Use nested Navigator with WillPopScope in Flutter

Question

In my application, I want to have a custom Navigation (only change part of the screen and keep an history of what I'm doing inside it). For that purpose, I am using a Navigator and it's working fine for simple navigation. However, I want to handle the back button of Android. There is a problem with it in Flutter apparently which forces me to handle the backbutton in the parent widget of the Navigator : https://github.com/flutter/flutter/issues/14083

Due to that, I need to retrieve the instance of my Navigator in the children and call pop() on it. I am trying to use a GlobalKey for this.

I am trying to make it work for a while now and made a sample project just to test this. Here is my code :

import 'package:flutter/material.dart';

void main() {
  runApp(MaterialApp(title: 'Navigation Basics', home: MainWidget()));
}

class MainWidget extends StatelessWidget {
  final GlobalKey<NavigatorState> navigatorKey = GlobalKey();

  @override
  Widget build(BuildContext context) {
    return SafeArea(
        child: WillPopScope(
            onWillPop: () => navigatorKey.currentState.maybePop(),
            child: Scaffold(
                body: Padding(
              child: Column(
                children: <Widget>[
                  Text("Toto"),
                  Row(
                    mainAxisAlignment: MainAxisAlignment.spaceBetween,
                    children: <Widget>[
                      Expanded(
                          child: RaisedButton(
                        child: Text('First'),
                        onPressed: () {
                          navigatorKey.currentState.pushNamed('/first');
                          // Navigator.push(
                          //   context,
                          //   MaterialPageRoute(builder: (context) => SecondRoute()),
                          // );
                        },
                      )),
                      Expanded(
                          child: RaisedButton(
                        child: Text('Second'),
                        onPressed: () {
                          navigatorKey.currentState.pushNamed('/second');
                        },
                      ))
                    ],
                  ),
                  Expanded(
                      child: Stack(
                    children: <Widget>[
                      Container(
                        decoration: BoxDecoration(color: Colors.red),
                      ),
                      ConstrainedBox(
                          constraints: BoxConstraints.expand(),
                          child: _getNavigator()),
                    ],
                  )),
                ],
              ),
              padding: EdgeInsets.only(bottom: 50),
            ))));
  }

  Navigator _getNavigator() {
    return Navigator(
        key: navigatorKey,
        initialRoute: '/',
        onGenerateRoute: (RouteSettings settings) {
          WidgetBuilder builder;
          switch (settings.name) {
            case '/':
              builder = (BuildContext _) => FirstRoute();
              break;
            case '/second':
              builder = (BuildContext _) => SecondRoute();
              break;
            default:
              throw new Exception('Invalid route: ${settings.name}');
          }
          return new MaterialPageRoute(builder: builder, settings: settings);
        });
  }
}

class FirstRoute extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      child: Column(
        children: <Widget>[
          RaisedButton(
            child: Text("GO TO FRAGMENT TWO"),
            onPressed: () => Navigator.of(context).pushNamed("/second"),
          )
        ],
      ),
      decoration: BoxDecoration(color: Colors.green),
    );
  }
}

class SecondRoute extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      child: Column(
        children: <Widget>[
          RaisedButton(
            child: Text("GO TO FRAGMENT ONE"),
            onPressed: () => Navigator.of(context).pop(),
          )
        ],
      ),
      decoration: BoxDecoration(color: Colors.blue),
    );
  }
}

This is however not working as I would like. The default Navigator seem to still be used : after opening the SecondRoute and pressing the Android back button, it just leaves the app instead of just going back to the first route.

How can I achieve what I want?

Answer 1

Following the documentation of onWillPop:

  /// Called to veto attempts by the user to dismiss the enclosing [ModalRoute].
  ///
  /// If the callback returns a Future that resolves to false, the enclosing
  /// route will not be popped.
  final WillPopCallback onWillPop;

your handler should indicate that the enclosing route should not be closed, hence returning false will resolve your issue.

changing your handler to this works:

    onWillPop: () async {
      navigatorKey.currentState.maybePop();
      return false;
    },