Programmatically removing/adding constraints animated

Question

I'm having difficulties removing and adding constraints programmatically with animation. I've simplified my existing project into a small project just for demonstration.

Setup is there are two (container) views: top and bottom. They contain two subviews: blue and red.

Top view has aspect ratio 2:3 constraint.

I have reference to topView, aspectRatioConstraint and heightConstraint which I initialize in viewDidLoad

  @IBOutlet weak var aspectRatioConstraint: NSLayoutConstraint!

  var heightConstraint: NSLayoutConstraint!

  @IBOutlet weak var topView: UIView!

  override func viewDidLoad() {
    super.viewDidLoad()
    heightConstraint = NSLayoutConstraint(item: topView, attribute: .height, relatedBy: .equal, toItem: topView, attribute: .height, multiplier: 1.0, constant: topView.frame.size.height)
    NSLayoutConstraint.activate([heightConstraint])
    topView.removeConstraint(aspectRatioConstraint)
  }

  @IBAction func startAnimation(_ sender: AnyObject) {
    UIView.animate(withDuration: 1.0, delay: 0.0, usingSpringWithDamping: 0.3, initialSpringVelocity: 0.1, options: .curveEaseIn, animations: {
      self.heightConstraint.constant = 20.0
      self.view.layoutIfNeeded()
      }, completion: nil)
  }

"+" button in navigation calls startAnimation method and it should resize heightConstraint over period of time, but it doesn't.

What am I doing wrong?

Sample project -> https://www.dropbox.com/sh/7i75i0zq9dormqb/AACzTYvWOoJL3MjcjgMIyOd9a?dl=0

Answer 1

Try this :

UIView.animate(withDuration: 1.0, delay: 0.0, usingSpringWithDamping: 0.3, initialSpringVelocity: 0.1, options: .curveEaseIn, animations: {
      NSLayoutConstraint.deactivate([self.heightConstraint])
      self.heightConstraint.constant = 20.0
      NSLayoutConstraint.activate([self.heightConstraint])
      self.view.layoutIfNeeded()
  }, completion: nil)

You have deactivate before updating some constraints