dispatch_sync

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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:

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.

The following approach:

I know where your confusion comes from:

As an optimization, this function invokes the block on the current thread when possible.

Careful, it says current thread.

Thread != Queue

A queue doesn‘t own a thread and a thread is not bound to a queue.

dispatch_sync

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原文地址：https://www.cnblogs.com/feng9exe/p/9174451.html