我有一个非常简单的UICollectionView，它使用组合布局来轻松实现动态单元格高度。不幸的是，这样做似乎禁用了使用UICollectionViewDataSourcePrefetching的内容预取。在下面的示例代码中，collectionView(_:prefetchItemsAt:)方法只在集合视图的初始显示时调用一次。没有滚动操作会导致进一步调用该方法。
要使预取正常工作，我可以做些什么？

class ViewController: UIViewController, 
    UICollectionViewDataSource,
    UICollectionViewDelegate,
    UICollectionViewDataSourcePrefetching
{
    @IBOutlet var collectionView: UICollectionView!

    override func viewDidLoad() {
        super.viewDidLoad()

        collectionView.collectionViewLayout = createLayout()
        collectionView.dataSource = self
        collectionView.delegate = self
        collectionView.prefetchDataSource = self
        collectionView.register(MyCell.self, forCellWithReuseIdentifier: "cell")
    }

    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return 100
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell", for: indexPath) as! MyCell
        cell.label.text = String(repeating: "\(indexPath) ", count: indexPath.item)

        return cell
    }

    // this is only called once. Why?
    func collectionView(_ collectionView: UICollectionView, prefetchItemsAt indexPaths: [IndexPath]) {
        print("prefetch for \(indexPaths)")
    }

    private func createLayout() -> UICollectionViewLayout {
        let layout = UICollectionViewCompositionalLayout { (_, _) -> NSCollectionLayoutSection? in

            let size = NSCollectionLayoutSize(widthDimension: .fractionalWidth(1.0),
                                              heightDimension: .estimated(44))
            let item = NSCollectionLayoutItem(layoutSize: size)

            let group = NSCollectionLayoutGroup.horizontal(layoutSize: size, subitem: item, count: 1)
            group.interItemSpacing = .fixed(16)

            let section = NSCollectionLayoutSection(group: group)
            section.contentInsets = NSDirectionalEdgeInsets(top: 0, leading: 16, bottom: 0, trailing: 16)
            section.interGroupSpacing = 8

            return section
        }

        return layout
    }
}

class MyCell: UICollectionViewCell {
    let label = UILabel()

    override init(frame: CGRect) {
        super.init(frame: frame)

        label.numberOfLines = 0
        label.lineBreakMode = .byWordWrapping
        backgroundColor = .orange
        contentView.addSubview(label)

        label.translatesAutoresizingMaskIntoConstraints = false
        label.topAnchor.constraint(equalTo: contentView.topAnchor).isActive = true
        label.leadingAnchor.constraint(equalTo: contentView.leadingAnchor).isActive = true
        label.trailingAnchor.constraint(equalTo: contentView.trailingAnchor).isActive = true
        label.bottomAnchor.constraint(equalTo: contentView.bottomAnchor).isActive = true
    }

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

}

（使用Xcode 11.5 / iOS 13.5。collectionView插座连接到故事板中的全屏示例）
编辑：进一步的测试表明heightDimension: .estimated(44)似乎是原因。用.absolute(44)替换它使预取再次工作，但这当然违背了拥有这样一个多行文本布局的目的。看起来像一个bug，如果有人想复制它，我已经提交了FB 7849272。
EDIT/2：目前，我可以通过使用普通的流布局来避免这种情况，并计算每个单元格的高度，这也使预取再次工作。然而，我很好奇，如果没有一个变通方案，同时仍然使用组合布局，所以我增加了奖金。