例都有一个基础值，称为*原始值*，其基本类型相同。这些值的类型在*原始值类型*中指定，必须表示整数、浮点数、字符串或单个字符。特别是，*原始值类型*必须遵循 `Equatable` 协议，并且遵循以下协议之一：`ExpressibleByIntegerLiteral` 用于整型字面量，`ExpressibleByFloatLiteral` 用于浮点型字面量，`ExpressibleByStringLiteral` 用于包含任意数量字符的字符串字面量，以及 `ExpressibleByUnicodeScalarLiteral` 或 `ExpressibleByExtendedGraphemeClusterLiteral` 用于仅包含单个字符的字符串字面量。每一个用例的名字和原始值必须唯一。 + + + +如果原始值类型被指定为 `Int`，并且你没有显式地为这些用例分配值，它们将隐式地被分配值 `0`、`1`、`2`，依此类推。每个未分配的 `Int` 类型的用例将隐式地被分配一个原始值，该值是从前一个用例的原始值自动递增的。 + +```swift +enum ExampleEnum: Int { + case a, b, c = 5, d +} +``` + + + +在上述示例中，`ExampleEnum.a` 的原始值为 `0`，而 `ExampleEnum.b` 的值为 `1`。由于 `ExampleEnum.c` 的值被显式设置为 `5`，因此 `ExampleEnum.d` 的值自动从 `5` 增加，结果为 `6`。 + +如果原始值类型被指定为 `String`，并且你没有显式地为各个用例分配值，则每个未分配的用例会隐式地分配一个与该成员名称相同文本的字符串。 + +```swift +enum GamePlayMode: String { + case cooperative, individual, competitive +} +``` + + + +在上述示例中，`GamePlayMode.cooperative` 的原始值是 `"cooperative"`，`GamePlayMode.individual` 的原始值是 `"individual"`，而 `GamePlayMode.competitive` 的原始值是 `"competitive"`。 + +具有原始值类型的枚举隐式遵循在 Swift 标准库中定义的 `RawRepresentable` 协议。因此，它们具有 `rawValue` 属性和一个可失败构造器，其签名为 `init?(rawValue: RawValue)`。你可以使用 `rawValue` 属性访问枚举用例的原始值，如 `ExampleEnum.b.rawValue`。你还可以使用原始值通过调用枚举的可失败构造器来查找相应的用例，如 `ExampleEnum(rawValue: 5)`，这将返回一个可选的用例。有关更多信息以及查看具有原始值类型的案例示例，请参见 。 + +### 访问枚举成员 + +要引用枚举类型的用例，请使用点（`.`）语法，如 `EnumerationType.enumerationCase` 所示。当枚举类型可以从上下文中推断时，可以省略它（仍然需要 `.`），参见 和 。 + +要检查枚举用例的值，请使用 `switch` 语句，如 中所示。在 `switch` 语句的用例分支中，枚举类型会与枚举用例进行模式匹配，详见 。 + + + + + + + +> 枚举声明的语法: +> +> *enum-declaration* → *attributes*_?_ *access-level-modifier*_?_ *union-style-enum* \ +> *enum-declaration* → *attributes*_?_ *access-level-modifier*_?_ *raw-value-style-enum* +> +> *union-style-enum* → **`indirect`**_?_ **`enum`** *enum-name* *generic-parameter-clause*_?_ *type-inheritance-clause*_?_ *generic-where-clause*_?_ **`{`** *union-style-enum-members*_?_ **`}`** \ +> *union-style-enum-members* → *union-style-enum-member* *union-style-enum-members*_?_ \ +> *union-style-enum-member* → *declaration* | *union-style-enum-case-clause* | *compiler-control-statement* \ +> *union-style-enum-case-clause* → *attributes*_?_ **`indirect`**_?_ **`case`** *union-style-enum-case-list* \ +> *union-style-enum-case-list* → *union-style-enum-case* | *union-style-enum-case* **`,`** *union-style-enum-case-list* \ +> *union-style-enum-case* → *enum-case-name* *tuple-type*_?_ \ +> *enum-name* → *identifier* \ +> *enum-case-name* → *identifier* +> +> *raw-value-style-enum* → **`enum`** *enum-name* *generic-parameter-clause*_?_ *type-inheritance-clause* *generic-where-clause*_?_ **`{`** *raw-value-style-enum-members* **`}`** \ +> *raw-value-style-enum-members* → *raw-value-style-enum-member* *raw-value-style-enum-members*_?_ \ +> *raw-value-style-enum-member* → *declaration* | *raw-value-style-enum-case-clause* | *compiler-control-statement* \ +> *raw-value-style-enum-case-clause* → *attributes*_?_ **`case`** *raw-value-style-enum-case-list* \ +> *raw-value-style-enum-case-list* → *raw-value-style-enum-case* | *raw-value-style-enum-case* **`,`** *raw-value-style-enum-case-list* \ +> *raw-value-style-enum-case* → *enum-case-name* *raw-value-assignment*_?_ \ +> *raw-value-assignment* → **`=`** *raw-value-literal* \ +> *raw-value-literal* → *numeric-literal* | *static-string-literal* | *boolean-literal* + + + +## 结构体声明 + +*结构体声明*将一个具名的结构体类型引入到你的程序中。结构体声明使用 `struct` 关键字声明，具有以下形式： + +```swift +struct <#structure name#>: <#adopted protocols#> { + <#declarations#> +} +``` + +结构体的主体包含零个或多个*声明*。这些*声明*可以包括存储属性和计算属性、类型属性、实例方法、类型方法、构造器、下标、类型别名，甚至其他结构体、类、actor 和枚举声明。结构体声明不能包含析构器或协议声明。有关包含各种类型声明的结构体的讨论和多个示例，请参见 。 + +结构体类型可以采用任意数量的协议，但不能从类、枚举或其他结构体继承。 + +有三种方法可以创建先前声明的结构体的实例： + +- 调用结构体中声明的某个构造器，参见 。 +- 如果没有声明构造器，则调用结构体的成员遍历构造器，参见 。 +- 如果没有声明构造器，且结构体声明的所有属性都给定了初始值，则调用结构体的默认构造器，参见 。 + +初始化结构体中声明属性的过程在 中描述。 + +结构体实例的属性可以使用点 (`.`) 语法访问，参见 。 + +结构体是值类型；当结构体的实例被赋值给变量或常量，或作为参数传递给函数调用时，会被复制。有关值类型的信息，请参见 。 + +你可以通过扩展声明来扩展结构体类型的行为，参见 。 + +> 结构体声明的语法: +> +> *struct-declaration* → *attributes*_?_ *access-level-modifier*_?_ **`struct`** *struct-name* *generic-parameter-clause*_?_ *type-inheritance-clause*_?_ *generic-where-clause*_?_ *struct-body* \ +> *struct-name* → *identifier* \ +> *struct-body* → **`{`** *struct-members*_?_ **`}`** +> +> *struct-members* → *struct-member* *struct-members*_?_ \ +> *struct-member* → *declaration* | *compiler-control-statement* + +## 类声明 + +*类声明*将一个具名的类类型引入到你的程序中。类声明使用 `class` 关键字声明，具有以下形式： + +```swift +class <#class name#>: <#superclass#>, <#adopted protocols#> { + <#declarations#> +} +``` + +类的主体包含零个或多个*声明*。这些*声明*可以包括存储属性和计算属性、实例方法、类型方法、构造器、一个析构器、下标、类型别名，甚至其他类、结构体、actor 和枚举声明。类声明不能包含协议声明。有关包含各种类型声明的类的讨论和多个示例，请参见 。 + +类类型只能继承自一个父类，即它的*超类*，但可以采用任意数量的协议。*超类*在*类名*和冒号之后首先出现，后面跟着任何*采用的协议*。泛型类可以继承其他泛型和非泛型类，但非泛型类只能继承其他非泛型类。当你在冒号后写泛型超类的名称时，必须包括该泛型类的全名，包括其泛型参数子句。 + +如在 中讨论的，类可以有指定构造器和便利构造器。类的指定构造器必须初始化所有声明的属性，并且必须在调用任何超类的指定构造器之前完成此操作。 + +类可以重写其超类的属性、方法、下标和构造器。重写的属性、方法、下标和指定构造器必须标记为 `override` 声明修饰符。 + + + +要要求子类实现超类的构造器，请使用 `required` 声明修饰符标记超类的构造器。子类对该构造器的实现也必须使用 `required` 声明修饰符进行标记。 + +尽管在*超类*中声明的属性和方法被当前类继承，但在*超类*中声明的指定构造器仅在子类满足 中描述的条件时才会被继承。Swift 类不从通用基类继承。 + +有两种方法可以创建一个先前声明的类的实例： + +- 调用类中声明的某个构造器，参见 。 +- 如果没有声明构造器，且类声明的所有属性都给定了初始值，则调用类的默认构造器，参见 。 + +使用点（`.`）语法访问类实例的属性，参见 。 + +类是引用类型；当类的实例被赋值给变量或常量，或作为参数传递给函数调用时，是引用而不是复制。有关引用类型的信息，请参见 。 + +你可以通过扩展声明扩展类类型的行为，参见 。 + +> 类声明的语法: +> +> *class-declaration* → *attributes*_?_ *access-level-modifier*_?_ **`final`**_?_ **`class`** *class-name* *generic-parameter-clause*_?_ *type-inheritance-clause*_?_ *generic-where-clause*_?_ *class-body* \ +> *class-declaration* → *attributes*_?_ **`final`** *access-level-modifier*_?_ **`class`** *class-name* *generic-parameter-clause*_?_ *type-inheritance-clause*_?_ *generic-where-clause*_?_ *class-body* \ +> *class-name* → *identifier* \ +> *class-body* → **`{`** *class-members*_?_ **`}`** +> +> *class-members* → *class-member* *class-members*_?_ \ +> *class-member* → *declaration* | *compiler-control-statement* + +## Actor 声明 + +*actor 声明*将一个具名的 actor 类型引入到你的程序中。actor 声明使用 `actor` 关键字声明，具有以下形式： + +```swift +actor <#actor name#>: <#adopted protocols#> { + <#declarations#> +} +``` + +actor 的主体包含零个或多个*声明*。这些*声明*可以包括存储属性和计算属性、实例方法、类型方法、构造器、一个析构器、下标、类型别名，甚至其他类、结构体和枚举声明。有关包含各种声明的 actor 的讨论和多个示例，请参见 。 + +actor 类型可以采用任意数量的协议，但不能从类、枚举、结构体或其他 actor 继承。然而，标记为 `@objc` 特性的 actor 隐式地遵循 `NSObjectProtocol` 协议，并作为 `NSObject` 的子类型暴露给 Objective-C 运行时。 + +有两种方法可以创建一个先前声明的 actor 的实例： + +- 调用 actor 中声明的某个构造方法，参见 。 +- 如果没有声明初始值，并且 actor 声明的所有属性都给定了初始值，则调用 actor 的默认构造器，参见 。 + +默认情况下，actor 的成员是与该 actor 隔离的。方法体或属性的 getter 等代码是在该 actor 上执行的。actor 内部的代码可以同步地与这些成员交互，因为代码已经在同一个 actor 上运行；但 actor 外部的代码必须使用 `await` 标记，以表明该代码是异步地在另一个 actor 上运行的。键路径不能引用 actor 的隔离成员。actor 隔离的存储属性可以作为 in-out 参数传递给同步函数，但不能传递给异步函数。 + +actor 还可以拥有非隔离成员，其声明使用 `nonisolated` 关键字标记。非隔离成员的执行方式类似于 actor 外部的代码：它无法与 actor 的任何隔离状态交互，调用者在使用时也不需要使用 `await` 进行标记。 + +actor 的成员只有在它们是非隔离或异步的情况下才能标记为 `@objc` 属性。 + +初始化 actor 中声明的属性的过程，参见 。 + +actor 实例的属性可以使用点 (`.`) 语法访问，参见 。 + +actor 是引用类型；当分配给变量或常量，或作为参数传递给函数调用时，actor 的实例是被引用而不是复制。有关引用类型的信息，请参见 。 + +你可以通过扩展声明扩展 actor 类型的行为，参见 。 + + + +> actor 声明的语法: +> +> *actor-declaration* → *attributes*_?_ *access-level-modifier*_?_ **`actor`** *actor-name* *generic-parameter-clause*_?_ *type-inheritance-clause*_?_ *generic-where-clause*_?_ *actor-body* \ +> *actor-name* → *identifier* \ +> *actor-body* → **`{`** *actor-members*_?_ **`}`** +> +> *actor-members* → *actor-member* *actor-members*_?_ \ +> *actor-member* → *declaration* | *compiler-control-statement* + +## 协议声明 + +*协议声明*将一个具名的协议类型引入到你的程序中。协议声明使用 `protocol` 关键字声明，具有以下形式： + +```swift +protocol <#protocol name#>: <#inherited protocols#> { + <#protocol member declarations#> +} +``` + +协议声明可以出现在全局作用域内，或嵌套在非泛型类型或非泛型函数内部。 + +协议的主体包含零个或多个*协议成员声明*，这些声明描述了任何采用该协议的类型必须满足的遵循性要求。特别是，协议可以声明遵循的类型必须实现某些属性、方法、构造器和下标。协议还可以声明特殊类型的类型别名，称为*关联类型*，可以指定协议中各种声明之间的关系。协议声明不能包含类、结构体、枚举或其他协议声明。*协议成员声明*将在下面详细讨论。 + +协议类型可以从任意数量的其他协议继承。当一个协议类型从其他协议继承时，这些其他协议的要求集合会被聚合，任何从当前协议继承的类型必须遵循所有这些要求。有关如何使用协议继承的示例，请参见 。 + +> 注意: +> 你还可以使用协议组合类型聚合多个协议的合规性要求，参见 和 。 + +你可以通过在该类型的扩展声明中采用协议，为先前声明的类型添加协议遵循性。在扩展中，你必须实现所采用协议的所有要求。如果该类型已经实现了所有要求，你可以将扩展声明的主体留空。 + +默认情况下，遵循协议的类型必须实现协议中声明的所有属性、方法和下标。也就是说，你可以使用 `optional` 声明修饰符来标记这些协议成员声明，以指定遵循类型的实现是可选的。`optional` 修饰符只能应用于标记为 `objc` 特性的成员，并且只能应用于标记为 `objc` 特性的协议成员。因此，只有类类型可以采用并遵循包含可选成员要求的协议。有关如何使用 `optional` 声明修饰符的信息，以及如何访问可选协议成员的指导——例如，当你不确定遵循类型是否实现它们时——请参见 。 + + + +枚举的用例可以满足类型成员的协议要求。具体来说，没有任何关联值的枚举用例满足类型 `Self` 的只读类型变量的协议要求，而具有关联值的枚举成员满足返回 `Self` 的函数的协议要求，该函数的参数及其实参标签与枚举成员的关联值匹配。例如： + +```swift +protocol SomeProtocol { + static var someValue: Self { get } + static func someFunction(x: Int) -> Self +} +enum MyEnum: SomeProtocol { + case someValue + case someFunction(x: Int) +} +``` + + + +要将协议的采用限制为类类型，只需在冒号后将 `AnyObject` 协议包含在*继承协议*列表中。例如，以下协议只能被类类型采用： + +```swift +protocol SomeProtocol: AnyObject { + /* 协议成员写在这里 */ +} +``` + + + +任何从标记为 `AnyObject` 要求的协议继承的协议，也只能被类类型采用。 + +> 注意: +> 如果一个协议标记了 `objc` 特性，则 `AnyObject` 要求隐式应用于该协议；无需显式的将该协议标记为 `AnyObject` 要求。 + +协议是具名类型，因此它们可以出现在代码中与其他具名类型相同的位置，如 中所讨论的。然而，你无法构造协议的实例，因为协议实际上并不提供它们所指定的要求的实现。 + +你可以使用协议来声明类或结构体的代理应该实现哪些方法，参见 。 + +> 协议声明的语法: +> +> *protocol-declaration* → *attributes*_?_ *access-level-modifier*_?_ **`protocol`** *protocol-name* *type-inheritance-clause*_?_ *generic-where-clause*_?_ *protocol-body* \ +> *protocol-name* → *identifier* \ +> *protocol-body* → **`{`** *protocol-members*_?_ **`}`** +> +> *protocol-members* → *protocol-member* *protocol-members*_?_ \ +> *protocol-member* → *protocol-member-declaration* | *compiler-control-statement* +> +> *protocol-member-declaration* → *protocol-property-declaration* \ +> *protocol-member-declaration* → *protocol-method-declaration* \ +> *protocol-member-declaration* → *protocol-initializer-declaration* \ +> *protocol-member-declaration* → *protocol-subscript-declaration* \ +> *protocol-member-declaration* → *protocol-associated-type-declaration* \ +> *protocol-member-declaration* → *typealias-declaration* + +### 协议属性声明 + +协议通过在协议声明体中包含一个*协议属性声明*，规定遵循该协议的类型必须实现一个属性。协议属性声明是一种特殊形式的变量声明，格式如下： + +```swift +var <#property name#>: <#type#> { get set } +``` + +与其他协议成员声明一样，这些属性声明仅声明遵循该协议的类型的 getter 和 setter 要求。因此，你不会在声明它的协议中直接实现 getter 或 setter。 + +遵循协议的类型可以通过多种方式满足 getter 和 setter 的要求。如果属性声明同时包含 `get` 和 `set` 关键字，遵循类型可以用存储变量属性或可读写的计算属性（即实现了 getter 和 setter 的属性）来实现。然而，这样的属性声明不能被实现为常量属性或只读计算属性。如果属性声明只包含 `get` 关键字，则可以实现为任何类型的属性。关于符合协议类型如何实现属性要求的示例，参见 。 + +在协议声明中声明类型属性要求时，使用 `static` 关键字标记属性声明。遵循该协议的结构体和枚举使用 `static` 关键字声明属性，而遵循该协议的类则可以使用 `static` 或 `class` 关键字声明属性。为结构体、枚举或类添加协议遵循的扩展使用与其扩展的类型相同的关键字。为类型属性要求提供默认实现的扩展使用 `static` 关键字。 + + + + + +另见 。 + +> 协议属性声明的语法: +> +> *protocol-property-declaration* → *variable-declaration-head* *variable-name* *type-annotation* *getter-setter-keyword-block* + +### 协议方法声明 + +协议通过在协议声明体中包含一个协议方法声明，规定遵循该协议的类型必须实现一个方法。协议方法声明的形式与函数声明相同，但有两个例外：它们不包含函数体，且不能在函数声明中提供任何默认参数值。关于遵循协议类型如何实现方法要求的示例，参见 。 + +在协议声明中声明类或静态方法的要求时，使用 `static` 修饰符标记方法声明。遵循该协议的结构体和枚举使用 `static` 关键字声明该方法，而遵循该协议的类则使用 `static` 或 `class` 关键字声明该方法。为结构体、枚举或类添加协议遵循的扩展使用与其扩展的类型相同的关键字。为类型方法要求提供默认实现的扩展使用 `static` 关键字。 + +另见 。 + + + +> 协议方法声明的语法: +> +> *protocol-method-declaration* → *function-head* *function-name* *generic-parameter-clause*_?_ *function-signature* *generic-where-clause*_?_ + +### 协议构造器声明 + +协议通过在协议声明的主体中包含协议构造器声明，要求遵循的类型必须实现一个构造器。协议构造器声明的形式与构造器声明相同，只是不包括构造器的主体。 + +遵循类型可以通过实现一个非可失败构造器或一个 `init!` 可失败构造器来满足非可失败协议构造器的要求。一个遵循类型可以通过实现任何类型的构造器来满足可失败协议构造器的要求。 + +当一个类实现一个构造器以满足协议的构造器要求时，如果该类尚未标记为 `final` 声明修饰符，则构造器必须标记为 `required` 声明修饰符。 + +另见 。 + +> 协议构造器声明的语法: +> +> *protocol-initializer-declaration* → *initializer-head* *generic-parameter-clause*_?_ *parameter-clause* *throws-clause*_?_ *generic-where-clause*_?_ \ +> *protocol-initializer-declaration* → *initializer-head* *generic-parameter-clause*_?_ *parameter-clause* **`rethrows`** *generic-where-clause*_?_ + +### 协议下标声明 + +协议声明遵循的类型必须通过在协议声明的主体中包含协议下标声明来实现下标。协议下标声明具有下标声明的特殊形式： + +```swift +subscript (<#parameters#>) -> <#return type#> { get set } +``` + +下标声明仅声明遵循协议的类型所需的最小 getter 和 setter 实现要求。如果下标声明同时包含 `get` 和 `set` 关键字，则遵循的类型必须实现 getter 和 setter 子句。如果下标声明仅包含 `get` 关键字，则遵循的类型必须实现*至少*一个 getter 子句，并且可以选择性地实现一个 setter 子句。 + +在协议声明中声明静态下标要求时，使用 `static` 声明修饰符标记下标声明。遵循该协议的结构体和枚举使用 `static` 关键字声明下标，而遵循该协议的类则使用 `static` 或 `class` 关键字声明下标。为结构体、枚举或类添加协议遵循性的扩展使用与其扩展的类型相同的关键字。为静态下标要求提供默认实现的扩展使用 `static` 关键字。 + +另见 。 + +> 协议下标声明的语法: +> +> *protocol-subscript-declaration* → *subscript-head* *subscript-result* *generic-where-clause*_?_ *getter-setter-keyword-block* + +### 协议关联类型声明 + +协议使用 `associatedtype` 关键字声明关联类型。关联类型为作为协议声明一部分使用的类型提供了别名。关联类型类似于泛型参数子句中的类型参数，但它们与声明它们的协议中的 `Self` 相关联。在该上下文中，`Self` 指的是遵循该协议的最终类型。有关更多信息和示例，请参见 。 + +你在协议声明中使用通用的 `where` 子句，以便为从另一个协议继承的关联类型添加约束，而无需重新声明关联类型。以下 `SubProtocol` 的声明是等效的： + +```swift +protocol SomeProtocol { + associatedtype SomeType +} + +protocol SubProtocolA: SomeProtocol { + // 此语法会产生警告。 + associatedtype SomeType: Equatable +} + +// 推荐使用此语法。 +protocol SubProtocolB: SomeProtocol where SomeType: Equatable { } +``` + + + + + + + +另见 。 + +> 协议关联类型声明的语法: +> +> *protocol-associated-type-declaration* → *attributes*_?_ *access-level-modifier*_?_ **`associatedtype`** *typealias-name* *type-inheritance-clause*_?_ *typealias-assignment*_?_ *generic-where-clause*_?_ + +## 构造器声明 + +*构造器声明*在你的程序中引入了一个类、结构体或枚举的构造器。构造器声明使用 `init` 关键字声明，有两种基本形式。 + +结构体、枚举和类类型可以有任意数量的构造器，但是类的构造器具有不同的规则和行为。与结构体和枚举不同，类有两种类型的构造器：指定构造器和便利构造器，参见 。 + +以下形式声明了结构体和枚举的构造器，以及类的指定构造器：： + +```swift +init(<#parameters#>) { + <#statements#> +} +``` + +类的指定构造器直接初始化类的所有属性。它不能调用当前类的其他构造器，如果该类有一个超类，则必须调用超类的一个指定构造器。如果该类从其超类继承了任何属性，则在当前类中设置或修改这些属性之前，必须调用超类的一个指定构造器。 + +指定构造器只能在类声明中声明，因此不能通过扩展声明添加到类中。 + +结构体和枚举中的构造器可以调用其他已声明的构造器，以委托部分或全部初始化过程。 + +要为一个类声明便利构造器，请使用 `convenience` 声明修饰符标记构造器声明。 + +```swift +convenience init(<#parameters#>) { + <#statements#> +} +``` + +便利构造器可以将构造过程委托给另一个便利构造器或一个指定构造器。但是，类的构造过程必须以一个将类中所有属性完全初始化的指定构造器的调用作为结束。便利构造器不能调用超类的构造器 + +你可以使用 `required` 声明修饰符标记指定和便利构造器，以要求每个子类实现该构造器。子类对该构造器的实现也必须标记为 `required` 声明修饰符。 + +默认情况下，超类中声明的构造器不会被子类继承。也就是说，如果子类用默认值初始化了所有存储属性，并且没有定义自己的构造器，它将继承超类的所有构造器。如果子类重写了超类的所有指定构造器，它将继承超类的便利构造器。 + +与方法、属性和下标一样，你需要使用 `override` 声明修饰符标记重写的指定构造器。 + +> 注意: +> 如果你使用 `required` 声明修饰符标记了一个构造器，则在子类中重写所需的构造器时，不要同时使用 `override` 修饰符标记该构造器。 + +就像函数和方法一样，构造器可以抛出或再抛出错误。与函数和方法一样，你在构造器的参数后使用 `throws` 或 `rethrows` 关键字来指示适当的行为。同样，构造器可以是异步的，你使用 `async` 关键字来指示这一点。 + +要查看各种类型声明中构造器的示例，请参见 。 + +### 可失败的构造器 + +*可失败的构造器*是一种生成一个可选实例或一个隐式解包的可选实例的构造器，具体取决于构造器声明的类型。因此，可失败的构造器可以返回 `nil` 以表示初始化失败。 + +要声明一个可失败的构造器并生成一个可选实例，需要在构造器声明中的 `init` 关键字后面加上问号（`init?`）。要声明一个可失败构造器并生成一个隐式解包的可选实例，则需要加上感叹号（`init!`）。下面的示例展示了一个 `init?` 可失败的构造器，它生成了一个结构体的可选实例。 + +```swift +struct SomeStruct { + let property: String + // 生成一个 `SomeStruct` 可选实例 + init?(input: String) { + if input.isEmpty { + // 丢弃 'self' 并返回 'nil' + return nil + } + property = input + } +} +``` + + + +调用 `init?` 可失败的构造器与调用不可失败的构造器的方式相同，只是你必须处理结果的可选性。 + +```swift +if let actualInstance = SomeStruct(input: "Hello") { + // 使用 'SomeStruct' 的实例执行操作 +} else { + // 'SomeStruct' 的初始化失败，初始化器返回了 'nil' +} +``` + + + +可失败的构造器可以在构造器主体的实现中的任何时刻返回 `nil`。 + +可失败的构造器可以委托给任何类型的构造器。不可失败的构造器可以委托给另一个不可失败的构造器或一个 `init!` 可失败的构造器。不可失败的构造器可以通过强制解包超类构造器的结果来委托给一个 `init?` 可失败的构造器——例如，通过写 `super.init()!`。 + +初始化失败会通过构造器委托传播。具体来说，如果一个可失败的构造器委托给一个失败并返回 `nil` 的构造器，那么委托的构造器也会失败并隐式返回 `nil`。如果一个不可失败的构造器委托给一个失败并返回 `nil` 的 `init!` 可失败构造器，那么会引发运行时错误（就像你使用`!`运算符来解包一个值为 `nil`的可选值一样）。 + +可失败的指定构造器可以在子类中被任何类型的指定构造器重写。不可失败的指定构造器只能在子类中被不可失败的指定构造器重写。 + +有关更多信息以及可失败构造器的示例，请参见 。 + +> 初始化声明的语法: +> +> *initializer-declaration* → *initializer-head* *generic-parameter-clause*_?_ *parameter-clause* **`async`**_?_ *throws-clause*_?_ *generic-where-clause*_?_ *initializer-body* \ +> *initializer-declaration* → *initializer-head* *generic-parameter-clause*_?_ *parameter-clause* **`async`**_?_ **`rethrows`** *generic-where-clause*_?_ *initializer-body* \ +> *initializer-head* → *attributes*_?_ *declaration-modifiers*_?_ **`init`** \ +> *initializer-head* → *attributes*_?_ *declaration-modifiers*_?_ **`init`** **`?`** \ +> *initializer-head* → *attributes*_?_ *declaration-modifiers*_?_ **`init`** **`!`** \ +> *initializer-body* → *code-block* + +## 析构器声明 + +*析构器声明*为类类型声明一个析构器。析构器不接受参数，具有以下形式： + +```swift +deinit { + <#statements#> +} +``` + +析构器在一个类对象不再有任何引用时，会在该对象被释放之前自动调用。析构器只能在类声明的主体内声明，不能在类的扩展中声明，并且每个类最多只能有一个析构器 + +子类继承其超类的析构器，该析构器在子类对象被释放之前隐式调用。子类对象在其继承链中的所有析构器执行完毕之前不会被释放。 + +析构器不会被直接调用。 + +在类声明中如何使用析构器的示例，请参见 。 + +> 析构器声明的语法: +> +> *deinitializer-declaration* → *attributes*_?_ **`deinit`** *code-block* + +## 扩展声明 + +*扩展声明*允许你扩展现有类型的行为。扩展声明使用 `extension` 关键字声明，具有以下形式： + +```swift +extension <#type name#> where <#requirements#> { + <#declarations#> +} +``` + +扩展声明的主体包含零个或多个*声明*。这些*声明*可以包括计算属性、计算类型属性、实例方法、类型方法、构造器、下标声明，甚至类、结构体和枚举声明。扩展声明不能包含析构器或协议声明、存储属性、属性观察者或其他扩展声明。协议扩展中的声明不能标记为 `final`。有关包含各种类型声明的扩展的讨论和多个示例，请参见 。 + +如果*类型名称*是类、结构体或枚举类型，则扩展该类型。如果*类型名称*是协议类型，则扩展所有遵循该协议的类型。 + +扩展声明可以扩展具有关联类型的泛型类型或协议，并可以包含*要求*。如果扩展类型的实例或遵循扩展协议的类型的实例满足*要求*，则该实例获得声明中指定的行为。 + +扩展声明可以包含构造器声明。也就是说，如果你正在扩展的类型在另一个模块中定义，则构造器声明必须委托给该模块中已定义的构造器，以确保该类型的成员得到正确初始化。 + +现有类型的属性、方法和构造器不能在该类型的扩展中被重写。 + +扩展声明可以通过指定*采用的协议*，为现有的类、结构体或枚举类型添加协议遵循： + +```swift +extension <#type name#>: <#adopted protocols#> where <#requirements#> { + <#declarations#> +} +``` + +扩展声明不能为现有类添加类继承，因此你只能在*类型名称*和冒号后指定协议列表。 + +### 条件遵循 + +你可以扩展一个泛型类型以有条件地遵循一个协议，从而使该类型的实例仅在满足某些要求时遵循该协议。你通过在扩展声明中包含*要求*来添加对协议的条件遵循。 + +#### 重写的要求在某些泛型上下文中不会被使用 + +在某些泛型上下文中，通过条件遵循协议而获得行为的类型，并不总是使用该协议要求的特定实现。为了说明这种行为，以下示例定义了两个协议和一个有条件地遵循这两个协议的泛型类型。 + + + +```swift +protocol Loggable { + func log() +} +extension Loggable { + func log() { + print(self) + } +} + +protocol TitledLoggable: Loggable { + static var logTitle: String { get } +} +extension TitledLoggable { + func log() { + print("\(Self.logTitle): \(self)") + } +} + +struct Pair: CustomStringConvertible { + let first: T + let second: T + var description: String { + return "(\(first), \(second))" + } +} + +extension Pair: Loggable where T: Loggable { } +extension Pair: TitledLoggable where T: TitledLoggable { + static var logTitle: String { + return "Pair of '\(T.logTitle)'" + } +} + +extension String: TitledLoggable { + static var logTitle: String { + return "String" + } +} +``` + + + +`Pair` 结构体在其泛型类型分别遵循 `Loggable` 或 `TitledLoggable` 时，也会相应地遵循 `Loggable` 和 `TitledLoggable`。在下面的示例中，`oneAndTwo` 是 `Pair` 的一个实例，由于 `String` 遵循 `TitledLoggable`，因此 `oneAndTwo` 也遵循 `TitledLoggable`。当直接调用 `oneAndTwo` 的 `log()` 方法时，将使用包含标题字符串的特定版本。 + +```swift +let oneAndTwo = Pair(first: "one", second: "two") +oneAndTwo.log() +// 打印 "Pair of 'String': (one, two)" +``` + + + +然而，当在泛型上下文中使用 `oneAndTwo` 或将其作为 `Loggable` 协议的一个实例时，特定的实现版本不会被使用。Swift 在选择调用哪个 `log()` 实现时，只参考 `Pair` 遵循 `Loggable` 所需的最低要求。因此，使用的是 `Loggable` 协议提供的默认实现。 + +```swift +func doSomething(with x: T) { + x.log() +} +doSomething(with: oneAndTwo) +// 打印 "(one, two)" +``` + + + +当在传递给 `doSomething(_:)` 的实例上调用 `log()` 时，自定义标题会从日志字符串中省略。 + +### 协议的遵循不应冗余 + +具体类型只能遵从某个协议一次。Swift 会将多余的协议遵从标记为错误。你可能会在两种情况下遇到这种错误。第一种情况是，当你以不同的要求多次显式地遵从同一个协议。第二种情况是，当你多次隐式地继承同一个协议。以下部分将讨论这些情况。 + +#### 解决显式冗余 + +对一个具体类型的多个扩展不能添加对同一协议的遵循，即使这些扩展的要求是互斥的。以下示例展示了这一限制。两个扩展声明试图为 `Serializable` 协议添加条件遵循，一个是针对包含 `Int` 元素的数组，另一个是针对包含 `String` 元素的数组。 + +```swift +protocol Serializable { + func serialize() -> Any +} + +extension Array: Serializable where Element == Int { + func serialize() -> Any { + // 实现 + } +} +extension Array: Serializable where Element == String { + func serialize() -> Any { + // 实现 + } +} +// 错误：'Array' 对协议 'Serializable' 的遵循是多余的 +``` + + + +如果你需要根据多个具体类型添加条件遵循，请创建一个每个类型都可以遵循的新协议，并在声明条件遵循时使用该协议作为要求。 + +```swift +protocol SerializableInArray { } +extension Int: SerializableInArray { } +extension String: SerializableInArray { } + +extension Array: Serializable where Element: SerializableInArray { + func serialize() -> Any { + // 实现 + } +} +``` + + + +#### 解决隐式冗余 + +当一个具体类型有条件地遵循一个协议时，该类型隐式地遵循任何具有相同要求的父协议。 + +如果你需要一个类型有条件地遵循两个继承自单一父协议的协议，请显式声明对父协议的遵循。这可以避免以不同的要求隐式地两次遵循父协议。 + +以下示例显式声明了 `Array` 对 `Loggable` 的条件遵循，以避免在声明其对 `TitledLoggable` 和新的 `MarkedLoggable` 协议的条件遵循时发生冲突。 + +```swift +protocol MarkedLoggable: Loggable { + func markAndLog() +} + +extension MarkedLoggable { + func markAndLog() { + print("----------") + log() + } +} + +extension Array: Loggable where Element: Loggable { } +extension Array: TitledLoggable where Element: TitledLoggable { + static var logTitle: String { + return "Array of '\(Element.logTitle)'" + } +} +extension Array: MarkedLoggable where Element: MarkedLoggable { } +``` + + + +在没有扩展显式声明对 `Loggable` 的条件遵循时，其他 `Array` 扩展会隐式创建这些声明，从而导致错误： + +```swift +extension Array: Loggable where Element: TitledLoggable { } +extension Array: Loggable where Element: MarkedLoggable { } +// 错误：'Array' 对协议 'Loggable' 的遵循是多余的 +``` + + + + + + + + + +> 扩展声明的语法: +> +> *extension-declaration* → *attributes*_?_ *access-level-modifier*_?_ **`extension`** *type-identifier* *type-inheritance-clause*_?_ *generic-where-clause*_?_ *extension-body* \ +> *extension-body* → **`{`** *extension-members*_?_ **`}`** +> +> *extension-members* → *extension-member* *extension-members*_?_ \ +> *extension-member* → *declaration* | *compiler-control-statement* + +## 下标声明 + +*下标*声明允许你为特定类型的对象添加下标支持，通常用于提供一种方便的语法来访问集合、列表或序列中的元素。下标声明使用 `subscript` 关键字声明，具有以下形式： + +```swift +subscript (<#parameters#>) -> <#return type#> { + get { + <#statements#> + } + set(<#setter name#>) { + <#statements#> + } +} +``` + +下标声明只能出现在类、结构体、枚举、扩展或协议声明的上下文中。 + +*参数*指定了在下标表达式中用于访问对应类型元素的一个或多个索引（例如，在表达式 `object[i]` 中的 `i`）。尽管用于访问元素的索引可以是任意类型，但每个参数都必须包含一个类型注释，以指定每个索引的类型。*返回类型*指定了被访问元素的类型。 + +与计算属性一样，下标声明支持读取和写入所访问元素的值。getter 用于读取值，setter 用于写入值。setter 子句是可选的，当只需要 getter 时，可以省略两个子句，直接返回请求的值。也就是说，如果提供了 setter 子句，则必须同时提供 getter 子句。 + +*setter 名称*和括号是可选的。如果你提供了 setter 名称，它将用作 setter 的参数名称。如果你不提供 setter 名称，setter 的默认参数名称是 `value`。setter 的参数类型与*返回类型*相同。 + +你可以在声明其类型的地方重载下标声明，只要*参数*或*返回类型*与要重载的下标不同。你也可以重写从超类继承的下标声明。在这样做时，必须使用 `override` 声明修饰符标记被重写的下标声明。 + +下标参数遵循与函数参数相同的规则，但有两个例外。默认情况下，使用下标的参数没有参数标签，这与函数、方法和构造器不同。然而，你可以使用与函数、方法和构造器相同的语法提供显式参数标签。此外，下标不能有 in-out 参数。下标参数可以具有默认值，参见 。 + +你还可以在协议声明的上下文中声明下标，参见 。 + +有关下标的更多信息以及下标声明的示例，请参见 。 + +### 类型下标声明 + +要声明由类型本身而非类型实例公开的下标，可以在下标声明中使用 `static` 声明修饰符。类可以使用 `class` 声明修饰符来标记类型计算属性，以允许子类重写超类的实现。在类声明中，`static` 关键字的效果与将声明标记为 `class` 和 `final` 声明修饰符相同。 + + + +> 下标声明的语法: +> +> *subscript-declaration* → *subscript-head* *subscript-result* *generic-where-clause*_?_ *code-block* \ +> *subscript-declaration* → *subscript-head* *subscript-result* *generic-where-clause*_?_ *getter-setter-block* \ +> *subscript-declaration* → *subscript-head* *subscript-result* *generic-where-clause*_?_ *getter-setter-keyword-block* \ +> *subscript-head* → *attributes*_?_ *declaration-modifiers*_?_ **`subscript`** *generic-parameter-clause*_?_ *parameter-clause* \ +> *subscript-result* → **`->`** *attributes*_?_ *type* + +## 宏声明 + +*宏声明*引入一个新的宏。它以 `macro` 关键字开始，具有以下形式： + +```swift +macro <#name#> = <#macro implementation#> +``` + +*宏实现*是另一个宏，用于指示执行此宏扩展的代码位置。执行宏扩展的代码是一个独立的 Swift 程序，该程序使用 [SwiftSyntax][] 模块与 Swift 代码进行交互。调用 Swift 标准库中的 `externalMacro(module:type:)` 宏，并传入包含宏实现的类型名称以及包含该类型的模块名称。 + +[SwiftSyntax]: http://github.com/apple/swift-syntax/ + +宏可以被重载，遵循与函数相同的模型。宏声明仅在文件作用域内出现。 + +有关 Swift 中宏的概述，请参见 。 + +> 宏声明的语法: +> +> *macro-declaration* → *macro-head* *identifier* *generic-parameter-clause*_?_ *macro-signature* *macro-definition*_?_ *generic-where-clause* \ +> *macro-head* → *attributes*_?_ *declaration-modifiers*_?_ **`macro`** \ +> *macro-signature* → *parameter-clause* *macro-function-signature-result*_?_ \ +> *macro-function-signature-result* → **`->`** *type* \ +> *macro-definition* → **`=`** *expression* + +## 操作符声明 + +*运算符声明*将新的中缀、前缀或后缀运算符引入到你的程序中，并使用 `operator` 关键字进行声明。 + +你可以声明三种不同优先级的运算符：中缀、前缀和后缀。运算符的*优先级*指定了运算符相对于其操作数的相对位置。 + +运算符声明有三种基本形式，每种形式对应一种结合性。运算符的结合性通过在 `operator` 关键字之前标注 `infix`、`prefix` 或 `postfix` 声明修饰符来指定。在每种形式中，运算符的名称只能包含 中定义的运算符字符。 + +以下形式声明了一个新的中缀运算符： + +```swift +infix operator <#operator name#>: <#precedence group#> +``` + +*中缀运算符*是一个二元运算符，它写在两个操作数之间，例如在表达式 `1 + 2` 中熟悉的加法运算符`+`。 + +中缀运算符可以选择性地指定优先级组。如果你省略运算符的优先级组，Swift 将使用默认优先级组 `DefaultPrecedence`，该组的优先级仅高于 `TernaryPrecedence`。有关更多信息，请参见 。 + +以下形式声明了一个新的前缀运算符： + +```swift +prefix operator <#operator name#> +``` + +*前缀运算符*是一种一元运算符，它直接写在操作数之前，例如表达式 `!a` 中的前缀逻辑非运算符（`!`）。 + +前缀运算符声明不指定优先级。前缀运算符是非结合的。 + +以下形式声明了一个新的后缀运算符： + +```swift +postfix operator <#operator name#> +``` + +*后缀运算符*是一种一元运算符，它紧跟在操作数后面，例如在表达式 `a!` 中的后缀强制解包运算符 `!`。 + +与前缀运算符一样，后缀运算符声明不指定优先级。后缀运算符是非结合的。 + +在声明一个新运算符后，你通过声明一个与运算符同名的静态方法来实现它。这个静态方法是运算符作为参数所接受的类型之一的成员——例如，一个将 `Double` 乘以 `Int` 的运算符是作为 `Double` 或 `Int` 结构上的静态方法实现的。如果你在实现前缀或后缀运算符，你还必须在方法声明中标记相应的 `prefix` 或 `postfix` 声明修饰符。要查看如何创建和实现新运算符的示例，请参见 。 + +> 操作符声明的语法: +> +> *operator-declaration* → *prefix-operator-declaration* | *postfix-operator-declaration* | *infix-operator-declaration* +> +> *prefix-operator-declaration* → **`prefix`** **`operator`** *operator* \ +> *postfix-operator-declaration* → **`postfix`** **`operator`** *operator* \ +> *infix-operator-declaration* → **`infix`** **`operator`** *operator* *infix-operator-group*_?_ +> +> *infix-operator-group* → **`:`** *precedence-group-name* + +## 优先级组声明 + +*优先级组声明*在程序中引入了一个新的中缀运算符优先级分组。运算符的优先级指定了在没有分组括号的情况下，运算符与其操作数的绑定紧密程度。 + +优先级组声明具有以下形式： + +```swift +precedencegroup <#precedence group name#> { + higherThan: <#lower group names#> + lowerThan: <#higher group names#> + associativity: <#associativity#> + assignment: <#assignment#> +} +``` + +*低级组名称*和*高级组名称*列表指定了新优先级组与现有优先级组的关系。`lowerThan` 优先级组属性只能用于引用当前模块外声明的优先级组。当两个运算符争夺其操作数时，如在表达式 `2 + 3 * 5` 中，具有较高相对优先级的运算符会更紧密地绑定到其操作数上。 + +> 注意: +> 使用*低级组名称*和*高级组名称*相关联的优先级组必须适合于单一的关系层次结构，但它们*不*必形成线性层次结构。这意味着可以有相对优先级未定义的优先级组。来自这些优先级组的运算符不能在没有分组括号的情况下相互使用。 + +Swift 定义了许多优先级组，以配合 Swift 标准库提供的运算符。例如，加法 (`+`) 和减法 (`-`) 运算符属于 `AdditionPrecedence` 组，而乘法 (`*`) 和除法 (`/`) 运算符属于 `MultiplicationPrecedence` 组。有关 Swift 标准库提供的优先级组的完整列表，请参见[运算符声明](https://developer.apple.com/documentation/swift/operator_declarations)。 + +运算符的*结合性*指定了在没有分组括号的情况下，具有相同优先级的运算符序列是如何分组的。通过写入上下文敏感的关键字之一来指定运算符的结合性：`left`、`right` 或 `none` ——如果你省略结合性，默认值为 `none`。左结合的运算符从左到右分组。例如，减法运算符（`-`）是左结合的，因此表达式`4 - 5 - 6` 被分组为 `(4 - 5) - 6`，并计算为 `-7`。右结合的运算符从右到左分组，而指定为 `none` 的运算符则完全不结合。相同优先级的非结合运算符不能相邻出现。例如，`<` 运算符的结合性为 `none`，这意味着 `1 < 2 < 3` 不是一个有效的表达式。 + +*赋值*优先级组的设置指定了运算符在包含可选链操作中的优先级。当设置为 `true` 时，对应优先级组中的运算符在可选链操作期间使用与 Swift 标准库中的赋值运算符相同的分组规则。否则，当设置为 `false` 或省略时，该优先级组中的运算符将遵循与不执行赋值的运算符相同的可选链规则。 + +> 优先级组声明的语法: +> +> *precedence-group-declaration* → **`precedencegroup`** *precedence-group-name* **`{`** *precedence-group-attributes*_?_ **`}`** +> +> *precedence-group-attributes* → *precedence-group-attribute* *precedence-group-attributes*_?_ \ +> *precedence-group-attribute* → *precedence-group-relation* \ +> *precedence-group-attribute* → *precedence-group-assignment* \ +> *precedence-group-attribute* → *precedence-group-associativity* +> +> *precedence-group-relation* → **`higherThan`** **`:`** *precedence-group-names* \ +> *precedence-group-relation* → **`lowerThan`** **`:`** *precedence-group-names* +> +> *precedence-group-assignment* → **`assignment`** **`:`** *boolean-literal* +> +> *precedence-group-associativity* → **`associativity`** **`:`** **`left`** \ +> *precedence-group-associativity* → **`associativity`** **`:`** **`right`** \ +> *precedence-group-associativity* → **`associativity`** **`:`** **`none`** +> +> *precedence-group-names* → *precedence-group-name* | *precedence-group-name* **`,`** *precedence-group-names* \ +> *precedence-group-name* → *identifier* + +## 声明修饰符 + +*声明修饰符* 是用于修改声明行为或意义的关键字或上下文相关关键字。你通过在声明的属性（如果有的话）和引入声明的关键字之间写入相应的关键字或上下文相关关键字来指定声明修饰符。 + +- `class`：将此修饰符应用于类的成员，以指示该成员是类本身的成员，而不是类实例的成员。具有此修饰符且没有 `final` 修饰符的超类成员可以被子类重写。 + +- `dynamic`：将此修饰符应用于可以用 Objective-C 表示的类的任何成员。当你使用 `dynamic` 修饰符标记成员声明时，对该成员的访问始终通过 Objective-C 运行时动态分派。对该成员的访问永远不会被编译器内联或去虚拟化。 + +因为带有 `dynamic` 修饰符的声明是通过 Objective-C 运行时进行调度的，因此它们必须标记为 `objc` 属性。 + +- `final`：将此修饰符应用于类或类的属性、方法或下标成员。它应用于类以指示该类不能被子类化。它应用于类的属性、方法或下标，以指示类成员在任何子类中不能被重写。有关如何使用 `final` 属性的示例，请参见 。 + +- `lazy`：将此修饰符应用于类或结构体的存储变量属性，以指示该属性的初始值在第一次访问该属性时最多计算并存储一次。有关如何使用 `lazy` 修饰符的示例，请参见 。 + +- `optional`：将此修饰符应用于协议的属性、方法或下标成员，表示实现该协议的类型不必实现这些成员。 + +你只能将 `optional` 修饰符应用于带有 `objc` 属性的协议。因此，只有类类型可以采用并遵循包含可选成员要求的协议。有关如何使用 `optional` 修饰符的更多信息，以及在不确定遵循类型是否实现了这些成员时如何访问可选协议成员的指导，请参见 。 + + + +- `required`：将此修饰符应用于类的指定或便利构造器，以指示每个子类必须实现该构造器。子类对该构造器的实现也必须标记为 `required` 修饰符。 + +- `static`：将此修饰符应用于结构体、类、枚举或协议的成员，以指示该成员属于类型本身，而不是该类型实例的成员。在类声明的作用域内，将 `static` 修饰符应用于成员声明上，与在该成员声明上写 `class` 和 `final` 修饰符具有相同的效果。然而，类的常量类型属性是一个例外：在这种情况下，`static` 具有其通常的、非类相关的含义，因为在这些声明上不能使用 `class` 或 `final`。 + +- `unowned`：将此修饰符应用于存储变量、常量或存储属性，以指示该变量或属性对作为其值存储的对象具有一个无主引用。如果在对象被释放后尝试访问该变量或属性，将会引发运行时错误。与弱引用类似，属性或值的类型必须是类类型；与弱引用不同，类型是非可选的。有关 `unowned` 修饰符的示例和更多信息，请参见 。 + +- `unowned(safe)`：`unowned` 的显式拼写。 + +- `unowned(unsafe)`：将此修饰符应用于存储变量、常量或存储属性，以指示该变量或属性对作为其值存储的对象具有一个无主引用。如果在对象被释放后尝试访问该变量或属性，你将访问对象曾经所在位置的内存，这是一种不安全的内存操作。与弱引用类似，属性或值的类型必须是类类型；与弱引用不同，该类型是非可选的。有关 `unowned` 修饰符的示例和更多信息，请参见 。 + +- `weak`：将此修饰符应用于存储变量或存储变量属性，以指示该变量或属性对作为其值存储的对象具有弱引用。变量或属性的类型必须是可选类类型。如果在对象被释放后访问该变量或属性，其值为 `nil`。有关 `weak` 修饰符的示例和更多信息，请参见 。 + +### 访问控制级别 + +Swift 提供五种访问控制级别：open、public、internal、file private 和 private。你可以使用以下访问级别修饰符之一标记声明，以指定声明的访问级别。访问控制的详细信息请参见 。 + +- `open`：将此修饰符应用于声明，以指示该声明可以被与该声明位于同一模块中的代码访问和子类化。标记为 `open` 访问级别修饰符的声明也可以被导入包含该声明的模块的模块中的代码访问和子类化。 + +- `public`：将此修饰符应用于声明，以指示该声明可以被与该声明位于同一模块中的代码访问和子类化。标记为 `public` 访问级别修饰符的声明也可以被导入包含该声明的模块的模块中的代码访问（但不能被子类化）。 + +- `package`：将此修饰符应用于声明，以指示该声明只能被与声明在同一包中的代码访问。包是你在使用的构建系统中定义的代码分发单元。当构建系统编译代码时，它通过将 `-package-name` 标志传递给 Swift 编译器来指定包名称。如果构建系统在构建它们时指定相同的包名称，则两个模块属于同一个包。 + +- `internal`：将此修饰符应用于声明，以指示该声明只能被与声明在同一模块中的代码访问。默认情况下，大多数声明隐式标记为 `internal` 访问级别修饰符。 + +- `fileprivate`：将此修饰符应用于声明，以指示该声明只能被与声明在同一源文件中的代码访问。 + +- `private`：将此修饰符应用于声明，以指示该声明只能被声明的直接封闭作用域内的代码访问。 + +出于访问控制的目的，扩展的行为如下： + +- 如果同一个文件中有多个扩展，并且这些扩展都扩展了相同的类型，那么所有这些扩展具有相同的访问控制作用域。这些扩展和它们扩展的类型可以在不同的文件中。 + +- 如果扩展与其扩展的类型在同一文件中，则扩展具有与其扩展的类型相同的访问控制作用域。 + +- 在类型声明中声明的私有成员可以从该类型的扩展中访问。在一个扩展中声明的私有成员可以从其他扩展和扩展类型的声明中访问。 + +每个上述访问级别修饰符可选择性地接受一个参数，该参数由括号中包含的 `set` 关键字组成——例如，`private(set)`。当你想要为变量或下标的setter指定一个小于或等于变量或下标本身的访问级别时，请使用这种形式的访问级别修饰符，如 中所讨论的。 + +> 声明修饰语的语法: +> +> *declaration-modifier* → **`class`** | **`convenience`** | **`dynamic`** | **`final`** | **`infix`** | **`lazy`** | **`optional`** | **`override`** | **`postfix`** | **`prefix`** | **`required`** | **`static`** | **`unowned`** | **`unowned`** **`(`** **`safe`** **`)`** | **`unowned`** **`(`** **`unsafe`** **`)`** | **`weak`** \ +> *declaration-modifier* → *access-level-modifier* \ +> *declaration-modifier* → *mutation-modifier* \ +> *declaration-modifier* → *actor-isolation-modifier* \ +> *declaration-modifiers* → *declaration-modifier* *declaration-modifiers*_?_ +> +> *access-level-modifier* → **`private`** | **`private`** **`(`** **`set`** **`)`** \ +> *access-level-modifier* → **`fileprivate`** | **`fileprivate`** **`(`** **`set`** **`)`** \ +> *access-level-modifier* → **`internal`** | **`internal`** **`(`** **`set`** **`)`** \ +> *access-level-modifier* → **`package`** | **`package`** **`(`** **`set`** **`)`** \ +> *access-level-modifier* → **`public`** | **`public`** **`(`** **`set`** **`)`** \ +> *access-level-modifier* → **`open`** | **`open`** **`(`** **`set`** **`)`** +> +> *mutation-modifier* → **`mutating`** | **`nonmutating`** +> +> *actor-isolation-modifier* → **`nonisolated`** + + diff --git a/swift-6.docc/ReferenceManual/Expressions.md b/swift-6.docc/ReferenceManual/Expressions.md new file mode 100644 index 000000000..b6e57f47e --- /dev/null +++ b/swift-6.docc/ReferenceManual/Expressions.md @@ -0,0 +1,1397 @@ +# 表达式 + +访问、修改和分配值。 + +Swift 中存在四种表达式：前缀表达式，中缀表达式，基本表达式和后缀表达式。表达式在返回一个值的同时还可以引发副作用。 + +通过前缀表达式和中缀表达式可以对简单表达式使用各种运算符。基本表达式从概念上讲是最简单的一种表达式，它是一种访问值的方式。后缀表达式则允许你建立复杂的表达式，例如函数调用和成员访问。每种表达式都在下面有详细论述。 + +> 表达式语法： +> +> *expression* → *try-operator*_?_ *await-operator*_?_ *prefix-expression* *infix-expressions*_?_ \ + +## 前缀表达式 + +前缀表达式由可选的前缀运算符和表达式组成。前缀运算符只接收一个参数，表达式则紧随其后。 + +关于这些运算符的更多信息，请参阅 和 . + +关于 Swift 标准库提供的运算符的更多信息，请参阅 [Operator Declarations](https://developer.apple.com/documentation/swift/operator_declarations). + +> 前缀表达式语法 +> +> *prefix-expression* → *prefix-operator*_?_ *postfix-expression* \ +> *prefix-expression* → *in-out-expression* + +### In-Out 表达式 + + *in-out表达式*将函数调用表达式传入的变量标记为输入输出实参。 + +```swift +&<#表达式#> +``` + +输入输出表达式也可以用于将非指针实参传入到需要指针的上下文中，如 . + +In-out 表达式也可以用于将非指针实参传入到需要指针的上下文中，如 . + +> Grammar of an in-out expression: +> +> *in-out-expression* → **`&`** *标识符* + +### Try 运算符 + +*try表达式*由 `try` 运算符加上紧随其后的可抛出错误的表达式组成，形式如下： + +```swift +try <#表达式#> +``` + +`try` 表达式的返回值是该表达式的值。 + +*可选 try* 表达式由 `try?` 运算符加上紧随其后的可抛出错误的表达式组成，形式如下： + +```swift +try? <#表达式#> +``` + +如果*表达式*没有抛出错误，可选 try 表达式的返回值是可选的该表达式的值，否则，返回值为 `nil`。 + +*强制 try 表达式*由 `try!` 运算符加上紧随其后的可抛出错误的表达式组成，形式如下： + +```swift +try! <#表达式#> +``` + +强制 try 表达式的返回值是该*表达式*的值。如果该*表达式*抛出了错误，将会引发运行时错误。 + +在中缀运算符左侧的表达式被标记上 `try`、`try?` 或者 `try!` 时，这个运算符对整个中缀表达式都产生作用。也就是说，你可以使用括号来明确运算符的作用范围。 + + + +```swift +// try 对两个函数调用都产生作用 +sum = try someThrowingFunction() + anotherThrowingFunction() + +// try 对两个函数调用都产生作用 +sum = try (someThrowingFunction() + anotherThrowingFunction()) + +// 错误：try 只对第一个函数调用产生作用 +sum = (try someThrowingFunction()) + anotherThrowingFunction() +``` + +`try` 表达式不能出现在中缀运算符的的右侧，除非中缀运算符是赋值运算符或者 `try` 表达式是被圆括号括起来的。 + +如果表达式中同时包含 `try` 和 `await` 运算符，`try` 运算符必须在前面。 + +更多关于 `try`、`try?` 和 `try!` 的信息，以及该如何使用的例子，请参阅 . + +> Try 表达式语法 +> +> *try运算符* → **`try`** | **`try`** **`?`** | **`try`** **`!`** + +### Await 运算符 + + *await表达式*由 await 运算符加上紧随其后的异步操作结果的表达式。形式如下： + +```swift +await <#表达式#> +``` + + `await` 表达式返回值就是该*表达式*的值。 + +被 `await` 标记的表达式被称为*潜在的暂停点*。 + +异步函数的执行可以在每个标记 `await` 的表达式的位置暂停。除此之外，并发代码的执行永远不会在其他位置暂停。这意味着在潜在暂停点之间的代码可以暂时打破不变量的状态进行安全更新，只要更新在下一个潜在暂停点之前完成。 + +await 表达式只能在异步的上下文中出现，比如传入 `async(priority:operation:)` 函数的尾随闭包中。它不能在 `defer` 语句的闭包中，或者在同步函数的自动闭包中出现。 + +在中缀运算符左侧的表达式被标记上 `await` 运算符时，这个运算符对整个中缀表达式都产生作用。也就是说，你可以使用括号来明确运算符的作用范围。 + + + +```swift +// await 对两个函数调用都产生作用 +sum = await someAsyncFunction() + anotherAsyncFunction() + +// await 对两个函数调用都产生作用 +sum = await (someAsyncFunction() + anotherAsyncFunction()) + +// 错误：await 只对第一个函数调用产生作用 +sum = (await someAsyncFunction()) + anotherAsyncFunction() +``` + +`await` 表达式不能出现在中缀运算符的的右侧，除非中缀运算符是赋值运算符或者 `await` 表达式是被圆括号括起来的。 + +如果表达式中同时包含 `try` 和 `await` 运算符，`try` 运算符必须在前面。 + + +> Await 表达式语法: +> +> *await运算符* → **`await`** + +## 中缀表达式 + +*中缀表达式*由中缀运算符和左右参数表达式组成。形式如下： + +```swift +<#left-左侧参数#> <#中缀运算符#> <#右侧参数t#> +``` + +关于这些运算符的更多信息，请参阅 和 . + +关于 Swift 标准库提供的运算符的更多信息，请参阅[Operator Declarations](https://developer.apple.com/documentation/swift/operator_declarations). + +> 注意 +> 在解析时，一个中缀表达式将作为一个扁平列表表示，然后根据运算符的优先级，再进一步进行组合。例如，> `2 + 3 * 5` 首先被看作具有五个元素的列表，即 `2`、`+`、`3`、`*`、`5`，随后根据运算符优先级组合为 `(2 + > (3 * 5))`。 + +> Grammar of an infix expression: +> +> *infix-expression* → *infix-operator* *prefix-expression* \ +> *infix-expression* → *assignment-operator* *try-operator*_?_ *await-operator*_?_ *prefix-expression* \ +> *infix-expression* → *conditional-operator* *try-operator*_?_ *await-operator*_?_ *prefix-expression* \ +> *infix-expression* → *type-casting-operator* \ +> *infix-expressions* → *infix-expression* *infix-expressions*_?_ + +### 赋值运算符 + + 赋值运算符会为某个给定的表达式赋值，形式如下； + +```swift +<#表达式#> = <#值#> +``` + +右边的值会被赋值给左边的表达式。如果左边表达式是一个元组，那么右边必须是一个具有同样元素个数的元组。（嵌套元组也是允许的。）右边的值中的每一部分都会被赋值给左边的表达式中的相应部分。例如： + +```swift +(a, _, (b, c)) = ("test", 9.45, (12, 3)) +// a 为 "test"，b 为 12，c 为 3，9.45 会被忽略 +``` + +赋值运算符不返回任何值。 + +> Grammar of an assignment operator: +> +> *assignment-operator* → **`=`** + +### 三元条件运算符 + +*三元条件运算符*会根据条件来对两个给定表达式中的一个进行求值，形式如下： + +```swift +<#条件#> ? <#表达式 (条件为真则使用)#> : <#表达式（条件为假则使用）#> +``` + +如果条件为真，那么对第一个表达式进行求值并返回结果。否则，对第二个表达式进行求值并返回结果。未使用的表达式不会进行求值。 + +关于使用三元条件运算符的例子，请参阅 . + +> Grammar of a conditional operator: +> +> *conditional-operator* → **`?`** *expression* **`:`** + +### 类型转换运算符 + +有 4 种类型转换运算符：is、as、as? 和 as!。它们有如下的形式： + +```swift +<#表达式#> is <#类型#> +<#表达式#> as <#类型#> +<#表达式#> as? <#类型#> +<#表达式#> as! <#类型#> +``` + +`is` 运算符在运行时检查表达式能否向下转化为指定的类型，如果可以则返回 `ture`，否则返回 false。 + +`as` 运算符在编译时执行向上转换和桥接。向上转换可将表达式转换成父类的实例而无需使用任何中间变量。以下表达式是等价的： + +```swift +func f(_ any: Any) { print("Function for Any") } +func f(_ int: Int) { print("Function for Int") } +let x = 10 +f(x) +// 打印 "Function for Int" + +let y: Any = x +f(y) +// 打印 "Function for Any" + +f(x as Any) +// 打印 "Function for Any" +``` + +桥接可将 Swift 标准库中的类型（例如 `String`）作为一个与之相关的 Foundation 类型（例如 `NSString`）来使用，而不需要新建一个实例。关于桥接的更多信息，请参阅[Working with Foundation Types](https://developer.apple.com/documentation/swift/imported_c_and_objective_c_apis/working_with_foundation_types). + +`as?` 运算符有条件地执行类型转换，返回目标类型的可选值。在运行时，如果转换成功，返回的可选值将包含转换后的值，否则返回 `nil`。如果在编译时就能确定转换一定会成功或是失败，则会导致编译报错。 + +`as!` 运算符执行强制类型转换，返回目标类型的非可选值。如果转换失败，则会导致运行时错误。表达式 `x as! T` 效果等同于 `(x as? T)!`。 + +关于类型转换的更多内容和例子，请参阅 . +要查看使用类型转换运算符的示例，请参阅 . + +> Grammar of a type-casting operator: +> +> *type-casting-operator* → **`is`** *type* \ +> *type-casting-operator* → **`as`** *type* \ +> *type-casting-operator* → **`as`** **`?`** *type* \ +> *type-casting-operator* → **`as`** **`!`** *type* + +## 基本表达式 + +*基本表达式是最基本的表达式。它们可以单独使用，也可以跟前缀表达式、中置表达式、后缀表达式组合使用。 + +> Grammar of a primary expression: +> +> *primary-expression* → *identifier* *generic-argument-clause*_?_ \ +> *primary-expression* → *literal-expression* \ +> *primary-expression* → *self-expression* \ +> *primary-expression* → *superclass-expression* \ +> *primary-expression* → *conditional-expression* \ +> *primary-expression* → *closure-expression* \ +> *primary-expression* → *parenthesized-expression* \ +> *primary-expression* → *tuple-expression* \ +> *primary-expression* → *implicit-member-expression* \ +> *primary-expression* → *wildcard-expression* \ +> *primary-expression* → *macro-expansion-expression* \ +> *primary-expression* → *key-path-expression* \ +> *primary-expression* → *selector-expression* \ +> *primary-expression* → *key-path-string-expression* + +### 字面量表达式 + +字面量表达式可由普通字面量（例如字符串或者数字），字典或者数组字面量，或者下面列表中的特殊字面量组成： + +> 注意： +> Prior to Swift 5.9, +> the following special literals were recognized: +> `#column`, +> `#dsohandle`, +> `#fileID`, +> `#filePath`, +> `#file`, +> `#function`, +> and `#line`. +> These are now implemented as macros in the Swift standard library: +> [`column()`](https://developer.apple.com/documentation/swift/column()), +> [`dsohandle()`](https://developer.apple.com/documentation/swift/dsohandle()), +> [`fileID()`](https://developer.apple.com/documentation/swift/fileID()), +> [`filePath()`](https://developer.apple.com/documentation/swift/filePath()), +> [`file()`](https://developer.apple.com/documentation/swift/file()), +> [`function()`](https://developer.apple.com/documentation/swift/function()), +> and [`line()`](https://developer.apple.com/documentation/swift/line()). + +数组字面量是值的有序集合，形式如下： + +```swift +[<#值 1#>, <#值 2#>, <#...#>] +``` + +数组中的最后一个表达式可以紧跟一个逗号。数组字面量的类型是 `[T]`，这个 `T` 就是数组中元素的类型。如果数组中包含多种类型，`T` 则是跟这些类型最近的的公共父类型。空数组字面量由一组方括号定义，可用来创建特定类型的空数组。 + +```swift +var emptyArray: [Double] = [] +``` + +字典字面量是一个包含无序键值对的集合，形式如下： + +```swift +[<#键 1#>: <#值 1#>, <#键 2#>: <#值 2#>, <#...#>] +``` + +字典中的最后一个表达式可以紧跟一个逗号。字典字面量的类型是 `[Key : Value]`，`Key` 表示键的类型，`Value` 表示值的类型。如果字典中包含多种类型，那么 `Key` 表示的类型则为所有键最接近的公共父类型，`Value` 与之相似。一个空的字典字面量由方括号中加一个冒号组成（`[:]`），从而与空数组字面量区分开，可以使用空字典字面量来创建特定类型的字典。 + +```swift +var emptyDictionary: [String: Double] = [:] +``` + +Xcode 使用 playground 字面量对程序编辑器中的颜色、文件或者图片创建可交互的展示。在 Xcode 之外的空白文本中，playground 字面量使用一种特殊的字面量语法来展示。 + +更多关于在 Xcode 中使用 playground 字面量的信息，请参阅[Add a color, file, or image literal](https://help.apple.com/xcode/mac/current/#/dev4c60242fc) +in Xcode Help. + +> Grammar of a literal expression: +> +> *literal-expression* → *literal* \ +> *literal-expression* → *array-literal* | *dictionary-literal* | *playground-literal* +> +> *array-literal* → **`[`** *array-literal-items*_?_ **`]`** \ +> *array-literal-items* → *array-literal-item* **`,`**_?_ | *array-literal-item* **`,`** *array-literal-items* \ +> *array-literal-item* → *expression* +> +> *dictionary-literal* → **`[`** *dictionary-literal-items* **`]`** | **`[`** **`:`** **`]`** \ +> *dictionary-literal-items* → *dictionary-literal-item* **`,`**_?_ | *dictionary-literal-item* **`,`** *dictionary-literal-items* \ +> *dictionary-literal-item* → *expression* **`:`** *expression* +> +> *playground-literal* → **`#colorLiteral`** **`(`** **`red`** **`:`** *expression* **`,`** **`green`** **`:`** *expression* **`,`** **`blue`** **`:`** *expression* **`,`** **`alpha`** **`:`** *expression* **`)`** \ +> *playground-literal* → **`#fileLiteral`** **`(`** **`resourceName`** **`:`** *expression* **`)`** \ +> *playground-literal* → **`#imageLiteral`** **`(`** **`resourceName`** **`:`** *expression* **`)`** + +### Self 表达式 + + `self` 表达式是对当前类型或者当前实例的显式引用，它有如下形式： + +```swift +self +self.<#成员名称#> +self[<#下标索引#>] +self(<#初始化器参数#>) +self.init(<#初始化器参数#>) +``` + +如果在构造器、下标、实例方法中，`self` 引用的是当前类型的实例。在一个类型方法中，`self` 引用的是当前的类型。 + +当访问成员时，`self` 可用来区分重名变量，例如函数的参数： + +```swift +class SomeClass { + var greeting: String + init(greeting: String) { + self.greeting = greeting + } +} +``` + +在 `mutating` 方法中，你可以对 `self` 重新赋值： + +```swift +struct Point { + var x = 0.0, y = 0.0 + mutating func moveBy(x deltaX: Double, y deltaY: Double) { + self = Point(x: x + deltaX, y: y + deltaY) + } +} +``` + +> Grammar of a self expression: +> +> *self-expression* → **`self`** | *self-method-expression* | *self-subscript-expression* | *self-initializer-expression* +> +> *self-method-expression* → **`self`** **`.`** *identifier* \ +> *self-subscript-expression* → **`self`** **`[`** *function-call-argument-list* **`]`** \ +> *self-initializer-expression* → **`self`** **`.`** **`init`** + +### 父类表达式 + +*父类表达式*可以使我们在某个类中访问它的父类，它有如下形式： + +```swift +super.<#成员名称#> +super[<#下标索引#>] +super.init(<#初始化器参数#>) +``` + +第一种形式用来访问父类的某个成员，第二种形式用来访问父类的下标，第三种形式用来访问父类的构造器。 + +子类可以通过父类表达式在它们的成员、下标和构造器中使用父类中的实现。 + +> Grammar of a superclass expression: +> +> *superclass-expression* → *superclass-method-expression* | *superclass-subscript-expression* | *superclass-initializer-expression* +> +> *superclass-method-expression* → **`super`** **`.`** *identifier* \ +> *superclass-subscript-expression* → **`super`** **`[`** *function-call-argument-list* **`]`** \ +> *superclass-initializer-expression* → **`super`** **`.`** **`init`** + +### 闭包表达式 + +*闭包表达式*会创建一个闭包，在其他语言中也叫 *lambda* 或匿名函数。跟函数一样，闭包包含了待执行的代码，不同的是闭包还会捕获所在环境中的常量和变量。它的形式如下： + +```swift +if <#condition 1#> { + <#expression used if condition 1 is true#> +} else if <#condition 2#> { + <#expression used if condition 2 is true#> +} else { + <#expression used if both conditions are false#> +} + +switch <#expression#> { +case <#pattern 1#>: + <#expression 1#> +case <#pattern 2#> where <#condition#>: + <#expression 2#> +default: + <#expression 3#> +} +``` + +如果闭包的主体中含有 try 表达式，则认为该闭包会引发异常。同理，若闭包主体含有 await 表达式，则认为该闭包是异步的。 + +闭包还有几种特殊的形式，能让闭包使用起来更加简洁： + + - 作为变量赋值的值。 + - 作为变量或常量声明的初始值。 + - 作为 `throw` 表达式引发的错误。 + - 作为函数、闭包或属性 getter 的返回值。 + - 作为条件表达式分支中的值。 + +条件表达式的分支是穷尽的，确保表达式总是会返回一个值，无论条件如何。这意味着每个 `if` 分支都需要一个对应的 `else` 分支。 + +无论条件如何，这意味着每个 `if` 分支都需要一个对应的 `else` 分支。 + +每个分支包含一个单独的表达式，该表达式在该分支的条件为真时用作条件表达式的值，或者是一个 `throw` 语句，或是一个永不返回的函数调用。 + +每个分支必须产生相同类型的值。由于每个分支的类型检查是独立的，有时你需要明确指定值的类型，例如当分支包含不同类型的字面量，或者当某个分支的值为 `nil` 时。当你需要提供这些信息时，可以在结果赋值的变量上添加类型注解，或者在分支值上添加 `as` 类型转换。 + +```swift +let number: Double = if someCondition { 10 } else { 12.34 } +let number = if someCondition { 10 as Double } else { 12.34 } +``` + +在结果构建器内，条件表达式只能作为变量或常量的初始值出现。这种行为意味着，当你在结果构建器中编写 `if` 或 `switch` —— 在变量或常量声明之外 —— 这段代码被理解为分支语句，并且结果构建器的一个方法会对这段代码进行转换。 + +即使条件表达式的某个分支会抛出异常，也不要将条件表达式放入 try 表达式中。 + +> Grammar of a conditional expression: +> +> *conditional-expression* → *if-expression* | *switch-expression* +> +> *if-expression* → **`if`** *condition-list* **`{`** *statement* **`}`** *if-expression-tail* \ +> *if-expression-tail* → **`else`** *if-expression* \ +> *if-expression-tail* → **`else`** **`{`** *statement* **`}`** +> +> *switch-expression* → **`switch`** *expression* **`{`** *switch-expression-cases* **`}`** \ +> *switch-expression-cases* → *switch-expression-case* *switch-expression-cases*_?_ \ +> *switch-expression-case* → *case-label* *statement* \ +> *switch-expression-case* → *default-label* *statement* + +### 闭包表达式 + +*闭包表达式*会创建一个闭包，在其他语言中也叫 *lambda* 或匿名函数。跟函数一样，闭包包含了待执行的代码，不同的是闭包还会捕获所在环境中的常量和变量。它的形式如下： + +```swift +{ (<#parameters#>) -> <#return type#> in + <#statements#> +} +``` + +闭包的参数声明形式跟函数一样，请参阅 . + +在闭包表达式中写入 `throws` 或 `async` 将显式地将闭包标记为丢掷或异步的。 + +```swift +{ (<#parameters#>) async throws -> <#return type#> in + <#statements#> +} +``` + +如果闭包的主体中含有 try 表达式，则认为该闭包会引发异常。同理，若闭包主体含有 await 表达式，则认为该闭包是异步的。 + +闭包还有几种特殊的形式，能让闭包使用起来更加简洁： + +- 闭包可以省略它的参数和返回值的类型。如果省略了参数名和所有的类型，也要省略 `in` 关键字。如果被省略的类型无法被编译器推断，那么就会导致编译错误。 + +- 闭包可以省略参数名，参数会被隐式命名为 `$` 加上其索引位置，例如 `$0`、`$1`、`$2` 分别表示第一个、第二个、第三个参数，以此类推。 + +- 如果闭包中只包含一个表达式，那么该表达式的结果就会被视为闭包的返回值。表达式结果的类型也会被推断为闭包的返回类型。 + +下面几个闭包表达式是等价的： + +```swift +myFunction { (x: Int, y: Int) -> Int in + return x + y +} + +myFunction { x, y in + return x + y +} + +myFunction { return $0 + $1 } + +myFunction { $0 + $1 } +``` + +关于如何将闭包作为参数来传递的内容，请参阅 . + +使用闭包表达式时，可以不必将其存储在一个变量或常量中，例如作为函数调用的一部分来立即使用一个闭包。在上面的例子中，传入 `myFunction` 的闭包表达式就是这种立即使用类型的闭包。因此，一个闭包是否逃逸与其使用时的上下文相关。一个会被立即调用或者作为函数的非逃逸参数传递的闭包表达式是非逃逸的，否则，这个闭包表达式是逃逸的。 + +关于逃逸闭包的内容，请参阅. + +#### 捕获列表 + +默认情况下，闭包会捕获附近作用域中的常量和变量，并使用强引用指向它们。你可以通过一个*捕获列表*来显式指定它的捕获行为。 + +捕获列表在参数列表之前，由中括号括起来，里面是由逗号分隔的一系列表达式。一旦使用了捕获列表，就必须使用 `in` 关键字，即使省略了参数名、参数类型和返回类型。 + +捕获列表中的项会在闭包创建时被初始化。每一项都会用闭包附近作用域中的同名常量或者变量的值初始化。例如下面的代码示例中，捕获列表包含 `a` 而不包含 `b`，这将导致这两个变量具有不同的行为。 + +```swift +var a = 0 +var b = 0 +let closure = { [a] in + print(a, b) +} + +a = 10 +b = 10 +closure() +// 打印 "0 10" +``` + +在示例中，变量 `b` 只有一个，然而，变量 `a` 有两个，一个在闭包外，一个在闭包内。闭包内的变量 `a` 会在闭包创建时用闭包外的变量 `a` 的值来初始化，除此之外它们并无其他联系。这意味着在闭包创建后，改变某个 `a` 的值都不会对另一个 `a` 的值造成任何影响。与此相反，闭包内外都是同一个变量 `b`，因此在闭包外改变其值，闭包内的值也会受影响。 + +如果闭包捕获的值具有引用语义则有所不同。例如，下面示例中，有两个变量 `x`，一个在闭包外，一个在闭包内，由于它们的值是引用语义，虽然这是两个不同的变量，它们却都引用着同一实例。 + +```swift +class SimpleClass { + var value: Int = 0 +} +var x = SimpleClass() +var y = SimpleClass() +let closure = { [x] in + print(x.value, y.value) +} + +x.value = 10 +y.value = 10 +closure() +// 打印 "10 10" +``` + +如果捕获列表中的值是类类型，你可以使用 `weak` 或者 `unowned` 来修饰它，闭包会分别用弱引用和无主引用来捕获该值。 + +```swift +myFunction { print(self.title) } // implicit strong capture +myFunction { [self] in print(self.title) } // explicit strong capture +myFunction { [weak self] in print(self!.title) } // weak capture +myFunction { [unowned self] in print(self.title) } // unowned capture +``` + +在捕获列表中，也可以将任意表达式的值绑定到一个常量上。该表达式会在闭包被创建时进行求值，闭包会按照指定的引用类型来捕获表达式的值。例如： + +```swift +// 以弱引用捕获 self.parent 并赋值给 parent +myFunction { [weak parent = self.parent] in print(parent!.title) } +``` + +关于闭包表达式的更多信息和例子，请参阅 . + +关于捕获列表的更多信息和例子，请参阅 . + +> Grammar of a closure expression: +> +> *closure-expression* → **`{`** *attributes*_?_ *closure-signature*_?_ *statements*_?_ **`}`** +> +> *closure-signature* → *capture-list*_?_ *closure-parameter-clause* **`async`**_?_ *throws-clause*_?_ *function-result*_?_ **`in`** \ +> *closure-signature* → *capture-list* **`in`** +> +> *closure-parameter-clause* → **`(`** **`)`** | **`(`** *closure-parameter-list* **`)`** | *identifier-list* \ +> *closure-parameter-list* → *closure-parameter* | *closure-parameter* **`,`** *closure-parameter-list* \ +> *closure-parameter* → *closure-parameter-name* *type-annotation*_?_ \ +> *closure-parameter* → *closure-parameter-name* *type-annotation* **`...`** \ +> *closure-parameter-name* → *identifier* +> +> *capture-list* → **`[`** *capture-list-items* **`]`** \ +> *capture-list-items* → *capture-list-item* | *capture-list-item* **`,`** *capture-list-items* \ +> *capture-list-item* → *capture-specifier*_?_ *identifier* \ +> *capture-list-item* → *capture-specifier*_?_ *identifier* **`=`** *expression* \ +> *capture-list-item* → *capture-specifier*_?_ *self-expression* \ +> *capture-specifier* → **`weak`** | **`unowned`** | **`unowned(safe)`** | **`unowned(unsafe)`** + +### 隐式成员表达式 + +*若类型*可被推断出来，可以使用*隐式成员表达式*来访问某个类型的成员（例如某个枚举成员或某个类型方法），形式如下： + +```swift +.<#成员名称#> +``` + +例如： + +```swift +var x = MyEnumeration.someValue +x = .anotherValue +``` + +如果推断的是可选类型，可以在隐式成员表达式里使用不可选类型的成员。 + +```swift +var someOptional: MyEnumeration? = .someValue +``` + +隐式成员表达式可以跟在后缀运算符或者其他在里介绍的语法后面。这被称为 链式隐式成员表达式。尽管链式后缀表达式大多都是相同类型，但其实只需要整个链式成员表达式可以转换为上下文的类型就行了。更具体的，如果隐式类型是可选的，则可以使用非可选类型的值，如果隐式类型是类类型，则可以使用其子类的值。例如： + +```swift +class SomeClass { + static var shared = SomeClass() + static var sharedSubclass = SomeSubclass() + var a = AnotherClass() +} +class SomeSubclass: SomeClass { } +class AnotherClass { + static var s = SomeClass() + func f() -> SomeClass { return AnotherClass.s } +} +let x: SomeClass = .shared.a.f() +let y: SomeClass? = .shared +let z: SomeClass = .sharedSubclass +``` +上面的代码中，`x` 的类型和上下文的隐式类型完全匹配，`y` 的类型是从 `SomeClass` 转换成 `SomeClass?`，`z` 的类型是从 `SomeSubclass` 转换成 `SomeClass`。 + +> Grammar of an implicit member expression: +> +> *implicit-member-expression* → **`.`** *identifier* \ +> *implicit-member-expression* → **`.`** *identifier* **`.`** *postfix-expression* + +### 圆括号表达式 + +圆括号表达式是由圆括号包围的表达式。你可以用圆括号说明成组的表达式的先后操作。成组的圆括号不会改变表达式的类型 - 例如 `(1)` 的类型就是简单的 `Int`。 + +> Grammar of a parenthesized expression: +> +> *parenthesized-expression* → **`(`** *expression* **`)`** + +### 元组表达式 + +元组表达式由圆括号和其中多个逗号分隔的子表达式组成。每个子表达式前面可以有一个标识符，用冒号隔开。元组表达式形式如下： + +```swift +(<#标识符 1#>: <#表达式 1#>, <#标识符 2#>: <#表达式 2#>, <#...#>) +``` + +元组表达式里的每一个标识符在表达式作用域里必须是唯一的。在嵌套的元组表达式中，同嵌套层级里的标识符也必须是唯一的。例如，`(a: 10, a: 20)` 是不合法的，因为标签 `a` 在同一层级出现了两次。然而，`(a: 10, b: (a: 1, x: 2))` 是合法的，尽管 `a` 出现了两次，但有一次在外层元组里，一次在内层元组里。 + +元组表达式可以一个表达式都没有，也可以包含两个或是更多的表达式。单个表达式用括号括起来就是括号表达式了。 + +> Note: Both an empty tuple expression and an empty tuple type +> are written `()` in Swift. +> Because `Void` is a type alias for `()`, +> you can use it to write an empty tuple type. +> However, like all type aliases, `Void` is always a type --- +> you can't use it to write an empty tuple expression. + +> Grammar of a tuple expression: +> +> *tuple-expression* → **`(`** **`)`** | **`(`** *tuple-element* **`,`** *tuple-element-list* **`)`** \ +> *tuple-element-list* → *tuple-element* | *tuple-element* **`,`** *tuple-element-list* \ +> *tuple-element* → *expression* | *identifier* **`:`** *expression* + +### 通配符表达式 + +*通配符表达式*可以在赋值过程中显式忽略某个值。例如下面的代码中，`10` 被赋值给 `x`，而 `20` 则被忽略： + +```swift +(x, _) = (10, 20) +// x 为 10，20 被忽略 +``` + +> Grammar of a wildcard expression: +> +> *wildcard-expression* → **`_`** + +### 宏扩展表达式 + +*宏扩展表达式*由一个宏名称和一个用逗号分隔的宏参数列表（在括号内）组成。宏在编译时被扩展。宏扩展表达式的形式如下： + +```swift +<#macro name#>(<#macro argument 1#>, <#macro argument 2#>) +``` + +如果宏不接受任何参数，宏扩展表达式可以省略宏名称后面的括号。 + +宏扩展表达式可以作为参数的默认值。当作为函数或方法参数的默认值使用时,宏会使用调用位置的源代码位置进行求值,而不是它们在函数定义中出现的位置。然而,当默认值是一个包含宏和其他代码的较大表达式时,这些宏会在函数定义的位置进行求值。 + +```swift +func f(a: Int = #line, b: Int = (#line), c: Int = 100 + #line) { + print(a, b, c) +} +f() // 打印 "4 1 101" +``` + +在上面的函数中,参数 `a` 的默认值是一个单独的宏表达式,因此该宏使用 `f(a:b:c:)` 被调用的源代码位置进行求值。相比之下,参数 `b` 和 `c` 的值是包含宏的表达式 —— 这些表达式中的宏使用 `f(a:b:c:)` 被定义的源代码位置进行求值。 + +当你使用宏作为默认值时,会在不展开宏的情况下进行类型检查,以检查以下要求: + +- 宏的访问级别与使用它的函数相同或限制更少。 +- 宏要么不接受参数,要么其参数是不带字符串插值的字面量。 +- 宏的返回类型与参数的类型匹配。 + +你使用宏表达式来调用独立宏。要调用附加宏，请使用在 . +中描述的自定义属性语法。独立宏和附加宏的扩展方式如下： + +1. Swift 解析源代码以生成抽象语法树（AST）。 + +2. 宏实现接收 AST 节点作为输入，并执行该宏所需的转换。 + +3. 宏实现生成的转换后 AST 节点会被添加到原始 AST 中。 + +每个宏的扩展是独立且自包含的。然而，为了优化性能，Swift 可能会启动一个外部进程来实现宏，并重用同一个进程来扩展多个宏。当你实现一个宏时，该代码不能依赖于你的代码之前扩展过的宏，也不能依赖于任何其他外部状态，例如当前时间。 + +对于嵌套宏和具有多个角色的附加宏，扩展过程会重复。嵌套的宏扩展表达式是从外到内进行扩展的。例如，在下面的代码中，`outerMacro(_:)` 首先被扩展，未扩展的对 `innerMacro(_:)` 的调用出现在 `outerMacro(_:)` 作为输入接收的抽象语法树中。 + +```swift +#outerMacro(12, #innerMacro(34), "some text") +``` + +一个具有多个角色的附加宏会针对每个角色进行一次扩展。每次扩展都接收相同的原始 AST 作为输入。Swift 通过收集所有生成的 AST 节点，并将它们放入 AST 中相应的位置，形成整体扩展。 + +有关 Swift 中宏的概述 请参阅 。 + +> Grammar of a macro-expansion expression: +> +> *macro-expansion-expression* → **`#`** *identifier* *generic-argument-clause*_?_ *function-call-argument-clause*_?_ *trailing-closures*_?_ + +### Key-Path 表达式 + +Key-path 表达式引用一个类型的属性或下标。在动态语言中使场景可以使用 Key-path 表达式，例如观察键值对。格式为： + +```swift +\<#类型名#>.<#路径#> +``` + +*类型名*是一个具体类型的名称，包含任何泛型参数，例如 `String`、`[Int]` 或 `Set`。 + +*路径*可由属性名称、下标、可选链表达式或者强制解包表达式组成。以上任意 key-path 组件可以以任何顺序重复多次。 + +在编译期，key-path 表达式会被一个 [`KeyPath`](https://developer.apple.com/documentation/swift/keypath) 类的实例替换。 + +对于所有类型，都可以通过传递 key-path 参数到下标方法 `subscript(keyPath:)` 来访问它的值。例如： + +```swift +struct SomeStructure { + var someValue: Int +} + +let s = SomeStructure(someValue: 12) +let pathToProperty = \SomeStructure.someValue + +let value = s[keyPath: pathToProperty] +// 值为 12 +``` + +在一些可以通过类型推断来确定所访问的具体类型的上下文中，可以省略 key-path 前的类型名字。下面的代码使用 `\.someProperty` 代替了 `SomeClass.someProperty` ： + +```swift +class SomeClass: NSObject { + @objc dynamic var someProperty: Int + init(someProperty: Int) { + self.someProperty = someProperty + } +} + +let c = SomeClass(someProperty: 10) +c.observe(\.someProperty) { object, change in + // ... +} +``` + +使用 `self` 作为路径可以创建一个恒等 key path (`\.self`)。恒等 key path 可以作为整个实例的引用，因此你仅需一步操作便可以利用它来访问以及修改其存储的所有数据。例如： + +```swift +var compoundValue = (a: 1, b: 2) +// Equivalent to compoundValue = (a: 10, b: 20) +compoundValue[keyPath: \.self] = (a: 10, b: 20) +``` + +通过点语法，可以让路径包含多个属性名称，以此来访问某实例的属性的属性。下面的代码使用 key-path 表达式 `\OuterStructure.outer.someValue` 来访问 `OuterStructure` 类型中 `outer` 属性的 `someValue` 属性。 + +```swift +struct OuterStructure { + var outer: SomeStructure + init(someValue: Int) { + self.outer = SomeStructure(someValue: someValue) + } +} + +let nested = OuterStructure(someValue: 24) +let nestedKeyPath = \OuterStructure.outer.someValue + +let nestedValue = nested[keyPath: nestedKeyPath] +// nestedValue is 24 +``` + +路径中也可以包含使用中括号的下标访问，只要下标访问的参数类型满足 `Hashable` 协议即可。下面的例子在 key path 中使用了下标来访问数组的第二个元素。 + +```swift +let greetings = ["hello", "hola", "bonjour", "안녕"] +let myGreeting = greetings[keyPath: \[String].[1]] +// myGreeting is 'hola' +``` + +T下标访问中使用的值可以是一个变量或者字面量，并且 key-path 表达式会使用值语义来捕获此值。下面的代码在 key-path 表达式和闭包中都使用了 `index` 变量来访问 `greetings` 数组的第三个元素。当 `index` 被修改时，key-path 表达式仍旧引用数组第三个元素，而闭包则使用了新的索引值。 + +```swift +var index = 2 +let path = \[String].[index] +let fn: ([String]) -> String = { strings in strings[index] } + +print(greetings[keyPath: path]) +// 打印 "bonjour" +print(fn(greetings)) +// 打印 "bonjour" + +// 将 'index' 设置为一个新的值不会影响到 'path' +index += 1 +print(greetings[keyPath: path]) +// 打印 "bonjour" + +// 'fn' 闭包使用了新值。 +print(fn(greetings)) +// 打印 "안녕" +``` + +路径可以使用可选链和强制解包。下面的代码在 key path 中使用了可选链来访问可选字符串的属性。 + +```swift +let firstGreeting: String? = greetings.first +print(firstGreeting?.count as Any) +// Prints "Optional(5)" + +// Do the same thing using a key path. +let count = greetings[keyPath: \[String].first?.count] +print(count as Any) +// Prints "Optional(5)" +``` + +可以混合使用各种 key path 组件来访问一些深度嵌套类型的值。下面的代码通过组合不同的组件，使用 key-path 表达式访问了一个字典数组中不同的值和属性。 + +```swift +let interestingNumbers = ["prime": [2, 3, 5, 7, 11, 13, 17], + "triangular": [1, 3, 6, 10, 15, 21, 28], + "hexagonal": [1, 6, 15, 28, 45, 66, 91]] +print(interestingNumbers[keyPath: \[String: [Int]].["prime"]] as Any) +// Prints "Optional([2, 3, 5, 7, 11, 13, 17])" +print(interestingNumbers[keyPath: \[String: [Int]].["prime"]![0]]) +// Prints "2" +print(interestingNumbers[keyPath: \[String: [Int]].["hexagonal"]!.count]) +// Prints "7" +print(interestingNumbers[keyPath: \[String: [Int]].["hexagonal"]!.count.bitWidth]) +// Prints "64" +``` + +你可以在平时提供函数或者闭包的上下文里使用 key path 表达式。特别地，你可以用根类型是 `SomeType` 和路径产生 `Value` 类型值的 key path 表达式来替换类型是 `(SomeType) -> Value` 的函数或者闭包。 + +```swift +struct Task { + var description: String + var completed: Bool +} +var toDoList = [ + Task(description: "Practice ping-pong.", completed: false), + Task(description: "Buy a pirate costume.", completed: true), + Task(description: "Visit Boston in the Fall.", completed: false), +] + +// 下面两种写法是等价的。 +let descriptions = toDoList.filter(\.completed).map(\.description) +let descriptions2 = toDoList.filter { $0.completed }.map { $0.description } +``` + +任何 key path 表达式的副作用发生的关键在于表达式在哪里被执行。例如，如果你在 key path 表达式中的一个下标里使用函数调用，该函数只会在表达式计算的时候调用一次，而不是每次这个 key path 被使用的时候。 + +```swift +func makeIndex() -> Int { + print("Made an index") + return 0 +} +// The line below calls makeIndex(). +let taskKeyPath = \[Task][makeIndex()] +// Prints "Made an index" + +// Using taskKeyPath doesn't call makeIndex() again. +let someTask = toDoList[keyPath: taskKeyPath] +``` + +关于更多如何使用 key path 与 Objective-C APIs 交互的信息，请参阅 [Using Objective-C Runtime Features in Swift](https://developer.apple.com/documentation/swift/using_objective_c_runtime_features_in_swift). +关于更多 key-value 编程和 key-value 观察的信息，请参阅 [Key-Value Coding Programming Guide](https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/KeyValueCoding/index.html#//apple_ref/doc/uid/10000107i) +和 [Key-Value Observing Programming Guide](https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/KeyValueObserving/KeyValueObserving.html#//apple_ref/doc/uid/10000177i). + +> Grammar of a key-path expression: +> +> *key-path-expression* → **`\`** *type*_?_ **`.`** *key-path-components* \ +> *key-path-components* → *key-path-component* | *key-path-component* **`.`** *key-path-components* \ +> *key-path-component* → *identifier* *key-path-postfixes*_?_ | *key-path-postfixes* +> +> *key-path-postfixes* → *key-path-postfix* *key-path-postfixes*_?_ \ +> *key-path-postfix* → **`?`** | **`!`** | **`self`** | **`[`** *function-call-argument-list* **`]`** + +### 选择器表达式 + +*选择器表达式*可以让你通过选择器来引用在 Objective-C 中方法（method）和属性（property）的 setter 和 getter 方法。 + +```swift +#selector(<#method name#>) +#selector(getter: <#property name#>) +#selector(setter: <#property name#>) +``` + +方法名和属性名必须是存在于 Objective-C 运行时中的方法和属性的引用。选择器表达式的返回值是一个 Selector 类型的实例。例如： + +```swift +class SomeClass: NSObject { + @objc let property: String + + @objc(doSomethingWithInt:) + func doSomething(_ x: Int) { } + + init(property: String) { + self.property = property + } +} +let selectorForMethod = #selector(SomeClass.doSomething(_:)) +let selectorForPropertyGetter = #selector(getter: SomeClass.property) +``` + +当为属性的 getter 创建选择器时，属性名可以是变量属性或者常量属性的引用。但是当为属性的 setter 创建选择器时，属性名只可以是对变量属性的引用。 + +方法名称可以包含圆括号来进行分组，并使用 as 操作符来区分具有相同方法名但类型不同的方法，例如: + +```swift +extension SomeClass { + @objc(doSomethingWithString:) + func doSomething(_ x: String) { } +} +let anotherSelector = #selector(SomeClass.doSomething(_:) as (SomeClass) -> (String) -> Void) +``` + +由于选择器是在编译时创建的，因此编译器可以检查方法或者属性是否存在，以及是否在运行时暴露给了 Objective-C 。 + +> Note: Although the *method name* and the *property name* are expressions, +> they're never evaluated. + +更多关于如何在 Swift 代码中使用选择器来与 Objective-C API 进行交互的信息，请参阅 [Using Objective-C Runtime Features in Swift](https://developer.apple.com/documentation/swift/using_objective_c_runtime_features_in_swift). + +> Grammar of a selector expression: +> +> *selector-expression* → **`#selector`** **`(`** *expression* **`)`** \ +> *selector-expression* → **`#selector`** **`(`** **`getter:`** *expression* **`)`** \ +> *selector-expression* → **`#selector`** **`(`** **`setter:`** *expression* **`)`** + +### Key-Path 字符串表达式 + +key-path 字符串表达式可以访问一个引用 Objective-C 属性的字符串，通常在 key-value 编程和 key-value 观察 APIs 中使用。其格式如下： + +```swift +#keyPath(<#property name#>) +``` + +属性名必须是一个可以在 Objective-C 运行时使用的属性的引用。在编译期，key-path 字符串表达式会被一个字符串字面量替换。例如： + +```swift +class SomeClass: NSObject { + @objc var someProperty: Int + init(someProperty: Int) { + self.someProperty = someProperty + } +} + +let c = SomeClass(someProperty: 12) +let keyPath = #keyPath(SomeClass.someProperty) + +if let value = c.value(forKey: keyPath) { + print(value) +} +// 打印 "12" +``` + +当在一个类中使用 key-path 字符串表达式时，可以省略类名，直接使用属性名来访问这个类的某个属性。 + +```swift +extension SomeClass { + func getSomeKeyPath() -> String { + return #keyPath(someProperty) + } +} +print(keyPath == c.getSomeKeyPath()) +// 打印 "true" +``` + +由于 key-path 字符串表达式在编译期才创建，编译期可以检查属性是否存在，以及属性是否暴露给 Objective-C 运行时。 + +关于更多如何使用 key path 与 Objective-C APIs 交互的信息，请参阅 [Using Objective-C Runtime Features in Swift](https://developer.apple.com/documentation/swift/using_objective_c_runtime_features_in_swift). +关于更多 key-value 编程和 key-value 观察的信息，请参阅 [Key-Value Coding Programming Guide](https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/KeyValueCoding/index.html#//apple_ref/doc/uid/10000107i) +和 [Key-Value Observing Programming Guide](https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/KeyValueObserving/KeyValueObserving.html#//apple_ref/doc/uid/10000177i). + +> Note: Although the *property name* is an expression, it's never evaluated. + +> Grammar of a key-path string expression: +> +> *key-path-string-expression* → **`#keyPath`** **`(`** *expression* **`)`** + +## 后缀表达式 + +*后缀表达式*就是在某个表达式的后面运用后缀运算符或其他后缀语法。从语法构成上来看，基本表达式也是后缀表达式。 + +关于这些运算符的更多信息，请参阅 和 . + +关于 Swift 标准库提供的运算符的更多信息，请参阅 [Operator Declarations](https://developer.apple.com/documentation/swift/operator_declarations). + +> Grammar of a postfix expression: +> +> *postfix-expression* → *primary-expression* \ +> *postfix-expression* → *postfix-expression* *postfix-operator* \ +> *postfix-expression* → *function-call-expression* \ +> *postfix-expression* → *initializer-expression* \ +> *postfix-expression* → *explicit-member-expression* \ +> *postfix-expression* → *postfix-self-expression* \ +> *postfix-expression* → *subscript-expression* \ +> *postfix-expression* → *forced-value-expression* \ +> *postfix-expression* → *optional-chaining-expression* + +### 函数调用表达式 + +*函数调用表达式*由函数名和在括号里以逗号分隔的参数列表组成。函数调用表达式形式如下： + +```swift +<#function name#>(<#argument value 1#>, <#argument value 2#>) +``` + +函数名可以是值为函数类型的任意表达式。 + +如果函数声明中指定了形参的名字，那么在调用的时候也必须得写出来，并通过冒号（`:`）分隔。这种函数调用表达式具有以下形式： + +```swift +<#function name#>(<#argument name 1#>: <#argument value 1#>, <#argument name 2#>: <#argument value 2#>) +``` + +函数调用表达式可以在函数调用表达式的尾部（右圆括号之后）加上多个尾随闭包，该闭包会作为函数的实参，在括号中最后一个实参后面添加。第一个闭包表达式时没有实参签名的，其他任意闭包表达式签名都有实参标签。如下两种写法是等价的，区别在是否使用尾随闭包语法： + +```swift +// someFunction 接受整型和闭包的实参 +someFunction(x: x, f: { $0 == 13 }) +someFunction(x: x) { $0 == 13 } + +// anotherFunction 接受一个整型和两个闭包的实参 +anotherFunction(x: x, f: { $0 == 13 }, g: { print(99) }) +anotherFunction(x: x) { $0 == 13 } g: { print(99) } +``` + +如果闭包是该函数的唯一实参，那么圆括号可以省略。 + +```swift +// someMethod 只接受一个闭包参数 +myData.someMethod() { $0 == 13 } +myData.someMethod { $0 == 13 } +``` + +为了支持实参中的尾随闭包，编译器从左到右检查形参列表，如下所示： + +| 尾随闭包 | 形参 | 行为 | +| ---------------- | --------- | ------ | +| 有标签 | 有标签 | 如果标签相同，闭包和形参匹配，否则跳过该形参 | +| 有标签 | 无标签 | 跳过该形参 | +| 无标签 | 有标签或无标签 | 如果形参在结构上类似于下面定义的函数类型，和闭包匹配，否则跳过该形参| + +尾随闭包作为其匹配形参的实参传递。 + +在扫描过程被跳过的形参不传递实参——例如，它们使用的是默认形参。当匹配后，扫描会继续下一个尾随闭包和形参。匹配过程结束后，所有的尾随闭包必须有对应的匹配。 + +如果形参不是输入输出参数，并且类似下面的情况，则算是结构上类似函数类型： + +- 函数类型的形参，例如 `(Bool) -> Int` +- 函数类型表达式的自动闭包形参，例如 `@autoclosure () -> ((Bool) -> Int)` +- 元素是函数类型的可变参数，例如 `((Bool) -> Int)...` +- 单层或多层可选类型的形参，例如 `Optional<(Bool) -> Int>` +- 由上面这些类型组合而成的形参，例如 `(Optional<(Bool) -> Int>)...` + +尾随闭包和结构上类似函数类型的形参匹配，但它并不是函数，所以闭包会按需包装。例如，如果形参类是是可选类型，闭包会自动包装成 `Optional` 。 + +为了简化 Swift 5.3 之前版本（从右到左匹配）的代码迁移 —— 编译器会同时检查从左到右和从右到左的顺序。如果不同的扫描方向产生了不同的结果，编译器则使用旧的从右到左的顺序，并生成警告。Swift 的未来版本将都使用从左到右的顺序。 + +```swift +typealias Callback = (Int) -> Int +func someFunction(firstClosure: Callback? = nil, + secondClosure: Callback? = nil) { + let first = firstClosure?(10) + let second = secondClosure?(20) + print(first ?? "-", second ?? "-") +} + +someFunction() // 打印 "- -" +someFunction { return $0 + 100 } // 歧义 +someFunction { return $0 } secondClosure: { return $0 } // 打印 "10 20" +``` + +在上面的例子中，Swift 5.3 中被标记为“歧义”的函数调用会打印”- 120“并产生一个编辑器警告。未来版本的 Swift 会打印”110 -“。 + + +如 所述，通过声明几种方法中的一种，类、结构体或枚举类型可以为函数调用语法启用语法糖。 + +#### 指针类型的隐式转换 + +在函数调用表达式里，如果实参和形参的类型不一致，编译器会尝试通过下面的规则进行隐式转换来匹配类型： + +- `inout SomeType` can become + `UnsafePointer` or `UnsafeMutablePointer` +- `inout Array` can become + `UnsafePointer` or `UnsafeMutablePointer` +- `Array` can become `UnsafePointer` +- `String` can become `UnsafePointer` + +下面两个函数调用是等价的： + +```swift +func unsafeFunction(pointer: UnsafePointer) { + // ... +} +var myNumber = 1234 + +unsafeFunction(pointer: &myNumber) +withUnsafePointer(to: myNumber) { unsafeFunction(pointer: $0) } +``` + +隐式转换创建的指针仅在函数调用期间有效。为了避免发生未定义行为，确保代码在函数调用结束后没有继续持有这些指针。 + +> Note: When implicitly converting an array to an unsafe pointer, +> Swift ensures that the array's storage is contiguous +> by converting or copying the array as needed. +> For example, you can use this syntax +> with an array that was bridged to `Array` +> from an `NSArray` subclass that makes no API contract about its storage. +> If you need to guarantee that the array's storage is already contiguous, +> so the implicit conversion never needs to do this work, +> use `ContiguousArray` instead of `Array`. + +使用 `&` 代替类似 `withUnsafePointer(to:)` 的显式函数可以在调用底层 C 函数的可读性更高，特别是当函数传入多个指针实参时。如果是其他 Swift 代码调用函数，避免使用 `&` 代替显式的不安全 API。 + +> Grammar of a function call expression: +> +> *function-call-expression* → *postfix-expression* *function-call-argument-clause* \ +> *function-call-expression* → *postfix-expression* *function-call-argument-clause*_?_ *trailing-closures* +> +> *function-call-argument-clause* → **`(`** **`)`** | **`(`** *function-call-argument-list* **`)`** \ +> *function-call-argument-list* → *function-call-argument* | *function-call-argument* **`,`** *function-call-argument-list* \ +> *function-call-argument* → *expression* | *identifier* **`:`** *expression* \ +> *function-call-argument* → *operator* | *identifier* **`:`** *operator* +> +> *trailing-closures* → *closure-expression* *labeled-trailing-closures*_?_ \ +> *labeled-trailing-closures* → *labeled-trailing-closure* *labeled-trailing-closures*_?_ \ +> *labeled-trailing-closure* → *identifier* **`:`** *closure-expression* + +### 构造器表达式 + +*构造器表达式*用于访问某个类型的构造器，形式如下： + +```swift +<#expression#>.init(<#initializer arguments#>) +``` + +你可以在函数调用表达式中使用构造器表达式来初始化某个类型的新实例。也可以使用构造器表达式来代理给父类构造器。 + +```swift +class SomeSubClass: SomeSuperClass { + override init() { + // 此处为子类构造过程 + super.init() + } +} +``` + +和函数类似，构造器表达式可以作为一个值。 例如： + +```swift +// 类型注解是必须的，因为 String 类型有多种构造器 +let initializer: (Int) -> String = String.init +let oneTwoThree = [1, 2, 3].map(initializer).reduce("", +) +print(oneTwoThree) +// 打印 "123" +``` + +如果通过名字来指定某个类型，可以不用构造器表达式而直接使用类型的构造器。在其他情况下，你必须使用构造器表达式。 + +```swift +let s1 = SomeType.init(data: 3) // 有效 +let s2 = SomeType(data: 1) // 有效 + +let s3 = type(of: someValue).init(data: 7) // 有效 +let s4 = type(of: someValue)(data: 5) // 错误 +``` + +> Grammar of an initializer expression: +> +> *initializer-expression* → *postfix-expression* **`.`** **`init`** \ +> *initializer-expression* → *postfix-expression* **`.`** **`init`** **`(`** *argument-names* **`)`** + +### 显式成员表达式 + +*显式成员表达式*允许我们访问命名类型、元组或者模块的成员，其形式如下： + +```swift +<#expression#>.<#member name#> +``` + +命名类型的某个成员在原始实现或者扩展中定义，例如： + +```swift +class SomeClass { + var someProperty = 42 +} +let c = SomeClass() +let y = c.someProperty // 访问成员 +``` + +元组的成员会隐式地根据表示它们出现顺序的整数来命名，以 0 开始，例如： + +```swift +var t = (10, 20, 30) +t.0 = t.1 +// 现在元组 (20, 20, 30) +``` + +对于模块的成员来说，只能直接访问顶级声明中的成员。 + +使用 `dynamicMemberLookup` 属性声明的类型包含可以在运行时查找的成员，具体请参阅 . + +为了区分只有参数名有所不同的方法或构造器，在圆括号中写出参数名，参数名后紧跟一个冒号，对于没有参数名的参数，使用下划线代替参数名。而对于重载方法，则需使用类型注解进行区分。例如： + +```swift +class SomeClass { + func someMethod(x: Int, y: Int) {} + func someMethod(x: Int, z: Int) {} + func overloadedMethod(x: Int, y: Int) {} + func overloadedMethod(x: Int, y: Bool) {} +} +let instance = SomeClass() + +let a = instance.someMethod // Ambiguous +let b = instance.someMethod(x:y:) // Unambiguous + +let d = instance.overloadedMethod // Ambiguous +let d = instance.overloadedMethod(x:y:) // Still ambiguous +let d: (Int, Bool) -> Void = instance.overloadedMethod(x:y:) // Unambiguous +``` + +如果点号（`.`）出现在行首，它会被视为显式成员表达式的一部分，而不是隐式成员表达式的一部分。例如如下代码所展示的被分为多行的链式方法调用： + +```swift +let x = [10, 3, 20, 15, 4] + .sorted() + .filter { $0 > 5 } + .map { $0 * 100 } +``` + +你可以将这种多行链式语法与编译器控制语句结合，以控制调用每个方法的时间。例如，以下代码在 iOS 上应用了不同的过滤规则： + +```swift +let numbers = [10, 20, 33, 43, 50] +#if os(iOS) + .filter { $0 < 40 } +#else + .filter { $0 > 25 } +#endif +``` + +在 `#if`、`#endif` 和其它编译指令之间的条件编译块可以包含一个隐式成员表达式，后跟零个或多个后缀，以形成一个后缀表达式。这些条件编译块还可以包含另一个条件编译块，或者这些表达式和块的组合体。 + +除了顶级代码（top-level code）以外，你还可以在任何能编写显式成员表达式的地方使用上述语法。 + +在条件编译块中，编译指令 `#if` 的分支必须包含至少一个表达式，其它分支可以为空。 + +> Grammar of an explicit member expression: +> +> *explicit-member-expression* → *postfix-expression* **`.`** *decimal-digits* \ +> *explicit-member-expression* → *postfix-expression* **`.`** *identifier* *generic-argument-clause*_?_ \ +> *explicit-member-expression* → *postfix-expression* **`.`** *identifier* **`(`** *argument-names* **`)`** \ +> *explicit-member-expression* → *postfix-expression* *conditional-compilation-block* +> +> *argument-names* → *argument-name* *argument-names*_?_ \ +> *argument-name* → *identifier* **`:`** + +### 后缀 Self 表达式 + +后缀 `self` 表达式由某个表达式或类型名紧跟 `.self` 组成，其形式如下： + +```swift +<#expression#>.self +<#type#>.self +``` + +第一种形式返回表达式的值。例如：`x.self` 返回 `x`。 + +第二种形式返回相应的类型。我们可以用它来获取某个实例的类型作为一个值来使用。例如，`SomeClass.self` 会返回 `SomeClass` 类型本身，你可以将其传递给相应函数或者方法作为参数。 + +> Grammar of a postfix self expression: +> +> *postfix-self-expression* → *postfix-expression* **`.`** **`self`** + +### 下标表达式 + +可通过*下标表达式*访问相应的下标，形式如下： + +```swift +<#expression#>[<#index expressions#>] +``` + +要获取下标表达式的值，可将索引表达式作为下标表达式的参数来调用下标 getter。下标 setter 的调用方式与之一样。 + +关于下标的声明，请参阅 . + +> Grammar of a subscript expression: +> +> *subscript-expression* → *postfix-expression* **`[`** *function-call-argument-list* **`]`** + +### 强制取值表达式 + +当你确定可选值不是 `nil` 时，可以使用*强制取值表达式*来强制解包，形式如下： + +```swift +<#expression#>! +``` + +I如果该表达式的值不是 `nil`，则返回解包后的值。否则，抛出运行时错误。 + +返回的值可以被修改，无论是修改值本身，还是修改值的成员。例如： + +```swift +var x: Int? = 0 +x! += 1 +// x 现在是 1 + +var someDictionary = ["a": [1, 2, 3], "b": [10, 20]] +someDictionary["a"]![0] = 100 +// someDictionary 现在是 ["a": [100, 2, 3], "b": [10, 20]] +``` + +> Grammar of a forced-value expression: +> +> *forced-value-expression* → *postfix-expression* **`!`** + +### 可选链表达式 + +可选链表达式提供了一种使用可选值的便捷方法，形式如下： + +```swift +<#expression#>? +``` + +后缀 `?` 运算符会根据表达式生成可选链表达式而不会改变表达式的值。 + +如果某个后缀表达式包含可选链表达式，那么它的执行过程会比较特殊。如果该可选链表达式的值是 `nil`，整个后缀表达式会直接返回 `nil`。如果该可选链表达式的值不是 `nil`，则返回可选链表达式解包后的值，并将该值用于后缀表达式中剩余的表达式。在这两种情况下，整个后缀表达式的值都会是可选类型。 + +如果某个后缀表达式中包含了可选链表达式，那么只有最外层的表达式会返回一个可选类型。例如，在下面的例子中，如果 c 不是 `nil`，那么它的值会被解包，然后通过 `.property` 访问它的属性，接着进一步通过 `.performAction()` 调用相应方法。整个 `c?.property.performAction()` 表达式返回一个可选类型的值，而不是多重可选类型。 + +```swift +var c: SomeClass? +var result: Bool? = c?.property.performAction() +``` + + +上面的例子跟下面的不使用可选链表达式的例子等价： + +```swift +var result: Bool? +if let unwrappedC = c { + result = unwrappedC.property.performAction() +} +``` + +可选链表达式解包后的值可以被修改，无论是修改值本身，还是修改值的成员。如果可选链表达式的值为 `nil`，则表达式右侧的赋值操作不会被执行。例如： + +```swift +func someFunctionWithSideEffects() -> Int { + return 42 // No actual side effects. +} +var someDictionary = ["a": [1, 2, 3], "b": [10, 20]] + +someDictionary["not here"]?[0] = someFunctionWithSideEffects() +// someFunctionWithSideEffects isn't evaluated +// someDictionary is still ["a": [1, 2, 3], "b": [10, 20]] + +someDictionary["a"]?[0] = someFunctionWithSideEffects() +// someFunctionWithSideEffects is evaluated and returns 42 +// someDictionary is now ["a": [42, 2, 3], "b": [10, 20]] +``` + +>可选链表达式语法： +> +> *可选链表达式* → *后缀表达式* **`?`** diff --git a/swift-6.docc/ReferenceManual/GenericParametersAndArguments.md b/swift-6.docc/ReferenceManual/GenericParametersAndArguments.md new file mode 100644 index 000000000..c71f72ca7 --- /dev/null +++ b/swift-6.docc/ReferenceManual/GenericParametersAndArguments.md @@ -0,0 +1,226 @@ + + +# 泛型形参和实参 + +将参数声明抽象化以脱离具体类型。 + +本节描述泛型类型、泛型函数和泛型构造器的形参和实参。在声明泛型类型、函数或构造器时，需要指定泛型可处理的类型形参。这些类型形参相当于占位符，当实例化泛型类型或调用泛型函数、初始化器时，会被具体的类型实参替换。 + +关于 Swift 语言中的“泛型”，参阅 . + + + +## 泛型形参子句 + +*泛型形参子句* 指定了泛型类型或函数的类型形参，以及对这些形参的任何相关约束和要求。泛型形参用尖括号（<>）括起来，格式如下： + +```swift +<<#generic parameter list#>> +``` + +多个*泛型形参*用逗号分开，每个形参的形式如下： + +```swift +<#type parameter#>: <#constraint#> +``` + +泛型形参由一个*类型形参*和一个可选的*约束*组成。*类型形参*只是一个占位符（如 `T`，`U`，`V`，`Key`，`Value`等），用来表示类型。在声明类型、函数或构造器时（包括函数或构造器的签名），你可以访问这些类型形参及其任何关联的类型。 + +*约束*指定了类型形参要继承自特定类，或遵循某个协议或协议组合。例如：在下面的泛型函数中，泛型形参 `T: Comparable` 表示任何替代类型形参 `T` 的类型实参都必须遵循 `Comparable` 协议。 + +```swift +func simpleMax(_ x: T, _ y: T) -> T { + if x < y { + return y + } + return x +} +``` + + + +以`Int`和`Double`，因为都遵循 `Comparable` 协议，所以该函数可以接受这两种类型的参数。与泛型类型不同，调用泛型函数或构造器时，你不需要指定泛型实参子句。泛型实参的类型会根据传给函数或构造器的参数类型推断出来。 + +```swift +simpleMax(17, 42) // T is inferred to be Int +simpleMax(3.14159, 2.71828) // T is inferred to be Double +``` + + + + + +### 泛型 Where 子句 + +你可以通过在类型或函数体的起始大括号前包含一个泛型`where`子句，来对类型形参及其相关类型指定额外的要求。泛型 `where`子句由`where`关键字组成，后跟一个或多个要求，多个*要求*用逗号进行分隔。 + +```swift +where <#requirements#> +``` + +泛型 `where` 子句中的*要求*指明类型形参要继承自某个类或遵循某个协议或协议组合。虽然 `where` 子句为表达类型形参的简单约束提供了语法糖（比如，`` 等同于 `where T: Comparable` 等），但它可以用于为类型形参及其关联类型提供更复杂的约束。比如，你可以指定类型形参的关联类型遵循某个协议：` where S.Iterator.Element: Equatable` 指定了`S`遵循 `Sequence`协议，并且`S`的关联类型`S.Iterator.Element`遵循`Equatable`协议。此约束确保序列中的每个元素都是符合`Equatable`协议的。 + +还可以使用 == 运算符来指定两个类型必须相同。例如， ` where S1.Iterator.Element == S2.Iterator.Element`，表示 S1 和 S2 都遵循 Sequence 协议，并且两个序列元素的类型必须相同。 + +任何替代类型形参的类型实参都必须满足对该类型形参指定的所有约束与要求。 + + `where`子句可以出现在包含类型形参的声明中，或作为声明的一部分，被嵌套另一个在含有类型形参的声明中。被嵌套的泛型 `where`子句依然可以指向包围它的声明中的类型形参，然而此时 `where`子句指定的要求仅用于它被声明的地方。 + +如果外层的声明也有一个 `where` 子句，那么两个子句的要求会合并。在下面的示例中，`startsWithZero()`仅在 `Element` 同时遵循`someProtocol`和`Numeric`协议时可用。 + +```swift +extension Collection where Element: SomeProtocol { + func startsWithZero() -> Bool where Element: Numeric { + return first == .zero + } +} +``` + + + + + +重载泛型函数或构造器，泛型形参子句中的类型形参必须有不同的约束或要求。当调用重载的泛型函数或构造器时，编译器会使用这些约束来决定调用哪个重载的函数或构造器。 + +更多关于泛型 `where` 从句的内容和关于泛型函数声明的例子，参阅 . + +> 泛型形参子句的语法格式： +> +> *泛型形参子句* → **`<`** *泛型形参列表* **`>`** \ +> *泛型形参列表* → *泛型形参* | *泛型形参* **`,`** *泛型形参列表* \ +> *泛型形参* → *类型名称* \ +> *泛型形参* → *类型名称* **`:`** *类型标识符* \ +> *泛型形参* → *类型名称* **`:`** *协议合成类型* +> +> *泛型 where 子句* → **`where`** *约束列表* \ +> *要求列表* → *要求* | *要求* **`,`** *要求列表* \ +> *要求* → *一致性要求* | *同类型要求* +> +> *一致性要求* → *类型标识符* **`:`** *类型标识符* \ +> *一致性要求* → *类型标识符* **`:`** *协议合成类型* \ +> *同类型要求* → *类型标识符* **`==`** *类型* + + + +## 泛型实参子句 + +*泛型实参* 指定了泛型类型的类型实参。泛型实参用尖括号（<>）包围，格式如下 + +```swift +<<#generic argument list#>> +``` + +多个泛型实参用逗号分开。类型实参是实际具体类型的名称，用来替代泛型类型中对应的类型参数，从而得到该泛型类型的特定版本。下面的示例展示了 Swift 标准库中的泛型字典类型的简化版本： + +```swift +struct Dictionary: Collection, ExpressibleByDictionaryLiteral { + /* ... */ +} +``` + + + +泛型`Dictionary`的特定版本`Dictionary`是通过具体的类型实参（`String`和`Int`）来替换泛型形参（`Key: Hashable` 和 `Value`）而形成的。每个类型实参必须满足它替换的泛型形参的所有约束，包括在泛型`where`子句中指定的额外要求。在上述例子中，`Key`类型形参要遵循`Hashable`协议，因此传入的实参`String`也要遵循`Hashable`协议。 + +还可以用本身就是泛型类型的特定版本的类型实参替代类型形参（假设已满足合适的约束和关联类型要求）。例如，可以将 `Array`中的类型形参 `Element` 替换 Array 泛型专用版本 `Array` ，以形成一个元素本身是整数数组的二维数组。 + +```swift +let arrayOfArrays: Array> = [[1, 2, 3], [4, 5, 6], [7, 8, 9]] +``` + + + +正如在中提到的，在指定泛型函数或构造器的类型实参时，不能使用泛型实参子句。 + +> 泛型实参的语法格式： +> +> *generic-argument-clause* → **`<`** *generic-argument-list* **`>`** \ +> *generic-argument-list* → *generic-argument* | *generic-argument* **`,`** *generic-argument-list* \ +> *generic-argument* → *type* + + diff --git a/swift-6.docc/ReferenceManual/LexicalStructure.md b/swift-6.docc/ReferenceManual/LexicalStructure.md new file mode 100644 index 000000000..06bdc0e3e --- /dev/null +++ b/swift-6.docc/ReferenceManual/LexicalStructure.md @@ -0,0 +1,851 @@ +# 词法结构 + +使用语法的最低层级组件。 + +Swift 的 *词法结构* 描述了哪些字符序列构成了语言中的合法标记（tokens）。这些合法标记构成了语言的最低层级构建块，并在后续章节中用于描述语言的其他部分。一个标记可以由标识符、关键字、标点符号、字面量或运算符组成。 + +在大多数情况下，这些标记是从 Swift 源文件的字符中生成的，生成过程考虑了输入文本中最长的可能子字符串，并受以下语法规则的约束。这种行为策略被称为 *最长匹配策略（longest match）* 或 *最大吞噬策略（maximal munch）*。 + +## 空白和注释 + +空白字符有两个用途：在源文件中分隔标记，并区分前缀、后缀和中缀运算符（参见 ），除此之外，空白字符会被忽略。以下字符被视为空白字符：空格 (U+0020)、换行符 (U+000A)、回车符 (U+000D)、水平制表符 (U+0009)、垂直制表符 (U+000B)、换页符 (U+000C) 和空字符 (U+0000)。 + + + +编译器将注释视为空白字符。单行注释以 `//` 开头，并持续到换行符 (U+000A) 或回车符 (U+000D)。多行注释以 `/*` 开头，以 `*/` 结束。多行注释可以嵌套，但注释符号必须头尾匹配。 + +注释中可以包含额外的格式和标记，如 [标记格式参考](https://developer.apple.com/library/content/documentation/Xcode/Reference/xcode_markup_formatting_ref/index.html) 中所述。 + +> 空白字符的语法: +> +> *whitespace* → *whitespace-item* *whitespace*_?_ \ +> *whitespace-item* → *line-break* \ +> *whitespace-item* → *inline-space* \ +> *whitespace-item* → *comment* \ +> *whitespace-item* → *multiline-comment* \ +> *whitespace-item* → U+0000、U+000B 或 U+000C +> +> *line-break* → U+000A \ +> *line-break* → U+000D \ +> *line-break* → U+000D 后跟 U+000A +> +> *inline-spaces* → *inline-space* *inline-spaces*_?_ \ +> *inline-space* → U+0009 或 U+0020 +> +> *comment* → **`//`** *comment-text* *line-break* \ +> *multiline-comment* → **`/*`** *multiline-comment-text* **`*/`** +> +> *comment-text* → *comment-text-item* *comment-text*_?_ \ +> *comment-text-item* → 除 U+000A 或 U+000D 之外的任意 Unicode 标量值 +> +> *multiline-comment-text* → *multiline-comment-text-item* *multiline-comment-text*_?_ \ +> *multiline-comment-text-item* → *multiline-comment* \ +> *multiline-comment-text-item* → *comment-text-item* \ +> *multiline-comment-text-item* → 除 **`/*`** 或 **`*/`** 之外的任意 Unicode 标量值 + +## 标识符 + +*标识符* 以大写或小写字母 A 到 Z、下划线 (`_`)、基本多语言平面（Basic Multilingual Plane）的非组合字母数字 Unicode 字符（Noncombining Alphanumeric Unicode Character），或基本多语言平面之外但不在私用区（Private Use Area）的字符开头。在第一个字符之后，还允许使用数字和组合 Unicode 字符（Combining Unicode Character）。 + +即使声明具有 `public` 访问级别修饰符，也应将以下内容视为仅内部使用：以下划线开头的标识符、第一个参数标签以下划线开头的下标操作，以及第一个参数标签以下划线开头的构造函数。这个约定允许框架作者以此方式标记某个 API 的一部分内容，以防止客户端与之交互或依赖，尽管某些限制要求这些声明是公开可访问的。此外，以两个下划线开头的标识符需保留给 Swift 编译器和标准库使用。 + +要将保留字用作标识符，可以在其前后加上反引号（\`）。例如，`class` 不是一个合法的标识符，但 `` `class` `` 是合法的。反引号不被视为标识符的一部分；`` `x` `` 和 `x` 具有相同的指代含义。 + + + +在没有显式参数名称的闭包中，参数会被隐式命名为 `$0`、`$1`、`$2` 等。这些名称在闭包的范围内是合法的标识符。 + +编译器会为具有属性包装器投射（Property Wrapper Projection）的属性合成以美元符号 (`$`) 开头的标识符。你的代码可以与这些标识符交互，但你不能声明带有该前缀的标识符。有关更多信息，请参阅 章节的 部分。 + + + + + +> 标识符的语法: +> +> *identifier* → *identifier-head* *identifier-characters*_?_ \ +> *identifier* → **`` ` ``** *identifier-head* *identifier-characters*_?_ **`` ` ``** \ +> *identifier* → *implicit-parameter-name* \ +> *identifier* → *property-wrapper-projection* \ +> *identifier-list* → *identifier* | *identifier* **`,`** *identifier-list* +> +> *identifier-head* → 大写或小写字母 A 到 Z \ +> *identifier-head* → **`_`** \ +> *identifier-head* → U+00A8、U+00AA、U+00AD、U+00AF、U+00B2–U+00B5 或 U+00B7–U+00BA \ +> *identifier-head* → U+00BC–U+00BE、U+00C0–U+00D6、U+00D8–U+00F6 或 U+00F8–U+00FF \ +> *identifier-head* → U+0100–U+02FF、U+0370–U+167F、U+1681–U+180D 或 U+180F–U+1DBF \ +> *identifier-head* → U+1E00–U+1FFF \ +> *identifier-head* → U+200B–U+200D、U+202A–U+202E、U+203F–U+2040、U+2054 或 U+2060–U+206F \ +> *identifier-head* → U+2070–U+20CF、U+2100–U+218F、U+2460–U+24FF 或 U+2776–U+2793 \ +> *identifier-head* → U+2C00–U+2DFF 或 U+2E80–U+2FFF \ +> *identifier-head* → U+3004–U+3007、U+3021–U+302F、U+3031–U+303F 或 U+3040–U+D7FF \ +> *identifier-head* → U+F900–U+FD3D、U+FD40–U+FDCF、U+FDF0–U+FE1F 或 U+FE30–U+FE44 \ +> *identifier-head* → U+FE47–U+FFFD \ +> *identifier-head* → U+10000–U+1FFFD、U+20000–U+2FFFD、U+30000–U+3FFFD 或 U+40000–U+4FFFD \ +> *identifier-head* → U+50000–U+5FFFD、U+60000–U+6FFFD、U+70000–U+7FFFD 或 U+80000–U+8FFFD \ +> *identifier-head* → U+90000–U+9FFFD、U+A0000–U+AFFFD、U+B0000–U+BFFFD 或 U+C0000–U+CFFFD \ +> *identifier-head* → U+D0000–U+DFFFD 或 U+E0000–U+EFFFD +> +> *identifier-character* → 数字 0 到 9 \ +> *identifier-character* → U+0300–U+036F、U+1DC0–U+1DFF、U+20D0–U+20FF 或 U+FE20–U+FE2F \ +> *identifier-character* → *identifier-head* \ +> *identifier-characters* → *identifier-character* *identifier-characters*_?_ +> +> *implicit-parameter-name* → **`$`** *decimal-digits* \ +> *property-wrapper-projection* → **`$`** *identifier-characters* + +## 关键字和标点符号 + +以下关键字是保留字，不能用作标识符，除非用反引号将它们转义，如上文 中所述。除了 `inout`、`var` 和 `let` 之外，其他关键字可以作为函数声明或函数调用中的参数名称，而无需使用反引号进行转义。当成员名称与关键字相同时，引用该成员时不需要使用反引号进行转义，除非在引用成员与使用关键字之间存在歧义——例如，`self`、`Type` 和 `Protocol` 在显式成员表达式中具有特殊含义，因此在这种情况下必须用反引号将它们转义。 + + + + + + + + + + + +- 用于声明的关键字：`associatedtype`、`borrowing`、`class`、`consuming`、`deinit`、`enum`、`extension`、`fileprivate`、`func`、`import`、`init`、`inout`、`internal`、`let`、`nonisolated`、`open`、`operator`、`private`、`precedencegroup`、`protocol`、`public`、`rethrows`、`static`、`struct`、`subscript`、`typealias` 和 `var`。 + + + +- 用于语句的关键字：`break`、`case`、`catch`、`continue`、`default`、`defer`、`do`、`else`、`fallthrough`、`for`、`guard`、`if`、`in`、`repeat`、`return`、`throw`、`switch`、`where` 和 `while`。 +- 用于表达式和类型的关键字：`Any`、`as`、`await`、`catch`、`false`、`is`、`nil`、`rethrows`、`self`、`Self`、`super`、`throw`、`throws`、`true` 和 `try`。 +- 用于模式的关键字：`_`。 +- 以井号 (`#`) 开头的关键字：`#available`、`#colorLiteral`、`#else`、`#elseif`、`#endif`、`#fileLiteral`、`#if`、`#imageLiteral`、`#keyPath`、`#selector`、`#sourceLocation`、`#unavailable`。 + +> 注意: +> 在 Swift 5.9 之前，以下关键字是保留字：`#column`、`#dsohandle`、`#error`、`#fileID`、`#filePath`、`#file`、`#function`、`#line` 和 `#warning`。 +> 它们现在已经在 Swift 标准库中实现为宏：[`column`](https://developer.apple.com/documentation/swift/column())、[`dsohandle`](https://developer.apple.com/documentation/swift/dsohandle())、[`error(_:)`](https://developer.apple.com/documentation/swift/error(_:))、[`fileID`](https://developer.apple.com/documentation/swift/fileID())、[`filePath`](https://developer.apple.com/documentation/swift/filePath())、[`file`](https://developer.apple.com/documentation/swift/file())、[`function`](https://developer.apple.com/documentation/swift/function())、[`line`](https://developer.apple.com/documentation/swift/line()) 以及 [`warning(_:)`](https://developer.apple.com/documentation/swift/warning(_:))。 + + + + + + + +- 在特定上下文中保留的关键字：`associativity`、`async`,`convenience`、`didSet`、`dynamic`、`final`、`get`、`indirect`、`infix`、`lazy`、`left`、`mutating`、`none`、`nonmutating`、`optional`、`override`、`package`、`postfix`、`precedence`、`prefix`、`Protocol`、`required`、`right`、`set`、`some`、`Type`、`unowned`、`weak` 和 `willSet`。除了在语法中的特定上下文出现之外，它们可以被当作标识符使用。 + + + +以下符号被保留为标点符号，不能用作自定义运算符：`(`、`)`、`{`、`}`、`[`、`]`、`.`、`,`、`:`、`;`、`=`、`@`、`#`、`&`（作为前缀运算符）、`->`、`` ` ``、`?` 和 `!`（作为后缀运算符）。 + +## 字面量 + +*字面量* 是某种类型值的源代码表示形式，例如数字或字符串。 + +以下是一些字面量的示例： + +```swift +42 // 整数字面量 +3.14159 // 浮点数字面量 +"Hello, world!" // 字符串字面量 +/Hello, .*/ // 正则表达式字面量 +true // 布尔字面量 +``` + + + + + +字面量本身没有类型。相反，字面量被解析为具有无限精度，Swift 的类型推断机制会尝试为字面量推断出一个类型。例如，在声明 `let x: Int8 = 42` 中，Swift 使用显式的类型注解（`: Int8`）来推断整数字面量 `42` 的类型为 `Int8`。如果没有适当的类型信息可用，Swift 会推断该字面量的类型为 Swift 标准库中定义的默认字面量类型之一，如下表所示。在为字面量值指定类型注解时，注解的类型必须是可以从该字面量值实例化的类型。也就是说，该类型必须遵循下表中列出的 Swift 标准库协议。 + +| 字面量 | 默认类型 | 协议 | +| ----- | ------ | ---- | +| 整数 | `Int` | `ExpressibleByIntegerLiteral` | +| 浮点数 | `Double` | `ExpressibleByFloatLiteral` | +| 字符串 | `String` | `ExpressibleByStringLiteral`，对于只包含单个 Unicode 标量的字符串字面量，使用 `ExpressibleByUnicodeScalarLiteral`，对于只包含单个扩展字形簇（extended grapheme cluster）的字符串字面量，使用 `ExpressibleByExtendedGraphemeClusterLiteral` | +| 正则表达式 | `Regex` | 无 | +| 布尔值 | `Bool` | `ExpressibleByBooleanLiteral` | + +例如，在声明 `let str = "Hello, world"` 中，字符串字面量 `"Hello, world"` 的默认推断类型是 `String`。同样，`Int8` 遵循 `ExpressibleByIntegerLiteral` 协议，因此可以在声明 `let x: Int8 = 42` 中用于整数字面量 `42` 的类型注解。 + + + +> 字面量的语法: +> +> *literal* → *numeric-literal* | *string-literal* | *regular-expression-literal* | *boolean-literal* | *nil-literal* +> +> *numeric-literal* → **`-`**_?_ *integer-literal* | **`-`**_?_ *floating-point-literal* \ +> *boolean-literal* → **`true`** | **`false`** \ +> *nil-literal* → **`nil`** + +### 整数字面量 + +*整数字面量* 表示具有未指定精度的整数值。默认情况下，整数字面量以十进制表示；你可以使用前缀指定其他进制。二进制字面量以 `0b` 开头，八进制字面量以 `0o` 开头，十六进制字面量以 `0x` 开头。 + +十进制字面量包含数字 `0` 到 `9`。二进制字面量包含 `0` 和 `1`，八进制字面量包含 `0` 到 `7`，而十六进制字面量则包含 `0` 到 `9` 以及大写或小写的 `A` 到 `F`。 + +负整数字面量通过在整数字面量前加上负号 (`-`) 来表示，如 `-42`。 + +为了提高可读性，数字之间允许使用下划线 (`_`)，但它们会被忽略，因此不会影响字面量的值。整数字面量可以以前导零 (`0`) 开头，但这些零同样会被忽略，不会影响字面量的进制或值。 + +除非另有说明，否则整数字面量的默认推断类型是 Swift 标准库类型 `Int`。Swift 标准库还定义了用于表示各种大小的有符号和无符号整数的类型，详细内容请参阅 。 + + + + + +> 整数字面量的语法: +> +> *integer-literal* → *binary-literal* \ +> *integer-literal* → *octal-literal* \ +> *integer-literal* → *decimal-literal* \ +> *integer-literal* → *hexadecimal-literal* +> +> *binary-literal* → **`0b`** *binary-digit* *binary-literal-characters*_?_ \ +> *binary-digit* → 数字 0 或 1 \ +> *binary-literal-character* → *binary-digit* | **`_`** \ +> *binary-literal-characters* → *binary-literal-character* *binary-literal-characters*_?_ +> +> *octal-literal* → **`0o`** *octal-digit* *octal-literal-characters*_?_ \ +> *octal-digit* → 数字 0 到 7 \ +> *octal-literal-character* → *octal-digit* | **`_`** \ +> *octal-literal-characters* → *octal-literal-character* *octal-literal-characters*_?_ +> +> *decimal-literal* → *decimal-digit* *decimal-literal-characters*_?_ \ +> *decimal-digit* → 数字 0 到 9 \ +> *decimal-digits* → *decimal-digit* *decimal-digits*_?_ \ +> *decimal-literal-character* → *decimal-digit* | **`_`** \ +> *decimal-literal-characters* → *decimal-literal-character* *decimal-literal-characters*_?_ +> +> *hexadecimal-literal* → **`0x`** *hexadecimal-digit* *hexadecimal-literal-characters*_?_ \ +> *hexadecimal-digit* → 数字 0 到 7, 字母 a 到 f 或字母 A 到 F \ +> *hexadecimal-literal-character* → *hexadecimal-digit* | **`_`** \ +> *hexadecimal-literal-characters* → *hexadecimal-literal-character* *hexadecimal-literal-characters*_?_ + +### 浮点数字面量 + +*浮点数字面量* 表示具有未指定精度的浮点值。 + +默认情况下，浮点数字面量以十进制形式表示（无前缀），但也可以以十六进制形式表示（带有 `0x` 前缀）。 + +十进制浮点数字面量由一串十进制数字序列组成，后面可以跟一个十进制小数部分、十进制指数部分或两者兼有。十进制小数部分由一个小数点 (`.`) 和紧随其后的一串十进制数字序列组成。指数部分以大写或小写的 `e` 为前缀，后面跟一串十进制数字，表示在 `e` 前面的值要乘以的 10 的幂。例如，`1.25e2` 表示 1.25 x 10²，结果为 `125.0`。类似地，`1.25e-2` 表示 1.25 x 10⁻²，结果为 `0.0125`。 + +十六进制浮点数字面量由一个 `0x` 前缀、一个可选的十六进制小数部分和一个十六进制指数部分组成。十六进制小数部分由一个小数点和紧随其后的一串十六进制数字序列组成。指数部分以大写或小写的 `p` 为前缀，后面跟一串十进制数字，表示在 `p` 前面的值要乘以的 2 的幂。例如，`0xFp2` 表示 15 x 2²，结果为 `60`。类似地，`0xFp-2` 表示 15 x 2⁻²，结果为 `3.75`。 + +负浮点数字面量通过在浮点数字面量前加上负号 (`-`) 来表示，如 `-42.5`。 + +为了提高可读性，数字之间允许使用下划线 (`_`)，但它们会被忽略，因此不会影响字面量的值。浮点数字面量可以以前导零 (`0`) 开头，但这些零同样会被忽略，不会影响字面量的进制或值。 + +除非另有说明，否则浮点数字面量的默认推断类型是 Swift 标准库类型 `Double`，它表示 64 位浮点数。Swift 标准库还定义了 `Float` 类型，它表示 32 位浮点数。 + +> 浮点数字面量的语法: +> +> *floating-point-literal* → *decimal-literal* *decimal-fraction*_?_ *decimal-exponent*_?_ \ +> *floating-point-literal* → *hexadecimal-literal* *hexadecimal-fraction*_?_ *hexadecimal-exponent* +> +> *decimal-fraction* → **`.`** *decimal-literal* \ +> *decimal-exponent* → *floating-point-e* *sign*_?_ *decimal-literal* +> +> *hexadecimal-fraction* → **`.`** *hexadecimal-digit* *hexadecimal-literal-characters*_?_ \ +> *hexadecimal-exponent* → *floating-point-p* *sign*_?_ *decimal-literal* +> +> *floating-point-e* → **`e`** | **`E`** \ +> *floating-point-p* → **`p`** | **`P`** \ +> *sign* → **`+`** | **`-`** + +### 字符串字面量 + +字符串字面量是由引号包围的一串字符序列。单行字符串字面量由双引号包围，其形式如下： + +```swift +"<#characters#>" +``` + +字符串字面量不能包含未转义的双引号（`"`）、未转义的反斜杠（`\`）、回车符或换行符。 + +多行字符串字面量由三个双引号包围，其形式如下： + +```swift +""" +<#characters#> +""" +``` + +与单行字符串字面量不同，多行字符串字面量可以包含未转义的双引号（`"`）、回车符和换行符。但它不能包含连续三个未转义的双引号。 + +开启多行字符串字面量的 `"""` 之后的换行符不属于字符串的一部分。结束字面量的 `"""` 之前的换行符也不属于字符串的一部分。要创建一个以换行符开始或结束的多行字符串字面量，请在其第一行或最后一行写一个空行。 + +多行字符串字面量可以使用任意组合的空格和制表符进行缩进，这些缩进不会包含在字符串中。结束字面量的 `"""` 确定了缩进的长度：字面量中的每一个非空行开头的缩进必须与结束 `"""` 之前的缩进完全相同。制表符和空格之间不会有相互转换。在该缩进之后可以包含额外的空格和制表符，这些空格和制表符会出现在字符串中。 + +多行字符串字面量中的换行符会被标准化为使用行分隔符。即使源文件中包含混合的回车符和换行符，字符串中的所有换行符也会变为一致。 + +在多行字符串字面量中，在行末尾写一个反斜杠 (`\`) 会将其后的换行符从字符串中忽略。任何在反斜杠和换行符之间的空白也会被忽略。你可以使用这种语法在源代码中硬折叠一个多行字符串字面量，而不会改变结果字符串的值。 + +特殊字符可以通过以下转义序列包含在单行和多行字符串字面量中： + +- 空字符 (`\0`) +- 反斜杠 (`\\`) +- 水平制表符 (`\t`) +- 换行符 (`\n`) +- 回车符 (`\r`) +- 双引号 (`\"`) +- 单引号 (`\'`) +- Unicode 标量 (`\u{`*n*`}`)，其中 *n* 是一个包含一到八位数字的十六进制数字 + + + +表达式的值可以通过在反斜杠 (`\`) 后面加上用括号括起来的表达式插入到字符串字面量中。插值表达式可以包含字符串字面量，但不能包含未转义的反斜杠、回车符或换行符。 + +例如，以下所有字符串字面量具有相同的值： + +```swift +"1 2 3" +"1 2 \("3")" +"1 2 \(3)" +"1 2 \(1 + 2)" +let x = 3; "1 2 \(x)" +``` + + + + + +由扩展定界符包围的字符串是由引号和一组或多组配对的井号（`#`）包围的一串字符序列。由扩展定界符包围的字符串具有以下形式： + +```swift +#"<#characters#>"# + +#""" +<#characters#> +"""# +``` + +由扩展定界符包围的字符串中的特殊字符在结果字符串中显示为普通字符而不是特殊字符。你可以使用扩展定界符来创建包含通常会产生特殊效果字符的字符串，这些特殊效果比如有生成字符串插值、开启转义序列或终止字符串。 + +以下示例展示了一个字符串字面量和一个由扩展定界符包围的字符串，它们创建了等价的字符串值： + +```swift +let string = #"\(x) \ " \u{2603}"# +let escaped = "\\(x) \\ \" \\u{2603}" +print(string) +// 打印 "\(x) \ " \u{2603}" +print(string == escaped) +// 打印 "true" +``` + + + +如果你使用多个井号来形成由扩展定界符包围的字符串，不要在井号之间放置空格： + + + +```swift +print(###"Line 1\###nLine 2"###) // 正确 +print(# # #"Line 1\# # #nLine 2"# # #) // 错误 +``` + + + +使用扩展定界符创建的多行字符串字面量具有与常规多行字符串字面量相同的缩进要求。 + +默认情况下，字符串字面量的推断类型为 `String`。 + +有关 `String` 类型的更多信息，请参见 和 [`String`](https://developer.apple.com/documentation/swift/string)。 + +字符串字面量通过 `+` 运算符连接时，连接操作在编译时完成。 +例如，以下示例中 `textA` 和 `textB` 的值是相同的——没有发生运行时的连接操作。 + +```swift +let textA = "Hello " + "world" +let textB = "Hello world" +``` + + + +> 字符串字面量的语法: +> +> *string-literal* → *static-string-literal* | *interpolated-string-literal* +> +> *string-literal-opening-delimiter* → *extended-string-literal-delimiter*_?_ **`"`** \ +> *string-literal-closing-delimiter* → **`"`** *extended-string-literal-delimiter*_?_ +> +> *static-string-literal* → *string-literal-opening-delimiter* *quoted-text*_?_ *string-literal-closing-delimiter* \ +> *static-string-literal* → *multiline-string-literal-opening-delimiter* *multiline-quoted-text*_?_ *multiline-string-literal-closing-delimiter* +> +> *multiline-string-literal-opening-delimiter* → *extended-string-literal-delimiter*_?_ **`"""`** \ +> *multiline-string-literal-closing-delimiter* → **`"""`** *extended-string-literal-delimiter*_?_ \ +> *extended-string-literal-delimiter* → **`#`** *extended-string-literal-delimiter*_?_ +> +> *quoted-text* → *quoted-text-item* *quoted-text*_?_ \ +> *quoted-text-item* → *escaped-character* \ +> *quoted-text-item* → 除 **`"`**、**`\`**、U+000A 或 U+000D 以外的任何 Unicode 标量值 +> +> *multiline-quoted-text* → *multiline-quoted-text-item* *multiline-quoted-text*_?_ \ +> *multiline-quoted-text-item* → *escaped-character* \ +> *multiline-quoted-text-item* → 除 **`\`** 以外的任何 Unicode 标量值 \ +> *multiline-quoted-text-item* → *escaped-newline* +> +> *interpolated-string-literal* → *string-literal-opening-delimiter* *interpolated-text*_?_ *string-literal-closing-delimiter* \ +> *interpolated-string-literal* → *multiline-string-literal-opening-delimiter* *multiline-interpolated-text*_?_ *multiline-string-literal-closing-delimiter* +> +> *interpolated-text* → *interpolated-text-item* *interpolated-text*_?_ \ +> *interpolated-text-item* → **`\(`** *expression* **`)`** | *quoted-text-item* +> +> *multiline-interpolated-text* → *multiline-interpolated-text-item* *multiline-interpolated-text*_?_ \ +> *multiline-interpolated-text-item* → **`\(`** *expression* **`)`** | *multiline-quoted-text-item* +> +> *escape-sequence* → **`\`** *extended-string-literal-delimiter* \ +> *escaped-character* → *escape-sequence* **`0`** | *escape-sequence* **`\`** | *escape-sequence* **`t`** | *escape-sequence* **`n`** | *escape-sequence* **`r`** | *escape-sequence* **`"`** | *escape-sequence* **`'`** \ +> *escaped-character* → *escape-sequence* **`u`** **`{`** *unicode-scalar-digits* **`}`** \ +> *unicode-scalar-digits* → 一到八位十六进制数字 +> +> *escaped-newline* → *escape-sequence* *inline-spaces*_?_ *line-break* + + + + + +### 正则表达式字面量 + +正则表达式字面量是一串被斜杠（`/`）包围的一串字符序列，形式如下： + +```swift +/<#regular expression#>/ +``` + +正则表达式字面量不能以未转义的制表符或空格开头，也不能包含未转义的斜杠（`/`）、回车符或换行符。 + +在正则表达式字面量中，反斜杠被视为正则表达式的一部分，而不仅仅是像在字符串字面量中那样作为转义字符。它表示后续的特殊字符应按字面理解，或者后续的非特殊字符应按特殊方式处理。例如，`/\(/` 匹配一个左括号，而 `/\d/` 匹配一个数字。 + + + +由扩展定界符包围的正则表达式字面量是一串被斜杠（`/`）和一组或多组配对的井号（`#`）包围的字符序列。使用扩展定界符包围的正则表达式字面量有以下形式： + +```swift +#/<#regular expression#>/# + +#/ +<#regular expression#> +/# +``` + +使用扩展定界符的正则表达式字面量可以以未转义的空格或制表符开头，可以包含未转义的斜杠（`/`），并且可以跨多行。在多行正则表达式字面量中，开始定界符必须在一行的末尾，结束定界符必须独立占一行。在多行正则表达式字面量内部，扩展正则表达式语法默认启用——具体来说，空白字符将被忽略，并允许使用注释。 + + + +如果使用多个井号来形成由扩展定界符包围的正则表达式字面量，不要在井号之间留有空格： + +```swift +let regex1 = ##/abc/## // 正确 +let regex2 = # #/abc/# # // 错误 +``` + + + +如果你需要创建一个空的正则表达式字面量，则必须使用扩展定界符语法。 + +> 正则表达式字面量的语法: +> +> *regular-expression-literal* → *regular-expression-literal-opening-delimiter* *regular-expression* *regular-expression-literal-closing-delimiter* \ +> *regular-expression* → 任何正则表达式 +> +> *regular-expression-literal-opening-delimiter* → *extended-regular-expression-literal-delimiter*_?_ **`/`** \ +> *regular-expression-literal-closing-delimiter* → **`/`** *extended-regular-expression-literal-delimiter*_?_ +> +> *extended-regular-expression-literal-delimiter* → **`#`** *extended-regular-expression-literal-delimiter*_?_ + +## 运算符 + +Swift 标准库定义了许多运算符供你使用，其中许多运算符在 和 中进行了讨论。本节描述了哪些字符可以用于定义自定义运算符。 + +自定义运算符可以用这些 ASCII 字符之一开头：`/`、`=`、`-`、`+`、`!`、`*`、`%`、`<`、`>`、`&`、`|`、`^`、`?` 或 `~`，或者是定义在下面语法中的 Unicode 字符（其中包括来自 *数学运算符（Mathematical Operators）*、*杂项符号（Miscellaneous Symbols）* 和 *装饰符号（Dingbats）* Unicode 块的字符等）之一。在第一个字符之后，还允许使用组合 Unicode 字符（Combining Unicode Character）。 + +你还可以定义以点（`.`）开头的自定义运算符。这些运算符可以包含额外的点。例如，`.+.` 被视为一个单一的运算符。如果一个运算符不是以点开头的，则它不能在其他地方包含点。例如，`+.+` 被视为 `+` 运算符后跟 `.+` 运算符。 + + + +虽然你可以定义包含问号 (`?`) 的自定义运算符，但它们不能仅由单个问号字符组成。此外，尽管运算符可以包含感叹号 (`!`)，但后缀运算符不能以问号或感叹号开头。 + + + + + +> 注意: 标记符号 `=`、`->`、`//`、`/*`、`*/`、`.`，以及前缀运算符 `<`、`&` 和 `?`，中缀运算符 `?`，后缀运算符 `>`、`!` 和 `?` 都是保留标记符号。 +> 这些标记符号不能被重载，也不能用作自定义运算符。 + +运算符周围的空白用来确定运算符是作为前缀运算符、后缀运算符还是中缀运算符使用。这种行为遵循以下规则： + +- 如果运算符两边都有空白或都没有空白，则它被视为中缀运算符。例如，表达式 `a+++b` 和 `a +++ b` 中的 `+++` 被视为中缀运算符。 +- 如果运算符左边有空白而右边没有空白，则它被视为前缀一元运算符。例如，表达式 `a +++b` 中的 `+++` 被视为前缀一元运算符。 +- 如果运算符右边有空白而左边没有空白，则它被视为后缀一元运算符。例如，表达式 `a+++ b` 中的 `+++` 被视为后缀一元运算符。 +- 如果运算符左边没有空白但紧跟在它后面的是一个点号 (`.`)，则它被视为后缀一元运算符。例如，表达式 `a+++.b` 中的 `+++` 被视为后缀一元运算符（解释为 `a+++ .b` 而非 `a +++ .b`）。 + +在这些规则中，运算符前的字符 `(`、`[` 和 `{`，运算符后的字符 `)`、`]` 和 `}`，以及字符 `,`、`;` 和 `:` 也被视为空白。 + +如果预定义的 `!` 或 `?` 运算符左边没有空白，无论右边是否有空白，它都会被视为后缀运算符。要使用 `?` 作为可选链运算符（Optional-Chaining Operator），它左边必须没有空白。要在三元条件运算符 (`?` `:`) 中使用它，则必须在左右两边都有空白。 + +如果中缀运算符的其中一个参数是正则表达式字面量，则该运算符的左右两边都必须有空白。 + +在某些构造中，以 `<` 或 `>` 开头的运算符可能会被拆分为两个或多个标记符号。拆分后剩余的部分会以同样的规则处理，并可能再次被拆分。这意味着在 `Dictionary>` 这样的构造中，你不需要添加空白来消除闭合 `>` 符号之间的歧义。在这个例子中，闭合的 `>` 符号不会被视为单个标记符号，也不会被错误解释为位移 `>>` 运算符。 + + + +要了解如何定义新的自定义操作符，请参阅 和 。要了解如何重载现有的操作符，请参阅 。 + + + +> 操作符的语法: +> +> *operator* → *operator-head* *operator-characters*_?_ \ +> *operator* → *dot-operator-head* *dot-operator-characters* +> +> *operator-head* → **`/`** | **`=`** | **`-`** | **`+`** | **`!`** | **`*`** | **`%`** | **`<`** | **`>`** | **`&`** | **`|`** | **`^`** | **`~`** | **`?`** \ +> *operator-head* → U+00A1–U+00A7 \ +> *operator-head* → U+00A9 或 U+00AB \ +> *operator-head* → U+00AC 或 U+00AE \ +> *operator-head* → U+00B0–U+00B1 \ +> *operator-head* → U+00B6、U+00BB、U+00BF、U+00D7 或 U+00F7 \ +> *operator-head* → U+2016–U+2017 \ +> *operator-head* → U+2020–U+2027 \ +> *operator-head* → U+2030–U+203E \ +> *operator-head* → U+2041–U+2053 \ +> *operator-head* → U+2055–U+205E \ +> *operator-head* → U+2190–U+23FF \ +> *operator-head* → U+2500–U+2775 \ +> *operator-head* → U+2794–U+2BFF \ +> *operator-head* → U+2E00–U+2E7F \ +> *operator-head* → U+3001–U+3003 \ +> *operator-head* → U+3008–U+3020 \ +> *operator-head* → U+3030 +> +> *operator-character* → *operator-head* \ +> *operator-character* → U+0300–U+036F \ +> *operator-character* → U+1DC0–U+1DFF \ +> *operator-character* → U+20D0–U+20FF \ +> *operator-character* → U+FE00–U+FE0F \ +> *operator-character* → U+FE20–U+FE2F \ +> *operator-character* → U+E0100–U+E01EF \ +> *operator-characters* → *operator-character* *operator-characters*_?_ +> +> *dot-operator-head* → **`.`** \ +> *dot-operator-character* → **`.`** | *operator-character* \ +> *dot-operator-characters* → *dot-operator-character* *dot-operator-characters*_?_ +> +> *infix-operator* → *operator* \ +> *prefix-operator* → *operator* \ +> *postfix-operator* → *operator* + + diff --git a/swift-6.docc/ReferenceManual/Patterns.md b/swift-6.docc/ReferenceManual/Patterns.md new file mode 100644 index 000000000..ad0f89360 --- /dev/null +++ b/swift-6.docc/ReferenceManual/Patterns.md @@ -0,0 +1,224 @@ +# 模式 + +模式代表单个值或者复合值的结构。例如，元组 `(1, 2)` 的结构是由逗号分隔的，包含两个元素的列表。因为模式代表一种值的结构，而不是特定的某个值，你可以利用模式来匹配各种各样的值。比如，`(x, y)` 可以匹配元组 `(1, 2)`，以及任何含两个元素的元组。除了利用模式匹配一个值以外，你可以从复合值中提取出部分或全部值，然后分别把各个部分的值和一个常量或变量绑定起来。 + +Swift 中的模式分为两类：一种能成功匹配任何类型的值，另一种在运行时匹配某个特定值时可能会失败。 + +第一类模式用于解构简单变量、常量和可选绑定中的值。此类模式包括通配符模式、标识符模式，以及包含前两种模式的值绑定模式和元组模式。你可以为这类模式指定一个类型注解，从而限制它们只能匹配某种特定类型的值。 + +第二类模式用于全模式匹配，这种情况下你试图匹配的值在运行时可能不存在。此类模式包括枚举用例模式、可选模式、表达式模式和类型转换模式。你在 `switch` 语句的 `case` 标签中，`do` 语句的 `catch` 子句中，或者在 `if`、`while`、`guard` 和 `for-in` 语句的 `case` 条件句中使用这类模式。 + +> 模式语法: +> +> *pattern* → *wildcard-pattern* *type-annotation*_?_ \ +> *pattern* → *identifier-pattern* *type-annotation*_?_ \ +> *pattern* → *value-binding-pattern* \ +> *pattern* → *tuple-pattern* *type-annotation*_?_ \ +> *pattern* → *enum-case-pattern* \ +> *pattern* → *optional-pattern* \ +> *pattern* → *type-casting-pattern* \ +> *pattern* → *expression-pattern* + +## 通配符模式 + +*通配符模式*由一个下划线（`_`）构成，用于匹配并忽略任何值。当你想忽略被匹配的值时可以使用该模式。例如，下面这段代码在闭区间 1...3 中迭代，每次迭代都忽略该区间的当前值： + +```swift +for _ in 1...3 { + // Do something three times. +} +``` + +> 通配符模式的语法: +> +> *wildcard-pattern* → **`_`** + +## 标识符模式 + +*标识符模式*匹配任何值，并将匹配的值和一个变量或常量绑定起来。例如，在下面的常量声明中，`someValue` 是一个标识符模式，匹配了 `Int` 类型的 `42`： + +```swift +let someValue = 42 +``` +当匹配成功时,值 `42` 被绑定（分配）到常量名称 `someValue` . + +如果一个变量或常量声明的左边是一个标识符模式，那么这个标识符模式是值绑定模式的子模式。 + +> 标识符模式语法 +> +> *identifier-pattern* → *identifier* + +## 值绑定模式 + +*值绑定模式*把匹配到的值绑定给一个变量或常量。把匹配到的值绑定给常量时，用关键字 `let`，绑定给变量时，用关键字 `var`。 + +在值绑定模式中的标识符模式会把新命名的变量或常量与匹配到的值做绑定。例如，你可以拆开一个元组，然后把每个元素绑定到相应的标识符模式中。 + +```swift +let point = (3, 2) +switch point { +// 将 point 中的元素绑定到 x 和 y +case let (x, y): + print("The point is at (\(x), \(y)).") +} +// 打印 "The point is at (3, 2)." +``` + +在上面的示例中， `let` 分配给元组模式`(x, y)` 中的每个标识符模式。因此， `switch` 语句中 `case let (x, y):` 和 `case (let x, let y):` 的匹配效果是一样的。 + +> 值绑定模式语法 +> +> *value-binding-pattern* → **`var`** *pattern* | **`let`** *pattern* + +## 元组模式 + +*元组模式*是由逗号分隔的，具有零个或多个模式的列表，并由一对圆括号括起来。元组模式匹配相应元组类型的值。 + +你可以使用类型注解去限制一个元组模式能匹配哪种元组类型。例如，在常量声明 let (x, y): (Int, Int) = (1, 2) 中的元组模式 (x, y): (Int, Int) 只匹配两个元素都是 Int 类型的元组。 + +当元组模式用作 `for-in` 语句或变量或常量声明中的模式时，它只能包含通配符模式、标识符模式、可选模式或包含这些模式的其他元组模式。例如，以下代码无效，因为元组模式 `(x, 0)` 中的元素 `0` 是表达式模式： + +```swift +let points = [(0, 0), (1, 0), (1, 1), (2, 0), (2, 1)] +// 下面的代码是错误的 +for (x, 0) in points { + /* ... */ +} +``` + +只包含一个元素的元组模式的圆括号没有效果，模式只匹配这个单个元素的类型。举例来说，下面的语句是等效的： + + +```swift +let a = 2 // a: Int = 2 +let (a) = 2 // a: Int = 2 +let (a): Int = 2 // a: Int = 2 +``` + +> 元组模式语法: +> +> *tuple-pattern* → **`(`** *tuple-pattern-element-list*_?_ **`)`** \ +> *tuple-pattern-element-list* → *tuple-pattern-element* | *tuple-pattern-element* **`,`** *tuple-pattern-element-list* \ +> *tuple-pattern-element* → *pattern* | *identifier* **`:`** *pattern* + +## 枚举用例模式 + +*枚举用例模式*匹配现有的某个枚举类型的某个用例。枚举用例模式出现在 `switch` 语句中的 `case` 标签中，以及 `if`、`while`、`guard` 和 `for-in` 语句的 `case` 条件中。 + +如果你准备匹配的枚举用例有任何关联的值，则相应的枚举用例模式必须指定一个包含每个关联值元素的元组模式。关于使用 `switch` 语句来匹配包含关联值的枚举用例的例子，请参阅. + +枚举用例模式同样会匹配那些被包装成可选值的用例。简化的语法能将可选模式过滤掉。注意，由于 `Optional` 是枚举实现的，`.none` 和 `.some` 都会作为枚举类型的用例出现在 switch 中。 + +```swift +enum SomeEnum { case left, right } +let x: SomeEnum? = .left +switch x { +case .left: + print("Turn left") +case .right: + print("Turn right") +case nil: + print("Keep going straight") +} +// 打印 "Turn left" +``` + +> 枚举用例模式语法: +> +> *enum-case-pattern* → *type-identifier*_?_ **`.`** *enum-case-name* *tuple-pattern*_?_ + +## 可选模式 + +可选模式匹配包装在一个 `Optional(Wrapped)` 或者 `ExplicitlyUnwrappedOptional(Wrapped)` 枚举中的 `Some(Wrapped)` 用例中的值。可选模式由一个标识符模式和紧随其后的一个问号组成，可以像枚举用例模式一样使用。 + +由于可选模式是 `Optional` 和 `ImplicitlyUnwrappedOptional` 枚举用例模式的语法糖，下面两种写法是等效的： + +```swift +let someOptional: Int? = 42 +// 使用枚举用例模式匹配 +if case .some(let x) = someOptional { + print(x) +} + +// 使用可选模式匹配 +if case let x? = someOptional { + print(x) +} +``` + +可选模式为 `for-in` 语句提供了一种迭代数组的简便方式，只为数组中非 `nil` 的元素执行循环体。 + +```swift +let arrayOfOptionalInts: [Int?] = [nil, 2, 3, nil, 5] +// 只匹配非 nil 的元素 +for case let number? in arrayOfOptionalInts { + print("Found a \(number)") +} +// Found a 2 +// Found a 3 +// Found a 5 +``` + +> 可选模式语法: +> +> *optional-pattern* → *identifier-pattern* **`?`** + +## 类型转换模式 + +有两种类型转换模式： `is` 模式和`as` 模式。 `is` 模式仅出现在`switch` 语句 case 标签中. `is` 和 `as` 模式具有以下形式: + +```swift +is <#类型#> +<#pattern#> as <#type#> +``` + +`is` 模式仅当一个值的类型在运行时和 `is` 模式右边的指定类型一致，或者是其子类的情况下，才会匹配这个值。`is` 模式和 `is` 运算符有相似表现，它们都进行类型转换，但是 `is` 模式没有返回类型。 + +`as` 模式仅当一个值的类型在运行时和 `as` 模式右边的指定类型一致，或者是其子类的情况下，才会匹配这个值。如果匹配成功，被匹配的值的类型被转换成 `as` 模式右边指定的类型。 + +关于使用 `switch` 语句配合 `is` 模式和 `as` 模式来匹配值的例子，请参阅. + +> 类型转换模式语法 +> +> *type-casting-pattern* → *is-pattern* | *as-pattern* \ +> *is-pattern* → **`is`** *type* \ +> *as-pattern* → *pattern* **`as`** *type* + +## 表达式模式 + +*表达式模式*代表表达式的值。表达式模式只出现在 `switch` 语句中的 `case` 标签中。 + +表达式模式代表的表达式会使用 Swift 标准库中的 `~=` 运算符与输入表达式的值进行比较。如果 `~=` 运算符返回 `true`，则匹配成功。默认情况下，`~=` 运算符使用 `==` 运算符来比较两个相同类型的值。它也可以将一个整型数值与一个 Range 实例中的一段整数区间做匹配，正如下面这个例子所示： + +```swift +let point = (1, 2) +switch point { +case (0, 0): + print("(0, 0) is at the origin.") +case (-2...2, -2...2): + print("(\(point.0), \(point.1)) is near the origin.") +default: + print("The point is at (\(point.0), \(point.1)).") +} +// 打印"(1, 2) is near the origin." +``` + +你可以重载 `~=` 运算符来提供自定义的表达式匹配行为。比如你可以重写上面的例子，将 `point` 表达式与字符串形式表示的点进行比较。 + +```swift +// 重载 ~= 运算符对字符串和整数进行比较 +func ~= (pattern: String, value: Int) -> Bool { + return pattern == "\(value)" +} +switch point { +case ("0", "0"): + print("(0, 0) is at the origin.") +default: + print("The point is at (\(point.0), \(point.1)).") +} +// 打印 "The point is at (1, 2)." +``` + +> 表达式模式语法: +> +> *expression-pattern* → *expression* diff --git a/swift-6.docc/ReferenceManual/Statements.md b/swift-6.docc/ReferenceManual/Statements.md new file mode 100644 index 000000000..08dc84732 --- /dev/null +++ b/swift-6.docc/ReferenceManual/Statements.md @@ -0,0 +1,1088 @@ + + +# 语句 + +对表达式进行分组，并控制执行流程。 + +在 Swift 中，有三种类型的语句：简单语句、编译器控制语句和控制流语句。简单语句是最常见的，用于构造表达式或者声明。编译器控制语句允许程序改变编译器的行为，包含编译配置语句和行控制语句。 + +控制流语句则用于控制程序执行的流程，Swift 中有多种类型的控制流语句：循环语句、分支语句和控制转移语句。循环语句用于重复执行代码块；分支语句用于执行满足特定条件的代码块；控制转移语句则用于改变代码的执行顺序。另外，Swift 提供了 `do` 语句，用于构建局部作用域，还用于错误的捕获和处理；还提供了 `defer` 语句，用于退出当前作用域之前执行清理操作。 + +是否将分号（`;`）添加到语句的末尾是可选的。但若要在同一行内写多条独立语句，则必须使用分号。 + +> 语句语法： +> +> *statement* → *expression* **`;`**_?_ \ +> *statement* → *declaration* **`;`**_?_ \ +> *statement* → *loop-statement* **`;`**_?_ \ +> *statement* → *branch-statement* **`;`**_?_ \ +> *statement* → *labeled-statement* **`;`**_?_ \ +> *statement* → *control-transfer-statement* **`;`**_?_ \ +> *statement* → *defer-statement* **`;`**_?_ \ +> *statement* → *do-statement* **`;`**_?_ \ +> *statement* → *compiler-control-statement* \ +> *statements* → *statement* *statements*_?_ + + + +## 循环语句 + +循环语句会根据特定的循环条件来重复执行代码块。Swift 提供三种类型的循环语句：`for-in` 语句、`while` 语句和 `repeat-while` 语句。 + +通过 break 语句和 continue 语句可以改变循环语句的控制流。有关这两条语句，可参阅 和 + 。 + +> 循环语句的语法： +> +> *loop-statement* → *for-in-statement* \ +> *loop-statement* → *while-statement* \ +> *loop-statement* → *repeat-while-statement* + +### For-In 语句 + +`for-in` 语句会为集合（或遵循[`Sequence`](https://developer.apple.com/documentation/swift/sequence)协议的任意类型）中的每一项执行一次代码块。 + +`for-in` 语句的形式如下： + +```swift +for <#item#> in <#collection#> { + <#statements#> +} +``` + +`for-in` 语句在循环开始前会调用集合表达式（`collection expression`）的 `makeIterator()` 方法来获取一个遵循了 `IteratorProtocol` 协议的迭代器类型。接下来循环开始，反复调用该迭代器的 `next()` 方法。如果其返回值不是 `nil`，它将会被赋给 `item`，然后执行循环体语句，执行完毕后回到循环开始处，继续重复这一过程；否则，既不会赋值也不会执行循环体语句，`for-in` 语句至此执行完毕。 + +> for-in 语句的语法： +> +> *for-in-statement* → **`for`** **`case`**_?_ *pattern* **`in`** *expression* *where-clause*_?_ *code-block* + +### While 语句 + +只要循环条件为真，`while` 语句就会重复执行代码块。 + +`while` 语句的形式如下： + +```swift +while <#condition#> { + <#statements#> +} +``` + + `while` 语句的执行流程如下： + +1. *条件* 判断 + + 如果为 `true`， 程序执行到第 2 步； + 如果为 `false`， 程序将结束执行 `while` 语句。 +2. 执行循环体中的语句，然后回到第 1 步 + +由于会在执行循环体中的语句前判断条件的值，因此循环体中的语句可能会被执行若干次，也可能一次也不会被执行。 + +条件的结果必须是 Bool 类型或者 Bool 的桥接类型。另外，条件语句也可以使用可选绑定，请参阅 。 + +> while 语句的语法： +> +> *while-statement* → **`while`** *condition-list* *code-block* +> +> *condition-list* → *condition* | *condition* **`,`** *condition-list* \ +> *condition* → *expression* | *availability-condition* | *case-condition* | *optional-binding-condition* +> +> *case-condition* → **`case`** *pattern* *initializer* \ +> *optional-binding-condition* → **`let`** *pattern* *initializer*_?_ | **`var`** *pattern* *initializer*_?_ + +### Repeat-While 语句 + +`repeat-while` 语句至少执行一次代码块，之后只要循环条件为真，就会重复执行代码块。 + +`repeat`-`while` 语句的形式如下： + +```swift +repeat { + <#statements#> +} while <#condition#> +``` + +`repeat`-`while`语句的执行流程如下： + +1. 执行循环体中的语句，然后转到第 2 步。 +2. 判断 *条件* 的值： + + 如果为 `true`，重复第 1 步； + 如果为 `false`，程序会结束执行 `repeat`-`while` 语句。 + +由于条件的值是在循环体中的语句执行后才进行判断，因此循环体中的语句至少会被执行一次。 + +条件的结果必须是 Bool 类型或者 Bool 的桥接类型。另外，条件语句也可以使用可选绑定，请参阅 + +> repeat-while 语句的语法： +> +> *repeat-while-statement* → **`repeat`** *code-block* **`while`** *expression* + +## 分支语句 + +分支语句会根据一个或者多个条件来执行指定部分的代码。分支语句中的条件将会决定程序如何分支以及执行哪部分代码。Swift 提供三种类型的分支语句：`if` 语句、 `guard` 语句和 `switch` 语句。 + +`if` 语句和 `switch` 语句中的控制流可以用 `break` 语句改变，关于`break`的用法，请参阅 。 + +> 分支语句的语法： +> +> *branch-statement* → *if-statement* \ +> *branch-statement* → *guard-statement* \ +> *branch-statement* → *switch-statement* + +### If 语句 + +`if` 语句会根据一个或多个条件来决定执行哪一块代码。 + +`if` 语句有两种基本形式，无论哪种形式，都必须有花括号。 + +第一种形式是当且仅当条件为真时执行代码，形式如下： + +```swift +if <#condition#> { + <#statements#> +} +``` + +第二种形式是在第一种形式的基础上添加 `else` 语句（通过引入 `else` 关键字），并且用于：当条件为真时执行一部分代码，当这同一个条件为假的时候执行另一部分代码。当只有一个 `else` 语句时，`if` 语句具有以下的形式： + +```swift +if <#condition#> { + <#statements to execute if condition is true#> +} else { + <#statements to execute if condition is false#> +} +``` + +`if` 语句的 `else` 语句也可包含另一个 `if` 语句，从而形成一条链来测试更多的条件，像下面这样： + +```swift +if <#condition 1#> { + <#statements to execute if condition 1 is true#> +} else if <#condition 2#> { + <#statements to execute if condition 2 is true#> +} else { + <#statements to execute if both conditions are false#> +} +``` + +if 语句中条件的结果必须是 Bool 类型或者 Bool 的桥接类型。另外，条件语句也可以使用可选绑定，请参阅 . + +> if 语句的语法: +> +> *if-statement* → **`if`** *condition-list* *code-block* *else-clause*_?_ \ +> *else-clause* → **`else`** *code-block* | **`else`** *if-statement* + +### Guard 语句 + +如果一个或者多个条件不成立，可用 `guard` 语句来退出当前作用域。 + +`guard` 语句的格式如下： + +```swift +guard <#condition#> else { + <#statements#> +} +``` + +语句中条件的结果必须是 Bool 类型或者 Bool 的桥接类型。另外，条件也可以是一条可选绑定，请参阅 。 + +在 `guard` 语句中进行可选绑定的任何常量或者变量，其可用范围从声明开始直到作用域结束。 + +`guard` 语句必须有 `else` 子句，而且必须在该子句中调用返回类型是 `Never` 的函数，或者使用下面的语句退出当前作用域： + +- `return` +- `break` +- `continue` +- `throw` + +关于控制转移语句，请参阅 。关于返回类型是 `Never` 的函数，请参阅。 + +> guard 语句的语法： +> +> *guard-statement* → **`guard`** *condition-list* **`else`** *code-block* + +### Switch 语句 + +`switch` 语句会根据控制表达式的值来决定执行哪部分代码。 + +`switch` 语句的形式如下： + +```swift +switch <#control expression#> { +case <#pattern 1#>: + <#statements#> +case <#pattern 2#> where <#condition#>: + <#statements#> +case <#pattern 3#> where <#condition#>, + <#pattern 4#> where <#condition#>: + <#statements#> +default: + <#statements#> +} +``` + +`switch` 语句会先计算 *控制表达式* 的值，然后与每一个 `case` 的模式进行匹配。如果匹配成功，程序将会执行对应的 `case` 中的语句。另外，每一个 `case` 的作用域都不能为空，也就是说在每一个 `case` 的冒号（`:`）后面必须至少有一条语句。如果你不想在匹配到的 `case` 中执行代码，只需在该 `case` 中写一条 `break` 语句即可。 + +可以用作控制表达式的值是十分灵活的。除了标量类型外，如 `Int`、`Character`，你可以使用任何类型的值，包括浮点数、字符串、元组、自定义类型的实例和可选类型。控制表达式的值还可以用来匹配枚举类型中的成员值或是检查该值是否包含在指定的 `Range` 中。关于如何在 `switch` 语句中使用这些类型，请参阅 中的 。 + +每个 case 的模式后面可以有一个 `where` 子句。`where` 子句由 `where` 关键字紧跟一个提供额外条件的表达式组成。因此，当且仅当控制表达式匹配一个 `case` 的模式且 `where` 子句的表达式为真时，`case` 中的语句才会被执行。在下面的例子中，控制表达式只会匹配包含两个相等元素的元组，例如 `(1, 1)`： + +```swift +case let (x, y) where x == y: +``` + + + +如上例所示， `case` 中的模式也可以使用 `let` 关键字绑定常量（也可以使用 `var` 关键字绑定变量）。这些常量（或变量）随后可以在相应的 `where` 子句中引用，并在 `case` 范围内的代码中使用。如果 `case` 包含多个匹配控制表达式的模式，则所有模式必须包含相同的常量或变量绑定，并且每个绑定的变量或常量在该 `case` 的所有模式中必须具有相同的类型。 + + + +`switch` 语句也可以包含默认分支，使用 `default` 关键字表示。只有所有 `case` 都无法匹配控制表达式时，默认分支中的代码才会被执行。一个 `switch` 语句只能有一个默认分支，而且必须在 `switch` 语句的最后面。 + +尽管模式匹配操作的实际执行顺序，特别是 `case` 中模式的求值顺序是未指定的，但在 switch 语句中，模式匹配的行为大概是按照源码中的顺序进行求值的——即它们在源代码中出现的顺序。因此，如果多个 `case` 包含求值为相同值的模式，从而可以匹配控制表达式的值，程序只会执行源代码中第一个匹配的 `case` 中的代码。 + + + + + +#### Switch 语句必须是详尽的 + +在 Swift 中，`switch` 语句中控制表达式的每一个可能的值都必须至少有一个 `case` 与之对应。在某些无法面面俱到的情况下（例如，表达式的类型是 `Int`），你可以使用 `default` 分支满足该要求。 + +#### 对未来枚举的`case`进行`switch` + +*非冻结枚举(`nonfrozen enumeration`)* 是一种特殊的枚举类型，它可能在未来会增加新的枚举 case，即使这时候你已经编译并且发布了你的应用，所以在 switch 非冻结枚举前需要深思熟虑。当一个库的作者们把一个枚举标记为非冻结的，这意味着他们保留了增加新的枚举 case 的权利，并且任何和这个枚举交互的代码都必须在无需重新编译的条件下能够处理那些未来可能新加入的 case 。只有演进模式的库代码、标准库代码、用 Swift 实现的 Apple 框架、C 以及 Objective-C 代码才能够声明非冻结枚举。更多关于冻结和非冻结枚举的内容，请参阅 . + +当你对非冻结枚举进行 switch 时，你总是需要有一个`default case`，即使每种枚举类型都已经有对应的 `case` 了。你可以在 `default` 前标注 `@unknown`，意思是这个 `case` 应该只匹配未来加入的枚举 `case`。如果你的 `default case` 中匹配了任何在编译时就能确定的枚举 `case`，Swift 会抛出一个警告。这可以很好地提醒你库的作者已经新增了一种 `case`，并且你还没有去处理。 + +以下就是一个例子，我们对标准库的 [`Mirror.AncestorRepresentation`](https://developer.apple.com/documentation/swift/mirror/ancestorrepresentation) 进行 switch 操作，每当有新的 case 加入，我们会得到一个警告，提示我们要去处理它。 + +```swift +let representation: Mirror.AncestorRepresentation = .generated +switch representation { +case .customized: + print("Use the nearest ancestor’s implementation.") +case .generated: + print("Generate a default mirror for all ancestor classes.") +case .suppressed: + print("Suppress the representation of all ancestor classes.") +@unknown default: + print("Use a representation that was unknown when this code was compiled.") +} +// Prints "Generate a default mirror for all ancestor classes." +``` + + + +#### 不存在隐式落入 + +当匹配到的 case 中的代码执行完毕后，switch 语句会直接退出，而不会继续执行下一个 case 。这就意味着，如果你想执行下一个 case，需要显式地在当前 case 中使用 fallthrough 语句。关于 fallthrough 语句的更多信息，请参阅 。 + +> switch 语句的语法： +> +> *switch-statement* → **`switch`** *expression* **`{`** *switch-cases*_?_ **`}`** \ +> *switch-cases* → *switch-case* *switch-cases*_?_ \ +> *switch-case* → *case-label* *statements* \ +> *switch-case* → *default-label* *statements* \ +> *switch-case* → *conditional-switch-case* +> +> *case-label* → *attributes*_?_ **`case`** *case-item-list* **`:`** \ +> *case-item-list* → *pattern* *where-clause*_?_ | *pattern* *where-clause*_?_ **`,`** *case-item-list* \ +> *default-label* → *attributes*_?_ **`default`** **`:`** +> +> *where-clause* → **`where`** *where-expression* \ +> *where-expression* → *expression* +> +> *conditional-switch-case* → *switch-if-directive-clause* *switch-elseif-directive-clauses*_?_ *switch-else-directive-clause*_?_ *endif-directive* \ +> *switch-if-directive-clause* → *if-directive* *compilation-condition* *switch-cases*_?_ \ +> *switch-elseif-directive-clauses* → *elseif-directive-clause* *switch-elseif-directive-clauses*_?_ \ +> *switch-elseif-directive-clause* → *elseif-directive* *compilation-condition* *switch-cases*_?_ \ +> *switch-else-directive-clause* → *else-directive* *switch-cases*_?_ + + + +## 带标签的语句 + +你可以在循环语句、if 语句、switch 语句或 do 语句前加上*语句标签*，它由标签名称跟一个冒号(`:`)组成。使用语句标签配合 `break` 和 `continue` 语句，以明确你希望如何在循环语句或 switch 语句中改变控制流程，关于`break` 和 `continue` 语句，请参阅 和 。 + +标签的作用域在该标签所标记的语句内。可以嵌套使用带标签的语句，但标签名称必须是唯一的。 + +关于使用带标签的语句的例子，请参阅 中的 。 + + + +> 带标签语句的语法： +> +> *labeled-statement* → *statement-label* *loop-statement* \ +> *labeled-statement* → *statement-label* *if-statement* \ +> *labeled-statement* → *statement-label* *switch-statement* \ +> *labeled-statement* → *statement-label* *do-statement* +> +> *statement-label* → *label-name* **`:`** \ +> *label-name* → *identifier* + +## 控制转移语句 + +控制转移语句能够无条件地把控制权从一片代码转移到另一片代码，从而改变代码执行的顺序。Swift 提供五种类型的控制转移语句：`break`语句、`continue` 语句、`fallthrough` 语句、`return` 语句和 `throw` 语句。 + +> 控制转移语句的语法： +> +> *control-transfer-statement* → *break-statement* \ +> *control-transfer-statement* → *continue-statement* \ +> *control-transfer-statement* → *fallthrough-statement* \ +> *control-transfer-statement* → *return-statement* \ +> *control-transfer-statement* → *throw-statement* + +### Break 语句 + +`break` 语句用于终止`循环`语句、`if` 语句或 `switch` 语句的执行。使用 `break` 语句时，可以只写 `break` 这个关键词，也可以在 `break` 后面跟上标签名，如下： + +```swift +break +break <#label name#> +``` + +当 `break` 语句后面带标签名时，可用于终止由这个标签标记的`循环`语句、`if` 语句或 `switch` 语句的执行。 + +而只写 `break` 时，则会终止 `switch` 语句或 `break` 语句所属的最内层循环语句的执行。不能使用 `break` 语句来终止未使用标签的 `if` 语句。 + +在这两种情况下，程序控制会被转移到包围的循环或 switch 语句后面的第一行代码（如果有的话）。 + +关于使用 break 语句的例子，请参阅中的 和 。 + +> break 语句的语法： +> +> *break-statement* → **`break`** *label-name*_?_ + +### Continue 语句 + +`continue` 语句用于终止循环中当前迭代的执行，但不会终止该循环的执行。使用 `continue` 语句时，可以只写 `continue` 这个关键词，也可以在 `continue` 后面跟上标签名，如下： + +```swift +continue +continue <#label name#> +``` + +当 `continue` 语句后面带标签名时，可用于终止由这个标签标记的循环中当前迭代的执行。 + +而当只写 `continue` 时，可用于终止 `continue` 语句所属的最内层循环中当前迭代的执行。 + +在这两种情况下，控制权都会被转移给循环语句的条件语句。 + +在 `for` 语句中，`continue` 语句执行后，增量表达式还是会被计算，这是因为每次循环体执行完毕后，增量表达式都会被计算。 + +关于使用 `continue` 语句的例子，请参阅中的 和 + +> continue 语句的语法： +> +> *continue-statement* → **`continue`** *label-name*_?_ + +### Fallthrough 语句 + +`fallthrough` 语句由 `fallthrough` 关键字组成，只能出现在 `switch` 语句的某个 `case` 块中。`fallthrough` 语句会导致程序执行从当前 `switch` 语句的一个 `case` 继续到下一个 `case`。即使下一个 `case` 标签的模式与 `switch` 语句的控制表达式的值不匹配，程序仍会继续执行该 `case`。 + +`fallthrough` 语句可出现在 `switch` 语句中的任意 `case` 中，但不能出现在最后一个 `case` 中。同时，`fallthrough` 语句也不能把控制权转移到使用了值绑定的 `case`。 + +关于在 `switch` 语句中使用 `fallthrough` 语句的例子，请参阅中的。 + +> fallthrough 语句的语法： +> +> *fallthrough-statement* → **`fallthrough`** + +### Return 语句 + +`return` 语句用于在函数或方法的实现中将控制权转移到调用函数或方法，接着程序将会从调用位置继续向下执行。 + +使用 `return` 语句时，可以只写 `return` 这个关键词，也可以在 `return` 后面跟上表达式，像下面这样： + +```swift +return +return <#expression#> +``` + +当 `return` 语句后面带表达式时，表达式的值将会返回给调用函数或方法。如果表达式的值的类型与函数或者方法声明的返回类型不匹配，Swift 则会在返回表达式的值之前将表达式的值的类型转换为返回类型。 + +> 注意： +> 正如 中所描述的， return 语句的一种特殊形式`return nil`可以在可失败的构造器中使用，以表示构造失败。 + + + +当只写 `return` 时，仅仅是从该函数或方法中返回，而不返回任何值（也就是说，函数或方法的返回类型为 `Void` 或者说 `()`）。 + +> return 语句的语法： +> +> *return-statement* → **`return`** *expression*_?_ + +### Throw 语句： + +`throw` 语句出现在抛出函数或者抛出方法体内，或者类型被 `throws` 关键字标记的闭包表达式体内。 + +`throw` 语句使程序在当前作用域结束执行，并向外围作用域传播错误。抛出的错误会一直传递，直到被 do 语句的 catch 子句处理掉。 + +`throw` 语句由 `throw` 关键字紧跟一个表达式组成，如下所示： + +```swift +throw <#expression#> +``` + + *表达式* 的值必须具有符合`Error`协议的类型。如果包含 `throw` 语句的 `do` 语句或函数声明了它抛出的错误类型，则该表达式的值必须是该类型的实例。 + +关于如何使用 `throw` 语句的例子，请参阅中的 + +> throw 语句的语法： +> +> *throw-statement* → **`throw`** *expression* + +## Defer 语句 + +`defer` 语句用于在将程序控制转移到 `defer` 语句所在作用域之外之前执行代码。 + +`defer` 语句的形式如下： + +```swift +defer { + <#statements#> +} +``` + +在 `defer` 语句中的语句无论程序控制如何转移都会被执行。在某些情况下，例如，手动管理资源时，比如关闭文件描述符，或者即使抛出了错误也需要执行一些操作时，就可以使用 `defer` 语句。 + +`defer` 语句中的语句在包含 `defer` 语句的作用域结束时执行。 + +```swift +func f(x: Int) { + defer { print("First defer") } + + if x < 10 { + defer { print("Second defer") } + print("End of if") + } + + print("End of function") +} +f(x: 5) +// Prints "End of if" +// Prints "Second defer" +// Prints "End of function" +// Prints "First defer" +``` + + + +在上面的代码中，`if` 语句中的 `defer` 会在函数 f 中声明的 `defer` 之前执行，因为 `if` 语句的作用域在函数的作用域之前结束。 + +如果多个 `defer` 语句出现在同一作用域中，它们的出现顺序与执行顺序相反。在给定作用域内，首先执行最后一个 `defer` 语句，这意味着该最后一个 `defer` 语句内部的语句可以引用将由其他 `defer` 语句清理的资源。 + +```swift +func f() { + defer { print("First defer") } + defer { print("Second defer") } + print("End of function") +} +f() +// Prints "End of function" +// Prints "Second defer" +// Prints "First defer" +``` + + + +`defer` 语句中的语句无法将控制权转移到 `defer` 语句外部。 + +> defer 语句的语法： +> +> *defer-statement* → **`defer`** *code-block* + +## Do 语句 + +`do` 语句用于引入一个新的作用域，该作用域中可以含有一个或多个 `catch` 子句，`catch` 子句中定义了一些匹配错误条件的模式。`do` 语句作用域内定义的常量和变量只能在 `do` 语句作用域内使用。 + +Swift 中的 `do` 语句与 C 中限定代码块界限的大括号（`{}`）很相似，也并不会降低程序运行时的性能。 + +`do` 语句的形式如下： + +```swift +do { + try <#expression#> + <#statements#> +} catch <#pattern 1#> { + <#statements#> +} catch <#pattern 2#> where <#condition#> { + <#statements#> +} catch <#pattern 3#>, <#pattern 4#> where <#condition#> { + <#statements#> +} catch { + <#statements#> +} +``` + +`do` 语句可以选择性地指定它抛出的错误类型，形式如下： + +```swift +do throws(<#type#>) { + try <#expression#> +} catch <#pattern> { + <#statements#> +} catch { + <#statements#> +} +``` + +如果 `do` 语句包含 `throws` 子句，则 `do` 块只能抛出指定类型的错误。类型必须是符合 `Error` 协议的具体类型、符合 `Error` 协议的不透明类型，或者是被封装的协议类型 `any Error`。如果 `do` 语句没有指定它抛出的错误类型，Swift 将按照以下方式推断错误类型： + +- 如果 `do` 代码块中的每个 `throws` 语句和 `try` 表达式都嵌套在一个全面的错误处理机制中，则 Swift 推断该 `do` 语句是非抛出的。 + +- 如果 `do` 代码块包含的代码抛出只有单一类型的错误，并且没有全面的错误处理（除了抛出 `Never`），则 Swift 推断该 `do` 语句抛出该具体错误类型。 + +- 如果 `do` 代码块包含的代码抛出超过一种类型的错误，并且没有全面的错误处理，则 Swift 推断该 `do` 语句抛出 `any Error`。 + +有关处理具有显式类型的错误的更多信息，可参阅 . + +如果 `do` 代码块中的任何语句抛出了错误，程序会跳转到第一个能模式匹配该错误的 `catch` 子句。如果没有任何子句匹配，错误会传递到外层作作用域。如果错误在最顶层依旧没有被处理，程序执行会因为运行时错误而停止。 + +如同 `switch` 语句，编译器会判断 `catch` 子句是否有遗漏。如果 `catch` 子句没有遗漏，则认为错误已被处理。否则，错误会自动传递到外层作用域，被某个 `catch` 子句处理掉或者被用 `throws` 关键字声明的抛出函数继续向外抛出。 + +拥有多个模式匹配的 `catch` 子句只需其中一个匹配到错误即可。如果 `catch` 子句拥有多个模式匹配，所有的模式必须包含相同的绑定常量或变量，并且每个 `catch` 子句里所有绑定的变量或常量的类型必须相同。 + + + +为了确保错误已经被处理，可以让 `catch` 子句使用匹配所有错误的模式，如通配符模式（`_`）。如果一个 `catch` 子句不指定一种具体模式，`catch` 子句会匹配任何错误，并绑定到名为 `error` 的局部常量。有关在 `catch` 子句中使用模式的更多信息， +可参阅 . + +关于如何在 `do` 语句中使用一系列 `catch` 子句的例子，请参阅. + +> do 语句的语法： +> +> *do-statement* → **`do`** *throws-clause*_?_ *code-block* *catch-clauses*_?_ \ +> *catch-clauses* → *catch-clause* *catch-clauses*_?_ \ +> *catch-clause* → **`catch`** *catch-pattern-list*_?_ *code-block* \ +> *catch-pattern-list* → *catch-pattern* | *catch-pattern* **`,`** *catch-pattern-list* \ +> *catch-pattern* → *pattern* *where-clause*_?_ + +## 编译器控制语句 + +编译器控制语句允许程序改变编译器的行为。Swift 有三种编译器控制语句：条件编译语句、线路控制语句和编译时诊断语句。 + +> 编译器控制语句的语法： +> +> *compiler-control-statement* → *conditional-compilation-block* \ +> *compiler-control-statement* → *line-control-statement* \ +> *compiler-control-statement* → *diagnostic-statement* + +### 条件编译代码块： + +条件编译代码块可以根据一个或多个配置来有条件地编译代码。 + +每一个条件编译代码块都以 `#if` 开始，`#endif` 结束。如下： + +```swift +#if <#compilation condition#> + <#statements#> +#endif +``` + +和`if`语句的条件不同， *编译条件* 是在编译时进行判断的。因此，只有当编译条件在编译时评估为 `true` 时，语句才会被编译和执。 + +编译条件可以包括 `true` 和 `false` 布尔字面量、与 `-D` 命令行标志一起使用的标识符，或者下表中列出的任何平台条件。 + +| Platform condition | Valid arguments | +| ------------------ | --------------- | +| `os()` | `macOS`, `iOS`, `watchOS`, `tvOS`, `visionOS`, `Linux`, `Windows` | +| `arch()` | `i386`, `x86_64`, `arm`, `arm64` | +| `swift()` | `>=` or `<` followed by a version number | +| `compiler()` | `>=` or `<` followed by a version number | +| `canImport()` | A module name | +| `targetEnvironment()` | `simulator`, `macCatalyst` | + + + + + +在 `swift()` 和 `compiler()` 之后的版本号包含有主版本号，可选副版本号，可选补丁版本号类似，并且用（`.`）来分隔。在比较符和版本号之间不能有空格，版本号与前面的函数相对应，比如 `compiler()` 对应的就是这个编译器的版本号，`swift()` 对应的就是你要编译的 Swift 语言的版本号。举个简单的例子，如果你在使用 `Swift 5` 的编译器，想编译 `Swift 4.2` ，可以看下面的例子： + +```swift +#if compiler(>=5) +print("Compiled with the Swift 5 compiler or later") +#endif +#if swift(>=4.2) +print("Compiled in Swift 4.2 mode or later") +#endif +#if compiler(>=5) && swift(<5) +print("Compiled with the Swift 5 compiler or later in a Swift mode earlier than 5") +#endif +// Prints "Compiled with the Swift 5 compiler or later" +// Prints "Compiled in Swift 4.2 mode or later" +// Prints "Compiled with the Swift 5 compiler or later in a Swift mode earlier than 5" +``` + + + + + +`canImport()` 条件传入的实参是模块的名字，这个模块可能并不是每个平台上都存在的。该模块的命名可以包含 . 符号。使用它可以检测是否可以导入这个模块，但实际上并没有导入。如果模块存在就返回 `true`，否则返回 `false` 。 + + + + + + + +`targetEnvironment()` 条件在特殊环境编译时返回 `true`；否则返回 `false`。 + +> 注意： `arch(arm)` 平台检测函数在 ARM 64 位设备上不会返回 `true` 。如果代码在 32 位的 iOS 模拟器上编译，`arch(i386)` 平台检测函数会返回 `true`。 + + + + + + + +你可以使用逻辑操作符 `&&`、`||` 和 `!` 来组合多个编译配置，还可以使用圆括号来进行分组。这些运算符与用于组合普通布尔表达式的逻辑运算符具有相同的结合性和优先级。 + +类似于 `if` 语句，你可以添加多个条件分支来测试不同的编译条件。你可以使用 `#elseif` 子句添加任意数量的附加分支。你还可以使用 `#else` 子句添加一个最终的附加分支。包含多个分支的条件编译块具有以下形式： + +```swift +#if <#compilation condition 1#> + <#statements to compile if compilation condition 1 is true#> +#elseif <#compilation condition 2#> + <#statements to compile if compilation condition 2 is true#> +#else + <#statements to compile if both compilation conditions are false#> +#endif +``` + +> 注意： 即使没有被编译，上面编译配置中的每个语句仍然会被解析。然而，唯一的例外是编译配置语句中包含 `swift()` 或 `compiler()` 条件：这时仅当编译器版本和语言版本匹配时，语句才会被解析。这种设定能确保旧的编译器不会尝试去解析新 Swift 版本的语法。 + +关于在条件编译块中如何包装显式成员表达式，请参见。 + +> 条件编译代码块的语法： +> +> *conditional-compilation-block* → *if-directive-clause* *elseif-directive-clauses*_?_ *else-directive-clause*_?_ *endif-directive* +> +> *if-directive-clause* → *if-directive* *compilation-condition* *statements*_?_ \ +> *elseif-directive-clauses* → *elseif-directive-clause* *elseif-directive-clauses*_?_ \ +> *elseif-directive-clause* → *elseif-directive* *compilation-condition* *statements*_?_ \ +> *else-directive-clause* → *else-directive* *statements*_?_ \ +> *if-directive* → **`#if`** \ +> *elseif-directive* → **`#elseif`** \ +> *else-directive* → **`#else`** \ +> *endif-directive* → **`#endif`** +> +> *compilation-condition* → *platform-condition* \ +> *compilation-condition* → *identifier* \ +> *compilation-condition* → *boolean-literal* \ +> *compilation-condition* → **`(`** *compilation-condition* **`)`** \ +> *compilation-condition* → **`!`** *compilation-condition* \ +> *compilation-condition* → *compilation-condition* **`&&`** *compilation-condition* \ +> *compilation-condition* → *compilation-condition* **`||`** *compilation-condition* +> +> *platform-condition* → **`os`** **`(`** *operating-system* **`)`** \ +> *platform-condition* → **`arch`** **`(`** *architecture* **`)`** \ +> *platform-condition* → **`swift`** **`(`** **`>=`** *swift-version* **`)`** | **`swift`** **`(`** **`<`** *swift-version* **`)`** \ +> *platform-condition* → **`compiler`** **`(`** **`>=`** *swift-version* **`)`** | **`compiler`** **`(`** **`<`** *swift-version* **`)`** \ +> *platform-condition* → **`canImport`** **`(`** *import-path* **`)`** \ +> *platform-condition* → **`targetEnvironment`** **`(`** *environment* **`)`** +> +> *operating-system* → **`macOS`** | **`iOS`** | **`watchOS`** | **`tvOS`** | **`visionOS`** | **`Linux`** | **`Windows`** \ +> *architecture* → **`i386`** | **`x86_64`** | **`arm`** | **`arm64`** \ +> *swift-version* → *decimal-digits* *swift-version-continuation*_?_ \ +> *swift-version-continuation* → **`.`** *decimal-digits* *swift-version-continuation*_?_ \ +> *environment* → **`simulator`** | **`macCatalyst`** + + + +### 行控制语句 + +行控制语句用于指定一个行号和文件名，这些行号和文件名可以与正在编译的源代码的行号和文件名不同。使用行控制语句可以更改 Swift 用于诊断和调试目的的源代码位置。 + +行控制语句形式如下： + +```swift +#sourceLocation(file: <#file path#>, line: <#line number#>) +#sourceLocation() +``` + +第一种的行控制语句会改变该语句之后的代码中的字面量表达式 `#line`、 `#file`、`#fileID` 和 `#filePath` 所表示的值，从行控制语句里行号的代码开始。*行号*是一个大于 0 的整形字面量，会改变 `#line` 表达式的值。*file path*会更改 `#file`、`#fileID` 和 `#filePath` 的值，并且是一个字符串字面量。指定的字符串成为 `#filePath` 的值，字符串的最后一个路径组件用于 `#fileID` 的值。关于 `#file`、 `#fileID` 和 `#filePath`，可参阅. + +行控制语句的第二种形式是 `#sourceLocation()`，会将源代码的定位信息重置回默认的行号和文件名。 + +> 行控制语句的语法: +> +> *line-control-statement* → **`#sourceLocation`** **`(`** **`file:`** *file-path* **`,`** **`line:`** *line-number* **`)`** \ +> *line-control-statement* → **`#sourceLocation`** **`(`** **`)`** \ +> *line-number* → A decimal integer greater than zero \ +> *file-path* → *static-string-literal* + +### 编译时诊断语句 + +在 `Swift 5.9` 之前，`#warning`和`#error`语句在编译期间会发出诊断。。现在，这一行为由 Swift 标准库中的 [warning(_:)][] 和 [error(_:)][] 宏提供。 + +[`warning(_:)`]: http://developer.apple.com/documentation/swift/documentation/swift/warning(_:) +[`error(_:)`]: http://developer.apple.com/documentation/swift/documentation/swift/error(_:) + +## 可用性条件 + +可用性条件可作为 `if`，`while`，`guard` 语句的条件，可以在运行时基于特定的平台参数来查询 API 的可用性。 + +可用性条件的形式如下： + +```swift +if #available(<#platform name#> <#version#>, <#...#>, *) { + <#statements to execute if the APIs are available#> +} else { + <#fallback statements to execute if the APIs are unavailable#> +} +``` + +使用可用性条件来执行一个代码块时，取决于使用的 API 在运行时是否可用，编译器会根据可用性条件提供的信息来决定是否执行相应的代码块。 + +可用性条件使用一系列逗号分隔的平台名称和版本。使用 `iOS`，`macOS`，`watchOS`，`tvOS`，`visionOS` 作为平台名称，并写上相应的版本号。`*` 参数是必须写的，用于处理未来的潜在平台。可用性条件确保了运行时的平台不低于条件中指定的平台版本时才执行代码块。 + +与布尔类型的条件不同，不能用逻辑运算符 `&&` 和 `||` 组合可用性条件。不要使用 `!` 来表示平台不可以用，可以使用“不可用性条件”，其形式如下： + +```swift +if #unavailable(<#platform name#> <#version#>, <#...#>) { + <#fallback statements to execute if the APIs are unavailable#> +} else { + <#statements to execute if the APIs are available#> +} +``` + +`#unavailable` 形式是语法糖，用于取反条件。在不可用性条件中，`*` 参数是隐式的，不能包含。它与可用性条件中的 `*` 参数具有相同的含义。 + +> 可用性条件的语法： +> +> *availability-condition* → **`#available`** **`(`** *availability-arguments* **`)`** \ +> *availability-condition* → **`#unavailable`** **`(`** *availability-arguments* **`)`** \ +> *availability-arguments* → *availability-argument* | *availability-argument* **`,`** *availability-arguments* \ +> *availability-argument* → *platform-name* *platform-version* \ +> *availability-argument* → **`*`** +> +> +> +> *platform-name* → **`iOS`** | **`iOSApplicationExtension`** \ +> *platform-name* → **`macOS`** | **`macOSApplicationExtension`** \ +> *platform-name* → **`macCatalyst`** | **`macCatalystApplicationExtension`** \ +> *platform-name* → **`watchOS`** | **`watchOSApplicationExtension`** \ +> *platform-name* → **`tvOS`** | **`tvOSApplicationExtension`** \ +> *platform-name* → **`visionOS`** | **`visionOSApplicationExtension`** \ +> *platform-version* → *decimal-digits* \ +> *platform-version* → *decimal-digits* **`.`** *decimal-digits* \ +> *platform-version* → *decimal-digits* **`.`** *decimal-digits* **`.`** *decimal-digits* + + + + + + + + + + diff --git a/swift-6.docc/ReferenceManual/SummaryOfTheGrammar.md b/swift-6.docc/ReferenceManual/SummaryOfTheGrammar.md new file mode 100644 index 000000000..de1eee022 --- /dev/null +++ b/swift-6.docc/ReferenceManual/SummaryOfTheGrammar.md @@ -0,0 +1,1083 @@ +# 语法总结 +完整语法一览。 + +## 词法结构 + +> 空白符语法： +> +> *空白符* → *空白符项* *空白符*可选 \ +> *空白符项* → *换行符* \ +> *空白符项* → *行内空格* \ +> *空白符项* → *注释* \ +> *空白符项* → *多行注释* \ +> *空白符项* → U+0000, U+000B, 或 U+000C +> +> *换行符* → U+000A \ +> *换行符* → U+000D \ +> *换行符* → U+000D 后接 U+000A +> +> *行内空格* → *行内空格符* *行内空格*可选 \ +> *行内空格符* → U+0009 或 U+0020 +> +> *注释* → **`//`** *注释文本* *换行符* \ +> *多行注释* → **`/*`** *多行注释文本* **`*/`** +> +> *注释文本* → *注释文本项* *注释文本*可选 \ +> *注释文本项* → 除 U+000A 或 U+000D 外的任意 Unicode 标量值 +> +> *多行注释文本* → *多行注释文本项* *多行注释文本*可选 \ +> *多行注释文本项* → *多行注释* \ +> *多行注释文本项* → *注释文本项* \ +> *多行注释文本项* → 除 **`/*`** 或 **`*/`** 外的任意 Unicode 标量值 + +> 标识符语法： +> +> *标识符* → *标识符头（Head）* *标识符字符集*可选 \ +> *标识符* → **`` ` ``** *标识符头（Head）* *标识符字符集*可选 **`` ` ``** \ +> *标识符* → *隐式参数名* \ +> *标识符* → *属性包装器呈现值* \ +> *标识符集* → *标识符* | *标识符* **`,`** *标识符集* +> +> *标识符头（Head）* → A 到 Z 的大写或小写字母 \ +> *标识符头（Head）* → **`_`** \ +> *标识符头（Head）* → U+00A8, U+00AA, U+00AD, U+00AF, U+00B2–U+00B5, 或 U+00B7–U+00BA \ +> *标识符头（Head）* → U+00BC–U+00BE, U+00C0–U+00D6, U+00D8–U+00F6, 或 U+00F8–U+00FF \ +> *标识符头（Head）* → U+0100–U+02FF, U+0370–U+167F, U+1681–U+180D, 或 U+180F–U+1DBF \ +> *标识符头（Head）* → U+1E00–U+1FFF \ +> *标识符头（Head）* → U+200B–U+200D, U+202A–U+202E, U+203F–U+2040, U+2054, 或 U+2060–U+206F \ +> *标识符头（Head）* → U+2070–U+20CF, U+2100–U+218F, U+2460–U+24FF, 或 U+2776–U+2793 \ +> *标识符头（Head）* → U+2C00–U+2DFF 或 U+2E80–U+2FFF \ +> *标识符头（Head）* → U+3004–U+3007, U+3021–U+302F, U+3031–U+303F, 或 U+3040–U+D7FF \ +> *标识符头（Head）* → U+F900–U+FD3D, U+FD40–U+FDCF, U+FDF0–U+FE1F, 或 U+FE30–U+FE44 \ +> *标识符头（Head）* → U+FE47–U+FFFD \ +> *标识符头（Head）* → U+10000–U+1FFFD, U+20000–U+2FFFD, U+30000–U+3FFFD, 或 U+40000–U+4FFFD \ +> *标识符头（Head）* → U+50000–U+5FFFD, U+60000–U+6FFFD, U+70000–U+7FFFD, 或 U+80000–U+8FFFD \ +> *标识符头（Head）* → U+90000–U+9FFFD, U+A0000–U+AFFFD, U+B0000–U+BFFFD, 或 U+C0000–U+CFFFD \ +> *标识符头（Head）* → U+D0000–U+DFFFD 或 U+E0000–U+EFFFD +> +> *标识符字符* → 数字 0 到 9 \ +> *标识符字符* → U+0300–U+036F, U+1DC0–U+1DFF, U+20D0–U+20FF, 或 U+FE20–U+FE2F \ +> *标识符字符* → *标识符头（Head）* \ +> *标识符字符集* → *标识符字符项* *标识符字符集*可选 +> +> *隐式参数名* → **`$`** *十进制数字* \ +> *属性包装器呈现值* → **`$`** *标识符字符集* + +> 字面量语法： +> +> *字面量* → *数字字面量* | *字符串字面量* | *正则表达式字面量* | *布尔字面量* | *空字面量* +> +> *数字字面量* → **`-`** 可选 *整型字面量* | **`-`** 可选 *浮点型字面量* \ +> *布尔字面量* → **`true`** | **`false`** \ +> *空字面量* → **`nil`** + +> 整型字面量语法： +> +> *整型字面量* → *二进制字面量* \ +> *整型字面量* → *八进制字面量* \ +> *整型字面量* → *十进制字面量* \ +> *整型字面量* → *十六进制字面量* +> +> *二进制字面量* → **`0b`** *二进制数字* *二进制字面量字符* 可选 \ +> *二进制数字* → 数字 0 或 1 \ +> *二进制字面量字符* → *二进制数字* | **`_`** \ +> *二进制字面量字符* → *二进制字面量字符* *二进制字面量字符* 可选 +> +> *八进制字面量* → **`0o`** *八进制数字* *八进制字面量字符* 可选 \ +> *八进制数字* → 数字 0 到 7 \ +> *八进制字面量字符* → *八进制数字* | **`_`** \ +> *八进制字面量字符* → *八进制字面量字符* *八进制字面量字符* 可选 +> +> *十进制字面量* → *十进制数字* *十进制字面量字符* 可选 \ +> *十进制数字* → 数字 0 到 9 \ +> *十进制数字* → *十进制数字* *十进制数字* 可选 \ +> *十进制字面量字符* → *十进制数字* | **`_`** \ +> *十进制字面量字符* → *十进制字面量字符* *十进制字面量字符* 可选 +> +> *十六进制字面量* → **`0x`** *十六进制数字* *十六进制字面量字符* 可选 \ +> *十六进制数字* → 数字 0 到 9，a 到 f，或 A 到 F \ +> *十六进制字面量字符* → *十六进制数字* | **`_`** \ +> *十六进制字面量字符* → *十六进制字面量字符* *十六进制字面量字符* 可选 + +> 浮点型字面量语法： +> +> *浮点型字面量* → *十进制字面量* *十进制分数* 可选 *十进制指数* 可选 \ +> *浮点型字面量* → *十六进制字面量* *十六进制分数* 可选 *十六进制指数* +> +> *十进制分数* → **`.`** *十进制字面量* \ +> *十进制指数* → *浮点数 e* *正负号* 可选 *十进制字面量* +> +> *十六进制分数* → **`.`** *十六进制数字* *十六进制字面量字符* 可选 \ +> *十六进制指数* → *浮点数 p* *正负号* 可选 *十进制字面量* +> +> *浮点数 e* → **`e`** | **`E`** \ +> *浮点数 p* → **`p`** | **`P`** \ +> *正负号* → **`+`** | **`-`** + +> 字符串字面量语法： +> +> *字符串字面量* → *静态字符串字面量* | *插值字符串字面量* +> +> *字符串开分隔定界符* → *字符串扩展分隔符* 可选 **`"`** \ +> *字符串闭分隔定界符* → **`"`** *字符串扩展分隔符* 可选 +> +> *静态字符串字面量* → *字符串开分隔定界符* *引用文本* 可选 *字符串闭分隔定界符* \ +> *静态字符串字面量* → *多行字符串开分隔定界符* *多行引用文本* 可选 *多行字符串闭分隔定界符* +> +> *多行字符串开分隔定界符* → *字符串扩展分隔符* 可选 **`"""`** \ +> *多行字符串闭分隔定界符* → **`"""`** *字符串扩展分隔符* 可选 \ +> *字符串扩展分隔符* → **`#`** *字符串扩展分隔符* 可选 +> +> *引用文本* → *引用文本项* *引用文本* 可选 \ +> *引用文本项* → *转义字符* \ +> *引用文本项* → 任何 Unicode 标量值，除了 **`"`**、**`\`**、U+000A 或 U+000D +> +> *多行引用文本* → *多行引用文本项* *多行引用文本* 可选 \ +> *多行引用文本项* → *转义字符* \ +> *多行引用文本项* → 任何 Unicode 标量值，除了 **`\`** \ +> *多行引用文本项* → *转义换行符* +> +> *插值字符串字面量* → *字符串开分隔定界符* *插值文本* 可选 *字符串闭分隔定界符* \ +> *插值字符串字面量* → *多行字符串开分隔定界符* *多行插值文本* 可选 *多行字符串闭分隔定界符* +> +> *插值文本* → *插值文本项* *插值文本* 可选 \ +> *插值文本项* → **`\(`** *表达式* **`)`** | *引用文本项* +> +> *多行插值文本* → *多行插值文本项* *多行插值文本* 可选 \ +> *多行插值文本项* → **`\(`** *表达式* **`)`** | *多行引用文本项* +> +> *转义序列* → **`\`** *字符串扩展分隔符* \ +> *转义字符* → *转义序列* **`0`** | *转义序列* **`\`** | *转义序列* **`t`** | *转义序列* **`n`** | *转义序列* **`r`** | *转义序列* **`"`** | *转义序列* **`'`** \ +> *转义字符* → *转义序列* **`u`** **`{`** *unicode-标量-数字* **`}`** \ +> *unicode-标量-数字* → 一到八位十六进制数字 +> +> *转义换行符* → *转义序列* *内联空格* 可选 *换行符* + +> 正则表达式字面量语法： +> +> *正则表达式字面量* → *正则表达式字面量开分隔定界符* *正则表达式* *正则表达式字面量闭分隔定界符* \ +> *正则表达式* → 任何正则表达式 +> +> *正则表达式字面量开分隔定界符* → *正则表达式扩展分隔符* 可选 **`/`** \ +> *正则表达式字面量闭分隔定界符* → **`/`** *正则表达式扩展分隔符* 可选 +> +> *正则表达式扩展分隔符* → **`#`** *正则表达式扩展分隔符* 可选 + +> 运算符语法： +> +> *运算符* → *运算符头* *运算符字符集* 可选 \ +> *运算符* → *点运算符头* *点运算符字符集* +> +> *运算符头* → **`/`** | **`=`** | **`-`** | **`+`** | **`!`** | **`*`** | **`%`** | **`<`** | **`>`** | **`&`** | **`|`** | **`^`** | **`~`** | **`?`** \ +> *运算符头* → U+00A1–U+00A7 \ +> *运算符头* → U+00A9 或 U+00AB \ +> *运算符头* → U+00AC 或 U+00AE \ +> *运算符头* → U+00B0–U+00B1 \ +> *运算符头* → U+00B6、U+00BB、U+00BF、U+00D7 或 U+00F7 \ +> *运算符头* → U+2016–U+2017 \ +> *运算符头* → U+2020–U+2027 \ +> *运算符头* → U+2030–U+203E \ +> *运算符头* → U+2041–U+2053 \ +> *运算符头* → U+2055–U+205E \ +> *运算符头* → U+2190–U+23FF \ +> *运算符头* → U+2500–U+2775 \ +> *运算符头* → U+2794–U+2BFF \ +> *运算符头* → U+2E00–U+2E7F \ +> *运算符头* → U+3001–U+3003 \ +> *运算符头* → U+3008–U+3020 \ +> *运算符头* → U+3030 +> +> *运算符字符* → *运算符头* \ +> *运算符字符* → U+0300–U+036F \ +> *运算符字符* → U+1DC0–U+1DFF \ +> *运算符字符* → U+20D0–U+20FF \ +> *运算符字符* → U+FE00–U+FE0F \ +> *运算符字符* → U+FE20–U+FE2F \ +> *运算符字符* → U+E0100–U+E01EF \ +> *运算符字符集* → *运算符字符* *运算符字符集* 可选 +> +> *点运算符头* → **`.`** \ +> *点运算符字符* → **`.`** | *运算符字符* \ +> *点运算符字符集* → *点运算符字符* *点运算符字符集* 可选 +> +> *中缀运算符* → *运算符* \ +> *前缀运算符* → *运算符* \ +> *后缀运算符* → *运算符* + +## 类型 + +> 类型语法： +> +> *类型* → *函数类型* \ +> *类型* → *数组类型* \ +> *类型* → *字典类型* \ +> *类型* → *类型标识符* \ +> *类型* → *元组类型* \ +> *类型* → *可选类型* \ +> *类型* → *隐式解析可选类型* \ +> *类型* → *协议合成类型* \ +> *类型* → *不透明类型* \ +> *类型* → *元类型* \ +> *类型* → *任意类型* \ +> *类型* → *自身类型* \ +> *类型* → **`(`** *type* **`)`** + +> 类型注释语法： +> +> *类型注释* → **`:`** *属性（Attributes）* 可选 **`inout`** 可选 *类型* + +> 类型标识符语法： +> +> *类型标识符* → *类型名* *泛型实参子句* 可选 | *类型名* *泛型实参子句* 可选 **`.`** *类型标识符* \ +> *类型名* → *标识符* + +> 元组类型语法： +> +> *元组类型* → **`(`** **`)`** | **`(`** *元组类型元素* **`,`** *元组类型元素集* **`)`** \ +> *元组类型元素集* → *元组类型元素* | *元组类型元素* **`,`** *元组类型元素集* \ +> *元组类型元素* → *元素名* *类型注释* | *类型* \ +> *元素名* → *标识符* + +> 函数类型语法： +> +> *函数类型* → *属性* 可选 *函数类型子句* **`async`** 可选 *throws* 可选 **`->`** *类型* +> +> *函数类型子句* → **`(`** **`)`** \ +> *函数类型子句* → **`(`** *函数类型参数集* **`...`** 可选 **`)`** +> +> *函数类型参数集* → *函数类型参数* | *函数类型参数* **`,`** *函数类型参数集* \ +> *函数类型参数* → *属性* 可选 **`inout`** 可选 *类型* | *参数标签* *类型注释* \ +> *参数标签* → *标识符* +> +> *throws 子句* → **`throws`** | **`throws`** **`(`** *类型* **`)`** + +> 数组类型语法： +> +> *数组类型* → **`[`** *类型* **`]`** + +> 字典类型语法： +> +> *字典类型* → **`[`** *类型* **`:`** *类型* **`]`** + +> 可选类型语法： +> +> *可选类型* → *类型* **`?`** + +> 隐式解析可选类型语法： +> +> *隐式解析可选类型* → *类型* **`!`** + +> 协议合成类型语法： +> +> *协议合成类型* → *类型标识符* **`&`** *协议合成延续* \ +> *协议合成延续* → *类型标识符* | *协议合成类型* + +> 不透明类型语法： +> +> *不透明类型* → **`some`** *类型* + +> 被包装的协议类型语法： +> +> *被包装的协议类型* → **`any`** *类型* + +> 元类型语法： +> +> *元类型* → *类型* **`.`** **`Type`** | *类型* **`.`** **`Protocol`** + +> 任意类型语法： +> +> *任意类型* → **`Any`** + +> 自身类型语法： +> +> *自身类型* → **`Self`** + +> 类型继承子句语法： +> +> *类型继承子句* → **`:`** *类型继承集* \ +> *类型继承集* → *属性* 可选 *类型标识符* | *属性* 可选 *类型标识符* **`,`** *类型继承集* + +## 表达式 + +> 表达式语法： +> +> *表达式* → *try 运算符* 可选 *await 运算符* 可选 *前缀表达式* *中缀表达式* 可选 + +> 前缀表达式语法： +> +> *前缀表达式* → *前缀运算符* 可选 *后缀表达式* \ +> *前缀表达式* → *输入输出表达式* + +> 输入输出表达式语法： +> +> *输入输出表达式* → **`&`** *基础表达式* + +> try 表达式语法： +> +> *try 运算符* → **`try`** | **`try`** **`?`** | **`try`** **`!`** + +> await 表达式语法： +> +> *await 运算符* → **`await`** + +> 中缀表达式语法： +> +> *中缀表达式* → *中缀运算符* *前缀表达式* \ +> *中缀表达式* → *赋值运算符* *try 运算符* 可选 *await 运算符* 可选 *前缀表达式* \ +> *中缀表达式* → *条件运算符* *try 运算符* 可选 *await 运算符* 可选 *前缀表达式* \ +> *中缀表达式* → *类型转换运算符* \ +> *中缀表达式* → *中缀表达式* *中缀表达式* 可选 + +> 赋值运算符语法： +> +> *赋值运算符* → **`=`** + +> 条件运算符语法： +> +> *条件运算符* → **`?`** *表达式* **`:`** + +> 类型转换运算符语法： +> +> *类型转换运算符* → **`is`** *类型* \ +> *类型转换运算符* → **`as`** *类型* \ +> *类型转换运算符* → **`as`** **`?`** *类型* \ +> *类型转换运算符* → **`as`** **`!`** *类型* + +> 基础表达式语法： +> +> *基础表达式* → *标识符* *泛型实参子句* 可选 \ +> *基础表达式* → *字面量表达式* \ +> *基础表达式* → *self 表达式* \ +> *基础表达式* → *父类表达式* \ +> *基础表达式* → *条件表达式* \ +> *基础表达式* → *闭包表达式* \ +> *基础表达式* → *圆括号表达式* \ +> *基础表达式* → *元组表达式* \ +> *基础表达式* → *隐式成员表达式* \ +> *基础表达式* → *通配符表达式* \ +> *基础表达式* → *宏展开表达式* \ +> *基础表达式* → *key-path 表达式* \ +> *基础表达式* → *选择器表达式* \ +> *基础表达式* → *key-path 字符串表达式* + +> 字面量表达式语法： +> +> *字面量表达式* → *字面量* \ +> *字面量表达式* → *数组字面量* | *字典字面量* | *playground 字面量* +> +> *数组字面量* → **`[`** *数组字面量项* 可选 **`]`** \ +> *数组字面量项* → *数组字面量项* **`,`** 可选 | *数组字面量项* **`,`** *数组字面量项* \ +> *数组字面量项* → *表达式* +> +> *字典字面量* → **`[`** *字典字面量项* **`]`** | **`[`** **`:`** **`]`** \ +> *字典字面量项* → *字典字面量项* **`,`** 可选 | *字典字面量项* **`,`** *字典字面量项* \ +> *字典字面量项* → *表达式* **`:`** *表达式* +> +> *playground 字面量* → **`#colorLiteral`** **`(`** **`red`** **`:`** *表达式* **`,`** **`green`** **`:`** *表达式* **`,`** **`blue`** **`:`** *表达式* **`,`** **`alpha`** **`:`** *表达式* **`)`** \ +> *playground 字面量* → **`#fileLiteral`** **`(`** **`resourceName`** **`:`** *表达式* **`)`** \ +> *playground 字面量* → **`#imageLiteral`** **`(`** **`resourceName`** **`:`** *表达式* **`)`** + +> self 表达式语法： +> +> *self 表达式* → **`self`** | *self 方法表达式* | *self 下标表达式* | *self 构造器表达式* +> +> *self 方法表达式* → **`self`** **`.`** *标识符* \ +> *self 下标表达式* → **`self`** **`[`** *函数调用参数表* **`]`** \ +> *self 构造器表达式* → **`self`** **`.`** **`init`** + +> 父类表达式语法： +> +> *父类表达式* → *父类方法表达式* | *父类下标表达式* | *父类构造器表达式* +> +> *父类方法表达式* → **`super`** **`.`** *标识符* \ +> *父类下标表达式* → **`super`** **`[`** *函数调用参数表* **`]`** \ +> *父类构造器表达式* → **`super`** **`.`** **`init`** + +> 条件表达式语法： +> +> *条件表达式* → *if 表达式* | *switch 表达式* +> +> *if 表达式* → **`if`** *条件集* **`{`** *语句* **`}`** *if 表达式后续* \ +> *if 表达式后续* → **`else`** *if 表达式* \ +> *if 表达式后续* → **`else`** **`{`** *语句* **`}`** +> +> *switch 表达式* → **`switch`** *表达式* **`{`** *switch表 达式 case* **`}`** \ +> *switch 表达式 case* → *switch 表达式 case* *switch 表达式 case* 可选 \ +> *switch case 表达式* → *case 标签* *语句* \ +> *switch case 表达式* → *default 标签* *语句* + +> 闭包表达式语法： +> +> *闭包表达式* → **`{`** *属性* 可选 *闭包签名* 可选 *语句* 可选 **`}`** +> +> *闭包签名* → *捕获列表* 可选 *闭包参数子句* **`async`** 可选 *throws* 可选 *函数结果* 可选 **`in`** \ +> *闭包签名* → *捕获列表* **`in`** +> +> *闭包参数子句* → **`(`** **`)`** | **`(`** *闭包参数集* **`)`** | *标识符集* \ +> *闭包参数集* → *闭包参数* | *闭包参数* **`,`** *闭包参数集* \ +> *闭包参数* → *闭包参数名* *类型注释* 可选 \ +> *闭包参数* → *闭包参数名* *类型注释* **`...`** \ +> *闭包参数名* → *标识符* +> +> *捕获列表* → **`[`** *捕获列表项* **`]`** \ +> *捕获列表项* → *捕获列表项* | *捕获列表项* **`,`** *捕获列表项* \ +> *捕获列表项* → *捕获说明符* 可选 *标识符* \ +> *捕获列表项* → *捕获说明符* 可选 *标识符* **`=`** *表达式* \ +> *捕获列表项* → *捕获说明符* 可选 *self 表达式* \ +> *捕获说明符* → **`weak`** | **`unowned`** | **`unowned(safe)`** | **`unowned(unsafe)`** + +> 隐式成员表达式语法： +> +> *隐式成员表达式* → **`.`** *标识符* \ +> *隐式成员表达式* → **`.`** *标识符* **`.`** *后缀表达式* + +> 圆括号表达式语法： +> +> *圆括号表达式* → **`(`** *表达式* **`)`** + +> 元组表达式语法： +> +> *元组表达式* → **`(`** **`)`** | **`(`** *元组元素* **`,`** *元组元素集* **`)`** \ +> *元组元素集* → *元组元素* | *元组元素* **`,`** *元组元素集* \ +> *元组元素* → *表达式* | *标识符* **`:`** *表达式* + +> 通配符表达式语法： +> +> *通配符表达式* → **`_`** + +> 宏展开表达式语法： +> +> *宏展开表达式* → **`#`** *标识符* *泛型参数子句* 可选 *函数调用参数子句* 可选 *尾随闭包* 可选 + +> key-path 表达式语法： +> +> *key-path 表达式* → **`\`** *类型* 可选 **`.`** *key-path 组件* \ +> *key-path 组件* → *key-path 组件* | *key-path 组件* **`.`** *key-path 组件* \ +> *key-path 组件* → *标识符* *key-path 后缀* 可选 | *key-path 后缀* +> +> *key-path 后缀* → *key-path 后缀* *key-path 后缀* 可选 \ +> *key-path 后缀* → **`?`** | **`!`** | **`self`** | **`[`** *函数调用参数集* **`]`** + +> 选择器表达式语法： +> +> *选择器表达式* → **`#selector`** **`(`** *表达式* **`)`** \ +> *选择器表达式* → **`#selector`** **`(`** **`getter:`** *表达式* **`)`** \ +> *选择器表达式* → **`#selector`** **`(`** **`setter:`** *表达式* **`)`** + +> key-path 字符串表达式语法： +> +> *key-path 字符串表达式* → **`#keyPath`** **`(`** *表达式* **`)`** + +> 后缀表达式语法： +> +> *后缀表达式* → *基本表达式* \ +> *后缀表达式* → *后缀表达式* *后缀运算符* \ +> *后缀表达式* → *函数调用表达式* \ +> *后缀表达式* → *构造器表达式* \ +> *后缀表达式* → *显式成员表达式* \ +> *后缀表达式* → *后缀 self 表达式* \ +> *后缀表达式* → *下标表达式* \ +> *后缀表达式* → *强制取值表达式* \ +> *后缀表达式* → *可选链式表达式* + +> 函数调用表达式语法： +> +> *函数调用表达式* → *后缀表达式* *函数调用参数子句* \ +> *函数调用表达式* → *后缀表达式* *函数调用参数子句* 可选 *尾随闭包* +> +> *函数调用参数子句* → **`(`** **`)`** | **`(`** *函数调用参数集* **`)`** \ +> *函数调用参数集* → *函数调用参数* | *函数调用参数* **`,`** *函数调用参数集* \ +> *函数调用参数* → *表达式* | *标识符* **`:`** *表达式* \ +> *函数调用参数* → *运算符* | *标识符* **`:`** *运算符* +> +> *尾随闭包* → *闭包表达式* *带标签的尾随闭包* 可选 \ +> *带标签的尾随闭包* → *带标签的尾随闭包* *带标签的尾随闭包* 可选 \ +> *带标签的尾随闭包* → *标识符* **`:`** *闭包表达式* + +> 构造器表达式语法： +> +> *构造器表达式* → *后缀表达式* **`.`** **`init`** \ +> *构造器表达式* → *后缀表达式* **`.`** **`init`** **`(`** *参数名* **`)`** + +> 显式成员表达式语法： +> +> *显式成员表达式* → *后缀表达式* **`.`** *十进制数字* \ +> *显式成员表达式* → *后缀表达式* **`.`** *标识符* *泛型参数子句* 可选 \ +> *显式成员表达式* → *后缀表达式* **`.`** *标识符* **`(`** *参数名* **`)`** \ +> *显式成员表达式* → *后缀表达式* *条件编译块* +> +> *参数名* → *参数名* *参数名* 可选 \ +> *参数名* → *标识符* **`:`** + +> 后缀 self表达式语法： +> +> *后缀 self 表达式* → *后缀表达式* **`.`** **`self`** + +> 下标表达式语法： +> +> *下标表达式* → *后缀表达式* **`[`** *函数调用参数集* **`]`** + +> 强制取值表达式语法： +> +> *强制取值表达式* → *后缀表达式* **`!`** + +> 可选链式表达式语法： +> +> *可选链式表达式* → *后缀表达式* **`?`** + +## 语句 + +> 语句语法： +> +> *语句* → *表达式* **`;`** 可选 \ +> *语句* → *声明* **`;`** 可选 \ +> *语句* → *循环语句* **`;`** 可选 \ +> *语句* → *分支语句* **`;`** 可选 \ +> *语句* → *标签语句* **`;`** 可选 \ +> *语句* → *控制转移语句* **`;`** 可选 \ +> *语句* → *延迟语句* **`;`** 可选 \ +> *语句* → *执行语句* **`;`** 可选 \ +> *语句* → *编译控制语句* \ +> *语句集* → *语句* *语句集* 可选 + +> 循环语句语法： +> +> *循环语句* → *for-in 语句* \ +> *循环语句* → *while 语句* \ +> *循环语句* → *repeat-while 语句* + +> for-in 语句语法： +> +> *for-in 语句* → **`for`** **`case`** 可选 *模式* **`in`** *表达式* *where 子句* 可选 *代码块* + +> while 语句语法： +> +> *while 语句* → **`while`** *条件集* *代码块* +> +> *条件集* → *条件* | *条件* **`,`** *条件集* \ +> *条件* → *表达式* | *可用性条件* | *case 条件* | *可选绑定条件* +> +> *case 条件* → **`case`** *模式* *构造器* \ +> *可选绑定条件* → **`let`** *模式* *构造器* 可选 | **`var`** *模式* *构造器* 可选 + +> repeat-while 语句语法： +> +> *repeat-while 语句* → **`repeat`** *代码块* **`while`** *表达式* + +> 分支语句语法： +> +> *分支语句* → *if 语句* \ +> *分支语句* → *guard 语句* \ +> *分支语句* → *switch 语句* + +> if 语句语法： +> +> *if 语句* → **`if`** *条件集* *代码块* *else 子句* 可选 \ +> *else 子句* → **`else`** *代码块* | **`else`** *if 语句* + +> guard 语句语法： +> +> *guard 语句* → **`guard`** *条件集* **`else`** *代码块* + +> switch 语句语法： +> +> *switch 语句* → **`switch`** *表达式* **`{`** *switch 语句* 可选 **`}`** \ +> *switch 语句* → *switch 语句* *switch 语句* 可选 \ +> *switch 语句* → *case 标签* *语句集* \ +> *switch 语句* → *default 标签* *语句集* \ +> *switch 语句* → *条件 switch 语句* +> +> *case 标签* → *属性* 可选 **`case`** *case 项集* **`:`** \ +> *case 项集* → *模式* *where 子句* 可选 | *模式* *where 子句* 可选 **`,`** *case 项集* \ +> *default 标签* → *属性* 可选 **`default`** **`:`** +> +> *where 子句* → **`where`** *where 表达式* \ +> *where 表达式* → *表达式* +> +> *条件 switch 语句* → *switch-if 指令子句* *switch-elseif 指令子句集* 可选 *switch-else 指令子句* 可选 *endif 指令* \ +> *switch-if 指令子句* → *if 指令* *编译条件* *switch-case 集* 可选 \ +> *switch-elseif 指令子句* → *elseif 指令子句* *switch-elseif 指令子句集* 可选 \ +> *switch-elseif 指令子句* → *elseif 指令* *编译条件* *switch-case 集* 可选 \ +> *switch-else 指令子句* → *else 指令* *switch-case 集* 可选 + +> 标签语句语法： +> +> *标签语句* → *语句标签* *循环语句* \ +> *标签语句* → *语句标签* *if 语句* \ +> *标签语句* → *语句标签* *switch 语句* \ +> *标签语句* → *语句标签* *do 语句* +> +> *语句标签* → *标签名* **`:`** \ +> *标签名* → *标识符* + +> 控制转移语句语法： +> +> *控制转移语句* → *break 语句* \ +> *控制转移语句* → *continue 语句* \ +> *控制转移语句* → *fallthrough 语句* \ +> *控制转移语句* → *return 语句* \ +> *控制转移语句* → *throw 语句* + +> break 语句语法： +> +> *break 语句* → **`break`** *标签名* 可选 + +> continue 语句语法： +> +> *continue 语句* → **`continue`** *标签名* 可选 + +> fallthrough 语句语法： +> +> *fallthrough 语句* → **`fallthrough`** + +> return 语句语法： +> +> *return 语句* → **`return`** *表达式* 可选 + +> throw 语句语法： +> +> *throw 语句* → **`throw`** *表达式* + +> defer 语句语法： +> +> *defer 语句* → **`defer`** *代码块* + +> do 语句语法： +> +> *do 语句* → **`do`** *throws 子句* 可选 *代码块* *catch 子句* 可选 \ +> *catch 子句集* → *catch 子句* *catch 子句* 可选 \ +> *catch 子句* → **`catch`** *catch 模式集* 可选 *代码块* \ +> *catch 模式集* → *catch 模式* | *catch 模式* **`,`** *catch 模式集* \ +> *catch 模式* → *模式* *where 子句* 可选 + +> 编译控制语句语法： +> +> *编译控制语句* → *条件编译块* \ +> *编译控制语句* → *行控制语句* \ +> *编译控制语句* → *诊断语句* + +> 条件编译块语法： +> +> *条件编译块* → *if 指令子句* *elseif 指令子句集* 可选 *else 指令子句* 可选 *endif 指令* +> +> *if 指令子句* → *if 指令* *编译条件* *语句集* 可选 \ +> *elseif 指令子句* → *elseif 指令子句* *elseif 指令子句集* 可选 \ +> *elseif 指令子句* → *elseif 指令* *编译条件* *语句集* 可选 \ +> *else 指令子句* → *else 指令* *语句集* 可选 \ +> *if 指令* → **`#if`** \ +> *elseif 指令* → **`#elseif`** \ +> *else 指令* → **`#else`** \ +> *endif 指令* → **`#endif`** +> +> *编译条件* → *平台条件* \ +> *编译条件* → *标识符* \ +> *编译条件* → *布尔字面量* \ +> *编译条件* → **`(`** *编译条件* **`)`** \ +> *编译条件* → **`!`** *编译条件* \ +> *编译条件* → *编译条件* **`&&`** *编译条件* \ +> *编译条件* → *编译条件* **`||`** *编译条件* +> +> *平台条件* → **`os`** **`(`** *操作系统* **`)`** \ +> *平台条件* → **`arch`** **`(`** *体系结构* **`)`** \ +> *平台条件* → **`swift`** **`(`** **`>=`** *swift 版本* **`)`** | **`swift`** **`(`** **`<`** *swift 版本* **`)`** \ +> *平台条件* → **`compiler`** **`(`** **`>=`** *swift 版本* **`)`** | **`compiler`** **`(`** **`<`** *swift 版本* **`)`** \ +> *平台条件* → **`canImport`** **`(`** *导入路径* **`)`** \ +> *平台条件* → **`targetEnvironment`** **`(`** *环境* **`)`** +> +> *操作系统* → **`macOS`** | **`iOS`** | **`watchOS`** | **`tvOS`** | **`visionOS`** | **`Linux`** | **`Windows`** \ +> *体系结构* → **`i386`** | **`x86_64`** | **`arm`** | **`arm64`** \ +> *swift 版本* → *十进制数字* *swift 版本后缀* 可选 \ +> *swift 版本后缀* → **`.`** *十进制数字* *swift 版本后缀* 可选 \ +> *环境* → **`simulator`** | **`macCatalyst`** + +> 行控制语句语法： +> +> *行控制语句* → **`#sourceLocation`** **`(`** **`file:`** *文件路径* **`,`** **`line:`** *行号* **`)`** \ +> *行控制语句* → **`#sourceLocation`** **`(`** **`)`** \ +> *行号* → 大于零的十进制整数 \ +> *文件路径* → *静态字符串字面量* + +> 可用性条件语法： +> +> *可用性条件* → **`#available`** **`(`** *可用性参数* **`)`** \ +> *可用性条件* → **`#unavailable`** **`(`** *可用性参数* **`)`** \ +> *可用性参数* → *可用性参数* | *可用性参数* **`,`** *可用性参数* \ +> *可用性参数* → *平台名* *平台版本* \ +> *可用性参数* → **`*`** +> +> *平台名* → **`iOS`** | **`iOSApplicationExtension`** \ +> *平台名* → **`macOS`** | **`macOSApplicationExtension`** \ +> *平台名* → **`macCatalyst`** | **`macCatalystApplicationExtension`** \ +> *平台名* → **`watchOS`** | **`watchOSApplicationExtension`** \ +> *平台名* → **`tvOS`** | **`tvOSApplicationExtension`** \ +> *平台名* → **`visionOS`** | **`visionOSApplicationExtension`** \ +> *平台版本* → *十进制数字* \ +> *平台版本* → *十进制数字* **`.`** *十进制数字* \ +> *平台版本* → *十进制数字* **`.`** *十进制数字* **`.`** *十进制数字* + +## 声明 + +> 声明语法： +> +> *声明* → *导入声明* \ +> *声明* → *常量声明* \ +> *声明* → *变量声明* \ +> *声明* → *类型别名声明* \ +> *声明* → *函数声明* \ +> *声明* → *枚举声明* \ +> *声明* → *结构体声明* \ +> *声明* → *类声明* \ +> *声明* → *actor 声明* \ +> *声明* → *协议声明* \ +> *声明* → *构造器声明* \ +> *声明* → *析构器声明* \ +> *声明* → *扩展声明* \ +> *声明* → *下标声明* \ +> *声明* → *运算符声明* \ +> *声明* → *优先级组声明* + +> 顶级声明语法： +> +> *顶级声明* → *语句集* 可选 + +> 代码块语法： +> +> *代码块* → **`{`** *语句集* 可选 **`}`** + +> 导入声明语法： +> +> *导入声明* → *属性* 可选 **`import`** *导入类型* 可选 *导入路径* +> +> *导入类型* → **`typealias`** | **`struct`** | **`class`** | **`enum`** | **`protocol`** | **`let`** | **`var`** | **`func`** \ +> *导入路径* → *标识符* | *标识符* **`.`** *导入路径* + +> 常量声明语法： +> +> *常量声明* → *属性* 可选 *声明修饰符* 可选 **`let`** *模式构造器集* +> +> *模式构造器集* → *模式构造器* | *模式构造器* **`,`** *模式构造器集* \ +> *模式构造器* → *模式* *构造器* 可选 \ +> *构造器* → **`=`** *表达式* + +> 变量声明语法： +> +> *变量声明* → *变量声明头* *模式构造器集* \ +> *变量声明* → *变量声明头* *变量名* *类型注解* *代码块* \ +> *变量声明* → *变量声明头* *变量名* *类型注解* *getter-setter* \ +> *变量声明* → *变量声明头* *变量名* *类型注解* *getter-setter 关键字（Keyword）块* \ +> *变量声明* → *变量声明头* *变量名* *构造器* *willSet-didSet 块* \ +> *变量声明* → *变量声明头* *变量名* *类型注解* *构造器* 可选 *willSet-didSet 块* +> +> *变量声明头* → *属性* 可选 *声明修饰符* 可选 **`var`** \ +> *变量名* → *标识符* +> +> *getter-setter* → *代码块* \ +> *getter-setter* → **`{`** *getter 子句* *setter 子句* 可选 **`}`** \ +> *getter-setter* → **`{`** *setter 子句* *getter 子句* **`}`** \ +> *getter 子句* → *属性* 可选 *可变性修饰符* 可选 **`get`** *代码块* \ +> *setter 子句* → *属性* 可选 *可变性修饰符* 可选 **`set`** *setter 名* 可选 *代码块* \ +> *setter 名* → **`(`** *标识符* **`)`** +> +> *getter-setter 关键字（Keyword）块* → **`{`** *getter 关键字子句* *setter 关键字子句* 可选 **`}`** \ +> *getter-setter 关键字（Keyword）块* → **`{`** *setter关键字子句* *getter 关键字子句* **`}`** \ +> *getter 关键字子句* → *属性* 可选 *可变性修饰符* 可选 **`get`** \ +> *setter 关键字子句* → *属性* 可选 *可变性修饰符* 可选 **`set`** +> +> *willSet-didSet 块* → **`{`** *willSet 子句* *didSet 子句* 可选 **`}`** \ +> *willSet-didSet 块* → **`{`** *didSet 子句* *willSet 子句* 可选 **`}`** \ +> *willSet 子句* → *属性* 可选 **`willSet`** *setter 名* 可选 *代码块* \ +> *didSet 子句* → *属性* 可选 **`didSet`** *setter 名* 可选 *代码块* + +> 类型别名声明语法： +> +> *类型别名声明* → *属性* 可选 *访问级别修饰符* 可选 **`typealias`** *类型别名名* *泛型参数子句* 可选 *类型别名赋值* \ +> *类型别名名* → *标识符* \ +> *类型别名赋值* → **`=`** *类型* + +> 函数声明语法： +> +> *函数声明* → *函数头* *函数名* *泛型参数子句* 可选 *函数签名* *泛型 where 子句* 可选 *函数体* 可选 +> +> *函数头* → *属性* 可选 *声明修饰符* 可选 **`func`** \ +> *函数名* → *标识符* | *运算符* +> +> *函数签名* → *参数子句* **`async`** 可选 *throws 子句* 可选 *函数结果* 可选 \ +> *函数签名* → *参数子句* **`async`** 可选 **`rethrows`** *函数结果* 可选 \ +> *函数结果* → **`->`** *属性* 可选 *类型* \ +> *函数体* → *代码块* +> +> *参数子句* → **`(`** **`)`** | **`(`** *参数集* **`)`** \ +> *参数集* → *参数* | *参数* **`,`** *参数集* \ +> *参数* → *外部参数名* 可选 *本地参数名* *参数类型注解* *默认参数子句* 可选 \ +> *参数* → *外部参数名* 可选 *本地参数名* *参数类型注解* \ +> *参数* → *外部参数名* 可选 *本地参数名* *参数类型注解* **`...`** +> +> *外部参数名* → *标识符* \ +> *本地参数名* → *标识符* \ +> *参数类型注解* → **`:`** *属性* 可选 *参数修饰符* 可选 *类型* \ +> *参数修饰符* → **`inout`** | **`borrowing`** | **`consuming`** \ +> *默认参数子句* → **`=`** *表达式* + +> 枚举声明语法： +> +> *枚举声明* → *属性* 可选 *访问级别修饰符* 可选 *联合式枚举* \ +> *枚举声明* → *属性* 可选 *访问级别修饰符* 可选 *原始值式枚举* +> +> *联合式枚举* → **`indirect`** 可选 **`enum`** *枚举名* *泛型参数子句* 可选 *类型继承子句* 可选 *泛型 where 子句* 可选 **`{`** *联合式枚举成员* 可选 **`}`** \ +> *联合式枚举成员* → *联合式枚举成员* *联合式枚举成员* 可选 \ +> *联合式枚举成员* → *声明* | *联合式枚举 case 子句* | *编译控制语句* \ +> *联合式枚举 case 子句* → *属性* 可选 **`indirect`** 可选 **`case`** *联合式枚举 case 集* \ +> *联合式枚举 case 集* → *联合式枚举 case* | *联合式枚举 case* **`,`** *联合式枚举 case 集* \ +> *联合式枚举 case* → *枚举case名* *元组类型* 可选 \ +> *枚举名* → *标识符* \ +> *枚举 case 名* → *标识符* +> +> *原始值式枚举* → **`enum`** *枚举名* *泛型参数子句* 可选 *类型继承子句* *泛型 where 子句* 可选 **`{`** *原始值式枚举成员* +> *原始值式枚举成员集* → *原始值式枚举成员* *原始值式枚举成员集*可选 \ +> *原始值式枚举成员集* → *声明* | *原始值式枚举 case 子句* | *编译控制语句* \ +> *原始值式枚举 case 子句* → *属性*可选 **`case`** *原始值式枚举 case 集* \ +> *原始值式枚举 case 集* → *原始值式枚举 case* | *原始值式枚举 case* **`,`** *原始值式枚举 case 集* \ +> *原始值式枚举 case* → *枚举 case 名* *原始值赋值*可选 \ +> *原始值赋值* → **`=`** *原始值字面量* \ +> *原始值字面量* → *数值字面量* | *静态字符串字面量* | *布尔字面量* + +> 结构声明语法： +> +> *结构声明* → *属性*可选 *访问级别修饰符*可选 **`struct`** *结构名* *泛型参数子句*可选 *类型继承子句*可选 *泛型 where 子句*可选 *结构主体* \ +> *结构名* → *标识符* \ +> *结构主体* → **`{`** *结构成员集*可选 **`}`** +> +> *结构成员集* → *结构成员* *结构成员集*可选 \ +> *结构成员* → *声明* | *编译控制语句* + +> 类声明语法： +> +> *类声明* → *属性*可选 *访问级别修饰符*可选 **`final`** 可选 **`class`** *类名* *泛型参数子句*可选 *类型继承子句*可选 *泛型 where 子句*可选 *类主体* \ +> *类声明* → *属性*可选 **`final`** *访问级别修饰符*可选 **`class`** *类名* *泛型参数子句*可选 *类型继承子句*可选 *泛型 where子句*可选 *类主体* \ +> *类名* → *标识符* \ +> *类主体* → **`{`** *类成员*可选 **`}`** +> +> *类成员* → *类成员* *类成员*可选 \ +> *类成员* → *声明* | *编译控制语句* + +> actor 声明语法： +> +> *actor 声明* → *属性*可选 *访问级别修饰符*可选 **`actor`** *actor 名* *泛型参数子句*可选 *类型继承子句*可选 *泛型 where 子句*可选 *actor 主体* \ +> *actor 名* → *标识符* \ +> *actor 主体* → **`{`** *actor 成员*可选 **`}`** +> +> *actor 成员* → *actor 成员* *actor 成员*可选 \ +> *actor 成员* → *声明* | *编译控制语句* + +> 协议声明语法： +> +> *协议声明* → *属性*可选 *访问级别修饰符*可选 **`protocol`** *协议名* *类型继承子句*可选 *泛型 where 子句*可选 *协议主体* \ +> *协议名* → *标识符* \ +> *协议主体* → **`{`** *协议成员*可选 **`}`** +> +> *协议成员* → *协议成员* *协议成员*可选 \ +> *协议成员* → *协议成员声明* | *编译控制语句* +> +> *协议成员声明* → *协议属性声明* \ +> *协议成员声明* → *协议方法声明* \ +> *协议成员声明* → *协议构造器声明* \ +> *协议成员声明* → *协议下标声明* \ +> *协议成员声明* → *协议关联类型声明* \ +> *协议成员声明* → *类型别名声明* + +> 协议属性声明语法： +> +> *协议属性声明* → *变量声明头* *变量名* *类型注解* *getter-setter 关键字块* + +> 协议方法声明语法： +> +> *协议方法声明* → *函数头* *函数名* *泛型参数子句*可选 *函数签名* *泛型 where 子句*可选 + +> 协议构造器声明语法： +> +> *协议构造器声明* → *构造器头* *泛型参数子句*可选 *参数子句* *抛出子句*可选 *泛型 where 子句*可选 \ +> *协议构造器声明* → *构造器头* *泛型参数子句*可选 *参数子句* **`rethrows`** *泛型 where 子句*可选 + +> 协议下标声明语法： +> +> *协议下标声明* → *下标头* *下标结果* *泛型 where 子句*可选 *getter-setter 关键字块* + +> 协议关联类型声明语法： +> +> *协议关联类型声明* → *属性*可选 *访问级别修饰符*可选 **`associatedtype`** *类型别名名* *类型继承子句*可选 *类型别名赋值*可选 *泛型 where 子句*可选 + +> 构造器声明语法： +> +> *构造器声明* → *构造器头* *泛型参数子句*可选 *参数子句* **`async`** 可选 *抛出子句*可选 *泛型 where 子句*可选 *构造器主体* \ +> *构造器声明* → *构造器头* *泛型参数子句*可选 *参数子句* **`async`** 可选 **`rethrows`** *泛型 where 子句*可选 *构造器主体* \ +> *构造器头* → *属性*可选 *声明修饰符*可选 **`init`** \ +> *构造器头* → *属性*可选 *声明修饰符*可选 **`init`** **`?`** \ +> *构造器头* → *属性*可选 *声明修饰符*可选 **`init`** **`!`** \ +> *构造器主体* → *代码块* + +> 析构器声明语法： +> +> *析构器声明* → *属性*可选 **`deinit`** *代码块* + +> 扩展声明语法： +> +> *扩展声明* → *属性*可选 *访问级别修饰符*可选 **`extension`** *类型标识符* *类型继承子句*可选 *泛型 where 子句*可选 *扩展主体* \ +> *扩展主体* → **`{`** *扩展成员*可选 **`}`** +> +> *扩展成员* → *扩展成员* *扩展成员*可选 \ +> *扩展成员* → *声明集* | *编译控制语句* + +> 下标声明语法： +> +> *下标声明* → *下标头* *下标结果* *泛型 where 子句*可选 *代码块* \ +> *下标声明* → *下标头* *下标结果* *泛型 where 子句*可选 *getter-setter 块* \ +> *下标声明* → *下标头* *下标结果* *泛型 where 子句*可选 *getter-setter 关键字块* \ +> *下标头* → *属性*可选 *声明修饰符*可选 **`subscript`** *泛型参数子句*可选 *参数子句* \ +> *下标结果* → **`->`** *特性*可选 *类型* + +> 宏声明语法： +> +> *宏声明* → *宏头* *标识符* *泛型参数子句*可选 *宏签名* *宏定义*可选 *泛型 where子句* \ +> *宏头* → *属性*可选 *声明修饰符*可选 **`macro`** \ +> *宏签名* → *参数子句* *宏函数签名结果*可选 \ +> *宏函数签名结果* → **`->`** *类型* \ +> *宏定义* → **`=`** *表达式* + +> 运算符声明语法： +> +> *运算符声明* → *前缀运算符声明* | *后缀运算符声明* | *中缀运算符声明* +> +> *前缀运算符声明* → **`prefix`** **`operator`** *运算符* \ +> *后缀运算符声明* → **`postfix`** **`operator`** *运算符* \ +> *中缀运算符声明* → **`infix`** **`operator`** *运算符* *中缀运算符组*可选 +> +> *中缀运算符组* → **`:`** *优先级组名* + +> 优先级组声明语法： +> +> *优先级组声明* → **`precedencegroup`** *优先级组名* **`{`** *优先级组属性*可选 **`}`** +> +> *优先级组属性* → *优先级组属性* *优先级组属性*可选 \ +> *优先级组属性* → *优先级组关系* \ +> *优先级组属性* → *优先级组赋值* \ +> *优先级组属性* → *优先级组结合* +> +> *优先级组关系* → **`higherThan`** **`:`** *优先级组名集* \ +> *优先级组关系* → **`lowerThan`** **`:`** *优先级组名集* +> +> *优先级组赋值* → **`assignment`** **`:`** *布尔字面量* +> +> *优先级组结合* → **`associativity`** **`:`** **`left`** \ +> *优先级组结合* → **`associativity`** **`:`** **`right`** \ +> *优先级组结合* → **`associativity`** **`:`** **`none`** +> +> *优先级组名集* → *优先级组名* | *优先级组名* **`,`** *优先级组名* \ +> *优先级组名* → *标识符* + +> 声明修饰符语法： +> +> *声明修饰符* → **`class`** | **`convenience`** | **`dynamic`** | **`final`** | **`infix`** | **`lazy`** | **`optional`** | **`override`** | **`postfix`** | **`prefix`** | **`required`** | **`static`** | **`unowned`** | **`unowned`** **`(`** **`safe`** **`)`** | **`unowned`** **`(`** **`unsafe`** **`)`** | **`weak`** \ +> *声明修饰符* → *访问级别修饰符* \ +> *声明修饰符* → *可变性修饰符* \ +> *声明修饰符* → *actor 隔离修饰符* \ +> *声明修饰符* → *声明修饰符* *声明修饰符*可选 +> +> *访问级别修饰符* → **`private`** | **`private`** **`(`** **`set`** **`)`** \ +> *访问级别修饰符* → **`fileprivate`** | **`fileprivate`** **`(`** **`set`** **`)`** \ +> *访问级别修饰符* → **`internal`** | **`internal`** **`(`** **`set`** **`)`** \ +> *访问级别修饰符* → **`package`** | **`package`** **`(`** **`set`** **`)`** \ +> *访问级别修饰符* → **`public`** | **`public`** **`(`** **`set`** **`)`** \ +> *访问级别修饰符* → **`open`** | **`open`** **`(`** **`set`** **`)`** +> +> *可变性修饰符* → **`mutating`** | **`nonmutating`** +> +> *actor 隔离修饰符* → **`nonisolated`** + +## 属性 + +> 属性语法： +> +> *属性* → **`@`** *属性名* *属性参数子句*可选 \ +> *属性名* → *标识符* \ +> *属性参数子句* → **`(`** *平衡令牌*可选 **`)`** \ +> *属性* → *属性* *属性*可选 +> +> *平衡令牌集* → *平衡令牌* *平衡令牌集*可选 \ +> *平衡令牌* → **`(`** *平衡令牌集*可选 **`)`** \ +> *平衡令牌* → **`[`** *平衡令牌集*可选 **`]`** \ +> *平衡令牌* → **`{`** *平衡令牌集*可选 **`}`** \ +> *平衡令牌* → 任何标识符、关键字、字面量或运算符 \ +> *平衡令牌* → 任何标点符号，除了 **`(`**, **`)`**, **`[`**, **`]`**, **`{`**, 或 **`}`** + +## 模式 + +> 模式语法： +> +> *模式* → *通配符模式* *类型注解*可选 \ +> *模式* → *标识符模式* *类型注解*可选 \ +> *模式* → *值绑定模式* \ +> *模式* → *元组模式* *类型注解*可选 \ +> *模式* → *枚举 case 模式* \ +> *模式* → *可选模式* \ +> *模式* → *类型转换模式* \ +> *模式* → *表达式模式* + +> 通配符模式语法： +> +> *通配符模式* → **`_`** + +> 标识符模式语法： +> +> *标识符模式* → *标识符* + +> 值绑定模式语法： +> +> *值绑定模式* → **`var`** *模式* | **`let`** *模式* + +> 元组模式语法： +> +> *元组模式* → **`(`** *元组模式元素集*可选 **`)`** \ +> *元组模式元素集* → *元组模式元素* | *元组模式元素* **`,`** *元组模式元素集* \ +> *元组模式元素* → *模式* | *标识符* **`:`** *模式* + +> 枚举 case 模式语法： +> +> *枚举 case 模式* → *类型标识符*可选 **`.`** *枚举 case 名* *元组模式*可选 + +> 可选模式语法： +> +> *可选模式* → *标识符模式* **`?`** + +> 类型转换模式语法： +> +> *类型转换模式* → *is 模式* | *as 模式* \ +> *is 模式* → **`is`** *类型* \ +> *as 模式* → *模式* **`as`** *类型* + +> 表达式模式语法： +> +> *表达式模式* → *表达式* + +## 泛型参数和参数 + +> 泛型参数子句语法： +> +> *泛型参数子句* → **`<`** *泛型参数集* **`>`** \ +> *泛型参数集* → *泛型参数* | *泛型参数* **`,`** *泛型参数集* \ +> *泛型参数* → *类型名* \ +> *泛型参数* → *类型名* **`:`** *类型标识符* \ +> *泛型参数* → *类型名* **`:`** *协议组合类型* +> +> *泛型 where子句* → **`where`** *需求集* \ +> *需求集* → *需求* | *需求* **`,`** *需求集* \ +> *需求* → *符合要求* | *同类型要求* +> +> *符合要求* → *类型标识符* **`:`** *类型标识符* \ +> *符合要求* → *类型标识符* **`:`** *协议组合类型* \ +> *同类型要求* → *类型标识符* **`==`** *类型* + +> 泛型参数子句语法： +> +> *泛型参数子句* → **`<`** *泛型参数集* **`>`** \ +> *泛型参数集* → *泛型参数* | *泛型参数* **`,`** *泛型参数集* \ +> *泛型参数* → *类型* diff --git a/swift-6.docc/ReferenceManual/Types.md b/swift-6.docc/ReferenceManual/Types.md new file mode 100644 index 000000000..63aa1c079 --- /dev/null +++ b/swift-6.docc/ReferenceManual/Types.md @@ -0,0 +1,535 @@ +# 类型 + +使用内置的命名类型和复合类型。 + +在 Swift 中，有两种类型:命名类型和复合类型。 +*命名类型*是在定义时可以赋予特定名称的类型。 +命名类型包括类、结构体、枚举和协议。 +例如，用户定义的名为 `MyClass` 的类的实例的类型为 `MyClass`。 +除了用户自定义的命名类型，Swift 标准库还定义了许多常用的命名类型，包括表示数组、字典和可选值的类型。 + +在很多编程语言中通常被认为是基本或原始的数据类型——例如表示数字、字符和字符串的类型——实际上是使用结构体在 Swift 标准库中定义和实现的命名类型。 +因为这些基本数据类型是命名类型，所以您可以使用扩展声明(在和中讨论)来扩展它们的行为，以满足您程序的需求。 + +复合类型指的是在 Swift 语言本身中定义的没有名称的类型。有两种复合类型:函数类型和元组类型。 +复合类型可能包含命名类型和其他复合类型。 +例如，元组类型 `(Int, (Int, Int))` 包含两个元素:第一个是命名类型 `Int`，第二个是另一个复合类型 `(Int, Int)`。 + +您可以在命名类型或复合类型周围加上括号。 +但是，在类型周围添加括号不会产生任何影响。 +例如，`(Int)` 等同于 `Int`。 + +本章讨论了 Swift 语言本身定义的类型，并描述了 Swift 的类型推断行为。 + +> 类型的语法: +> > *type* → *function-type* \ > *type* → *array-type* \ > *type* → *dictionary-type* \ > *type* → *type-identifier* \ > *type* → *tuple-type* \ > *type* → *optional-type* \ > *type* → *implicitly-unwrapped-optional-type* \ > *type* → *protocol-composition-type* \ > *type* → *opaque-type* \ > *type* → *boxed-protocol-type* \ > *type* → *metatype-type* \ > *type* → *any-type* \ > *type* → *self-type* \ > *type* → **`(`** *type* **`)`** + +## 类型注解 + +*类型注解*显式指定变量或表达式的类型。 +类型注解以冒号 (`:`) 开头，以类型结尾，如下例所示: + +```swift +let someTuple: (Double, Double) = (3.14159, 2.71828) +func someFunction(a: Int) { /* ... */ } +``` + +在第一个示例中，表达式 `someTuple` 被指定为元组类型 `(Double, Double)`。 +在第二个示例中，函数 `someFunction` 的参数 `a` 被指定为类型 `Int`。 + +类型注解可以在类型之前包含一个可选的类型属性列表。 + +> 类型注解的语法: +> > *type-annotation* → **`:`** *attributes*_?_ *type* + +## 类型标识符 + +*类型标识符*指的是命名类型，或命名类型、复合类型的类型别名。 + +大多数情况下，类型标识符直接指的是与标识符同名的命名类型。 +例如，`Int` 是一个类型标识符，直接指的是命名类型 `Int`，类型标识符 `Dictionary` 直接指的是命名类型 `Dictionary`。 + +有两种情况，类型标识符不是指与其同名的类型。 +第一种情况，类型标识符指的是命名类型或复合类型的类型别名。 +例如，在下面的示例中，类型注解中使用的 `Point` 指的是元组类型 `(Int, Int)`。 + +```swift +typealias Point = (Int, Int) +let origin: Point = (0, 0) +``` + +第二种情况，类型标识符使用点(.)语法来引用声明在其他模块中或嵌套在其他类型中的命名类型。例如，以下代码中的类型标识符引用了在 ExampleModule 模块中声明的命名类型 MyType。 + +```swift +var someValue: ExampleModule.MyType +``` + +元组类型是一个用逗号分隔的类型列表，用括号括起来。你可以将元组类型用作函数的返回类型，以使函数能够返回包含多个值的单个元组。你还可以命名元组类型的元素，并使用这些名称来引用各个元素的值。元素名称由一个标识符后跟一个冒号(:)组成。有关演示这两个特性的示例，请参阅。当元组类型的元素有名称时，该名称是类型的一部分。 + +```swift +var someTuple = (top: 10, bottom: 12) // someTuple 的类型为 (top: Int, bottom: Int) +someTuple = (top: 4, bottom: 42) // OK: 名称匹配 +someTuple = (9, 99) // OK: 名称被推断 +someTuple = (left: 5, right: 5) // 错误: 名称不匹配 +``` + +除了 Void 是空元组类型 () 的类型别名外，所有元组类型都包含两个或更多类型。 + +函数类型表示函数、方法或闭包的类型，由一个参数类型和返回类型组成，用箭头(->)分隔: + +```swift +(<#参数类型#>) -> <#返回类型#> +``` + +参数类型是一个用逗号分隔的类型列表。由于返回类型可以是元组类型，函数类型支持返回多个值的函数和方法。 + +函数类型 () -> T (其中 T 是任何类型)的参数可以应用 autoclosure 属性，在调用时隐式创建闭包。这提供了一种语法上方便的方式，在调用函数时延迟表达式的计算，而无需编写显式闭包。有关 autoclosure 函数类型参数的示例，请参阅。 + +函数类型可以在其参数类型中有可变参数。在语法上，可变参数由一个基本类型名后跟三个点(...)组成，如 Int...。可变参数被视为包含基本类型名元素的数组。例如，可变参数 Int... 被视为 [Int]。有关使用可变参数的示例，请参阅。 + +要指定一个输入输出参数，需要在参数类型前加上 `inout` 关键字。你不能在可变参数或返回类型上使用 `inout` 关键字。输入输出参数在 中有讨论。 + +如果函数类型只有一个参数，且参数类型为元组，则在写该函数类型时必须给元组加括号。例如，`((Int, Int)) -> Void` 是一个接受单个参数的函数类型，该参数的类型为 `(Int, Int)`，并且不返回任何值。相反，如果不加括号，`(Int, Int) -> Void` 是一个接受两个 `Int` 参数并且不返回任何值的函数类型。同样，因为 `Void` 是 `()` 的类型别名，所以函数类型 `(Void) -> Void` 与 `(()) -> ()` 相同 --- 一个接受单个参数为空元组的函数。这些类型与 `() -> ()` 不同 --- 一个不接受任何参数的函数。 + +函数和方法中的参数名不是相应函数类型的一部分。例如: + +```swift +func someFunction(left: Int, right: Int) {} +func anotherFunction(left: Int, right: Int) {} +func functionWithDifferentLabels(top: Int, bottom: Int) {} + +var f = someFunction // f 的类型是 (Int, Int) -> Void，而不是 (left: Int, right: Int) -> Void。 +f = anotherFunction // OK +f = functionWithDifferentLabels // OK + +func functionWithDifferentArgumentTypes(left: Int, right: String) {} +f = functionWithDifferentArgumentTypes // 错误 + +func functionWithDifferentNumberOfArguments(left: Int, right: Int, top: Int) {} +f = functionWithDifferentNumberOfArguments // 错误 +``` + +因为参数标签不是函数类型的一部分，所以在写函数类型时要省略它们。 + +```swift +var operation: (lhs: Int, rhs: Int) -> Int // 错误 +var operation: (_ lhs: Int, _ rhs: Int) -> Int // OK +var operation: (Int, Int) -> Int // OK +``` + +如果一个函数类型包含多个箭头(`->`)时，表示该函数类型实际上是一个高阶函数，它接受一些参数并返回另一个函数。在这种情况下，函数类型是从右向左进行分组的。例如，函数类型`(Int) -> (Int) -> Int`被理解为`(Int) -> ((Int) -> Int)` --- 即一个接受`Int`参数并返回另一个接受`Int`参数并返回`Int`结果的函数。 + +可能抛出或重新抛出错误的函数类型必须包含`throws`关键字。你可以在`throws`后面加上括号中的类型来指定函数抛出的错误类型。抛出的错误类型必须符合`Error`协议。写`throws`而不指定类型等同于写`throws(any Error)`。省略`throws`等同于写`throws(Never)`。函数抛出的错误类型可以是任何符合`Error`协议的类型，包括泛型类型、装箱协议类型和不透明类型。 + +函数抛出的错误类型是该函数类型的一部分，错误类型之间的子类型关系意味着相应的函数类型也是子类型关系。例如，如果你声明了一个自定义的`MyError`类型，一些函数类型之间的关系如下，从超类型到子类型: + +1. 抛出任何错误的函数，标记为`throws(any Error)` +2. 抛出特定错误的函数，标记为`throws(MyError)` +3. 不抛出错误的函数，标记为`throws(Never)` + +由于这些子类型关系: + +- 你可以在需要抛出函数的地方使用不抛出函数。 +- 你可以在需要抛出函数的地方使用抛出具体错误类型的函数。 +- 你可以在需要抛出更一般错误类型的函数的地方使用抛出更特定错误类型的函数。 + +如果使用关联类型或泛型类型参数指定抛出的错误类型，那么该关联类型或泛型类型参数隐式地需要符合`Error`协议。 + +异步函数的函数类型必须用`async`关键字标记。`async`关键字是函数类型的一部分，同步函数是异步函数的子类型。因此，你可以在需要异步函数的地方使用同步函数。 + +非逃逸函数类型的参数不能存储在`Any`类型的属性、变量或常量中，因为这可能会导致值逃逸。值逃逸指的是将一个本应在特定作用域内使用的闭包逃逸到了该作用域之外，从而可能导致意外行为或内存泄漏。因此，Swift 对非逃逸闭包做了这个限制。 + +一个非逃逸函数不能作为另一个非逃逸函数的参数传入。这个限制有助于 Swift 在编译时而不是运行时执行更多对内存访问冲突的检查。例如: + +```swift +let external: (() -> Void) -> Void = { _ in () } +func takesTwoFunctions(first: (() -> Void) -> Void, second: (() -> Void) -> Void) { + first { first {} } // 错误 + second { second {} } // 错误 + + first { second {} } // 错误 + second { first {} } // 错误 + + first { external {} } // 正确 + external { first {} } // 正确 +} +``` + +在上述代码中，`takesTwoFunctions(first:second:)` 的两个参数都是函数。由于它们都没有标记为 `@escaping`，因此它们都是非逃逸的。 + +上述代码中标记为“错误”的四个函数调用会导致编译器错误。因为 `first` 和 `second` 参数是非逃逸函数，所以它们不能被传递给另一个非逃逸函数参数。另一方面，标记为“正确”的两个函数调用不会导致编译器错误。这些函数调用没有违反限制，因为 `external` 不是 `takesTwoFunctions(first:second:)` 的参数之一。 + +如果你需要避免这个限制，可以将其中一个参数标记为逃逸的，或者使用 `withoutActuallyEscaping(_:do:)` 函数临时将其中一个非逃逸函数参数转换为逃逸函数。有关避免内存访问冲突的信息，请参阅 。 + +> 函数类型的语法: +> > *function-type* → *attributes*_?_ *function-type-argument-clause* **`async`**_?_ *throws-clause*_?_ **`->`** *type* +> > *function-type-argument-clause* → **`(`** **`)`** \ +> *function-type-argument-clause* → **`(`** *function-type-argument-list* **`...`**_?_ **`)`** +> > *function-type-argument-list* → *function-type-argument* | *function-type-argument* **`,`** *function-type-argument-list* \ +> *function-type-argument* → *attributes*_?_ *parameter-modifier*_?_ *type* | *argument-label* *type-annotation* \ +> *argument-label* → *identifier* +> > *throws-clause* → **`throws`** | **`throws`** **`(`** *type* **`)`** + +func polymorphicF(a: Int) -> T { return a } 是一个泛型函数，可以返回任何类型的值。而 monomorphicF(a: Int) -> Int { return a } 是一个单态函数，只能返回 Int 类型的值。 + +var myMonomorphicF = monomorphicF 这种赋值是允许的。 + +但是，以下赋值是不允许的: + +var myPolymorphicF = polymorphicF --> + +## 数组类型 + +Swift 语言为 Swift 标准库 `Array` 类型提供了以下语法糖: + +```swift +[<#类型#>] +``` + +换句话说，以下两个声明是等价的: + +```swift +let someArray: Array = ["Alex", "Brian", "Dave"] +let someArray: [String] = ["Alex", "Brian", "Dave"] +``` + +在这两种情况下，常量 `someArray` 都被声明为一个字符串数组。可以通过使用方括号指定有效的索引值来访问数组的元素: +`someArray[0]` 指的是索引为 0 的元素 `"Alex"`。 + +你可以通过嵌套方括号对来创建多维数组，其中基本元素类型的名称包含在最内层的方括号对中。 +例如，你可以使用三组方括号创建一个三维整数数组: + +```swift +var array3D: [[[Int]]] = [[[1, 2], [3, 4]], [[5, 6], [7, 8]]] +``` + +当访问多维数组的元素时，最左边的下标索引指的是最外层数组中该索引处的元素。下一个向右的下标索引指的是嵌套一层的数组中该索引处的元素。以此类推。这意味着在上面的示例中，`array3D[0]` 指的是 `[[1, 2], [3, 4]]`、`array3D[0][1]` 指的是 `[3, 4]`、`array3D[0][1][1]` 指的是值 4。 + +有关 Swift 标准库 `Array` 类型的详细讨论，请参阅 。 + +> 数组类型的语法: +> > *array-type* → **`[`** *type* **`]`** + +## 字典类型 + +Swift 语言为 Swift 标准库 `Dictionary` 类型提供了以下语法糖: + +```swift +[<#键类型#>: <#值类型#>] +``` + +换句话说，以下两个声明是等价的: + +```swift +let someDictionary: [String: Int] = ["Alex": 31, "Paul": 39] +let someDictionary: Dictionary = ["Alex": 31, "Paul": 39] +``` + +在这两种情况下，常量 `someDictionary` 都被声明为一个以字符串作为键、整数作为值的字典。 + +可以通过在方括号中指定相应的键来访问字典的值:`someDictionary["Alex"]` 指的是与键 `"Alex"` 关联的值。 +下标返回字典值类型的可选值。 +如果指定的键不在字典中，下标将返回 `nil`。 + +字典的键类型必须符合 Swift 标准库 `Hashable` 协议。 + +有关 Swift 标准库 `Dictionary` 类型的详细讨论，请参阅 。 + +> 字典类型的语法: +> > *dictionary-type* → **`[`** *type* **`:`** *type* **`]`** + +## 可选类型 + +## 可选类型 + +Swift 语言将后缀 `?` 定义为命名类型 `Optional` 的语法糖，该类型定义在 Swift 标准库中。换句话说，以下两个声明是等价的: + +```swift +var optionalInteger: Int? +var optionalInteger: Optional +``` + +在这两种情况下，变量 `optionalInteger` 都被声明为可选整数类型。注意，类型和 `?` 之间不能有空格。 + +`Optional` 类型是一个枚举，有两种情况 `none` 和 `some(Wrapped)`，用于表示值可能存在或不存在。任何类型都可以显式声明为(或隐式转换为)可选类型。如果在声明可选变量或属性时没有提供初始值，它的值会自动默认为 `nil`。 + +如果一个可选类型的实例包含值，你可以使用后缀操作符 `!` 访问该值，如下所示: + +```swift +optionalInteger = 42 +optionalInteger! // 42 +``` + +使用 `!` 操作符解包值为 `nil` 的可选值会导致运行时错误。 + +你也可以使用可选链和可选绑定来有条件地对可选表达式执行操作。如果值为 `nil`，则不执行任何操作，因此也不会产生运行时错误。 + +有关更多信息和示例说明如何使用可选类型，请参阅 。 + +> 可选类型的语法: +> > *optional-type* → *type* **`?`** + +## 隐式解包可选类型 + +隐式解包是指在访问可选值时自动将其解包。Swift 语言将后缀 `!` 定义为命名类型 `Optional` 的语法糖，该类型定义在 Swift 标准库中，附加行为是在访问时自动解包。如果尝试使用值为 `nil` 的隐式解包可选值，你会得到一个运行时错误。 + +除了隐式解包行为外，以下两个声明是等价的: + +```swift +var implicitlyUnwrappedString: String! +var explicitlyUnwrappedString: Optional +``` + +注意，类型和 `!` 之间不能有空格。嵌套在元组类型或泛型类型(如字典或数组的元素类型)中的可选类型不能标记为隐式解包，因为隐式解包会改变包含该类型声明的含义。例如: + +```swift +let tupleOfImplicitlyUnwrappedElements: (Int!, Int!) // 错误 +let implicitlyUnwrappedTuple: (Int, Int)! // 正确 + +let arrayOfImplicitlyUnwrappedElements: [Int!] // 错误 +let implicitlyUnwrappedArray: [Int]! // 正确 +``` + +由于隐式解包可选值与可选值具有相同的 `Optional` 类型，因此在任何可以使用可选值的地方，也可以使用隐式解包可选值。例如，你可以将隐式解包可选值赋值给可选变量、常量和属性，反之亦然。 + +与可选值一样，如果在声明隐式解包可选变量或属性时没有提供初始值，它的值会自动默认为 `nil`。 + +使用可选链来有条件地对隐式解包可选表达式执行操作。如果值为 `nil`，则不执行任何操作，因此也不会产生运行时错误。 + +有关隐式解包可选类型的更多信息，请参阅 。 + +> 隐式解包可选类型的语法: +> > *implicitly-unwrapped-optional-type* → *type* **`!`** + +## 协议组合类型 + +一个*协议组合类型*定义了一种类型，该类型符合指定协议列表中的每个协议。它也可以定义一种从给定类继承并符合指定协议列表中每个协议的类型。 + +协议组合类型具有以下形式: + +```swift +<#协议 1#> & <#协议 2#> +``` + +协议组合类型允许你指定一个值的类型符合多个协议的要求，而无需显式定义一个新的命名协议来继承你希望该类型符合的每个协议。你可以使用协议组合类型`ProtocolA & ProtocolB & ProtocolC`而不是声明一个新的协议继承自`ProtocolA`、`ProtocolB`和`ProtocolC`。同样，你可以使用`SuperClass & ProtocolA`而不是声明一个新的协议，该协议是`SuperClass`的子类并符合`ProtocolA`。 + +协议组合列表中的每一项都是以下之一，列表最多可以包含一个类: + +- 类的名称 +- 协议的名称 +- 底层类型是协议组合类型、协议或类的类型别名 + +当协议组合类型包含类型别名时，同一协议可能会在定义中出现多次，重复项会被忽略。例如，下面代码中`PQR`的定义等同于`P & Q & R`。 + +```swift +typealias PQ = P & Q +typealias PQR = PQ & Q & R +``` + +一个*不透明类型*定义了一种符合协议或协议组合的类型，而不指定底层具体类型。不透明类型出现在函数或下标的返回类型中，或属性的类型中。不透明类型不能作为元组类型或泛型类型的一部分出现，例如数组的元素类型或可选类型的包装类型。 + +不透明类型具有以下形式: + +```swift +some <#约束#> +``` + +*约束*是类类型、协议类型、协议组合类型或`Any`。只有当值是符合列出的协议或协议组合的类型实例，或者从列出的类继承时，才能将该值用作不透明类型的实例。与不透明值交互的代码只能以*约束*定义的接口中的方式使用该值。 + +在编译时，不透明类型的值具有特定的具体类型，Swift 可以使用该底层类型进行优化。但是，不透明类型形成了一个边界，关于该底层类型的信息不能跨越该边界。 + +协议声明不能包含不透明类型。类不能将不透明类型用作非 final 方法的返回类型。使用不透明类型作为返回类型的函数必须返回共享单个底层类型的值。返回类型可以包括函数的泛型类型参数中的类型。例如，函数`someFunction()`可以返回类型为`T`或`Dictionary`的值。 + +一个*装箱协议类型*定义了一种符合协议或协议组合的类型，该类型在程序运行时可以变化。装箱协议类型具有以下形式: + +```swift +any <#约束#> +``` + +*约束*是协议类型、协议组合类型、协议类型的元类型或协议组合类型的元类型。 + +在运行时，一个装箱协议类型是指可以存储任何符合该协议约束的类型值的类型。装箱协议类型的实例可以包含满足协议约束的任何类型的值。这种行为与不透明类型的工作方式形成对比，在不透明类型中，编译时已知某个特定的符合类型。 + +在使用装箱协议类型时，需要通过一个额外的间接层级来访问值，这种间接访问的方式称为装箱。装箱通常需要单独的内存分配用于存储，并且访问时需要额外的间接层级，这在运行时会产生性能开销。 + +对 `Any` 或 `AnyObject` 类型应用 `any` 没有任何效果，因为这些类型已经是装箱协议类型。 + +元类型指的是描述一个类型本身的类型，包括类类型、结构体类型、枚举类型和协议类型。 + +类、结构体或枚举类型的元类型是该类型的名称后跟 .Type。协议类型的元类型(而不是运行时符合该协议的具体类型)是该协议的名称后跟 .Protocol。例如，类型 SomeClass 的元类型是 SomeClass.Type，协议 SomeProtocol 的元类型是 SomeProtocol.Protocol。 + +你可以使用后缀 self 表达式以值的形式访问一个类型。例如，SomeClass.self 返回 SomeClass 本身，而不是 SomeClass 的实例。SomeProtocol.self 返回 SomeProtocol 本身，而不是运行时符合 SomeProtocol 的类型的实例。你可以使用 type(of:) 函数和一个类型的实例来访问该实例的动态运行时类型作为一个值，如下例所示: + +```swift +class SomeBaseClass { + class func printClassName() { + print("SomeBaseClass") + } +} + +class SomeSubClass: SomeBaseClass { + override class func printClassName() { + print("SomeSubClass") + } +} + +let someInstance: SomeBaseClass = SomeSubClass() +// someInstance 的编译时类型是 SomeBaseClass, +// someInstance 的运行时类型是 SomeSubClass +type(of: someInstance).printClassName() // 打印 "SomeSubClass" +``` + +使用初始化表达式可以从该类型的元类型值构造该类型的实例。对于类实例，被调用的初始化方法必须使用 required 关键字标记，或者该类被标记为 final 类。 + +```swift +class AnotherSubClass: SomeBaseClass { + let string: String + required init(string: String) { + self.string = string + } + override class func printClassName() { + print("AnotherSubClass") + } +} + +let metatype: AnotherSubClass.Type = AnotherSubClass.self +let anotherInstance = metatype.init(string: "some string") +``` + + + +> 元类型语法: +> > *metatype-type* → *type* **`.`** **`Type`** | *type* **`.`** **`Protocol`** + +## Any 类型 + +`Any` 类型可以包含来自所有其他类型的值。 +`Any` 可以用作以下任何类型实例的具体类型: + +- 类、结构或枚举 +- 元类型，如 `Int.self` +- 具有任何类型组件的元组 +- 闭包或函数类型 + +```swift +let mixed: [Any] = ["one", 2, true, (4, 5.3), { () -> Int in return 6 }] +``` + + + +当您使用 `Any` 作为实例的具体类型时，您需要在访问其属性或方法之前将实例转换为已知类型。 +具有 `Any` 具体类型的实例保持其原始动态类型，并且可以使用类型转换运算符 --- `as`、`as?` 或 `as!` 转换为该类型。 +例如，使用 `as?` 有条件地将异构数组中的第一个对象向下转换为 `String`，如下所示: + +```swift +if let first = mixed.first as? String { + print("第一项 '\(first)' 是字符串。") +} +// 打印 "第一项 'one' 是字符串。" +``` + + + +有关转换的更多信息，请参阅 。 + +`AnyObject` 协议类似于 `Any` 类型。 +所有类型都隐式符合 `AnyObject`。 +与由语言定义的 `Any` 不同， `AnyObject` 由 Swift 标准库定义。 +有关更多信息，请参阅 和 [`AnyObject`](https://developer.apple.com/documentation/swift/anyobject)。 + +> Any 类型语法: +> > *any-type* → **`Any`** + +## Self 类型 + +`Self` 类型不是特定类型，而是让您方便地引用当前类型而无需重复或知道该类型的名称。 + +在协议声明或协议成员声明中， `Self` 类型指的是最终符合该协议的类型。 + +在结构、类或枚举声明中， `Self` 类型指的是由声明引入的类型。 +在类型成员的声明中， `Self` 类型指的是该类型。 +在类声明的成员中， `Self` 只能出现在以下位置: + +- 作为方法的返回类型 +- 作为只读下标的返回类型 +- 作为只读计算属性的类型 +- 在方法体内 + +例如，下面的代码显示了一个实例方法 `f`，其返回类型是 `Self`。 + + + + + + + +上面的例子最后一部分展示了，`Self` 指的是 `z` 值的运行时类型 `子类`，而不是变量本身的编译时类型 `超类`。 + +在嵌套类型声明中，`Self` 类型指的是最内层类型声明引入的类型。 + +`Self` 类型指的是与 Swift 标准库中的 `type(of:)` 函数相同的类型。写 `Self.someStaticMember` 来访问当前类型的成员与写 `type(of: self).someStaticMember` 是一样的。 + +> Self 类型的语法: +> > *self-type* → **`Self`** + +*类型继承子句* 用于指定命名类型继承自哪个类以及符合哪些协议。类型继承子句以冒号 (:) 开头，后跟一系列类型标识符。 + +类可以继承自单个超类并符合任意数量的协议。在定义类时，超类的名称必须出现在类型标识符列表的首位，后跟该类必须符合的任意数量的协议。如果该类不继承自其他类，则列表可以从协议开始。有关类继承的扩展讨论和几个示例，请参阅 。 + +其他命名类型只能继承自或符合一系列协议。协议类型可以继承自任意数量的其他协议。当一个协议类型继承自其他协议时，来自那些其他协议的要求会被聚合在一起，任何继承自当前协议的类型都必须符合所有这些要求。 + +对于为枚举案例分配原始值的枚举定义，类型继承子句可以是指定这些原始值类型的单个命名类型。有关使用类型继承子句指定其原始值类型的枚举定义示例，请参阅 。 + +> 类型继承子句的语法: +> > *type-inheritance-clause* → **`:`** *type-inheritance-list* +> > *type-inheritance-list* → *attributes*_?_ *type-identifier* | *attributes*_?_ *type-identifier* **`,`** *type-inheritance-list* + +Swift 广泛使用 *类型推断*，允许你在代码中省略许多变量和表达式的类型或部分类型。 + +例如，你可以写 `var x = 0`，而不是 `var x: Int = 0`，完全省略类型 —— 编译器正确推断 `x` 命名的是 `Int` 类型的值。 + +同样，当完整类型可以从上下文推断出来时，你可以省略部分类型。例如，写下 `let dict: Dictionary = ["A": 1]`更简洁，编译器会推断 `dict` 的类型是 `Dictionary`。 + +在上面的两个示例中，类型信息从表达式树的叶子传递到根部。也就是说，`var x: Int = 0` 中 `x` 的类型是通过首先检查 `0` 的类型，然后将此类型信息传递至根 (变量 `x`) 来推断的。 + +在 Swift 中，类型信息也可以从根向下流向叶子节点。例如，在下面的例子中，常量 `eFloat` 使得数字字面量 (`: Float`) 导致数字字面量 `2.71828` 被推断为 `Float` 类型，而不是 `Double` 类型。 + +```swift +let e = 2.71828 // e 的类型被推断为 Double +let eFloat: Float = 2.71828 // eFloat 的类型为 Float +``` + + + +Swift 的类型推断是在单个表达式或语句层级进行的。这意味着推断一个省略的类型或表达式中部分类型所需的所有信息，必须可从类型检查该表达式或其子表达式中获得。 + + + + diff --git a/swift-6-beta.docc/The-Swift-Programming-Language.md b/swift-6.docc/The-Swift-Programming-Language.md similarity index 90% rename from swift-6-beta.docc/The-Swift-Programming-Language.md rename to swift-6.docc/The-Swift-Programming-Language.md index 35d60e4c3..00bb5b387 100644 --- a/swift-6-beta.docc/The-Swift-Programming-Language.md +++ b/swift-6.docc/The-Swift-Programming-Language.md @@ -1,4 +1,4 @@ -# The Swift Programming Language (6 beta) +# The Swift Programming Language (6.0) 中文版 @Metadata { @TechnologyRoot @@ -12,13 +12,13 @@ ## Topics -### Welcome to Swift +### 欢迎来到 Swift - - - -### Language Guide +### 语言指南 - - @@ -50,7 +50,7 @@ - - -### Language Reference +### 语言参考 - - @@ -63,10 +63,6 @@ - - -### Revision History - -- - + + + diff --git a/swift_docs_structure.json b/swift_docs_structure.json new file mode 100644 index 000000000..7f6a0692a --- /dev/null +++ b/swift_docs_structure.json @@ -0,0 +1,176 @@ +{ + "Welcome to Swift": [ + { + "title": "About Swift", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/aboutswift" + }, + { + "title": "Version Compatibility", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/compatibility" + }, + { + "title": "A Swift Tour", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/guidedtour" + } + ], + "Language Guide": [ + { + "title": "The Basics", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/thebasics" + }, + { + "title": "Basic Operators", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/basicoperators" + }, + { + "title": "Strings and Characters", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/stringsandcharacters" + }, + { + "title": "Collection Types", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/collectiontypes" + }, + { + "title": "Control Flow", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/controlflow" + }, + { + "title": "Functions", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/functions" + }, + { + "title": "Closures", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/closures" + }, + { + "title": "Enumerations", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/enumerations" + }, + { + "title": "Structures and Classes", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/classesandstructures" + }, + { + "title": "Properties", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/properties" + }, + { + "title": "Methods", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/methods" + }, + { + "title": "Subscripts", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/subscripts" + }, + { + "title": "Inheritance", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/inheritance" + }, + { + "title": "Initialization", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/initialization" + }, + { + "title": "Deinitialization", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/deinitialization" + }, + { + "title": "Optional Chaining", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/optionalchaining" + }, + { + "title": "Error Handling", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/errorhandling" + }, + { + "title": "Concurrency", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/concurrency" + }, + { + "title": "Macros", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/macros" + }, + { + "title": "Type Casting", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/typecasting" + }, + { + "title": "Nested Types", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/nestedtypes" + }, + { + "title": "Extensions", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/extensions" + }, + { + "title": "Protocols", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/protocols" + }, + { + "title": "Generics", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/generics" + }, + { + "title": "Opaque and Boxed Protocol Types", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/opaquetypes" + }, + { + "title": "Automatic Reference Counting", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/automaticreferencecounting" + }, + { + "title": "Memory Safety", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/memorysafety" + }, + { + "title": "Access Control", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/accesscontrol" + }, + { + "title": "Advanced Operators", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/advancedoperators" + } + ], + "Language Reference": [ + { + "title": "About the Language Reference", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/aboutthelanguagereference" + }, + { + "title": "Lexical Structure", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/lexicalstructure" + }, + { + "title": "Types", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/types" + }, + { + "title": "Expressions", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/expressions" + }, + { + "title": "Statements", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/statements" + }, + { + "title": "Declarations", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/declarations" + }, + { + "title": "Attributes", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/attributes" + }, + { + "title": "Patterns", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/patterns" + }, + { + "title": "Generic Parameters and Arguments", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/genericparametersandarguments" + }, + { + "title": "Summary of the Grammar", + "url": "https://docs.swift.org/swift-book/documentation/the-swift-programming-language/summaryofthegrammar" + } + ] +}