UIKit collection views and compositional layout

A collection view can render lists, grids, carousels, and mixed sections. Layout describes geometry. Registrations describe reusable presentation. A diffable snapshot describes current identity and order.

Build layout per section

let layout = UICollectionViewCompositionalLayout { section, environment in
    if section == 0 {
        return NSCollectionLayoutSection.list(
            using: UICollectionLayoutListConfiguration(appearance: .insetGrouped),
            layoutEnvironment: environment
        )
    }

    let columns = environment.container.effectiveContentSize.width > 700 ? 4 : 2
    return attachmentGrid(columns: columns)
}

The environment reports available container space. The layout does not branch on device name.

Register configuration, not a decoration subclass

let noteRegistration = UICollectionView.CellRegistration<UICollectionViewListCell, NoteID> {
    [weak self] cell, _, id in
    guard let note = self?.notesByID[id] else { return }
    var content = UIListContentConfiguration.subtitleCell()
    content.text = note.title
    content.secondaryText = note.body
    cell.contentConfiguration = content
}

Subclass when the cell owns meaningful custom behavior or hierarchy. Content and background configurations cover many presentation-only cases.

Keep three responsibilities separate

snapshot: which sections and items exist
layout: where those items go
registration: how one item appears now

Stable IDs connect them. Cell configuration retrieves current model data. Supplementary views such as section headers follow the same reuse rules.

Validate changing environments

Test compact and wide windows, Dynamic Type, long localization, right-to-left order, empty sections, rapid snapshots, selection, focus, keyboard navigation, image cancellation, and memory pressure.

Validation boundary

Layout, registrations, diffable updates, reuse, focus, performance, and accessibility remain Not verified without an Apple runtime.

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References

UICollectionView renders flexible collections.
UICollectionViewCompositionalLayout composes section layouts.
UICollectionView.CellRegistration configures reusable cells.