title: Posting NSNotification on the main thread
body: <p>I found the following code snippet which allows <code>NSNotification</code> to be posted on the main thread from any background thread. I would like to know if this is a safe and acceptable practice please?</p>
  
  <pre><code>dispatch_async(dispatch_get_main_queue(),^{
      [[NSNotificationCenter defaultCenter] postNotificationName:@"ImageRetrieved" 
                                                          object:nil 
                                                        userInfo:imageDict];
  });
  </code></pre>