具有分页效果的滚动 card banner

CardBannerFlowLayout.swift

class CardBannerFlowLayout: UICollectionViewFlowLayout {
    
    var padding: CGFloat = 20
    var cardSize: CGSize {
        return CGSize(width: screen_width - 2 * padding, height: 168)
    }
    let cardSpacing: CGFloat = 10.0
    var initialNonCardLength: CGFloat {
        return padding + cardSpacing
    }
    
    override func prepare() {
        super.prepare()
        
        itemSize = cardSize
        scrollDirection = .horizontal
        collectionView?.contentInset = UIEdgeInsets(top: 0, left: padding, bottom: 0, right: padding)
        minimumLineSpacing = cardSpacing
        collectionView?.decelerationRate = .fast
        collectionView?.showsHorizontalScrollIndicator = false
        collectionView?.backgroundColor = .clear
    }
    
    override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
        var adjustmentOffset = CGFloat.greatestFiniteMagnitude
        // 屏幕中点在 collectionView 座标系上的 x 座标
        let screenCenter = proposedContentOffset.x + screen_width / 2
        // scrollView 滚动自然停止时所显示的区域 frame
        let visibleRect = CGRect(x: proposedContentOffset.x, y: 0, width: screen_width, height: collectionView!.bounds.height)
        let visibleAttrs = layoutAttributesForElements(in: visibleRect)
        visibleAttrs?.forEach {
            // 比较屏幕中心点和 item 中心点在 collectionView 座标系上的位置
            let attrOffset = $0.center.x - screenCenter
            if abs(attrOffset) < abs(adjustmentOffset) {
                adjustmentOffset = attrOffset
            }
        }

        let compensate = screen_width - initialNonCardLength // 单个卡片移动距离
        if velocity.x > 0 { // 左滑 增加 proposedContentOffset.x
            if adjustmentOffset < 0 { // item 中心在屏幕中心左侧
                adjustmentOffset += compensate
            }
        } else if velocity.x < 0 { // 右滑 减少 proposedContentOffset.x
            if adjustmentOffset > 0 {
                adjustmentOffset -= compensate
            }
        }
        
        return CGPoint(x: proposedContentOffset.x + adjustmentOffset, y: proposedContentOffset.y)
    }

    override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
        return true
    }
}