different behavior of table view reordering with or without NSFetchedResutlsController delegate

I have a table view backed up by a NSFetchedResutlsController. I want to support reordering on the table view. I use a property called 'index' in my NSManagedObject entity to record the order for each object, and recalculate the index whenever a reorder action is performed by the user in tableView:moveRowAtIndexPath:toIndexPath: (shown below, the Person is a subclass of NSManagedObject)

- (void) tableView:(UITableView *)tab开发者_如何学编程leView moveRowAtIndexPath:(NSIndexPath *)fromIndexPath toIndexPath:(NSIndexPath *)toIndexPath
{
  NSMutableArray *userList = [self.personResult.fetchedObjects mutableCopy];
  Person *user = [[userList objectAtIndex:fromIndexPath.row] retain];
  [userList removeObjectAtIndex:fromIndexPath.row];
  [userList insertObject:user atIndex:toIndexPath.row];
  [user release];

  int indexCounter = 0;
  for (Person *user in userList)
  {
    user.index = [NSNumber numberWithInt:indexCounter++];
  }
}

What makes me confused is, if I don't set a delegate for the NSFetchedResutlsController, the order of the objects in the NSFetchedResutlsController will not be updated. So when I click on the cell to view detail information, it will display contents according to the previous order. However, when I set a delegate with empty implementation for the NSFetchedResutlsController (as shown below), the order of the objects in the NSFetchedResutlsController do get updated.

- (void) controllerWillChangeContent:(NSFetchedResultsController *)controller
{
}

- (void) controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath
{
}

- (void) controllerDidChangeContent:(NSFetchedResultsController *)controller
{
}

I checked the initial value of the delegate property and it is set to nil. So I wonder if the implementation of the framework (maybe Core Data or something) will be different according to the existence of the delegate.

Any idea on this problem?