关于dispatch_semaphore_t的浅见

- (void)semaphoreaTest {
    NSLog(@"semaphore---begin");
    
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
    
    dispatch_async(queue, ^{
        [NSThread sleepForTimeInterval:2];
        dispatch_semaphore_signal(semaphore);
    });
    
    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
    
    NSLog(@"semaphore---end");
}

关于dispatch_semaphore_t，网上的文章讲的都是

image.png image.png

dispatch_semaphore_wait先会使semaphore count - 1，如果 - 1后的semaphore count >= 0，则放行，不然则阻塞当前线程，而dispatch_semaphore_signal只起到了semaphore count + 1的作用。

事实是这样吗，dispatch_semaphore_signal官方文档解释是：

Increment the counting semaphore. If the previous value was less than zero, this function wakes a thread currently waiting in dispatch_semaphore_wait

意思是dispatch_semaphore_signal会先让semaphore count + 1，如果+1前的semaphore count是 < 0的，则直接唤醒正在dispatch_semaphore_wait中等待的线程，所以我理解的是

dispatch_semaphore_wait只是起到了通过semaphore count判断是否需要阻塞当前线程并记录当前的线程的作用，当dispatch_semaphore_signal需要唤醒线程时，会根据dispatch_semaphore_wait记录的线程进行唤醒。

再加一个案例解释，火车票线程安全访问问题：

- (void)semaphoreThreadSafe {
    [self logCurrentThreadInfo:nil];
    NSLog(@"semaphore---begin");
    
    semaphore = dispatch_semaphore_create(1);
    ticketSurplusCount = 50;
    
    //queue1 1号窗口
    dispatch_queue_t queue1 = dispatch_queue_create("one.window.queue", DISPATCH_QUEUE_SERIAL);
    
    //queue2 2号窗口
    dispatch_queue_t queue2 = dispatch_queue_create("two.window.queue", DISPATCH_QUEUE_SERIAL);
    
    dispatch_async(queue1, ^{
        [self saleTicketSafe];
    });
    dispatch_async(queue2, ^{
        [self saleTicketSafe];
    });
}

- (void)saleTicketSafe {
    while (1) {
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        if (ticketSurplusCount > 0) {  // 如果还有票，继续售卖
            ticketSurplusCount--;
            NSLog(@"%@", [NSString stringWithFormat:@"剩余票数：%ld 窗口：%@", ticketSurplusCount, [NSThread currentThread]]);
            [NSThread sleepForTimeInterval:0.2];
            dispatch_semaphore_signal(semaphore);
        } else { // 如果已卖完，关闭售票窗口
            NSLog(@"所有火车票均已售完");
            
            // 相当于解锁
            dispatch_semaphore_signal(semaphore);
            break;
        }
    }
}

线程1：1号窗口所在的线程
线程2：2号窗口所在线程
semaphore count：信号量计数
● 线程1 访问 ticketSurplusCount，通过dispatch_semaphore_wait将semaphore count - 1，此时semaphore count = 0，不需要阻塞线程1；
● 线程1 还没对ticketSurplusCount访问完成时，线程2 突然也要访问，经过dispatch_semaphore_wait时，semaphore count - 1，此时semaphore count = -1，需要阻塞 线程2，线程2变为阻塞(等待)状态；
● 此时线程1对ticketSurplusCount访问完成，调用dispatch_semaphore_signal使semaphore count + 1，现在的semaphore count = 0，然后由于semaphore count在 + 1前是 < 0 的，所以需要唤醒线程2
● 线程2现在可以正常访问了