flutter context踩坑

下面代码有三个button，内容都是一样的，只不过包装方式不同。

第一个是没有包装的，第二个用Builder包了一层，第三个封装成了StatelessWidget。

运行一下，第一个button会报如下错误：

是说调用的Scaffod.of（context)找不到Scaffold。

import 'package:flutter/material.dart';

void main() => runApp(BuilderApp());

class BuilderApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: BuilderHomePage(),
    );
  }
}

class BuilderHomePage extends StatefulWidget {
  @override
  _BuilderHomePageState createState() => _BuilderHomePageState();
}

class _BuilderHomePageState extends State<BuilderHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Hello"),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            RaisedButton(
              onPressed: () {
                Scaffold.of(context).showSnackBar(
                    new SnackBar(content: Text("Hello  SnackBar1")));
              },
              child: Text("Test"),
            ),
            Builder(
              builder: (BuildContext context) {
                return RaisedButton(
                  onPressed: () {
                    Scaffold.of(context).showSnackBar(
                        new SnackBar(content: Text("Hello  SnackBar2")));
                  },
                  child: Text("Builder SnackBar"),
                );
              },
            ),
            WidgetTest()
          ],
        ),
      ),
    );
  }
}


class WidgetTest extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      child:     RaisedButton(
        onPressed: () {
          Scaffold.of(context).showSnackBar(
              new SnackBar(content: Text("Hello  SnackBar3")));
        },
        child: Text("No Builder SnackBar"),
      ),
    );
  }
}