Bk/performance optimizations

Example/MagazineLayoutExample/PerformanceDemoViewController.swift

@@ -26,7 +26,7 @@ final class PerformanceDemoViewController: UIViewController {
   override func viewDidLoad() {
     super.viewDidLoad()
 
-    title = "Performance (10K Items)"
+    title = "Performance (100K Items)"
     view.backgroundColor = .systemBackground
 
     navigationItem.rightBarButtonItems = [
@@ -71,8 +71,8 @@ final class PerformanceDemoViewController: UIViewController {
   ]
 
   private func loadInitialData() {
-    // Create 10,000 items
-    items = (0..<10_000).map { index in
+    // Create 100,000 items
+    items = (0..<100_000).map { index in
       let color = colors[index % colors.count]
       let item = PerformanceItem(id: nextItemID, color: color)
       nextItemID += 1

Example/MagazineLayoutExample/RootMenuViewController.swift

@@ -198,7 +198,7 @@ private enum DemoOption: String, CaseIterable {
     case .messageThread:
       return "Bottom-to-top layout with pagination"
     case .performance:
-      return "10,000 items with traditional data source"
+      return "100,000 items with traditional data source"
     }
   }
 

MagazineLayout/LayoutCore/SectionModel.swift

@@ -38,7 +38,10 @@ struct SectionModel {
     numberOfRows = 0
 
     updateIndexOfFirstInvalidatedRowIfNecessary(toProposedIndex: 0)
-    calculateElementFramesIfNecessary()
+
+    if !MagazineLayout._enableExperimentalOptimizations {
+      calculateElementFramesIfNecessary()
+    }
   }
 
   // MARK: Internal
@@ -257,15 +260,22 @@ struct SectionModel {
   }
 
   mutating func removeFooter() -> Bool {
-    guard let indexOfFooter = indexOfFooterRow() else {
+    guard footerModel != nil else {
       return false
     }
-    updateIndexOfFirstInvalidatedRowIfNecessary(toProposedIndex: indexOfFooter)
+    // `indexOfFooterRow()` is `nil` if the section hasn't been laid out yet (deferred layout
+    // calculation). In that case the whole section is already invalidated from row 0, so there's no
+    // additional row to invalidate.
+    if let indexOfFooter = indexOfFooterRow() {
+      updateIndexOfFirstInvalidatedRowIfNecessary(toProposedIndex: indexOfFooter)
+    }
     footerModel = nil
     return true
   }
 
   mutating func updateItemHeight(toPreferredHeight preferredHeight: CGFloat, atIndex index: Int) {
+    calculateElementFramesIfNecessary()
+
     // Accessing this array using an unsafe, untyped (raw) pointer avoids expensive copy-on-writes
     // and Swift retain / release calls.
     itemModels.withUnsafeMutableBufferPointer { directlyMutableItemModels in
@@ -292,6 +302,8 @@ struct SectionModel {
   }
 
   mutating func updateHeaderHeight(toPreferredHeight preferredHeight: CGFloat) {
+    calculateElementFramesIfNecessary()
+
     headerModel?.preferredHeight = preferredHeight
 
     if let indexOfHeaderRow = indexOfHeaderRow(), let headerModel = headerModel {
@@ -312,6 +324,8 @@ struct SectionModel {
   }
 
   mutating func updateFooterHeight(toPreferredHeight preferredHeight: CGFloat) {
+    calculateElementFramesIfNecessary()
+
     footerModel?.preferredHeight = preferredHeight
 
     if let indexOfFooterRow = indexOfFooterRow(), let footerModel = footerModel {
@@ -393,7 +407,10 @@ struct SectionModel {
   }
 
   private func indexOfFooterRow() -> Int? {
-    guard footerModel != nil else { return nil }
+    // `numberOfRows` is 0 until the section's element frames have been calculated. With deferred
+    // layout calculation, the footer's row index isn't known yet in that state, so we return `nil`
+    // rather than a bogus `numberOfRows - 1` (which would be -1).
+    guard footerModel != nil, numberOfRows > 0 else { return nil }
     return numberOfRows - 1
   }
 

MagazineLayout/Public/MagazineLayout.swift

@@ -42,6 +42,9 @@ public final class MagazineLayout: UICollectionViewLayout {
 
   // MARK: Public
 
+  /// A temporary flag to enable safely testing some optimizations.
+  public static var _enableExperimentalOptimizations = false
+
   /// The vertical layout direction of items in the collection view. This property changes the behavior of
   /// scroll-position-preservation when performing batch updates or when the collection view's bounds changes.
   public var verticalLayoutDirection = MagazineLayoutVerticalLayoutDirection.topToBottom
@@ -60,12 +63,14 @@ public final class MagazineLayout: UICollectionViewLayout {
 
   override public var collectionViewContentSize: CGSize {
     guard collectionView != nil else { return .zero }
-    return layoutState.contentSize
+    return updatedLayoutState().contentSize
   }
 
   override public func prepare() {
     super.prepare()
 
+    _delegateMagazineLayout = currentCollectionView.delegate as? UICollectionViewDelegateMagazineLayout
+
     // Save the previous collection view width if necessary
     if prepareActions.contains(.cachePreviousWidth) {
       cachedCollectionViewWidth = currentCollectionView.bounds.width
@@ -78,9 +83,15 @@ public final class MagazineLayout: UICollectionViewLayout {
       hasPinnedHeaderOrFooter = false
     }
 
+    var reusableIndexPath = IndexPath(item: 0, section: 0)
+
     // Update layout metrics if necessary
     if prepareActions.contains(.updateLayoutMetrics) {
       for sectionIndex in 0..<modelState.numberOfSections {
+        if Self._enableExperimentalOptimizations {
+          reusableIndexPath.section = sectionIndex
+        }
+
         let sectionMetrics = metricsForSection(atIndex: sectionIndex)
         modelState.updateMetrics(to: sectionMetrics, forSectionAtIndex: sectionIndex)
 
@@ -103,25 +114,37 @@ public final class MagazineLayout: UICollectionViewLayout {
         }
 
         let numberOfItems = modelState.numberOfItems(inSectionAtIndex: sectionIndex)
+        var indexPath = IndexPath(item: 0, section: sectionIndex)
         for itemIndex in 0..<numberOfItems {
-          let indexPath = IndexPath(item: itemIndex, section: sectionIndex)
-          modelState.updateItemSizeMode(to: sizeModeForItem(at: indexPath), forItemAt: indexPath)
+          if Self._enableExperimentalOptimizations {
+            reusableIndexPath.item = itemIndex
+            modelState.updateItemSizeMode(to: sizeModeForItem(at: indexPath), forItemAt: reusableIndexPath)
+          } else {
+            indexPath = IndexPath(item: itemIndex, section: sectionIndex)
+            modelState.updateItemSizeMode(to: sizeModeForItem(at: indexPath), forItemAt: indexPath)
+          }
         }
       }
     }
 
     // Recreate section models from scratch if necessary
     if prepareActions.contains(.recreateSectionModels) {
       layoutStateBeforeRecreateSectionModels = LayoutState(
-        modelState: layoutState.modelState.copy(),
+        modelState: modelState.copy(),
         bounds: currentCollectionView.bounds,
         contentInset: contentInset,
         scale: scale,
         verticalLayoutDirection: verticalLayoutDirection)
 
       var sections = [SectionModel]()
       for sectionIndex in 0..<currentCollectionView.numberOfSections {
-        let sectionModel = sectionModelForSection(atIndex: sectionIndex)
+        if Self._enableExperimentalOptimizations {
+          reusableIndexPath.section = sectionIndex
+        }
+
+        let sectionModel = sectionModelForSection(
+          atIndex: sectionIndex,
+          reusableIndexPath: &reusableIndexPath)
         sections.append(sectionModel)
       }
 
@@ -133,14 +156,15 @@ public final class MagazineLayout: UICollectionViewLayout {
 
   override public func prepare(forCollectionViewUpdates updateItems: [UICollectionViewUpdateItem]) {
     let layoutStateBeforeCollectionViewUpdates = LayoutState(
-      modelState: layoutState.modelState.copy(),
+      modelState: modelState.copy(),
       bounds: currentCollectionView.bounds,
       contentInset: contentInset,
       scale: scale,
       verticalLayoutDirection: verticalLayoutDirection)
     self.layoutStateBeforeCollectionViewUpdates = layoutStateBeforeCollectionViewUpdates
 
     var updates = [CollectionViewUpdate<SectionModel, ItemModel>]()
+    var reusableIndexPath = IndexPath(item: 0, section: 0)
 
     for updateItem in updateItems {
       let updateAction = updateItem.updateAction
@@ -154,7 +178,12 @@ public final class MagazineLayout: UICollectionViewLayout {
         }
 
         if indexPath.item == NSNotFound {
-          let sectionModel = sectionModelForSection(atIndex: indexPath.section)
+          if Self._enableExperimentalOptimizations {
+            reusableIndexPath.section = indexPath.section
+          }
+          let sectionModel = sectionModelForSection(
+            atIndex: indexPath.section,
+            reusableIndexPath: &reusableIndexPath)
           updates.append(.sectionReload(sectionIndex: indexPath.section, newSection: sectionModel))
         } else {
           let itemModel = itemModelForItem(at: indexPath)
@@ -182,7 +211,12 @@ public final class MagazineLayout: UICollectionViewLayout {
         }
 
         if indexPath.item == NSNotFound {
-          let sectionModel = sectionModelForSection(atIndex: indexPath.section)
+          if Self._enableExperimentalOptimizations {
+            reusableIndexPath.section = indexPath.section
+          }
+          let sectionModel = sectionModelForSection(
+            atIndex: indexPath.section,
+            reusableIndexPath: &reusableIndexPath)
           updates.append(.sectionInsert(sectionIndex: indexPath.section, newSection: sectionModel))
         } else {
           let itemModel = itemModelForItem(at: indexPath)
@@ -227,6 +261,7 @@ public final class MagazineLayout: UICollectionViewLayout {
 
     if let layoutStateBeforeCollectionViewUpdates{
       let targetContentOffsetAnchor = layoutStateBeforeCollectionViewUpdates.targetContentOffsetAnchor
+      let layoutState = updatedLayoutState()
       let targetYOffset = layoutState.yOffset(
         for: targetContentOffsetAnchor,
         isPerformingBatchUpdates: true)
@@ -248,7 +283,7 @@ public final class MagazineLayout: UICollectionViewLayout {
 
     if currentCollectionView.bounds.size != oldBounds.size {
       layoutStateBeforeAnimatedBoundsChange = LayoutState(
-        modelState: layoutState.modelState.copy(),
+        modelState: modelState.copy(),
         bounds: oldBounds,
         contentInset: contentInset,
         scale: scale,
@@ -670,13 +705,15 @@ public final class MagazineLayout: UICollectionViewLayout {
       withOriginalAttributes: originalAttributes) as! MagazineLayoutInvalidationContext
     context.invalidateLayoutMetrics = false
 
+    let layoutState = updatedLayoutState()
+
     switch preferredAttributes.representedElementCategory {
     case .cell:
       let targetContentOffsetAnchor = (
         layoutStateBeforeRecreateSectionModels ??
           layoutStateBeforeCollectionViewUpdates ??
           layoutStateBeforeAnimatedBoundsChange ??
-          self.layoutState
+          layoutState
       ).targetContentOffsetAnchor
       let targetYOffsetBefore = layoutState.yOffset(
         for: targetContentOffsetAnchor,
@@ -823,7 +860,7 @@ public final class MagazineLayout: UICollectionViewLayout {
       return super.targetContentOffset(forProposedContentOffset: proposedContentOffset)
     }
 
-    let yOffset = layoutState.yOffset(
+    let yOffset = updatedLayoutState().yOffset(
       for: layoutStateBefore.targetContentOffsetAnchor,
       isPerformingBatchUpdates: layoutStateBeforeCollectionViewUpdates != nil)
 
@@ -885,6 +922,8 @@ public final class MagazineLayout: UICollectionViewLayout {
   private var cachedCollectionViewWidth: CGFloat?
   private var previousContentInset: UIEdgeInsets?
 
+  private var _delegateMagazineLayout: UICollectionViewDelegateMagazineLayout?
+
   private var currentCollectionView: UICollectionView {
     guard let collectionView = collectionView else {
       preconditionFailure("`collectionView` should not be `nil`")
@@ -917,10 +956,6 @@ public final class MagazineLayout: UICollectionViewLayout {
       height: currentCollectionView.bounds.height - contentInset.top - contentInset.bottom + refreshControlHeight)
   }
 
-  private var delegateMagazineLayout: UICollectionViewDelegateMagazineLayout? {
-    return currentCollectionView.delegate as? UICollectionViewDelegateMagazineLayout
-  }
-
   private var scale: CGFloat {
     collectionView?.traitCollection.nonZeroDisplayScale ?? 1
   }
@@ -929,18 +964,31 @@ public final class MagazineLayout: UICollectionViewLayout {
     currentCollectionView.adjustedContentInset
   }
 
-  private var layoutState: LayoutState {
+  private var modelState: ModelState {
+    if MagazineLayout._enableExperimentalOptimizations {
+      _layoutState.modelState
+    } else {
+      updatedLayoutState().modelState
+    }
+  }
+
+  private var delegateMagazineLayout: UICollectionViewDelegateMagazineLayout? {
+    if MagazineLayout._enableExperimentalOptimizations {
+      _delegateMagazineLayout
+    } else {
+      currentCollectionView.delegate as? UICollectionViewDelegateMagazineLayout
+    }
+  }
+
+  /// Relatively expensive compared to just grabbing the `_layoutState`; only use if you need updated metrics.
+  private func updatedLayoutState() -> LayoutState {
     _layoutState.bounds = currentCollectionView.bounds
     _layoutState.contentInset = contentInset
     _layoutState.scale = scale
     _layoutState.verticalLayoutDirection = verticalLayoutDirection
     return _layoutState
   }
 
-  private var modelState: ModelState {
-    layoutState.modelState
-  }
-
   private func metricsForSection(atIndex sectionIndex: Int) -> MagazineLayoutSectionMetrics {
     guard let delegateMagazineLayout = delegateMagazineLayout else {
       return MagazineLayoutSectionMetrics.defaultSectionMetrics(
@@ -1045,9 +1093,23 @@ public final class MagazineLayout: UICollectionViewLayout {
     }
   }
 
-  private func sectionModelForSection(atIndex sectionIndex: Int) -> SectionModel {
-    let itemModels = (0..<currentCollectionView.numberOfItems(inSection: sectionIndex)).map {
-      itemModelForItem(at: IndexPath(item: $0, section: sectionIndex))
+  private func sectionModelForSection(
+    atIndex sectionIndex: Int,
+    reusableIndexPath: inout IndexPath)
+    -> SectionModel
+  {
+    let numberOfItems = currentCollectionView.numberOfItems(inSection: sectionIndex)
+    var itemModels = [ItemModel]()
+    itemModels.reserveCapacity(numberOfItems)
+
+    for itemIndex in 0..<numberOfItems {
+      if Self._enableExperimentalOptimizations {
+        reusableIndexPath.item = itemIndex
+        itemModels.append(itemModelForItem(at: reusableIndexPath))
+      } else {
+        itemModels.append(itemModelForItem(at: IndexPath(item: itemIndex, section: sectionIndex)))
+      }
+
     }
 
     return SectionModel(

MagazineLayout/Public/MagazineLayoutInvalidationContext.swift

@@ -20,15 +20,12 @@ import UIKit
 /// Used to indicate that collection view properties and/or delegate layout metrics changed.
 public final class MagazineLayoutInvalidationContext: UICollectionViewLayoutInvalidationContext {
 
-  /// A temporary flag to enable safely testing a change to how layout invalidation works.
-  public static var _invalidateLayoutMetricsDefaultValue = true
-
   /// Indicates whether to recompute the positions and sizes of elements based on the current collection view and delegate layout
   /// metrics.
   ///
-  /// Defaults to `false`. Set to `true` when delegate-provided layout values (e.g. item size
+  /// Set to `true` when delegate-provided layout values (e.g. item size
   /// modes, header/footer visibility, section metrics) have changed and the layout needs to
   /// re-query the delegate.
-  public var invalidateLayoutMetrics = _invalidateLayoutMetricsDefaultValue
+  public var invalidateLayoutMetrics = !MagazineLayout._enableExperimentalOptimizations
 
 }