Login and register control in swift with firebase

Question

I'm coding in Swift with Firebase. I'm trying to understand something about authentication but it seems like that i can't get the point.

I need a function that check if a user is already logged in our platform. I need another function that check if the username and the password are both correct (if the user is in db)

Is there someone that could help me?

     func getLogin(username: String, pass: String) -> Int {
         print("getLogin in")
         var counter: Int = 0
         db.collection("users").whereField("username", isEqualTo: username).whereField("password", isEqualTo: pass).getDocuments() { (querySnapshot, err) in
             if let err = err {
                 print("Error getting documents: (err)")
                 counter = -1
                 print("\(counter)")
             } else {
                 for document in querySnapshot!.documents {
                     print("(document.documentID) => (document.data())")
                     counter += 1
                 }
                 print("\(counter)")
             }
         }
         return counter
     }

func getUser(username: String) -> Int {
         print("getUser in")
         var counter: Int = 0
         db.collection("users").document(username).whereField("username", isEqualTo: 
         username).getDocuments() { (querySnapshot, err) in
             for document in querySnapshot!.documents {
                 counter += 1
                 print("(document.documentID) => \(document.data())")
             }
         }
         return counter
     }