title: Create a custom animatable property
body: <p>On <code>UIView</code> you can change the backgroundColour animated. And on a <code>UISlideView</code> you can change the value animated.</p>
  
  <p>Can you add a custom property to your own <code>UIView</code> subclass so that it can be animated?</p>
  
  <p>If I have a <code>CGPath</code> within my <code>UIView</code> then I can animate the drawing of it by changing the percentage drawn of the path.</p>
  
  <p>Can I encapsulate that animation into the subclass.</p>
  
  <p>i.e. I have a <code>UIView</code> with a <code>CGPath</code> that creates a circle.</p>
  
  <p>If the circle is not there it represents 0%. If the circle is full it represents 100%. I can draw any value by changing the percentage drawn of the path. I can also animate the change (within the UIView subclass) by animating the percentage of the <code>CGPath</code> and redrawing the path.</p>
  
  <p>Can I set some property (i.e. percentage) on the <code>UIView</code> so that I can stick the change into a <code>UIView animateWithDuration</code> block and it animate the change of the percentage of the path?</p>
  
  <p>I hope I have explained what I would like to do well.</p>
  
  <p>Essentially, all I want to do is something like...</p>
  
  <pre><code>[UIView animateWithDuration:1.0
   animations:^{
       myCircleView.percentage = 0.7;
   }
   completion:nil];
  </code></pre>
  
  <p>and the circle path animate to the given percentage.</p>