title: When NSFetchedResultsChangeUpdate change type happens?
body: <p>I wonder when <code>NSFetchedResultsChangeUpdate</code> change type in <code>controller:didChangeObject:atIndexPath:forChangeType:newIndexPath:</code> method is passed?</p>
  
  <p>I've created <code>NSFetchedResultsController</code> with non-nil <code>sectionNameKeyPath</code> and fetch request that have <code>sortDescriptor</code>. Added few objects that matched fetched request predicate. Then I've tried both changing of sort property and <code>sectionNameKeyPath</code> property of one of that objects. Actually it worked fine: changed object moved around a tableView, BUT every time I've received  <code>NSFetchedResultsChangeMove</code> change type! Changing of other props (not used for sorting or sections) of that object did not trigger <code>controller:didChangeObject:atIndexPath:forChangeType:newIndexPath:</code> at all!</p>
  
  <p>The reason why I'm interested in <code>NSFetchedResultsChangeUpdate</code> is that I want <code>NSFetchedResultsController</code> to notify me when certain property of already fetched object changes in bounds matching <code>NSFetchedResultsController.fetchRequest.predicate</code>. For example, predicate matches object of certain entity with <code>prop == 1 OR prop == 2</code>. But I also want to be notified when one of such matched objects changes it's <code>prop</code> value from 1 to 2.</p>
  
  <p>Just don't want to create two <code>NSFetchedResultsController</code>s since I believe there is a more elegant way. + <code>NSFetchedResultsChangeUpdate</code> change type is the only one mystery still undiscovered for me about <code>NSFetchedResultsController</code> - never caught it.</p>