How to open multiple webview pages using bottom navigation

```dart
import 'package:flutter/material.dart';
import 'package:flutter_webview_plugin/flutter_webview_plugin.dart';
import 'package:insight_bank/generated/i18n.dart';
import 'package:insight_bank/navigation/router.dart';
import 'package:insight_bank/util/layouts.dart';
import 'package:insight_bank/util/logger.dart';
import 'package:insight_bank/util/utils.dart';
import 'package:insight_bank/widgets/presentation/common/common_widgets.dart';

class WebViewPage extends StatefulWidget {
final String url;
final String title;
final bool hasAppBar;
final bool hasScrollBar;
final Widget bottomNavigationBar;

const WebViewPage(
this.url, {
this.title,
this.hasAppBar = true,
this.hasScrollBar = true,
this.bottomNavigationBar,
Key key,
}) : super(key: key);

@override
_WebViewPageState createState() => new _WebViewPageState();
}

class _WebViewPageState extends State {
final Logger _logger = Logger.tag('Webview');

String _title;
final _webViewPlugin = FlutterWebviewPlugin();

@override
void initState() {
super.initState();
_title = widget.title;
Logger.tagD('.... url: ${widget.url}');
_webViewPlugin.onStateChanged.listen((state) async {
if (state.type == WebViewState.startLoad) {
_logger.d('web page [${widget.url}] loading...');
}
if (state.type == WebViewState.finishLoad) {
String script = 'window.document.title';
final title = await _webViewPlugin.evalJavascript(script);
if (_title != title) {
_logger.d('web page title updated to $title');
setState(() {
_title = title;
});
}
}
});
_webViewPlugin.onDestroy.listen((_) {
if (Navigator.canPop(context)) {
Navigator.of(context).pop();
}
});
}

@override
Widget build(BuildContext context) {
_title = _title ?? S.of(context).hintLoading;

final appBar = widget.hasAppBar
? AppBar(
backgroundColor: L.scaffoldBackgroundColor,
title: TextWidget.title(_title),
leading: IconButton(
icon: Icon(Icons.arrow_back),
color: L.black,
onPressed: () => Router.pageBack(context),
),
)
: PreferHeightWidget(
height: L.s(L.statusBarHeightInPixel),
color: L.primaryColor,
);

return MaterialApp(
theme: Theme.of(context),
home: WebviewScaffold(
key: ObjectKey(widget.url),
appBar: appBar,
url: widget.url,
scrollBar: widget.hasScrollBar,
hidden: true,
withJavascript: true,
bottomNavigationBar: widget.bottomNavigationBar,
resizeToAvoidBottomInset: true,
initialChild: Container(
color: L.primaryColor,
child: Center(
child: CircularProgressIndicator(valueColor: AlwaysStoppedAnimation(L.white)),
),
),
),
);
}
}
```

I have a HomePage widget which contains a BottomNavigationBar, and there are 3 tabs in it.
The first page is a normal flutter page, and the 2nd and 3rd pages are webview pages with different urls. But when I tap on 2nd page at first, 2nd web page will be load and 3rd will NOT, vice versa.

The logs shows that the 3rd page's initState was called, but there is no actual page loading.

从 issue 中所示的 WebViewPage 和 HomePage 设置开始，然后检查 WebviewScaffold 如何处理 bottomNavigationBar 和多个 FlutterWebviewPlugin 实例。使用不同的 URL 重现三个标签页之间的切换，并验证选中时两个网页都能加载。

由索引模型根据 Issue 内容生成。