Как ограничить число для ввода в uitextfield swift?

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class ViewController: UIViewController, UITextFieldDelegate {

    @IBOutlet weak var numberTextField: UITextField!

    override func viewDidLoad() {
        super.viewDidLoad()
        numberTextField.delegate = self
    }

    func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
        let allowedCharacters = CharacterSet.decimalDigits
        let characterSet = CharacterSet(charactersIn: string)
        return allowedCharacters.isSuperset(of: characterSet) && (textField.text?.count ?? 0) < 4
    }
}