2nd/10_Wiki/Topic_Programming/Architecture/BLoC.md

---
id: wiki-2026-0508-bloc
title: BLoC
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [BLoC, Business Logic Component, flutter_bloc]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [flutter, state-management, dart, reactive]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: dart
  framework: flutter,flutter_bloc
---

# BLoC

## 매 한 줄
> **"매 UI ↔ business logic 의 strict separation — Event in · State out · Stream 으로 의 mediate"**. 2018 Felix Angelov · Paolo Soares Google I/O 의 introduce, 2026 의 flutter_bloc 9.x · Cubit · Hydrated 의 Flutter ecosystem 의 dominant state-management.

## 매 핵심

### 매 contract
- **Event**: input (button tap · API tick).
- **State**: output (immutable snapshot).
- **Bloc**: `mapEventToState` (legacy) → `on<Event>` handler (modern 8.x+).
- **Cubit**: simpler — 매 event 의 X, method 의 emit 의 directly.

### 매 component
- `BlocProvider` — DI · scope.
- `BlocBuilder` — rebuild 의 state change 의 reactive.
- `BlocListener` — side-effect (snackbar · navigation).
- `BlocConsumer` — builder + listener.
- `RepositoryProvider` — data layer DI.

### 매 응용
1. 매 Flutter 의 production app (BMW · Nubank · Alibaba).
2. Form validation (매 reactive validation per field).
3. Authentication flow.
4. Hydrated state (매 disk-persisted, app restart 의 survive).

## 💻 패턴

### Bloc (event-driven)
```dart
sealed class CounterEvent {}
class Increment extends CounterEvent {}
class Decrement extends CounterEvent {}

class CounterBloc extends Bloc<CounterEvent, int> {
  CounterBloc() : super(0) {
    on<Increment>((event, emit) => emit(state + 1));
    on<Decrement>((event, emit) => emit(state - 1));
  }
}

// usage
BlocProvider(
  create: (_) => CounterBloc(),
  child: BlocBuilder<CounterBloc, int>(
    builder: (ctx, count) => Text('$count'),
  ),
)
```

### Cubit (simpler · method-based)
```dart
class CounterCubit extends Cubit<int> {
  CounterCubit() : super(0);
  void increment() => emit(state + 1);
  void decrement() => emit(state - 1);
}

// trigger
context.read<CounterCubit>().increment();
```

### Async event handler with state machine
```dart
sealed class LoginState {}
class LoginInitial   extends LoginState {}
class LoginLoading   extends LoginState {}
class LoginSuccess   extends LoginState { final User user; LoginSuccess(this.user); }
class LoginFailure   extends LoginState { final String msg; LoginFailure(this.msg); }

class LoginBloc extends Bloc<LoginEvent, LoginState> {
  final AuthRepository auth;
  LoginBloc(this.auth) : super(LoginInitial()) {
    on<SubmitLogin>(_onSubmit);
  }

  Future<void> _onSubmit(SubmitLogin e, Emitter<LoginState> emit) async {
    emit(LoginLoading());
    try {
      final user = await auth.login(e.email, e.password);
      emit(LoginSuccess(user));
    } catch (err) {
      emit(LoginFailure(err.toString()));
    }
  }
}
```

### BlocListener (side-effect)
```dart
BlocListener<LoginBloc, LoginState>(
  listener: (ctx, state) {
    if (state is LoginSuccess) Navigator.pushNamed(ctx, '/home');
    if (state is LoginFailure) ScaffoldMessenger.of(ctx).showSnackBar(
      SnackBar(content: Text(state.msg)),
    );
  },
  child: LoginForm(),
)
```

### Hydrated Bloc (auto-persist)
```dart
class SettingsCubit extends HydratedCubit<Settings> {
  SettingsCubit() : super(const Settings(theme: 'light'));

  void setTheme(String t) => emit(state.copyWith(theme: t));

  @override
  Settings fromJson(Map<String, dynamic> json) => Settings.fromJson(json);

  @override
  Map<String, dynamic> toJson(Settings state) => state.toJson();
}
```

### Stream-debounced search Bloc
```dart
class SearchBloc extends Bloc<SearchQuery, SearchState> {
  SearchBloc(this.api) : super(SearchInitial()) {
    on<SearchQuery>(
      _onSearch,
      transformer: (events, mapper) => events
          .debounceTime(const Duration(milliseconds: 300))
          .switchMap(mapper),
    );
  }

  Future<void> _onSearch(SearchQuery e, Emitter<SearchState> emit) async {
    emit(SearchLoading());
    final results = await api.search(e.term);
    emit(SearchLoaded(results));
  }
}
```

### Test (bloc_test)
```dart
blocTest<CounterBloc, int>(
  'emits [1] when Increment is added',
  build: () => CounterBloc(),
  act: (b) => b.add(Increment()),
  expect: () => [1],
);
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| Simple state · single screen | Cubit |
| Complex event flow · undo/redo | Bloc |
| Persisted state | HydratedBloc/Cubit |
| Server-driven · realtime | Bloc + StreamSubscription |
| Multi-screen shared | BlocProvider 의 root |
| 매 small project | Provider · Riverpod |

**기본값**: Cubit 의 first, Bloc 의 event 의 explicit 의 require 의 case.

## 🔗 Graph
- 부모: [[State Management]]
- 응용: [[Clean Architecture]]
- Adjacent: [[Provider]] · [[Riverpod]]

## 🤖 LLM 활용
**언제**: Bloc · Cubit · Event · State 의 boilerplate 의 generate, blocTest 의 draft.
**언제 X**: 매 actual event sequencing · concurrency strategy (debounce/restartable/sequential) — 매 domain 의 understand 의 require.

## ❌ 안티패턴
- **State 의 mutable**: 매 immutable + copyWith 의 mandatory.
- **BlocBuilder 의 entire screen 의 wrap**: granular rebuild 의 lose — multiple builder 의 split.
- **Bloc 의 navigation 의 directly do**: side-effect 의 BlocListener 의 isolate.
- **Singleton bloc 의 manual instantiate**: BlocProvider 의 use — lifecycle 의 leak prevent.

## 🧪 검증 / 중복
- Verified (bloclibrary.dev, flutter_bloc 9.x docs, Felix Angelov GitHub).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — Bloc + Cubit + Hydrated + bloc_test patterns |