swift的一个横向滚动居中变大的layout

class CenterLayout: UICollectionViewFlowLayout {

//    let kZoomFactor = 0.1

   // let kActiveDistanse = 200.0

    let kMaxCellWidth:CGFloat = 168

    let kMinCellWidth:CGFloat = 152

    let kMaxCellHeight:CGFloat = 504

    let kMinCellHeight:CGFloat = 456

    override init() {

        super.init()

        self.initParams()

    }

    requiredinit?(coder aDecoder:NSCoder) {

        super.init(coder: aDecoder)

        self.initParams()

    }

    func initParams() {

        self.itemSize = CGSize.init(width: kMaxCellWidth, height: kMaxCellHeight)

        self.scrollDirection = .horizontal

        self.minimumLineSpacing = 7.5

        self.minimumInteritemSpacing = KScreenHeight

        let insert = (KScreenWidth-kMaxCellWidth)/2

        self.sectionInset=UIEdgeInsetsMake(0,insert, 0,insert)

    }

    overridefuncshouldInvalidateLayout(forBoundsChange newBounds:CGRect) ->Bool{

        return true

    }

    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {

        letarray =super.layoutAttributesForElements(in: rect)

        let pPoint = self.collectionView?.contentOffset ?? CGPoint.zero

        let pSize = self.collectionView?.bounds.size ?? CGSize.init(width: kMinCellWidth, height: kMinCellHeight)

        letvisibleRect =CGRect.init(origin: pPoint, size: pSize)

        array?.forEach({ (attributes)in

            ifrect.intersects(attributes.frame){

                vardistance =Double(visibleRect.midX-attributes.center.x)

                distance =abs(distance)

                letmaxDistance =KScreenWidth/2

                letmaxZoom =max((maxDistance-CGFloat(distance))/maxDistance,0)

                let fWidth = kMinCellWidth + (kMaxCellWidth - kMinCellWidth) * maxZoom

                let fHeight = kMinCellHeight + (kMaxCellHeight - kMinCellHeight) * maxZoom

                attributes.size=CGSize.init(width: fWidth, height: fHeight)

            }

        })

        returnarray;

}

    overridefunctargetContentOffset(forProposedContentOffset proposedContentOffset:CGPoint, withScrollingVelocity velocity:CGPoint) ->CGPoint{

        var offsetAdjustment = CGFloat.greatestFiniteMagnitude

        lethorizontalCenter = proposedContentOffset.x+(self.collectionView!.bounds.size.width/2.0)

        lettargetRect =CGRect.init(x: proposedContentOffset.x, y: 0.0, width:self.collectionView!.bounds.size.width, height:self.collectionView!.bounds.size.height)

        letarray =super.layoutAttributesForElements(in: targetRect)

        array?.forEach({ (layoutAttributes)in

            letitemHorizontalCenter = layoutAttributes.center.x

            if(abs(itemHorizontalCenter-horizontalCenter)

                offsetAdjustment = itemHorizontalCenter-horizontalCenter;

            }

        })

       letptTarget =CGPoint.init(x: proposedContentOffset.x+offsetAdjustment, y: proposedContentOffset.y)

        return  ptTarget

    }

}