Flutter之系统主题有关的组件MateriaApp

修改代码
 return MaterialApp(
    title: 'Flutter Demo',
    theme: ThemeData(
      // This is the theme of your application.
      //
      // Try running your application with "flutter run". You'll see the
      // application has a blue toolbar. Then, without quitting the app, try
      // changing the primarySwatch below to Colors.green and then invoke
      // "hot reload" (press "r" in the console where you ran "flutter run",
      // or simply save your changes to "hot reload" in a Flutter IDE).
      // Notice that the counter didn't reset back to zero; the application
      // is not restarted.
      primarySwatch: Colors.orange,
    ),
    home: MyHomePage(title: 'Flutter Demo Home Page'),
  );
}

改变当前的AppBar的主题

appBar: AppBar(
  // Here we take the value from the MyHomePage object that was created by
  // the App.build method, and use it to set our appbar title.
  title: Text(widget.title),
    toolbarOpacity:0.5
),

setstate 这个方法只能在state 和stateFullWidget里面使用