判断ScrollView的上下滚动 动态改变view的大小

//背景view
- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView {
    contentOffsetY = scrollView.contentOffset.y;
}
- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
    newContentOffsetY = scrollView.contentOffset.y;
    if (newContentOffsetY > oldContentOffsetY && oldContentOffsetY > contentOffsetY) {
        // 向上滚动
        [self scrollToTop:scrollView];
    } else if (newContentOffsetY < oldContentOffsetY && oldContentOffsetY < contentOffsetY) {
        // 向下滚动
         [self scrollToBottom:scrollView];
    }
//如果不做这步判断，快速拖动时，会出现闪一下的bug
    if (scrollView.dragging) {
        if ((scrollView.contentOffset.y - contentOffsetY) > 0.f) {
            [self scrollToTop:scrollView];
        } else if ((contentOffsetY - scrollView.contentOffset.y) > 0.f) {
            [self scrollToBottom:scrollView];
        }
    }
}

- (void)scrollToBottom:(UIScrollView *)scrollView {
    // 向下滚动
    CGRect rect = self.bgView.frame;
    rect.size.height = (- scrollView.contentOffset.y);
    rect.origin.y = TOP_HEIGHT;
    self.bgView.frame = rect;
}

- (void)scrollToTop:(UIScrollView *)scrollView {
    // 向上滚动
    CGRect rect = self.bgView.frame;
    rect.size.height = (- scrollView.contentOffset.y);
    rect.origin.y = TOP_HEIGHT;
    self.bgView.frame = rect;
}
- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate {

    oldContentOffsetY = scrollView.contentOffset.y;
}