在当前队列上同步分派 [英] Sync dispatch on current queue

问题描述

我知道你可能会发现这是一个奇怪的问题，但我只是在学习GCD，我想完全了解它的所有方面。所以这里是：

I know you might find this an odd question, but I'm just learning GCD and I want to fully understand all its aspects. So here it is:

是否有任何理由在CURRENT QUEUE上分派任务SYNC？

Is there ever any reason to dispatch a task SYNC on the CURRENT QUEUE?

例如：

    dispatch_queue_t concurrentQueue = dispatch_get_global_queue(...);
    dispatch_async(concurrentQueue, ^{
       //this is work task 0

       //first do something here, then suddenly:

       dispatch_sync(concurrentQueue, ^{
               //work task 1
       });

       //continue work task 0
    });

我理解一件事：if代替 concurrentQueue 我使用 serial 队列，然后我在该串行队列上得到死锁，因为工作任务1 >工作任务0 完成（由于保证执行顺序的串行队列），并且同时工作任务0 无法继续执行，因为它等待SYNC dispath函数返回（请纠正我，如果我错了，这将使我一个总的noob）。

I understand one thing: if instead of concurrentQueue I use a serial queue, then I get a deadlock on that serial queue, because work task 1 cannot start until the work task 0 is finished (because of the serial queue that guarantees order of execution), and in the same time work task 0 cannot continue its execution because it waits for the SYNC dispath function to return (please correct me if I'm wrong, that would make me a total noob).

回到原始思想，上述代码和同一代码之间有什么区别，而不是调用 dispatch_sync 函数，我只需写工作任务1 代码直接？

So coming back to the original idea, is there any difference whatsoever between the code above and the same code where instead of calling the dispatch_sync function I simply write work task 1 code directly?

推荐答案

否。我不能想到在你已经在同一并发队列上 dispatch_sync 的原因。如果你这样做，GCD将立即调用你的块，在线，在同一个线程，就好像你直接调用。 （我检查过。）正如你所指出的，在一个串行队列上这样做会使你死锁。

No. I can't think of a reason to ever dispatch_sync on the same concurrent queue you're already on. If you do that, GCD will just immediately invoke your block, in-line, on the same thread, as if you had called it directly. (I checked.) And as you pointed out, doing that on a serial queue will deadlock you.

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