diffable 数据源节标题在更新期间闪烁

我目前面临的问题是，当将新快照应用于当前数据源时，页眉、页脚和装饰视图不是集合视图子视图的一部分，这可以被视为奇怪的闪烁。以前有人遇到过这个问题吗？

我通过以下方式更新数据源：

    var snapshot = NSDiffableDataSourceSnapshot<Section, Item>()
    snapshot.appendSections(Sections.allCases)
    items.forEach { snapshot.appendItems([$0], toSection: ItemSectionMapper.getSection(for: $0)) }
    self.dataSource?.apply(snapshot)

EDIT: It only seems to happen on iOS 14 devices.

编辑2： 以下是示例项目中同一问题的屏幕录制：https://i.stack.imgur.com/YzTWU.jpg https://i.stack.imgur.com/YzTWU.jpg

下面是它的代码：

import UIKit

// MARK: - Cell -

final class Cell: UICollectionViewCell {
    static let reuseIdentifier = "Cell"

    var isExpanded = false {
        didSet { label.numberOfLines = numberOfLines }
    }

    var numberOfLines: Int { isExpanded ? 0 : 3 }

    lazy var label: UILabel = {
        let label = UILabel()
        label.numberOfLines = numberOfLines
        label.frame.size = contentView.bounds.size
        label.autoresizingMask = [.flexibleWidth, .flexibleHeight]
        return label
    }()

    override init(frame: CGRect) {
        super.init(frame: frame)
        contentView.addSubview(label)
    }

    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override func sizeThatFits(_ size: CGSize) -> CGSize {
        label.sizeThatFits(size)
    }
}

final class Header: UICollectionReusableView {
    static let elementKind = "Header"
    
    lazy var label: UILabel = {
        let label = UILabel()
        label.numberOfLines = 1
        label.frame.size = bounds.size
        label.autoresizingMask = [.flexibleWidth, .flexibleHeight]
        return label
    }()
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        addSubview(label)
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    override func sizeThatFits(_ size: CGSize) -> CGSize {
        label.sizeThatFits(size)
    }
}

// MARK: - UIViewController -

class ViewController: UIViewController {
    struct Item: Hashable {
        let text: String
        var isExpanded = false
        private let uuid = UUID()
    }

    var items: [Item] = [
        .init(
            text: """
            Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor
            incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis
            nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.
            Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore
            eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident.
            """
        ),
        .init(
            text: """
            Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor
            incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis
            nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.
            Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore
            eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident.
            """,
            isExpanded: true
        )
    ]

    lazy var collectionView: UICollectionView = {
        let collectionView = UICollectionView(frame: view.bounds, collectionViewLayout: createCollectionViewLayout())
        collectionView.register(Cell.self, forCellWithReuseIdentifier: Cell.reuseIdentifier)
        collectionView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
        collectionView.contentInset.top = 44
        collectionView.backgroundColor = .white
        collectionView.delegate = self
        return collectionView
    }()

    lazy var dataSource = UICollectionViewDiffableDataSource<Int, Item>(collectionView: collectionView) { collectionView, indexPath, itemIdentifier in
        guard let cell = collectionView.dequeueReusableCell(withReuseIdentifier: Cell.reuseIdentifier, for: indexPath) as? Cell else { fatalError() }
        cell.isExpanded = itemIdentifier.isExpanded
        cell.label.text = itemIdentifier.text
        return cell
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        dataSource.supplementaryViewProvider = { (collectionView, kind, indexPath) -> UICollectionReusableView? in
            guard let view = collectionView.dequeueReusableSupplementaryView(ofKind: kind, withReuseIdentifier: Header.elementKind, for: indexPath) as? Header else { fatalError() }
            view.label.text = "Test"
            return view
        }
        view.addSubview(collectionView)
        collectionView.register(Header.self, forSupplementaryViewOfKind: Header.elementKind, withReuseIdentifier: Header.elementKind)
        updateSnapshot()
    }

    private func createCollectionViewLayout() -> UICollectionViewCompositionalLayout {
        let layoutSize = NSCollectionLayoutSize.init(
            widthDimension: .fractionalWidth(1.0),
            heightDimension: .estimated(200)
        )

        let section = NSCollectionLayoutSection(group:
            .vertical(
                layoutSize: layoutSize,
                subitems: [.init(layoutSize: layoutSize)]
            )
        )
        
        let header = NSCollectionLayoutBoundarySupplementaryItem(layoutSize: .init(widthDimension: .fractionalWidth(1), heightDimension: .estimated(20)), elementKind: Header.elementKind, alignment: .top)
        section.boundarySupplementaryItems = [header]
        section.contentInsets = .init(top: 0, leading: 16, bottom: 0, trailing: 16)
        section.interGroupSpacing = 20

        return .init(section: section)
    }

    private func updateSnapshot() {
        var snapshot = NSDiffableDataSourceSnapshot<Int, Item>()
        snapshot.appendSections([0])
        snapshot.appendItems(items)
        dataSource.apply(snapshot, animatingDifferences: true)
    }
}

// MARK: - UICollectionViewDelegate -

extension ViewController: UICollectionViewDelegate {
    public func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        guard let itemIdentifier = dataSource.itemIdentifier(for: indexPath) else { return }
        items[indexPath.row] = .init(text: itemIdentifier.text, isExpanded: !itemIdentifier.isExpanded)
        updateSnapshot()
    }
}

感谢@JWK

这种行为似乎出乎意料，尽管我相信它的发生只是因为整个部分都用动画更新了apply(_:animatingDifferences:completion:)。您可以尝试以下几种解决方法：

1. Set animatingDifferences to false打电话时apply(_:animatingDifferences:completion:)。如果您想要动画，则不太理想。
2. 添加另一个部分而不是使用boundarySupplementaryItems。不扩展的部分不应影响视觉效果。您可能需要引入另一个单元并使用UICollectionViewCompositionalLayout's init(sectionProvider:)为此（提供正确的NSCollectionLayoutSection对于每个部分）。
3. 如果您使用的是 iOS 14+，我认为您可以通过设置来免费获得所需的行为UICollectionViewListCell's accessories财产给UICellAccessory.OutlineDisclosureOptions(style: .header)。有一个示例项目以及其他有用的示例here https://developer.apple.com/documentation/uikit/views_and_controls/collection_views/implementing_modern_collection_views.