多线程售卖火车票：使用 semaphore 加锁，保证线程安全

dispatch_semaphore_t semaphoreLock;
@property(nonatomic, assign)NSInteger ticketSurplusCount;

/**
 * 线程安全：使用 semaphore 加锁
 * 初始化火车票数量、卖票窗口(线程安全)、并开始卖票
 */
- (void)initTicketStatusSave {
    //打印当前线程
    NSLog(@"currentThread---%@",[NSThread currentThread]);
    NSLog(@"semaphore---begin");
    semaphoreLock = dispatch_semaphore_create(1);
    self.ticketSurplusCount = 50;
    // queue1 代表北京火车票售卖窗口
    dispatch_queue_t queue1 = dispatch_queue_create("net.bujige.testQueue1", DISPATCH_QUEUE_SERIAL);
    // queue2 代表上海火车票售卖窗口
    dispatch_queue_t queue2 = dispatch_queue_create("net.bujige.testQueue2", DISPATCH_QUEUE_SERIAL);
    // queue3 代表深圳火车票售卖窗口
    dispatch_queue_t queue3 = dispatch_queue_create("net.bujige.testQueue3", DISPATCH_QUEUE_SERIAL);
    //queue4 代表广州火车票售卖窗口
    dispatch_queue_t queue4 = dispatch_queue_create("net.bujige.testQueue4", DISPATCH_QUEUE_SERIAL);
    // queue5 代表天津火车票售卖窗口
    dispatch_queue_t queue5 = dispatch_queue_create("net.bujige.testQueue5", DISPATCH_QUEUE_SERIAL);
    __weak typeof(self) weakSelf = self;
    dispatch_async(queue1, ^{
        NSLog(@"北京火车票售卖窗口");
        [weakSelf saleTicketSafe];
    });
    dispatch_async(queue2, ^{
        NSLog(@"上海火车票售卖窗口");
        [weakSelf saleTicketSafe];
    });
    dispatch_async(queue3, ^{
        NSLog(@"深圳火车票售卖窗口");
        [weakSelf saleTicketSafe];
    });
    dispatch_async(queue4, ^{
        NSLog(@"广州火车票售卖窗口");
        [weakSelf saleTicketSafe];
    });
    dispatch_async(queue5, ^{
        NSLog(@"天津火车票售卖窗口");
        [weakSelf saleTicketSafe];
    });
}
 
/**
 * 售卖火车票(线程安全)
 */
- (void)saleTicketSafe {
    while (1) {
        //相当于加锁
        dispatch_semaphore_wait(semaphoreLock, DISPATCH_TIME_FOREVER);
        if (self.ticketSurplusCount > 0) {  //如果还有票，继续售卖
            self.ticketSurplusCount--;
            NSLog(@"%@", [NSString stringWithFormat:@"剩余票数：%ld 窗口：%@", (long)self.ticketSurplusCount, [NSThread currentThread]]);
            [NSThread sleepForTimeInterval:0.2];
        } 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(semaphoreLock);
            break;
        }
        // 相当于解锁
        dispatch_semaphore_signal(semaphoreLock);
    }
}
输出结果：
2020-07-06 16:59:37.783410+0800 GCD[3557:1214746] 剩余票数：6 窗口：<NSThread: 0x283442ec0>{number = 9, name = (null)}
2020-07-06 16:59:37.990147+0800 GCD[3557:1214742] 剩余票数：5 窗口：<NSThread: 0x28345ec80>{number = 5, name = (null)}
2020-07-06 16:59:38.195653+0800 GCD[3557:1214741] 剩余票数：4 窗口：<NSThread: 0x283459c00>{number = 4, name = (null)}
2020-07-06 16:59:38.399338+0800 GCD[3557:1214745] 剩余票数：3 窗口：<NSThread: 0x28345eec0>{number = 3, name = (null)}
2020-07-06 16:59:38.605101+0800 GCD[3557:1214740] 剩余票数：2 窗口：<NSThread: 0x2834422c0>{number = 8, name = (null)}
2020-07-06 16:59:38.808133+0800 GCD[3557:1214746] 剩余票数：1 窗口：<NSThread: 0x283442ec0>{number = 9, name = (null)}
2020-07-06 16:59:39.009541+0800 GCD[3557:1214742] 剩余票数：0 窗口：<NSThread: 0x28345ec80>{number = 5, name = (null)}
2020-07-06 16:59:39.215021+0800 GCD[3557:1214741] 所有火车票均已售完
2020-07-06 16:59:39.215652+0800 GCD[3557:1214745] 所有火车票均已售完
2020-07-06 16:59:39.216028+0800 GCD[3557:1214740] 所有火车票均已售完
2020-07-06 16:59:39.216446+0800 GCD[3557:1214746] 所有火车票均已售完
2020-07-06 16:59:39.216697+0800 GCD[3557:1214742] 所有火车票均已售完