1. FutureBuilder

Center(
    child: FutureBuilder(
      // initialData: "加载中",
      // future: Future.delayed(Duration(seconds: 2), () => "123"),
      future: Future.delayed(Duration(seconds: 2), () => throw "网络错误error"),
      builder: (BuildContext context, AsyncSnapshot<dynamic> snapshot){
        // snapshot.connectionState
        if(snapshot.hasError) {
          return Text("${snapshot.error}");
        }
        // initialData
        if(snapshot.hasData) {
          return Text("数据 ${snapshot.data}");
        }
        return CircularProgressIndicator();
      },
    ),
  ),
);

2. StreamBuild（StreamController）

默认： 单次监听 （无监听时 会缓存事件）
多监听： StreamController.broadcast(); （无监听时 不会缓存事件）

其他Stream构建方式

Stream<DateTime> getDate() async*{
    while(true) {
        await Future.delayed(Duration(seconds: 1));
        yield DateTime.now();
    }
}

StreamController _streamController = StreamController();
// StreamController _streamController = StreamController.broadcast();
int index = 0;

@override
Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Test'),
      ),
      body: Center(
        child: StreamBuilder(
          stream: _streamController.stream,
          builder: (BuildContext context, AsyncSnapshot<dynamic> snapshot){
            // snapshot.connectionState
            if(snapshot.hasError) {
              return Text("${snapshot.error}");
            }
            // initialData
            if(snapshot.hasData) {
              return Text("数据 ${snapshot.data}");
            }
            return CircularProgressIndicator();
          },
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          _streamController.sink.add("event ${index++}");
        },
      ),
    );
}