GCD執行緒控制

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(queue, ^{
     dispatch_group_t _group = dispatch_group_create();
     dispatch_group_async(_group, queue, ^{
          dispatch_group_enter(_group);
                    //執行第一個任務
                    dispatch_group_leave(_group);
                });
                //等待上面請求完成之後再執行裡面的步驟
                dispatch_group_notify(_group, queue, ^{
                    //執行第二個任務
     });
});