This UITextField is save to UserDefaults when editingDidEnd, can restore text from UserDefaults, and can edit key in Inspector of Interface-Builder.

RestorableTextField.swift

import UIKit

class RestorableTextField: UITextField {
    @IBInspectable var defaultsKey: String = ""

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        
        self.addTarget(self, action: #selector(saveToUserDefaults(_:)), for: .editingDidEnd)
    }
    
    @objc func saveToUserDefaults(_ sender:RestorableTextField) {
        guard defaultsKey != "" else { return }
        UserDefaults.standard.set(sender.text ?? "", forKey: defaultsKey)
        // this method is unnecessary and shouldn't be used.
        // UserDefaults.standard.synchronize()
    }
    
    func restoreByUserDefaults() {
        guard defaultsKey != "" else { return }
        guard let text = UserDefaults.standard.string(forKey: defaultsKey) else { return }
        guard text != "" else { return }
        
        self.text = text
    }
}

Swift 4.