线程处理

    dispatch_group_t group = dispatch_group_create();

    dispatch_group_async(group, dispatch_get_global_queue(0,0), ^{

        //并行执行的线程1

    });

    dispatch_group_async(group, dispatch_get_global_queue(0,0), ^{

        //并行执行的线程2

    });

    dispatch_group_notify(group, dispatch_get_global_queue(0,0), ^{

        //执行完线程1以及线程2 汇总结果

    });