main.dart

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

class CounterBloc {
  int initialCount = 20;

  BehaviorSubject<int> _subjectCounter;

  CounterBloc() {
    _subjectCounter = new BehaviorSubject<int>.seeded(this.initialCount);
  }
  Observable<int> get counterObservable => _subjectCounter.stream;

  void increment() {
    initialCount++;
    _subjectCounter.sink.add(initialCount);
  }

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

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

class StreamText extends StatelessWidget {
  final CounterBloc counterBloc;

  const StreamText({Key key, this.counterBloc}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return StreamBuilder(
      stream: counterBloc.counterObservable,
      builder: (BuildContext context, AsyncSnapshot<int> snapshot) =>
          Text('${snapshot.data}'),
    );
  }
}

class MyApp extends StatelessWidget {
  final CounterBloc _counterBloc = CounterBloc();

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        home: SafeArea(
            child: Column(
      children: <Widget>[
        StreamText(
          counterBloc: _counterBloc,
        ),
        RaisedButton(
          onPressed: _counterBloc.increment,
          child: Text('Sum'),
        )
      ],
    )));
  }
}