WillPopScope 返回拦截

当一个界面中有WillPopScope组件时，在页面返回时会触发回调，决定是否返回。可用于二次确认退出的场景。

WillPopScope使用

   <br />【child】 : 子组件   【Widget】<br />【onWillPop】 : 返回回调  【WillPopCallback】<br />![75.gif](https://cdn.nlark.com/yuque/0/2020/gif/326147/1589444794889-6813ea18-12b3-4f33-8299-6e979d6873b6.gif#align=left&display=inline&height=209&margin=%5Bobject%20Object%5D&name=75.gif&originHeight=209&originWidth=387&size=245477&status=done&style=none&width=387)

import 'package:flutter/material.dart';
class CustomWillPopScope extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      child: WillPopScope(child: (BackButton()),
          onWillPop: ()=>_willPop(context)),
    );
  }
  Future<bool> _willPop(context) async{
    return await showDialog(
      context: context,
      builder: (context) =>  AlertDialog(
        shape:  RoundedRectangleBorder(
            borderRadius: BorderRadius.all(Radius.circular(10))),
        title:  Text('提示'),
        content:  Text('你确定要离开此页吗?'),
        actions: <Widget>[
          FlatButton(
            onPressed: () => Navigator.of(context).pop(true),
            child:  Text('确定'),
          ),
           FlatButton(
            onPressed: () => Navigator.of(context).pop(false),
            child:  Text('取消'),
          ),
        ],
      ),
    ) ?? false;
  }
}