title: Unit testing for code like dispatch_async.
body: <p>Need to do unit testing for the following code, dispatch_async means code won't be executed by app logic sequence, any idea on how to make it run timely?  </p>
  
  <p>Thank you.  </p>
  
  <pre><code> dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
          [AdTracker dosomething];
      });
  </code></pre>