Is there any way to using UICollectionViewCell's self-sizing only in vertical direction?

I am writing a demo about the difference using compositional layout & traditional flow layout with UICollectionView.

I found that if I want to set the cell’s width equal to the UICollectionView’s width , but the cell’s height is determined by the cell’s content (self-sizing only in vertical).

1. This can be easily with compositional layout

   let height = NSCollectionLayoutDimension.estimated(144)
   let itemSize = NSCollectionLayoutSize(widthDimension: NSCollectionLayoutDimension.fractionalWidth(1), heightDimension: height)
   

2. But, if I use traditional flow layout, I can only set flow layout’s estimatedItemSize like this:

   layout.estimatedItemSize = CGSizeMake(collectionView.width, 140)
   

   But, after layout, the cell’s final width will always determined by the actual content, not the width equal to collectionView’s width.

Is there any best practice about this kind vertical-only self-sizing?

There are a variety of ways. One is to implement preferredLayoutAttributesFitting(_:), using required for width, but a low priority for height:

class Cell: UICollectionViewCell {
    @IBOutlet weak var label: UILabel!

    override func preferredLayoutAttributesFitting(
        _ layoutAttributes: UICollectionViewLayoutAttributes
    ) -> UICollectionViewLayoutAttributes {
        let attributes = super.preferredLayoutAttributesFitting(layoutAttributes)

        let size = CGSize(width: superview!.bounds.width - 40, height: 40)
        attributes.frame.size = contentView.systemLayoutSizeFitting(
            size,
            withHorizontalFittingPriority: .required,
            verticalFittingPriority: .defaultLow
        )
        return attributes
    }
}

That yields:

The full MRE:

class Cell: UICollectionViewCell {
    @IBOutlet weak var label: UILabel!

    override func preferredLayoutAttributesFitting(
        _ layoutAttributes: UICollectionViewLayoutAttributes
    ) -> UICollectionViewLayoutAttributes {
        let attributes = super.preferredLayoutAttributesFitting(layoutAttributes)

        let size = CGSize(width: superview!.bounds.width - 40, height: 40)
        attributes.frame.size = contentView.systemLayoutSizeFitting(
            size,
            withHorizontalFittingPriority: .required,
            verticalFittingPriority: .defaultLow
        )
        return attributes
    }
}

class ViewController: UIViewController {
    @IBOutlet weak var collectionView: UICollectionView!

    // some strings of increasing lengths

    let strings = (0...15).map {
        let formatter = NumberFormatter()
        formatter.numberStyle = .spellOut
        var value = 0.0
        for i in 0 ... $0 {
            value += pow(10.0, Double(i))
        }
        return formatter.string(for: value)!
    }

    var dataSource: UICollectionViewDiffableDataSource<Section, String>!

    override func viewDidLoad() {
        super.viewDidLoad()

        dataSource = .init(collectionView: collectionView) { [weak self] collectionView, indexPath, itemIdentifier in
            let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "Cell", for: indexPath) as! Cell
            cell.label.text = self?.strings[indexPath.item]
            return cell
        }
        collectionView.dataSource = dataSource
        applySnapshot()
    }

    func applySnapshot() {
        var snapshot = NSDiffableDataSourceSnapshot<Section, String>()
        snapshot.appendSections([.main])
        snapshot.appendItems(strings)
        dataSource.apply(snapshot)
    }
}

extension ViewController {
    enum Section: Hashable {
        case main
    }
}