The return type 'Transaction' isn't a 'Future<_>', as required by the closure's context

Question

Future<void> init() async {
    if (!_initialized) {
      _firebaseMessaging.requestNotificationPermissions();

      User user = FirebaseAuth.instance.currentUser;
      _uid = user.uid;

      _firebaseMessaging.onTokenRefresh.listen((event) {
        var db = FirebaseFirestore.instance;
        var ref = FirebaseFirestore.instance.doc("users/$_uid");
        db.runTransaction(
            (transaction) => transaction.update(ref, {constants.fcm: event}));
      });

I'm getting a return type error in this line:

db.runTransaction(
                (transaction) => transaction.update(ref, {constants.fcm: event}));

The return type 'Transaction' isn't a 'Future<_>', as required by the closure's context

Answer 1

You're not returning anything from the method explicit, so Dart tries to return the result of the last line of code ( db.runTransaction ), which returns a Transaction .

Since the method is marked to return Future<void> , which is not the same as a Transaction , you get the error you posted.

Why did you mark the method as returning a Future ? Does it really need to return anything? If so, what?