监听与订阅（Stream）

import 'dart:async';

import 'package:flutter/material.dart';

class StreamDemo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('StreamDemo'),
      ),
      body: StreamDemoHome(),
    );
  }
}

class StreamDemoHome extends StatefulWidget {
  @override
  _StreamDemoHomeState createState() => _StreamDemoHomeState();
}

class _StreamDemoHomeState extends State<StreamDemoHome> {
  StreamSubscription _streamSubscription;

  @override
  void initState() {
    super.initState();

    print('Create a stream');
    Stream<String> _streamDemo = Stream.fromFuture(fetchData());

    print('Start listening on a stream');
    // 添加订阅
    _streamSubscription = _streamDemo.listen(onData, onError: onError, onDone: onDone);

    print('Initialize completed.');
  }

  void onDone() {
    print('Done!');
  }

  void onError(error) {
    print('Error: $error');
  }

  void onData(String data) {
    print('$data');
  }

  // 暂停订阅
  void _pauseStream(){
    print('Pause subscription');
    _streamSubscription.pause();
  }

  // 恢复订阅
  void _resumeStream(){
    print('Resume subscription');
    _streamSubscription.resume();
  }

  // 取消订阅
  void _cancelStream(){
    print('Cancel subscription');
    _streamSubscription.cancel();
  }

  Future<String> fetchData() async {
    await Future.delayed(Duration(seconds: 5));
    throw 'Something happen';
    // return 'hello~';
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      child: Center(
        child: Row(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            FlatButton(
              child: Text('Pause'),
              onPressed: _pauseStream,
            ),
            FlatButton(
              child: Text('Resume'),
              onPressed: _resumeStream,
            ),
            FlatButton(
              child: Text('Cancel'),
              onPressed: _cancelStream,
            ),
          ],
        ),
      ),
    );
  }
}

控制器（controller）

StreamController<String> _streamDemo;

@override
void initState() {
  super.initState();

  print('Create a stream');
  _streamDemo = StreamController<String>();

  print('Start listening on a stream');
  // 添加订阅
  _streamSubscription = _streamDemo.stream.listen(onData, onError: onError, onDone: onDone);

  print('Initialize completed.');
}

void _addDataToStream() async {
    print('Add data to stream.');

    String data = await fetchData();
    _streamDemo.add(data);
  }

sink

一般来说，对于stream，我们使用stream输出，sink输入

StreamSink _sinkDemo;

@override
  void initState() {
    super.initState();

    print('Create a stream');
    _streamDemo = StreamController<String>();
    _sinkDemo = _streamDemo.sink;

    print('Start listening on a stream');
    // 添加订阅
    // 当有数据时，通过stream流出
    _streamSubscription = _streamDemo.stream.listen(onData, onError: onError, onDone: onDone);

    print('Initialize completed.');
  }

void _addDataToStream() async {
    print('Add data to stream.');

    String data = await fetchData();
    _sinkDemo.add(data);
  }

多次订阅（broadcast）

@override
  void initState() {
    super.initState();

    print('Create a stream');
    // Stream<String> _streamDemo = Stream.fromFuture(fetchData());
    _streamDemo = StreamController.broadcast();
    _sinkDemo = _streamDemo.sink;

    print('Start listening on a stream');
    // 添加订阅
    _streamSubscription = _streamDemo.stream.listen(onData, onError: onError, onDone: onDone);

    _streamSubscription = _streamDemo.stream.listen(onDataTwo, onError: onError, onDone: onDone);

    print('Initialize completed.');
  }

StreamBuilder

根据Stream上的数据渲染组件

StreamBuilder(
  stream: _streamDemo.stream,
  initialData: '...',
  builder: (context, snapshot){
    print(snapshot);
    return Text('${snapshot.data}');
  },
),