Flutter笔记--异步UI更新

 FutureBuilder       

        这一节主要介绍Flutter中异步UI更新涉及的知识。在应用层面开发中，很多时候我们会依赖一些异步数据来动态更新UI，比如在打开一个页面时我们需要先从互联网上获取数据，在获取数据的过程中我们显示一个加载框，等获取到数据时我们再渲染带有数据的页面；Flutter中FutureBuilder能够满足这种场景。

     FutureBuilder会依赖一个Future，它会根据所依赖的Future的状态来动态构建自身。我们看一下FutureBuilder构造函数：

FutureBuilder({
  this.future,
  this.initialData,
  @required this.builder,
})

future：FutureBuilder依赖的Future，通常是一个异步耗时任务。
initialData：初始数据，用户设置默认数据。
builder：Widget构建器；该构建器会在Future执行的不同阶段被多次调用，构建器签名如下：
Function (BuildContext context, AsyncSnapshot snapshot)
         snapshot会包含当前异步任务的状态信息及结果信息 ，比如我们可以通过snapshot.connectionState获取异步任务的状态信息、通过snapshot.hasError判断异步任务是否有错误等等，完整的定义读者可以查看AsyncSnapshot类定义。

官方建议 它的实例尽量越早比较好，在State的initState，State.didUpdateWidget， or State.didChangeDependencies，不要在build里面去实例化。

栗子：

class _DemoState extends State<Demo> {

@override
Widget build(BuildContext context) {
  return FutureBuilder<String>(
    future: downloadData(), // function where you call your api
    builder: (BuildContext context, AsyncSnapshot<String> snapshot) {  // AsyncSnapshot<Your object type>
      if( snapshot.connectionState == ConnectionState.waiting){
          return  Center(child: Text('Please wait its loading...'));
      }else{
          if (snapshot.hasError)
            return Center(child: Text('Error: ${snapshot.error}'));
          else
            return Center(child: new Text('${snapshot.data}'));  // snapshot.data  :- get your object which is pass from your downloadData() function
      }
    },
  );
}
Future<String> downloadData()async{
  //   var response =  await http.get('https://getProjectList');    
  return Future.value("Data download successfully"); // return your response
}
}

官网:FutureBuilder

StreamBuilder

         StreamBuilder控件用用最新交互流数据来构建自身 ，通常我们在开发 Flutter 页面，数据发生更新，都是直接通过 setState 方式对整个页面进行更新。如果页面某些数据需要每秒都更新，比如说下载文件时显示的进度条，对整个页面都刷新是很影响页面性能，有必要局部刷新。

 栗子:

Stream<int> _bids = (() async* {
  await Future<void>.delayed(Duration(seconds: 1));
  yield 1;
  await Future<void>.delayed(Duration(seconds: 1));
})();

Widget build(BuildContext context) {
  return DefaultTextStyle(
    style: Theme.of(context).textTheme.headline2,
    textAlign: TextAlign.center,
    child: Container(
      alignment: FractionalOffset.center,
      color: Colors.white,
      child: StreamBuilder<int>(
        stream: _bids,
        builder: (BuildContext context, AsyncSnapshot<int> snapshot) {
          List<Widget> children;
          if (snapshot.hasError) {
            children = <Widget>[
              Icon(
                Icons.error_outline,
                color: Colors.red,
                size: 60,
              ),
              Padding(
                padding: const EdgeInsets.only(top: 16),
                child: Text('Error: ${snapshot.error}'),
              )
            ];
          } else {
            switch (snapshot.connectionState) {
              case ConnectionState.none:
                children = <Widget>[
                  Icon(
                    Icons.info,
                    color: Colors.blue,
                    size: 60,
                  ),
                  const Padding(
                    padding: EdgeInsets.only(top: 16),
                    child: Text('Select a lot'),
                  )
                ];
                break;
              case ConnectionState.waiting:
                children = <Widget>[
                  SizedBox(
                    child: const CircularProgressIndicator(),
                    width: 60,
                    height: 60,
                  ),
                  const Padding(
                    padding: EdgeInsets.only(top: 16),
                    child: Text('Awaiting bids...'),
                  )
                ];
                break;
              case ConnectionState.active:
                children = <Widget>[
                  Icon(
                    Icons.check_circle_outline,
                    color: Colors.green,
                    size: 60,
                  ),
                  Padding(
                    padding: const EdgeInsets.only(top: 16),
                    child: Text('\$${snapshot.data}'),
                  )
                ];
                break;
              case ConnectionState.done:
                children = <Widget>[
                  Icon(
                    Icons.info,
                    color: Colors.blue,
                    size: 60,
                  ),
                  Padding(
                    padding: const EdgeInsets.only(top: 16),
                    child: Text('\$${snapshot.data} (closed)'),
                  )
                ];
                break;
            }
          }

          return Column(
            mainAxisAlignment: MainAxisAlignment.center,
            crossAxisAlignment: CrossAxisAlignment.center,
            children: children,
          );
        },
      ),
    ),
  );
}

官网:StreamBuilder