GCD

gistfile1.m

//创建一个下载队列
dispatch_queue_t downloadQueue = dispatch_queue_create("flickr downloader", NULL);

dispatch_async(downloadQueue, ^{
    //在这里做一些需要资源的任务
  
    //分发两个行队列，分别执行do_first_work,do_second_work
    dispatch_group_t group = dispatch_group_create();
    dispatch_group_async(group,dispatch_get_global_queue(0,0),^{
        do_frist_work();
    }
    dispatch_group_async(group,dispatch_get_global_queue(0,0),^{
        do_second_work();
    }
    
    //指定group全部完成的时候调用的block
    dispatch_group_notify(group,dispatch_get_global_queue(0,0),^{
          dispatch_async(dispatch_get_main_queue(), ^{
            //返回主线程,更新UI
        });
    }
});
dispatch_release(downloadQueue);