从并发队列调用 dispatch_sync - 它会完全阻塞吗？
Calling dispatch_sync from a concurrent queue - does it block entirely?
假设我从并发队列中调用 dispatch_sync - 它会阻塞整个队列还是只阻塞那个执行线程？
dispatch_sync 将阻塞调用者线程直到执行完成，并发队列有多个线程，因此它只会阻塞该队列中的一个线程，其他线程仍将执行。
Apple 对此是这样说的：
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.
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.
As an optimization, this function invokes the block on the current
thread when possible.
假设我从并发队列中调用 dispatch_sync - 它会阻塞整个队列还是只阻塞那个执行线程？
dispatch_sync 将阻塞调用者线程直到执行完成，并发队列有多个线程，因此它只会阻塞该队列中的一个线程，其他线程仍将执行。
Apple 对此是这样说的：
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.
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.
As an optimization, this function invokes the block on the current thread when possible.