From apple's modernizing collection view

CustomCellListViewController.swift

private class CustomListCell: ItemListCell {
    
    private func defaultListContentConfiguration() -> UIListContentConfiguration { return .subtitleCell() }
    private lazy var listContentView = UIListContentView(configuration: defaultListContentConfiguration())
    
    private let categoryIconView = UIImageView()
    private let categoryLabel = UILabel()
    private var customViewConstraints: (categoryLabelLeading: NSLayoutConstraint,
                                        categoryLabelTrailing: NSLayoutConstraint,
                                        categoryIconTrailing: NSLayoutConstraint)?
    
    private func setupViewsIfNeeded() {
        // We only need to do anything if we haven't already setup the views and created constraints.
        guard customViewConstraints == nil else { return }
        
        contentView.addSubview(listContentView)
        contentView.addSubview(categoryLabel)
        contentView.addSubview(categoryIconView)
        listContentView.translatesAutoresizingMaskIntoConstraints = false
        let defaultHorizontalCompressionResistance = listContentView.contentCompressionResistancePriority(for: .horizontal)
        listContentView.setContentCompressionResistancePriority(defaultHorizontalCompressionResistance - 1, for: .horizontal)
        categoryLabel.translatesAutoresizingMaskIntoConstraints = false
        categoryIconView.translatesAutoresizingMaskIntoConstraints = false
        let constraints = (
            categoryLabelLeading: categoryLabel.leadingAnchor.constraint(greaterThanOrEqualTo: listContentView.trailingAnchor),
            categoryLabelTrailing: categoryIconView.leadingAnchor.constraint(equalTo: categoryLabel.trailingAnchor),
            categoryIconTrailing: contentView.trailingAnchor.constraint(equalTo: categoryIconView.trailingAnchor)
        )
        NSLayoutConstraint.activate([
            listContentView.topAnchor.constraint(equalTo: contentView.topAnchor),
            listContentView.bottomAnchor.constraint(equalTo: contentView.bottomAnchor),
            listContentView.leadingAnchor.constraint(equalTo: contentView.leadingAnchor),
            categoryLabel.centerYAnchor.constraint(equalTo: contentView.centerYAnchor),
            categoryIconView.centerYAnchor.constraint(equalTo: contentView.centerYAnchor),
            constraints.categoryLabelLeading,
            constraints.categoryLabelTrailing,
            constraints.categoryIconTrailing
        ])
        customViewConstraints = constraints
    }
    
    private var separatorConstraint: NSLayoutConstraint?
    private func updateSeparatorConstraint() {
        guard let textLayoutGuide = listContentView.textLayoutGuide else { return }
        if let existingConstraint = separatorConstraint, existingConstraint.isActive {
            return
        }
        let constraint = separatorLayoutGuide.leadingAnchor.constraint(equalTo: textLayoutGuide.leadingAnchor)
        constraint.isActive = true
        separatorConstraint = constraint
    }
    
    /// - Tag: UpdateConfiguration
    override func updateConfiguration(using state: UICellConfigurationState) {
        setupViewsIfNeeded()
        
        // Configure the list content configuration and apply that to the list content view.
        var content = defaultListContentConfiguration().updated(for: state)
        content.imageProperties.preferredSymbolConfiguration = .init(font: content.textProperties.font, scale: .large)
        content.image = state.item?.image
        content.text = state.item?.title
        content.secondaryText = state.item?.description
        content.axesPreservingSuperviewLayoutMargins = []
        listContentView.configuration = content
        
        // Get the list value cell configuration for the current state, which we'll use to obtain the system default
        // styling and metrics to copy to our custom views.
        let valueConfiguration = UIListContentConfiguration.valueCell().updated(for: state)
        
        // Configure custom image view for the category icon, copying some of the styling from the value cell configuration.
        categoryIconView.image = state.item?.category.icon
        categoryIconView.tintColor = valueConfiguration.imageProperties.resolvedTintColor(for: tintColor)
        categoryIconView.preferredSymbolConfiguration = .init(font: valueConfiguration.secondaryTextProperties.font, scale: .small)
        
        // Configure custom label for the category name, copying some of the styling from the value cell configuration.
        categoryLabel.text = state.item?.category.name
        categoryLabel.textColor = valueConfiguration.secondaryTextProperties.resolvedColor()
        categoryLabel.font = valueConfiguration.secondaryTextProperties.font
        categoryLabel.adjustsFontForContentSizeCategory = valueConfiguration.secondaryTextProperties.adjustsFontForContentSizeCategory
        
        // Update some of the constraints for our custom views using the system default metrics from the configurations.
        customViewConstraints?.categoryLabelLeading.constant = content.directionalLayoutMargins.trailing
        customViewConstraints?.categoryLabelTrailing.constant = valueConfiguration.textToSecondaryTextHorizontalPadding
        customViewConstraints?.categoryIconTrailing.constant = content.directionalLayoutMargins.trailing
        updateSeparatorConstraint()
    }
}

ItemListCell.swift

// This list cell subclass is an abstract class with a property that holds the item the cell is displaying,
// which is added to the cell's configuration state for subclasses to use when updating their configuration.
private class ItemListCell: UICollectionViewListCell {
    private var item: Item? = nil
    
    func updateWithItem(_ newItem: Item) {
        guard item != newItem else { return }
        item = newItem
        setNeedsUpdateConfiguration()
    }
    
    override var configurationState: UICellConfigurationState {
        var state = super.configurationState
        state.item = self.item
        return state
    }
}

UICollectionViewCell.swift

// thank you apple!

@available(iOS 6.0, *)
open class UICollectionViewCell : UICollectionReusableView {

    
    /// Requests the cell update its configuration for its current state. This method is called automatically
    /// when the cell's `configurationState` may have changed, as well as in other circumstances where an
    /// update may be required. Multiple requests may be coalesced into a single update at the appropriate time.
    @available(iOS 14.0, *)
    open func setNeedsUpdateConfiguration()
}