GCD-semaphore

主要应用：

1. 保持线程同步
2. 为线程加锁

保持线程同步

/**
 * semaphore 线程同步
 */
- (void)semaphoreSync {
    NSLog(@"currentThread---%@",[NSThread currentThread]);  // 打印当前线程
    NSLog(@"semaphore---begin");
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
    
    __block int number = 0;
    dispatch_async(queue, ^{
        // 追加任务1
        [NSThread sleepForTimeInterval:2];              // 模拟耗时操作
        NSLog(@"1---%@",[NSThread currentThread]);      // 打印当前线程
        number = 100;
        dispatch_semaphore_signal(semaphore);
    });
    
    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
    NSLog(@"semaphore---end,number = %zd",number);
}

为线程加锁

/**
 * 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 {
    NSLog(@"currentThread---%@",[NSThread currentThread]);  // 打印当前线程
    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);
    __weak typeof(self) weakSelf = self;
    dispatch_async(queue1, ^{
        [weakSelf saleTicketSafe];
    });
    dispatch_async(queue2, ^{
        [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：%d 窗口：%@", 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);
    }
}