dispatch_sync
Submits a block object for execution on a dispatch queue and waits until that block completes.
提交块对象以在调度队列上执行，并等待该块完成。
Declaration
void dispatch_sync(dispatch_queue_t queue, dispatch_block_t block);
Parameters
The queue on which to submit the block. This parameter cannot be NULL.
提交块的队列。
此参数不能为NULL。
The block to be invoked on the target dispatch queue. This parameter cannot be NULL.
要在目标调度队列上调用的块。
此参数不能为NULL。
Discussion
Submits a block to a dispatch queue for synchronous execution. Unlike dispatch_async, this function does not return until the block has finished. Calling this function and targeting the current queue results in deadlock.
将块提交到调度队列以进行同步执行。 与dispatch_async不同，此功能在块完成之前不会返回。 调用此函数并以当前队列为目标队列会导致死锁。
Unlike with dispatch_async, no retain is performed on the target queue. Because calls to this function are synchronous, it "borrows" the reference of the caller. Moreover, no Block_copy is performed on the block.
与dispatch_async不同，不对目标队列执行持有。 因为调用此函数是同步的，所以它“借用”调用者的引用。 此外，不对块执行Block_copy。
As an optimization, this function invokes the block on the current thread when possible.
作为优化，可能的话此函数会在当前线程上调用块.
