Bronze solution without using NSCharacterSet

iOS Programming: The Big Nerd Ranch Guide (6th Edi 4. Text Input and Delegation
1

Here it is a simpler solution without using NSCharacterSet:

func textField(_ textField: UITextField,
                      shouldChangeCharactersIn range: NSRange,
                      replacementString string: String) -> Bool {
        if textField.text != "" || string != "" {
            let res = (textField.text ?? "") + string
            return Double(res) != nil
        }
        return true
    }