UICollectionView下拉放大顶部图片

记录一下 下次直接用 不用再算了

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
    CGFloat y = scrollView.contentOffset.y;
    CGFloat w = kScreenWidth * 9 / 16.f;
    if (y < w && y < 0) {
        CGFloat newHight = fabs(y) + w;
        CGFloat newWidth = newHight * 16 / 9.f;
        CGFloat x = -(newWidth - kScreenWidth) / 2.f;
        self.topImgView.frame = CGRectMake(x, y, newWidth, newHight);
    }
}