如何在flutter中导航回上一个屏幕后显示弹出窗口？

import 'package:flutter/material.dart';

class Screen1 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Screen 1'),
      ),
      body: Center(
        child: RaisedButton(
          child: Text('Go to Screen 2'),
          onPressed: () {
            Navigator.push(
              context,
              MaterialPageRoute(builder: (context) => Screen2()),
            );
          },
        ),
      ),
    );
  }
}

class Screen2 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Screen 2'),
      ),
      body: Center(
        child: RaisedButton(
          child: Text('Go back and show dialog'),
          onPressed: () {
            Navigator.pop(context); // 导航回上一个屏幕
            showDialog(
              context: context,
              builder: (BuildContext context) {
                return AlertDialog(
                  title: Text('Dialog'),
                  content: Text('This is a dialog.'),
                  actions: <Widget>[
                    FlatButton(
                      child: Text('Close'),
                      onPressed: () {
                        Navigator.of(context).pop(); // 关闭弹出窗口
                      },
                    ),
                  ],
                );
              },
            );
          },
        ),
      ),
    );
  }
}

void main() {
  runApp(MaterialApp(
    home: Screen1(),
  ));
}