UICollectionViewでタグが左寄せに並んでいるようなレイアウトを実現する

FlowLayoutHorizontalLeft.swift

class FlowLayoutHorizontalLeft: UICollectionViewFlowLayout {

    private var cache: [IndexPath: UICollectionViewLayoutAttributes] = [:]

    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        super.layoutAttributesForElements(in: rect)?
            .lazy
            .filter { $0.representedElementCategory == .cell }
            .map { self.layoutAttributesForItem(at: $0.indexPath) }
            .compactMap { $0 }
    }

    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        let current = super.layoutAttributesForItem(at: indexPath)
        let previous = cache[IndexPath(item: indexPath.item - 1, section: indexPath.section)]
        switch (current, previous) {
        case let (current?, previous?) where current.center.y == previous.center.y:
            current.center.x = (previous.center.x - previous.bounds.midX + previous.bounds.width)
                + current.bounds.midX + minimumInteritemSpacing
            cache[indexPath] = current
            return current

        case let (current, _):
            cache[indexPath] = current
            return current
        }
    }

    override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
        let shouldInvalidate = super.shouldInvalidateLayout(forBoundsChange: newBounds)
        if shouldInvalidate {
            cache.removeAll(keepingCapacity: true)
        }
        return shouldInvalidate
    }
}