GCD 信号量 dispatch_semaphore_t

1、GCD知识讲解

   1）dispatch_semaphore_create(long value) //创建一个信号量，总量为value，value不能小于0

   2）dispatch_semaphore_wait(dispatch_semaphore_t dsema, dispatch_time_t timeout) //如果信号总量为0，进入等待状态，信号量大于0时，继续执行代码，同时将         信号量减1

   3）dispatch_semaphore_signal(dispatch_semaphore_t desma) //发送信号量，信号量就会加1

2、例子

dispatch_semaphore_t lock = dispatch_semaphore_create(1);
        
        for(int i=0;i<9;i++){
            dispatch_semaphore_wait(lock, DISPATCH_TIME_FOREVER);
            dispatch_async(dispatch_get_global_queue(0, 0), ^{
                NSLog(@"我是第%d进入",i);
                dispatch_semaphore_signal(lock);
            });
        }