the keyboard doesn't show up.

@IBOutlet weak var nameTextField: UITextField!
@IBOutlet weak var textFieldBottomConstraint: NSLayoutConstraint!

override func viewDidLoad() {
    super.viewDidLoad()

    NotificationCenter.default.addObserver(self, selector: #selector(ViewController.keyboardWillShow(_:)), name: Notification.Name.UIKeyboardWillShow, object: nil)
    NotificationCenter.default.addObserver(self, selector: #selector(ViewController.keyboardWillHide(_:)), name: .UIKeyboardWillHide, object: nil)

    func keyboardWillHide(_ notification: Notification) {
        textFieldBottomConstraint.constant = 40

        UIView.animate(withDuration: 0.8) {
            self.view.layoutIfNeeded()
        }
    }

}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if segue.identifier == "startAdventure" {

        do {
            if let name = nameTextField.text {
                if name == "" {
                    throw AdventureError.nameNotProvided
                } else {
                    guard let pageController = segue.destination as? PageController else { return }

                    pageController.page = Adventure.story(withName: name)
                }
            }
        } catch AdventureError.nameNotProvided {
            let alertController = UIAlertController(title: "Name Not Provided", message: "Provide a name to start the story", preferredStyle: .alert)

            let action = UIAlertAction(title: "OK", style: .default, handler: nil)
            alertController.addAction(action)

            present(alertController, animated: true, completion: nil)
        } catch let error {
            fatalError("\(error.localizedDescription)")
        }
    }
}

func keyboardWillShow(_ notification: Notification) {
    if let info = notification.userInfo, let keyboardFrame = info [UIKeyboardFrameEndUserInfoKey] as? NSValue {
        let frame = keyboardFrame.cgRectValue
        textFieldBottomConstraint.constant = frame.size.height + 10

        UIView.animate(withDuration: 0.8) {
            self.view.layoutIfNeeded()
        }
    }
}

func keyboardWillHide(_ notification: Notification) {
    textFieldBottomConstraint.constant = 40

    UIView.animate(withDuration: 0.8) {
        self.view.layoutIfNeeded()
    }
}

deinit {
    NotificationCenter.default.removeObserver(self)
}