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

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

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; Tasks 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