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<!DOCTYPE HTML>
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<i class="fa fa-check"></i> <b>1.</b> 欢迎使用 Swift
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<i class="fa fa-check"></i> <b>1.2.</b> Swift 初见
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<i class="fa fa-check"></i> <b>2.2.</b> 基本运算符
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<i class="fa fa-check"></i> <b>2.4.</b> 集合类型
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<i class="fa fa-check"></i> <b>2.5.</b> 控制流
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<i class="fa fa-check"></i> <b>2.6.</b> 函数
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<i class="fa fa-check"></i> <b>2.7.</b> 闭包
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<i class="fa fa-check"></i> <b>2.8.</b> 枚举
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<i class="fa fa-check"></i> <b>2.9.</b> 类和结构体
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<i class="fa fa-check"></i> <b>2.10.</b> 属性
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<i class="fa fa-check"></i> <b>2.11.</b> 方法
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<i class="fa fa-check"></i> <b>2.12.</b> 附属脚本
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<i class="fa fa-check"></i> <b>2.14.</b> 构造过程
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<i class="fa fa-check"></i> <b>2.15.</b> 析构过程
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<i class="fa fa-check"></i> <b>2.16.</b> 自动引用计数
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<i class="fa fa-check"></i> <b>2.17.</b> 可选链
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<i class="fa fa-check"></i> <b>2.18.</b> 类型检查
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<i class="fa fa-check"></i> <b>2.19.</b> 类型嵌套
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<i class="fa fa-check"></i> <b>2.20.</b> 扩展
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<i class="fa fa-check"></i> <b>2.21.</b> 协议
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<i class="fa fa-check"></i> <b>2.22.</b> 泛型
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<i class="fa fa-check"></i> <b>2.23.</b> 高级操作符
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<section class="normal" id="section-gitbook_126">
<blockquote>
<p>翻译：Lin-H</p>
<p>校对：shinyzhu</p>
</blockquote>
<h1 id="-">类型嵌套</h1>
<hr>
<p>本页包含内容：</p>
<ul>
<li><a href="#nested_types_in_action">类型嵌套实例</a></li>
<li><a href="#referring_to_nested_types">类型嵌套的引用</a></li>
</ul>
<p>枚举类型常被用于实现特定类或结构体的功能。也能够在有多种变量类型的环境中，方便地定义通用类或结构体来使用，为了实现这种功能，Swift允许你定义类型嵌套，可以在枚举类型、类和结构体中定义支持嵌套的类型。</p>
<p>要在一个类型中嵌套另一个类型，将需要嵌套的类型的定义写在被嵌套类型的区域{}内，而且可以根据需要定义多级嵌套。</p>
<p><a name="nested_types_in_action"></a></p>
<h2 id="-">类型嵌套实例</h2>
<p>下面这个例子定义了一个结构体<code>BlackjackCard</code>(二十一点)，用来模拟<code>BlackjackCard</code>中的扑克牌点数。<code>BlackjackCard</code>结构体包含2个嵌套定义的枚举类型<code>Suit</code> 和 <code>Rank</code>。</p>
<p>在<code>BlackjackCard</code>规则中，<code>Ace</code>牌可以表示1或者11，<code>Ace</code>牌的这一特征用一个嵌套在枚举型<code>Rank</code>的结构体<code>Values</code>来表示。</p>
<pre><code>struct BlackjackCard {
// 嵌套定义枚举型Suit
enum Suit: Character {
case Spades = "♠", Hearts = "♡", Diamonds = "♢", Clubs = "♣"
}
// 嵌套定义枚举型Rank
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.toRaw(), second: nil)
}
}
}
// BlackjackCard 的属性和方法
let rank: Rank, suit: Suit
var description: String {
var output = "suit is \(suit.toRaw()),"
output += " value is \(rank.values.first)"
if let second = rank.values.second {
output += " or \(second)"
}
return output
}
}
</code></pre><p>枚举型的<code>Suit</code>用来描述扑克牌的四种花色，并分别用一个<code>Character</code>类型的值代表花色符号。</p>
<p>枚举型的<code>Rank</code>用来描述扑克牌从<code>Ace</code>~10,<code>J</code>,<code>Q</code>,<code>K</code>,13张牌，并分别用一个<code>Int</code>类型的值表示牌的面值。(这个<code>Int</code>类型的值不适用于<code>Ace</code>,<code>J</code>,<code>Q</code>,<code>K</code>的牌)。</p>
<p>如上文所提到的，枚举型<code>Rank</code>在自己内部定义了一个嵌套结构体<code>Values</code>。这个结构体包含两个变量，只有<code>Ace</code>有两个数值，其余牌都只有一个数值。结构体<code>Values</code>中定义的两个属性：</p>
<p><code>first</code>, 为<code>Int</code>
<code>second</code>, 为 <code>Int?</code>, 或 “optional <code>Int</code>”</p>
<p><code>Rank</code>定义了一个计算属性<code>values</code>，这个计算属性会根据牌的面值，用适当的数值去初始化<code>Values</code>实例，并赋值给<code>values</code>。对于<code>J</code>,<code>Q</code>,<code>K</code>,<code>Ace</code>会使用特殊数值，对于数字面值的牌使用<code>Int</code>类型的值。</p>
<p><code>BlackjackCard</code>结构体自身有两个属性—<code>rank</code>与<code>suit</code>，也同样定义了一个计算属性<code>description</code>，<code>description</code>属性用<code>rank</code>和<code>suit</code>的中内容来构建对这张扑克牌名字和数值的描述，并用可选类型<code>second</code>来检查是否存在第二个值，若存在，则在原有的描述中增加对第二数值的描述。</p>
<p>因为<code>BlackjackCard</code>是一个没有自定义构造函数的结构体，在<a href="https://github.com/CocoaChina-editors/Welcome-to-Swift/blob/master/The%20Swift%20Programming%20Language/02Language%20Guide/14Initialization.md" target="_blank">Memberwise Initializers for Structure Types</a>中知道结构体有默认的成员构造函数，所以你可以用默认的<code>initializer</code>去初始化新的常量<code>theAceOfSpades</code>:</p>
<pre><code>let theAceOfSpades = BlackjackCard(rank: .Ace, suit: .Spades)
println("theAceOfSpades: \(theAceOfSpades.description)")
// 打印出 "theAceOfSpades: suit is ♠, value is 1 or 11"
</code></pre><p>尽管<code>Rank</code>和<code>Suit</code>嵌套在<code>BlackjackCard</code>中，但仍可被引用，所以在初始化实例时能够通过枚举类型中的成员名称单独引用。在上面的例子中<code>description</code>属性能正确得输出对<code>Ace</code>牌有1和11两个值。</p>
<p><a name="referring_to_nested_types"></a></p>
<h2 id="-">类型嵌套的引用</h2>
<p>在外部对嵌套类型的引用，以被嵌套类型的名字为前缀，加上所要引用的属性名：</p>
<pre><code>let heartsSymbol = BlackjackCard.Suit.Hearts.toRaw()
// 红心的符号 为 "♡"
</code></pre><p>对于上面这个例子，这样可以使<code>Suit</code>, <code>Rank</code>, 和 <code>Values</code>的名字尽可能的短，因为它们的名字会自然的由被定义的上下文来限定。</p>
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