在更新过程中，可扩散数据源部分标题闪烁

如何解决在更新过程中，可扩散数据源部分标题闪烁

我当前面临的问题是，在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的子视图的一部分，这可能会引起怪异的闪烁。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)

编辑： 这似乎只发生在iOS 14设备上。

EDIT2： 这是示例项目中相同问题的屏幕录像： https://imgur.com/a/0rS9aZU

下面的代码：

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,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()
    }
}

对@JWK表示敬意

解决方法

这种行为似乎是出乎意料的，尽管我相信这是因为整个部分都使用apply(_:animatingDifferences:completion:)中的动画进行了更新。您可以尝试以下几种解决方法：

1. 在调用animatingDifferences时将false设置为apply(_:animatingDifferences:completion:)。如果您想要动画，则不理想。
2. 添加另一部分，而不使用boundarySupplementaryItems。不展开的部分不应在视觉上受到影响。您可能需要引入另一个单元格，并为此使用UICollectionViewCompositionalLayout的{​​{1}}（以便为每个部分提供正确的init(sectionProvider:)）。
3. 如果您使用的是iOS 14+，我认为您可以通过将NSCollectionLayoutSection的{​​{1}}属性设置为UICollectionViewListCell来免费获得所需的行为。有一个示例项目以及其他有用的示例here。