Simple bloc

main_bloc.dart

import 'dart:async';
import 'package:rxdart/rxdart.dart';
import 'package:flutter/material.dart';

void main() => runApp(MyApp());

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'),
    );
  }
}

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

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  final bloc = new TestBloc();

  @override
  void dispose() {
    bloc.dispose();
    super.dispose();
  }

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

class TestStateless extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    print('build TestStateless');
    return Text('TestStateless');
  }
}

class TestStateful extends StatefulWidget {
  @override
  _TestStatefulState createState() => _TestStatefulState();
}

class _TestStatefulState extends State<TestStateful> {
  @override
  Widget build(BuildContext context) {
    print('build TestStateful');
    return Text('_TestStatefulState');
  }
}

class TestBloc {
  //Model
  int _counter = 0;

  //Input to bloc
  var subject = BehaviorSubject<int>();

  //Output from bloc
  Stream<int> get counter =>
      subject.map((data) => this._counter += data);

  void increment() {
    subject.add(1);
  }

  void dispose() {
    subject.close();
  }
}

pubspec.yaml

dependencies:
  flutter:
    sdk: flutter
  cupertino_icons: ^0.1.2
  rxdart: ^0.19.0