dart-dependency-injection

name	dart-dependency-injection
description	Dependency injection patterns for Dart/Flutter applications. Use when configuring get_it, registering dependencies, using constructor injection, setting up composition root, and testing with mocks.

Dart Dependency Injection

Composition Root

• get_it calls (getIt.get<T>()) only at app init (main() or setup function)
• Never inside class constructors or methods
• Exception: global UI services (e.g. audio player for button click sounds) -- document clearly, keep to absolute minimum
• Resolve dependencies at composition root, pass to constructors when creating instances

// Composition root (main.dart or setup function)
final service = getIt<IService>();
final repository = getIt<IRepository>(param1: service);
final useCase = getIt<IUseCase>(param1: repository);
final controller = MyController(useCase: useCase);

Constructor Injection

• All dependencies through constructors: MyClass(this.service, this.repository)
• Use abstract classes as interfaces, not concrete implementations
• Enables swapping implementations in tests without overriding get_it

Registration

• Order: datasources -> repositories -> use cases -> controllers (leaf nodes first)
• Factory: stateless services, new instance per resolution
• Singleton: stateful services (repositories, API clients, databases), shared instance
• Prefer instance-based singletons -- avoid static methods and lazy proxy wrappers

Lifecycle & Scoping

• Dispose singletons that hold resources (database, StreamController)
• Global singletons for app-wide services
• Scoped providers (e.g. RepositoryProvider) for feature-specific dependencies in Flutter

Circular Dependencies

• If A depends on B and B on A: extract shared interface, introduce mediator, or split responsibility
• Circular registration causes runtime errors or infinite loops

Testing

• Constructor injection enables easy mocking: MyClass(mockService, mockRepository)
• No need to override get_it or register mocks globally for unit tests
• Separate setup functions or parameters to switch implementations per environment (dev/staging/prod)

test('should do something', () {
  final mockRepo = MockIRepository();
  when(() => mockRepo.get()).thenAnswer((_) async => result);
  final useCase = MyUseCase(mockRepo);
  // test useCase
});