SwiftUI Expert Skill

Overview

Use this skill to build, review, or improve SwiftUI features with correct state management, modern API usage, Swift concurrency best practices, 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.

For architectural decisions (MVVM, MVC, VIPER, Clean Architecture, Coordinator, two-layer view patterns), defer to the architecture-design-skill.

Workflow Decision Tree

1) Review existing SwiftUI code

• Check property wrapper usage against the selection guide (see references/state-management.md)
• Verify modern API usage (see references/modern-apis.md)
• Verify view composition follows extraction rules (see references/view-composition.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 (see references/liquid-glass.md)

2) Improve existing SwiftUI code

• Audit state management for correct wrapper selection (prefer @Observable over ObservableObject)
• Replace deprecated APIs with modern equivalents (see references/modern-apis.md)
• Extract complex views into separate subviews (see references/view-composition.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 (see references/animation-basics.md, references/animation-transitions.md)
• Suggest image downsampling when UIImage(data:) is used (optional, 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)
• Use modern APIs (no deprecated modifiers or patterns, see references/modern-apis.md)
• Use @Observable for shared state (with @MainActor if not using default actor isolation)
• Structure views for optimal diffing: extract subviews early, keep views small (see references/view-composition.md)
• Separate business logic into testable models when needed (see references/state-management.md#when-to-use-a-viewmodel)
• Use correct animation patterns (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

• Always prefer @Observable over ObservableObject for new code
• Mark @Observable classes with @MainActor unless using default actor isolation
• Always mark @State and @StateObject as private (makes dependencies clear)
• Never declare passed values as @State or @StateObject (they only accept initial values)
• Use @State with @Observable classes (not @StateObject)
• @Binding only when child needs to modify parent state
• @Bindable for injected @Observable objects needing bindings
• Use let for read-only values; var + .onChange() for reactive reads
• Legacy: @StateObject for owned ObservableObject; @ObservedObject for injected
• Nested ObservableObject doesn’t work (pass nested objects directly); @Observable handles nesting fine

When to Use a ViewModel

Use a separate model class (often called a ViewModel) when:

• Testability: you need to unit-test state transitions or business logic
• Complex state: multiple interdependent state properties that evolve together
• Async coordination: orchestrating multiple async operations with loading/error state
• Reuse across views: the same logic is needed in more than one view

For simple views with straightforward display logic, keeping @State directly in the view is fine and often cleaner.

This is a practical guide, not an MVVM mandate — see architecture-design-skill if you need to decide on an app-wide architectural pattern.

View Composition

• Prefer modifiers over conditional views for state changes (maintains view identity)
• Extract complex views into separate struct subviews for performance (not @ViewBuilder functions)
• Keep views small; each view should have one clear responsibility
• 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 in containers
• Use specialized built-in components when they fit: Label, GroupBox, Form, Section
• Build custom view styles (ButtonStyle, LabelStyle) to reuse styling across instances
• Action handlers should reference methods, not contain inline logic
• Use relative layout over hard-coded constants
• Views should work in any context (don’t assume screen size or presentation style)

Reusability Recommendation

Prefer passing primitive/value types to presentational (content) views rather than full model objects. This makes views more reusable, simplifies previews, and reduces dependencies.

// Preferred for pure display views
struct UserRow: View {
    let name: String
    let avatarURL: URL?
    let isOnline: Bool
}

// Acceptable when the view needs the full model or has bindings to it
struct UserEditor: View {
    @Bindable var user: UserModel
}

This is a recommendation for reusability, not a strict rule. Views that edit or coordinate model state often need the model directly.

Modern APIs

• Use foregroundStyle() instead of foregroundColor()
• Use clipShape(.rect(cornerRadius:)) instead of cornerRadius()
• Use Tab API instead of tabItem() (iOS 18+)
• Use Button instead of onTapGesture() (unless need location/count)
• Use NavigationStack instead of NavigationView
• Use navigationDestination(for:) for type-safe navigation
• Use two-parameter or no-parameter onChange() variant
• Use ImageRenderer for rendering SwiftUI views
• Use .sheet(item:) instead of .sheet(isPresented:) for model-based content
• Sheets should own their actions and call dismiss() internally
• Use ScrollViewReader for programmatic scrolling with stable IDs
• Avoid UIScreen.main.bounds for sizing
• Avoid GeometryReader when alternatives exist (e.g., containerRelativeFrame())

Swift Best Practices

• Use modern Text formatting (.format parameters, not String(format:))
• Use localizedStandardContains() for user-input filtering (not contains())
• Prefer static member lookup (.blue vs Color.blue)
• Use .task modifier for automatic cancellation of async work
• Use .task(id:) for value-dependent tasks

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 (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 (Modern)

Legacy (Pre-iOS 17):

Modern API Replacements

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

• Using @Observable instead of ObservableObject for new code
• @Observable classes marked with @MainActor (if needed)
• Using @State with @Observable classes (not @StateObject)
• @State and @StateObject properties are private
• Passed values NOT declared as @State or @StateObject
• @Binding only where child modifies parent state
• @Bindable for injected @Observable needing bindings
• Nested ObservableObject avoided (or passed directly to child views)
• ViewModel used only when testability or complex state warrants it

Modern APIs (see references/modern-apis.md)

• Using foregroundStyle() instead of foregroundColor()
• Using clipShape(.rect(cornerRadius:)) instead of cornerRadius()
• Using Tab API instead of tabItem() (iOS 18+)
• Using Button instead of onTapGesture() (unless need location/count)
• Using NavigationStack instead of NavigationView
• Avoiding UIScreen.main.bounds
• Using alternatives to GeometryReader when possible
• Button images include text labels for accessibility

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

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

ScrollView (see references/scroll-patterns.md)

• Using ScrollViewReader with stable IDs for programmatic scrolling
• Using .scrollIndicators(.hidden) instead of initializer parameter

Text & Formatting (see references/text-formatting.md)

• Using modern Text formatting (not String(format:))
• Using localizedStandardContains() for search filtering

View Composition (see references/view-composition.md)

• Using modifiers instead of conditionals for state changes
• Complex views extracted to separate subviews (not @ViewBuilder functions)
• Views kept small with one responsibility
• Container views use @ViewBuilder let content: Content
• Specialized built-ins used where appropriate (Label, GroupBox, Form)
• Custom styles used to share visual patterns across instances
• Presentational views prefer primitive parameters (reusability recommendation)

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/view-composition.md)

• Avoiding layout thrash (deep hierarchies, excessive GeometryReader)
• Gating frequent geometry updates by thresholds
• Business logic separated into testable models (when warranted)
• 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, data flow, when to use a ViewModel
• references/view-composition.md — View extraction, styles, specialized views, reusability
• references/performance-patterns.md — Performance optimization techniques and anti-patterns
• references/list-patterns.md — ForEach identity, stability, and list best practices
• references/modern-apis.md — Modern API usage and deprecated replacements
• references/animation-basics.md — Core animation concepts, implicit/explicit animations
• references/animation-transitions.md — Transitions, custom transitions, Animatable protocol
• references/animation-advanced.md — Transactions, phase/keyframe animations, completion handlers
• references/navigation-patterns.md — Sheet presentation and navigation patterns
• references/scroll-patterns.md — ScrollView patterns and programmatic scrolling
• references/text-formatting.md — Modern text formatting and string operations
• references/image-optimization.md — AsyncImage, image downsampling, and optimization
• references/liquid-glass.md — iOS 26+ Liquid Glass API (adopt only when explicitly requested)

Philosophy

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

• We don’t enforce specific architectures (e.g., MVVM, VIPER) — see architecture-design-skill
• We do encourage separating business logic for testability when warranted
• We prioritize modern APIs over deprecated ones
• We emphasize thread safety with @MainActor and @Observable
• We optimize for performance and maintainability
• We follow Apple’s Human Interface Guidelines and API design patterns
• We use “prefer”/”avoid” for recommendations; “always”/”never” only for correctness of API