* contentSize: The size of the content view. 其实就是scrollview可以滚动的区域，表示内容视图，比如frame = (0 ,0 ,320 ,480) contentSize = (320 ,960)，代表你的scrollview可以上下滚动，滚动区域为frame大小的两倍。
* contentOffset:The point at which the origin of the content view is offset from the origin of the scroll view. 是scrollview当前显示区域顶点相对于frame顶点的偏移量，比如上个例子你拉到最下面，contentoffset就是(0 ,480)，也就是y偏移了480
* contentInset:The distance that the content view is inset from the enclosing scroll view.是scrollview的contentview的顶点相对于scrollview的位置，例如你的contentInset = (0 ,100)，那么你的contentview就是从scrollview的(0 ,100)开始显示
//上拉加载更多
- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate{
[_refreshHeaderView egoRefreshScrollViewDidEndDragging:scrollView];
float offset=scrollView.contentOffset.y;
float contentHeight=scrollView.contentSize.height;
float sub=contentHeight-offset;
if ((scrollView.height-sub)>20) {//如果上拉距离超过20p，则加载更多数据
//[self loadMoreData];//此处在view底部加载更多数据
}
}
