ios - 如何使用自定义 UICollectionViewLayout 更改 UICollectionView 上的选定项目

Question

我有一个带有自定义 collectionViewLayout 的 UICollectionView，就像卡片轮播一样。

触摸时它可以正常工作，但是在尝试从代码中更改所选卡片时它不会正确对齐。

图片显示了问题：

我首先尝试使用 scrollToItem，以及 setContentOffset 的一些变体，例如：

collectionView.scrollToItem(at: indexPath, at: .left, animated: true)    
collectionView.setContentOffset(CGPoint(x: (attri!.frame.origin.x - sectionLeftInset), y: 0), animated: true)
collectionView.setContentOffset(CGPoint(x: CGFloat((213 * currentPage)), y: 0), animated: true)

但它永远不适合。

编辑：添加 UICollectionViewLayout 的代码

import UIKit

class CarouselLayout: UICollectionViewLayout {
    
    // MARK: - Private Properties
    private var cachedItemsAttributes: [IndexPath: UICollectionViewLayoutAttributes] = [:]
    private let itemSize = CGSize(width: 213, height: 135)
    private let spacing: CGFloat = 30
    private let spacingWhenFocused: CGFloat = 60
    
    // MARK: - Public Properties
    
    override var collectionViewContentSize: CGSize {
        let leftmostEdge = cachedItemsAttributes.values.map { $0.frame.minX }.min() ?? 0
        let rightmostEdge = cachedItemsAttributes.values.map { $0.frame.maxX }.max() ?? 0
        return CGSize(width: rightmostEdge - leftmostEdge, height: itemSize.height)
    }

    private var continuousFocusedIndex: CGFloat {
        guard let collectionView = collectionView else { return 0 }
        let offset = collectionView.bounds.width / 2 + collectionView.contentOffset.x - itemSize.width / 2
        return offset / (itemSize.width + spacing)
    }
    
    // MARK: - Public Methods
    
    override open func prepare() {
        super.prepare()
        guard let collectionView = self.collectionView else { return }
        updateInsets()
        guard cachedItemsAttributes.isEmpty else { return }
        collectionView.decelerationRate = UIScrollView.DecelerationRate.fast
        let itemsCount = collectionView.numberOfItems(inSection: 0)
        for item in 0..<itemsCount {
            let indexPath = IndexPath(item: item, section: 0)
            cachedItemsAttributes[indexPath] = createAttributesForItem(at: indexPath)
        }
    }
    
    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        return cachedItemsAttributes
            .map { $0.value }
            .filter { $0.frame.intersects(rect) }
            .map { self.shiftedAttributes(from: $0) }
    }

    override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
        guard let collectionView = collectionView else { return super.targetContentOffset(forProposedContentOffset: proposedContentOffset) }
        let collectionViewMidX: CGFloat = collectionView.bounds.size.width / 2
        guard let closestAttribute = findClosestAttributes(toXPosition: proposedContentOffset.x + collectionViewMidX) else { return super.targetContentOffset(forProposedContentOffset: proposedContentOffset) }
        return CGPoint(x: closestAttribute.center.x - collectionViewMidX, y: proposedContentOffset.y)
    }

    // MARK: - Invalidate layout
    
    override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
        if newBounds.size != collectionView?.bounds.size { cachedItemsAttributes.removeAll() }
        return true
    }
    
    override func invalidateLayout(with context: UICollectionViewLayoutInvalidationContext) {
        if context.invalidateDataSourceCounts { cachedItemsAttributes.removeAll() }
        super.invalidateLayout(with: context)
    }
    
    // MARK: - Items
    
    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        guard let attributes = cachedItemsAttributes[indexPath] else { fatalError("No attributes cached") }
        return shiftedAttributes(from: attributes)
    }
    
    private func createAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        let attributes = UICollectionViewLayoutAttributes(forCellWith: indexPath)
        guard let collectionView = collectionView else { return nil }
        attributes.frame.size = itemSize
        attributes.frame.origin.y = (collectionView.bounds.height - itemSize.height) / 2
        attributes.frame.origin.x = CGFloat(indexPath.item) * (itemSize.width + spacing)
        return attributes
    }
    
    private func shiftedAttributes(from attributes: UICollectionViewLayoutAttributes) -> UICollectionViewLayoutAttributes {
        guard let attributes = attributes.copy() as? UICollectionViewLayoutAttributes else { fatalError("Couldn't copy attributes") }
        let roundedFocusedIndex = round(continuousFocusedIndex)
        guard attributes.indexPath.item != Int(roundedFocusedIndex) else { return attributes }
        let shiftArea = (roundedFocusedIndex - 0.5)...(roundedFocusedIndex + 0.5)
        let distanceToClosestIdentityPoint = min(abs(continuousFocusedIndex - shiftArea.lowerBound), abs(continuousFocusedIndex - shiftArea.upperBound))
        let normalizedShiftFactor = distanceToClosestIdentityPoint * 2
        let translation = (spacingWhenFocused - spacing) * normalizedShiftFactor
        let translationDirection: CGFloat = attributes.indexPath.item < Int(roundedFocusedIndex) ? -1 : 1
        attributes.transform = CGAffineTransform(translationX: translationDirection * translation, y: 0)
        return attributes
    }

    // MARK: - Private Methods
    
    private func findClosestAttributes(toXPosition xPosition: CGFloat) -> UICollectionViewLayoutAttributes? {
        guard let collectionView = collectionView else { return nil }
        let searchRect = CGRect(
            x: xPosition - collectionView.bounds.width, y: collectionView.bounds.minY,
            width: collectionView.bounds.width * 2, height: collectionView.bounds.height
        )
        return layoutAttributesForElements(in: searchRect)?.min(by: { abs($0.center.x - xPosition) < abs($1.center.x - xPosition) })
    }
    
    private func updateInsets() {
        guard let collectionView = collectionView else { return }
        collectionView.contentInset.left = (collectionView.bounds.size.width - itemSize.width) / 2
        collectionView.contentInset.right = (collectionView.bounds.size.width - itemSize.width) / 2
    }
}

标签： iosswiftxcodeuicollectionviewuicollectionviewlayout