define ImageHeight 197

if !defined(ABS) #define NSABS_IMPL(A,L) ({ typeof(A) NSX_PASTE(__a,L) = (A); (NSX_PASTE(__a,L) < 0) ? - NSX_PASTE(__a,L) : NSX_PASTE(__a,L); }) #define ABS(A) NSABS_IMPL(A,COUNTER)

endif

//主窗口的宽、高#define kMainScreenWidth MainScreenWidth()

define kMainScreenHeight MainScreenHeight()

• (void)updateImgFrame{
  CGFloat yOffset = self.tableView.contentOffset.y;
  if (yOffset < 0) { //向下滑动，图片高度增加相同的高度，宽度按比例增加，ABS函数返回一个数的绝对值
  CGFloat factor = ((ABS(yOffset)+ImageHeight) * kMainScreenWidth)/ImageHeight;
  //-(factor-ImageWidth)/2 使变宽的图片居中显示
  CGRect f = CGRectMake(-(factor-kMainScreenWidth)/2, 0, factor, ImageHeight+ABS(yOffset));
  self.headBgImageView.frame = f;
  }else { //向上滑动，图片只需跟随向上即可
  CGRect f = self.headBgImageView.frame;
  f.origin.y = - yOffset;
  self.headBgImageView.frame = f;
  }
  }