Fix SwiftUI item view onAppear not getting called

CHANGELOG.md

@@ -19,6 +19,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fixed Storyboard support by removing the `fatalError` in `init?(coder: NSCoder)`
 - Fixed an issue that could cause the calendar to layout unnecessarily due to a trait collection change notification
 - Fixed an issue that could cause off-screen items to appear or disappear instantly, rather than animating in or out during animated content changes
+- Fixed an issue that caused a SwiftUI view being used as a calendar item to not receive calls to `onAppear`
 
 ### Changed
 - Removed all deprecated code, simplifying the public API in preparation for a 2.0 release

Example/HorizonCalendarExample/HorizonCalendarExample/Demo View Controllers/SwiftUIScreenDemoViewController.swift

@@ -121,8 +121,7 @@ struct SwiftUIScreenDemo: View {
       } else {
         isSelected = false
       }
-      return SwiftUIDayView(dayNumber: day.day, isSelected: isSelected)
-        .calendarItemModel
+      return SwiftUIDayView(dayNumber: day.day, isSelected: isSelected).calendarItemModel
     }
 
     .dayRangeItemProvider(for: selectedDateRanges) { dayRangeLayoutContext in

Sources/Public/AnyCalendarItemModel.swift

@@ -43,6 +43,8 @@ public protocol AnyCalendarItemModel {
   /// - Note: There is no reason to invoke this function from your feature code; it should only be invoked internally.
   func _isContentEqual(toContentOf other: AnyCalendarItemModel) -> Bool
 
+  mutating func _setSwiftUIWrapperViewContentIDIfNeeded(_ id: AnyHashable)
+
 }
 
 // MARK: - _CalendarItemViewDifferentiator

Sources/Public/CalendarItemModel.swift

@@ -82,10 +82,23 @@ public struct CalendarItemModel<ViewRepresentable>: AnyCalendarItemModel where
     return content == other.content
   }
 
+  public mutating func _setSwiftUIWrapperViewContentIDIfNeeded(_ id: AnyHashable) {
+    guard
+      var content = content as? SwiftUIWrapperViewContentIDUpdatable,
+      content.id == AnyHashable(PlaceholderID.placeholderID)
+    else {
+      return
+    }
+    content.id = id
+    self.content = content as? ViewRepresentable.Content
+  }
+
   // MARK: Private
 
   private let invariantViewProperties: ViewRepresentable.InvariantViewProperties
-  private let content: ViewRepresentable.Content?
+
+  // This is only mutable because we need to update the ID for `SwiftUIWrapperView`'s content.
+  private var content: ViewRepresentable.Content?
 
 }
 
@@ -169,9 +182,20 @@ extension View {
   /// - Warning: Using a SwiftUI view with the calendar will cause `SwiftUIView.HostingController`(s) to be added to the
   /// closest view controller in the responder chain in relation to the `CalendarView`.
   public var calendarItemModel: CalendarItemModel<SwiftUIWrapperView<Self>> {
-    CalendarItemModel<SwiftUIWrapperView<Self>>(
-      invariantViewProperties: .init(initialContent: self),
-      content: .init(content: self))
+    let contentAndID = SwiftUIWrapperView.ContentAndID(
+      content: self,
+      id: PlaceholderID.placeholderIDAnyHashable)
+    return CalendarItemModel<SwiftUIWrapperView<Self>>(
+      invariantViewProperties: .init(initialContentAndID: contentAndID),
+      content: contentAndID)
   }
 
 }
+
+// MARK: - PlaceholderID
+
+/// This exists only to facilitate internally updating the ID of a `SwiftUIWrapperView`'s content.
+private enum PlaceholderID: Hashable {
+  case placeholderID
+  static let placeholderIDAnyHashable = AnyHashable(PlaceholderID.placeholderID)
+}

Sources/Public/CalendarView.swift

@@ -806,7 +806,9 @@ public final class CalendarView: UIView {
   }
 
   private func configureView(_ view: ItemView, with visibleItem: VisibleItem) {
-    view.calendarItemModel = visibleItem.calendarItemModel
+    var calendarItemModel = visibleItem.calendarItemModel
+    calendarItemModel._setSwiftUIWrapperViewContentIDIfNeeded(visibleItem.itemType)
+    view.calendarItemModel = calendarItemModel
     view.itemType = visibleItem.itemType
     view.frame = visibleItem.frame.alignedToPixels(forScreenWithScale: scale)
 

Sources/Public/ItemViews/SwiftUIWrapperView.swift

@@ -30,8 +30,10 @@ public final class SwiftUIWrapperView<Content: View>: UIView {
 
   // MARK: Lifecycle
 
-  public init(content: Content) {
-    self.content = content
+  public init(contentAndID: ContentAndID) {
+    self.contentAndID = contentAndID
+    hostingController = HostingController(
+      rootView: .init(content: contentAndID.content, id: contentAndID.id))
 
     super.init(frame: .zero)
 
@@ -69,16 +71,16 @@ public final class SwiftUIWrapperView<Content: View>: UIView {
 
   // MARK: Fileprivate
 
-  fileprivate var content: Content {
+  fileprivate var contentAndID: ContentAndID {
     didSet {
-      hostingController.rootView = content
+      hostingController.rootView = .init(content: contentAndID.content, id: contentAndID.id)
       configureGestureRecognizers()
     }
   }
 
   // MARK: Private
 
-  private lazy var hostingController = HostingController<Content>(rootView: content)
+  private let hostingController: HostingController<IDWrapperView<Content>>
 
   private weak var hostingControllerView: UIView?
 
@@ -126,8 +128,8 @@ extension SwiftUIWrapperView: CalendarItemViewRepresentable {
 
     // MARK: Lifecycle
 
-    init(initialContent: Content) {
-      self.initialContent = initialContent
+    init(initialContentAndID: ContentAndID) {
+      self.initialContentAndID = initialContentAndID
     }
 
     // MARK: Public
@@ -142,24 +144,29 @@ extension SwiftUIWrapperView: CalendarItemViewRepresentable {
 
     // MARK: Fileprivate
 
-    fileprivate let initialContent: Content
+    fileprivate let initialContentAndID: ContentAndID
 
   }
 
-  public struct ContentWrapper: Equatable {
+  public struct ContentAndID: Equatable, SwiftUIWrapperViewContentIDUpdatable {
 
     // MARK: Lifecycle
 
-    public init(content: Content) {
+    public init(content: Content, id: AnyHashable) {
       self.content = content
+      self.id = id
     }
 
     // MARK: Public
 
-    public static func == (_: ContentWrapper, _: ContentWrapper) -> Bool {
+    public static func == (_: ContentAndID, _: ContentAndID) -> Bool {
       false
     }
 
+    // MARK: Internal
+
+    var id: AnyHashable
+
     // MARK: Fileprivate
 
     fileprivate let content: Content
@@ -168,17 +175,26 @@ extension SwiftUIWrapperView: CalendarItemViewRepresentable {
 
   public static func makeView(
     withInvariantViewProperties invariantViewProperties: InvariantViewProperties)
-    -> SwiftUIWrapperView<Content>
+  -> SwiftUIWrapperView<Content>
   {
-    SwiftUIWrapperView<Content>(content: invariantViewProperties.initialContent)
+    SwiftUIWrapperView<Content>(contentAndID: invariantViewProperties.initialContentAndID)
   }
 
-  public static func setContent(_ content: ContentWrapper, on view: SwiftUIWrapperView<Content>) {
-    view.content = content.content
+  public static func setContent(
+    _ contentAndID: ContentAndID,
+    on view: SwiftUIWrapperView<Content>)
+  {
+    view.contentAndID = contentAndID
   }
 
 }
 
+// MARK: - SwiftUIWrapperViewContentIDUpdatable
+
+protocol SwiftUIWrapperViewContentIDUpdatable {
+  var id: AnyHashable { get set }
+}
+
 // MARK: UIResponder Next View Controller Helper
 
 extension UIResponder {
@@ -188,35 +204,48 @@ extension UIResponder {
   }
 }
 
-// MARK: - SwiftUIWrapperView.HostingController
+// MARK: - IDWrapperView
 
+/// A wrapper view that uses the `id(_:)` modifier on the wrapped view so that each one has its own identity, even if it was reused.
 @available(iOS 13.0, *)
-extension SwiftUIWrapperView {
+private struct IDWrapperView<Content: View>: View {
 
-  /// The `UIHostingController` type used by `SwiftUIWrapperView` to embed SwiftUI views in a UIKit view hierarchy.
-  public final class HostingController<Content: View>: UIHostingController<Content> {
+  let content: Content
+  let id: AnyHashable
 
-    // MARK: Lifecycle
+  var body: some View {
+    content
+      .id(id)
+  }
 
-    fileprivate override init(rootView: Content) {
-      super.init(rootView: rootView)
+}
 
-      // This prevents the safe area from affecting layout.
-      _disableSafeArea = true
-    }
+// MARK: - HostingController
 
-    @MainActor required dynamic init?(coder aDecoder: NSCoder) {
-      fatalError("init(coder:) has not been implemented")
-    }
+/// The `UIHostingController` type used by `SwiftUIWrapperView` to embed SwiftUI views in a UIKit view hierarchy. This
+/// exists to disable safe area insets and set the background color to clear.
+@available(iOS 13.0, *)
+private final class HostingController<Content: View>: UIHostingController<Content> {
 
-    public override func viewDidLoad() {
-      super.viewDidLoad()
+  // MARK: Lifecycle
 
-      // Override the default `.systemBackground` color since `CalendarView` subviews should be
-      // clear.
-      view.backgroundColor = .clear
-    }
+  override init(rootView: Content) {
+    super.init(rootView: rootView)
+
+    // This prevents the safe area from affecting layout.
+    _disableSafeArea = true
+  }
+
+  @MainActor required dynamic init?(coder aDecoder: NSCoder) {
+    fatalError("init(coder:) has not been implemented")
+  }
+
+  override func viewDidLoad() {
+    super.viewDidLoad()
 
+    // Override the default `.systemBackground` color since `CalendarView` subviews should be
+    // clear.
+    view.backgroundColor = .clear
   }
 
 }

Tests/ItemViewReuseManagerTests.swift

@@ -521,4 +521,6 @@ private struct MockCalendarItemModel: AnyCalendarItemModel, Equatable {
     false
   }
 
+  mutating func _setSwiftUIWrapperViewContentIDIfNeeded(_ id: AnyHashable) { }
+
 }