如何停止水平CollectionView Swift并使其居中

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {
    let totalCellWidth = cellWidth * numberOfItems // 计算所有cell的总宽度
    let totalSpacingWidth = cellSpacing * (numberOfItems - 1) // 计算所有间距的总宽度
    let leftInset = (collectionView.frame.width - CGFloat(totalCellWidth + totalSpacingWidth)) / 2 // 计算左边距
    let rightInset = leftInset // 右边距等于左边距
    return UIEdgeInsets(top: 0, left: leftInset, bottom: 0, right: rightInset)
}

let layout = UICollectionViewFlowLayout()
layout.scrollDirection = .horizontal
collectionView.collectionViewLayout = layout

func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>) {
    let layout = collectionView.collectionViewLayout as! UICollectionViewFlowLayout
    let cellWidthIncludingSpacing = cellWidth + cellSpacing // cell的宽度加上间距
    var offset = targetContentOffset.pointee // 当前的偏移量
    let index = (offset.x + scrollView.contentInset.left) / cellWidthIncludingSpacing // 计算当前显示的cell索引
    let roundedIndex = round(index) // 四舍五入取整
    offset = CGPoint(x: roundedIndex * cellWidthIncludingSpacing - scrollView.contentInset.left, y: -scrollView.contentInset.top) // 调整偏移量使得CollectionView停止在居中位置
    targetContentOffset.pointee = offset
}