alexander-kastil/swiftui-expert-skill

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.

When to Use This Skill

Use this skill when:

• Building new SwiftUI features
• Refactoring existing SwiftUI views
• Reviewing SwiftUI code quality
• Adopting modern SwiftUI patterns
• Working with SwiftUI state management
• Implementing iOS 26+ Liquid Glass styling

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-structure.md)
• Check performance patterns are applied (see references/performance-patterns.md)
• Verify list patterns use stable identity (see references/list-patterns.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 (prefer @Observable over ObservableObject)
• Replace deprecated APIs with modern equivalents (see references/modern-apis.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)
• 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)
• 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-structure.md)
• Separate business logic into testable models (see references/layout-best-practices.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

Modern APIs

• Use foregroundStyle() instead of foregroundColor()
• Use clipShape(.rect(cornerRadius:)) instead of cornerRadius()
• Use Tab API instead of tabItem()
• 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

View Composition

• Prefer modifiers over conditional views for state changes (maintains view identity)
• Extract complex views into separate subviews for better readability and performance
• Keep views small for optimal performance
• 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
• Separate business logic into testable models (not about enforcing architectures)
• 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)

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

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)

| Wrapper | Use When | |---------|----------| | @State | Internal view state (must be private), or owned @Observable class | | @Binding | Child modifies parent's state | | @Bindable | Injected @Observable needing bindings | | let | Read-only value from parent | | var | Read-only value watched via .onChange() |

Legacy (Pre-iOS 17): | Wrapper | Use When | |---------|----------| | @StateObject | View owns an ObservableObject (use @State with @Observable instead) | | @ObservedObject | View receives an ObservableObject |

Modern API Replacements

| Deprecated | Modern Alternative | |------------|-------------------| | foregroundColor() | foregroundStyle() | | cornerRadius() | clipShape(.rect(cornerRadius:)) | | tabItem() | Tab API | | onTapGesture() | Button (unless need location/count) | | NavigationView | NavigationStack | | onChange(of:) { value in } | onChange(of:) { old, new in } or onChange(of:) { } | | fontWeight(.bold) | bold() | | GeometryReader | containerRelativeFrame() or visualEffect() | | showsIndicators: false | .scrollIndicators(.hidden) | | String(format: "%.2f", value) | Text(value, format: .number.precision(.fractionLength(2))) | | string.contains(search) | string.localizedStandardContains(search) (for user input) |

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)

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()
• [ ] 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/sheet-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 Structure (see references/view-structure.md)

• [ ] Using modifiers instead of conditionals for state changes
• [ ] Complex views extracted to separate subviews
• [ ] Views kept small for performance
• [ ] 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 separated into testable models
• [ ] Action handlers reference methods (not inline logic)
• [ ] Using relative layout (not hard-coded constants)
• [ ] Views work in any context (context-agnostic)

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 (prefer @Observable)
• 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/modern-apis.md - Modern API usage and deprecated replacements
• references/sheet-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

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 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