How can a method which accepts parameters be called without passing it's needed values? - flutter

Question

I found this code snippet below on GitHub:
import 'package:flutter/widgets.dart';
import 'package:firebase_auth/firebase_auth.dart';
enum Status { Uninitialized, Authenticated, Authenticating, Unauthenticated }
class UserRepository with ChangeNotifier {
final FirebaseAuth auth;
FirebaseUser _user;
Status _status = Status.Uninitialized;
UserRepository.instance({this.auth}) {
auth.onAuthStateChanged.listen(onAuthStateChanged);
}
Status get status => _status;
FirebaseUser get user => _user;
Future<bool> signIn(String email, String password) async {
try {
_status = Status.Authenticating;
notifyListeners();
await auth.signInWithEmailAndPassword(email: email, password: password);
return true;
} catch (e) {
_status = Status.Unauthenticated;
notifyListeners();
return false;
}
}
Future signOut() async {
auth.signOut();
_status = Status.Unauthenticated;
notifyListeners();
return Future.delayed(Duration.zero);
}
Future<void> onAuthStateChanged(FirebaseUser firebaseUser) async {
if (firebaseUser == null) {
_status = Status.Unauthenticated;
} else {
_user = firebaseUser;
_status = Status.Authenticated;
}
notifyListeners();
}
}
At the top where UserRepository was instantiated,
UserRepository.instance({this.auth}) {
auth.onAuthStateChanged.listen(onAuthStateChanged);
}
on auth.onAuthStateChanged.listen he passes the onAuthStatechanged method. This method as you can see below the code snippet takes in parameter FirebaseUser firebaseUser but this is never passed when called.
My question is, how can this work then if it receives no value when called?
Full disclosure: This code isn't mine, it was/is available on GitHub. I only posted it here for whoever has an answer to my question to fully understand.

Answer 1

"auth.onAuthStateChanged.listen" itself is a function which takes
Future<void> Function(FirebaseUser) as an argument. Function eating function thats all.
OG author could've passed a unnamed function right there like this
auth.onAuthStateChanged.listen((FirebaseUser firebaseUser){});
but that would be less readable