How to deactivate or override the Android “BACK” button, in Flutter?

Question

Is there a way to deactivate the Android back button when on a specific page?

class WakeUpApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: "Time To Wake Up ?",
      home: new WakeUpHome(),
      routes: <String, WidgetBuilder>{
        '/pageOne': (BuildContext context) => new pageOne(),
        '/pageTwo': (BuildContext context) => new pageTwo(),
      },
    );
  }
}

On pageOne I have a button to go to pageTwo:

new FloatingActionButton(
  onPressed: () {    
    Navigator.of(context).pushNamed('/pageTwo');
  },
)

My problem is that if I press the Back arrow at the bottom of the android screen, I go back to pageOne. I would like this button to not show up at all. Ideally, I would like to have no possible way out of this screen unless the user for example keeps his finger pressed on the screen for 5 seconds. (I am trying to write an App for toddlers, and would like only the parents to be able to navigate out of the particular screen).

Answer 1

The answer is WillPopScope. It will prevent the page from being popped by the system. You'll still be able to use Navigator.of(context).pop()

@override
Widget build(BuildContext context) {
  return new WillPopScope(
    onWillPop: () async => false,
    child: new Scaffold(
      appBar: new AppBar(
        title: new Text("data"),
        leading: new IconButton(
          icon: new Icon(Icons.ac_unit),
          onPressed: () => Navigator.of(context).pop(),
        ),
      ),
    ),
  );
}

Answer 2

As Rémi Rousselet pointed out, WillPopScope is usually the way to go. However, if you are developing a stateful widget that should react to the back button directly, you may use this:

https://pub.dartlang.org/packages/back_button_interceptor

Note: I am the author of this package.

Answer 3

While Remi's answer is right, usually you don't want to simply block the back button but want a user to confirm the exit.

You can do it similar way by getting an answer from the confirmation dialog, because onWillPop is a future.

@override
Widget build(BuildContext context) {
  return WillPopScope(
    child: Scaffold(...),
    onWillPop: () => showDialog<bool>(
      context: context,
      builder: (c) => AlertDialog(
        title: Text('Warning'),
        content: Text('Do you really want to exit'),
        actions: [
          FlatButton(
            child: Text('Yes'),
            onPressed: () => Navigator.pop(c, true),
          ),
          FlatButton(
            child: Text('No'),
            onPressed: () => Navigator.pop(c, false),
          ),
        ],
      ),
    ),
  );
}

Answer 4

You can use Future.value(bool) to handle the back button.

bool _allow = true;

@override
Widget build(BuildContext context) {
  return WillPopScope(
    child: Scaffold(appBar: AppBar(title: Text("Back"))),
    onWillPop: () {
      return Future.value(_allow); // if true allow back else block it
    },
  );
}

Answer 5

I'm posting this here in case anyone out there finds this and wishes they would find a simple example https://gist.github.com/b-cancel/0ca372017a25f0c120b14dfca3591aa5

import 'package:flutter/material.dart';

import 'dart:async';

void main() => runApp(new BackButtonOverrideDemoWidget());

class BackButtonOverrideDemoWidget extends StatefulWidget{
  @override
  _BackButtonOverrideDemoWidgetState createState() => new _BackButtonOverrideDemoWidgetState();
}

class _BackButtonOverrideDemoWidgetState extends State<BackButtonOverrideDemoWidget> with WidgetsBindingObserver{

  //-------------------------Test Variable

  bool isBackButtonActivated = false;

  //-------------------------Required For WidgetsBindingObserver

  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addObserver(this);
  }

  @override
  void dispose() {
    WidgetsBinding.instance.removeObserver(this);
    super.dispose();
  }

  //-------------------------Function That Triggers when you hit the back key

  @override
  didPopRoute(){
    bool override;
    if(isBackButtonActivated)
      override = false;
    else
      override = true;
    return new Future<bool>.value(override);
  }

  //-------------------------Build Method

  @override
  Widget build(BuildContext context) {
    return new Directionality(
      textDirection: TextDirection.ltr,
      child: new Container(
          color: (isBackButtonActivated) ? Colors.green : Colors.red,
          child: new Center(
              child: new FlatButton(
                color: Colors.white,
                onPressed: () {
                  isBackButtonActivated = !isBackButtonActivated;
                  setState(() {});
                },
                child: (isBackButtonActivated) ?
                new Text("DeActive the Back Button") : new Text("Activate the Back Button"),
              )
          )
      ),
    );
  }
}

Answer 6

I used mixin and WillPopScope widget just couldn't get the job done for me. This is best approach I found, much better than WillPopScope in my opinion.
final bool canPop = ModalRoute.of(context)?.canPop ?? false;
Used it like this inside appbar:

leading: ModalRoute.of(context)?.canPop ?? false
    ? IconButton(
        onPressed: () {
          Navigator.pop(context);
        },
        icon: (Platform.isAndroid)
            ? const Icon(Icons.arrow_back)
            : const Icon(Icons.arrow_back_ios),
      )
    : Container(),

Answer 7

The answer maybe you knew that's use WillPopScope,but unfortunately on IOS you can't swipe to back the previous page, so let's custom your MaterialPageRoute:

class CustomMaterialPageRoute<T> extends MaterialPageRoute<T> {
  @protected
  bool get hasScopedWillPopCallback {
    return false;
  }
  CustomMaterialPageRoute({
    @required WidgetBuilder builder,
    RouteSettings settings,
    bool maintainState = true,
    bool fullscreenDialog = false,
  }) : super( 
          builder: builder,
          settings: settings,
          maintainState: maintainState,
          fullscreenDialog: fullscreenDialog,
        );
}

Now you can use WillPopScope and swipe to back will work on IOS. The details answer is here: https://github.com/flutter/flutter/issues/14203#issuecomment-540663717

Answer 8

Here's an alternative solution that works if you're coding with null safety. You need to disable the the default back button, and replace it with an IconButton. In this example, I'm pushing an AlertDialog when the user clicks the back button to confirm before exiting. You can replace this function and send the user to any other page

return WillPopScope(
  onWillPop: () async => false,
  child: Scaffold(
    appBar: AppBar(
      automaticallyImplyLeading: true,
      title: Text(),
      leading: IconButton(
        icon: Icon(Icons.arrow_back),
        onPressed: () => showDialog<bool>(
          context: context,
          builder: (c) => AlertDialog(
            title: Text('Warning'),
            content: Text('Are you sure you want to exit?'),
            ),
            actions: [
              TextButton(
                  child: Text('Yes'),
                  onPressed: () {
                    Navigator.pop(c, true);
                    Navigator.pop(context);
                  }),
              TextButton(
                child: Text('No'),
                onPressed: () => Navigator.pop(c, false),
              ),
            ],
          ),
        ),
      ),
    ),

Answer 9

Trying this will kill your app state

@override
  Widget build(BuildContext context) {
    return WillPopScope(
      ////////////////
      onWillPop: () => showDialog<bool>(
        context: context,
        builder: (c) => AlertDialog(
          title: Text(
            'Warning',
            textAlign: TextAlign.center,
          ),
          content: Text('Are you sure to exit?'),
          actions: [
            TextButton(
              style: TextButton.styleFrom(
                primary: Colors.green,
              ),
              onPressed: () async {
                exit(0);// kill app
              },
              child: Text('Yes'),
            ),
            TextButton(
              style: TextButton.styleFrom(
                primary: Colors.red,
              ),
              onPressed: () => Navigator.pop(c, false),
              child: Text('No'),
            )
          ],
        ),
      ),
      /////////////////////
      child: Scaffold(),
    );
  }