Implement willDisplay and didEndDisplaying APIs (#121)

Closes #96

Implements `willDisplay` and `didEndDisplaying`.

---------

Co-authored-by: Jesse Squires <jesse@jessesquires.com>

Author: nuomi1

CHANGELOG.md

@@ -7,6 +7,11 @@ NEXT
 
 - TBA
 
+0.1.4
+-----
+
+- Implemented `willDisplay()` and `didEndDisplaying()` APIs for both `CellViewModel` and `SupplementaryViewModel`. ([@nuomi1](https://github.com/nuomi1), [#121](https://github.com/jessesquires/ReactiveCollectionsKit/pull/121))
+
 0.1.3
 -----
 

Sources/CellViewModel.swift

@@ -36,6 +36,14 @@ public protocol CellViewModel: DiffableViewModel, ViewRegistrationProvider {
     /// Handles the selection event for this cell, optionally using the provided `coordinator`.
     /// - Parameter coordinator: An event coordinator object, if one was provided to the `CollectionViewDriver`.
     func didSelect(with coordinator: CellEventCoordinator?)
+
+    /// Tells the view model that its cell is about to be displayed in the collection view.
+    /// This corresponds to the delegate method `collectionView(_:willDisplay:forItemAt:)`.
+    func willDisplay()
+
+    /// Tells the view model that its cell was removed from the collection view.
+    /// This corresponds to the delegate method `collectionView(_:didEndDisplaying:forItemAt:)`.
+    func didEndDisplaying()
 }
 
 extension CellViewModel {
@@ -51,6 +59,12 @@ extension CellViewModel {
     public func didSelect(with coordinator: CellEventCoordinator?) {
         coordinator?.didSelectCell(viewModel: self)
     }
+
+    /// Default implementation. Does nothing.
+    public func willDisplay() { }
+
+    /// Default implementation. Does nothing.
+    public func didEndDisplaying() { }
 }
 
 extension CellViewModel {
@@ -124,10 +138,20 @@ public struct AnyCellViewModel: CellViewModel {
         self._didSelect(coordinator)
     }
 
-    /// :nodoc: "override" extension
+    /// :nodoc:
+    public func willDisplay() {
+        self._willDisplay()
+    }
+
+    /// :nodoc:
+    public func didEndDisplaying() {
+        self._didEndDisplaying()
+    }
+
+    /// :nodoc: "override" the extension
     public let cellClass: AnyClass
 
-    /// :nodoc: "override" extension
+    /// :nodoc: "override" the extension
     public let reuseIdentifier: String
 
     // MARK: Private
@@ -139,6 +163,8 @@ public struct AnyCellViewModel: CellViewModel {
     private let _contextMenuConfiguration: UIContextMenuConfiguration?
     private let _configure: (CellType) -> Void
     private let _didSelect: (CellEventCoordinator?) -> Void
+    private let _willDisplay: () -> Void
+    private let _didEndDisplaying: () -> Void
 
     // MARK: Init
 
@@ -162,6 +188,8 @@ public struct AnyCellViewModel: CellViewModel {
         self._didSelect = { coordinator in
             viewModel.didSelect(with: coordinator)
         }
+        self._willDisplay = viewModel.willDisplay
+        self._didEndDisplaying = viewModel.didEndDisplaying
         self.cellClass = viewModel.cellClass
         self.reuseIdentifier = viewModel.reuseIdentifier
     }

Sources/CollectionViewDriver.swift

@@ -286,22 +286,60 @@ public final class CollectionViewDriver: NSObject {
 // MARK: UICollectionViewDelegate
 
 extension CollectionViewDriver: UICollectionViewDelegate {
+    // MARK: Managing the selected cells
+
     /// :nodoc:
     public func collectionView(_ collectionView: UICollectionView,
                                didSelectItemAt indexPath: IndexPath) {
         self.viewModel.cellViewModel(at: indexPath).didSelect(with: self._cellEventCoordinator)
     }
 
+    // MARK: Managing cell highlighting
+
     /// :nodoc:
     public func collectionView(_ collectionView: UICollectionView,
                                shouldHighlightItemAt indexPath: IndexPath) -> Bool {
         self.viewModel.cellViewModel(at: indexPath).shouldHighlight
     }
 
+    // MARK: Managing context menus
+
     /// :nodoc:
     public func collectionView(_ collectionView: UICollectionView,
                                contextMenuConfigurationForItemAt indexPath: IndexPath,
                                point: CGPoint) -> UIContextMenuConfiguration? {
         self.viewModel.cellViewModel(at: indexPath).contextMenuConfiguration
     }
+
+    // MARK: Tracking the addition and removal of views
+
+    /// :nodoc:
+    public func collectionView(_ collectionView: UICollectionView,
+                               willDisplay cell: UICollectionViewCell,
+                               forItemAt indexPath: IndexPath) {
+        self.viewModel._safeCellViewModel(at: indexPath)?.willDisplay()
+    }
+
+    /// :nodoc:
+    public func collectionView(_ collectionView: UICollectionView,
+                               willDisplaySupplementaryView view: UICollectionReusableView,
+                               forElementKind elementKind: String,
+                               at indexPath: IndexPath) {
+        self.viewModel._safeSupplementaryViewModel(ofKind: elementKind, at: indexPath)?.willDisplay()
+    }
+
+    /// :nodoc:
+    public func collectionView(_ collectionView: UICollectionView,
+                               didEndDisplaying cell: UICollectionViewCell,
+                               forItemAt indexPath: IndexPath) {
+        self.viewModel._safeCellViewModel(at: indexPath)?.didEndDisplaying()
+    }
+
+    /// :nodoc:
+    public func collectionView(_ collectionView: UICollectionView,
+                               didEndDisplayingSupplementaryView view: UICollectionReusableView,
+                               forElementOfKind elementKind: String,
+                               at indexPath: IndexPath) {
+        self.viewModel._safeSupplementaryViewModel(ofKind: elementKind, at: indexPath)?.didEndDisplaying()
+    }
 }

Sources/CollectionViewModel.swift

@@ -152,6 +152,42 @@ public struct CollectionViewModel: Hashable, DiffableViewModel {
 
     // MARK: Internal
 
+    func _safeSectionViewModel(at index: Int) -> SectionViewModel? {
+        guard index < self.count else {
+            return nil
+        }
+        return self.sectionViewModel(at: index)
+    }
+
+    func _safeCellViewModel(at indexPath: IndexPath) -> AnyCellViewModel? {
+        guard let section = self._safeSectionViewModel(at: indexPath.section),
+              indexPath.item < section.cells.count else {
+            return nil
+        }
+        return self.cellViewModel(at: indexPath)
+    }
+
+    func _safeSupplementaryViewModel(ofKind kind: String, at indexPath: IndexPath) -> AnySupplementaryViewModel? {
+        guard let section = self._safeSectionViewModel(at: indexPath.section) else {
+            return nil
+        }
+
+        if kind == section.header?._kind {
+            return section.header
+        }
+
+        if kind == section.footer?._kind {
+            return section.footer
+        }
+
+        let supplementaryViews = section.supplementaryViews.filter { $0._kind == kind }
+        guard indexPath.item < supplementaryViews.count else {
+            return nil
+        }
+
+        return self.supplementaryViewModel(ofKind: kind, at: indexPath)
+    }
+
     func allRegistrations() -> Set<ViewRegistration> {
         var all = Set<ViewRegistration>()
         self.sections.forEach {

Sources/SupplementaryViewModel.swift

@@ -26,6 +26,22 @@ public protocol SupplementaryViewModel: DiffableViewModel, ViewRegistrationProvi
     /// Configures the provided view for display in the collection.
     /// - Parameter view: The view to configure.
     func configure(view: ViewType)
+
+    /// Tells the view model that its supplementary view is about to be displayed in the collection view.
+    /// This corresponds to the delegate method `collectionView(_:willDisplaySupplementaryView:forElementKind:at:)`.
+    func willDisplay()
+
+    /// Tells the view model that its supplementary view was removed from the collection view.
+    /// This corresponds to the delegate method `collectionView(_:didEndDisplayingSupplementaryView:forElementOfKind:at:)`.
+    func didEndDisplaying()
+}
+
+extension SupplementaryViewModel {
+    /// Default implementation. Does nothing.
+    public func willDisplay() { }
+
+    /// Default implementation. Does nothing.
+    public func didEndDisplaying() { }
 }
 
 extension SupplementaryViewModel {
@@ -91,10 +107,20 @@ public struct AnySupplementaryViewModel: SupplementaryViewModel {
         self._configure(view)
     }
 
-    /// :nodoc: "override" extension
+    /// :nodoc:
+    public func willDisplay() {
+        self._willDisplay()
+    }
+
+    /// :nodoc:
+    public func didEndDisplaying() {
+        self._didEndDisplaying()
+    }
+
+    /// :nodoc: "override" the extension
     public let viewClass: AnyClass
 
-    /// :nodoc: "override" extension
+    /// :nodoc: "override" the extension
     public let reuseIdentifier: String
 
     // MARK: Private
@@ -103,6 +129,8 @@ public struct AnySupplementaryViewModel: SupplementaryViewModel {
     private let _id: UniqueIdentifier
     private let _registration: ViewRegistration
     private let _configure: (ViewType) -> Void
+    private let _willDisplay: () -> Void
+    private let _didEndDisplaying: () -> Void
 
     // MARK: Init
 
@@ -121,6 +149,8 @@ public struct AnySupplementaryViewModel: SupplementaryViewModel {
             precondition(view is T.ViewType, "View must be of type \(T.ViewType.self). Found \(view.self)")
             viewModel.configure(view: view as! T.ViewType)
         }
+        self._willDisplay = viewModel.willDisplay
+        self._didEndDisplaying = viewModel.didEndDisplaying
         self.viewClass = viewModel.viewClass
         self.reuseIdentifier = viewModel.reuseIdentifier
     }

Tests/Fakes/FakeCellNibView.swift

@@ -44,6 +44,16 @@ struct FakeCellNibViewModel: CellViewModel {
         self.expectationDidSelect?.fulfillAndLog()
     }
 
+    var expectationWillDisplay: XCTestExpectation?
+    func willDisplay() {
+        self.expectationWillDisplay?.fulfillAndLog()
+    }
+
+    var expectationDidEndDisplaying: XCTestExpectation?
+    func didEndDisplaying() {
+        self.expectationDidEndDisplaying?.fulfillAndLog()
+    }
+
     nonisolated static func == (left: Self, right: Self) -> Bool {
         left.id == right.id
     }