平滑的下拉加载更多数据

- (void)setContentSize:(CGSize)contentSize
{
    if (!CGSizeEqualToSize(self.contentSize, CGSizeZero))
    {
        if (contentSize.height > self.contentSize.height)
        {
            CGPoint offset = self.contentOffset;
            offset.y += (contentSize.height - self.contentSize.height);
            self.contentOffset = offset;
        }
    }
    [super setContentSize:contentSize];
}