flutter_router_redirection.dart

import 'package:flutter/material.dart';
import 'package:go_router/go_router.dart';
import 'package:provider/provider.dart';

class LoginInfo extends ChangeNotifier {
  String get userName => _userName;
  String _userName = '';

  bool get loggedIn => _userName.isNotEmpty;

  void login(String userName) {
    _userName = userName;
    notifyListeners();
  }

  void logout() {
    _userName = '';
    notifyListeners();
  }
}

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

class App extends StatelessWidget {
  App({Key? key}) : super(key: key);

  final LoginInfo _loginInfo = LoginInfo();

  static const String title = 'Redirection';

  @override
  Widget build(BuildContext context) => ChangeNotifierProvider<LoginInfo>.value(
        value: _loginInfo,
        child: MaterialApp.router(
          routerConfig: _router,
          title: title,
          debugShowCheckedModeBanner: false,
        ),
      );

  late final GoRouter _router = GoRouter(
    routes: <GoRoute>[
      GoRoute(
        path: '/',
        builder: (BuildContext context, GoRouterState state) =>
            const HomeScreen(),
      ),
      GoRoute(
        path: '/login',
        builder: (BuildContext context, GoRouterState state) =>
            const LoginScreen(),
      ),
    ],

    // redirect to the login page if the user is not logged in
    redirect: (BuildContext context, GoRouterState state) {
      // if the user is not logged in, they need to login
      final bool loggedIn = _loginInfo.loggedIn;
      final bool loggingIn = state.subloc == '/login';
      if (!loggedIn) {
        return '/login';
      }

      // if the user is logged in but still on the login page, send them to
      // the home page
      if (loggingIn) {
        return '/';
      }

      // no need to redirect at all
      return null;
    },

    // changes on the listenable will cause the router to refresh it's route
    refreshListenable: _loginInfo,
  );
}

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

  @override
  Widget build(BuildContext context) => Scaffold(
        appBar: AppBar(
            backgroundColor: const Color(0xFFFF6200),
            title: const Text(App.title)),
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              ElevatedButton(
                onPressed: () {
                  // log a user in, letting all the listeners know
                  context.read<LoginInfo>().login('test-user');

                  // router will automatically redirect from /login to / using
                  // refreshListenable
                },
                child: const Text('Login'),
              ),
            ],
          ),
        ),
      );
}

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

  @override
  Widget build(BuildContext context) {
    final LoginInfo info = context.read<LoginInfo>();

    return Scaffold(
      appBar: AppBar(
        backgroundColor: const Color(0xFFFF6200),
        title: const Text(App.title),
        actions: <Widget>[
          IconButton(
            onPressed: info.logout,
            tooltip: 'Logout: ${info.userName}',
            icon: const Icon(Icons.logout),
          )
        ],
      ),
      body: const Center(
        child: Text('HomeScreen'),
      ),
    );
  }
}