错误代码

// ignore: must_be_immutable
class CustomButton extends StatelessWidget {
  CustomButton({this.label});

  String label;

  @override
  Widget build(BuildContext context) {
    return new RaisedButton(onPressed: () {},
        child: new Text(label, textDirection: TextDirection.ltr,));
  }
}



class MyButton extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new Scaffold(appBar: new AppBar(title: new Text("demo"),),
        body: new CustomButton(label: "hello"));
  }
}

void main() {
  runApp(new MyButton());
}

日志

09-19 11:29:07.530 13216-13272/com.flutter.flutter_app I/flutter: The following assertion was thrown building MyButton:
09-19 11:29:07.530 13216-13272/com.flutter.flutter_app I/flutter: MediaQuery.of() called with a context that does not contain a MediaQuery.
09-19 11:29:07.530 13216-13272/com.flutter.flutter_app I/flutter: No MediaQuery ancestor could be found starting from the context that was passed to MediaQuery.of().
09-19 11:29:07.530 13216-13272/com.flutter.flutter_app I/flutter: This can happen because you do not have a WidgetsApp or MaterialApp widget (those widgets introduce
09-19 11:29:07.530 13216-13272/com.flutter.flutter_app I/flutter: a MediaQuery), or it can happen if the context you use comes from a widget above those widgets.

分析

因为这里我们使用了RaisedButton是一个Material的组件 所以用Scaffold包裹，包裹后直接返回给MyButton，这里就会报错，日志中说到

This can happen because you do not have a WidgetsApp or MaterialApp widget

说明这个Scaffold必须使用MaterialApp进行包裹，所以修改代码

正确代码

// ignore: must_be_immutable
class CustomButton extends StatelessWidget {
  CustomButton({this.label});

  String label;

  @override
  Widget build(BuildContext context) {
    return new RaisedButton(onPressed: () {},
        child: new Text(label, textDirection: TextDirection.ltr,));
  }
}


//因为RaisedButton是一个Material的组件  所以用Scaffold包裹 ，但是最外层必须返回一个MaterialApp
class MyButton extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(home: new Scaffold(appBar: new AppBar(title: new Text("demo"),),
        body: new CustomButton(label: "hello")));
  }
}

void main() {
  runApp(new MyButton());
}