ConstraintNumber.swift

ObjC

-(BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {
  BOOL result = YES;
  NSString *prospectiveText = [textField.text stringByReplacingCharactersInRange:range withString:string];

  if ([textField isEqual:self.numberTextField]) {
    if (string.length > 0) {
      NSCharacterSet *disallowedCharacterSet = [NSCharacterSet characterSetWithCharactersInString:@"0123456789"].invertedSet;
      BOOL replacementStringIsLegal = [string rangeOfCharacterFromSet:disallowedCharacterSet].location == NSNotFound;

      BOOL resultingStringLengthIsLegal = prospectiveText.length <= 6;

      NSScanner *scanner = [NSScanner scannerWithString:prospectiveText];
      BOOL resultingTextIsNumeric = [scanner scanDecimal:nil] && scanner.atEnd;

      result = replacementStringIsLegal &&
                resultingStringLengthIsLegal &&
                resultingTextIsNumeric;
    }
  }

  return result;
}

Swift

func textField(textField: UITextField, shouldChangeCharactersInRange range: NSRange, replacementString string: String) -> Bool {
  var result = true
  let prospectiveText = (textField.text as NSString).stringByReplacingCharactersInRange(range, withString: string)

  if textField == numberField {
    if count(string) > 0 {
      let disallowedCharacterSet = NSCharacterSet(charactersInString: "0123456789.-").invertedSet
      let replacementStringIsLegal = string.rangeOfCharacterFromSet(disallowedCharacterSet) == nil

      let resultingStringLengthIsLegal = count(prospectiveText) <= 6

      let scanner = NSScanner(string: prospectiveText)
      let resultingTextIsNumeric = scanner.scanDecimal(nil) && scanner.atEnd

      result = replacementStringIsLegal &&
               resultingStringLengthIsLegal &&
               resultingTextIsNumeric
    }
  }
  return result
}