创建一个嵌套导航

Apps accumulate dozens and then hundreds of routes over time. Some of your routes make sense as top-level (global) routes. For example, “/”, “profile”, “contact”, “social_feed” are all possible top-level routes within your app. But, imagine that you defined every possible route in your top-level Navigator widget. The list would be very long, and many of these routes would be better handled nested within another widget.

Consider an Internet of Things (IoT) setup flow for a wireless light bulb that you control with your app. This setup flow consists of 4 pages: find nearby bulbs, select the bulb that you want to add, add the bulb, and then complete the setup. You could orchestrate this behavior from your top-level Navigator widget. However, it makes more sense to define a second, nested Navigator widget within your SetupFlow widget, and let the nested Navigator take ownership over the 4 pages in the setup flow. This delegation of navigation facilitates greater local control, which is generally preferable when developing software.

The following animation shows the app’s behavior:

Gif showing the nested "setup" flow

In this recipe, you implement a four-page IoT setup flow that maintains its own navigation nested beneath the top-level Navigator widget.

Prepare for navigation

This IoT app has two top-level screens, along with the setup flow. Define these route names as constants so that they can be referenced within code.

const routeHome = '/';
const routeSettings = '/settings';
const routePrefixDeviceSetup = '/setup/';
const routeDeviceSetupStart = '/setup/$routeDeviceSetupStartPage';
const routeDeviceSetupStartPage = 'find_devices';
const routeDeviceSetupSelectDevicePage = 'select_device';
const routeDeviceSetupConnectingPage = 'connecting';
const routeDeviceSetupFinishedPage = 'finished';

The home and settings screens are referenced with static names. The setup flow pages, however, use two paths to create their route names: a /setup/ prefix followed by the name of the specific page. By combining the two paths, your Navigator can determine that a route name is intended for the setup flow without recognizing all the individual pages associated with the setup flow.

The top-level Navigator isn’t responsible for identifying individual setup flow pages. Therefore, your top-level Navigator needs to parse the incoming route name to identify the setup flow prefix. Needing to parse the route name means that you can’t use the routes property of your top-level Navigator. Instead, you must provide a function for the onGenerateRoute property.

Implement onGenerateRoute to return the appropriate widget for each of the three top-level paths.

onGenerateRoute: (settings) {
 late Widget page;
 if (settings.name == routeHome) {
   page = HomeScreen();
 } else if (settings.name == routeSettings) {
   page = SettingsScreen();
 } else if (settings.name!.startsWith(routePrefixDeviceSetup)) {
   final subRoute = settings.name!.substring(
     routePrefixDeviceSetup.length,
   );
   page = SetupFlow(
     setupPageRoute: subRoute,
   );
 } else {
   throw Exception('Unknown route: ${settings.name}');
 }

 return MaterialPageRoute<dynamic>(
   builder: (context) {
     return page;
   },
   settings: settings,
 );
},

Notice that the home and settings routes are matched with exact route names. However, the setup flow route condition only checks for a prefix. If the route name contains the setup flow prefix, then the rest of the route name is ignored and passed on to the SetupFlow widget to process. This splitting of the route name is what allows the top-level Navigator to be agnostic toward the various subroutes within the setup flow.

Create a stateful widget called SetupFlow that accepts a route name.

class SetupFlow extends StatefulWidget {
 const SetupFlow({
   Key? key,
   required this.setupPageRoute,
 }) : super(key: key);

 final String setupPageRoute;

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

class SetupFlowState extends State<SetupFlow> {
  //...
}

Display an app bar for the setup flow

The setup flow displays a persistent app bar that appears across all pages.

Return a Scaffold widget from your SetupFlow widget’s build() method, and include the desired AppBar widget.

@override
Widget build(BuildContext context) {
 return Scaffold(
   appBar: _buildFlowAppBar(),
   body: SizedBox(),
 );
}

PreferredSizeWidget _buildFlowAppBar() {
 return AppBar(
   title: Text('Bulb Setup'),
 );
}

The app bar displays a back arrow and exits the setup flow when the back arrow is pressed. However, exiting the flow causes the user to lose all progress. Therefore, the user is prompted to confirm whether they want to exit the setup flow.

Prompt the user to confirm exiting the setup flow, and ensure that the prompt appears when the user presses the hardware back button on Android.

Future<void> _onExitPressed() async {
 final isConfirmed = await _isExitDesired();

 if (isConfirmed && mounted) {
   _exitSetup();
 }
}

Future<bool> _isExitDesired() async {
 return await showDialog<bool>(
         context: context,
         builder: (context) {
           return AlertDialog(
             title: Text('Are you sure?'),
             content: Text('If you exit device setup, your progress will be lost.'),
             actions: [
               TextButton(
                 onPressed: () {
                   Navigator.of(context).pop(true);
                 },
                 child: Text('Leave'),
               ),
               TextButton(
                 onPressed: () {
                   Navigator.of(context).pop(false);
                 },
                 child: Text('Stay'),
               ),
             ],
           );
         }) ??
     false;
}

void _exitSetup() {
 Navigator.of(context).pop();
}

@override
Widget build(BuildContext context) {
 return WillPopScope(
   onWillPop: _isExitDesired,
   child: Scaffold(
     appBar: _buildFlowAppBar(),
     body: SizedBox(),
   ),
 );
}

PreferredSizeWidget _buildFlowAppBar() {
 return AppBar(
   leading: IconButton(
     onPressed: _onExitPressed,
     icon: Icon(Icons.chevron_left),
   ),
   title: Text('Bulb Setup'),
 );
}

When the user taps the back arrow in the app bar, or presses the back button on Android, an alert dialog pops up to confirm that the user wants to leave the setup flow. If the user presses Leave, then the setup flow pops itself from the top-level navigation stack. If the user presses Stay, then the action is ignored.

You might notice that the Navigator.pop() is invoked by both the Leave and Stay buttons. To be clear, this pop() action pops the alert dialog off the navigation stack, not the setup flow.

Generate nested routes

The setup flow’s job is to display the appropriate page within the flow.

Add a Navigator widget to SetupFlow, and implement the onGenerateRoute property.

final _navigatorKey = GlobalKey<NavigatorState>();

void _onDiscoveryComplete() {
 _navigatorKey.currentState!.pushNamed(
   routeDeviceSetupSelectDevicePage,
 );
}

void _onDeviceSelected(String deviceId) {
 _navigatorKey.currentState!.pushNamed(
   routeDeviceSetupConnectingPage,
 );
}

void _onConnectionEstablished() {
 _navigatorKey.currentState!.pushNamed(
   routeDeviceSetupFinishedPage,
 );
}

@override
Widget build(BuildContext context) {
 return WillPopScope(
   onWillPop: _isExitDesired,
   child: Scaffold(
     appBar: _buildFlowAppBar(),
     body: Navigator(
       key: _navigatorKey,
       initialRoute: widget.setupPageRoute,
       onGenerateRoute: _onGenerateRoute,
     ),
   ),
 );
}

Route _onGenerateRoute(RouteSettings settings) {
 late Widget page;
 switch (settings.name) {
   case routeDeviceSetupStartPage:
     page = WaitingPage(
       message: 'Searching for nearby bulb...',
       onWaitComplete: _onDiscoveryComplete,
     );
     break;
   case routeDeviceSetupSelectDevicePage:
     page = SelectDevicePage(
       onDeviceSelected: _onDeviceSelected,
     );
     break;
   case routeDeviceSetupConnectingPage:
     page = WaitingPage(
       message: 'Connecting...',
       onWaitComplete: _onConnectionEstablished,
     );
     break;
   case routeDeviceSetupFinishedPage:
     page = FinishedPage(
       onFinishPressed: _exitSetup,
     );
     break;
 }

 return MaterialPageRoute<dynamic>(
   builder: (context) {
     return page;
   },
   settings: settings,
 );
}

The _onGenerateRoute function works the same as for a top-level Navigator. A RouteSettings object is passed into the function, which includes the route’s name. Based on that route name, one of four flow pages is returned.

The first page, called find_devices, waits a few seconds to simulate network scanning. After the wait period, the page invokes its callback. In this case, that callback is _onDiscoveryComplete. The setup flow recognizes that, when device discovery is complete, the device selection page should be shown. Therefore, in _onDiscoveryComplete, the _navigatorKey instructs the nested Navigator to navigate to the select_device page.

The select_device page asks the user to select a device from a list of available devices. In this recipe, only one device is presented to the user. When the user taps a device, the onDeviceSelected callback is invoked. The setup flow recognizes that, when a device is selected, the connecting page should be shown. Therefore, in _onDeviceSelected, the _navigatorKey instructs the nested Navigator to navigate to the ”connecting” page.

The connecting page works the same way as the find_devices page. The connecting page waits for a few seconds and then invokes its callback. In this case, the callback is _onConnectionEstablished. The setup flow recognizes that, when a connection is established, the final page should be shown. Therefore, in _onConnectionEstablished, the _navigatorKey instructs the nested Navigator to navigate to the finished page.

The finished page provides the user with a Finish button. When the user taps Finish, the _exitSetup callback is invoked, which pops the entire setup flow off the top-level Navigator stack, taking the user back to the home screen.

Congratulations! You implemented nested navigation with four subroutes.

Interactive example

Run the app:

  • On the Add your first bulb screen, click the FAB, shown with a plus sign, +. This brings you to the Select a nearby device screen. A single bulb is listed.
  • Click the listed bulb. A Finished! screen appears.
  • Click the Finished button to return to the first screen.
// Copyright 2020, the Flutter project authors. Please see the AUTHORS file
// for details. 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:flutter/material.dart';

const routeHome = '/';
const routeSettings = '/settings';
const routePrefixDeviceSetup = '/setup/';
const routeDeviceSetupStart = '/setup/$routeDeviceSetupStartPage';
const routeDeviceSetupStartPage = 'find_devices';
const routeDeviceSetupSelectDevicePage = 'select_device';
const routeDeviceSetupConnectingPage = 'connecting';
const routeDeviceSetupFinishedPage = 'finished';

void main() {
  runApp(
    MaterialApp(
      theme: ThemeData(
        brightness: Brightness.dark,
        appBarTheme: const AppBarTheme(
          backgroundColor: Colors.blue,
        ),
        floatingActionButtonTheme: const FloatingActionButtonThemeData(
          backgroundColor: Colors.blue,
        ),
      ),
      onGenerateRoute: (settings) {
        late Widget page;
        if (settings.name == routeHome) {
          page = const HomeScreen();
        } else if (settings.name == routeSettings) {
          page = const SettingsScreen();
        } else if (settings.name!.startsWith(routePrefixDeviceSetup)) {
          final subRoute =
              settings.name!.substring(routePrefixDeviceSetup.length);
          page = SetupFlow(
            setupPageRoute: subRoute,
          );
        } else {
          throw Exception('Unknown route: ${settings.name}');
        }

        return MaterialPageRoute<dynamic>(
          builder: (context) {
            return page;
          },
          settings: settings,
        );
      },
      debugShowCheckedModeBanner: false,
    ),
  );
}

@immutable
class SetupFlow extends StatefulWidget {
  static SetupFlowState of(BuildContext context) {
    return context.findAncestorStateOfType<SetupFlowState>()!;
  }

  const SetupFlow({
    Key? key,
    required this.setupPageRoute,
  }) : super(key: key);

  final String setupPageRoute;

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

class SetupFlowState extends State<SetupFlow> {
  final _navigatorKey = GlobalKey<NavigatorState>();

  @override
  void initState() {
    super.initState();
  }

  void _onDiscoveryComplete() {
    _navigatorKey.currentState!.pushNamed(routeDeviceSetupSelectDevicePage);
  }

  void _onDeviceSelected(String deviceId) {
    _navigatorKey.currentState!.pushNamed(routeDeviceSetupConnectingPage);
  }

  void _onConnectionEstablished() {
    _navigatorKey.currentState!.pushNamed(routeDeviceSetupFinishedPage);
  }

  Future<void> _onExitPressed() async {
    final isConfirmed = await _isExitDesired();

    if (isConfirmed && mounted) {
      _exitSetup();
    }
  }

  Future<bool> _isExitDesired() async {
    return await showDialog<bool>(
            context: context,
            builder: (context) {
              return AlertDialog(
                title: const Text('Are you sure?'),
                content: const Text(
                    'If you exit device setup, your progress will be lost.'),
                actions: [
                  TextButton(
                    onPressed: () {
                      Navigator.of(context).pop(true);
                    },
                    child: const Text('Leave'),
                  ),
                  TextButton(
                    onPressed: () {
                      Navigator.of(context).pop(false);
                    },
                    child: const Text('Stay'),
                  ),
                ],
              );
            }) ??
        false;
  }

  void _exitSetup() {
    Navigator.of(context).pop();
  }

  @override
  Widget build(BuildContext context) {
    return WillPopScope(
      onWillPop: _isExitDesired,
      child: Scaffold(
        appBar: _buildFlowAppBar(),
        body: Navigator(
          key: _navigatorKey,
          initialRoute: widget.setupPageRoute,
          onGenerateRoute: _onGenerateRoute,
        ),
      ),
    );
  }

  Route _onGenerateRoute(RouteSettings settings) {
    late Widget page;
    switch (settings.name) {
      case routeDeviceSetupStartPage:
        page = WaitingPage(
          message: 'Searching for nearby bulb...',
          onWaitComplete: _onDiscoveryComplete,
        );
        break;
      case routeDeviceSetupSelectDevicePage:
        page = SelectDevicePage(
          onDeviceSelected: _onDeviceSelected,
        );
        break;
      case routeDeviceSetupConnectingPage:
        page = WaitingPage(
          message: 'Connecting...',
          onWaitComplete: _onConnectionEstablished,
        );
        break;
      case routeDeviceSetupFinishedPage:
        page = FinishedPage(
          onFinishPressed: _exitSetup,
        );
        break;
    }

    return MaterialPageRoute<dynamic>(
      builder: (context) {
        return page;
      },
      settings: settings,
    );
  }

  PreferredSizeWidget _buildFlowAppBar() {
    return AppBar(
      leading: IconButton(
        onPressed: _onExitPressed,
        icon: const Icon(Icons.chevron_left),
      ),
      title: const Text('Bulb Setup'),
    );
  }
}

class SelectDevicePage extends StatelessWidget {
  const SelectDevicePage({
    Key? key,
    required this.onDeviceSelected,
  }) : super(key: key);

  final void Function(String deviceId) onDeviceSelected;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Padding(
          padding: const EdgeInsets.symmetric(horizontal: 24.0),
          child: Column(
            mainAxisSize: MainAxisSize.min,
            children: [
              Text(
                'Select a nearby device:',
                style: Theme.of(context).textTheme.headline6,
              ),
              const SizedBox(height: 24),
              SizedBox(
                width: double.infinity,
                height: 54,
                child: ElevatedButton(
                  style: ButtonStyle(
                    backgroundColor: MaterialStateColor.resolveWith((states) {
                      return const Color(0xFF222222);
                    }),
                  ),
                  onPressed: () {
                    onDeviceSelected('22n483nk5834');
                  },
                  child: const Text(
                    'Bulb 22n483nk5834',
                    style: TextStyle(
                      fontSize: 24,
                    ),
                  ),
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }
}

class WaitingPage extends StatefulWidget {
  const WaitingPage({
    Key? key,
    required this.message,
    required this.onWaitComplete,
  }) : super(key: key);

  final String message;
  final VoidCallback onWaitComplete;

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

class _WaitingPageState extends State<WaitingPage> {
  @override
  void initState() {
    super.initState();
    _startWaiting();
  }

  Future<void> _startWaiting() async {
    await Future<dynamic>.delayed(const Duration(seconds: 3));

    if (mounted) {
      widget.onWaitComplete();
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Padding(
          padding: const EdgeInsets.symmetric(horizontal: 24.0),
          child: Column(
            mainAxisSize: MainAxisSize.min,
            children: [
              const CircularProgressIndicator(),
              const SizedBox(height: 32),
              Text(widget.message),
            ],
          ),
        ),
      ),
    );
  }
}

class FinishedPage extends StatelessWidget {
  const FinishedPage({
    Key? key,
    required this.onFinishPressed,
  }) : super(key: key);

  final VoidCallback onFinishPressed;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Padding(
          padding: const EdgeInsets.symmetric(horizontal: 24.0),
          child: Column(
            mainAxisSize: MainAxisSize.min,
            children: [
              Container(
                width: 250,
                height: 250,
                decoration: const BoxDecoration(
                  shape: BoxShape.circle,
                  color: Color(0xFF222222),
                ),
                child: const Center(
                  child: Icon(
                    Icons.lightbulb,
                    size: 175,
                    color: Colors.white,
                  ),
                ),
              ),
              const SizedBox(height: 32),
              const Text(
                'Bulb added!',
                textAlign: TextAlign.center,
                style: TextStyle(
                  fontSize: 24,
                  fontWeight: FontWeight.bold,
                ),
              ),
              const SizedBox(height: 32),
              ElevatedButton(
                style: ButtonStyle(
                  padding: MaterialStateProperty.resolveWith((states) {
                    return const EdgeInsets.symmetric(
                        horizontal: 24, vertical: 12);
                  }),
                  backgroundColor: MaterialStateColor.resolveWith((states) {
                    return const Color(0xFF222222);
                  }),
                  shape: MaterialStateProperty.resolveWith((states) {
                    return const StadiumBorder();
                  }),
                ),
                onPressed: onFinishPressed,
                child: const Text(
                  'Finish',
                  style: TextStyle(
                    fontSize: 24,
                  ),
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }
}

@immutable
class HomeScreen extends StatelessWidget {
  const HomeScreen({
    Key? key,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: _buildAppBar(context),
      body: Center(
        child: Padding(
          padding: const EdgeInsets.symmetric(horizontal: 24.0),
          child: Column(
            mainAxisSize: MainAxisSize.min,
            children: [
              Container(
                width: 250,
                height: 250,
                decoration: const BoxDecoration(
                  shape: BoxShape.circle,
                  color: Color(0xFF222222),
                ),
                child: Center(
                  child: Icon(
                    Icons.lightbulb,
                    size: 175,
                    color: Theme.of(context).scaffoldBackgroundColor,
                  ),
                ),
              ),
              const SizedBox(height: 32),
              const Text(
                'Add your first bulb',
                textAlign: TextAlign.center,
                style: TextStyle(
                  fontSize: 24,
                  fontWeight: FontWeight.bold,
                ),
              ),
            ],
          ),
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          Navigator.of(context).pushNamed(routeDeviceSetupStart);
        },
        child: const Icon(Icons.add),
      ),
    );
  }

  PreferredSizeWidget _buildAppBar(BuildContext context) {
    return AppBar(
      title: const Text('Welcome'),
      actions: [
        IconButton(
          icon: const Icon(Icons.settings),
          onPressed: () {
            Navigator.pushNamed(context, routeSettings);
          },
        ),
      ],
    );
  }
}

class SettingsScreen extends StatelessWidget {
  const SettingsScreen({
    Key? key,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: _buildAppBar(),
      body: SingleChildScrollView(
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: List.generate(8, (index) {
            return Container(
              width: double.infinity,
              height: 54,
              margin: const EdgeInsets.only(left: 16, right: 16, top: 16),
              decoration: BoxDecoration(
                borderRadius: BorderRadius.circular(8),
                color: const Color(0xFF222222),
              ),
            );
          }),
        ),
      ),
    );
  }

  PreferredSizeWidget _buildAppBar() {
    return AppBar(
      title: const Text('Settings'),
    );
  }
}