在不耗时的情况下 A事件 B事件 完成后去执行C

    dispatch_group_t group = dispatch_group_create();
    dispatch_group_async(group, dispatch_get_global_queue(0,0), ^{
        // 并行执行的线程一
        [self upStudentMessage:self.studentMode];
        NSLog(@"11111111 %d",self.IsUpSussecc);

    });
    dispatch_group_async(group, dispatch_get_global_queue(0,0), ^{
        // 并行执行的线程二
        [self upStudentPicture:self.studentMode];
        NSLog(@"22222222 %d",self.IsUpSussecc);


    });
    dispatch_group_notify(group, dispatch_get_global_queue(0,0), ^{
        // 汇总结果
        NSLog(@"3333333 %d",self.IsUpSussecc);

        if (self.IsUpSussecc ) {

            dispatch_async(self.DB_queue, ^{

                NSLog(@"实时上传成功 更新学生状态");
                [self updateStudent:self.studentMode StudentState:0]; //  上传成功 更新学生状态
            });

        }else{

            dispatch_async(self.DB_queue, ^{

                NSLog(@"实时上传失败 更新学生状态");
                [self updateStudent:self.studentMode StudentState:1]; // 上传失败 更新学生状态
            });

        }
    });