NSFetchedResultsController didChangeObject with a filtering NSPredicate

Question

I have a UITableView backed by an NSFetchedResultsController that shows items which have been bookmarked by the user. Items can be un-bookmarked from a button within the row, which leads to the problem. After an item is un-bookmarked, it should disappear from the table view because it no longer matches the predicate, but because my row counts per section have been altered by the update, I get a variation of this error:

CoreData: error: Serious application error. An exception was caught from the delegate NSFetchedResultsController during a call to -controllerDidChangeContent:. Invalid update: invalid number of rows in section 0. The number of rows contained in an existing section after the update (3) must be equal to the number of rows contained in that section before the update (4), plus or minus the number of rows inserted or deleted from that section (0 inserted, 0 deleted) and plus or minus the number of rows moved into or out of that section (0 moved in, 0 moved out). with userInfo (null)

Here's my very simple didChangeObject method:

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

[super controller:controller didChangeObject:anObject atIndexPath:indexPath forChangeType:type newIndexPath:newIndexPath];
[self.tableView reloadRowsAtIndexPaths:@[indexPath] withRowAnimation:UITableViewRowAnimationAutomatic];

}

Is there some way that I can instruct the NSFetchedResultsController not to sweat the mismatched counts? Or do I need a different approach entirely?

Answer 1

Your didChangeObject delegate method looks very incomplete, in particular it does not check which event occurred (insertion, deletion or update).

You can find a template in the NSFetchedResultsControllerDelegate protocol documenation. The method looks normally similar to this:

- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject
       atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type
      newIndexPath:(NSIndexPath *)newIndexPath
{
    UITableView *tableView = self.tableView;
    switch(type) {
        case NSFetchedResultsChangeInsert:
            [tableView insertRowsAtIndexPaths:@[newIndexPath] withRowAnimation:UITableViewRowAnimationAutomatic];
            break;

        case NSFetchedResultsChangeDelete:
            [tableView deleteRowsAtIndexPaths:@[indexPath] withRowAnimation:UITableViewRowAnimationAutomatic];
            break;

        case NSFetchedResultsChangeUpdate:
            [tableView reloadRowsAtIndexPaths:@[indexPath] withRowAnimation:UITableViewRowAnimationAutomatic];
            break;

        case NSFetchedResultsChangeMove:
            [tableView deleteRowsAtIndexPaths:@[indexPath] withRowAnimation:UITableViewRowAnimationAutomatic];
            [tableView insertRowsAtIndexPaths:@[newIndexPath] withRowAnimation:UITableViewRowAnimationAutomatic];
            break;
    }
}

You should also implement the controller:didChangeSection:atIndex:forChangeType: delegate method.

And I do not understand what the

[super controller:controller didChangeObject:anObject atIndexPath:indexPath forChangeType:type newIndexPath:newIndexPath];

call is for!