Optimize data structures and defer layout work#160
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bryankeller
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Jun 9, 2026
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| if MagazineLayout._enableExperimentalOptimizations { | ||
| newHeaderLocationsForFlattenedIndices.removeAll(keepingCapacity: true) | ||
| newFooterLocationsForFlattenedIndices.removeAll(keepingCapacity: true) | ||
| newBackgroundLocationsForFlattenedIndices.removeAll(keepingCapacity: true) | ||
| newItemLocationsForFlattenedIndices.removeAll(keepingCapacity: true) | ||
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| for sectionIndex in 0..<sectionModels.count { | ||
| if sectionModels[sectionIndex].headerModel != nil { | ||
| let elementLocation = ElementLocation(elementIndex: 0, sectionIndex: sectionIndex) | ||
| newHeaderLocationsForFlattenedIndices.append(elementLocation) | ||
| } | ||
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| if sectionModels[sectionIndex].footerModel != nil { | ||
| let elementLocation = ElementLocation(elementIndex: 0, sectionIndex: sectionIndex) | ||
| newFooterLocationsForFlattenedIndices.append(elementLocation) | ||
| } | ||
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| if sectionModels[sectionIndex].backgroundModel != nil { | ||
| backgroundLocationsForFlattenedIndices[flattenedBackgroundIndex] = ElementLocation( | ||
| elementIndex: 0, | ||
| sectionIndex: sectionIndex) | ||
| flattenedBackgroundIndex += 1 | ||
| if sectionModels[sectionIndex].backgroundModel != nil { | ||
| let elementLocation = ElementLocation(elementIndex: 0, sectionIndex: sectionIndex) | ||
| newBackgroundLocationsForFlattenedIndices.append(elementLocation) | ||
| } | ||
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| for itemIndex in 0..<sectionModels[sectionIndex].numberOfItems { | ||
| let elementLocation = ElementLocation(elementIndex: itemIndex, sectionIndex: sectionIndex) | ||
| newItemLocationsForFlattenedIndices.append(elementLocation) | ||
| } |
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same logic as the existing else branch, but using the new arrays instead of dictionaries
bryankeller
commented
Jun 9, 2026
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| headerLocationsForFlattenedIndices.removeAll() | ||
| footerLocationsForFlattenedIndices.removeAll() | ||
| backgroundLocationsForFlattenedIndices.removeAll() | ||
| itemLocationsForFlattenedIndices.removeAll() | ||
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| var flattenedHeaderIndex = 0 | ||
| var flattenedFooterIndex = 0 | ||
| var flattenedBackgroundIndex = 0 | ||
| var flattenedItemIndex = 0 | ||
| for sectionIndex in 0..<sectionModels.count { | ||
| if sectionModels[sectionIndex].headerModel != nil { | ||
| headerLocationsForFlattenedIndices[flattenedHeaderIndex] = ElementLocation( | ||
| elementIndex: 0, | ||
| sectionIndex: sectionIndex) | ||
| flattenedHeaderIndex += 1 | ||
| } | ||
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| if sectionModels[sectionIndex].footerModel != nil { | ||
| footerLocationsForFlattenedIndices[flattenedFooterIndex] = ElementLocation( | ||
| elementIndex: 0, | ||
| sectionIndex: sectionIndex) | ||
| flattenedFooterIndex += 1 | ||
| } | ||
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| if sectionModels[sectionIndex].backgroundModel != nil { | ||
| backgroundLocationsForFlattenedIndices[flattenedBackgroundIndex] = ElementLocation( | ||
| elementIndex: 0, | ||
| sectionIndex: sectionIndex) | ||
| flattenedBackgroundIndex += 1 | ||
| } | ||
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| for itemIndex in 0..<sectionModels[sectionIndex].numberOfItems { | ||
| itemLocationsForFlattenedIndices[flattenedItemIndex] = ElementLocation( | ||
| elementIndex: itemIndex, | ||
| sectionIndex: sectionIndex) | ||
| flattenedItemIndex += 1 | ||
| for itemIndex in 0..<sectionModels[sectionIndex].numberOfItems { | ||
| itemLocationsForFlattenedIndices[flattenedItemIndex] = ElementLocation( | ||
| elementIndex: itemIndex, | ||
| sectionIndex: sectionIndex) | ||
| flattenedItemIndex += 1 | ||
| } |
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existing code, just indented and put in an else branch
bryankeller
commented
Jun 9, 2026
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| private var newItemIndicesForRowIndices = [ClosedRange<Int>?]() | ||
| private var newRowIndicesForItemIndices = [Int?]() | ||
| private var newItemRowHeightsForRowIndices = [CGFloat]() |
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more dictionary -> array flattening. I also simplified some types, like using a ClosedRange<Int> to represent the item indices in a particular row of content, rather than using an array for that. The indices in a row are always contiguous, so no reason to incur array allocation overhead just to say "row 5 contains item indices 9...10"
bryankeller
commented
Jun 9, 2026
Comment on lines
+678
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| if MagazineLayout._enableExperimentalOptimizations { | ||
| newItemIndicesForRowIndices.grow(toInclude: rowIndex, fillingWith: nil) | ||
| if let range = newItemIndicesForRowIndices[rowIndex] { | ||
| newItemIndicesForRowIndices[rowIndex] = range.lowerBound...itemIndex | ||
| } else { | ||
| newItemIndicesForRowIndices[rowIndex] = itemIndex...itemIndex | ||
| } | ||
| newRowIndicesForItemIndices.grow(toInclude: itemIndex, fillingWith: nil) | ||
| newRowIndicesForItemIndices[itemIndex] = rowIndex | ||
| } else { | ||
| itemIndicesForRowIndices[rowIndex] = itemIndicesForRowIndices[rowIndex] ?? [] | ||
| itemIndicesForRowIndices[rowIndex]?.append(itemIndex) | ||
| rowIndicesForItemIndices[itemIndex] = rowIndex | ||
| } |
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like all of these if/else cases, the false branch is existing unchanged code, and the true branch is the equivalent logic but operating on the new arrays rather than the old dictionaries.
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bryankeller
commented
Jun 9, 2026
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| @@ -936,7 +936,7 @@ public final class MagazineLayout: UICollectionViewLayout { | |||
| } | |||
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| if context.invalidateLayoutMetrics && shouldInvalidateLayoutMetrics { | |||
| prepareActions.formUnion([.updateLayoutMetrics]) | |||
| prepareActions.formUnion(.updateLayoutMetrics) | |||
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prevent unnecessary array allocations
brynbodayle
approved these changes
Jun 9, 2026
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Details
Another round of perf fixes - these are the trickiest ones. There are two main improvements:
All changes are gated behind the new experiment optimizations flag.
How Has This Been Tested
Tested in demo app and Airbnb app. Additionally, all unit tests pass.
Types of changes