Flutter bloc one shot effects

Flutter BLoC one-shot UI effects

For SnackBar, navigation after action, toast, dialog triggered by a BLoC state change. Based on the BLoC Todos tutorial (lastDeletedTodo + listenWhen).

Pattern

A nullable payload field on the data-bearing state, plus a BlocListener with listenWhen that fires exactly when the payload changes.

@freezed
sealed class TodosState with _$TodosState {
  const factory TodosState.initial() = TodosInitial;
  const factory TodosState.loading() = TodosLoading;
  const factory TodosState.loaded({
    required List<Todo> todos,
    Todo? lastDeletedTodo,       // entity payload
    DateTime? lastRefreshedAt,   // nonce, when payload can repeat
    Exception? actionError,      // inline error that preserves data
  }) = TodosLoaded;
  const factory TodosState.error({required Exception e}) = TodosError;
}

// Effect-carrying emit: fresh constructor, not copyWith.
// Each emit sets only the relevant marker; other effect fields reset to null.
emit(TodosState.loaded(todos: next, lastDeletedTodo: deleted));
emit(TodosState.loaded(todos: s.todos, actionError: e));

// Data-only update (optimistic UI): copyWith is fine.
// Never set an effect field via copyWith.
emit(s.copyWith(todos: optimisticTodos));

MultiBlocListener(
  listeners: [
    BlocListener<TodosBloc, TodosState>(
      listenWhen: (prev, curr) {
        if (curr is! TodosLoaded || curr.lastDeletedTodo == null) return false;
        if (prev is! TodosLoaded) return true;
        return curr.lastDeletedTodo != prev.lastDeletedTodo;
      },
      listener: (context, state) {
        final s = state as TodosLoaded;
        ScaffoldMessenger.of(context).showSnackBar(
          SnackBar(content: Text('${s.lastDeletedTodo!.title} deleted')),
        );
      },
    ),
  ],
  child: BlocBuilder<TodosBloc, TodosState>(...),
)

Rules

• UI effects only inside BlocListener / BlocConsumer.listener. Never in build or BlocBuilder.builder.
• One listener per effect type, grouped via MultiBlocListener.
• Effect = nullable payload field on the data-bearing state. Not a separate state variant. Not a sticky boolean.
• The payload must produce != on every emit that should re-fire the listener. Exception works by identity (new instance every time); plain String does not (value equality). Add a DateTime nonce when equality is value-based.
• Effect-carrying emits go through a fresh constructor, not copyWith. copyWith carries old markers forward.
• Builder reads persistent data only. Never check effect fields in build. If a render branch is needed, model it as a regular field with its own name, not as an effect marker.
• After await, check if (!context.mounted) return;.
• BLoC / Cubit holds no BuildContext and never calls UI directly.

Terminal state vs payload marker

If the current screen is gone after the effect (Navigator.pop, pushReplacement, sheet close), a payload marker is wrong. Use a separate terminal state variant.

@freezed
sealed class AddItemState with _$AddItemState {
  const factory AddItemState.loaded({...}) = AddItemLoaded;
  const factory AddItemState.success() = AddItemSuccess;  // terminal
  const factory AddItemState.error({required Exception e}) = AddItemError;
}

// listener: no mounted check needed when no await precedes pop
if (state is AddItemSuccess) Navigator.of(context).pop(true);

Checklist

• UI effects only inside BlocListener / BlocConsumer.listener.
• Effect is a nullable payload field, not a separate state variant.
• Payload produces != on every emit (verify equality semantics).
• All effect-carrying emits go through a fresh constructor.
• listenWhen checks both the field change and the cross-variant transition.
• Builder does not read effect fields.
• After await, context.mounted is checked.

References

• BLoC Todos tutorial
• bloc#4073, sticky status discussion