Bk/add layout state #148
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| // Created by Bryan Keller on 10/31/25. | ||
| // Copyright © 2025 Airbnb Inc. All rights reserved. | ||
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| // 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. | ||
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| import UIKit | ||
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| // MARK: - LayoutState | ||
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| /// Represents the state of the layout, including metrics and the current `ModelState`. | ||
| struct LayoutState { | ||
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| // MARK: Internal | ||
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| let modelState: ModelState | ||
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| var bounds: CGRect | ||
| var contentInset: UIEdgeInsets | ||
| var scale: CGFloat | ||
| var verticalLayoutDirection: MagazineLayoutVerticalLayoutDirection | ||
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| var minContentOffset: CGPoint { | ||
| CGPoint(x: -contentInset.left, y: -contentInset.top) | ||
| } | ||
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| var maxContentOffset: CGPoint { | ||
| let x = contentSize.width - bounds.width + contentInset.right | ||
| let y = contentSize.height - bounds.height + contentInset.bottom | ||
| return CGPoint(x: max(x, minContentOffset.x), y: max(y, minContentOffset.y)) | ||
| } | ||
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| var contentSize: CGSize { | ||
| // This is a workaround for `layoutAttributesForElementsInRect:` not getting invoked enough | ||
| // times if `collectionViewContentSize.width` is not smaller than the width of the collection | ||
| // view, minus horizontal insets. This results in visual defects when performing batch | ||
| // updates. To work around this, we subtract 0.0001 from our content size width calculation; | ||
| // this small decrease in `collectionViewContentSize.width` is enough to work around the | ||
| // incorrect, internal collection view `CGRect` checks, without introducing any visual | ||
| // differences for elements in the collection view. | ||
| // See https://openradar.appspot.com/radar?id=5025850143539200 for more details. | ||
| let width = bounds.width - contentInset.left - contentInset.right - 0.0001 | ||
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| let numberOfSections = modelState.numberOfSections | ||
| let height: CGFloat = | ||
| if numberOfSections <= 0 { | ||
| 0 | ||
| } else { | ||
| modelState.sectionMaxY(forSectionAtIndex: numberOfSections - 1) | ||
| } | ||
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| return CGSize(width: width, height: height) | ||
| } | ||
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| var targetContentOffsetAnchor: TargetContentOffsetAnchor { | ||
| var visibleItemLocationFramePairs = [ElementLocationFramePair]() | ||
| for itemLocationFramePair in modelState.itemLocationFramePairs(forItemsIn: bounds) { | ||
| visibleItemLocationFramePairs.append(itemLocationFramePair) | ||
| } | ||
| visibleItemLocationFramePairs.sort { $0.elementLocation < $1.elementLocation } | ||
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| let firstVisibleItemLocationFramePair = visibleItemLocationFramePairs.first { | ||
| // When scrolling up, only calculate a target content offset based on visible, already-sized | ||
| // cells. Otherwise, scrolling will be jumpy. | ||
| modelState.isItemHeightSettled(indexPath: $0.elementLocation.indexPath) | ||
| } ?? visibleItemLocationFramePairs.first // fallback to the first item if we can't find one with a settled height | ||
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| let lastVisibleItemLocationFramePair = visibleItemLocationFramePairs.reversed().first { | ||
| // When scrolling down, only calculate a target content offset based on visible, already-sized | ||
| // cells. Otherwise, scrolling will be jumpy. | ||
| modelState.isItemHeightSettled(indexPath: $0.elementLocation.indexPath) | ||
| } ?? visibleItemLocationFramePairs.last // fallback to the last item if we can't find one with a settled height | ||
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| guard | ||
| let firstVisibleItemLocationFramePair, | ||
| let lastVisibleItemLocationFramePair, | ||
| let firstVisibleItemID = modelState.idForItemModel( | ||
| at: firstVisibleItemLocationFramePair.elementLocation.indexPath), | ||
| let lastVisibleItemID = modelState.idForItemModel( | ||
| at: lastVisibleItemLocationFramePair.elementLocation.indexPath) | ||
| else { | ||
| switch verticalLayoutDirection { | ||
| case .topToBottom: return .top | ||
| case .bottomToTop: return .bottom | ||
| } | ||
| } | ||
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| let top = minContentOffset.y.alignedToPixel(forScreenWithScale: scale) | ||
| let bottom = maxContentOffset.y.alignedToPixel(forScreenWithScale: scale) | ||
| let isAtTop = bounds.minY <= top | ||
| let isAtBottom = bounds.minY >= bottom | ||
| let position: Position | ||
| if isAtTop, isAtBottom { | ||
| switch verticalLayoutDirection { | ||
| case .topToBottom: | ||
| position = .atTop | ||
| case .bottomToTop: | ||
| position = .atBottom | ||
| } | ||
| } else if isAtTop { | ||
| position = .atTop | ||
| } else if isAtBottom { | ||
| position = .atBottom | ||
| } else { | ||
| position = .inMiddle | ||
| } | ||
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| switch verticalLayoutDirection { | ||
| case .topToBottom: | ||
| switch position { | ||
| case .atTop: | ||
| return .top | ||
| case .inMiddle, .atBottom: | ||
| let top = bounds.minY + contentInset.top | ||
| let distanceFromTop = firstVisibleItemLocationFramePair.frame.minY - top | ||
| return .topItem( | ||
| id: firstVisibleItemID, | ||
| distanceFromTop: distanceFromTop.alignedToPixel(forScreenWithScale: scale)) | ||
| } | ||
| case .bottomToTop: | ||
| switch position { | ||
| case .atTop, .inMiddle: | ||
| let bottom = bounds.maxY - contentInset.bottom | ||
| let distanceFromBottom = lastVisibleItemLocationFramePair.frame.maxY - bottom | ||
| return .bottomItem( | ||
| id: lastVisibleItemID, | ||
| distanceFromBottom: distanceFromBottom.alignedToPixel(forScreenWithScale: scale)) | ||
| case .atBottom: | ||
| return .bottom | ||
| } | ||
| } | ||
| } | ||
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Comment on lines
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Contributor
Author
There was a problem hiding this comment. logic pulled out of a function that used to live in MagazineLayout.swift |
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| func yOffset(for targetContentOffsetAnchor: TargetContentOffsetAnchor) -> CGFloat { | ||
| switch targetContentOffsetAnchor { | ||
| case .top: | ||
| return minContentOffset.y | ||
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| case .bottom: | ||
| return maxContentOffset.y | ||
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| case .topItem(let id, let distanceFromTop): | ||
| guard let indexPath = modelState.indexPathForItemModel(withID: id) else { return bounds.minY } | ||
| let itemFrame = modelState.frameForItem(at: ElementLocation(indexPath: indexPath)) | ||
| let proposedYOffset = itemFrame.minY - contentInset.top - distanceFromTop | ||
| // Clamp between minYOffset...maxYOffset | ||
| return min(max(proposedYOffset, minContentOffset.y), maxContentOffset.y) | ||
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| case .bottomItem(let id, let distanceFromBottom): | ||
| guard let indexPath = modelState.indexPathForItemModel(withID: id) else { return bounds.minY } | ||
| let itemFrame = modelState.frameForItem(at: ElementLocation(indexPath: indexPath)) | ||
| let proposedYOffset = itemFrame.maxY - bounds.height + contentInset.bottom - distanceFromBottom | ||
| // Clamp between minYOffset...maxYOffset | ||
| return min(max(proposedYOffset, minContentOffset.y), maxContentOffset.y) | ||
| } | ||
| } | ||
| } | ||
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| // MARK: - Position | ||
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| private enum Position { | ||
| case atTop | ||
| case inMiddle | ||
| case atBottom | ||
| } | ||
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all of this used to live in
MagazineLayout.swift- I moved it here without changing any math or comments