iOS 下拉图片放大

详情页，tableview的headview，下拉图片放大

 func scrollViewDidScroll(_ scrollView: UIScrollView) {
        let offsetY = scrollView.contentOffset.y
        if offsetY < 0 {
            let totalOffset = tableView.tableHeaderView?.frame.height + abs(offsetY)
            let f = totalOffset / tableView.tableHeaderView?.frame.height
            headerView.bgImageView.frame = CGRect(x: -UIScreen.main.bounds.width * (f - 1) * 0.5, y: offsetY, width: UIScreen.main.bounds.width * f, height: totalOffset)
        }
    }