title: layer.addSublayer vs layer.insertSublayer animation
body: <p>I am animating a CAGradient as so</p>
  
  <pre><code>let gradientChangeAnimation = CABasicAnimation(keyPath: "colors")
      gradientChangeAnimation.duration = 5.0
      gradientChangeAnimation.toValue = [
          UIColor(red: 244/255, green: 88/255, blue: 53/255, alpha: 1).cgColor,
          UIColor(red: 196/255, green: 70/255, blue: 107/255, alpha: 1).cgColor
      ]
      gradientChangeAnimation.fillMode = kCAFillModeForwards
      gradientChangeAnimation.isRemovedOnCompletion = false
      gradientLayer.add(gradientChangeAnimation, forKey: "colorChange")
  </code></pre>
  
  <p>And it works fine when I add it to the view using this method</p>
  
  <pre><code>self.view.layer.addSublayer(gradientLayer)
  </code></pre>
  
  <p>However, it is now blocking all of my UI buttons and icons, so I attempt to insert it underneath these items with</p>
  
  <pre><code>self.view.layer.insertSublayer(gradientLayer, at: 0)
  </code></pre>
  
  <p>But when I do this, the animation stops working completely</p>
  
  <p>Why is this happening?</p>