title: Animation stops working when view appears again
body: <pre><code>extension UIView {
  
      func startBlinking() {
          UIView.animate(withDuration: 0.8, delay: 0.0, options: [.allowUserInteraction, .curveEaseInOut, .autoreverse, .repeat], animations: { self.alpha = 0 }, completion: nil)
      }
  }
  </code></pre>
  <p>How do I use it?</p>
  <pre><code>override func viewDidAppear(_ animated: Bool) {
      super.viewDidAppear(animated)
      monthLabel.startBlinking()
  }
  </code></pre>
  <p>When method is called first time, then it blinks... but the second time when view did appear the label disappears and... that's it.</p>
  <p>Why doesn't it work again?</p>
  <p>My logs:</p>
  <pre>did load
  did appear
  start blinking
  did appear
  start blinking</pre>