NSFetchedResultsControllerDelegate incomprehensible behavior in GCD managed environment

Question

I have CoreData and table with related NSFetchedResultsController. Controller has context, which created in main queue and works readonly. Of course, tableviewcontroller implement NSFetchedResultsControllerDelegate protocol.

Take a look at on of the method, that it implements:

- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath {
    switch (type) {
        case NSFetchedResultsChangeInsert:
            NSLog(@"Inserted in %@", [NSString stringWithUTF8String:dispatch_queue_get_label(dispatch_get_current_queue())]);
            [_tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;

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

        case NSFetchedResultsChangeUpdate:
            NSLog(@"Updated in %@", [NSString stringWithUTF8String:dispatch_queue_get_label(dispatch_get_current_queue())]);
            [_tableView reloadRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;

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

In background I download data and update database in my application. Updating of database always the same. It is updated in method of my data manager:

- (void)saveDataInBackgroundInForeignContext:(void (^)(NSManagedObjectContext *))saveBlock completion:(void (^)(void))completion {
    AppDelegate *delegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
    dispatch_async([delegate.dispatcher queueForDataSavingInModel:self.modelName], ^{
        [self saveDataInForeignContext:saveBlock];

        dispatch_sync(dispatch_get_main_queue(), ^{
            completion();
        });
    });
}


- (void)saveDataInForeignContext:(void (^)(NSManagedObjectContext *))saveBlock {
    NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
    if (coordinator != nil) {

        NSManagedObjectContext *localContext = [[NSManagedObjectContext alloc] init];
        [localContext setPersistentStoreCoordinator:coordinator];

        [localContext setMergePolicy:NSMergeByPropertyObjectTrumpMergePolicy];
        [self.managedObjectContext setMergePolicy:NSMergeByPropertyStoreTrumpMergePolicy];
        [[NSNotificationCenter defaultCenter] addObserver:self.managedObjectContext
                                                 selector:@selector(mergeChangesFromContextDidSaveNotification:)
                                                     name:NSManagedObjectContextDidSaveNotification
                                                   object:localContext];

        saveBlock(localContext);

        if (localContext.hasChanges) {

            [self updateLastUpdateDateInConformedUpdatedObjects:localContext];

            NSError *error = nil;
            BOOL success = [localContext save:&error];
            if (!success) {
                NSLog(@"Saving in foreign context failed. %@", error.userInfo);
            }
        }

        [localContext release];
    }
}

in saveBlock I modify context depends on data from server. So, in results I have strange behavior:

Pay attention to NSLog in controller:didChangeObject:atIndexPath method in first listing. And lets take a look on logs:

2012-11-16 02:59:33.376 [27824:5303] Inserted in ru.idecide.saving.calls // WTF WHY?!
2012-11-16 03:05:56.219 [27824:c07] Updated in com.apple.main-thread

ru.idecide.saving.calls - queue of saving data.

This doesn't really matter, everything work, but method insertRowsAtIndexPaths have effect on UI in 2-3 seconds after calling on inserting and (obviously) immediately on updating. Why does it happen and what can I do to avoid it?

Answer 1

The problem is here:

    [[NSNotificationCenter defaultCenter] addObserver:self.managedObjectContext
                                             selector:@selector(mergeChangesFromContextDidSaveNotification:)
                                                 name:NSManagedObjectContextDidSaveNotification
                                               object:localContext];

You're linking your main-thread context directly to your background-queue context. When localContext, running on your background thread, posts a notification, the notification is delivered to its observers (self.managedObjectContext) on the same queue - the background queue.

You need transfer the notification to the main thread before delivering it to self.managedObjectContext. Give yourself a new method to receive the notification on the background queue and forward it to the main thread:

- (void)backgroundContextDidSave:(NSNotification *)note {
    dispatch_async(dispatch_get_main_queue(), ^{
        [self.managedObjectContext mergeChangesFromContextDidSaveNotification:note];
    });
}

Then use that method selector when you register for notifications:

    [[NSNotificationCenter defaultCenter] addObserver:self.managedObjectContext
                                             selector:@selector(backgroundContextDidSave:)
                                                 name:NSManagedObjectContextDidSaveNotification
                                               object:localContext];