NSOperationQueue *operationQueue = [[NSOperationQueue alloc] init];
    [operationQueue setMaxConcurrentOperationCount:0];
    for (int i = 0; i < 30; i++){
        NSOperation *operation = [NSBlockOperation blockOperationWithBlock:^{
            sleep(3);
            NSLog(@"operation finished: %d\n", i);
        }];
        [operation setQueuePriority:i];
        [operationQueue addOperation:operation];
    }
    [operationQueue setMaxConcurrentOperationCount:1];