折扣換算

此功能如下：

1. 將原價乘以件數再計算折扣後的特價金額
2. 畫面一出現立刻跳出數字鍵盤並將原價輸入框預設為第一輸入者becomeFirstResponder
3. 點擊空白處將鍵盤收回
4. 再次點擊時將textField裡面的數字清空
5. 按下現省GO收回鍵盤並算出特價
6. 只要任一格沒填數字特價就會顯示0元
7. 限制不同的textField內的數字位數

畫面如下：

部分程式碼：

class DiscountViewController: UIViewController, UITextFieldDelegate {
override func viewDidLoad() {
super.viewDidLoad()
originalPrice.becomeFirstResponder()
}
func textFieldDidBeginEditing(_ textField: UITextField) {
textField.text = ""
}
override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
self.view.endEditing(true)
}
func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
guard let text = textField.text else { return true }
var limitLength = 6
//件數
if textField == quantity{
limitLength = 3
}
//折扣
if textField == discount {
limitLength = 1
}
let newLength = text.count + string.count - range.length
if newLength > limitLength {
return false
}
//原價
return newLength <= 6
}