Flutter WebView exception on device back button pressed

Question

I'm pretty new to Flutter so please forgive me if this is a basic question.

I'm trying to create a Flutter app that shows only a full screen webview but I need the device back button to go back in history. The first task I achieved with ease but the second is being a struggle.

I followed the instructions in this answer, but I couldn't remove all the clutter of a 360+ lines answer just to a back button and make it work.

My code is really simple:

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

void main() => runApp(MyApp());

WebViewController _controller;

Future<void> _onWillPop(BuildContext context) async {
  if (await _controller.canGoBack()) {
    _controller.goBack();
  } else {
    exit(0);
    return Future.value(false);
  }
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    SystemChrome.setEnabledSystemUIOverlays([]);
    return WillPopScope(
      onWillPop: () => _onWillPop(context),
      child: MaterialApp(title: 'Test App', home: WebView(
      initialUrl: 'https://google.com',
      javascriptMode: JavascriptMode.unrestricted,
      onWebViewCreated: (WebViewController c) => _controller = c,
    )),
    );
  }
}

But when I try to press the device back button, it throws the following error:

java.lang.RuntimeException: Cannot execute operation because FlutterJNI is not attached to native.
    at io.flutter.embedding.engine.FlutterJNI.ensureAttachedToNative(FlutterJNI.java:227)
    at io.flutter.embedding.engine.FlutterJNI.markTextureFrameAvailable(FlutterJNI.java:554)
    at io.flutter.embedding.engine.renderer.FlutterRenderer.markTextureFrameAvailable(FlutterRenderer.java:274)
    at io.flutter.embedding.engine.renderer.FlutterRenderer.access$300(FlutterRenderer.java:38)
    at io.flutter.embedding.engine.renderer.FlutterRenderer$SurfaceTextureRegistryEntry$1.onFrameAvailable(FlutterRenderer.java:145)
    at android.graphics.SurfaceTexture$1.handleMessage(SurfaceTexture.java:211)
    at android.os.Handler.dispatchMessage(Handler.java:107)
    at android.os.Looper.loop(Looper.java:224)
    at android.app.ActivityThread.main(ActivityThread.java:7520)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:539)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950)

There are at least 3 threads on flutter's github but none of the solutions worked for me. what am I missing here?

Answer 1

Here is the working solution (100% working)-

  // Use any webview plugin
  // WebView _controller;

  Future<void> _handleBack(context) async {
    var status = await _controller.canGoBack();
    if (status) {
      _controller.goBack();
    } else {
      showDialog(
          context: context,
          builder: (context) => AlertDialog(
                title: Text('Do you want to exit'),
                actions: <Widget>[
                  FlatButton(
                    onPressed: () {
                      Navigator.of(context).pop();
                    },
                    child: Text('No'),
                  ),
                  FlatButton(
                    onPressed: () {
                      SystemNavigator.pop();
                    },
                    child: Text('Yes'),
                  ),
                ],
              ));
    }
  }

Here is calling WillPopScope

  @override
  Widget build(BuildContext context) {
    return WillPopScope(
            onWillPop: () => _handleBack(context),
            child: SafeArea(
              top: true,
              child: Scaffold(......)))

Answer 2

In Android MainActivity.kt, just add this:

override fun onDestroy() { flutterEngine?.platformViewsController?.onFlutterViewDestroyed() super.onDestroy() }

Answer 3

You can copy paste run full code below
You can use Completer
code snippet

WebViewController controller;

final Completer<WebViewController> _controllerCompleter =
          Completer<WebViewController>();
...
onWebViewCreated: (WebViewController c) {
                _controllerCompleter.future.then((value) => controller = value);
                _controllerCompleter.complete(c);
              },

working demo

full code

import 'dart:io';
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:webview_flutter/webview_flutter.dart';
import 'dart:async';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  WebViewController controller;

  final Completer<WebViewController> _controllerCompleter =
      Completer<WebViewController>();

  Future<void> _onWillPop(BuildContext context) async {
    print("onwillpop");
    if (await controller.canGoBack()) {
      controller.goBack();
    } else {
      exit(0);
      return Future.value(false);
    }
  }

  @override
  Widget build(BuildContext context) {
    return WillPopScope(
      onWillPop: () => _onWillPop(context),
      child: Scaffold(
        body: WebView(
          initialUrl: 'https://flutter.dev/',
          javascriptMode: JavascriptMode.unrestricted,
          onWebViewCreated: (WebViewController c) {
            _controllerCompleter.future.then((value) => controller = value);
            _controllerCompleter.complete(c);
          },
        ),
      ),
    );
  }
}