uikit-fundamentals

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+).

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. Lifecycle order matters. viewDidLoad → viewIsAppearing → viewDidAppear → viewWillDisappear → viewDidDisappear. Put geometry-dependent code in viewIsAppearing, not viewWillAppear.
2. viewIsAppearing replaces most viewWillAppear uses. Available from iOS 13+, called with up-to-date trait collection and geometry. Prefer it for layout adjustments.
3. Views are not view controllers. Keep UIView subclasses focused on rendering. Business logic belongs in the view controller or a separate object.
4. Responder chain flows up. First responder → superview chain → view controller → parent VC → window → application. Use becomeFirstResponder() / resignFirstResponder() deliberately.
5. Trait changes (iOS 17+). Use registerForTraitChanges([UITraitUserInterfaceStyle.self]) { (self: Self, _) in ... } instead of the deprecated traitCollectionDidChange(_:).
6. Size classes drive adaptive layout. .compact and .regular for horizontal and vertical axes. Check traitCollection.horizontalSizeClass rather than checking device type.

References

• references/uikit-fundamentals-patterns.md — Lifecycle, view hierarchy, responder chain, traits