iOS系统相册中查看图片，UIScrollView左右翻页时顶端

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
    //停止滚动时调用一次
}

- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
    //不停地调用，此处四舍五入，可以保证滑动中间的时候数字再变
    int index = roundf(scrollView.contentOffsetX/kScreenWidth);//四舍五入
    NSString *title = [NSString stringWithFormat:@"%ld/%ld", index +1, self.images.count];
}