为什么在WebView上url改变后initState不能工作？

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> {
  WebView _webView;
  String _currentUrl;

  @override
  void initState() {
    super.initState();
    _webView = WebView(
      initialUrl: 'https://example.com',
      onPageFinished: (String url) {
        setState(() {
          _currentUrl = url;
        });
        // 在页面加载完成后执行相应操作
        // ...
      },
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('WebView'),
      ),
      body: _webView,
    );
  }
}