title: cancel a background thread in Objective-C
body: <p>How do I cancel a background thread in Objective-C?</p>
  
  <p>I am calling background threads like so:</p>
  
  <pre><code>[self performSelectorInBackground:@selector(setupThumbnails) withObject:nil];
  </code></pre>
  
  <p>It is called when a swipe happens, for each swipe I want to cancel the previous background thread request.</p>