custom_lint

Tools for building custom lint rules.

License

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Index

• Index
• Tutorial
• About
• Usage
  • Creating a custom lint package
  • Using our custom lint package in an application
  • Obtaining the list of lints in the CI
  • Using the Dart debugger
  • Testing your plugins using expect_lint

Tutorial

You can read the latest blog post or watch the advanced use case with custom_lint video.

About

Lint rules are a powerful way to improve the maintainability of a project. The more, the merrier! But while Dart offers a wide variety of lint rules by default, it cannot reasonably include every possible lint. For example, Dart does not include lints related to third-party packages.

Custom_lint fixes that by allowing package authors to write custom lint rules.

Custom_lint is similar to analyzer_plugin, but goes deeper by trying to provide a better developer experience.

That includes:

• A command-line to obtain the list of lints in your CI without having to write a command line yourself.
• A simplified project setup: No need to deal with the analyzer server or error handling. Custom_lint takes care of that for you, so that you can focus on writing lints.
• Debugger support. Inspect your lints using the Dart debugger and place breakpoints.
• Supports hot-reload/hot-restart: Updating the source code of a linter plugin will dynamically restart it, without having to restart your IDE/analyzer server.
• Built-in support for // ignore: and // ignore_for_file:.
• Built-in testing mechanism using // expect_lint. See Testing your plugins using expect_lint
• Support for print(...) and exceptions: If your plugin somehow throws or print debug messages, custom_lint will generate a log file with the messages/errors.

Usage

Using custom_lint is split in two parts:

• how to define a custom_lint package
• how users can install our package in their application to see our newly defined lints

Creating a custom lint package

To create a custom lint, you will need two things:

• updating your pubspec.yaml to include custom_lint_builder as a dependency:

  # pubspec.yaml
  name: my_custom_lint_package
  environment:
    sdk: ">=2.16.0 <3.0.0"
  
  dependencies:
    # we will use analyzer for inspecting Dart files
    analyzer:
    analyzer_plugin:
    # custom_lint_builder will give us tools for writing lints
    custom_lint_builder:

• create a lib/<my_package_name>.dart file in your project with the following:

  // This is the entrypoint of our custom linter
  PluginBase createPlugin() => _ExampleLinter();
  
  /// A plugin class is used to list all the assists/lints defined by a plugin.
  class _ExampleLinter extends PluginBase {
    /// We list all the custom warnings/infos/errors
    @override
    List<LintRule> getLintRules(CustomLintConfigs configs) => [
          MyCustomLintCode(),
        ];
  }
  
  class MyCustomLintCode extends DartLintRule {
    MyCustomLintCode() : super(code: _code);
  
    /// Metadata about the warning that will show-up in the IDE.
    /// This is used for `// ignore: code` and enabling/disabling the lint
    static const _code = LintCode(
      name: 'my_custom_lint_code',
      problemMessage: 'This is the description of our custom lint',
    );
  
    @override
    void run(
      CustomLintResolver resolver,
      ErrorReporter reporter,
      CustomLintContext context,
    ) {
      // Our lint will highlight all variable declarations with our custom warning.
      context.registry.addVariableDeclaration((node) {
        // "node" exposes metadata about the variable declaration. We could
        // check "node" to show the lint only in some conditions.
  
        // This line tells custom_lint to render a waring at the location of "node".
        // And the warning shown will use our `code` variable defined above as description.
        reporter.reportErrorForNode(code, node);
      });
    }
  }

That's it for defining a custom lint package!

If you're looking for a more advanced example, see the example. This example implements:

• a lint appearing on all variables of a specific type
• a quick fix for that lint
• an "assist" for providing refactoring options.

Let's now use it in an application.

Using our custom lint package in an application

For users to run custom_lint packages, there are a few steps:

• The application must contain an analysis_options.yaml with the following:

  analyzer:
    plugins:
      - custom_lint

• The application also needs to add custom_lint and our package(s) as dev dependency in their application:

  # The pubspec.yaml of an application using our lints
  name: example_app
  environment:
    sdk: ">=2.16.0 <3.0.0"
  
  dev_dependencies:
    custom_lint:
    my_custom_lint_package:

That's all! After running pub get (and possibly restarting their IDE), users should now see our custom lints in their Dart files:

Obtaining the list of lints in the CI

Unfortunately, running dart analyze does not pick up our newly defined lints. We need a separate command for this.

To do that, users of our custom lint package can run the following inside their terminal:

$ dart run custom_lint
  lib/main.dart:0:0 • This is the description of our custom lint • my_custom_lint_code

If you are working on a Flutter project, run flutter pub run custom_lint instead.

Using the Dart debugger

To debug plugins in custom_lint, you need to connect to plugins using "attach" mode in your IDE (cmd+shift+p + Debug: attach to Dart process in VSCode).

When using this command, you will need a VM service URI provided by custom_lint.

There are two possible ways to obtain one:

• if you started your plugin using custom_lint --watch, it should be visible in the console output.
• if your plugin is started by your IDE, you can open the custom_lint.log file that custom_lint created next to the pubspec.yaml of your analyzed projects.

In both cases, what you're looking for is logs similar to:

The Dart VM service is listening on http://127.0.0.1:60671/9DS43lRMY90=/
The Dart DevTools debugger and profiler is available at: http://127.0.0.1:60671/9DS43lRMY90=/devtools/#/?uri=ws%3A%2F%2F127.0.0.1%3A60671%2F9DS43lRMY90%3D%2Fws

What you'll want is the first URI. In this example, that is http://127.0.0.1:60671/9DS43lRMY90=/. You can then pass this to your IDE, which should now be able to attach to the plugin.

Testing your plugins using expect_lint

Custom_lint comes with an official testing mechanism for asserting that your plugins correctly work.

Testing your plugins is straightforward: Simply write a file that should contain lints from your plugin (such as the example folder). Then, using a syntax similar to // ignore, write a // expect_lint: code in the line before your lint:

// expect_lint: riverpod_final_provider
var provider = Provider(...);

When doing this, there are two possible cases:

• The line after the expect_lint correctly contains the expected lint. In that case, the lint is ignored (similarly to if we used // ignore)
• The next line does not contain the lint. In that case, the expect_lint comment will have an error.

This allows testing your plugins by simply running custom_lint on your test/example folder. Then, if any expected lint is missing, the command will fail. But if your plugin correctly emits the lint, the command will succeed.

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Built and maintained by Invertase.