Keeping Flutter dart stream open to watch for changes to a model

Issue

I’m new to Flutter/Dart and trying to create a basic authentication service. My plain is to make my main application listen to a stream which will emit the user when authentication actions take place. This is how a lot of the Firebase examples work, the only difference is that I want to connect directly to my own API.

To do this I’ve created an AuthService, but can’t actually seem to get it working.

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Provider<AuthService>(
      create: (_) => AuthService(),
      child: MaterialApp(
        home: AuthWidget()
      )
    );
  }
}

class AuthWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {

    final authService = Provider.of<AuthService>(context);
    authService.getUser();

    // wrap the application in a Stream that watches for the authentication state
    // to change. On authentication state change,
    return StreamBuilder<User>(
        stream: authService.onAuthStateChanged,
        builder: (context, snapshot) {
          
          // check to ensure that the connectionState is active
          if (snapshot.connectionState == ConnectionState.active) {
            final user = snapshot.data;
            return user == null ? Login() : Home();
          }

          return Login();
        }
    );
  }
}


class AuthService {

  User currentUser;

  Future<User> getUser() {
    return Future.value(currentUser);
  }

  void login(User user) {
    currentUser = user;
  }

  void logout() {
    currentUser = null;
  }

  Stream<User> get onAuthStateChanged {
    return Stream.fromFuture(getUser());
  }
}

What I want to achieve

At the top level MyApp, I’d like to be able to listen for changes in authentication using the AuthProvider. I should be able to access the authentication provider anywhere, e.g.

final auth = Provider.of<AuthService>(context, listen: false);

// log the user in
auth.login(user);

// log the user out
auth.logout(user);

// get the current authenticated user
auth.getUser();

And use this to manage the application. The problem I am having with my existing code is that the Stream status is instantly marked as done, so no further changes are ever emitted from the onAuthStateChanged. I actually need my AuthWidget to continue listening throughout the lifetime of the application, so that I can respond to login/logout events and change the UI accordingly.

Any help is greatly appreciated!

Solution

Ok, I managed to come up with a solution in the end using slightly different techniques. Thanks to this article by Aaron K Saunders for the pointers – https://dev.to/aaronksaunders/simple-login-flow-in-flutter-then-firebase-part-one-29n6

Essentially my AuthService now implements a ChangeNotifier and I can use this around the app to check auth, show different routes, get current user etc.

Answered By – Chris

Answer Checked By – Gilberto Lyons (FlutterFixes Admin)