如何在flutter中使用Image.network()放置图像占位符？

dependencies:
  flutter:
    sdk: flutter
  flutter/material.dart

import 'package:flutter/material.dart';

class MyImageWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Image.network(
      'https://example.com/image.jpg',
      loadingBuilder: (BuildContext context, Widget child, ImageChunkEvent loadingProgress) {
        if (loadingProgress == null) {
          return child;
        }
        return Center(
          child: CircularProgressIndicator(
            value: loadingProgress.expectedTotalBytes != null
                ? loadingProgress.cumulativeBytesLoaded / loadingProgress.expectedTotalBytes
                : null,
          ),
        );
      },
    );
  }
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('My App'),
        ),
        body: Center(
          child: MyImageWidget(),
        ),
      ),
    );
  }
}