title: Create loading-circle
body: <p>I'd like to draw a circle part by part. The circle should act as a loading-circle and should update itself after something is completed and should colorize a part of the circle. </p>
  
  <p>I've tried already to do that with a for-loop in the <code>drawRect-method</code>, but the circle doesn't update properly. </p>
  
  <p>Are there any important methods I have to use to update the circle "smooth"?</p>