Custom annotations

Learn to write your own annotation class.

Writing a custom annotation

You can write your own annotation. An annotation is simply a class with a const constructor. For example class Todo { final String msg; const Todo(this.msg); } is an annotation class.

You use it as @Todo('fix'). After the @ sign comes the class name and any arguments. Dart binds this label to the element.

Custom annotations are usually combined with code generation tools. On their own they have no runtime effect; they carry label information.

Applying an annotation

You can apply your Todo annotation to a method. Example: @Todo('add tests') void run() { ... }. The annotation is written right before the method.

You can pass arguments to the annotation: @Todo('message'). This corresponds to const Todo('message').

Multiple annotations can attach to one element. Tools can scan these labels and produce reports, for example list all Todo tags.

Exercise The Todo annotation

Define a Todo annotation class with a final String msg field and a const Todo(this.msg) constructor. Write a void run() { print('running'); } method marked with @Todo('test'). In main call run().

Show example solution

class Todo {
  final String msg;
  const Todo(this.msg);
}

@Todo('test')
void run() {
  print('running');
}

void main() {
  run();
}

Test yourself

What does a custom annotation require?
- A method returning a Future
- A class with a const constructor
- At least one abstract method
- An enum
Show answer

A class with a const constructor — an annotation is a class with a const constructor.

const constructor

class Todo {
  final String msg;
  ____ Todo(this.msg);
}

Show answer

class Todo {
  final String msg;
  const Todo(this.msg);
}

Custom annotation parts

class Todo

Annotation class
const Todo(this.msg)

const constructor
@Todo('test')

Application
final String msg

Field

Next lesson: @override

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