带有 groupPagingCentered 的 UICollectionViewCompositionalLayout 未开始居中

Question

我的布局代码非常简单，您将在有关新组合布局的每个教程或文章中看到这一点。

  func createLayout() -> UICollectionViewLayout {
    let itemSize = NSCollectionLayoutSize(widthDimension: .fractionalWidth(1.0), heightDimension: .fractionalHeight(1.0))
    let item = NSCollectionLayoutItem(layoutSize: itemSize)
    item.contentInsets = .init(top: 0, leading: 5, bottom: 0, trailing: 5)

    let groupSize = NSCollectionLayoutSize(widthDimension: .fractionalWidth(0.93), heightDimension: .fractionalHeight(1.0))
    let group = NSCollectionLayoutGroup.horizontal(layoutSize: groupSize, subitems: [item])

    let section = NSCollectionLayoutSection(group: group)
    section.orthogonalScrollingBehavior = .groupPagingCentered

    let layout = UICollectionViewCompositionalLayout(section: section)
    return layout
  }

当我启动应用程序时，单元格未正确居中。只有当我将单元格拖动最微小的量时，它才会弹到正确的位置。

前：

当我拖动它一点点后：

我还没有看到任何关于同样问题的问题。Twitter 或博客上没有人谈论它。不确定我在这里做错了什么？

Answer 1

也许为时已晚，但这里有一个解决方法：

func createLayout() -> UICollectionViewLayout {
    let layout = UICollectionViewCompositionalLayout { (sectionIndex, environment) -> NSCollectionLayoutSection? in
        let sideInset: CGFloat = 5

        let itemSize = NSCollectionLayoutSize(widthDimension: .fractionalWidth(1.0), heightDimension: .fractionalHeight(1.0))
        let item = NSCollectionLayoutItem(layoutSize: itemSize)
        item.contentInsets = .init(top: 0, leading: sideInset, bottom: 0, trailing: sideInset)

        let groupWidth = environment.container.contentSize.width * 0.93
        let groupSize = NSCollectionLayoutSize(widthDimension: .absolute(groupWidth), heightDimension: .fractionalHeight(1.0))
        let group = NSCollectionLayoutGroup.horizontal(layoutSize: groupSize, subitems: [item])

        let section = NSCollectionLayoutSection(group: group)

        // add leading and trailing insets to the section so groups are aligned to the center
        let sectionSideInset = (environment.container.contentSize.width - groupWidth) / 2
        section.contentInsets = NSDirectionalEdgeInsets(top: 0, leading: sectionSideInset, bottom: 0, trailing: sectionSideInset)

        // note this is not .groupPagingCentered
        section.orthogonalScrollingBehavior = .groupPaging

        return section
    }

    return layout
}

Answer 2

这是预期的行为。“居中”意味着单元格在滚动后捕捉到中心。在进行任何滚动之前，整个组会一直滚动到右侧。您的组只有0.93分数宽度，因此差异很小。当分数宽度较小时，效果就不那么难看了。