textField Editing Changed not reacting fast enough (Asynchronous calls) - ios

Question

I have a textfield that queries a firebase database for existing users and then display a UIImage according to if the user is available or not. The problem is that once the async code loads, the textfield doesn't react on changed value.
example. If i type 12345 as a username, i don't query the database. Everything ok. If i add a 6 it queries firebase and it shows me the user is free. if i press backspace and have 12345 the textFieldChanged is triggered again, and database is not queried. All OK.
but the problem is, when i have 12345, and i type 6 and very fast back so i have 12345, the query is running and shows me the available icon (because the back was pressed very fast). Is this because of the Simulator or is it a real problem and can i be fixed easily ?
my code:
#IBAction func textFieldChanged(_ sender: UITextField) {
if let username = usernameInputText.text, username.count > 5 {
checkIfUserExists(username: username) { doesExist in //(2)
if doesExist! {
self.completeSignupButton.isEnabled = false
self.ifAvailableImageView.image = UIImage(named: "Close")
} else {
self.completeSignupButton.isEnabled = true
self.ifAvailableImageView.image = UIImage(named: "Check")
}
}
} else {
ifAvailableImageView.image = UIImage(named: "Close")
self.completeSignupButton.isEnabled = false
}
}
func checkIfUserExists(username: String, completion: #escaping (Bool?) -> Void) {
spinner.startAnimating()
self.ifAvailableImageView.image = nil
let docRef = db.collection("users").document(username)
docRef.getDocument { (document, error) in
if error != nil {
self.spinner.stopAnimating()
completion(nil)
} else {
self.spinner.stopAnimating()
if let document = document {
if document.exists {
completion(true)
} else {
completion(false)
}
}
}
}
}

Answer 1

You can just compare the username being processed with the current text in the text field and not process the result if it not the same because you only want to process the latest one.
#IBAction func textFieldChanged(_ sender: UITextField) {
if let username = usernameInputText.text, username.count > 5 {
checkIfUserExists(username: username) { doesExist in //(2)
// Check if current text and the completion being processed are for the same username
if username != sender.text {
return
}
if doesExist! {
self.completeSignupButton.isEnabled = false
self.ifAvailableImageView.image = UIImage(named: "Close")
} else {
self.completeSignupButton.isEnabled = true
self.ifAvailableImageView.image = UIImage(named: "Check")
}
}
} else {
ifAvailableImageView.image = UIImage(named: "Close")
self.completeSignupButton.isEnabled = false
}
}