如何使用Flutter在网页视图中点击url后显示加载屏幕？

dependencies:
  flutter:
    sdk: flutter
  webview_flutter: ^2.0.0

import 'package:webview_flutter/webview_flutter.dart';

class MyWebView extends StatefulWidget {
  @override
  _MyWebViewState createState() => _MyWebViewState();
}

class _MyWebViewState extends State<MyWebView> {
  final Completer<WebViewController> _controller =
      Completer<WebViewController>();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Web View'),
      ),
      body: WebView(
        initialUrl: 'https://example.com',
        javascriptMode: JavascriptMode.unrestricted,
        onWebViewCreated: (WebViewController webViewController) {
          _controller.complete(webViewController);
        },
        navigationDelegate: (NavigationRequest request) {
          showDialog(
            context: context,
            builder: (BuildContext context) {
              return AlertDialog(
                title: Text('Loading...'),
                content: LinearProgressIndicator(),
              );
            },
          );
          return NavigationDecision.navigate;
        },
      ),
    );
  }
}

class HomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Flutter Web View'),
      ),
      body: Center(
        child: RaisedButton(
          child: Text('Open Web View'),
          onPressed: () {
            Navigator.push(
              context,
              MaterialPageRoute(builder: (context) => MyWebView()),
            );
          },
        ),
      ),
    );
  }
}