How can I declare that a text field can only contain an integer?

Question

In swift, I am trying to make a text field that will allow a button to be enabled, but only when the text field contains an integer. How can I do this?

Answer 1

IN SWIFT 4

        func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
             if textField == mobileTF{                   
             let allowingChars = "0123456789"
             let numberOnly = NSCharacterSet.init(charactersIn: allowingChars).inverted
             let validString = string.rangeOfCharacter(from: numberOnly) == nil
                    return validString
              }
         return true
         }