stream multiplexing

stream_multiplex.dart

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

void main() {
  runApp(new MyApp());
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

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

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

typedef S ReduceFn<S, T>(S stream, T input);
StreamTransformer makeReduceTransformer(reduceFn, seed) {
  var state = seed;
  return new StreamTransformer.fromHandlers(handleData: (s, sink) {
    state = reduceFn(state, s);
    sink.add(state);
  });
}

class _MyHomePageState extends State<MyHomePage> {
  Stream<int> _counterStream;
  Stream<String> _fizzBuzzStream;
  StreamController<Null> _clickStreamController =
      new StreamController<Null>.broadcast(sync: true);

  @override
  void initState() {
    super.initState();
    int stateFunc(int previous, int input) {
      return previous + input;
    }

    final internalStateStream = _clickStreamController.stream
        .map((_) => 1)
        .transform(makeReduceTransformer(stateFunc, 0));
    StreamController<int> fizzBuzzController =
        new StreamController<int>(sync: true);
    StreamController<int> stateStreamController =
        new StreamController<int>(sync: true);

    internalStateStream.listen((val) {
      fizzBuzzController.add(val);
      stateStreamController.add(val);
    });

    _fizzBuzzStream = fizzBuzzController.stream.map((int val) {
      var output = new StringBuffer('$val:');
      if (val % 3 == 0) {
        output.write('fizz');
      }
      if (val % 5 == 0) {
        output.write('buzz');
      }
      return output.toString();
    });

    _counterStream = stateStreamController.stream;
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
        body: new Center(
          child: new Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              new Text(
                'You have pushed the button this many times:',
              ),
              new StreamBuilder(
                stream: _counterStream,
                builder: (context, snapshot) => new Text(
                      '${snapshot.data ?? 0}',
                      style: Theme.of(context).textTheme.display1,
                    ),
              ),
              new StreamBuilder(
                stream: _fizzBuzzStream,
                builder: (context, snapshot) => new Text(
                      snapshot.data ?? '--',
                      style: Theme.of(context).textTheme.display2,
                    ),
              ),
            ],
          ),
        ),
        floatingActionButton: new FloatingActionButton(
          onPressed: () => _clickStreamController.add(null),
          tooltip: 'Increment',
          child: new Icon(Icons.add),
        ));
  }
}