uikit-collections

name: uikit-collections description: Use when building or reviewing UITableView, UICollectionView, diffable data sources, compositional layout, or cell registration.

UIKit Collections

Review and write UITableView and UICollectionView code for correct data source management, modern cell registration, and compositional layouts.

Responsibility

Owns: UITableView, UICollectionView, UITableViewDiffableDataSource, UICollectionViewDiffableDataSource, NSDiffableDataSourceSnapshot, NSDiffableDataSourceSectionSnapshot, UICollectionViewCompositionalLayout, NSCollectionLayoutSection/Group/Item, UICollectionView.CellRegistration, UITableView.CellRegistration, supplementary views, self-sizing cells, swipe actions, prefetching.

Does NOT own: SwiftUI List/LazyVGrid (swiftui-patterns skill), Core Data fetch results controllers (core-data skill), view controller lifecycle (uikit-fundamentals skill).

Core Principles

1. Diffable data sources are the default. Never use the legacy numberOfRows / cellForRow pattern in new code. Diffable data sources eliminate index-out-of-range crashes.
2. Cell registration over register(_:forCellReuseIdentifier:). Use UICollectionView.CellRegistration / UITableView.CellRegistration for type-safe cell configuration.
3. Compositional layout for collections. Use NSCollectionLayoutSection with NSCollectionLayoutGroup and NSCollectionLayoutItem — never UICollectionViewFlowLayout in new code.
4. Section snapshots for hierarchy. Use NSDiffableDataSourceSectionSnapshot for expandable/collapsible outlines.
5. Apply snapshots on the main queue. Always call apply(_:animatingDifferences:) from the main thread.
6. Self-sizing cells. Set estimatedItemSize to .automatic or provide estimated dimensions. Ensure cells have unambiguous height constraints.

References

• references/uikit-collections-patterns.md — Diffable data sources, compositional layout, cell registration