iOS UIKit Advanced

Use When

• Advanced UIKit for production iOS apps — UICollectionViewDiffableDataSource with NSDiffableDataSourceSnapshot, UICollectionViewCompositionalLayout (sections with orthogonal scrolling, badges, headers), UIViewControllerTransitioningDelegate for...
• The task needs reusable judgment, domain constraints, or a proven workflow rather than ad hoc advice.

Do Not Use When

• The task is unrelated to ios-uikit-advanced or would be better handled by a more specific companion skill.
• The request only needs a trivial answer and none of this skill's constraints or references materially help.

Required Inputs

• Gather relevant project context, constraints, and the concrete problem to solve; load references only as needed.
• Confirm the desired deliverable: design, code, review, migration plan, audit, or documentation.

Workflow

• Read this SKILL.md first, then load only the referenced deep-dive files that are necessary for the task.
• Apply the ordered guidance, checklists, and decision rules in this skill instead of cherry-picking isolated snippets.
• Produce the deliverable with assumptions, risks, and follow-up work made explicit when they matter.

Quality Standards

• Keep outputs execution-oriented, concise, and aligned with the repository's baseline engineering standards.
• Preserve compatibility with existing project conventions unless the skill explicitly requires a stronger standard.
• Prefer deterministic, reviewable steps over vague advice or tool-specific magic.

Anti-Patterns

• Treating examples as copy-paste truth without checking fit, constraints, or failure modes.
• Loading every reference file by default instead of using progressive disclosure.

Outputs

• A concrete result that fits the task: implementation guidance, review findings, architecture decisions, templates, or generated artifacts.
• Clear assumptions, tradeoffs, or unresolved gaps when the task cannot be completed from available context alone.
• References used, companion skills, or follow-up actions when they materially improve execution.

Evidence Produced

Category	Artifact	Format	Example
Correctness	UIKit diffable data source test plan	Markdown doc covering snapshot updates, identifier stability, and cell reuse tests	docs/ios/uikit-tests.md

References

• Use the references/ directory for deep detail after reading the core workflow below.

Production-grade UIKit patterns for polished, crash-free iOS interfaces. No placeholder patterns — every section is ready to ship.

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SECTION 1: Diffable Data Source — Eliminating Index-Out-of-Bounds Crashes

The old beginUpdates/endUpdates pattern crashes when data changes occur during animation. Diffable data source computes diffs automatically and applies them safely.

enum Section { case main, featured }

struct Item: Hashable {
    let id: UUID
    let title: String
    // Hashable identity is all diffable needs — diff is computed automatically
}

class ContactsViewController: UIViewController {
    private var dataSource: UITableViewDiffableDataSource<Section, Item>!

    override func viewDidLoad() {
        super.viewDidLoad()
        configureDataSource()
    }

    private func configureDataSource() {
        dataSource = UITableViewDiffableDataSource<Section, Item>(
            tableView: tableView
        ) { tableView, indexPath, item in
            let cell = tableView.dequeueReusableCell(withIdentifier: "Cell", for: indexPath)
            var config = cell.defaultContentConfiguration()
            config.text = item.title
            cell.contentConfiguration = config
            return cell
        }
    }

    func applySnapshot(items: [Item], animatingDifferences: Bool = true) {
        var snapshot = NSDiffableDataSourceSnapshot<Section, Item>()
        snapshot.appendSections([.main])
        snapshot.appendItems(items, toSection: .main)
        dataSource.apply(snapshot, animatingDifferences: animatingDifferences)
    }

    // Reload specific items without full reload (iOS 15+)
    func updateItem(_ item: Item) {
        var snapshot = dataSource.snapshot()
        snapshot.reconfigureItems([item])  // preserves cell height animation
        dataSource.apply(snapshot, animatingDifferences: true)
    }
}

Multi-section snapshots:

var snapshot = NSDiffableDataSourceSnapshot<Section, Item>()
snapshot.appendSections([.featured, .main])
snapshot.appendItems(featuredItems, toSection: .featured)
snapshot.appendItems(regularItems, toSection: .main)
// Move items between sections:
snapshot.moveItem(item, afterItem: otherItem)
snapshot.moveSection(.featured, afterSection: .main)
dataSource.apply(snapshot)

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Additional Guidance

Extended guidance for ios-uikit-advanced was moved to references/skill-deep-dive.md to keep this entrypoint compact and fast to load.

Use that deep dive for:

• SECTION 2: Compositional Layout — Complex Collection Layouts
• SECTION 3: Custom View Controller Transitions
• SECTION 4: UIViewPropertyAnimator — Interruptible Animations
• SECTION 5: Context Menus
• SECTION 6: Bottom Sheets with UISheetPresentationController
• SECTION 7: NSFetchedResultsController with Diffable Data Source
• SECTION 8: Prefetching for Smooth Scrolling
• SECTION 9: Production Anti-Patterns
• SECTION 10: Advanced Interactions Reference
• Quick-Reference Checklist