Thanks you, here my code for RegisterView class RegisterView : UIView, UITextFieldDelegate {     @IBOutlet weak var username : UITextField?     @IBOutlet weak var password : UITextField?     @IBOutlet weak var repassword : UITextField?     @IBOutlet weak var email : UITextField?     @IBOutlet weak var displayname : UITextField?     var contentView : UIView?     public static var nibName: String {             return String(describing: self.self)         }          public static var nib: UINib {             let bundle = Bundle(for: self.self)             return UINib(nibName: self.nibName, bundle: bundle)         }     override init(frame: CGRect) {         super.init(frame: frame)         self.setupView()     }     required init?(coder : NSCoder) {         super.init(coder: coder)         setupView()     }     private func setupView () {         guard let view = Self.nib.instantiate(withOwner: self, options: nil).first as? UIView else { fatalError("Error loading \(self) from nib") }                 addSubview(view)                 view.translatesAutoresizingMaskIntoConstraints = false         if #available(iOS 11.0, *) {             view.leadingAnchor.constraint(equalTo: self.safeAreaLayoutGuide.leadingAnchor, constant: 0).isActive = true             view.topAnchor.constraint(equalTo: self.safeAreaLayoutGuide.topAnchor, constant: 0).isActive = true             view.trailingAnchor.constraint(equalTo: self.safeAreaLayoutGuide.trailingAnchor, constant: 0).isActive = true             view.bottomAnchor.constraint(equalTo: self.safeAreaLayoutGuide.bottomAnchor, constant: 0).isActive = true         }         self.displayname?.delegate = self         self.email?.delegate = self         self.username?.delegate = self         self.password?.delegate = self         self.repassword?.delegate = self         super.layoutSubviews()     }          func textFieldShouldReturn(_ textField: UITextField) -> Bool {         textField.resignFirstResponder()         return true     }     func textFieldShouldEndEditing(_ textField: UITextField) -> Bool {         textField.resignFirstResponder()         return true     }     func textFieldShouldBeginEditing(_ textField: UITextField) -> Bool {         return true     }     public func register(username: String, email : String, displayName : String, password : String) {     } } And here's the code for add RegisterView into parent view (I'm planning to switch between RegisterView and LoginView in parent view)         self.processView?.showTextOnly(text: "register button clicked")                  if (self.registerView != nil) {             self.upperView.removeChildView()             self.upperView.addSubview(registerView!)                         return         }         self.registerView = RegisterView();         self.registerView?.frame = upperView.bounds         self.registerView?.autoresizingMask = [.flexibleWidth, .flexibleHeight]         self.upperView.addSubview(registerView!)         self.registerView?.center = CGPoint(x : upperView.frame.width / 2,                                       y : upperView.frame.height / 2)     }