Flutter中如何使用WillPopScope的示例代码

WillPopScope(
  onWillPop: () async => showDialog(
    context: context,
    builder: (context) =>
      AlertDialog(title: Text('你确定要退出吗？'), actions: <Widget>[
       RaisedButton(
         child: Text('退出'),
         onPressed: () => Navigator.of(context).pop(true)),
       RaisedButton(
         child: Text('取消'),
         onPressed: () => Navigator.of(context).pop(false)),
      ])),
  child: Container(
   alignment: Alignment.center,
   child: Text('点击后退按钮，询问是否退出。'),
  ))

DateTime _lastQuitTime;
WillPopScope(
  onWillPop: () async {
   if (_lastQuitTime == null ||
     DateTime.now().difference(_lastQuitTime).inSeconds > 1) {
    print('再按一次 Back 按钮退出');
    Scaffold.of(context)
      .showSnackBar(SnackBar(content: Text('再按一次 Back 按钮退出')));
    _lastQuitTime = DateTime.now();
    return false;
   } else {
    print('退出');
    Navigator.of(context).pop(true);
    return true;
   }
  },
  child: Container(
   alignment: Alignment.center,
   child: Text('点击后退按钮，询问是否退出。'),
  ))

class MyHomePage extends StatefulWidget {
 MyHomePage({Key key, this.title}) : super(key: key);

 final String title;

 @override
 _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {

 GlobalKey<NavigatorState> _key = GlobalKey();

 @override
 Widget build(BuildContext context) {
  return Scaffold(
   body: WillPopScope(
     onWillPop: () async {
      if (_key.currentState.canPop()) {
       _key.currentState.pop();
       return false;
      }
      return true;
     },
     child: Column(
      children: <Widget>[
       Expanded(
        child: Navigator(
         key: _key,
         onGenerateRoute: (RouteSettings settings) =>
           MaterialPageRoute(builder: (context) {
          return OnePage();
         }),
        ),
       ),
       Container(
        height: 50,
        color: Colors.blue,
        alignment: Alignment.center,
        child: Text('底部Bar'),
       )
      ],
     )),
  );
 }
}

class OnePage extends StatelessWidget {
 @override
 Widget build(BuildContext context) {
  return Scaffold(
   body: Center(
    child: Container(
     child: RaisedButton(
      child: Text('去下一个页面'),
      onPressed: () {
       Navigator.push(context, MaterialPageRoute(builder: (context) {
        return TwoPage();
       }));
      },
     ),
    ),
   ),
  );
 }
}

class TwoPage extends StatelessWidget {
 @override
 Widget build(BuildContext context) {
  return Scaffold(
   body: Center(
    child: Container(
     child: Text('这是第二个页面'),
    ),
   ),
  );
 }
}