self.scrollView.delegate = self;


- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
   CGPoint offset = scrollView.contentOffset;
   if (offset.y < 100.f) {
      self.navigationController.navigationBar.alpha = 1.f - offset.y / 100.f;
      self.navigationItem.leftBarButtonItem = ...;
      self.navigationItem.rightBarButtonItem = ...;
   }
   else {
      self.navigationController.navigationBar.alpha = MIN(1.f, (offset.y - 100.f) / 100.f);
      self.navigationItem.leftBarButtonItem = ...;
      self.navigationItem.rightBarButtonItem = ...;
  }
}

就是一个判断滚动条位置，然后元素背景过度！去研究css3过度属性

应该是组合模式 先透明度 最后换图片