向下滑动会有跳动现象，需修改一下OFFSET，同时数量少于一屏时要返回

[bestoml]

• (void)resetContentOffsetIfNeeded {
  CGPoint contentOffset = self.contentOffset;

  if(self.contentSize.height <= self.bounds.size.height) {
  return;
  }
  float bottomEdge = (self.contentSize.height - self.bounds.size.height);
  //scroll over top
  if (contentOffset.y < 0.0) {
  float ori = contentOffset.y;
  contentOffset.y = self.contentSize.height / 3.0 + contentOffset.y;
  NSLog(@"content y offset %f:%f",ori,contentOffset.y);
  }
  //scroll over bottom
  else if (contentOffset.y >= bottomEdge) {
  float ori = contentOffset.y;
  float offsetOver = contentOffset.y - bottomEdge;
  contentOffset.y = (self.contentSize.height / 3.0)*2 - self.bounds.size.height +offsetOver;
  NSLog(@"content y offset %f:%f bounds:%f",ori,contentOffset.y,self.bounds.size.height);
  }
  [self setContentOffset: contentOffset];
  }