Macro

Learn the Dart 3 macro experiment.

Macros (Dart 3)

Dart 3 introduced an experimental macro system. Macros generate code at compile time: when you annotate a class, the compiler injects methods/fields automatically.

Unlike previous approaches, macros are embedded in the compiler; they need no separate build_runner step. This speeds up the dev loop.

Since they are still experimental, the API may change. For production, source_gen (code generation) is still recommended; macros are promising.

Marking with annotations

Macros are triggered by annotations (metadata). When you add @MyMacro() to a class, the compiler runs the macro definition and injects the generated code.

Annotations in Dart are classes with a const constructor. Example: class Generated { const Generated(); }. Then @Generated() class Service {}.

Even without macros, annotations can be read at runtime via mirrors; but this does not work in Flutter/AOT.

Exercise Define and apply an annotation

Define the annotation class class Generated { const Generated(); }. Define class Service {} marked with @Generated(). Inside main add print(Service().runtimeType);.

Show example solution

class Generated {
  const Generated();
}

@Generated()
class Service {}

void main() {
  print(Service().runtimeType);
}

Test yourself

When do macros generate code?
- At runtime
- At compile time
- Only in tests
- On the server
Show answer

At compile time — macros generate code at compile time.
Match the approaches

Macro

Compile time
source_gen

Build step
dart:mirrors

Runtime
runtimeType

Basic type

Apply an annotation

@____()
class Service {}

Show answer

@Generated()
class Service {}

Next lesson: Code generation

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