GCD信号量机制

1.创建信号量，创建的初始值决定线程并发数

dispatch_semaphore_t semaphore = dispatch_semaphore_create(2); 

2.等待信号

dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER); 

// 由于是异步执行的，所以每次循环Block里面的dispatch_semaphore_signal根本还没有执行就会执行dispatch_semaphore_wait，从而semaphore-1.当循环10此后，semaphore等于0，则会阻塞线程，直到执行了Block的dispatch_semaphore_signal 才会继续执行

3.发信号，信号量值会加1

dispatch_semaphore_signal(semaphore);

4.信号量可使多个异步线程同步执行

    dispatch_semaphore_t semaphore1 = dispatch_semaphore_create(0);

    dispatch_semaphore_t semaphore2 = dispatch_semaphore_create(0);

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

            NSLog(@"thread 2");

            sleep(2);

            dispatch_semaphore_signal(semaphore2);

        });

        dispatch_semaphore_wait(semaphore2, DISPATCH_TIME_FOREVER);

        NSLog(@"thread 1");

        sleep(2);

        dispatch_semaphore_signal(semaphore1);

    });

    dispatch_semaphore_wait(semaphore1, DISPATCH_TIME_FOREVER);

    NSLog(@"main thread");