dispatch_semaphore_create(1) 創(chuàng)建一個 semaphore 信號總量為 1

dispatch_semaphore_wait(lock, DISPATCH_TIME_FOREVER) 如果信號總量為0，進(jìn)入等待狀態(tài)珊皿，信號量大于0時歼捐，繼續(xù)執(zhí)行代碼，同時將信號總量 -1

dispatch_semaphore_signal(lock); 發(fā)送信號量币喧，信號量 +1

使用時注意使用signal以確保編譯器release掉dispatch_semaphore_t時的值與初始值一致， 否則會EXC_BAD_INSTRUCTION ,見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);

        });
    }

獲取通訊錄

        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);

?

使用 Dispatch Semaphore 控制并發(fā)線程數(shù)量

 void dispatch_async_limit(dispatch_queue_t queue,NSUInteger limitSemaphoreCount, dispatch_block_t block) {
//控制并發(fā)數(shù)的信號量
    static dispatch_semaphore_t limitSemaphore;
   
    //專門控制并發(fā)等待的線程
    static dispatch_queue_t receiverQueue;
    
    //使用 dispatch_once而非 lazy 模式，防止可能的多線程搶占問題
    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 放在主線程會阻塞主線程
    dispatch_async(receiverQueue, ^{
        //可用信號量后才能繼續(xù)，否則等待
        dispatch_semaphore_wait(limitSemaphore, DISPATCH_TIME_FOREVER);
        dispatch_async(queue, ^{
            !block ? : block();
            //在該工作線程執(zhí)行完成后釋放信號量
            dispatch_semaphore_signal(limitSemaphore);
        });
    });
}

??????
?????????????