title: How to setup KVO on a Collection (NSArray or NSSet) of NSManagedObjects
body: <p>I have an iPad app that has a <code>UITableViewController</code> that implements the <code>NSFetchedResultsControllerDelegate</code>. (Mostly using the <code>CoreDataTableViewController</code> <a href="http://www.timroadley.com/Tutorials/CoreDataBasicsPart2/CoreDataTableViewController.zip" rel="noreferrer">code</a>  from the Stanford iOS classes.)</p>
  
  <p>I have a secondary model object (<code>self.locations</code>) that is an array of <code>Location</code> objects which is a subclass of <code>NSManagedObjects</code>. This array drives the content of a <code>UISegmentedControl</code> that filters my main <code>fetchedResultsContoller</code>.</p>
  
  <p>It is possible to modify the contents of <code>self.locations</code> via a popover. I want to set up some sort of observing so that my main <code>UITableViewController</code> can watch for changes in the objects stored in <code>self.locations</code> and reload the <code>UISegmentedControl</code> if necessary. </p>
  
  <p>This might also result in a reload of the main data in the table, so I want to be careful to not reload on every little modification.</p>
  
  <p>I think I understand how to setup KVO on a single <code>NSManagedObject</code> but I'm not sure how to do it on object contained in an array. I understand that I can use another NSFetchedResultsController, but my <code>self.locations</code> object does not drive a second UITableView, so I'm not sure it makes sense. </p>