ios-----队列监听

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, queue, ^{ /加载图片1 / });
dispatch_group_async(group, queue, ^{ /加载图片2 / });
dispatch_group_async(group, queue, ^{ /加载图片3 / });
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
// 子线程加载完毕，执行操作
});