可以减少在WIdget中“test-then-else”这样的三元表达式来加载不同UI显示，还有自带淡入淡出动画。

class _MyHomeState extends State
    
      {  var _lst = 
     
      [];  @override  void initState() {    getData();    super.initState();  }  getData() async {    await Future.delayed(Duration(seconds: 4));    setState(() {      _lst = List.generate(100, (int index) => "Item ${index + 1}");    });  }  @override  Widget build(BuildContext context) {    return Scaffold(      appBar: AppBar(        title: Text('Flutter Demo'),      ),      body: AnimatedCrossFade(        duration: Duration(milliseconds: 600),        firstChild: Center(          child: CircularProgressIndicator(),        ),        secondChild: ListView(          children: _lst              .map((String s) => ListTile(                    key: Key(s),                    title: Text(s),                  ))              .toList(),        ),        crossFadeState:            _lst.isEmpty ? CrossFadeState.showFirst : CrossFadeState.showSecond,      ),    );  }}