class ViewController: UIViewController {

  @IBOutlet weak var txtFieldDemo: UITextField! {
        didSet{
            txtFieldDemo.delegate = self
            txtFieldDemo.addTarget(self, action: #selector(textFieldDidChange(textField:)), for: .editingChanged)
        }
    }
}

extension ViewController: UITextFieldDelegate{
   func textFieldDidBeginEditing(_ textField: UITextField) {
        textField.underlined()
    }

    func textFieldDidEndEditing(_ textField: UITextField) {
        textField.removeUnderline()
    }

    @objc func textFieldDidChange(textField: UITextField){
        textField.hideUnderline()
        textField.underlined()
    }
}