I am trying to check when a text field changes, equivalent too the function used for textView - textViewDidChange so far I have done this: 
  fu         
        You should follow this steps:
Sample code:
import UIKit
class ViewController: UIViewController, UITextFieldDelegate {
    @IBOutlet var yourTextFiled : UITextField!
    override func viewDidLoad() {
        super.viewDidLoad()
        yourTextFiled.delegate = self
    }
    func textFieldDidEndEditing(_ textField: UITextField) {
        // your code
    }
    func textFieldShouldReturn(_ textField: UITextField) -> Bool {
        // your code
    }
    .
    .
    .
}