class CounterBloc implements Bloc {
  final _countController = BehaviorSubject<int>(seedValue: 0);
  final _incrementController = PublishSubject<void>();

  CounterBloc() {
    _incrementController
        .scan<int>((sum, _v, _i) => sum + 1, 0)
        .pipe(_countController);
  }

  ValueObservable<int> get count => _countController;
  Sink<void> get increment => _incrementController.sink;

  @override
  void dispose() async {
    await _incrementController.close();
    await _countController.close();
  }
}

void main() => runApp(
      // Create and provide the bloc.
      BlocProvider<CounterBloc>(
        creator: (_context, _bag) => CounterBloc(),
        child: App(),
      ),
    );

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    // Access the bloc with O(1) computation complexity.
    final bloc = BlocProvider.of<CounterBloc>(context);
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: StreamBuilder<int>(
            stream: bloc.count,
            initialData: bloc.count.value,
            builder: (context, snap) => Text(
                  'count: ${snap.data}',
                  style: Theme.of(context).textTheme.title,
                ),
          ),
        ),
        floatingActionButton: FloatingActionButton(
          child: const Icon(Icons.add),
          onPressed: () => bloc.increment.add(null),
        ),
      ),
    );
  }
}