1. 뷰 컨트롤러는 UITextFieldDelegate아래와 같이을 준수해야 합니다.

   class MyViewController: UIViewController, UITextFieldDelegate {
   
   }
   

2. 텍스트 필드의 대리자를 설정합니다. myTextField.delegate = self

3. 뷰 컨트롤러에서 메소드를 구현하십시오. textField(_:shouldChangeCharactersInRange:replacementString:)

모두 함께:

class MyViewController: UIViewController,UITextFieldDelegate  //set delegate to class

@IBOutlet var mytextField: UITextField             //  textfield variable

override func viewDidLoad() {
    super.viewDidLoad()
    mytextField.delegate = self                  //set delegate
}


func textField(textField: UITextField, shouldChangeCharactersInRange range: NSRange,
                       replacementString string: String) -> Bool
{
    let maxLength = 4
    let currentString: NSString = textField.text
    let newString: NSString =
             currentString.stringByReplacingCharactersInRange(range, withString: string)
    return newString.length <= maxLength
}

Swift 4의 경우

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
    let maxLength = 1
    let currentString: NSString = (textField.text ?? "") as NSString
    let newString: NSString =
        currentString.replacingCharacters(in: range, with: string) as NSString
    return newString.length <= maxLength
}

지정된 텍스트 필드에 지정된 문자 집합 만 입력하도록 허용

func textField(textField: UITextField, shouldChangeCharactersInRange range: NSRange, replacementString string: String) -> Bool {
  var result = true

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

  return result
}

최대 길이의 숫자 입력 만받는 iOS 텍스트 필드를 프로그래밍하는 방법