SwiftUI UICollectionview可表示的垂直居中单元格

Question

我已经创建了一个UIViewController的可表示视图，我正在尝试将单元格垂直居中在collectionview中。

但是，单元格显示的位置略低于UICollectionview的中心。

我相信问题是，尽管有可见的带有高度的单元格，collectionView.contentSize.height为0。 （我已经注释了对应的行，它是从底部开始的6行）

为什么它会返回零？

struct MyCollectionView : UIViewRepresentable {

    @Binding var data:[WordPairCollectionStruct]

    var didSelectItem: ((_ indexPath: IndexPath)->()) = {_ in }

    func makeUIView(context: Context) -> UICollectionView {

        let layout = UICollectionViewFlowLayout()
        layout.estimatedItemSize = UICollectionViewFlowLayout.automaticSize

        let collectionView = UICollectionView(frame: .zero, collectionViewLayout: layout)
        collectionView.register(UINib(nibName: "WordPairCell", bundle: nil), forCellWithReuseIdentifier: "myCell")
        collectionView.translatesAutoresizingMaskIntoConstraints = false
        collectionView.dataSource = context.coordinator
        collectionView.delegate = context.coordinator
        collectionView.backgroundColor = .blue
        collectionView.alwaysBounceVertical = true

        return collectionView
    }

    func updateUIView(_ uiView: UICollectionView, context: Context) {
       // uiView.reloadData()
    }

    func makeCoordinator() -> Coordinator {
        return Coordinator(data: data)
    }

    class Coordinator: NSObject, UICollectionViewDelegate, UICollectionViewDataSource,UICollectionViewDelegateFlowLayout {

        var data: [WordPairCollectionStruct]

        init(data: [WordPairCollectionStruct]) {

            self.data = data
        }

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

        func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
            let cell: WordPairCell = collectionView.dequeueReusableCell(withReuseIdentifier: "myCell", for: indexPath) as! WordPairCell

            cell.wordText.text = data[indexPath.row].word

            return cell
        }

        func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
            print("Tapped \(indexPath.row)")
        }

        func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {

            let flowLayout = collectionViewLayout as! UICollectionViewFlowLayout
            let numberOfItems = CGFloat(collectionView.numberOfItems(inSection: section))
            let combinedItemWidth = (numberOfItems * flowLayout.itemSize.width) + ((numberOfItems - 1)  * flowLayout.minimumInteritemSpacing)
            let padding = (collectionView.frame.width - combinedItemWidth) / 2

            print("** \(collectionView.contentSize.height)")
            // Line of interest **********
            let top = max((collectionView.frame.height - collectionView.contentSize.height) / 2, 0)

            return UIEdgeInsets(top: top, left: 0, bottom: 0, right: 0)
        }

    }
}