gistfile1.m

        self.backgroundManagedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
        [self.backgroundManagedObjectContext setPersistentStoreCoordinator:persistentStoreCoordinator];
        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(backgroundContextDidSave:) name:NSManagedObjectContextDidSaveNotification object:self.backgroundManagedObjectContext];


- (void)backgroundContextDidSave:(NSNotification *)didSaveNotification {
    [self.managedObjectContext performBlock:^{
        NSDate *start = [NSDate date];
        [self.managedObjectContext mergeChangesFromContextDidSaveNotification:didSaveNotification];
        DLog(@"Merging background changes took %f", [[NSDate date] timeIntervalSinceDate:start]);
    }];
}