swiftui-expert-skill

name	swiftui-expert-skill
description	Use when writing, reviewing, or refactoring SwiftUI code for iOS or macOS, including state management, view composition, performance, Liquid Glass adoption, or Instruments `.trace` capture/analysis for hangs, hitches, CPU hotspots, or excessive view updates.

SwiftUI Expert Skill

Operating Rules

• Consult references/latest-apis.md at the start of every task to avoid deprecated APIs
• Prefer native SwiftUI APIs over UIKit/AppKit bridging unless bridging is necessary
• Focus on correctness and performance; do not enforce specific architectures (MVVM, VIPER, etc.)
• Encourage separating business logic from views for testability without mandating how
• Follow Apple's Human Interface Guidelines and API design patterns
• Only adopt Liquid Glass when explicitly requested by the user (see references/liquid-glass.md)
• Present performance optimizations as suggestions, not requirements
• Use #available gating with sensible fallbacks for version-specific APIs

Task Workflow

Review existing SwiftUI code

• Read the code under review and identify which topics apply
• Flag deprecated APIs (compare against references/latest-apis.md)
• Run the Topic Router below for each relevant topic
• Validate #available gating and fallback paths for iOS 26+ features

Improve existing SwiftUI code

• Audit current implementation against the Topic Router topics
• Replace deprecated APIs with modern equivalents from references/latest-apis.md
• Refactor hot paths to reduce unnecessary state updates
• Extract complex view bodies into separate subviews
• Suggest image downsampling when UIImage(data:) is encountered (optional optimization, see references/image-optimization.md)

Implement new SwiftUI feature

• Design data flow first: identify owned vs injected state
• Structure views for optimal diffing (extract subviews early)
• Apply correct animation patterns (implicit vs explicit, transitions)
• Use Button for all tappable elements; add accessibility grouping and labels
• Gate version-specific APIs with #available and provide fallbacks

Record a new Instruments trace

Trigger when the user asks to "record a trace", "profile the app", "capture a session", etc. Full reference: references/trace-recording.md.

1. Confirm target — attach to a running app, launch an app, or record all processes? If the user didn't say, ask. List connected devices when useful:

   python3 "${SKILL_DIR}/scripts/record_trace.py" --list-devices
   

2. Pick a template based on target kind — the SwiftUI template populates the SwiftUI lane on any real device: a physical iOS/iPadOS device or the host Mac. The only exception is the iOS Simulator, where the SwiftUI lane comes back empty — switch to --template "Time Profiler" in that case (still gives Time Profiler + Hangs + Animation Hitches). Always check --list-devices: simulators kind → Time Profiler; devices kind (real devices and the host Mac) → default SwiftUI. Full decision table in references/trace-recording.md.
3. Start the recording. For agent-driven sessions where the user says "I'll tell you when I'm done", start in the background and use a stop-file:

   python3 "${SKILL_DIR}/scripts/record_trace.py" \
       --device "<name|udid>" --attach "<AppName>" \
       --stop-file /tmp/stop-trace --output ~/Desktop/session.trace
   

   For interactive sessions, just tell the user to press Ctrl+C when done.
4. Signal stop — when the user says they've finished exercising the app, touch /tmp/stop-trace. The script cleanly SIGINTs xctrace and waits up to 60s for finalisation.
5. Analyse the resulting trace (flow into the "Trace-driven improvement" workflow below).

Trace-driven improvement (Instruments .trace provided)

Trigger whenever the user's request references a .trace file. A target SwiftUI source file is optional — if given, cite specific lines; if not, recommend where to look based on view names and symbols the trace already reveals.

Full reference: references/trace-analysis.md. Summary of the composition pattern:

1. Scope the analysis. Ask yourself: does the user want the whole trace, or a slice?
   • "focus on X / after X / between X and Y / during X" → resolve to a window first (see step 2).
   • No scoping cue → analyse the whole trace.
2. Resolve a window (only if the user scoped). The parser exposes two discovery modes:

   # Find a log that marks the start/end of the region of interest:
   python3 "${SKILL_DIR}/scripts/analyze_trace.py" --trace <path> \
       --list-logs --log-message-contains "loaded feed" --log-limit 5
   # Or list os_signpost intervals (paired begin/end), filterable by name:
   python3 "${SKILL_DIR}/scripts/analyze_trace.py" --trace <path> \
       --list-signposts --signpost-name-contains "ImageDecode"
   

   Both modes accept --window START_MS:END_MS to scope discovery. Pick the time_ms (for logs) or start_ms/end_ms (for signposts) that match the user's description. Build a window like --window 10400:11700.
3. Run the main analysis (with or without --window):

   python3 "${SKILL_DIR}/scripts/analyze_trace.py" --trace <path> \
       --json-only --top 10 [--window START_MS:END_MS]
   

4. Interpret with references/trace-analysis.md — key diagnostics:
   • main_running_coverage_pct inside each correlation (<25% = blocked; ≥75% = CPU-bound).
   • swiftui-causes.top_sources reveals why updates keep happening — high-edge-count sources like UserDefaultObserver.send() or wide EnvironmentWriter entries are structural invalidation bugs. Fixing one often collapses many downstream hot views.
5. When a specific view shows as expensive, ask who's invalidating it. Use --fanin-for "<view name>" to get the ranked list of source nodes driving the updates.
6. Optionally ground in source. If the user pointed at a file, read it and match view names / user-code symbols against identifiers there. If not, recommend which files to open based on the view names SwiftUI reported.

Topic Router

Consult the reference file for each topic relevant to the current task:

Topic	Reference
State management	references/state-management.md
View composition	references/view-structure.md
Performance	references/performance-patterns.md
Lists and ForEach	references/list-patterns.md
Layout	references/layout-best-practices.md
Sheets and navigation	references/sheet-navigation-patterns.md
ScrollView	references/scroll-patterns.md
Focus management	references/focus-patterns.md
Animations (basics)	references/animation-basics.md
Animations (transitions)	references/animation-transitions.md
Animations (advanced)	references/animation-advanced.md
Accessibility	references/accessibility-patterns.md
Swift Charts	references/charts.md
Charts accessibility	references/charts-accessibility.md
Image optimization	references/image-optimization.md
Liquid Glass (iOS 26+)	references/liquid-glass.md
macOS scenes	references/macos-scenes.md
macOS window styling	references/macos-window-styling.md
macOS views	references/macos-views.md
Text patterns	references/text-patterns.md
Deprecated API lookup	references/latest-apis.md
Previews	references/previews.md
Instruments trace analysis	references/trace-analysis.md

Correctness Checklist

These are hard rules -- violations are always bugs:

• @State properties are private
• @Binding only where a child modifies parent state
• Passed values never declared as @State or @StateObject (they ignore updates)
• @StateObject for view-owned objects; @ObservedObject for injected
• iOS 17+: @State with @Observable; @Bindable for injected observables needing bindings
• ForEach uses stable identity (never .indices for dynamic content)
• Constant number of views per ForEach element
• .animation(_:value:) always includes the value parameter
• @FocusState properties are private
• No redundant @FocusState writes inside tap gesture handlers on .focusable() views
• iOS 26+ APIs gated with #available and fallback provided
• import Charts present in files using chart types
• Previews use self-contained mock data; no dependency on live services or network

References

• references/latest-apis.md -- Read first for every task. Deprecated-to-modern API transitions (iOS 15+ through iOS 26+)
• references/state-management.md -- Property wrappers, data flow, @Observable migration
• references/view-structure.md -- View extraction, container patterns, @ViewBuilder
• references/performance-patterns.md -- Hot-path optimization, update control, _logChanges()
• references/list-patterns.md -- ForEach identity, Table (iOS 16+), inline filtering pitfalls
• references/layout-best-practices.md -- Layout patterns, GeometryReader alternatives
• references/accessibility-patterns.md -- VoiceOver, Dynamic Type, grouping, traits
• references/animation-basics.md -- Implicit/explicit animations, timing, performance
• references/animation-transitions.md -- View transitions, matchedGeometryEffect, Animatable
• references/animation-advanced.md -- Phase/keyframe animations (iOS 17+), @Animatable macro (iOS 26+)
• references/charts.md -- Swift Charts marks, axes, selection, styling, Chart3D (iOS 26+)
• references/charts-accessibility.md -- Charts VoiceOver, Audio Graph, fallback strategies
• references/sheet-navigation-patterns.md -- Sheets, NavigationSplitView, Inspector
• references/scroll-patterns.md -- ScrollViewReader, programmatic scrolling
• references/focus-patterns.md -- Focus state, focusable views, focused values, default focus, common pitfalls
• references/image-optimization.md -- AsyncImage, downsampling, caching
• references/liquid-glass.md -- iOS 26+ Liquid Glass effects and fallback patterns
• references/macos-scenes.md -- Settings, MenuBarExtra, WindowGroup, multi-window
• references/macos-window-styling.md -- Toolbar styles, window sizing, Commands
• references/macos-views.md -- HSplitView, Table, PasteButton, AppKit interop
• references/previews.md -- #Preview macro, (iOS 18+), preview traits, mock data patterns for self-contained previews