如何在集合视图组合布局中为某些布局组指定某些单元格

Question

我想使用具有合成布局的集合视图制作时间轴，其中记录按天分组。每个组包含当天并排排列的所有记录。日子之间的间隔是几天，例如每天2px。

尝试使用构图布局设计此图形，我的倾向是我需要2个组。一个具有自定义布局的主组，因为单元格之间的间隔时间使它们分开，而嵌套日组则包含所有记录。这将是水平的，因为记录以一致的间距并排放置。

这是我尝试的布局

 func createLayout() -> UICollectionViewLayout {
    let layout = UICollectionViewCompositionalLayout {
        (sectionIndex: Int, layoutEnvironment: NSCollectionLayoutEnvironment) -> NSCollectionLayoutSection? in
        
        let dayItemSize = NSCollectionLayoutSize(widthDimension: .absolute(self.cellWidth), heightDimension: .fractionalHeight(1))
        let dayItem = NSCollectionLayoutItem(layoutSize: dayItemSize)
        
        let numberOfPostsInDay = 5 // self.days[?].numberOfPosts how would I set this?
        let dayItemsContentWidth = CGFloat(numberOfPostsInDay) * self.cellWidth
        
        let dayGroupSize = NSCollectionLayoutSize(widthDimension: .absolute(10),heightDimension: .fractionalHeight(1.0))
        let dayGroup = NSCollectionLayoutGroup.horizontal(layoutSize: dayGroupSize, subitems: [dayItem])
        
        // Determine main content width
        let numberOfDays = self.endDate.days(from: self.startDate)
        let contentWidth = numberOfDays * CGFloat(5) // 5 pixels per day on the timeline?
        let mainGroupSize = NSCollectionLayoutSize(widthDimension: .absolute(contentWidth), heightDimension: .fractionalHeight(1.0))

        let mainGroup = NSCollectionLayoutGroup.custom(layoutSize: mainGroupSize) { env in
            var frame = CGRect(x: 0,y: 0,width: self.cellWidth,height: self.collectionView.frame.height)
            let firstDay = self.days[i].date
            var items = [NSCollectionLayoutGroupCustomItem(frame: frame)]
            i = 1
            for day in self.days {
                let dayDiff = day.date.days(from: firstDay)
                frame.origin.x = self.cellWidth * CGFloat(dayDiff)
                items.append(NSCollectionLayoutGroupCustomItem(frame: frame))
            }
            return items
        }
        

        let leftInset = layoutEnvironment.container.contentSize.width / 2 - dayItemSize.widthDimension.dimension
        let rightInset = leftInset
        let section = NSCollectionLayoutSection(group: mainGroup)
        section.contentInsets = NSDirectionalEdgeInsets(top: 0, leading: leftInset, bottom: 0, trailing: rightInset)
        return section
    }
    let config = UICollectionViewCompositionalLayoutConfiguration()
    config.scrollDirection = .horizontal
    layout.configuration = config
    return layout
}

我正在使用字典-self.days，其中包含带日期的一天，然后是该天的帖子数。

我的问题是NSCollectionLayoutGroupCustomItemProvider闭包仅提供环境，不提供有关indexPath或其布局项的任何内容。我的印象是，布局基本上会一遍又一遍地调用此回调，直到有足够的单元格来填充该部分为止。我不确定是否可以指定哪个单元格可以进入哪个组？我需要使用这些部分吗？