Predictive back support for root routes (#120385)
This PR aims to support Android's predictive back gesture when popping the entire Flutter app. Predictive route transitions between routes inside of a Flutter app will come later. <img width="200" src="https://user-images.githubusercontent.com/389558/217918109-945febaa-9086-41cc-a476-1a189c7831d8.gif" /> ### Trying it out If you want to try this feature yourself, here are the necessary steps: 1. Run Android 33 or above. 1. Enable the feature flag for predictive back on the device under "Developer options". 1. Create a Flutter project, or clone [my example project](https://github.com/justinmc/flutter_predictive_back_examples). 1. Set `android:enableOnBackInvokedCallback="true"` in android/app/src/main/AndroidManifest.xml (already done in the example project). 1. Check out this branch. 1. Run the app. Perform a back gesture (swipe from the left side of the screen). You should see the predictive back animation like in the animation above and be able to commit or cancel it. ### go_router support go_router works with predictive back out of the box because it uses a Navigator internally that dispatches NavigationNotifications! ~~go_router can be supported by adding a listener to the router and updating SystemNavigator.setFrameworkHandlesBack.~~ Similar to with nested Navigators, nested go_routers is supported by using a PopScope widget. <details> <summary>Full example of nested go_routers</summary> ```dart // Copyright 2014 The Flutter Authors. All rights reserved. // Use of this source code is governed by a BSD-style license that can be // found in the LICENSE file. import 'package:go_router/go_router.dart'; import 'package:flutter/material.dart'; import 'package:flutter/scheduler.dart'; void main() => runApp(_MyApp()); class _MyApp extends StatelessWidget { final GoRouter router = GoRouter( routes: <RouteBase>[ GoRoute( path: '/', builder: (BuildContext context, GoRouterState state) => _HomePage(), ), GoRoute( path: '/nested_navigators', builder: (BuildContext context, GoRouterState state) => _NestedGoRoutersPage(), ), ], ); @override Widget build(BuildContext context) { return MaterialApp.router( routerConfig: router, ); } } class _HomePage extends StatelessWidget { @override Widget build(BuildContext context) { return Scaffold( appBar: AppBar( title: const Text('Nested Navigators Example'), ), body: Center( child: Column( mainAxisAlignment: MainAxisAlignment.center, children: <Widget>[ const Text('Home Page'), const Text('A system back gesture here will exit the app.'), const SizedBox(height: 20.0), ListTile( title: const Text('Nested go_router route'), subtitle: const Text('This route has another go_router in addition to the one used with MaterialApp above.'), onTap: () { context.push('/nested_navigators'); }, ), ], ), ), ); } } class _NestedGoRoutersPage extends StatefulWidget { @override State<_NestedGoRoutersPage> createState() => _NestedGoRoutersPageState(); } class _NestedGoRoutersPageState extends State<_NestedGoRoutersPage> { late final GoRouter _router; final GlobalKey<NavigatorState> _nestedNavigatorKey = GlobalKey<NavigatorState>(); // If the nested navigator has routes that can be popped, then we want to // block the root navigator from handling the pop so that the nested navigator // can handle it instead. bool get _popEnabled { // canPop will throw an error if called before build. Is this the best way // to avoid that? return _nestedNavigatorKey.currentState == null ? true : !_router.canPop(); } void _onRouterChanged() { // Here the _router reports the location correctly, but canPop is still out // of date. Hence the post frame callback. SchedulerBinding.instance.addPostFrameCallback((Duration duration) { setState(() {}); }); } @override void initState() { super.initState(); final BuildContext rootContext = context; _router = GoRouter( navigatorKey: _nestedNavigatorKey, routes: [ GoRoute( path: '/', builder: (BuildContext context, GoRouterState state) => _LinksPage( title: 'Nested once - home route', backgroundColor: Colors.indigo, onBack: () { rootContext.pop(); }, buttons: <Widget>[ TextButton( onPressed: () { context.push('/two'); }, child: const Text('Go to another route in this nested Navigator'), ), ], ), ), GoRoute( path: '/two', builder: (BuildContext context, GoRouterState state) => _LinksPage( backgroundColor: Colors.indigo.withBlue(255), title: 'Nested once - page two', ), ), ], ); _router.addListener(_onRouterChanged); } @override void dispose() { _router.removeListener(_onRouterChanged); super.dispose(); } @override Widget build(BuildContext context) { return PopScope( popEnabled: _popEnabled, onPopped: (bool success) { if (success) { return; } _router.pop(); }, child: Router<Object>.withConfig( restorationScopeId: 'router-2', config: _router, ), ); } } class _LinksPage extends StatelessWidget { const _LinksPage ({ required this.backgroundColor, this.buttons = const <Widget>[], this.onBack, required this.title, }); final Color backgroundColor; final List<Widget> buttons; final VoidCallback? onBack; final String title; @override Widget build(BuildContext context) { return Scaffold( backgroundColor: backgroundColor, body: Center( child: Column( mainAxisAlignment: MainAxisAlignment.center, children: <Widget>[ Text(title), //const Text('A system back here will go back to Nested Navigators Page One'), ...buttons, TextButton( onPressed: onBack ?? () { context.pop(); }, child: const Text('Go back'), ), ], ), ), ); } } ``` </details> ### Resources Fixes https://github.com/flutter/flutter/issues/109513 Depends on engine PR https://github.com/flutter/engine/pull/39208✔ Design doc: https://docs.google.com/document/d/1BGCWy1_LRrXEB6qeqTAKlk-U2CZlKJ5xI97g45U7azk/edit# Migration guide: https://github.com/flutter/website/pull/8952
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