NSFetchedResultsController trying to limit number of records displayed

Question

When creating a NSFetchRequest to pass over to my NSFetchedResultsController I am setting the fetchLimit property to 3.

Now initially this appears to work fine. I can modify the first three returned objects in any way which changes their order and they all reshuffle correctly. The problem appears when I either alter a object which initially fell outside the first three in a way which now brings it into the first three, or when simply adding a new object so it will appear within the first three.

What I expected to happen: Inserted object pushes the rest down and one drops off the bottom.

What actually happens: Inserted object pushes the rest down and the record count grows to 4?!

Can anyone explain this, or how I should deal with this?

Answer 1

I have made some progress working around this, basically by ignoring numberOfObjects and returning the actual length I want the table fixed at. This takes a bit of trickery in controller:didChangeObject:... but seems to be working so far.

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    return kTableSize;
    //return [[[self.fetchedResultsController sections] objectAtIndex:section] numberOfObjects];
}

- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath
{
    UITableView *tableView = self.myTableView;

    switch(type) {

        case NSFetchedResultsChangeInsert:

            // Only modify table if insert will effect visible rows
            if (newIndexPath.row < kTableSize) {
                // Delete last row to maintain fixed length
                [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:[NSIndexPath indexPathForRow:(kTableSize - 1) inSection:newIndexPath.section]] withRowAnimation:UITableViewRowAnimationAutomatic];

                [tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationAutomatic];
            }
            break;

        case NSFetchedResultsChangeDelete:

            // Only modify table if delete will effect visible rows
            if (indexPath.row < kTableSize) {
                [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationAutomatic];

                // Insert extra row to maintain fixed length
                [tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:[NSIndexPath indexPathForRow:(kTableSize - 1) inSection:newIndexPath.section]] withRowAnimation:UITableViewRowAnimationAutomatic];
            }
            break;

        case NSFetchedResultsChangeUpdate:

            // Only modify table if update will effect visible rows
            if (indexPath.row < kTableSize) {
                [tableView reloadRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationAutomatic];
            }
            break;

        case NSFetchedResultsChangeMove:

            // Only modify table if move will effect visible rows
            if ((indexPath.row < kTableSize) || (newIndexPath.row < kTableSize)) {


                // Delete old row or last row of table
                if (indexPath.row < kTableSize) {
                    [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationAutomatic];
                } else {
                    [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:[NSIndexPath indexPathForRow:(kTableSize - 1) inSection:newIndexPath.section]] withRowAnimation:UITableViewRowAnimationAutomatic];
                }

                // Insert new row or a row at bottom of table
                if (newIndexPath.row < kTableSize) {
                    [tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationAutomatic];
                } else {
                    [tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:[NSIndexPath indexPathForRow:(kTableSize - 1) inSection:newIndexPath.section]] withRowAnimation:UITableViewRowAnimationAutomatic];
                }
            }
            break;
    }
}

Also need to take care in tableView:cellForRowAtIndexPath: to make sure we don't try and access an object which doesn't exist if there are less objects than the table length.