Support item expanding and collapsing

Example/Sources/List/ListViewController.swift

@@ -81,9 +81,9 @@ final class ListViewController: ExampleViewController, CellEventCoordinator {
 
     private func makeViewModel() -> CollectionViewModel {
         // Create People Section
-        let peopleCellViewModels = self.model.people.map {
+        let peopleCellViewModels = self.model.people.enumerated().map { index, person in
             let menuConfig = UIContextMenuConfiguration.configFor(
-                itemId: $0.id,
+                itemId: person.id,
                 favoriteAction: { [unowned self] in
                     self.toggleFavorite(id: $0)
                 },
@@ -92,9 +92,12 @@ final class ListViewController: ExampleViewController, CellEventCoordinator {
                 }
             )
 
+            let children = makeViewModel(for: person.subPeople)
+
             return PersonCellViewModelList(
-                person: $0,
-                contextMenuConfiguration: menuConfig
+                person: person,
+                contextMenuConfiguration: menuConfig,
+                children: children
             ).eraseToAnyViewModel()
         }
         let peopleHeader = HeaderViewModel(title: "People", style: .small)
@@ -134,6 +137,16 @@ final class ListViewController: ExampleViewController, CellEventCoordinator {
         // Create final view model
         return CollectionViewModel(id: "list_view", sections: [peopleSection, colorSection])
     }
+
+    private func makeViewModel(for people: [PersonModel]) -> [AnyCellViewModel] {
+        let children: [AnyCellViewModel] = people.map {
+            PersonCellViewModelList(person: $0,
+                                    contextMenuConfiguration: nil,
+                                    children: makeViewModel(for: $0.subPeople)).eraseToAnyViewModel()
+        }
+
+        return children
+    }
 }
 
 extension ListViewController: UIScrollViewDelegate {

Example/Sources/List/PersonCellViewModelList.swift

@@ -23,6 +23,8 @@ struct PersonCellViewModelList: CellViewModel {
 
     let contextMenuConfiguration: UIContextMenuConfiguration?
 
+    let children: [AnyCellViewModel]
+
     func configure(cell: UICollectionViewListCell) {
         var contentConfiguration = UIListContentConfiguration.subtitleCell()
         contentConfiguration.text = self.person.name
@@ -35,7 +37,7 @@ struct PersonCellViewModelList: CellViewModel {
         let flagEmoji = UICellAccessory.customView(
             configuration: .init(customView: label, placement: .leading())
         )
-        var accessories = [flagEmoji, .disclosureIndicator()]
+        var accessories = [flagEmoji, .disclosureIndicator(), .outlineDisclosure()]
 
         if self.person.isFavorite {
             let imageView = UIImageView(image: UIImage(systemName: "star.fill"))

Example/Sources/PersonModel/PersonModel.swift

@@ -19,6 +19,7 @@ struct PersonModel: Hashable {
     let birthdate: Date
     let nationality: String
     var isFavorite = false
+    var subPeople: [PersonModel] = []
 
     var birthDateText: String {
         self.birthdate.formatted(date: .long, time: .omitted)
@@ -45,7 +46,13 @@ extension Date {
 extension PersonModel {
     static func makePeople() -> [PersonModel] {
         [
-            PersonModel(name: "Noam Chomsky", birthdate: Date(year: 1_928, month: 12, day: 7), nationality: "🇺🇸"),
+            PersonModel(name: "Noam Chomsky", birthdate: Date(year: 1_928, month: 12, day: 7), nationality: "🇺🇸", subPeople: [
+                .init(name: "Steve Jobs", birthdate: Date(year: 1955, month: 2, day: 24), nationality: "🇺🇸", subPeople: [
+                    .init(name: "Another Steve Jobs", birthdate: Date(year: 1955, month: 2, day: 24), nationality: "🇺🇸", subPeople: [
+                        .init(name: "Yet Another Steve Jobs", birthdate: Date(year: 1955, month: 2, day: 24), nationality: "🇺🇸")
+                    ])
+                ])
+            ]),
             PersonModel(name: "Emma Goldman", birthdate: Date(year: 1_869, month: 6, day: 27), nationality: "🇷🇺"),
             PersonModel(name: "Mikhail Bakunin", birthdate: Date(year: 1_814, month: 5, day: 30), nationality: "🇷🇺"),
             PersonModel(name: "Ursula K. Le Guin", birthdate: Date(year: 1_929, month: 10, day: 21), nationality: "🇺🇸"),

Sources/CellViewModel.swift

@@ -38,6 +38,10 @@ public protocol CellViewModel: DiffableViewModel, ViewRegistrationProvider {
     /// This corresponds to the delegate method `collectionView(_:contextMenuConfigurationForItemAt:point:)`.
     var contextMenuConfiguration: UIContextMenuConfiguration? { get }
 
+    /// Returns an array of children for the cell.
+    /// These children correspond to the items that have been appended to this item as part of a `NSDiffableDataSourceSectionSnapshot`.
+    var children: [AnyCellViewModel] { get }
+
     /// Configures the provided cell for display in the collection.
     /// - Parameter cell: The cell to configure.
     @MainActor
@@ -90,6 +94,9 @@ extension CellViewModel {
     /// Default implementation. Returns `true`.
     public var shouldHighlight: Bool { true }
 
+    /// Default implementation. Returns `[]`.
+    public var children: [AnyCellViewModel] { [] }
+
     /// Default implementation. Returns `nil`.
     public var contextMenuConfiguration: UIContextMenuConfiguration? { nil }
 
@@ -198,6 +205,9 @@ public struct AnyCellViewModel: CellViewModel {
     /// :nodoc:
     public var shouldHighlight: Bool { self._shouldHighlight }
 
+    /// :nodoc:
+    public var children: [AnyCellViewModel] { self._children }
+
     /// :nodoc:
     public var contextMenuConfiguration: UIContextMenuConfiguration? { self._contextMenuConfiguration }
 
@@ -251,6 +261,7 @@ public struct AnyCellViewModel: CellViewModel {
     private let _shouldDeselect: Bool
     private let _shouldHighlight: Bool
     private let _contextMenuConfiguration: UIContextMenuConfiguration?
+    private let _children: [AnyCellViewModel]
     private let _configure: @Sendable @MainActor (CellType) -> Void
     private let _didSelect: @Sendable @MainActor (CellEventCoordinator?) -> Void
     private let _didDeselect: @Sendable @MainActor (CellEventCoordinator?) -> Void
@@ -276,6 +287,7 @@ public struct AnyCellViewModel: CellViewModel {
         self._shouldSelect = viewModel.shouldSelect
         self._shouldDeselect = viewModel.shouldDeselect
         self._shouldHighlight = viewModel.shouldHighlight
+        self._children = viewModel.children
         self._contextMenuConfiguration = viewModel.contextMenuConfiguration
         self._configure = {
             viewModel._configureGeneric(cell: $0)

Sources/CollectionViewDriver.swift

@@ -328,45 +328,45 @@ extension CollectionViewDriver: UICollectionViewDelegate {
     /// :nodoc:
     public func collectionView(_ collectionView: UICollectionView,
                                shouldSelectItemAt indexPath: IndexPath) -> Bool {
-        self.viewModel.cellViewModel(at: indexPath).shouldSelect
+        self.viewModel.cellViewModel(at: indexPath, in: collectionView).shouldSelect
     }
 
     /// :nodoc:
     public func collectionView(_ collectionView: UICollectionView,
                                didSelectItemAt indexPath: IndexPath) {
-        self.viewModel.cellViewModel(at: indexPath).didSelect(with: self._cellEventCoordinator)
+        self.viewModel.cellViewModel(at: indexPath, in: collectionView).didSelect(with: self._cellEventCoordinator)
     }
 
     /// :nodoc:
     public func collectionView(_ collectionView: UICollectionView,
                                shouldDeselectItemAt indexPath: IndexPath) -> Bool {
-        self.viewModel.cellViewModel(at: indexPath).shouldDeselect
+        self.viewModel.cellViewModel(at: indexPath, in: collectionView).shouldDeselect
     }
 
     /// :nodoc:
     public func collectionView(_ collectionView: UICollectionView,
                                didDeselectItemAt indexPath: IndexPath) {
-        self.viewModel.cellViewModel(at: indexPath).didDeselect(with: self._cellEventCoordinator)
+        self.viewModel.cellViewModel(at: indexPath, in: collectionView).didDeselect(with: self._cellEventCoordinator)
     }
 
     // MARK: Managing cell highlighting
 
     /// :nodoc:
     public func collectionView(_ collectionView: UICollectionView,
                                shouldHighlightItemAt indexPath: IndexPath) -> Bool {
-        self.viewModel.cellViewModel(at: indexPath).shouldHighlight
+        self.viewModel.cellViewModel(at: indexPath, in: collectionView).shouldHighlight
     }
 
     /// :nodoc:
     public func collectionView(_ collectionView: UICollectionView,
                                didHighlightItemAt indexPath: IndexPath) {
-        self.viewModel.cellViewModel(at: indexPath).didHighlight()
+        self.viewModel.cellViewModel(at: indexPath, in: collectionView).didHighlight()
     }
 
     /// :nodoc:
     public func collectionView(_ collectionView: UICollectionView,
                                didUnhighlightItemAt indexPath: IndexPath) {
-        self.viewModel.cellViewModel(at: indexPath).didUnhighlight()
+        self.viewModel.cellViewModel(at: indexPath, in: collectionView).didUnhighlight()
     }
 
     // MARK: Managing context menus
@@ -375,7 +375,7 @@ extension CollectionViewDriver: UICollectionViewDelegate {
     public func collectionView(_ collectionView: UICollectionView,
                                contextMenuConfigurationForItemAt indexPath: IndexPath,
                                point: CGPoint) -> UIContextMenuConfiguration? {
-        self.viewModel.cellViewModel(at: indexPath).contextMenuConfiguration
+        self.viewModel.cellViewModel(at: indexPath, in: collectionView).contextMenuConfiguration
     }
 
     // MARK: Tracking the addition and removal of views
@@ -384,7 +384,7 @@ extension CollectionViewDriver: UICollectionViewDelegate {
     public func collectionView(_ collectionView: UICollectionView,
                                willDisplay cell: UICollectionViewCell,
                                forItemAt indexPath: IndexPath) {
-        self.viewModel._safeCellViewModel(at: indexPath)?.willDisplay()
+        self.viewModel._safeCellViewModel(at: indexPath, in: collectionView)?.willDisplay()
     }
 
     /// :nodoc:
@@ -399,7 +399,7 @@ extension CollectionViewDriver: UICollectionViewDelegate {
     public func collectionView(_ collectionView: UICollectionView,
                                didEndDisplaying cell: UICollectionViewCell,
                                forItemAt indexPath: IndexPath) {
-        self.viewModel._safeCellViewModel(at: indexPath)?.didEndDisplaying()
+        self.viewModel._safeCellViewModel(at: indexPath, in: collectionView)?.didEndDisplaying()
     }
 
     /// :nodoc:

Sources/CollectionViewModel.swift

@@ -65,16 +65,19 @@ public struct CollectionViewModel: DiffableViewModel {
 
     // MARK: Accessing Cells
 
-    /// Returns the cell for the specified `id`.
+    /// Returns the cell for the specified `id` by traversing each section and all of the children of each cell.
     ///
     /// - Parameter id: The identifier for the cell.
     /// - Returns: The cell, if it exists.
     public func cellViewModel(for id: UniqueIdentifier) -> AnyCellViewModel? {
         for section in self.sections {
-            for cell in section.cells where cell.id == id {
-                return cell
+            for cell in section.cells {
+                if let foundViewModel = cellViewModel(in: cell, with: id) {
+                    return foundViewModel
+                }
             }
         }
+
         return nil
     }
 
@@ -84,14 +87,42 @@ public struct CollectionViewModel: DiffableViewModel {
     /// - Returns: The cell at `indexPath`.
     ///
     /// - Precondition: The specified `indexPath` must be valid.
-    public func cellViewModel(at indexPath: IndexPath) -> AnyCellViewModel {
+    public func cellViewModel(at indexPath: IndexPath, in collectionView: UICollectionView) -> AnyCellViewModel {
         precondition(indexPath.section < self.count)
         let section = self.sectionViewModel(at: indexPath.section)
 
         let cells = section.cells
         precondition(indexPath.item < cells.count)
 
-        return cells[indexPath.item]
+        guard let diffableDataSource = collectionView.dataSource as? DiffableDataSource
+        else {
+            return cells[indexPath.item]
+        }
+
+        let snapshot = diffableDataSource.snapshot(for: section.id)
+        let id = snapshot.visibleItems[indexPath.item]
+
+        guard let cellViewModel = cellViewModel(for: id)
+        else {
+            return cells[indexPath.item]
+        }
+
+        return cellViewModel
+    }
+
+    /// Recursively traverse the children array of each child to locate a matching cell view model
+    private func cellViewModel(in viewModel: AnyCellViewModel, with id: UniqueIdentifier) -> AnyCellViewModel? {
+        if viewModel.id == id {
+            return viewModel
+        }
+
+        for child in viewModel.children {
+            if let foundChildViewModel = cellViewModel(in: child, with: id) {
+                return foundChildViewModel
+            }
+        }
+
+        return nil
     }
 
     // MARK: Accessing Supplementary Views
@@ -168,12 +199,12 @@ public struct CollectionViewModel: DiffableViewModel {
         return self.sectionViewModel(at: index)
     }
 
-    func _safeCellViewModel(at indexPath: IndexPath) -> AnyCellViewModel? {
+    func _safeCellViewModel(at indexPath: IndexPath, in collectionView: UICollectionView) -> AnyCellViewModel? {
         guard let section = self._safeSectionViewModel(at: indexPath.section),
               indexPath.item < section.cells.count else {
             return nil
         }
-        return self.cellViewModel(at: indexPath)
+        return self.cellViewModel(at: indexPath, in: collectionView)
     }
 
     func _safeSupplementaryViewModel(ofKind kind: String, at indexPath: IndexPath) -> AnySupplementaryViewModel? {