Why my solution of moving my textfield doesn't work?

// Creation of the textField and the animate-able constraints
let textField = UITextField()
private var textFieldXConstraint: NSLayoutConstraint?
private var textFieldYConstraint: NSLayoutConstraint?

let startButton = UIButton()

override func viewDidLoad() {
    super.viewDidLoad()

    NSNotificationCenter.defaultCenter().addObserver(self, selector: #selector(ViewController.keyboardWillShow(_:)), name: UIKeyboardWillShowNotification, object: nil)

    NSNotificationCenter.defaultCenter().addObserver(self, selector: #selector(ViewController.keyboardWillDissappear(_:)), name: UIKeyboardWillHideNotification, object: nil)

    textField.delegate = self

    startButton.enabled = false
    startButton.alpha = 0.3
    startButton.setTitle("Start Adventure", forState: UIControlState.Normal)

    startButton.addTarget(self, action: #selector(ViewController.startButtonDidTouch), forControlEvents: UIControlEvents.TouchUpInside)
}

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

override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
    if segue.identifier == "startAdventure" {
        if let pageController = segue.destinationViewController as? PageController, let text = textField.text {
            pageController.page = Adventure.story(text)
        }
    }
}

// MARK: Helper methods

func keyboardWillShow(notification: NSNotification) {

    if let userInfo = notification.userInfo, keyboardFrameValue = userInfo[UIKeyboardFrameEndUserInfoKey] as? NSValue {

        let keyboardFrame = keyboardFrameValue.CGRectValue()

        UIView.animateWithDuration(0.8, animations: {
            self.textFieldYConstraint?.constant -= keyboardFrame.size.height
            self.view.layoutIfNeeded()
        })
    }
}

func keyboardWillDissappear(notification: NSNotification) {

    UIView.animateWithDuration(0.8, animations: {
        self.textFieldYConstraint?.constant += 100
        print(self.textFieldYConstraint!)
        self.view.layoutIfNeeded()
    })

}

// This happen when you press the start adventure button
func startButtonDidTouch() {
    performSegueWithIdentifier("startAdventure", sender: self)
}

// MARK: UI Creation
override func viewWillLayoutSubviews() {
    view.addSubview(startButton)
    startButton.translatesAutoresizingMaskIntoConstraints = false

    startButton.tintColor = UIColor.whiteColor()

    NSLayoutConstraint.activateConstraints([
        startButton.centerXAnchor.constraintEqualToAnchor(view.centerXAnchor),
        startButton.bottomAnchor.constraintEqualToAnchor(view.bottomAnchor, constant: -48.0)
        ])

    view.addSubview(textField)
    textField.translatesAutoresizingMaskIntoConstraints = false

    textField.minimumFontSize = 24.0
    textField.placeholder = "Write your name"
    textField.borderStyle = .RoundedRect

    textFieldXConstraint = NSLayoutConstraint(item: textField, attribute: NSLayoutAttribute.CenterX, relatedBy: NSLayoutRelation.Equal, toItem: view, attribute: NSLayoutAttribute.CenterX, multiplier: 1, constant: 0)

    textFieldYConstraint = NSLayoutConstraint(item: textField, attribute: NSLayoutAttribute.BottomMargin, relatedBy: NSLayoutRelation.LessThanOrEqual, toItem: startButton, attribute: NSLayoutAttribute.TopMargin, multiplier: 1, constant: -24)

    if textFieldXConstraint != nil && textFieldYConstraint != nil {
        view.addConstraint(textFieldXConstraint!)
        view.addConstraint(textFieldYConstraint!)
    }
}

// MARK: TextField Helper Methods

// called when 'return' key pressed. return NO to ignore.
func textFieldShouldReturn(textField: UITextField) -> Bool {
    textField.resignFirstResponder()
    return true
}

// called when users tap out of textfield
override func touchesBegan(touches: Set<UITouch>, withEvent event: UIEvent?) {
    self.view.endEditing(true)
}

// Checks if there is a name to activate the button
func textField(textField: UITextField, shouldChangeCharactersInRange range: NSRange, replacementString string: String) -> Bool {
    let lenght = (textField.text?.characters.count)! - (range.length) + (string.characters.count)

    if lenght > 0 {
        startButton.enabled = true
        startButton.alpha = 1.0
    } else {
        startButton.enabled = false
        startButton.alpha = 0.3
    }

    return true
}

// MARK: deinit

deinit {
    NSNotificationCenter.defaultCenter().removeObserver(self, name: UIKeyboardWillShowNotification, object: nil)

    NSNotificationCenter.defaultCenter().removeObserver(self, name: UIKeyboardWillHideNotification, object: nil)
}