peterbamuhigire/ios-uikit-advanced

iOS UIKit Advanced

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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.

References

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

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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