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");