Bk/optimize view reuse (#313)

* Optimize view reuse

* Use ObjectIdentifier for view differentiator

* Update tests

* Update CHANGELOG.md

CHANGELOG.md

@@ -16,6 +16,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 - Rewrote accessibility code to avoid posting notifications, which causes poor Voice Over performance and odd focus bugs
+- Rewrote `ItemViewReuseManager` to perform fewer set operations, improving CPU usage by ~15% when scrolling quickly on an iPhone XR
 
 ## [v2.0.0](https://github.com/airbnb/HorizonCalendar/compare/v1.16.0...v2.0.0) - 2023-12-19
 

Sources/Internal/ItemViewReuseManager.swift

@@ -23,139 +23,68 @@ final class ItemViewReuseManager {
 
   // MARK: Internal
 
-  func viewsForVisibleItems(
-    _ visibleItems: Set<VisibleItem>,
-    recycleUnusedViews: Bool,
-    viewHandler: (
-      ItemView,
-      VisibleItem,
-      _ previousBackingVisibleItem: VisibleItem?,
-      _ isReusedViewSameAsPreviousView: Bool)
-      -> Void)
+  func reusedViewContexts(
+    visibleItems: Set<VisibleItem>,
+    reuseUnusedViews: Bool)
+    -> [ReusedViewContext]
   {
-    var visibleItemsDifferencesItemViewDifferentiators = [
-      _CalendarItemViewDifferentiator: Set<VisibleItem>
-    ]()
-
-    // For each reuse ID, track the difference between the new set of visible items and the previous
-    // set of visible items. The remaining previous visible items after subtracting the current
-    // visible items are the previously visible items that aren't currently visible, and are
-    // therefore free to be reused.
+    var contexts = [ReusedViewContext]()
+
+    var previousViewsForVisibleItems = viewsForVisibleItems
+    viewsForVisibleItems.removeAll(keepingCapacity: true)
+
     for visibleItem in visibleItems {
-      let differentiator = visibleItem.calendarItemModel._itemViewDifferentiator
+      let viewDifferentiator = visibleItem.calendarItemModel._itemViewDifferentiator
 
-      var visibleItemsDifference: Set<VisibleItem>
-      if let difference = visibleItemsDifferencesItemViewDifferentiators[differentiator] {
-        visibleItemsDifference = difference
-      } else if
-        let previouslyVisibleItems = visibleItemsForItemViewDifferentiators[differentiator]
+      let context: ReusedViewContext =
+        if let view = previousViewsForVisibleItems.removeValue(forKey: visibleItem)
       {
-        visibleItemsDifference = previouslyVisibleItems.subtracting(visibleItems)
+        ReusedViewContext(
+          view: view,
+          visibleItem: visibleItem,
+          isViewReused: true,
+          isReusedViewSameAsPreviousView: true)
+      } else if !(unusedViewsForViewDifferentiators[viewDifferentiator]?.isEmpty ?? true) {
+        ReusedViewContext(
+          view: unusedViewsForViewDifferentiators[viewDifferentiator]!.remove(at: 0),
+          visibleItem: visibleItem,
+          isViewReused: true,
+          isReusedViewSameAsPreviousView: false)
       } else {
-        visibleItemsDifference = []
+        ReusedViewContext(
+          view: ItemView(initialCalendarItemModel: visibleItem.calendarItemModel),
+          visibleItem: visibleItem,
+          isViewReused: false,
+          isReusedViewSameAsPreviousView: false)
       }
 
-      let context = reusedViewContext(
-        for: visibleItem,
-        recycleUnusedViews: recycleUnusedViews,
-        unusedPreviouslyVisibleItems: &visibleItemsDifference)
-      viewHandler(
-        context.view,
-        visibleItem,
-        context.previousBackingVisibleItem,
-        context.isReusedViewSameAsPreviousView)
-
-      visibleItemsDifferencesItemViewDifferentiators[differentiator] = visibleItemsDifference
-    }
-  }
-
-  // MARK: Private
-
-  private var visibleItemsForItemViewDifferentiators = [
-    _CalendarItemViewDifferentiator: Set<VisibleItem>
-  ]()
-  private var viewsForVisibleItems = [VisibleItem: ItemView]()
-
-  private func reusedViewContext(
-    for visibleItem: VisibleItem,
-    recycleUnusedViews: Bool,
-    unusedPreviouslyVisibleItems: inout Set<VisibleItem>)
-    -> ReusedViewContext
-  {
-    let differentiator = visibleItem.calendarItemModel._itemViewDifferentiator
-
-    let view: ItemView
-    let previousBackingVisibleItem: VisibleItem?
-    let isReusedViewSameAsPreviousView: Bool
-
-    if let previouslyVisibleItems = visibleItemsForItemViewDifferentiators[differentiator] {
-      if previouslyVisibleItems.contains(visibleItem) {
-        // New visible item was also an old visible item, so we can just use the same view again.
-
-        guard let previousView = viewsForVisibleItems[visibleItem] else {
-          preconditionFailure("""
-              `viewsForVisibleItems` must have a key for every member in
-              `visibleItemsForItemViewDifferentiators`'s values.
-            """)
-        }
+      contexts.append(context)
 
-        view = previousView
-        previousBackingVisibleItem = visibleItem
-        isReusedViewSameAsPreviousView = true
+      viewsForVisibleItems[visibleItem] = context.view
+    }
 
-        visibleItemsForItemViewDifferentiators[differentiator]?.remove(visibleItem)
-        viewsForVisibleItems.removeValue(forKey: visibleItem)
-      } else {
-        if recycleUnusedViews, let previouslyVisibleItem = unusedPreviouslyVisibleItems.first {
-          // An unused, previously-visible item is available, so reuse it.
-
-          guard let previousView = viewsForVisibleItems[previouslyVisibleItem] else {
-            preconditionFailure("""
-                `viewsForVisibleItems` must have a key for every member in
-                `visibleItemsForItemViewDifferentiators`'s values.
-              """)
-          }
-
-          view = previousView
-          previousBackingVisibleItem = previouslyVisibleItem
-          isReusedViewSameAsPreviousView = false
-
-          unusedPreviouslyVisibleItems.remove(previouslyVisibleItem)
-
-          visibleItemsForItemViewDifferentiators[differentiator]?.remove(previouslyVisibleItem)
-          viewsForVisibleItems.removeValue(forKey: previouslyVisibleItem)
-        } else {
-          // No previously-visible item is available for reuse (or view recycling is disabled), so
-          // create a new view.
-          view = ItemView(initialCalendarItemModel: visibleItem.calendarItemModel)
-          previousBackingVisibleItem = nil
-          isReusedViewSameAsPreviousView = false
-        }
+    if reuseUnusedViews {
+      for (visibleItem, unusedView) in previousViewsForVisibleItems {
+        let viewDifferentiator = visibleItem.calendarItemModel._itemViewDifferentiator
+        unusedViewsForViewDifferentiators[viewDifferentiator, default: .init()].append(unusedView)
       }
-    } else {
-      // No previously-visible item is available for reuse, so create a new view.
-      view = ItemView(initialCalendarItemModel: visibleItem.calendarItemModel)
-      previousBackingVisibleItem = nil
-      isReusedViewSameAsPreviousView = false
     }
 
-    let newVisibleItems = visibleItemsForItemViewDifferentiators[differentiator] ?? []
-    visibleItemsForItemViewDifferentiators[differentiator] = newVisibleItems
-    visibleItemsForItemViewDifferentiators[differentiator]?.insert(visibleItem)
-    viewsForVisibleItems[visibleItem] = view
-
-    return ReusedViewContext(
-      view: view,
-      previousBackingVisibleItem: previousBackingVisibleItem,
-      isReusedViewSameAsPreviousView: isReusedViewSameAsPreviousView)
+    return contexts
   }
 
+  // MARK: Private
+
+  private var viewsForVisibleItems = [VisibleItem: ItemView]()
+  private var unusedViewsForViewDifferentiators = [_CalendarItemViewDifferentiator: [ItemView]]()
+
 }
 
 // MARK: - ReusedViewContext
 
-private struct ReusedViewContext {
+struct ReusedViewContext {
   let view: ItemView
-  let previousBackingVisibleItem: VisibleItem?
+  let visibleItem: VisibleItem
+  let isViewReused: Bool
   let isReusedViewSameAsPreviousView: Bool
 }

Sources/Public/AnyCalendarItemModel.swift

@@ -54,7 +54,6 @@ public protocol AnyCalendarItemModel {
 ///
 /// - Note: There is no reason to create an instance of this enum from your feature code; it should only be invoked internally.
 public struct _CalendarItemViewDifferentiator: Hashable {
-  let viewRepresentableTypeDescription: String
-  let viewTypeDescription: String
+  let viewType: ObjectIdentifier
   let invariantViewProperties: AnyHashable
 }

Sources/Public/CalendarItemModel.swift

@@ -46,8 +46,7 @@ public struct CalendarItemModel<ViewRepresentable>: AnyCalendarItemModel where
     content: ViewRepresentable.Content)
   {
     _itemViewDifferentiator = _CalendarItemViewDifferentiator(
-      viewRepresentableTypeDescription: String(reflecting: ViewRepresentable.self),
-      viewTypeDescription: String(reflecting: ViewRepresentable.ViewType.self),
+      viewType: ObjectIdentifier(ViewRepresentable.self),
       invariantViewProperties: invariantViewProperties)
 
     self.invariantViewProperties = invariantViewProperties
@@ -115,8 +114,7 @@ extension CalendarItemModel where ViewRepresentable.Content == Never {
   ///   and `font`, assuming none of those values change in response to `content` updates.
   public init(invariantViewProperties: ViewRepresentable.InvariantViewProperties) {
     _itemViewDifferentiator = _CalendarItemViewDifferentiator(
-      viewRepresentableTypeDescription: String(reflecting: ViewRepresentable.self),
-      viewTypeDescription: String(reflecting: ViewRepresentable.ViewType.self),
+      viewType: ObjectIdentifier(ViewRepresentable.self),
       invariantViewProperties: invariantViewProperties)
 
     self.invariantViewProperties = invariantViewProperties

Sources/Public/CalendarView.swift

@@ -773,29 +773,30 @@ public final class CalendarView: UIView {
     var viewsToHideForVisibleItems = visibleViewsForVisibleItems
     visibleViewsForVisibleItems.removeAll(keepingCapacity: true)
 
-    reuseManager.viewsForVisibleItems(
-      visibleItems,
-      recycleUnusedViews: !UIAccessibility.isVoiceOverRunning,
-      viewHandler: { view, visibleItem, previousBackingVisibleItem, isReusedViewSameAsPreviousView in
-        UIView.conditionallyPerformWithoutAnimation(when: !isReusedViewSameAsPreviousView) {
-          if view.superview == nil {
-            let insertionIndex = subviewInsertionIndexTracker.insertionIndex(
-              forSubviewWithCorrespondingItemType: visibleItem.itemType)
-            scrollView.insertSubview(view, at: insertionIndex)
-          }
-
-          view.isHidden = false
-
-          configureView(view, with: visibleItem)
+    let contexts = reuseManager.reusedViewContexts(
+      visibleItems: visibleItems,
+      reuseUnusedViews: !UIAccessibility.isVoiceOverRunning)
+
+    for context in contexts {
+      UIView.conditionallyPerformWithoutAnimation(when: !context.isReusedViewSameAsPreviousView) {
+        if context.view.superview == nil {
+          let insertionIndex = subviewInsertionIndexTracker.insertionIndex(
+            forSubviewWithCorrespondingItemType: context.visibleItem.itemType)
+          scrollView.insertSubview(context.view, at: insertionIndex)
         }
 
-        visibleViewsForVisibleItems[visibleItem] = view
+        context.view.isHidden = false
 
-        if let previousBackingVisibleItem {
-          // Don't hide views that were reused
-          viewsToHideForVisibleItems.removeValue(forKey: previousBackingVisibleItem)
-        }
-      })
+        configureView(context.view, with: context.visibleItem)
+      }
+
+      visibleViewsForVisibleItems[context.visibleItem] = context.view
+
+      if context.isViewReused {
+        // Don't hide views that were reused
+        viewsToHideForVisibleItems.removeValue(forKey: context.visibleItem)
+      }
+    }
 
     // Hide any old views that weren't reused. This is faster than adding / removing subviews.
     // If VoiceOver is running, we remove the view to save memory (since views aren't reused).