title: Block version of performSelectorOnMainThread:withObject:waitUntilDone:
body: <p>Is there a way that I can execute a block rather than a selector corresponding to this and similar methods?</p>
  
  <p>I have observers that may receive events that aren't generated on the main thread. I want the action to be performed on the main thread if it is primarily UI oriented. Right now, I need to write two methods to do this, where one is the event observer, and the second is the code that needs to be executed on the main thread.</p>
  
  <p>I would like to encapsulate this all into one method, if I could.</p>