NSDiffableDataSourceSnapshot -> "deleteItems" does not work for cells that are collapsed

Code Block language
class OutlineViewController: UIViewController {
  
  enum Section {
    case main
  }
  
  class OutlineItem: Hashable {
    let title: String
    let subitems: [OutlineItem]
    let outlineViewController: UIViewController.Type?
    
    init(title: String,
         viewController: UIViewController.Type? = nil,
         subitems: [OutlineItem] = []) {
      self.title = title
      self.subitems = subitems
      self.outlineViewController = viewController
    }
    func hash(into hasher: inout Hasher) {
      hasher.combine(identifier)
    }
    static func == (lhs: OutlineItem, rhs: OutlineItem) -> Bool {
      return lhs.identifier == rhs.identifier
    }
    private let identifier = UUID()
  }
  
  var dataSource: UICollectionViewDiffableDataSource<Section, OutlineItem>! = nil
  var outlineCollectionView: UICollectionView! = nil
  
  override func viewDidLoad() {
    super.viewDidLoad()
    navigationItem.title = "Modern Collection Views"
    configureCollectionView()
    configureDataSource()
    
    navigationItem.rightBarButtonItem = UIBarButtonItem(barButtonSystemItem: .trash, target: self, action: #selector(removeCells))
  }
  
  @objc func removeCells() {
    
    guard let item = menuItems.first(where: { $0.title == "Outlines" }) else {
      return
    }
    
    let items = [item] + item.subitems
    
    var snapshot = dataSource.snapshot()
    snapshot.deleteItems(items)
    
    dataSource.apply(snapshot, animatingDifferences: true)
  }
  
  private lazy var menuItems: [OutlineItem] = {
    return [
      OutlineItem(title: "Lists", subitems: [
        OutlineItem(title: "Simple List", viewController: SimpleListViewController.self),
        OutlineItem(title: "Reorderable List", viewController: ReorderableListViewController.self),
        OutlineItem(title: "List Appearances", viewController: ListAppearancesViewController.self),
        OutlineItem(title: "List with Custom Cells", viewController: CustomCellListViewController.self)
      ]),
      OutlineItem(title: "Outlines", subitems: [
        OutlineItem(title: "Emoji Explorer", viewController: EmojiExplorerViewController.self),
        OutlineItem(title: "Emoji Explorer - List", viewController: EmojiExplorerListViewController.self)
      ]),
      OutlineItem(title: "Cell Configurations", subitems: [
        OutlineItem(title: "Custom Configurations", viewController: CustomConfigurationViewController.self)
      ])
    ]
  }()
  
}
extension OutlineViewController {
  
  func configureCollectionView() {
    let collectionView = UICollectionView(frame: view.bounds, collectionViewLayout: generateLayout())
    view.addSubview(collectionView)
    collectionView.autoresizingMask = [.flexibleHeight, .flexibleWidth]
    collectionView.backgroundColor = .systemGroupedBackground
    self.outlineCollectionView = collectionView
    collectionView.delegate = self
  }
  
  func configureDataSource() {
    
    let containerCellRegistration = UICollectionView.CellRegistration<UICollectionViewListCell, OutlineItem> { (cell, indexPath, menuItem) in
      var contentConfiguration = cell.defaultContentConfiguration()
      contentConfiguration.text = menuItem.title
      contentConfiguration.textProperties.font = .preferredFont(forTextStyle: .headline)
      cell.contentConfiguration = contentConfiguration
      
      let disclosureOptions = UICellAccessory.OutlineDisclosureOptions(style: .header)
      cell.accessories = [.outlineDisclosure(options: disclosureOptions)]
      cell.backgroundConfiguration = UIBackgroundConfiguration.clear()
    }
    
    let cellRegistration = UICollectionView.CellRegistration<UICollectionViewListCell, OutlineItem> { cell, indexPath, menuItem in
      var contentConfiguration = cell.defaultContentConfiguration()
      contentConfiguration.text = menuItem.title
      cell.contentConfiguration = contentConfiguration
      cell.backgroundConfiguration = UIBackgroundConfiguration.clear()
    }
    
    dataSource = UICollectionViewDiffableDataSource<Section, OutlineItem>(collectionView: outlineCollectionView) {
      (collectionView: UICollectionView, indexPath: IndexPath, item: OutlineItem) -> UICollectionViewCell? in
      if item.subitems.isEmpty {
        return collectionView.dequeueConfiguredReusableCell(using: cellRegistration, for: indexPath, item: item)
      } else {
        return collectionView.dequeueConfiguredReusableCell(using: containerCellRegistration, for: indexPath, item: item)
      }
    }
    
    let snapshot = initialSnapshot()
    self.dataSource.apply(snapshot, to: .main, animatingDifferences: false)
  }
  
  func generateLayout() -> UICollectionViewLayout {
    let listConfiguration = UICollectionLayoutListConfiguration(appearance: .sidebar)
    let layout = UICollectionViewCompositionalLayout.list(using: listConfiguration)
    return layout
  }
  
  func initialSnapshot() -> NSDiffableDataSourceSectionSnapshot<OutlineItem> {
    var snapshot = NSDiffableDataSourceSectionSnapshot<OutlineItem>()
    
    func addItems(_ menuItems: [OutlineItem], to parent: OutlineItem?) {
      snapshot.append(menuItems, to: parent)
      for menuItem in menuItems where !menuItem.subitems.isEmpty {
        addItems(menuItem.subitems, to: menuItem)
      }
    }
    
    addItems(menuItems, to: nil)
    return snapshot
  } 
}

When you're using outline functionality (representing a hierarchy of items, including expand/collapse) with a diffable data source, you need to use NSDiffableDataSourceSectionSnapshot, which is a snapshot for a single section that is capable of representing parent-child relationships and expand/collapse state.

When you're using section snapshots, generally speaking you only want to use the top-level NSDiffableDataSourceSnapshot (for the entire data source) to initially populate the section identifiers, and then use the section snapshots afterwards to populate & update the items within each section.

The top-level NSDiffableDataSourceSnapshot doesn't support outline functionality, and doesn't know about parent-child relationships between items or expand/collapse state. (The top-level snapshot is essentially a "flattened" representation of all the data, containing only sections and expanded items.) Thus, you won't be able to query the top-level snapshot for information about items that are inside a collapsed parent, as those collapsed items don't exist in the top-level snapshot. Use the APIs on NSDiffableDataSourceSectionSnapshot to manage those items instead.