flutter_bloc: Rebuild Optimization, Persistence & Debugging

BlocSelector: Fine-Grained Rebuilds

// Rebuilds ONLY when itemCount specifically changes, ignoring
// unrelated changes to the rest of CartState
BlocSelector<CartBloc, CartState, int>(
  selector: (state) => state.itemCount,
  builder: (context, count) => Text('$count'),
)

// buildWhen/listenWhen: custom condition for whether to react at all
BlocBuilder<CartBloc, CartState>(
  buildWhen: (previous, current) => previous.itemCount != current.itemCount,
  builder: (context, state) => Text('${state.itemCount}'),
)

BlocProvider.value(): Reusing an Existing Instance

// Passes an EXISTING Bloc into a new route -- avoids recreating it
// or accidentally having it auto-disposed while still needed elsewhere
Navigator.push(context, MaterialPageRoute(
  builder: (_) => BlocProvider.value(
    value: context.read<CartBloc>(),
    child: CheckoutScreen(),
  ),
))

Persisting State with HydratedBloc

// Automatically persists state to local storage and restores it
// on app restart -- no manual serialization/storage code needed
class CartCubit extends HydratedCubit<CartState> {
  CartCubit() : super(CartState.empty());

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

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

Global Debugging with BlocObserver

// Observes events/state changes/errors across EVERY Bloc in the app
class AppBlocObserver extends BlocObserver {
  @override
  void onChange(BlocBase bloc, Change change) {
    print('${bloc.runtimeType} $change');
    super.onChange(bloc, change);
  }
}

void main() {
  Bloc.observer = AppBlocObserver();
  runApp(MyApp());
}

Constructor Injection for Testability

LoginBloc(this.authRepository) accepting a repository via the constructor lets a unit test pass LoginBloc(MockAuthRepository()) to fully control its behavior -- the same dependency-injection testability benefit seen with mocking libraries elsewhere.