tableView 每次滚动定值距离

#pragma mark - ScrollViewDelegate
- (void)scrollViewWillEndDragging:(UIScrollView *)scrollView withVelocity:(CGPoint)velocity targetContentOffset:(inout CGPoint *)targetContentOffset
{
    CGPoint point = [self nearestContentOffsetForOffset:*targetContentOffset];
    targetContentOffset->x = point.x;
    targetContentOffset->y = point.y;
}
- (CGPoint)nearestContentOffsetForOffset:(CGPoint)offset
{
    //每次滚动的距离   
    CGFloat pageSize = CellWidth/CellNumber;
    //四舍五入
    NSInteger page = round(offset.y / pageSize);
    CGFloat targetY = pageSize * page;
    return CGPointMake(offset.x,targetY);
}