Step4.swift

init(center: NotificationCenter = .default) {
        _center = center
        //4. Tell the notification center to listen to the system keyboardWillShow and keyboardWillHide notification
        _center.addObserver(self, selector: #selector(keyBoardWillShow(notification:)), name: UIResponder.keyboardWillShowNotification, object: nil)
        _center.addObserver(self, selector: #selector(keyBoardWillHide(notification:)), name: UIResponder.keyboardWillHideNotification, object: nil)
    }