Flutter:添加url后如何显示加载画面？

dependencies:
  flutter:
    sdk: flutter
  webview_flutter: ^2.0.0

import 'package:flutter/material.dart';
import 'package:webview_flutter/webview_flutter.dart';

class MyWebView extends StatefulWidget {
  final String url;

  MyWebView({required this.url});

  @override
  _MyWebViewState createState() => _MyWebViewState();
}

class _MyWebViewState extends State<MyWebView> {
  bool _isLoading = true;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('WebView'),
      ),
      body: Stack(
        children: [
          WebView(
            initialUrl: widget.url,
            onPageFinished: (String url) {
              setState(() {
                _isLoading = false;
              });
            },
          ),
          if (_isLoading)
            Center(
              child: CircularProgressIndicator(),
            ),
        ],
      ),
    );
  }
}

Navigator.push(
  context,
  MaterialPageRoute(
    builder: (context) => MyWebView(url: 'https://www.example.com'),
  ),
);