title: Terminate a UIView transition animation
body: <p>I am slowly animating a UIImageView from one image to another. Like so:</p>
  
  <pre><code>[UIView transitionWithView:self.backgroundView duration:30 options:UIViewAnimationOptionTransitionCrossDissolve animations:^{
      self.backgroundView.image = endImage;
  } completion:nil];
  </code></pre>
  
  <p>Is there some way to terminate the animation partway through, in response to an event?</p>