List CollectionViewCell with Sub.swift

 let containerCellRegistration = UICollectionView.CellRegistration<UICollectionViewListCell, OutlineItem> { cell, _, menuItem in

    var contentConfiguration = cell.defaultContentConfiguration()
    contentConfiguration.text = menuItem.title

    if menuItem.imageName != nil {
        contentConfiguration.image = UIImage(systemName: menuItem.imageName!)
    }

    contentConfiguration.textProperties.font = .preferredFont(forTextStyle: .headline)
    cell.contentConfiguration = contentConfiguration

    let disclosureOptions = UICellAccessory.OutlineDisclosureOptions(style: .header)
    cell.accessories = [.outlineDisclosure(options: disclosureOptions)]

    let background = UIBackgroundConfiguration.clear()
    cell.backgroundConfiguration = background
}

let cellRegistration = UICollectionView.CellRegistration<UICollectionViewListCell, OutlineItem> { cell, _, menuItem in
    var content = UIListContentConfiguration.cell()
    content.text = menuItem.title
    cell.contentConfiguration = content

    let background = UIBackgroundConfiguration.clear()
    cell.backgroundConfiguration = background

    cell.accessories = self.splitViewWantsToShowDetail() ? [] : [.disclosureIndicator()]
}

dataSource = UICollectionViewDiffableDataSource<Section, OutlineItem>(collectionView: outlineCollectionView) {
    (collectionView: UICollectionView, indexPath: IndexPath, item: OutlineItem) -> UICollectionViewCell? in
    // Return the cell.
    if item.subitems.isEmpty {
        return collectionView.dequeueConfiguredReusableCell(using: cellRegistration, for: indexPath, item: item)
    } else {
        return collectionView.dequeueConfiguredReusableCell(using: containerCellRegistration, for: indexPath, item: item)
    }
}

// Load our initial data.
let snapshot = initialSnapshot()
dataSource.apply(snapshot, to: .main, animatingDifferences: false)