iOS Dispatch_sync 阻塞线程的原因

大家的知道在主队列上使用dispatch_sync()，

- (void)testSyncMainThread {

    dispatch_queue_t main = dispatch_get_main_queue();
    NSLog(@"*********1");
    dispatch_sync(main, ^(){
            NSLog(@"*********2");
    });
    NSLog(@"*********3");
}

会造成线程阻塞，但是查找网上说的原因，发现基本上说法都是一样的，感觉不是不太好理解，

我查看一下API,

/*!

* @function dispatch_sync

*

* @abstract

* Submits a block for synchronous execution on a dispatch queue.

*

* @discussion

* Submits a workitem to a dispatch queue like dispatch_async(), however

* dispatch_sync() will not return until the workitem has finished.

*

* Work items submitted to a queue with dispatch_sync() do not observe certain

* queue attributes of that queue when invoked (such as autorelease frequency

* and QOS class).

*

* Calls to dispatch_sync() targeting the current queue will result

* in dead-lock. Use of dispatch_sync() is also subject to the same

* multi-party dead-lock problems that may result from the use of a mutex.

* Use of dispatch_async() is preferred.

*

* Unlike dispatch_async(), no retain is performed on the target queue. Because

* calls to this function are synchronous, the dispatch_sync() "borrows" the

* reference of the caller.

*

* As an optimization, dispatch_sync() invokes the workitem on the thread which

* submitted the workitem, except when the passed queue is the main queue or

* a queue targetting it (See dispatch_queue_main_t,

* dispatch_set_target_queue()).

*

* @param queue

* The target dispatch queue to which the block is submitted.

* The result of passing NULL in this parameter is undefined.

*

* @param block

* The block to be invoked on the target dispatch queue.

* The result of passing NULL in this parameter is undefined.

*/

结合API，我的理解：

第一步：当代码执行到 dispatch_sync()时，会首先把dispatch_sync()函数放到当前线程的队列（就是主队列中），而且这个线程并不会立即返回，而是会等待block中的任务都完成时才会返回，

第二步：再把block中的任务依次放到主队列中等待执行，

由于是同步执行，而且block中的任务在主队列的线性表中的前面有个dispatch_sync()等待完成，而dispatch_sync()的完成又依赖于block的执行完成，就这样造成了死锁。

最后：在其他队列上模拟这种死锁情况

 dispatch_queue_t lwMainQueue = dispatch_queue_create("lw.queue", DISPATCH_QUEUE_SERIAL);
    dispatch_sync(lwMainQueue, ^{
        NSLog(@"*********1");
        dispatch_sync(lwMainQueue, ^{NSLog(@"*********2");
        });
    NSLog(@"*********3");
    });

效果是一样的