GCD线程控制

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);
                    //执行第一个任务
                    dispatch_group_leave(_group);
                });
                //等待上面请求完成之后再执行里面的步骤
                dispatch_group_notify(_group, queue, ^{
                    //执行第二个任务
     });
});