今天由于项目有这个需求，所以mark下这段代码！

int _lastPosition;- (void)scrollViewDidScroll:(UIScrollView *)scrollView{      int currentPostion = scrollView.contentOffset.y;      if (currentPostion - _lastPosition > 25) {          _lastPosition = currentPostion;          NSLog(@"ScrollDown UP");         // [self.navigationController setNavigationBarHidden:NO animated:YES];    }      else if (_lastPosition - currentPostion > 25)      {          _lastPosition = currentPostion;          NSLog(@"ScrollDown now");      }  }