title: Finding which class responds to a specific message
body: <p>In a <a href="https://stackoverflow.com/questions/29829792/serious-application-error-model-object-getting-nsfetchedresultscontroller-messa">previous question</a>, I mentioned that a <code>Core Data</code> class is receiving a message it doesn't understand (I believe it has something to do with <code>NSFetchedResultsController</code>), called </p>
  
  <pre><code>-(int) sectionOffset
  </code></pre>
  
  <p>In trying to find the cause of the bug, I tried to find out which class was the intended receiver, to no avail: there's no mention to this message in the docs or even in google.</p>
  
  <p>As a last resort, I was planning to iterate over all <code>NSObject</code>'s subclasses and ask (via <code>instanceRespondsToSelector:</code>) if it understands the message. Hopefully, that should give me a clue of what happening <a href="https://stackoverflow.com/questions/29829792/serious-application-error-model-object-getting-nsfetchedresultscontroller-messa">here</a>.</p>
  
  <p>So, is there a way to iterate through all the subclasses of a certain class? Is there a better way to do this?</p>