WebViewController? _webviewController;




WillPopScope(
  onWillPop: () => _exitApp(context),
  child: xxx,   
}



Future<bool> _exitApp(BuildContext context) async {
 if (await _webviewController!.canGoBack()) {
   print("onwill goback");
   _webviewController!.goBack();
   return Future.value(false);
 } else {
   debugPrint("_exit will not go back");
   return Future.value(true);
 }
}

import 'dart:async';

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

void main() => runApp(MaterialApp(home: WebViewExample()));

class WebViewExample extends StatefulWidget {
 @override
 _WebViewExampleState createState() => _WebViewExampleState();
}

class _WebViewExampleState extends State<WebViewExample> {

 WebViewController? _webviewController;

 Future<bool> _exitApp(BuildContext context) async {
   if (await _webviewController!.canGoBack()) {
     print("onwill goback");
     _webviewController!.goBack();
     return Future.value(false);
   } else {
     debugPrint("_exit will not go back");
     return Future.value(true);
   }
 }

 @override
 Widget build(BuildContext context) {
   return WillPopScope(
     onWillPop: () => _exitApp(context),
     child: Scaffold(
       appBar: AppBar(
         title: const Text('Flutter WebView example'),
         // This drop down menu demonstrates that Flutter widgets can be shown over the web view.
       ),
       // We're using a Builder here so we have a context that is below the Scaffold
       // to allow calling Scaffold.of(context) so we can show a snackbar.
       body: Builder(builder: (BuildContext context) {
         return WebView(
           initialUrl: 'http://droidyue.com',
           javascriptMode: JavascriptMode.unrestricted,
           onWebViewCreated: (WebViewController webViewController) {
             _webviewController = webViewController;
           },
         );
       }),
     ),
   );
 }
}