Improve performance of layout model IDs

MagazineLayout.xcodeproj/project.pbxproj

@@ -38,6 +38,7 @@
 		93A1C04D21ACED1100DED67D /* TestingSupport.swift in Sources */ = {isa = PBXBuildFile; fileRef = 93A1C00E21ACED0100DED67D /* TestingSupport.swift */; };
 		93A1C04E21ACED1100DED67D /* ElementLocationFramePairsTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 93A1C00F21ACED0100DED67D /* ElementLocationFramePairsTests.swift */; };
 		93A1C04F21ACED1100DED67D /* ModelStateUpdateTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 93A1C01021ACED0100DED67D /* ModelStateUpdateTests.swift */; };
+		93A868552EE0D7870027691E /* IDGenerator.swift in Sources */ = {isa = PBXBuildFile; fileRef = 93A868542EE0D7870027691E /* IDGenerator.swift */; };
 		FCAC642622085AF100973F4C /* MagazineLayoutHeaderVisibilityMode.swift in Sources */ = {isa = PBXBuildFile; fileRef = FCAC642522085AF100973F4C /* MagazineLayoutHeaderVisibilityMode.swift */; };
 		FCAC642822085B0E00973F4C /* FooterModel.swift in Sources */ = {isa = PBXBuildFile; fileRef = FCAC642722085B0E00973F4C /* FooterModel.swift */; };
 		FD244FEF28B41F9900046C0D /* UITraitCollection+DisplayScale.swift in Sources */ = {isa = PBXBuildFile; fileRef = FD244FEE28B41F9900046C0D /* UITraitCollection+DisplayScale.swift */; };
@@ -94,6 +95,7 @@
 		93A1C02B21ACED0100DED67D /* ModelState.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = ModelState.swift; sourceTree = "<group>"; };
 		93A1C05321ACEDFC00DED67D /* LICENSE */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = text; path = LICENSE; sourceTree = SOURCE_ROOT; };
 		93A1C05421ACEDFC00DED67D /* README.md */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = net.daringfireball.markdown; path = README.md; sourceTree = SOURCE_ROOT; };
+		93A868542EE0D7870027691E /* IDGenerator.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = IDGenerator.swift; sourceTree = "<group>"; };
 		FCAC642522085AF100973F4C /* MagazineLayoutHeaderVisibilityMode.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = MagazineLayoutHeaderVisibilityMode.swift; sourceTree = "<group>"; };
 		FCAC642722085B0E00973F4C /* FooterModel.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = FooterModel.swift; sourceTree = "<group>"; };
 		FD23F5F021AF4A1B00AA78D4 /* Info.plist */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = text.plist.xml; path = Info.plist; sourceTree = "<group>"; };
@@ -228,6 +230,7 @@
 				93540AAF282E25D90008BD6F /* ScreenPixelAlignment.swift */,
 				FDABC2572B23D0A000C9B8EF /* TargetContentOffsetAnchor.swift */,
 				FD244FEE28B41F9900046C0D /* UITraitCollection+DisplayScale.swift */,
+				93A868542EE0D7870027691E /* IDGenerator.swift */,
 			);
 			path = Types;
 			sourceTree = "<group>";
@@ -363,6 +366,7 @@
 				93A1C04421ACED0100DED67D /* SectionModel.swift in Sources */,
 				93A1C03E21ACED0100DED67D /* ElementLocation.swift in Sources */,
 				93A1C04821ACED0100DED67D /* ModelState.swift in Sources */,
+				93A868552EE0D7870027691E /* IDGenerator.swift in Sources */,
 				9398462A2296864200E442DA /* RowOffsetTracker.swift in Sources */,
 				FD4DFF0821B0C737001F46CE /* MagazineLayoutCollectionViewCell.swift in Sources */,
 				93443FD22EB582AE00D60F56 /* LayoutState.swift in Sources */,

MagazineLayout/LayoutCore/BackgroundModel.swift

@@ -14,23 +14,19 @@
 // limitations under the License.
 
 import CoreGraphics
-import Foundation
 
 /// Represents the layout information for a background in a section.
 struct BackgroundModel {
 
   // MARK: Lifecycle
 
   init() {
-    id = NSUUID().uuidString
     originInSection = .zero
     size = .zero
   }
 
   // MARK: Internal
 
-  let id: String
-
   var originInSection: CGPoint
   var size: CGSize
 

MagazineLayout/LayoutCore/ItemModel.swift

@@ -14,23 +14,22 @@
 // limitations under the License.
 
 import CoreGraphics
-import Foundation
 
 /// Represents the layout information for an item in a section.
 struct ItemModel {
 
   // MARK: Lifecycle
 
-  init(sizeMode: MagazineLayoutItemSizeMode, height: CGFloat) {
-    id = UUID()
+  init(idGenerator: IDGenerator, sizeMode: MagazineLayoutItemSizeMode, height: CGFloat) {
+    id = idGenerator.next()
     self.sizeMode = sizeMode
     originInSection = .zero
     size = CGSize(width: 0, height: height)
   }
 
   // MARK: Internal
 
-  let id: UUID
+  let id: UInt64
 
   var sizeMode: MagazineLayoutItemSizeMode
   var originInSection: CGPoint

MagazineLayout/LayoutCore/ModelState.swift

@@ -40,7 +40,7 @@ final class ModelState {
     sectionModels[sectionIndex].numberOfItems
   }
 
-  func idForItemModel(at indexPath: IndexPath) -> UUID? {
+  func idForItemModel(at indexPath: IndexPath) -> UInt64? {
     guard
       indexPath.section < sectionModels.count,
       indexPath.item < sectionModels[indexPath.section].numberOfItems else
@@ -52,7 +52,7 @@ final class ModelState {
     return sectionModels[indexPath.section].idForItemModel(atIndex: indexPath.item)
   }
 
-  func indexPathForItemModel(withID id: UUID) -> IndexPath? {
+  func indexPathForItemModel(withID id: UInt64) -> IndexPath? {
     for sectionIndex in 0..<sectionModels.count {
       guard let index = sectionModels[sectionIndex].indexForItemModel(withID: id) else {
         continue
@@ -63,7 +63,7 @@ final class ModelState {
     return nil
   }
 
-  func idForSectionModel(atIndex index: Int) -> UUID? {
+  func idForSectionModel(atIndex index: Int) -> UInt64? {
     guard index < sectionModels.count else {
       // This occurs when getting layout attributes for initial / final animations
       return nil
@@ -72,7 +72,7 @@ final class ModelState {
     return sectionModels[index].id
   }
 
-  func indexForSectionModel(withID id: UUID) -> Int? {
+  func indexForSectionModel(withID id: UInt64) -> Int? {
     for sectionIndex in 0..<sectionModels.count {
       guard sectionModels[sectionIndex].id == id else { continue }
       return sectionIndex

MagazineLayout/LayoutCore/SectionModel.swift

@@ -14,21 +14,21 @@
 // limitations under the License.
 
 import CoreGraphics
-import Foundation
 
 /// Represents the layout information for a section.
 struct SectionModel {
 
   // MARK: Lifecycle
 
   init(
+    idGenerator: IDGenerator,
     itemModels: [ItemModel],
     headerModel: HeaderModel?,
     footerModel: FooterModel?,
     backgroundModel: BackgroundModel?,
     metrics: MagazineLayoutSectionMetrics)
   {
-    id = UUID()
+    id = idGenerator.next()
     self.itemModels = itemModels
     self.headerModel = headerModel
     self.footerModel = footerModel
@@ -43,7 +43,7 @@ struct SectionModel {
 
   // MARK: Internal
 
-  let id: UUID
+  let id: UInt64
 
   private(set) var headerModel: HeaderModel?
   private(set) var footerModel: FooterModel?
@@ -55,11 +55,11 @@ struct SectionModel {
     return itemModels.count
   }
 
-  func idForItemModel(atIndex index: Int) -> UUID {
+  func idForItemModel(atIndex index: Int) -> UInt64 {
     return itemModels[index].id
   }
 
-  func indexForItemModel(withID id: UUID) -> Int? {
+  func indexForItemModel(withID id: UInt64) -> Int? {
     return itemModels.firstIndex { $0.id == id }
   }
 

MagazineLayout/LayoutCore/Types/IDGenerator.swift

@@ -0,0 +1,29 @@
+// Created by Bryan Keller on 12/3/25.
+// Copyright © 2025 Airbnb Inc. All rights reserved.
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+/// Generates sequential `UInt64` IDs for section and item models. 18,446,744,073,709,551,615 ought to be enough for any layout.
+final class IDGenerator {
+
+  // MARK: Internal
+
+  func next() -> UInt64 {
+    defer { id &+= 1 }
+    return id
+  }
+
+  // MARK: Private
+
+  private var id: UInt64 = 0
+}

MagazineLayout/LayoutCore/Types/TargetContentOffsetAnchor.swift

@@ -21,6 +21,6 @@ import UIKit
 enum TargetContentOffsetAnchor: Equatable {
   case top
   case bottom
-  case topItem(id: UUID, distanceFromTop: CGFloat)
-  case bottomItem(id: UUID, distanceFromBottom: CGFloat)
+  case topItem(id: UInt64, distanceFromTop: CGFloat)
+  case bottomItem(id: UInt64, distanceFromBottom: CGFloat)
 }

MagazineLayout/Public/MagazineLayout.swift

@@ -827,6 +827,7 @@ public final class MagazineLayout: UICollectionViewLayout {
   // MARK: Private
 
   private let _flipsHorizontallyInOppositeLayoutDirection: Bool
+  private let idGenerator = IDGenerator()
 
   private lazy var _layoutState = LayoutState(
     modelState: ModelState(currentVisibleBoundsProvider: { [weak self] in
@@ -1042,6 +1043,7 @@ public final class MagazineLayout: UICollectionViewLayout {
     }
 
     return SectionModel(
+      idGenerator: idGenerator,
       itemModels: itemModels,
       headerModel: headerModelForHeader(inSectionAtIndex: sectionIndex),
       footerModel: footerModelForFooter(inSectionAtIndex: sectionIndex),
@@ -1052,6 +1054,7 @@ public final class MagazineLayout: UICollectionViewLayout {
   private func itemModelForItem(at indexPath: IndexPath) -> ItemModel {
     let itemSizeMode = sizeModeForItem(at: indexPath)
     return ItemModel(
+      idGenerator: idGenerator,
       sizeMode: itemSizeMode,
       height: initialItemHeight(from: itemSizeMode))
   }

Tests/LayoutStateTargetContentOffsetTests.swift

@@ -224,25 +224,28 @@ final class LayoutStateTargetContentOffsetTests: XCTestCase {
 
   // MARK: Private
 
+  private let idGenerator = IDGenerator()
+
   private func modelState(bounds: CGRect) -> ModelState {
     let modelState = ModelState(currentVisibleBoundsProvider: { bounds })
     let sections = [
       SectionModel(
+        idGenerator: idGenerator,
         itemModels: [
-          ItemModel(widthMode: .halfWidth, preferredHeight: nil),
-          ItemModel(widthMode: .halfWidth, preferredHeight: 70),
-          ItemModel(widthMode: .halfWidth, preferredHeight: 90),
-          ItemModel(widthMode: .halfWidth, preferredHeight: 80),
-          ItemModel(widthMode: .fullWidth(respectsHorizontalInsets: true), preferredHeight: nil),
-          ItemModel(widthMode: .fullWidth(respectsHorizontalInsets: true), preferredHeight: 135),
-          ItemModel(widthMode: .fullWidth(respectsHorizontalInsets: true), preferredHeight: 135),
-          ItemModel(widthMode: .halfWidth, preferredHeight: 55),
-          ItemModel(widthMode: .halfWidth, preferredHeight: 105),
-          ItemModel(widthMode: .halfWidth, preferredHeight: 80),
-          ItemModel(widthMode: .halfWidth, preferredHeight: 95),
-          ItemModel(widthMode: .thirdWidth, preferredHeight: 200),
-          ItemModel(widthMode: .thirdWidth, preferredHeight: 200),
-          ItemModel(widthMode: .thirdWidth, preferredHeight: nil),
+          ItemModel(idGenerator: idGenerator, widthMode: .halfWidth, preferredHeight: nil),
+          ItemModel(idGenerator: idGenerator, widthMode: .halfWidth, preferredHeight: 70),
+          ItemModel(idGenerator: idGenerator, widthMode: .halfWidth, preferredHeight: 90),
+          ItemModel(idGenerator: idGenerator, widthMode: .halfWidth, preferredHeight: 80),
+          ItemModel(idGenerator: idGenerator, widthMode: .fullWidth(respectsHorizontalInsets: true), preferredHeight: nil),
+          ItemModel(idGenerator: idGenerator, widthMode: .fullWidth(respectsHorizontalInsets: true), preferredHeight: 135),
+          ItemModel(idGenerator: idGenerator, widthMode: .fullWidth(respectsHorizontalInsets: true), preferredHeight: 135),
+          ItemModel(idGenerator: idGenerator, widthMode: .halfWidth, preferredHeight: 55),
+          ItemModel(idGenerator: idGenerator, widthMode: .halfWidth, preferredHeight: 105),
+          ItemModel(idGenerator: idGenerator, widthMode: .halfWidth, preferredHeight: 80),
+          ItemModel(idGenerator: idGenerator, widthMode: .halfWidth, preferredHeight: 95),
+          ItemModel(idGenerator: idGenerator, widthMode: .thirdWidth, preferredHeight: 200),
+          ItemModel(idGenerator: idGenerator, widthMode: .thirdWidth, preferredHeight: 200),
+          ItemModel(idGenerator: idGenerator, widthMode: .thirdWidth, preferredHeight: nil),
         ],
         headerModel: nil,
         footerModel: nil,
@@ -264,11 +267,12 @@ final class LayoutStateTargetContentOffsetTests: XCTestCase {
     let modelState = ModelState(currentVisibleBoundsProvider: { bounds })
     let sections = [
       SectionModel(
+        idGenerator: idGenerator,
         itemModels: [
           // Create items that are 500px tall, larger than the 400px bounds height
-          ItemModel(widthMode: .fullWidth(respectsHorizontalInsets: true), preferredHeight: 500),
-          ItemModel(widthMode: .fullWidth(respectsHorizontalInsets: true), preferredHeight: 500),
-          ItemModel(widthMode: .fullWidth(respectsHorizontalInsets: true), preferredHeight: 500),
+          ItemModel(idGenerator: idGenerator, widthMode: .fullWidth(respectsHorizontalInsets: true), preferredHeight: 500),
+          ItemModel(idGenerator: idGenerator, widthMode: .fullWidth(respectsHorizontalInsets: true), preferredHeight: 500),
+          ItemModel(idGenerator: idGenerator, widthMode: .fullWidth(respectsHorizontalInsets: true), preferredHeight: 500),
         ],
         headerModel: nil,
         footerModel: nil,
@@ -291,8 +295,15 @@ final class LayoutStateTargetContentOffsetTests: XCTestCase {
 // MARK: - ItemModel
 
 private extension ItemModel {
-  init(widthMode: MagazineLayoutItemWidthMode, preferredHeight: CGFloat?) {
-    self.init(sizeMode: .init(widthMode: widthMode, heightMode: .dynamic), height: 150)
+  init(
+    idGenerator: IDGenerator,
+    widthMode: MagazineLayoutItemWidthMode,
+    preferredHeight: CGFloat?)
+  {
+    self.init(
+      idGenerator: idGenerator,
+      sizeMode: .init(widthMode: widthMode, heightMode: .dynamic),
+      height: 150)
     self.preferredHeight = preferredHeight
   }
 }

Tests/ModelStateEmptySectionLayoutTests.swift

@@ -45,12 +45,14 @@ final class ModelStateEmptySectionLayoutTests: XCTestCase {
 
     let initialSections = [
       SectionModel(
+        idGenerator: idGenerator,
         itemModels: [],
         headerModel: nil,
         footerModel: nil,
         backgroundModel: nil,
         metrics: metrics0),
       SectionModel(
+        idGenerator: idGenerator,
         itemModels: [],
         headerModel: nil,
         footerModel: nil,
@@ -82,6 +84,7 @@ final class ModelStateEmptySectionLayoutTests: XCTestCase {
 
     let initialSections = [
       SectionModel(
+        idGenerator: idGenerator,
         itemModels: [],
         headerModel: HeaderModel(
           heightMode: .static(height: 45),
@@ -94,6 +97,7 @@ final class ModelStateEmptySectionLayoutTests: XCTestCase {
         backgroundModel: BackgroundModel(),
         metrics: metrics0),
       SectionModel(
+        idGenerator: idGenerator,
         itemModels: [],
         headerModel: HeaderModel(
           heightMode: .static(height: 65),
@@ -145,6 +149,8 @@ final class ModelStateEmptySectionLayoutTests: XCTestCase {
 
   // MARK: Private
 
+  private let idGenerator = IDGenerator()
+
   private var modelState: ModelState!
 
 }