How to execute tasks in separate threads in Objective C

This code snippet shows how you can execute tasks  in two separate threads:

    dispatch_queue_t download_queue = dispatch_queue_create("Home View Controller", NULL);

    //first thread

    dispatch_async(download_queue, ^{               

        NSError *error = nil;

        NSArray *array = [[NetworkService getThingsByAccountId:accountId error:&error] positions];        

        //second thread, we use the main thread to run ui related activities

        dispatch_async(dispatch_get_main_queue(),^{

            [spinner stopAnimating];

                ViewController *controller = [[ViewController alloc] initWithNibName:@"ViewController" array:array];

                [self.navigationController pushViewController:controller animated:YES];

                [controller release];                    

        });

    });

    dispatch_release(download_queue);