How to implement delayed/batched table view update from NSFetchedResultsControllerDelegate?

Question

The documentation says:

You should consider carefully whether you want to update the table view as each change is made. If a large number of modifications are made simultaneously—for example, if you are reading data from a background thread— /.../ you could just implement controllerDidChangeContent: (which is sent to the delegate when all pending changes have been processed) to reload the table view.

This is exactly what I'm doing: I'm processing incoming changes in a background thread with a different ManagedObjectContext, and merge the results into the main thread MOC with mergeChangesFromContextDidSaveNotification:. So far so good.

I chose to not implement controller:didChangeObject:... and would instead like to do the batched update that the document suggests.

Question/problem: the document doesn't elaborate how to actually implement the batched update? Should I just call [tableview reloadData] in controllerDidChangeContent: or is there a less intrusive way that saves me from a full reload?

One thought I have: I could take note of mergeChangesFrom... notification that contains the changed objects, figure out their indexpaths, and just call tableview:ReloadRowsAtIndexPaths: for them. But is there any authoritative info, recommendations or examples? Or just [tableview reloadData]?

(Aside: controller:didChangeObject:... started behaving really erratically when it received a set of batched updates, even though the same updating code [that I now put in background thread] was fine before when it was running on the main thread, but of course locking up the UI.)

Answer 1

I would just call reloadData in controllerDidChangeContent:.

For animating individual changes to the table, Apple's boiler plate code (iOS SDK 4.3.x) looks like this:

- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller
{
    [self.tableView beginUpdates];
}

- (void)controller:(NSFetchedResultsController *)controller didChangeSection:(id <NSFetchedResultsSectionInfo>)sectionInfo
           atIndex:(NSUInteger)sectionIndex forChangeType:(NSFetchedResultsChangeType)type
{
    switch(type)
    {
        case NSFetchedResultsChangeInsert:
            [self.tableView insertSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];
            break;

        case NSFetchedResultsChangeDelete:
            [self.tableView deleteSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];
            break;
    }
}

- (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:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;

        case NSFetchedResultsChangeDelete:
            [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;

        case NSFetchedResultsChangeUpdate:
            [self configureCell:[tableView cellForRowAtIndexPath:indexPath] atIndexPath:indexPath];
            break;

        case NSFetchedResultsChangeMove:
            [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
            [tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath]withRowAnimation:UITableViewRowAnimationFade];
            break;
    }
}

- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller
{
    [self.tableView endUpdates];
}