title: Cut transparent hole in UIView
body: <p>Looking to create a view that has a transparent frame inside of it so that the views behind the view can be seen through this transparent frame, but areas outside of this will not show through. So essentially a window within the view.</p>
  
  <p>Hoping to be able to do something like this:</p>
  
  <pre><code> CGRect hole = CGRectMake(100, 100, 250, 250);
  CGContextRef context = UIGraphicsGetCurrentContext();
  
  CGContextSetFillColorWithColor(context, [UIColor blackColor].CGColor);
  CGContextFillRect(context, rect);
  
  CGContextAddRect(context, hole);
  CGContextClip(context);
  
  CGContextSetFillColorWithColor(context, [UIColor clearColor].CGColor);
  CGContextFillRect(context, rect);
  </code></pre>
  
  <p>but the clear does not override the black so whole background is black. Any ideas along these lines?</p>