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.

@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:

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.

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

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.

---

5. Bloc Implementation

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

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.

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.

// 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