Uikit Expert

UIKit Expert Skill

Overview

Favor native APIs and Apple's documented guidance. Operating principles: facts over architecture (no MVVM/VIPER/Coordinator mandates, but do separate business logic for testability); correctness first, then performance; reserve "always"/"never" for correctness, use "suggest"/"consider" for optional optimizations.

Workflows

All three modes draw on the same Core Guidelines below; each topic links to its reference file there. Pick the entry point:

• Review — Walk the Review Checklist as pass/fail gates. For any failure, fix per the matching Core Guideline. Prioritize correctness gates (lifecycle, Auto Layout, memory, concurrency) over style; treat performance items as optional.
• Improve — For each issue, apply the modern replacement from the Deprecated → Modern table and the matching Core Guideline. Present performance work (downsampling, prefetching, constraint-churn fixes) as optional optimizations, never mandates.
• Implement new — Build in this order, applying the relevant Core Guideline at each step:

1. Design data flow — owned state, injected dependencies, model layer
2. Lifecycle — one-time setup in viewDidLoad, geometry in viewIsAppearing
3. Auto Layout — batch activation, zero churn
4. Collection views — DiffableDataSource + CompositionalLayout + CellRegistration
5. Navigation — all 4 appearance slots, concurrent-transition guards
6. Animation, memory ([weak self], Task cancellation), concurrency (@MainActor)
7. Interop, image loading, keyboard, Dynamic Type / VoiceOver / dark mode
8. Gate iOS 26+ features with #available and sensible fallbacks

Core Guidelines

View Controller Lifecycle — see references/view-controller-lifecycle.md

• Use viewDidLoad for one-time setup: subviews, constraints, delegates — NOT geometry
• Use viewIsAppearing (back-deployed iOS 13+) for geometry-dependent work, trait-based layout, scroll-to-item
• viewDidLayoutSubviews fires multiple times — use only for lightweight layer frame adjustments
• viewWillAppear is limited to transition coordinator animations and balanced notification registration
• Always call super in every lifecycle override
• Child VC containment: addChild → addSubview → didMove(toParent:) — in that exact order
• Verify deallocation with deinit logging during development

Auto Layout — see references/auto-layout.md

• Always set translatesAutoresizingMaskIntoConstraints = false on programmatic views
• Use NSLayoutConstraint.activate([]) — never individual .isActive = true
• Create constraints once, toggle isActive or modify .constant — never remove and recreate
• Never change priority from/to .required (1000) at runtime — use 999
• Animate constraints: update constant → call layoutIfNeeded() inside animation block on superview
• iOS 26+: use .flushUpdates option to simplify constraint animation
• Avoid deeply nested UIStackViews in reusable cells
• Set constraint.identifier on key constraints so Unable to simultaneously satisfy constraints logs are readable

let label = UILabel()
label.translatesAutoresizingMaskIntoConstraints = false   // required on programmatic views
view.addSubview(label)
NSLayoutConstraint.activate([                              // one batch = one solve pass
    label.centerXAnchor.constraint(equalTo: view.centerXAnchor),
    label.centerYAnchor.constraint(equalTo: view.centerYAnchor),
])

Collection Views & Data Sources — see references/modern-collection-views.md, references/cell-configuration.md, references/list-performance.md

• Use UICollectionViewDiffableDataSource with stable identifiers (UUID/database ID, not full model structs)
• Use reconfigureItems for content updates, reloadItems only when cell type changes
• Use applySnapshotUsingReloadData for initial population (bypasses diffing)
• Use UICollectionViewCompositionalLayout for any non-trivial layout
• Use UICollectionView.CellRegistration — no string identifiers, no manual casting
• Use UIContentConfiguration for cell content and UIBackgroundConfiguration for cell backgrounds
• Use configurationUpdateHandler for state-driven styling (selection, highlight)

// Store the registration once — never create it inside the cell provider (crashes on iOS 15+).
private lazy var cellRegistration = UICollectionView.CellRegistration<PhotoCell, Photo> { cell, _, photo in
    cell.configure(with: photo)
}

// Data source is keyed by the stable ID, so the provider receives an id — look up the model, then configure with it.
dataSource = .init(collectionView: collectionView) { [weak self] cv, indexPath, id in
    guard let self, let photo = self.photo(for: id) else { return nil }
    return cv.dequeueConfiguredReusableCell(using: self.cellRegistration, for: indexPath, item: photo)
}

var snapshot = NSDiffableDataSourceSnapshot<Section, Photo.ID>()
snapshot.appendSections([.main])
snapshot.appendItems(photos.map(\.id))
dataSource.apply(snapshot, animatingDifferences: true)

Navigation — see references/navigation-patterns.md

• Configure all 4 UINavigationBarAppearance slots (standard, scrollEdge, compact, compactScrollEdge)
• Set appearance on navigationItem (per-VC) in viewDidLoad, not on navigationBar in viewWillAppear
• Use setViewControllers(_:animated:) for deep links — not sequential push calls
• Guard against concurrent transitions — check transitionCoordinator before push/pop
• Set prefersLargeTitles once on the bar; use largeTitleDisplayMode per VC

Animation — see references/animation-patterns.md

• UIView.animate — simple one-shot animations; check finished in completion
• UIViewPropertyAnimator — gesture-driven, interruptible; respect state machine (inactive → active → stopped)
• CABasicAnimation — layer-only properties (cornerRadius, shadow, 3D transforms); set model value first
• iOS 17+ spring API: UIView.animate(springDuration:bounce:) aligns with SwiftUI
• Constraint animation: flush layout → update constant → animate layoutIfNeeded() on superview

Memory Management — see references/memory-management.md

• Default to [weak self] in all escaping closures
• Timer: use block-based API with [weak self], invalidate in viewWillDisappear
• CADisplayLink: use weak proxy pattern (no block-based API available)
• NotificationCenter: [weak self] in closure, remove observer in deinit
• Nested closures: re-capture [weak self] in stored inner closures
• Delegates: always weak var delegate: SomeDelegate? with AnyObject constraint
• Verify deallocation with deinit — if never called, a retain cycle exists

Concurrency — see references/concurrency-main-thread.md

• UIViewController is @MainActor — all subclass methods are implicitly main-actor
• Store Task references, cancel in viewDidDisappear — not deinit
• Check Task.isCancelled before UI updates after await
• Task.detached does NOT inherit actor isolation — explicit MainActor.run needed for UI
• Never call DispatchQueue.main.sync from background — use await MainActor.run

UIKit–SwiftUI Interop — see references/uikit-swiftui-interop.md

• UIHostingController: full child VC containment (addChild → addSubview → didMove), retain as stored property
• sizingOptions = .intrinsicContentSize (iOS 16+) for Auto Layout containers
• UIViewRepresentable: set mutable state in updateUIView, not makeUIView; guard against update loops
• UIHostingConfiguration (iOS 16+) for SwiftUI content in collection view cells

Image Loading — see references/image-loading.md

• Decoded bitmap size = width × height × 4 bytes (a 12MP photo = ~48MB RAM)
• Downsample with ImageIO at display size — never load full bitmap and resize
• iOS 15+: use byPreparingThumbnail(of:) or prepareForDisplay() for async decoding
• Cell reuse: cancel Task in prepareForReuse, clear image, verify identity on completion