set the value for a variable once or initialize a variable - Swift - swift

Question

Im using Firebase Firestore as my database, and I want to limit the number of calls to the database so my plan is to initialize an object once and have it ready for me to use anywhere throughout my app. What I want to do is basically give the publicUsername a value collected from the database once, so once publicUsername has a value, I don't want to call to the database anymore. I then want to call publicUsername through my app such as when I want to show the user their username I can just call the publicUsername value as so: usernameLabel.text = publicUsername.
Here are 2 methods I've tried using, but no luck!:
let publicUsername = Uzer().uzername
class Uzer {
var uzername = ""
init() {
let uid = Auth.auth().currentUser?.uid
let database = Firestore.firestore().collection("Users").document(uid!)
database.getDocument { (docSnapshot, error) in
guard let docSnapshot = docSnapshot, docSnapshot.exists else {return}
let mydata = docSnapshot.data()
let username = mydata!["Username"] as? String ?? ""
self.uzername = username
}
}
}
OR
let publicUsername = User().username
class Users {
var username = ""
func returnUsername () -> String {
if username == "" {
let uid = Auth.auth().currentUser?.uid
let database = Firestore.firestore().collection("Users").document(uid!)
database.getDocument { (docSnapshot, error) in
guard let docSnapshot = docSnapshot, docSnapshot.exists else {return}
let mydata = docSnapshot.data()
let username = mydata!["Username"] as? String ?? ""
self.username = username
}
return username
} else {
return username
}
}
}