IOS学习笔记之十二：Dispatch_sync死锁

Dispatch_sync(dispatch_get_main_queue,^{})死锁的问题

dispatch_sync的意思像上边解释的一样，就是向特定的queue中插入一个block，等到这个block被执行了，再继续下面的任务，但是如果这个被安排任务的queue是currentQueue,那么将会成为死锁，因为sync会block currentqueue所以现在线程被block了，所以不会再继续执行下去。下面是搜到的英文解释。

dispatch_sync does two things:

1. queue a block
2. blocks the current thread until the block has finished running

Given that the main thread is a serial queue (which means it uses only one thread), the following statement:

dispatch_sync(dispatch_get_main_queue(), ^(){/*...*/});

will cause the following events:

1. dispatch_sync queues the block in the main queue.
2. dispatch_sync blocks the thread of the main queue until the block finishes executing.
3. dispatch_sync waits forever because the thread where the block is supposed to run is blocked.

The key to understanding this is that dispatch_sync does not execute blocks, it only queues them. Execution will happen on a future iteration of the run loop.

还有，之所以要在mainqueue中去刷新界面，是因为有可能很多线程在后台运行，而UI的刷新永远是在主线程中去完成，所以UI的刷新如果被放到了其他的后台线程中，那么刷新就无法执行，所以必须要放在主线程中去做。