title: Bad access in using "setNeedsDisplay" method in background thread
body: <p>There is a object dragging and video recording in my app. I am using a <code>ScreenCapture</code> view in my app. I am taking screenshots of main screen and making a video. </p>
  
  <p>When I am using the following code in <code>drawRect()</code> method of my <code>ScreenCapture</code> view, the dragging becomes slow for iOS 6 and works fine for iOS 5: </p>
  
  <pre><code>[self performSelector:@selector(setNeedsDisplay) withObject:nil
        afterDelay:delayRemaining &gt; 0.0 ? delayRemaining : 0.01];
  </code></pre>
  
  <p>And when i replace that line of code with following code, the dragging works fine but gives a bad access:</p>
  
  <pre><code>[self performSelectorInBackground:@selector(setNeedsDisplay) withObject:nil];
  </code></pre>
  
  <p>What is the reason for this?</p>