UIAlertController with text field and disable buttons if textfield is empty

alert.swift

 let alertController = UIAlertController(title: nil, message:  alertMessageString, preferredStyle: .alert)
                let cancelAction = UIAlertAction(title: alertLeftBtuTitle, style: .default, handler: nil)
                alertController.addAction(cancelAction)
                let okAction = UIAlertAction(title: alertRightBtnText , style: UIAlertActionStyle.default, handler: {
                    (action) -> Void in
                    print("ok tapped")
                })
                okAction.isEnabled = false
                alertController.addAction(okAction)

alertController.addTextField(configurationHandler: {(textField: UITextField!) in
                        textField.placeholder = "Reject reason:"
                        textField.isSecureTextEntry = true
                        NotificationCenter.default.addObserver(forName: .UITextFieldTextDidChange, object: textField, queue: OperationQueue.main, using:
                            {_ in
                                let textCount = textField.text?.trimmingCharacters(in: .whitespacesAndNewlines).count ?? 0
                                let textIsNotEmpty = textCount > 0
                                // If the text contains non whitespace characters, enable the OK Button
                                okAction.isEnabled = textIsNotEmpty
                        })
                    }
                             
DispatchQueue.main.async {
                    self.present(alertController, animated: true, completion: nil)
}