为什么这个dispatch_sync（）调用冻结？ [英] Why is this dispatch_sync() call freezing?

问题描述

我使用Kiwi测试框架来测试我的应用程序中的身份验证方法。在调用dispatch_sync时，测试冻结，如下所示：

I'm using the Kiwi testing framework to test an authentication method in my app. The test freezes at a call to dispatch_sync which looks like this:

dispatch_queue_t main = dispatch_get_main_queue();
dispatch_sync(main, ^
                  {
                      [[NSNotificationCenter defaultCenter] postNotificationName:kNotificationAuthenticationSuccess object:nil userInfo:ret];
                  });

我想知道为什么它冻结在那里，如果有人有任何提示。

I'd like to know why it freezes there, if anyone has any hints.

推荐答案

有关冻结提示的第二部分：

For the second part of your question regarding the hint on the freeze:

当在队列上调用 dispatch_sync 时，始终验证此队列是否不是当前队列（ dispatch_get_current_queue code>）。因为 dispatch_sync 会将您的块排队作为第一个参数传递的队列，然后将等待此块执行，然后继续。

When calling dispatch_sync on a queue, always verify that this queue is not already the current queue (dispatch_get_current_queue()). Because dispatch_sync will queue your block on the queue passed as the first parameter, and then will wait for this block to be executed before continuing.

所以如果 dispatch_get_current_queue（）和队列在队列中是相同的，队列在你的情况下，主队列将阻塞调用dispatch_sync直到...主队列作为执行块，但它不能，因为队列被阻塞，和这里有一个美丽的死锁

So if the dispatch_get_current_queue() and the queue on which you enqueue your block are the same, namely the main queue in your case, the main queue will block on the call to dispatch_sync until… the main queue as executed the block, but it can't, as the queue is blocked, and you have a beautiful deadlock here.

一个解决方案（直到iOS6）：

One solution ( up until iOS6):

dispatch_queue_t main = dispatch_get_main_queue();
dispatch_block_t block = ^
              {
                  [[NSNotificationCenter defaultCenter] postNotificationName:kNotificationAuthenticationSuccess object:nil userInfo:ret];
              };
if (dispatch_get_current_queue() == main)
  block(); // execute the block directly, as main is already the current active queue
else
  dispatch_sync(main, block); // ask the main queue, which is not the current queue, to execute the block, and wait for it to be executed before continuing

---

注意， dispatch_get_current_queue（）只能用于调试目的，在生产。事实上，自iOS6.1.3起， dispatch_get_current_queue 已被弃用。

---

Be careful, dispatch_get_current_queue() is only to be used for debugging purposes and never in production. In fact, dispatch_get_current_queue is deprecated since iOS6.1.3.

如果您处于主要队列的特定情况（仅与主线程相关联），您可以改为测试 [NSThread isMainThread] （由@ meaning-matters建议）。

If you are in the specific case of the main queue (which is associated with the main thread only), you may instead test [NSThread isMainThread] as suggested by @meaning-matters.

顺便说一句，你确定需要 dispatch_sync 吗？
我想稍后发送通知，避免阻塞，直到它被发送，在你的情况下是可以接受的，所以你也可以考虑使用 dispatch_async 使用 dispatch_sync 并需要队列比较条件），这将避免死锁问题。

By the way, are you sure you need to dispatch_sync in your case? I guess sending your notification a bit later, avoiding to block until it has been sent, is acceptable in your case, so you may also consider using dispatch_async (instead of using dispatch_sync and needing the queue comparison condition), which would avoid the deadlock issue too.

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