issues with UIView animation swift 4 Xcode9

Question

I am trying to make an animation of a UILabel from off the screen to the center with a spring animation block. The first part of my code below in viewDidLoad() works perfectly, but when I add the animation block, it's like the code in the closure of the animation gets read first and it doesn't animate because the label is already in the place where I want the label to animate to. I have also tried to put this exact code in viewDidAppear() but the same thing happens.

@IBOutlet weak var follow: UILabel!
@IBOutlet weak var followX: NSLayoutConstraint!

override func viewDidLoad() {
    super.viewDidLoad()

    self.follow.translatesAutoresizingMaskIntoConstraints = false

    self.followX.constant = self.view.frame.width/2 + follow.frame.width/2
    self.view.layoutIfNeeded()

    UIView.animate(withDuration: 2, delay: 2, usingSpringWithDamping: 5, initialSpringVelocity: 5, options: .curveEaseOut, animations: {
        self.followX.constant = 0
        self.view.layoutIfNeeded()
    })

}

Answer 1

This is the correct way to do it (although I would try to move that animation to viewWillAppear or viewDidAppear ):

@IBOutlet weak var follow: UILabel!
@IBOutlet weak var followX: NSLayoutConstraint!

override func viewDidLoad() {
    super.viewDidLoad()

    self.follow.translatesAutoresizingMaskIntoConstraints = false

    self.followX.constant = self.view.frame.width/2 + follow.frame.width/2
    self.view.layoutIfNeeded()

    self.followX.constant = 0
    self.view.setNeedsLayout()

    UIView.animate(withDuration: 2, delay: 2, usingSpringWithDamping: 5, initialSpringVelocity: 5, options: .curveEaseOut, animations: {
        self.view.layoutIfNeeded()
    })

}

To work with autolayout:

1. You first make sure the to begin animation is ready:

    self.followX.constant = self.view.frame.width/2 + follow.frame.width/2 self.view.layoutIfNeeded() 

2. Then you change constraints to the final state of the animation:

    self.followX.constant = 0 

3. Then you tell autolayout that the constraints were changed:

    self.view.setNeedsLayout() 

4. And finally you call UIView.animate with layoutIfNeeded() to animate the change:

    UIView.animate(withDuration: 2, delay: 2, usingSpringWithDamping: 5, initialSpringVelocity: 5, options: .curveEaseOut, animations: { self.view.layoutIfNeeded() }) 

Answer 2

override func viewDidLayoutSubviews() {
   if(once)
   {
      once = false
      self.follow.translatesAutoresizingMaskIntoConstraints = false
      self.followX.constant = self.view.frame.width/2 +   follow.frame.width/2
      self.view.layoutIfNeeded()
   }

}

and in viewDidAppear

 override func viewDidAppear() {

   self.followX.constant = 0
   UIView.animate(withDuration: 2, delay: 0, usingSpringWithDamping: 5, initialSpringVelocity: 5, options: .curveEaseOut, animations: {
      self.view.layoutIfNeeded()
  })

 }