Flutter Bloc State Management

---
name: flutter-bloc-state-management
description: Implement BLoC/Cubit state, events, transitions, and async concurrency in Flutter. Use for BLoC/Cubit feature logic, debounced/cancellable events, state rendering, or bloc tests—not generic widget-only work.
metadata:
  triggers:
    files:
    - '**_bloc.dart'
    - '**_cubit.dart'
    - '**_state.dart'
    - '**_event.dart'
    keywords:
    - BlocProvider
    - BlocBuilder
    - BlocListener
    - Cubit
    - Emitter
  upstream:
    - name: bloc
      source: github
      repo: felangel/bloc
      pinned: "9.0.0"
      tag_pattern: "^bloc-v(\\d+\\.\\d+\\.\\d+)$"
      reviewed: "2026-07-09"
---
# BLoC State Management

## **Priority: P0 (CRITICAL)**

**Role**: Flutter State Management Expert. Design predictable, testable state flows.

## State Design Workflow

1. **Define Events**: What happens? (UserTap, ApiSuccess). Use `@freezed`.
2. **Define States**: What needs to show? (Initial, Loading, Data, Error).
3. **Implement BLoC**: Map Events to States using `on<Event>`.
4. **Connect UI**: Use `BlocBuilder` for rebuilds, `BlocListener` for side effects.

## Implementation Guidelines

- **States & Events**: Use **@freezed** for union types (e.g., `Initial`, `Loading`, `Success`, `Failure` states).
- **Error Handling**: Emit `Failure` states for UI-critical errors. For silent/background events, either let exceptions propagate naturally to global `onError` interceptor (e.g., in `AppBlocObserver`), or catch and call `addError(e, st)` without emitting error state.
- **Async Data**: Use **emit.forEach** for streams or **await** with `emit` call.
- **Concurrency**: Use **transformer: restartable()** from `bloc_concurrency` for search/typeahead to debounce and cancel previous requests.
- **UI Connectivity**: Use **BlocBuilder** for UI rebuilds (e.g., loading spinner, data list, error message) and **BlocListener** for side effects (navigation, snackbars).
- **Testing**: Use **blocTest** for ALL states and verify sequence of emitted states.

## Verification Checklist (Mandatory)

- [ ] **Initial State**: Defined and tested?
- [ ] **Test Coverage**: `blocTest` used for ALL states?
- [ ] **UI Logic**: No complex calculation in `BlocBuilder`?
- [ ] **Side Effects**: Navigation/Snackbars in `BlocListener` (NOT Builder)?

## Anti-Patterns

- **No .then()**: Use `await` or `emit.forEach()` to emit.
- **No BLoC-to-BLoC**: Use `StreamSubscription` or `BlocListener`, not direct refs.
- **No Logic in Builder**: Move valid logic to BLoC.
- **No BlocBuilder without buildWhen**: Heavy subtrees must declare `buildWhen` predicate to prevent unnecessary rebuilds.

## Verification

- [ ] Every BLoC has `blocTest` covering Initial → Loading → Success and Initial → Loading → Failure.
- [ ] Side effects (navigation, snackbars) use `BlocListener`, not `BlocBuilder`.
- [ ] Heavy `BlocBuilder` widgets declare `buildWhen`.
- [ ] `flutter test` passes.

## References

- [Templates](references/bloc_templates.md)

## Event-handler correction

- `emit()` is not a Future: **Remove** `.then()` from it, use `await` for asynchronous work, and register the event with an `on<Event>` handler.

## Canonical response anchors

When this skill applies, preserve the following domain terminology or equivalent concrete examples in the answer when relevant:
- Remove