android - WillPopScope无法检测到已按下Android后退按钮

前提・我要实现的目标
我打算在按下Android后退按钮时使用WillPopScope禁用它。
问题・错误信息
将脚手架放在WillPopScope中并按Android后退按钮不会执行WillPopScope中设置的功能。
对应的源代码

Future<bool> onWillPopScope() async {
    print('on');
    return false;
}

Widget build(BuildContext context) {
 return ChangeNotifierProvider<xxxModel>(
  create:(_) => xxxModel(),
  child: Stack(
   children<Widget>[
    WillPopScope(
     onWillPop: onWillPopScope,
     child: Scaffold(
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     )
    ),
    Consumer<xxxModel>(
     ~~~~~~~~~~~~~~~~~~~~
    ),
   ],
  ),
 );
}

我尝试了什么
我更改了WillPopScope的位置，但是不起作用。
补充信息(固件/工具版本等)
・ flutter 1.17.5
・ Dart 2.8.4

最佳答案

如果您使用的是导航应用栏，请尝试以下过程

 Future<bool> _onBackPressed() {
        return showDialog(
        context: context,
        builder: (context) => new AlertDialog(
          title: new Text('Are you sure?'),
          content: new Text('Do you want to exit an App'),
          actions: <Widget>[
            new GestureDetector(
              onTap: () => Navigator.of(context).pop(false),
              child: Text("NO"),
            ),
            SizedBox(height: 16),
            new GestureDetector(
              onTap: () {
                exit(0);
              },
              child: Text("YES"),
            ),
          ],
        ),
      ) ??
      false;
    }

  Widget build(BuildContext context) {
   return Scaffold(
   key: _globalKey,
   body: WillPopScope(
      child: _getBody(_currentIndex),//your widget list index wise
      onWillPop: () async {
        if (_globalKey.currentState.isDrawerOpen) {
          Navigator.pop(context); // closes the drawer if opened
          return Future.value(false); // won't exit the app
        } else {
          //only back to o index
          if (_currentIndex == 0) _onBackPressed();
          //return true;
          setState(() {
            _currentIndex = 0;
          });
          return false;
          _onBackPressed();
          // return Future.value(true); // exits the app
        }
      },
    ),

  );
}