ios - UICollectionViewLayout:仅将 DecorationView 添加到特定单元格

Question

我开发了一个自定义 CollectionViewLayout，它使用 DecorationView 显示单元格后面的阴影。

但是，我只想将此装饰添加到某些单元格。 UICollectionView 是垂直，但它可能在单元格内包含嵌入的水平 UICollectionView。不应装饰嵌入 UICollectionView 的单元格，如图所示:

这是我用来添加阴影的代码。 UICollectionViewLayout 没有提供如何检索单元格类的方法，因此它可以决定是否添加阴影:

  override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
    let parent = super.layoutAttributesForElements(in: rect)
    guard let attributes = parent, !attributes.isEmpty else {
      return parent
    }

    let sections = attributes.map{$0.indexPath.section}
    let unique = Array(Set(sections))


    // Need to detect, which sections contain an embedded UICollectionView and exclude them from the UNIQUE set


    let backgroundShadowAttributes: [UICollectionViewLayoutAttributes] = unique.compactMap{ section in
      let indexPath = IndexPath(item: 0, section: section)
      return self.layoutAttributesForDecorationView(ofKind: backgroundViewClass.reuseIdentifier(),
                                                    at: indexPath)
    }

    return attributes + backgroundShadowAttributes + separators
  }

有什么方法可以有条件地指定应该装饰哪些 View ？

Answer 1

用这段代码完成: 直接询问数据源是否显示特定部分的阴影的协议(protocol):

protocol SectionBackgroundFlowLayoutDataSource {
  func shouldDisplayBackgroundFor(section: Int) -> Bool
}

并利用 func layoutAttributesForElements(in rect: CGRect) 方法中的协议(protocol):

  override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
    let parent = super.layoutAttributesForElements(in: rect)
    guard let attributes = parent, !attributes.isEmpty else {
      return parent
    }

    attributes.forEach(configureRoundCornersAttributes)

    // Display shadows for every section by default
    var sectionsWithShadow = Set(attributes.map{$0.indexPath.section})
    if let dataSource = collectionView?.dataSource as? SectionBackgroundFlowLayoutDataSource {
    // Ask DataSource for sections with shadows, if it supports the protocol
      sectionsWithShadow = sectionsWithShadow.filter{dataSource.shouldDisplayBackgroundFor(section: $0)}
    }

    let backgroundShadowAttributes: [UICollectionViewLayoutAttributes] = sectionsWithShadow.compactMap{ section in
      let indexPath = IndexPath(item: 0, section: section)
      return self.layoutAttributesForDecorationView(ofKind: backgroundViewClass.reuseIdentifier(),
                                                    at: indexPath)
    }

    return attributes + backgroundShadowAttributes + separators
  }

func shouldDisplayBackgroundFor(section: Int) -> Bool 可能比 cellForItemAtIndexPath 返回更快，因为它不需要完整的单元格配置。