#pragma mark <UIScrollViewDelegate>
-(void)scrollViewDidScroll:(UIScrollView *)scrollView{
    self.naviView.alpha = scrollView.contentOffset.y/400;
    
// change the position of section header when scrolled by changing scroll's contentInset
    CGFloat sectionHeaderHeight = 64;
    CGFloat ceilPositon = CGRectGetHeight(_subTitleView.frame)-sectionHeaderHeight;
    if (scrollView.contentOffset.y < sectionHeaderHeight){
        scrollView.contentInset = UIEdgeInsetsMake(0, 0, 0, 0);
    }
    // the "scrollView.contentOffset.y" must larger than its position to ceiling
    if (scrollView.contentOffset.y>=ceilPositon && scrollView.contentOffset.y>=sectionHeaderHeight) {
        scrollView.contentInset = UIEdgeInsetsMake(sectionHeaderHeight, 0, 0, 0);
    }
}

http://blog.csdn.net/axllll/article/details/50502475