dispatch_semaphore_create(1) 创建一个 semaphore 信号总量为 1

dispatch_semaphore_wait(lock, DISPATCH_TIME_FOREVER) 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，同时将信号总量 -1

dispatch_semaphore_signal(lock); 发送信号量，信号量 +1

使用时注意使用signal以确保编译器release掉dispatch_semaphore_t时的值与初始值一致， 否则会EXC_BAD_INSTRUCTION ,见http://is.gd/EaJgk5

  dispatch_semaphore_t lock = dispatch_semaphore_create(1);
    

    for (int i = 0; i < 10; i++)
    {
        dispatch_semaphore_wait(lock, DISPATCH_TIME_FOREVER);

        dispatch_async(dispatch_get_global_queue(0, 0), ^{
           

            NSLog(@"%d", i);
            
            dispatch_semaphore_signal(lock);

        });
    }

获取通讯录

        dispatch_semaphore_t sema = dispatch_semaphore_create(0);
        ABAddressBookRequestAccessWithCompletion(abRef, ^(bool granted, CFErrorRef error)
                                                 {
                                                     isOpen =  granted;
                                                     dispatch_semaphore_signal(sema);
                                                 });
        
        dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
        
        CFRelease(abRef);

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使用 Dispatch Semaphore 控制并发线程数量

 void dispatch_async_limit(dispatch_queue_t queue,NSUInteger limitSemaphoreCount, dispatch_block_t block) {
//控制并发数的信号量
    static dispatch_semaphore_t limitSemaphore;
   
    //专门控制并发等待的线程
    static dispatch_queue_t receiverQueue;
    
    //使用 dispatch_once而非 lazy 模式，防止可能的多线程抢占问题
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        limitSemaphore = dispatch_semaphore_create(limitSemaphoreCount);
        receiverQueue = dispatch_queue_create("receiver", DISPATCH_QUEUE_SERIAL);
    });
    
    // 如不加 receiverQueue 放在主线程会阻塞主线程
    dispatch_async(receiverQueue, ^{
        //可用信号量后才能继续，否则等待
        dispatch_semaphore_wait(limitSemaphore, DISPATCH_TIME_FOREVER);
        dispatch_async(queue, ^{
            !block ? : block();
            //在该工作线程执行完成后释放信号量
            dispatch_semaphore_signal(limitSemaphore);
        });
    });
}

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