NSFetchedResultsController: delete 1 row crashes the app

Question

Deleting a row from a UITableView fed by an NSFetchedResultsController causes my app to crash.

Error is:

* Assertion failure in -[UI

Answer 1

If you want to delete a row you need to delete managedObject only.

if (editingStyle == UITableViewCellEditingStyleDelete)
{
    [self.managedObjectContext deleteObject:[self.fetchedResultsController objectAtIndexPath:indexPath]];

    NSError *error = nil;
    if (![self.managedObjectContext save:&error]) {
        // handle error
    }
}

Deleting the managed object triggers the NSFetchResultController delegate methods, and they will update the tableView.

Edit

You should implement NSFetchResultController delegate method

- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath{
switch(type) {
 case NSFetchedResultsChangeDelete:
            [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;
}


////
   default:
   break;
    }

Because when you work with data source like NSFetchedResultsController, all changes must come from there and your table only reflects them.

Answer 2

Don't call

[self.tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];

if you use a fetched results controller. Only delete the object with

[self.managedObjectContext deleteObject:sd];

The fetched results controller delegate method didChangeObject: is then called automatically, and that calls deleteRowsAtIndexPaths:.

So in your case, the row was deleted twice, and that caused the exception.

Note that you don't need beginUpdates/endUpdates here.