1:No Directionality widget found.

 Widget build(BuildContext context){
    return Container(
      color: Colors.yellow,
      alignment: Alignment(0.0,0.0),
      child: Container(
        color: Colors.blue,
        child: AspectRatio(
          aspectRatio: 2/1,
          child: Icon(Icons.add,),
        ),
      )
    );
  }
}

2:哪個(gè)是啟動(dòng)文件？
3:創(chuàng)建新的dart文件怎么調(diào)用？
在main.dart中引入頭文件. ' home: StateManagerDemo(),'進(jìn)行引用。

class App extends StatelessWidget{
  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: StateManagerDemo(),
      theme: ThemeData(
        primaryColor: Colors.blue,
      ),
    );
  }
}

4:StatelessWidget對(duì)應(yīng)final不可變的類型。
5:重新加載相當(dāng)于realoadData

setState(() { });