title: Animate the fractionComplete of UIViewPropertyAnimator for blurring the background
body: <p>So I'm using the new UIViewPropertyAnimator and UIVisualEffectView to achieve the same thing as the Spotlight search when you scrolling down on the home screen and it blurs the background.</p>
  
  <p>I'm using the fractionComplete property to set the procent of how much to blur when panning a UIView.</p>
  
  <pre><code>    animator = UIViewPropertyAnimator(duration: 1, curve: .linear) {
          self.blurEffectView.effect = nil
      }
  </code></pre>
  
  <p>And the amount of blurriness is changed with a value between 0.0 - 1.0.</p>
  
  <pre><code>        animator?.fractionComplete = blurValue
  </code></pre>
  
  <p>But when I cancel the pan gesture I want the blur to animate back from where it is to no blur (e.g ~ -> 1.0) with a duration of something like 0.4 milliseconds.</p>
  
  <p>Right now I just set the fractionComplete to 1.0 when the pan gesture is cancelled. Instead I want to animate it.<br>
  I have tried the UIView.animate(withDuration.. but it doesn't affect the UIViewPropertyAnimators fractionComplete, and thats the only way to blur an UIVisualEffectView.</p>
  
  <p>Any ideas?</p>