Basic BLoC example

main.dart

import 'package:flutter_web/material.dart';
import 'package:flutter_web_ui/ui.dart' as ui;
import 'dart:async';

void main() async {
  await ui.webOnlyInitializePlatform();
  runApp(MyApp());
}

abstract class MyHomePageAction {
  MyHomePageAction();
  factory MyHomePageAction.increment(int originalValue) => Increment(originalValue);
}

class Increment extends MyHomePageAction {
  int originalValue;
  Increment(this.originalValue) : super();
}

class MyHomePageBloc {
  int _counter = 0;
  final StreamController<int> _counterStream = StreamController.broadcast();
  final StreamController<MyHomePageAction> _eventStream = StreamController();
  Stream<int> get state => _counterStream.stream;
  StreamSink<MyHomePageAction> get eventSink => _eventStream.sink;

  MyHomePageBloc() {
    _counterStream.add(_counter);
    _eventStream.stream.listen((_) {
      _onAddClicked();
    });
  }

  void _onAddClicked() {
    _counter++;
    _counterStream.add(_counter);
  }
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(
        title: 'Flutter Demo Home Page',
        bloc: MyHomePageBloc(),
      ),
    );
  }
}

class MyHomePage extends StatelessWidget {
  final String title;
  final MyHomePageBloc bloc;
  MyHomePage({Key key, this.title, this.bloc}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(this.title),
      ),
      body: Center(
        child: StreamBuilder(
          stream: bloc.state,
          initialData: 0,
          builder: (BuildContext context, AsyncSnapshot<int> snapshot) {
            return Column(
              mainAxisAlignment: MainAxisAlignment.center,
              children: <Widget>[
                Text(
                  'You have pushed the button this many times:',
                ),
                Text(
                  '${snapshot.data}',
                  style: Theme.of(context).textTheme.display1,
                ),
              ],
            );
          },
        ),
      ),
      floatingActionButton: StreamBuilder(
          stream: bloc.state,
          initialData: 0,
          builder: (BuildContext context, AsyncSnapshot<int> snapshot) {
            return FloatingActionButton(
              onPressed: () =>
                  bloc.eventSink.add(MyHomePageAction.increment(snapshot.data)),
              tooltip: 'Increment',
              child: Icon(Icons.add),
            );
          }),
    );
  }
}