How do I logout a user from Firebase when closing the app in Flutter

Question

I have an app that can't persist the user state of firebase auth for security reasons but I can't find a way to logout the user when the app is closed or force closed.

I'm using a listener to navigate the user to next page but, with this, when I open the app it automatically navigate to the logged page.

 Future<StreamSubscription<User?>> listenUserChanges(
      GlobalKey<NavigatorState> navigatorKey) async {
    final sub = auth.userChanges().listen((event) async {
      if (event != null) {
        final token = await getUserId();
        await storage.setToken(token);
        navigatorKey.currentState!.pushReplacementNamed('/logged');
      } else {
        await storage.removeToken();
      }
    });
    return sub;
  }

Solution

Use WidgetsBindingObserver to do your stuff. Wrap the MaterialApp with AppLifeCycleManager()

class AppLifeCycleManager extends StatefulWidget {
  final Widget child;
  final MyUserRepository myUserRepo;

  const AppLifeCycleManager(
      {Key? key, required this.child, required this.myUserRepo})
      : super(key: key);

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

class _AppLifeCycleManagerState extends State<AppLifeCycleManager>
    with WidgetsBindingObserver {
  @override
  void initState() {
    super.initState();

    widget.myUserRepo.setOnlineCustomer();
    WidgetsBinding.instance!.addObserver(this);

  }

  @override
  Future<void> didChangeAppLifecycleState(AppLifecycleState state) async {
    super.didChangeAppLifecycleState(state);

    switch (state) {
      case AppLifecycleState.paused:
        widget.myUserRepo.setInactiveCustomer();
        break;
      case AppLifecycleState.resumed:
        widget.myUserRepo.setOnlineCustomer();
        break;
      case AppLifecycleState.inactive:
        widget.myUserRepo.setInactiveCustomer();
        break;
      case AppLifecycleState.detached:
        widget.myUserRepo.setInactiveCustomer();
        break;
    }
  }

  @override
  Widget build(BuildContext context) {
    return widget.child;
  }

  @override
  void dispose() {
    widget.myUserRepo.setInactiveCustomer();
    WidgetsBinding.instance!.removeObserver(this);
    super.dispose();
  }
}