uikit-fundamentals

name: uikit-fundamentals description: Use when reviewing UIKit lifecycle, view hierarchy, responder chain, traits, size classes, or modernization away from shared state.

UIKit Fundamentals

Review and write UIKit code for correct view controller lifecycle, view hierarchy management, responder chain handling, and trait-based adaptation.

Responsibility

Owns: UIViewController lifecycle, UIView hierarchy and layout passes, responder chain and first responder, trait collections and size classes, UITraitChangeRegistration (iOS 17+), appearance customization, viewIsAppearing (iOS 13+), and UIKit app modernization for multi-window environments.

Does NOT own: Auto Layout constraints (auto-layout skill), collection/table views (uikit-collections skill), navigation flows (uikit-navigation skill), Core Animation layers (core-animation skill), SwiftUI bridging (uikit-interop skill).

Core Principles

1. Apple modernization guidance wins. For UIScreen.main, mainScreen, interfaceOrientation, application-vs-scene lifecycle, and safe-area modernization, read references/apple-uikit-modernization/ first and apply the task-specific replacement there.
2. Lifecycle order matters. viewDidLoad → viewIsAppearing → viewDidAppear → viewWillDisappear → viewDidDisappear. Put geometry-dependent code in viewIsAppearing, not viewWillAppear.
3. viewIsAppearing replaces most viewWillAppear uses. Available from iOS 13+, called with up-to-date trait collection and geometry. Prefer it for layout adjustments.
4. Views are not view controllers. Keep UIView subclasses focused on rendering. Business logic belongs in the view controller or a separate object.
5. Responder chain flows up. First responder → superview chain → view controller → parent VC → window → application. Use becomeFirstResponder() / resignFirstResponder() deliberately.
6. Trait changes (iOS 17+). Use registerForTraitChanges([UITraitUserInterfaceStyle.self]) { (self: Self, _) in ... } instead of the deprecated traitCollectionDidChange(_:).
7. Size classes drive adaptive layout. .compact and .regular for horizontal and vertical axes. Check traitCollection.horizontalSizeClass rather than checking device type.
8. No global scene/window guesses. Do not replace deprecated shared-state reads with UIApplication.shared or another global walk. Prefer context closest to the consumer: the view, view controller, window scene, or an explicit parameter.
9. Modernization edits must be complete. If a replacement requires a new overload plus a deprecated forwarding wrapper or trait-change registration, make the whole pattern atomic. Partial migrations are worse than no migration.

References

• references/apple-uikit-modernization/ — Apple-authoritative modernization tasks for UIScreen, safe areas, orientation, and scene lifecycle.
• references/uikit-fundamentals-patterns.md — Lifecycle, view hierarchy, responder chain, traits