Stackoverflow question: UICollectionViewCompositionalLayout multi column layout with auto-sizing cells - https://stackoverflow.com/a/64985964/2569338

GridViewController.swift

/*
 GOAL: 
 - UICollectionViewCompositionalLayout multi column layout with auto-sizing cells where each cell has the same height in a given row (uses the max cell height in a row)
 REALITY:
 - The groups aren't filled :/
*/

import Foundation
import UIKit

final class GridViewController: UIViewController {

    private enum Section {
        case main
    }

    private var dataSource: UICollectionViewDiffableDataSource<Section, Int>! = nil
    private var collectionView: UICollectionView! = nil

    override func viewDidLoad() {
        super.viewDidLoad()
        navigationItem.title = "Grid"
        configureHierarchy()
        configureDataSource()
    }
}

extension GridViewController {
    private func createLayout() -> UICollectionViewLayout {
        let itemSize = NSCollectionLayoutSize(
            widthDimension: .fractionalWidth(1.0),
            heightDimension: .estimated(44.0)
        )
        let item = NSCollectionLayoutItem(layoutSize: itemSize)

        // Inner group
        let innerGroupSize = NSCollectionLayoutSize(
            widthDimension: .fractionalWidth(1.0),
            heightDimension: .estimated(44.0)
        )
        let innerGroup = NSCollectionLayoutGroup.horizontal(
            layoutSize: innerGroupSize,
            subitem: item,
            count: 5
        )

        let outerGroupSize = NSCollectionLayoutSize(
            widthDimension: .fractionalWidth(1.0),
            heightDimension: .estimated(44.0)
        )
        let outerGroup = NSCollectionLayoutGroup.vertical(
            layoutSize: outerGroupSize,
            subitems: [innerGroup]
        )

        let section = NSCollectionLayoutSection(group: outerGroup)
        let layout = UICollectionViewCompositionalLayout(section: section)
        return layout
    }
}

extension GridViewController {
    private func configureHierarchy() {
        collectionView = UICollectionView(frame: view.bounds, collectionViewLayout: createLayout())
        collectionView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
        collectionView.backgroundColor = .black
        view.addSubview(collectionView)
    }
    private func configureDataSource() {
        let cellRegistration = UICollectionView.CellRegistration<TextCell, Int> { (cell, indexPath, identifier) in
            // Populate the cell
            let texts = [
                String(identifier),
                String.random(Int.random(in: 0...16))?.uppercased(),
            ]
            cell.label.text = texts
                .compactMap { $0 }
                .joined(separator: "\n")
            cell.layer.borderColor = UIColor.black.cgColor
            cell.layer.borderWidth = 1
            cell.label.textAlignment = .center
            cell.label.font = UIFont.preferredFont(forTextStyle: .title1)
        }

        dataSource = UICollectionViewDiffableDataSource<Section, Int>(collectionView: collectionView) {
            $0.dequeueConfiguredReusableCell(using: cellRegistration, for: $1, item: $2)
        }

        var snapshot = NSDiffableDataSourceSnapshot<Section, Int>()
        snapshot.appendSections([.main])
        snapshot.appendItems(Array(0..<94))
        dataSource.apply(snapshot, animatingDifferences: false)
    }
}

final class TextCell: UICollectionViewCell {
    static let reuseIdentifier = "text-cell-reuse-identifier"

    let label = UILabel()

    override init(frame: CGRect) {
        super.init(frame: frame)
        contentView.backgroundColor = UIColor.cornflowerBlue
        configure()
    }

    required init?(coder: NSCoder) {
        fatalError("not implemnted")
    }

    private func configure() {
        label.translatesAutoresizingMaskIntoConstraints = false
        // label.font = UIFont.monospacedDigitSystemFont(ofSize: 14, weight: .regular)
        label.adjustsFontForContentSizeCategory = true
        label.numberOfLines = 0
        contentView.addSubview(label)
        let inset = CGFloat(16)
        NSLayoutConstraint.activate([
            label.leadingAnchor.constraint(equalTo: contentView.leadingAnchor, constant: inset),
            label.trailingAnchor.constraint(equalTo: contentView.trailingAnchor, constant: -inset),
            label.topAnchor.constraint(equalTo: contentView.topAnchor, constant: inset),
            label.bottomAnchor.constraint(equalTo: contentView.bottomAnchor, constant: -inset)
            ])
    }
}

extension String {
    static func random(_ length: Int, minimumLength: Int = 0) -> String? {
        guard length > 0 else { return nil }
        let letters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
        var randomString = ""
        for _ in minimumLength..<length {
            randomString.append(letters.randomElement()!)
        }
        return randomString
    }
}