Custom TextField in swift

CustomTextField.swift

import Foundation
import UIKit

class CustomTextField: UITextField, UITextFieldDelegate{
    
    required init(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        self.delegate = self
    }
    
    func textField(textField: UITextField, shouldChangeCharactersInRange range: NSRange, replacementString string: String) -> Bool {
        if (string == "X" ){
            //ignore input
            return false
        }
        return true
    }
    
}