title: Animated pie chart in iOS
body: <p>I'm trying to create an animated pie chart in iOS that acts basically like this:</p>
  
  <p><img src="https://i.stack.imgur.com/UDmAy.png" alt="Pie chart progression"></p>
  
  <p>In a nutshell, it starts as a grey circle, and as the animation progresses the arrow moves around the circle until it gets to the percentage I specify.</p>
  
  <p>I've used the sample code that Zachary Waldowski posted in this SO question:</p>
  
  <p><a href="https://stackoverflow.com/a/9150991/271976" title="Animated CAShapeLayer Pie">Animated CAShapeLayer Pie</a></p>
  
  <p>That's gotten me to the point where I can create the basic animation.  The maroon pie piece grows to hit the correct size.  What I'm struggling with is how to map the arrow to the animation so that it gets dragged along as the pie piece grows.</p>
  
  <p>Any thoughts on how I can accomplish this?</p>