[riverpod] Auth async provider as Listener (for RoRouter.refreshListenable)

auth.dart

part 'auth.g.dart';

// The key to become Listenable, is to implement Listenable...
// And notify listener when state changed of course.
@Riverpod(keepAlive: true)
class Auth extends _$Auth implements Listenable {
  VoidCallback? _callback;
  
  @override
  void addListener(VoidCallback listener) {
    _callback = listener;
  }

  @override
  void removeListener(VoidCallback listener) {
    if (_callback == listener) {
      _callback = null;
    }
  }

  @override
  FutureOr<User?> build() async {
    // Listen to self so we can notify listener's callback.
    ref.listenSelf((_, __) {
      if (state.isLoading) return;
      _callback?.call();
    });

    // Listen to Firebase auth state.
    fb.FirebaseAuth.instance.authStateChanges().listen(_firebaseAuthListener);
    return null;
  }

  signIn(String email, String password) async {
    await fb.FirebaseAuth.instance.signInWithEmailAndPassword(email: email, password: password);
  }

  signOut() async {
    await fb.FirebaseAuth.instance.signOut();
  }

  void _firebaseAuthListener(fb.User? fbUser) async {
    update((_) async {
      // Convert Firebase user to domain user.
      return fbUser == null ? null : DomainUser(fbUser);
    });
  }
}

main.dart

viud main() {
  runApp(
    ProviderScope(
      child: App(),
    ),
  );
}

class App extends ConsumerWidget with ScreenMixin {
  const App({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context, WidgetRef ref) {
    return ref.watch(routerProvider()).when(
          data: (router) {
            return MaterialApp.router(
              routerConfig: router,
              // ...
            );
          },
          error: //...
          loading: //...
    );
  }
}

router.dart

part 'router.g.dart';

@riverpod
FutureOr<Raw<GoRouter>> router(RouterRef ref) async {
  final user = await ref.watch(authProvider.future);

  return GoRouter(
    refreshListenable: ref.watch(authProvider.notifier),
    routes: [
      //...
    ],
    redirect: (context, state) {
      final signedIn = user != null;
      // redirect according to signed-in state.
    }
  );
}