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Improve bounds-change handling for bottomToTop layout #135

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Merged
bryankeller merged 1 commit into from
Apr 17, 2025
Merged

Improve bounds-change handling for bottomToTop layout #135

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25 changes: 16 additions & 9 deletions MagazineLayout/Public/MagazineLayout.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -613,15 +613,22 @@ public final class MagazineLayout: UICollectionViewLayout {

invalidationContext.invalidateLayoutMetrics = false

// If our layout direction is bottom to top we want to adjust scroll position relative to the
// bottom
if
case .bottomToTop = verticalLayoutDirection,
newBounds.maxY < currentCollectionView.contentSize.height + contentInset.bottom
{
invalidationContext.contentOffsetAdjustment = CGPoint(
x: 0.0,
y: currentCollectionView.bounds.maxY - newBounds.maxY)
// If our layout direction is `bottomToTop`, we need to handle the case of a non-animated bounds
// change by using the invalidation context's `contentOffsetAdjustment`. The calculation to get
// this right is odd, and is dependent on how close we were to the bottom of the collection view.
if case .bottomToTop = verticalLayoutDirection {
if newBounds.height < currentCollectionView.bounds.height {
invalidationContext.contentOffsetAdjustment = CGPoint(
x: 0.0,
y: currentCollectionView.bounds.midY - newBounds.midY)
} else if newBounds.height > currentCollectionView.bounds.height {
let distanceFromBottom = currentCollectionView.contentSize.height - currentCollectionView.bounds.maxY
let midYDelta = newBounds.midY - currentCollectionView.bounds.midY
let heightDelta = newBounds.height - currentCollectionView.bounds.height
invalidationContext.contentOffsetAdjustment = CGPoint(
x: 0.0,
y: midYDelta - min(distanceFromBottom, heightDelta))
}
}

return invalidationContext
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