skills/ HoangNguyen0403/agent-skills-standard

flutter-bloc-state-management

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.

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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

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:

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