custom_lint

License

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
• FAQs
  • Q. How do I get all the classes present in my source code?
  • Q. How do I get all the global variables present in my source code?
  • Q. How do I get the path to my currently opened dart file?
  • Q. I want to insert code at the end of the file. How can I get the required offset?

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 mainta∏inability 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_pachage_name>.dart file in your project with the following:

  // This is the entrypoint of our custom linter
  PluginBase createPlugin() => _ExampleLinter();
  
  // This class is the one that will analyze Dart files and return lints
  class _ExampleLinter extends PluginBase {
    @override
    Stream<Lint> getLints(ResolvedUnitResult resolvedUnitResult) async* {
      // A basic lint that shows at the top of the file.
      yield Lint(
        code: 'my_custom_lint_code',
        message: 'This is the description of our custom lint',
        // Where your lint will appear within the Dart file.
        // The following code will make appear at the top of the file (offset 0),
        // and be 10 characters long.
        location: resolvedUnitResult.lintLocationFromOffset(0, length: 10),
      );
    }
  }
  

That's it for defining a custom lint package!

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.

FAQs #

Q. How do I get all the classes present in my source code? #

There are two ways of doing that. You can either use the Element Tree or the Abstract Syntax Tree (AST) to find all the classes. Here's an example with each approach.

• Element Tree Approach

  class _CustomLint extends PluginBase {
    @override
    Stream<Lint> getLints(ResolvedUnitResult resolvedUnitResult) async* {
      final library = resolvedUnitResult.libraryElement;
      final classes = library.topLevelElements.whereType<ClassElement>();
      ....
      ....
    }
  }
  

  The variable classes is of type List<ClassElement> and will contain all the ClassElements.

• AST Approach

  class ClassDeclarationVisitor extends GeneralizingAstVisitor<void> {
    ClassDeclarationVisitor({
      required this.onClassDeclarationVisit,
    });
  
    void Function(ClassDeclaration node) onClassDeclarationVisit;
  
    @override
    void visitClassDeclaration(ClassDeclaration node) {
      onClassDeclarationVisit(node);
      super.visitClassDeclaration(node);
    }
  }
  
  class _CustomLint extends PluginBase {
    @override
    Stream<Lint> getLints(ResolvedUnitResult resolvedUnitResult) async* {
      final classDeclarations = <ClassDeclaration>[];
      resolvedUnitResult.unit.visitChildren(
        ClassDeclarationVisitor(
          onClassDeclarationVisit: classDeclarations.add,
        ),
      );
      ....
      ....
    }
  }
  

  This way, the variable classDeclarations will contain list of ClassDeclaration.

Q. How do I get all the global variables present in my source code? #

Same as above. Just replace ClassElement with VariableElement in the Element Tree Approach or replace ClassDeclaration with VariableDeclaration in the AST approach.

Q. How do I get the path to my currently opened dart file? #

You can use the instance of ResolvedUnitResult to call the path getter.

class _CustomLints extends PluginBase {
  @override
  Stream<Lint> getLints(ResolvedUnitResult resolvedUnitResult) async* {
    final path = resolvedUnitResult.path; // Gives the path of the currently opened Dart file
    ...
    ...
  }
}

Q. I want to insert code at the end of the file. How can I get the required offset? #

To find the offset for the end of the file (or simply put, find the end line of a Dart file), you can use the instance of ResolvedUnitResult to call the end getter.

class _CustomLints extends PluginBase {
  @override
  Stream<Lint> getLints(ResolvedUnitResult resolvedUnitResult) async* {
    final endLineOffset = resolvedUnitResult.unit.end; // Gives the offset for the last line in a Dart file
  }
}

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