如何在Flutter中设置AlertDialog上的背景图像？

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('AlertDialog with Background Image'),
        ),
        body: Center(
          child: ElevatedButton(
            onPressed: () {
              showDialog(
                context: context,
                builder: (BuildContext context) {
                  return AlertDialog(
                    backgroundColor: Colors.transparent, // 设置背景透明
                    contentPadding: EdgeInsets.zero, // 去除默认的内边距
                    content: Container(
                      decoration: BoxDecoration(
                        image: DecorationImage(
                          image: AssetImage('assets/background_image.jpg'), // 设置背景图像
                          fit: BoxFit.cover,
                        ),
                      ),
                      child: Column(
                        mainAxisSize: MainAxisSize.min,
                        children: [
                          Text(
                            'This is an AlertDialog',
                            style: TextStyle(
                              color: Colors.white,
                              fontSize: 20,
                            ),
                          ),
                          ElevatedButton(
                            onPressed: () {
                              Navigator.of(context).pop();
                            },
                            child: Text('Close'),
                          ),
                        ],
                      ),
                    ),
                  );
                },
              );
            },
            child: Text('Show AlertDialog'),
          ),
        ),
      ),
    );
  }
}