Self-sizing cells with UICollectionViewCompositionalLayout

问题

I have a view controller that displays a collection view with self-sizing cells. The collection view has one section that scrolls horizontally. It looks like this:

Problem

The collection view behaves unexpectedly when the view controller is presented using the default pageSheet style on iOS 13+.

1. When pulling upward on the sheet, cells may appear to resize like the cell labeled "Rectify" below:

2. When pulling upward on the sheet, the content may shift horizontally. Sometimes, cells may disappear too:

Question

Is there a way to fix this behavior while still using UICollectionViewCompositionalLayout and the pageSheet presentation style?

Code Summary

The code is pretty straightforward. Just 3 classes, which can be dropped into the ViewController.swift file using the Single View App project template in Xcode.

1. A UICollectionViewCell class called Cell. The cell has a UILabel and overrides sizeThatFits(_:).
2. A UIViewController called ViewController used only to present BugViewController.
3. BugViewController, which configures the data source and presents the collection view. This is where the problem occurs.

Code

import UIKit

// MARK: - Cell -

final class Cell: UICollectionViewCell {
    static let reuseIdentifier = "Cell"

    lazy var label: UILabel = {
        let label = UILabel()
        label.textAlignment = .center
        label.frame.size = contentView.bounds.size
        label.autoresizingMask = [.flexibleWidth, .flexibleHeight]
        return label
    }()

    override init(frame: CGRect) {
        super.init(frame: frame)
        contentView.addSubview(label)
        contentView.backgroundColor = .tertiarySystemFill
    }

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

    override func sizeThatFits(_ size: CGSize) -> CGSize {
        .init(width: label.sizeThatFits(size).width + 32, height: 32)
    }
}

// MARK: - ViewController -

final class ViewController: UIViewController {
    private let button: UIButton = {
        let button = UIButton(type: .system)
        button.setTitle("Tap Me!".uppercased(), for: .normal)
        button.addTarget(self, action: #selector(presentBugViewController), for: .touchUpInside)
        button.sizeToFit()
        return button
    }()

    override func viewDidLoad() {
        super.viewDidLoad()
        view.addSubview(button)
    }

    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        button.center = view.center
    }

    @objc func presentBugViewController() {
        present(BugViewController(), animated: true)
    }
}

// MARK: - BugViewController -

final class BugViewController: UIViewController {
    private let models = [
        "Better Call Saul",
        "Mad Men",
        "Rectify",
        "Tiger King: Murder, Mayhem, and Madness",
        "Master of None",
        "BoJack Horseman"
    ]

    private lazy var collectionView: UICollectionView = {
        let collectionView = UICollectionView(frame: view.bounds, collectionViewLayout: createCollectionViewLayout())
        collectionView.register(Cell.self, forCellWithReuseIdentifier: Cell.reuseIdentifier)
        collectionView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
        collectionView.contentInset.top = 44
        collectionView.backgroundColor = .white
        return collectionView
    }()

    private lazy var dataSource = UICollectionViewDiffableDataSource<Int, String>(collectionView: collectionView) { collectionView, indexPath, itemIdentifier in
        guard let cell = collectionView.dequeueReusableCell(withReuseIdentifier: Cell.reuseIdentifier, for: indexPath) as? Cell else { fatalError() }
        cell.label.text = itemIdentifier
        return cell
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        view.addSubview(collectionView)

        var snapshot = NSDiffableDataSourceSnapshot<Int, String>()
        snapshot.appendSections([0])
        snapshot.appendItems(models)
        dataSource.apply(snapshot)
    }

    private func createCollectionViewLayout() -> UICollectionViewCompositionalLayout {
        let layoutSize = NSCollectionLayoutSize.init(
            widthDimension: .estimated(200),
            heightDimension: .absolute(32)
        )

        let section = NSCollectionLayoutSection(group:
            .horizontal(
                layoutSize: layoutSize,
                subitems: [.init(layoutSize: layoutSize)]
            )
        )
        section.interGroupSpacing = 8
        section.orthogonalScrollingBehavior = .continuous

        return .init(section: section)
    }
}

Notes

• The collection view in my app actually has many sections and scrolls vertically. That is why I'm using a vertically scrolling collection view and a section with orthogonalScrollingBehavior in the example code.

Failed Attempts

• I've tried using Auto Layout constraints instead of sizeThatFits(_:).
• I've tried not using UICollectionViewDiffableDataSource.

Workarounds

Modifying the cell with a child scroll view and passing in an array of strings (as opposed to one at a time) does avoid this problem. But, it's a dirty hack that I'd like to avoid if possible.

来源：https://stackoverflow.com/questions/61499162/self-sizing-cells-with-uicollectionviewcompositionallayout