Bloc Skill

Design, implement, and test state management using the bloc and flutter_bloc libraries.

When to Use

Use this skill when:

• Creating a new Cubit or Bloc for a feature.
• Modeling state (choosing between sealed classes and a single state class with status enum).
• Wiring BlocBuilder, BlocListener, BlocConsumer, or BlocProvider in the widget tree.
• Writing unit tests for a Cubit or Bloc.
• Deciding between Cubit and Bloc.
• Refactoring existing state management to follow bloc conventions.

1. Cubit vs Bloc

| Situation | Use | |---|---| | Simple state, no events needed | Cubit | | Complex flows, event traceability needed | Bloc | | Advanced event processing (debounce, throttle) | Bloc with event transformers |

Default to Cubit. Refactor to Bloc only when requirements grow.

2. Naming Conventions

Events (Bloc only)

• Named in past tense: LoginButtonPressed, UserProfileLoaded.
• Format: BlocSubject + optional noun + verb.
• Initial load event: BlocSubjectStarted (e.g., AuthenticationStarted).
• Base event class: BlocSubjectEvent.

States

• Named as nouns (states are snapshots in time).
• Base state class: BlocSubjectState.
• Sealed subclasses: BlocSubject + Initial | InProgress | Success | Failure.
• Example: LoginInitial, LoginInProgress, LoginSuccess, LoginFailure.
• Single-class approach: BlocSubjectState + BlocSubjectStatus enum (initial, loading, success, failure).

3. Modeling State

When to use a sealed class with subclasses

• States are well-defined and mutually exclusive.
• Type-safe exhaustive switch is desired.
• Subclass-specific properties exist.

dart
@immutable
sealed class LoginState extends Equatable {
  const LoginState();
}

final class LoginInitial extends LoginState {
  @override
  List<Object?> get props => [];
}

final class LoginInProgress extends LoginState {
  @override
  List<Object?> get props => [];
}

final class LoginSuccess extends LoginState {
  const LoginSuccess(this.user);
  final User user;
  @override
  List<Object?> get props => [user];
}

final class LoginFailure extends LoginState {
  const LoginFailure(this.message);
  final String message;
  @override
  List<Object?> get props => [message];
}

Handle all states exhaustively in the UI:

dart
switch (state) {
  case LoginInitial():  ...
  case LoginInProgress(): ...
  case LoginSuccess(:final user): ...
  case LoginFailure(:final message): ...
}

When to use a single class with a status enum

• Many shared properties across states.
• Simpler, more flexible; previous data must be retained after failure.

dart
enum LoginStatus { initial, loading, success, failure }

@immutable
class LoginState extends Equatable {
  const LoginState({
    this.status = LoginStatus.initial,
    this.user,
    this.errorMessage,
  });

  final LoginStatus status;
  final User? user;
  final String? errorMessage;

  LoginState copyWith({
    LoginStatus? status,
    User? user,
    String? errorMessage,
  }) {
    return LoginState(
      status: status ?? this.status,
      user: user ?? this.user,
      errorMessage: errorMessage ?? this.errorMessage,
    );
  }

  @override
  List<Object?> get props => [status, user, errorMessage];
}

State rules (both approaches)

• Extend Equatable and pass all relevant fields to props.
• Copy List/Map properties with List.of/Map.of inside props.
• Annotate with @immutable.
• Always emit a new instance; never reuse the same state object.
• Duplicate states are ignored by bloc — ensure meaningful state changes.

4. Cubit Implementation

dart
class LoginCubit extends Cubit<LoginState> {
  LoginCubit(this._authRepository) : super(const LoginState());

  final AuthRepository _authRepository;

  Future<void> login(String email, String password) async {
    emit(state.copyWith(status: LoginStatus.loading));
    try {
      final user = await _authRepository.login(email, password);
      emit(state.copyWith(status: LoginStatus.success, user: user));
    } catch (e) {
      emit(state.copyWith(status: LoginStatus.failure, errorMessage: e.toString()));
    }
  }
}

Rules:

• Only call emit inside the Cubit/Bloc.
• Public methods return void or Future<void> only.
• Keep business logic out of UI.
• When overriding storage in a HydratedCubit, pass it as a named parameter: super(initialState, storage: storage).

5. Bloc Implementation

dart
sealed class LoginEvent {}
final class LoginSubmitted extends LoginEvent {
  LoginSubmitted({required this.email, required this.password});
  final String email;
  final String password;
}

class LoginBloc extends Bloc<LoginEvent, LoginState> {
  LoginBloc(this._authRepository) : super(LoginInitial()) {
    on<LoginSubmitted>(_onLoginSubmitted);
  }

  final AuthRepository _authRepository;

  Future<void> _onLoginSubmitted(
    LoginSubmitted event,
    Emitter<LoginState> emit,
  ) async {
    emit(LoginInProgress());
    try {
      final user = await _authRepository.login(event.email, event.password);
      emit(LoginSuccess(user));
    } catch (e) {
      emit(LoginFailure(e.toString()));
    }
  }
}

Rules:

• Trigger state changes via bloc.add(Event()), not custom public methods.
• Keep event handler methods private (_onEventName).
• Internal/repository events must be private and may use custom transformers.

6. Architecture

Three layers — each must stay in its own boundary:

Presentation  →  Business Logic (Cubit/Bloc)  →  Data (Repository → DataProvider)

• Data Layer: Repositories wrap data providers. Providers perform raw CRUD (HTTP, DB). Repositories expose clean domain objects.
• Business Logic Layer: Cubits/Blocs receive repository data and emit states. Inject repositories via constructor.
• Presentation Layer: Renders UI based on state. Handles user input by calling cubit methods or adding bloc events.

Rules:

• Blocs must not access data providers directly — only via repositories.
• No direct bloc-to-bloc communication. Use BlocListener in the UI to bridge blocs.
• For shared data, inject the same repository into multiple blocs.
• Initialize BlocObserver in main.dart.

7. Flutter Bloc Widgets

| Widget | Use | |---|---| | BlocProvider | Provide a bloc to a subtree | | MultiBlocProvider | Provide multiple blocs without nesting | | BlocBuilder | Rebuild UI on state change | | BlocListener | Side effects only (navigation, dialogs, snackbars) | | MultiBlocListener | Listen to multiple blocs without nesting | | BlocConsumer | Rebuild UI + side effects together | | BlocSelector | Rebuild only when a selected slice of state changes | | RepositoryProvider | Provide a repository to the widget tree | | MultiRepositoryProvider | Provide multiple repositories without nesting |

dart
BlocProvider(
  create: (context) => LoginCubit(context.read<AuthRepository>()),
  child: LoginView(),
);

BlocBuilder<LoginCubit, LoginState>(
  builder: (context, state) {
    return switch (state.status) {
      LoginStatus.loading => const CircularProgressIndicator(),
      LoginStatus.success => const HomeView(),
      LoginStatus.failure => Text(state.errorMessage ?? 'Error'),
      LoginStatus.initial => const LoginForm(),
    };
  },
);

BlocListener<LoginCubit, LoginState>(
  listener: (context, state) {
    if (state.status == LoginStatus.failure) {
      ScaffoldMessenger.of(context).showSnackBar(
        SnackBar(content: Text(state.errorMessage ?? 'Login failed')),
      );
    }
  },
  child: LoginForm(),
);

Rules:

• Use context.read<T>() in callbacks (not in build).
• Use context.watch<T>() in build only when necessary; prefer BlocBuilder.
• Never call context.watch or context.select at the root of build — scope with Builder.
• Handle all possible states in the UI (initial, loading, success, failure).

8. Testing

Use bloc_test package. Mock repositories with mocktail.

dart
import 'package:bloc_test/bloc_test.dart';
import 'package:mocktail/mocktail.dart';
import 'package:test/test.dart';

class MockAuthRepository extends Mock implements AuthRepository {}

void main() {
  group('LoginCubit', () {
    late AuthRepository authRepository;
    late LoginCubit loginCubit;

    setUp(() {
      authRepository = MockAuthRepository();
      loginCubit = LoginCubit(authRepository);
    });

    tearDown(() => loginCubit.close());

    test('initial state should be LoginState with status initial', () {
      expect(loginCubit.state, const LoginState());
    });

    blocTest<LoginCubit, LoginState>(
      'should emit [loading, success] when login succeeds',
      build: () {
        when(() => authRepository.login(any(), any()))
            .thenAnswer((_) async => fakeUser);
        return loginCubit;
      },
      act: (cubit) => cubit.login('email@test.com', 'password'),
      expect: () => [
        const LoginState(status: LoginStatus.loading),
        LoginState(status: LoginStatus.success, user: fakeUser),
      ],
    );

    blocTest<LoginCubit, LoginState>(
      'should emit [loading, failure] when login throws',
      build: () {
        when(() => authRepository.login(any(), any()))
            .thenThrow(Exception('error'));
        return loginCubit;
      },
      act: (cubit) => cubit.login('email@test.com', 'wrong'),
      expect: () => [
        const LoginState(status: LoginStatus.loading),
        isA<LoginState>().having((s) => s.status, 'status', LoginStatus.failure),
      ],
    );
  });
}

Rules:

• Always call tearDown(() => cubit.close()).
• Use blocTest for state emission assertions.
• Use group() named after the class under test.
• Name test cases with "should" to describe expected behavior.
• Register fallback values for custom types: registerFallbackValue(MyEvent()).

9. Common Pitfalls

| Pitfall | Fix | |---|---| | Emitting the same state instance twice | Always create a new state object; bloc ignores duplicate emissions via ==. | | Calling context.watch inside callbacks | Use context.read in callbacks; watch is only valid inside build. | | Forgetting Equatable props | Add every field to props; missing fields cause silent state update bugs. | | Mutable state fields | Keep state @immutable; use copyWith or new sealed subclass instances. | | Business logic in widgets | Move all logic into the Cubit/Bloc; widgets only dispatch events or call methods. |

dart
// BAD — mutating state in-place
state.items.add(newItem);
emit(state);

// GOOD — emit a new state with copied list
emit(state.copyWith(items: [...state.items, newItem]));

References

• Bloc GitHub Repository