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Improve logic
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@bryankeller
bryankeller committed Feb 10, 2024
1 parent 65c9a43 commit bcef59d
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Showing 3 changed files with 144 additions and 59 deletions.
71 changes: 59 additions & 12 deletions MagazineLayout/LayoutCore/Types/TargetContentOffsetAnchor.swift
View file
Original file line number Diff line number Diff line change
Expand Up @@ -22,25 +22,28 @@ import UIKit
enum TargetContentOffsetAnchor: Equatable {
case top
case bottom
case topItem(id: String, distanceFromTop: CGFloat)
case bottomItem(id: String, distanceFromBottom: CGFloat)
case topItem(id: String, itemEdge: ItemEdge, distanceFromTop: CGFloat)
case bottomItem(id: String, itemEdge: ItemEdge, distanceFromBottom: CGFloat)

static func targetContentOffsetAnchor(
verticalLayoutDirection: MagazineLayoutVerticalLayoutDirection,
topInset: CGFloat,
bottomInset: CGFloat,
bounds: CGRect,
contentHeight: CGFloat,
scale: CGFloat,
firstVisibleItemID: String,
lastVisibleItemID: String,
firstVisibleItemFrame: CGRect,
lastVisibleItemFrame: CGRect)
-> Self
{
let top = (-topInset).alignedToPixel(forScreenWithScale: scale)
let bottom = (contentHeight + bottomInset - bounds.height).alignedToPixel(forScreenWithScale: scale)
let position: Position
if bounds.minY <= -topInset {
if bounds.minY <= top {
position = .atTop
} else if bounds.minY >= contentHeight + bottomInset - bounds.height {
} else if bounds.minY >= bottom {
position = .atBottom
} else {
position = .inMiddle
Expand All @@ -52,14 +55,38 @@ enum TargetContentOffsetAnchor: Equatable {
case .atTop:
return .top
case .inMiddle, .atBottom:
let distanceFromTop = firstVisibleItemFrame.minY - (bounds.minY + topInset)
return .topItem(id: firstVisibleItemID, distanceFromTop: distanceFromTop)
let top = bounds.minY + topInset
let topDistanceFromTop = firstVisibleItemFrame.value(for: .top) - top
let bottomDistanceFromTop = firstVisibleItemFrame.value(for: .bottom) - top
if abs(topDistanceFromTop) < abs(bottomDistanceFromTop) {
return .topItem(
id: firstVisibleItemID,
itemEdge: .top,
distanceFromTop: topDistanceFromTop.alignedToPixel(forScreenWithScale: scale))
} else {
return .topItem(
id: firstVisibleItemID,
itemEdge: .bottom,
distanceFromTop: bottomDistanceFromTop.alignedToPixel(forScreenWithScale: scale))
}
}
case .bottomToTop:
switch position {
case .atTop, .inMiddle:
let distanceFromBottom = lastVisibleItemFrame.maxY - (bounds.maxY - bottomInset)
return .bottomItem(id: lastVisibleItemID, distanceFromBottom: distanceFromBottom)
let bottom = bounds.maxY - bottomInset
let topDistanceFromBottom = lastVisibleItemFrame.value(for: .top) - bottom
let bottomDistanceFromBottom = lastVisibleItemFrame.value(for: .bottom) - bottom
if abs(topDistanceFromBottom) < abs(bottomDistanceFromBottom) {
return .bottomItem(
id: lastVisibleItemID,
itemEdge: .top,
distanceFromBottom: topDistanceFromBottom.alignedToPixel(forScreenWithScale: scale))
} else {
return .bottomItem(
id: lastVisibleItemID,
itemEdge: .bottom,
distanceFromBottom: bottomDistanceFromBottom.alignedToPixel(forScreenWithScale: scale))
}
case .atBottom:
return .bottom
}
Expand All @@ -82,15 +109,35 @@ enum TargetContentOffsetAnchor: Equatable {
case .bottom:
return contentHeight - bounds.height + bottomInset

case .topItem(let id, let distanceFromTop):
case .topItem(let id, let itemEdge, let distanceFromTop):
guard let indexPath = indexPathForItemID(id) else { return bounds.minY }
let itemFrame = frameForItemAtIndexPath(indexPath)
return itemFrame.minY - topInset - distanceFromTop
return itemFrame.value(for: itemEdge) - topInset - distanceFromTop

case .bottomItem(let id, let distanceFromBottom):
case .bottomItem(let id, let itemEdge, let distanceFromBottom):
guard let indexPath = indexPathForItemID(id) else { return bounds.minY }
let itemFrame = frameForItemAtIndexPath(indexPath)
return itemFrame.maxY - bounds.height + bottomInset - distanceFromBottom
return itemFrame.value(for: itemEdge) - bounds.height + bottomInset - distanceFromBottom
}
}

}

// MARK: ItemEdge

enum ItemEdge {
case top
case bottom
}

// MARK: CGRect + Item Edge Value

private extension CGRect {

func value(for itemEdge: ItemEdge) -> CGFloat {
switch itemEdge {
case .top: minY
case .bottom: maxY
}
}

Expand Down
110 changes: 71 additions & 39 deletions MagazineLayout/Public/MagazineLayout.swift
View file
Original file line number Diff line number Diff line change
Expand Up @@ -99,13 +99,6 @@ public final class MagazineLayout: UICollectionViewLayout {
override public func prepare() {
super.prepare()

// Save the most recent content inset. We read this later in `invalidateLayout:`. Unlike the
// bounds and content size, the content insets are updated _before_ `invalidateLayout` is
// called, leaving us no choice other than tracking it separately here.
lastContentInset = contentInset

guard !prepareActions.isEmpty else { return }

// Save the previous collection view width if necessary
if prepareActions.contains(.cachePreviousWidth) {
cachedCollectionViewWidth = currentCollectionView.bounds.width
Expand Down Expand Up @@ -255,6 +248,9 @@ public final class MagazineLayout: UICollectionViewLayout {
modelState.applyUpdates(updates)
hasDataSourceCountInvalidationBeforeReceivingUpdateItems = false

lastSizedElementMinY = nil
lastSizedElementPreferredHeight = nil

super.prepare(forCollectionViewUpdates: updateItems)
}

Expand All @@ -266,7 +262,14 @@ public final class MagazineLayout: UICollectionViewLayout {

targetContentOffsetAnchor = nil
preInvalidationContentSize = nil
preInvalidationContentInset = nil

if let stagedContentOffsetAdjustment {
let context = MagazineLayoutInvalidationContext()
context.invalidateLayoutMetrics = false
context.contentOffsetAdjustment = stagedContentOffsetAdjustment
invalidateLayout(with: context)
}
stagedContentOffsetAdjustment = nil

super.finalizeCollectionViewUpdates()
}
Expand All @@ -276,15 +279,19 @@ public final class MagazineLayout: UICollectionViewLayout {

targetContentOffsetAnchor = targetContentOffsetAnchor(
bounds: oldBounds,
contentHeight: preInvalidationContentSize?.height ?? collectionViewContentSize.height,
topInset: preInvalidationContentInset?.top ?? contentInset.top,
bottomInset: preInvalidationContentInset?.bottom ?? contentInset.bottom)
contentHeight: preInvalidationContentSize?.height ?? currentCollectionView.contentSize.height,
// There doesn't seem to be a reliable way to get the correct content insets here. We can try
// track them in invalidateLayout or prepare, but then there are edge cases where we need to
// track multiple past inset values. It's a huge mess, and I don't think it's worth solving.
// The downside is that if your insets change on rotation, you won't always land in the exact
// correct spot if you're in the middle of the content. Being at the top or bottom works fine.
topInset: 0,
bottomInset: 0)
}

override public func finalizeAnimatedBoundsChange() {
targetContentOffsetAnchor = nil
preInvalidationContentSize = nil
preInvalidationContentInset = nil

super.finalizeAnimatedBoundsChange()
}
Expand Down Expand Up @@ -665,6 +672,8 @@ public final class MagazineLayout: UICollectionViewLayout {
withOriginalAttributes originalAttributes: UICollectionViewLayoutAttributes)
-> UICollectionViewLayoutInvalidationContext
{
let originalAttributes = originalAttributes.copy() as! UICollectionViewLayoutAttributes

switch preferredAttributes.representedElementCategory {
case .cell:
modelState.updateItemHeight(
Expand Down Expand Up @@ -715,29 +724,60 @@ public final class MagazineLayout: UICollectionViewLayout {
forPreferredLayoutAttributes: preferredAttributes,
withOriginalAttributes: originalAttributes) as! MagazineLayoutInvalidationContext

// If layout information is discarded above our current scroll position (on rotation, for
// example), we need to compensate for preferred size changes to items as we're scrolling up,
// otherwise, the collection view will appear to jump each time an element is sized.
// Since size adjustments can occur for multiple items in the same soon-to-be-visible row, we
// need to account for this by considering the preferred height for previously sized elements in
// the same row so that we only adjust the content offset by the exact amount needed to create
// smooth scrolling.
let isScrolling = currentCollectionView.isDragging || currentCollectionView.isDecelerating
let isSizingElementAboveTopEdge = originalAttributes.frame.minY < currentCollectionView.contentOffset.y

if isScrolling && isSizingElementAboveTopEdge {
let top = currentCollectionView.bounds.minY + contentInset.top
let bottom = currentCollectionView.bounds.maxY
let isSizingElementAboveTopEdge = originalAttributes.frame.minY < top
let isSizingElementBelowBottomEdge = originalAttributes.frame.minY > bottom

let contentOffsetAdjustment: CGPoint
if verticalLayoutDirection == .topToBottom, isSizingElementAboveTopEdge {
// If layout information is discarded above our current scroll position (on rotation, for
// example), we need to compensate for preferred size changes to items as we're scrolling up,
// otherwise, the collection view will appear to jump each time an element is sized.
// Since size adjustments can occur for multiple items in the same soon-to-be-visible row, we
// need to account for this by considering the preferred height for previously sized elements
// in the same row so that we only adjust the content offset by the exact amount needed to
// create smooth scrolling.
let isSameRowAsLastSizedElement = lastSizedElementMinY?.isEqual(
to: currentElementY,
threshold: 1 / scale)
?? false

if isSameRowAsLastSizedElement {
let lastSizedElementPreferredHeight = self.lastSizedElementPreferredHeight ?? 0
if preferredAttributes.size.height > lastSizedElementPreferredHeight {
context.contentOffsetAdjustment.y = preferredAttributes.size.height - lastSizedElementPreferredHeight
contentOffsetAdjustment = CGPoint(
x: 0,
y: preferredAttributes.size.height - lastSizedElementPreferredHeight)
} else {
contentOffsetAdjustment = .zero
}
} else {
context.contentOffsetAdjustment.y = preferredAttributes.size.height - originalAttributes.size.height
contentOffsetAdjustment = CGPoint(
x: 0,
y: preferredAttributes.size.height - originalAttributes.size.height)
}
} else if
verticalLayoutDirection == .bottomToTop,
!isSizingElementBelowBottomEdge,
modelState.isPerformingBatchUpdates
{
contentOffsetAdjustment = CGPoint(
x: 0,
y: preferredAttributes.size.height - originalAttributes.size.height)
} else {
contentOffsetAdjustment = .zero
}

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`.
// This needs to be set in `finalizeCollectionViewUpdates`, otherwise it doesn't get applied.
stagedContentOffsetAdjustment = contentOffsetAdjustment
} else if isScrolling {
context.contentOffsetAdjustment = contentOffsetAdjustment
}

lastSizedElementMinY = currentElementY
Expand All @@ -754,11 +794,6 @@ public final class MagazineLayout: UICollectionViewLayout {
return
}

if collectionViewContentSize.width > 0, collectionViewContentSize.height > 0 {
preInvalidationContentSize = preInvalidationContentSize ?? collectionViewContentSize
}
preInvalidationContentInset = preInvalidationContentInset ?? lastContentInset ?? contentInset

let shouldInvalidateLayoutMetrics = !context.invalidateEverything &&
!context.invalidateDataSourceCounts

Expand Down Expand Up @@ -829,10 +864,7 @@ public final class MagazineLayout: UICollectionViewLayout {
private let _flipsHorizontallyInOppositeLayoutDirection: Bool
private let verticalLayoutDirection: MagazineLayoutVerticalLayoutDirection

private var lastContentInset: UIEdgeInsets?
private var cachedCollectionViewWidth: CGFloat?
private var preInvalidationContentSize: CGSize?
private var preInvalidationContentInset: UIEdgeInsets?

// These properties are used to prevent scroll jumpiness due to self-sizing after rotation; see
// comment in `invalidationContext(forPreferredLayoutAttributes:withOriginalAttributes:)` for more
Expand Down Expand Up @@ -870,6 +902,8 @@ public final class MagazineLayout: UICollectionViewLayout {

private var isPerformingAnimatedBoundsChange = false
private var targetContentOffsetAnchor: TargetContentOffsetAnchor?
private var preInvalidationContentSize: CGSize?
private var stagedContentOffsetAdjustment: CGPoint?

// Used to provide the model state with the current visible bounds for the sole purpose of
// supporting pinned headers and footers.
Expand Down Expand Up @@ -1241,18 +1275,15 @@ public final class MagazineLayout: UICollectionViewLayout {
bottomInset: CGFloat)
-> TargetContentOffsetAnchor?
{
let insetBounds = bounds.inset(by: .init(top: topInset, left: 0, bottom: bottomInset, right: 0))
var visibleItemLocationFramePairs = [ElementLocationFramePair]()
for itemLocationFramePair in modelState.itemLocationFramePairs(forItemsIn: bounds) {
for itemLocationFramePair in modelState.itemLocationFramePairs(forItemsIn: insetBounds) {
visibleItemLocationFramePairs.append(itemLocationFramePair)
}
visibleItemLocationFramePairs.sort { $0.elementLocation < $1.elementLocation }

let firstVisibleItemLocationFramePair = visibleItemLocationFramePairs.first {
$0.frame.minY >= bounds.minY + topInset
}
let lastVisibleItemLocationFramePair = visibleItemLocationFramePairs.last {
$0.frame.maxY <= bounds.minY + bounds.height - bottomInset
}
let firstVisibleItemLocationFramePair = visibleItemLocationFramePairs.first
let lastVisibleItemLocationFramePair = visibleItemLocationFramePairs.last

guard
let firstVisibleItemLocationFramePair,
Expand All @@ -1273,6 +1304,7 @@ public final class MagazineLayout: UICollectionViewLayout {
bottomInset: bottomInset,
bounds: bounds,
contentHeight: contentHeight,
scale: scale,
firstVisibleItemID: firstVisibleItemID,
lastVisibleItemID: lastVisibleItemID,
firstVisibleItemFrame: firstVisibleItemLocationFramePair.frame,
Expand Down
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