use viewdidapper if statement regarding textfield input (Swift4)

I would like to use viewdidappear to automatically change things depending on whatever is in the textfield. I tried doing that it is not working. I dont know why it is not working.

      override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        //...Do stuff

        if ana.text == "X" {
            ana.backgroundColor = .gray
        }}