title: UIView transition animation not executing
body: <p>I'm trying to use an animation transition for when a UIView appears on screen.  The UIView appears correctly but the animation doesn't occur when it appears.</p>
  
  <pre><code>  override func viewWillAppear(_ animated: Bool) {
      super.viewWillAppear(animated)
  
      let coreView = UIView(frame: CGRect(x: 10, y: 10, width: 100, height: 100))
      coreView.backgroundColor = UIColor.cyan
      coreView.layer.borderColor = UIColor.darkGray.cgColor
      coreView.layer.borderWidth = 8
      coreView.layer.cornerRadius = 15
      coreView.isHidden = true
      self.view.addSubview(coreView)
  
      //The transition occurs here
      UIView.transition(with: coreView, duration: 2, options: [.curveEaseInOut, .transitionCurlDown], animations: {
  
        coreView.isHidden = false
      }, completion: {_ in})
  
      }
  </code></pre>