Write, review, or improve UIKit code following best practices for view controller lifecycle, Auto Layout, collection views, navigation, animation, memory management, and modern iOS 18–26 APIs. Use when building new UIKit features, refactoring existing views or view controllers, reviewing code qualit

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 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) 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 Keyboard & Scroll — see references/keyboard scroll.md Use UIKeyboardLayoutGuide (iOS 15+) — pin content bottom to view.keyboardLayoutGuide.topAnchor iPad: set followsUndockedKeyboard = true for floating keyboards Replace all manual keyboard notification handling with the layout guide Adaptive Layout & Accessibility — see references/adaptive appearance.md Use registerForTraitChanges (iOS 17+) instead of deprecated traitCollectionDidChange Dynamic Type: UIFont.preferredFont(forTextStyle:) + adjustsFontForContentSizeCategory = true Dark mode: use semantic colors ( .label , .systemBackground ); re resolve CGColor on trait changes VoiceOver: set accessibilityLabel , accessibilityTraits , accessibilityHint on custom views Use UIAccessibilityCustomAction for complex list item actions Quick Reference — Deprecated → Modern APIs Deprecated / Legacy Modern Replacement Since traitCollectionDidChange registerForTraitChanges( :handler:) iOS 17 Keyboard notifications UIKeyboardLayoutGuide iOS 15 cell.textLabel / detailTextLabel UIListContentConfiguration iOS 14 register + string dequeue UICollectionView.CellRegistration iOS 14 reloadItems on snapshot reconfigureItems iOS 15 barTintColor / isTranslucent UINavigationBarAppearance (4 slots) iOS 13 UICollectionViewFlowLayout (complex) UICollectionViewCompositionalLayout iOS 13 Manual layoutIfNeeded() in animations .flushUpdates option iOS 26 Legacy app lifecycle UIScene + SceneDelegate Mandatory iOS 26 ObservableObject + manual invalidation @Observable + UIObservationTrackingEnabled iOS 18 Review Checklist One gate per domain — the correctness items to verify (details in the matching Core Guideline above): [ ] Lifecycle — no geometry in viewDidLoad (use viewIsAppearing ); every override calls super ; child VC = addChild → addSubview → didMove ; deinit confirms dealloc [ ] Auto Layout — translatesAutoresizingMaskIntoConstraints = false ; NSLayoutConstraint.activate([]) ; no churn (toggle isActive / .constant ); no setNeedsLayout() inside layoutSubviews (infinite loop) [ ] Collection views — diffable + stable identifiers; reconfigureItems not reloadItems ; no duplicate snapshot IDs ( BUG IN CLIENT crash); CellRegistration not string dequeue [ ] Navigation — all 4 appearance slots; on navigationItem not navigationBar ; concurrent transition guard [ ] Animation — API fits use case; constraint animation flush → update → animate; CAAnimation sets model value first [ ] Memory — [weak self] in escaping closures; Task s cancelled in viewDidDisappear ; delegates weak ; no strong self recapture in nested closures [ ] Concurrency — Task.isCancelled checked after await ; no DispatchQueue.main.sync from background; no redundant @MainActor on VC subclasses [ ] Images — downsampled to display size; cell load cancels in prepareForReuse + verifies identity; NSCache sized by decoded bytes [ ] Interop — UIHostingController retained + full containment; updateUIView guards update loops [ ] Keyboard — UIKeyboardLayoutGuide , not notifications [ ] Adaptive/A11y — registerForTraitChanges ; Dynamic Type; CGColor re resolved on trait change; accessibilityLabel / Traits on custom views [ ] iOS 26+ — available guards with fallbacks; UIScene lifecycle; consider UIObservationTrackingEnabled References references/view controller lifecycle.md — Lifecycle ordering, viewIsAppearing, child VC containment references/auto layout.md — Batch activation, constraint churn, priority, animation, debugging references/modern collection views.md — Diffable data sources, compositional layout, CellRegistration references/cell configuration.md — UIContentConfiguration, UIBackgroundConfiguration, configurationUpdateHandler references/list performance.md — Prefetching, cell reuse, reconfigureItems, scroll performance references/navigation patterns.md — Bar appearance, concurrent transitions, large titles, deep links references/animation patterns.md — UIView.animate, UIViewPropertyAnimator, CAAnimation, springs references/memory management.md — Retain cycles, [weak self], Timer/CADisplayLink/nested closure traps references/concurrency main thread.md — @MainActor, Task lifecycle, Swift 6, GCD migration references/uikit swiftui interop.md — UIHostingController, UIViewRepresentable, sizing, state bridging references/image loading.md — Downsampling, decoded bitmap math, cell reuse race condition references/keyboard scroll.md — UIKeyboardLayoutGuide, scroll view insets, iPad floating keyboard references/adaptive appearance.md — Trait changes, Dynamic Type, dark mode, VoiceOver, accessibility references/modern uikit apis.md — Observation framework, updateProperties(), .flushUpdates, UIScene, Liquid Glass

uikit-expert