Flutter Firebase：我在获取Firestore数据时遇到问题

I'm trying to check if a user is already registered or if i need to create a new account. The idea was that i wanted to get a list of users and check if my new email matches up. If not create a new user I have a DatabaseService class which I use to get my data, and a AuthService which i use to perform login and registration. I my DatabaseService class I have a function to return a list of User objects:

List<User>  getUserList() {
print(databaseReference);
List<User> userList = [];
databaseReference
    .collection("users")
    .getDocuments()
    .then((QuerySnapshot snapshot) {
      print('s');
  snapshot.documents.forEach((doc) {
    print(doc.documentID);
    userList.add(User(
      uid: doc.documentID,
      email: doc.data['email'],
      name: doc.data['name'],
      imgUrl: doc.data['imgUrl'],
    ));
    print(userList);
  });
});
return userList;

}

This works fine when I call it from my LoginPage. I get a list with all my auhtorized users. But when i run it out of my loginIfRegistered function in my AuthService class. I just get an empty list. this is the function:

  Future loginIfRegistered(String email, String password, List<User> userList) async {
print('CHECKING IF ALREADY REGISTERED');
bool alreadyRegistered = false;

updateUserList();

for (var user in userList) {
  if (user.email == email) {
    alreadyRegistered = true;
  }
}

if (userList.length == 0) {
  alreadyRegistered = false;
}


if (alreadyRegistered) {
// sign in  
print('ALREADY REGISTERED');
  try {
    return await _auth.signInWithEmailAndPassword(
        email: email, password: password);
  } catch (e) {
    print(e.toString());
    return e;
  }
} else {
  print('NOT REGISTERED');
  // register
  return null;
}

}

But when I pass the userList as a parameter into my function (Future loginIfRegistered(String email, String password, List<User> userList) async {...}), and initialize it on my LoginPage there is no problem. I also tried getting the list at the top of my AuthService and it worked aswell, but I don't want it to be just updated when AuthService is initialized but rather when i hit the SignIn button.

I also tried adding a updateUserList function in AuthService which is called in loginIfRegistered() but that also doesn't work.

So why do I get an empty list if i call getUserList()?

To recap: I want to have a login page which also registers you if you don't already have an account. When I press the LogIn Button the function SigninIfRegisters (in AuthService) is run which needs to know all users in my firestore Database (I made a collection for all my users, not out of Firebaseauth or something like that (is that a good idea?)). Therefore i made a function in DatabaseService which gives me a List<User> this works fine if called out of the LoginPage but not out of any functions in AuthService. If i instantiate List<User> not in any function in AuthService it also works.

请有人解释为什么？？？