useUITextFieldDelegateToValidate_2.swift

    override func viewDidLoad() {
        super.viewDidLoad()
        
        self.view.backgroundColor = UIColor.white
        
        // 身份證號
        let idLbl: UILabel = UILabel()
        idLbl.frame = CGRect(x: 0, y: 40, width: self.view.frame.size.width / 3, height: 50)
        idLbl.font = UIFont.systemFont(ofSize: 20)
        idLbl.textColor = UIColor.black
        idLbl.textAlignment = NSTextAlignment.left
        idLbl.adjustsFontSizeToFitWidth = true
        idLbl.text = "\t" + "身份證號："
        self.view.addSubview(idLbl)
        
        // 身份證輸入框
        let idTxtFld: UITextField = UITextField()
        idTxtFld.frame = CGRect(x: idLbl.frame.origin.x + idLbl.frame.size.width, y: 40, width: self.view.frame.size.width / 3 * 2, height: 50)
        idTxtFld.font = UIFont.systemFont(ofSize: 20)
        idTxtFld.backgroundColor = UIColor.clear
        idTxtFld.textAlignment = NSTextAlignment.left
        idTxtFld.clearButtonMode = UITextFieldViewMode.unlessEditing // 清除按鈕
        idTxtFld.placeholder = "請輸入證件號碼"
        idTxtFld.delegate = self
        self.view.addSubview(idTxtFld)
        
        // 註冊tab事件，點選瑩幕任一處可關閉瑩幕小鍵盤
        let tap: UITapGestureRecognizer = UITapGestureRecognizer(target: self, action: #selector(ViewController.dismissKeyboard))
        self.view.addGestureRecognizer(tap)
    }