如何在flutter中为网页视图添加刷新指示器？

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

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Web View'),
      ),
      body: Stack(
        children: [
          WebView(
            initialUrl: 'https://example.com',
            onPageStarted: (String url) {
              setState(() {
                isLoading = true;
              });
            },
            onPageFinished: (String url) {
              setState(() {
                isLoading = false;
              });
            },
          ),
          if (isLoading)
            Center(
              child: CircularProgressIndicator(),
            ),
        ],
      ),
    );
  }
}

class _MyWebViewState extends State<MyWebView> {
  // ...

  Future<void> _refreshWebView() async {
    webView.reload();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Web View'),
      ),
      body: RefreshIndicator(
        onRefresh: _refreshWebView,
        child: Stack(
          children: [
            WebView(
              // ...
            ),
            if (isLoading)
              Center(
                child: CircularProgressIndicator(),
              ),
          ],
        ),
      ),
    );
  }
}