在等待超时时发送信号量信号

Question

当dispatch_semaphore_wait运行到超时时，它会自动发出信号（增加计数），还是手动完成？

Answer 1

dispatch_semaphore_wait()递减计数信号量，并等待结果值小于零。 如果发生超时，则此减量会反转，因此您无需手动调整计数。

对于我来说，这对文档来说并不明显，但与负数一致表明线程正在等待信号量的事实一致。 另请参阅源代码中的此注释：

// If the internal value is negative, then the absolute of the value is
// equal to the number of waiting threads. ...

您也可以通过打印信号量的debugDescription来验证它，输出显示当前值：

let sem = dispatch_semaphore_create(0)

NSLog("%@", sem.debugDescription)
// <OS_dispatch_semaphore: semaphore[0x100514a70] = { ..., value = 0, orig = 0 }>
// --> Initial value is 0

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, Int64(NSEC_PER_SEC)),
    dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) {
        NSLog("%@", sem.debugDescription)
        // <OS_dispatch_semaphore: semaphore[0x100514a70] = { ..., value = -1, orig = 0 }>
        // --> One thread is waiting, value is -1.
}

let ret = dispatch_semaphore_wait(sem, dispatch_time(DISPATCH_TIME_NOW, 2*Int64(NSEC_PER_SEC)))
NSLog("%@", sem.debugDescription)
// <OS_dispatch_semaphore: semaphore[0x100514a70] = { ..., value = 0, orig = 0 }>
// --> Time out, value is 0 again.