为什么在使用全局分派队列时不调用NSURLConnection委托方法?

When I do the following:

当我做以下事情时:

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, NULL), ^{
  create NSURLRequest;
  create NSURLConnectionDelegate;
  create NSURLConnection;
  start NSURLConnection;
});
dispatch