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Improve contentOffsetAdjustment behavior #141

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Merged
merged 1 commit into from
Oct 10, 2025
Merged

Improve contentOffsetAdjustment behavior #141

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33 changes: 15 additions & 18 deletions MagazineLayout/Public/MagazineLayout.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -233,6 +233,7 @@ public final class MagazineLayout: UICollectionViewLayout {
// Calculate the target offset before applying updates, since the target offset should be based
// on the pre-update state.
targetContentOffsetAnchor = currentTargetContentOffsetAnchor
contentHeightBeforeUpdates = collectionViewContentSize.height

modelState.applyUpdates(updates)
hasDataSourceCountInvalidationBeforeReceivingUpdateItems = false
Expand All @@ -249,15 +250,16 @@ public final class MagazineLayout: UICollectionViewLayout {
itemLayoutAttributesForPendingAnimations.removeAll()
supplementaryViewLayoutAttributesForPendingAnimations.removeAll()

targetContentOffsetAnchor = nil

if let stagedContentOffsetAdjustment {
if let targetContentOffsetAnchor {
let targetYOffset = yOffset(for: targetContentOffsetAnchor)
let context = MagazineLayoutInvalidationContext()
context.invalidateLayoutMetrics = false
context.contentOffsetAdjustment = stagedContentOffsetAdjustment
context.contentOffsetAdjustment.y = targetYOffset - currentCollectionView.contentOffset.y
invalidateLayout(with: context)
}
stagedContentOffsetAdjustment = nil

targetContentOffsetAnchor = nil
contentHeightBeforeUpdates = nil

targetContentOffsetCompensatingYOffsetForAppearingItem = nil

Expand Down Expand Up @@ -718,10 +720,9 @@ public final class MagazineLayout: UICollectionViewLayout {
}

if let attributes = itemLayoutAttributesForPendingAnimations[preferredAttributes.indexPath] {
attributes.frame.size.height = modelState.frameForItem(
attributes.frame = modelState.frameForItem(
at: ElementLocation(indexPath: preferredAttributes.indexPath),
.afterUpdates).height
attributes.frame.origin.y -= (contentOffsetAdjustment?.y ?? 0)
.afterUpdates)
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@bryankeller bryankeller Oct 10, 2025

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much simpler - the math before was unnecessarily complex

}

case .supplementaryView:
Expand Down Expand Up @@ -764,15 +765,11 @@ public final class MagazineLayout: UICollectionViewLayout {
forPreferredLayoutAttributes: preferredAttributes,
withOriginalAttributes: originalAttributes) as! MagazineLayoutInvalidationContext

if let contentOffsetAdjustment {
if modelState.isPerformingBatchUpdates {
// If we're in the middle of a batch update, we need to adjust our content offset. Doing it
// here in the middle of a batch update gets ignored for some reason. Instead, we delay
// slightly and do it in `finalizeCollectionViewUpdates`.
stagedContentOffsetAdjustment = contentOffsetAdjustment
} else {
context.contentOffsetAdjustment = contentOffsetAdjustment
}
if let contentOffsetAdjustment, !modelState.isPerformingBatchUpdates {
// If we're in the middle of a batch update, we need to adjust our content offset. Doing it
// here in the middle of a batch update gets ignored for some reason. Instead, we delay
// slightly and do it in `finalizeCollectionViewUpdates`.
context.contentOffsetAdjustment = contentOffsetAdjustment
}

context.invalidateLayoutMetrics = false
Expand Down Expand Up @@ -914,7 +911,7 @@ public final class MagazineLayout: UICollectionViewLayout {

private var isPerformingAnimatedBoundsChange = false
private var targetContentOffsetAnchor: TargetContentOffsetAnchor?
private var stagedContentOffsetAdjustment: CGPoint?
private var contentHeightBeforeUpdates: CGFloat?
private var previousContentInset: UIEdgeInsets?

// Used to provide the model state with the current visible bounds for the sole purpose of
Expand Down