如何在选中时突出显示UITextField？

func textFieldDidBeginEditing(_ textField: UITextField) {
    textField.layer.borderColor = UIColor.blue.cgColor
    textField.layer.borderWidth = 2.0
}

func textFieldDidBeginEditing(_ textField: UITextField) {
    textField.backgroundColor = UIColor.yellow
}

func textFieldDidBeginEditing(_ textField: UITextField) {
    textField.layer.shadowColor = UIColor.black.cgColor
    textField.layer.shadowOpacity = 0.5
    textField.layer.shadowOffset = CGSize(width: 0, height: 2)
    textField.layer.shadowRadius = 4.0
}