如何:使UICollectionViewCells居中

class CenteredCollectionViewFlowLayout: UICollectionViewFlowLayout {
    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        guard let attributes = super.layoutAttributesForElements(in: rect) else { return nil }
        
        let collectionViewWidth = collectionView?.bounds.width ?? 0
        let contentInset = collectionView?.contentInset ?? .zero
        let sectionInset = self.sectionInset
        
        var leftMargin: CGFloat = 0
        var maxY: CGFloat = -1.0
        
        for attribute in attributes {
            if attribute.frame.origin.y >= maxY {
                leftMargin = sectionInset.left
            }
            
            attribute.frame.origin.x = leftMargin
            
            leftMargin += attribute.frame.width + minimumInteritemSpacing
            maxY = max(attribute.frame.maxY, maxY)
        }
        
        return attributes
    }
}