异步汇合

 //多个异步请求完成后汇总结结果（任务执行不区分先后顺序）

    dispatch_queue_t queue = dispatch_get_global_queue(0,0);

    dispatch_group_t group = dispatch_group_create();

    dispatch_group_async(group, queue, ^{/*任务a */ });

    dispatch_group_async(group, queue, ^{/*任务b */ });

    dispatch_group_async(group, queue, ^{/*任务c */ });

    dispatch_group_async(group, queue, ^{/*任务d */ });

    dispatch_group_notify(group,dispatch_get_main_queue(), ^{

       // 在a、b、c、d异步执行完成后，会回调这里

    });