SwiftUI Expert Skill

Overview

Use this skill to build, review, or improve SwiftUI features with correct state management, optimal view composition, and iOS 26+ Liquid Glass styling. Prioritize native APIs, Apple design guidance, and performance-conscious patterns. This skill focuses on facts and best practices without enforcing specific architectural patterns.

Workflow Decision Tree

1) Review existing SwiftUI code

• Check property wrapper usage against the selection guide (see references/state-management.md)
• Verify view composition follows extraction rules (see references/view-structure.md)
• Check performance patterns are applied (see references/performance-patterns.md)
• Verify list patterns use stable identity (see references/list-patterns.md)
• Check animation patterns for correctness (see references/animation-basics.md, references/animation-transitions.md)
• Inspect Liquid Glass usage for correctness and consistency (see references/liquid-glass.md)
• Validate iOS 26+ availability handling with sensible fallbacks

2) Improve existing SwiftUI code

• Audit state management for correct wrapper selection (see references/state-management.md)
• Extract complex views into separate subviews (see references/view-structure.md)
• Refactor hot paths to minimize redundant state updates (see references/performance-patterns.md)
• Ensure ForEach uses stable identity (see references/list-patterns.md)
• Improve animation patterns (use value parameter, proper transitions, see references/animation-basics.md, references/animation-transitions.md)
• Suggest image downsampling when UIImage(data:) is used (as optional optimization, see references/image-optimization.md)
• Adopt Liquid Glass only when explicitly requested by the user

3) Implement new SwiftUI feature

• Design data flow first: identify owned vs injected state (see references/state-management.md)
• Structure views for optimal diffing (extract subviews early, see references/view-structure.md)
• Keep business logic in services and models for testability (see references/layout-best-practices.md)
• Use correct animation patterns (implicit vs explicit, transitions, see references/animation-basics.md, references/animation-transitions.md, references/animation-advanced.md)
• Apply glass effects after layout/appearance modifiers (see references/liquid-glass.md)
• Gate iOS 26+ features with #available and provide fallbacks

Core Guidelines

State Management

• @State must be private; use for internal view state
• @Binding only when a child needs to modify parent state
• @StateObject when view creates the object; @ObservedObject when injected
• iOS 17+: Use @State with @Observable classes; use @Bindable for injected observables needing bindings
• Use let for read-only values; var + .onChange() for reactive reads
• Never pass values into @State or @StateObject — they only accept initial values
• Nested ObservableObject doesn't propagate changes — pass nested objects directly; @Observable handles nesting fine

View Composition

• Extract complex views into separate subviews for better readability and performance
• Prefer modifiers over conditional views for state changes (maintains view identity)
• Keep view body simple and pure (no side effects or complex logic)
• Use @ViewBuilder functions only for small, simple sections
• Prefer @ViewBuilder let content: Content over closure-based content properties
• Keep business logic in services and models; views should orchestrate UI flow
• Action handlers should reference methods, not contain inline logic
• Views should work in any context (don't assume screen size or presentation style)

Performance

• Pass only needed values to views (avoid large "config" or "context" objects)
• Eliminate unnecessary dependencies to reduce update fan-out
• Check for value changes before assigning state in hot paths
• Avoid redundant state updates in onReceive, onChange, scroll handlers
• Minimize work in frequently executed code paths
• Use LazyVStack/LazyHStack for large lists
• Use stable identity for ForEach (never .indices for dynamic content)
• Ensure constant number of views per ForEach element
• Avoid inline filtering in ForEach (prefilter and cache)
• Avoid AnyView in list rows
• Consider POD views for fast diffing (or wrap expensive views in POD parents)
• Suggest image downsampling when UIImage(data:) is encountered (as optional optimization)
• Avoid layout thrash (deep hierarchies, excessive GeometryReader)
• Gate frequent geometry updates by thresholds
• Use Self._printChanges() to debug unexpected view updates

Animations

• Use .animation(_:value:) with value parameter (deprecated version without value is too broad)
• Use withAnimation for event-driven animations (button taps, gestures)
• Prefer transforms (offset, scale, rotation) over layout changes (frame) for performance
• Transitions require animations outside the conditional structure
• Custom Animatable implementations must have explicit animatableData
• Use .phaseAnimator for multi-step sequences (iOS 17+)
• Use .keyframeAnimator for precise timing control (iOS 17+)
• Animation completion handlers need .transaction(value:) for reexecution
• Implicit animations override explicit animations (later in view tree wins)

Liquid Glass (iOS 26+)

Only adopt when explicitly requested by the user.

• Use native glassEffect, GlassEffectContainer, and glass button styles
• Wrap multiple glass elements in GlassEffectContainer
• Apply .glassEffect() after layout and visual modifiers
• Use .interactive() only for tappable/focusable elements
• Use glassEffectID with @Namespace for morphing transitions

Quick Reference

Property Wrapper Selection

Wrapper	Use When
@State	Internal view state (must be private)
@Binding	Child modifies parent's state
@StateObject	View owns an ObservableObject
@ObservedObject	View receives an ObservableObject
@Bindable	iOS 17+: Injected @Observable needing bindings
let	Read-only value from parent
var	Read-only value watched via .onChange()

Liquid Glass Patterns

// Basic glass effect with fallback
if #available(iOS 26, *) {
    content
        .padding()
        .glassEffect(.regular.interactive(), in: .rect(cornerRadius: 16))
} else {
    content
        .padding()
        .background(.ultraThinMaterial, in: RoundedRectangle(cornerRadius: 16))
}

// Grouped glass elements
GlassEffectContainer(spacing: 24) {
    HStack(spacing: 24) {
        GlassButton1()
        GlassButton2()
    }
}

// Glass buttons
Button("Confirm") { }
    .buttonStyle(.glassProminent)

Review Checklist

State Management

• @State properties are private
• @Binding only where child modifies parent state
• @StateObject for owned, @ObservedObject for injected
• iOS 17+: @State with @Observable, @Bindable for injected
• Passed values NOT declared as @State or @StateObject
• Nested ObservableObject avoided (or passed directly to child views)

Sheets & Navigation (see references/sheet-navigation-patterns.md)

• Using .sheet(item:) for model-based sheets
• Sheets own their actions and dismiss internally

ScrollView (see references/scroll-patterns.md)

• Using ScrollViewReader with stable IDs for programmatic scrolling

View Structure (see references/view-structure.md)

• Using modifiers instead of conditionals for state changes
• Complex views extracted to separate subviews
• Container views use @ViewBuilder let content: Content

Performance (see references/performance-patterns.md)

• View body kept simple and pure (no side effects)
• Passing only needed values (not large config objects)
• Eliminating unnecessary dependencies
• State updates check for value changes before assigning
• Hot paths minimize state updates
• No object creation in body
• Heavy computation moved out of body

List Patterns (see references/list-patterns.md)

• ForEach uses stable identity (not .indices)
• Constant number of views per ForEach element
• No inline filtering in ForEach
• No AnyView in list rows

Layout (see references/layout-best-practices.md)

• Avoiding layout thrash (deep hierarchies, excessive GeometryReader)
• Gating frequent geometry updates by thresholds
• Business logic kept in services and models (not in views)
• Action handlers reference methods (not inline logic)
• Using relative layout (not hard-coded constants)
• Views work in any context (context-agnostic)

Animations (see references/animation-basics.md, references/animation-transitions.md, references/animation-advanced.md)

• Using .animation(_:value:) with value parameter
• Using withAnimation for event-driven animations
• Transitions paired with animations outside conditional structure
• Custom Animatable has explicit animatableData implementation
• Preferring transforms over layout changes for animation performance
• Phase animations for multi-step sequences (iOS 17+)
• Keyframe animations for precise timing (iOS 17+)
• Completion handlers use .transaction(value:) for reexecution

Liquid Glass (iOS 26+)

• #available(iOS 26, *) with fallback for Liquid Glass
• Multiple glass views wrapped in GlassEffectContainer
• .glassEffect() applied after layout/appearance modifiers
• .interactive() only on user-interactable elements
• Shapes and tints consistent across related elements

References

• references/state-management.md - Property wrappers and data flow
• references/view-structure.md - View composition, extraction, and container patterns
• references/performance-patterns.md - Performance optimization techniques and anti-patterns
• references/list-patterns.md - ForEach identity, stability, and list best practices
• references/layout-best-practices.md - Layout patterns, context-agnostic views, and testability
• references/animation-basics.md - Core animation concepts, implicit/explicit animations, timing, performance
• references/animation-transitions.md - Transitions, custom transitions, Animatable protocol
• references/animation-advanced.md - Transactions, phase/keyframe animations (iOS 17+), completion handlers (iOS 17+)
• references/sheet-navigation-patterns.md - Sheet presentation and navigation patterns
• references/scroll-patterns.md - ScrollView patterns and programmatic scrolling
• references/image-optimization.md - AsyncImage, image downsampling, and optimization
• references/liquid-glass.md - iOS 26+ Liquid Glass API

Philosophy

This skill focuses on facts and best practices, not architectural opinions:

• We don't enforce specific architectures (e.g., MVVM, VIPER)
• We do encourage separating business logic for testability
• We optimize for performance and maintainability
• We follow Apple's Human Interface Guidelines and API design patterns