swiftui-expert-skill

SwiftUI Expert Skill

Operating Rules

• Treat each View type as an invalidation boundary: give it only the data it reads and keep frequently changing dependencies close to the smallest affected subtree
• Search references/latest-apis.md when writing, reviewing, or migrating API usage; look up only the APIs relevant to the task
• Replace hard-deprecated APIs with modern equivalents. During feature work, flag soft-deprecated APIs and leave them in place (see references/soft-deprecation.md)
• 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
• For layout and rendering inputs, read the value nearest the SwiftUI view that consumes it; do not substitute process-global screen state

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); replace hard-deprecated APIs, and flag soft-deprecated APIs without rewriting them unless the user asked to migrate
• Run the Topic Router below for each relevant topic
• Validate #available gating and fallback paths for version-specific features
• For broad codebase reviews, first identify smaller focus areas and present them one at a time; if the user requests a whole-codebase review, divide it into a TODO list

Improve existing SwiftUI code

• Audit current implementation against the Topic Router topics
• Replace hard-deprecated APIs with modern equivalents from references/latest-apis.md; flag soft-deprecated APIs and do not rewrite them during feature work
• 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 (gives Time Profiler + Hangs, but no 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.

Hangs: if xctrace help lists triage (Xcode 27.2 does), start with xctrace triage <trace> --style hangs, then drill in with xctrace show. The bundled parser remains the tool for hitches, SwiftUI updates, the cause graph, and log or signpost windows.

Summary of the parser's 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.
7. Return a prioritised plan. Cite evidence (coverage %, hot symbol, overlapping view, log timestamp, cause-graph edges) and route each recommendation to a Topic Router reference.
8. Only edit code if the user asked for edits.

Topic Router

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

Topic	Reference
State management	references/state-management.md
Environment and @Entry, including "Storing a closure/class type in '@Entry var'" warnings	references/environment-patterns.md
View composition	references/view-structure.md
View modifiers and identity	references/modifier-patterns.md
Performance	references/performance-patterns.md
Lists and ForEach	references/list-patterns.md
Resizable layout, safe areas, two-column reflow, foldable grids, arrangements, and reserved regions	references/layout-best-practices.md
iPhone Duo, foldable, or large-display screens (read first to choose the technique)	references/iphone-duo.md
Sheets, navigation, NavigationSplitView on large displays, and tab bar/sidebar (sidebarAdaptable)	references/sheet-navigation-patterns.md
ScrollView, scroll position, and scroll geometry	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 and display scale	references/image-optimization.md
Toolbars	references/toolbar-patterns.md
Document-based apps	references/document-apps.md
WebKit	references/webkit-integration.md
Styled text editing	references/styled-text-editing.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
Localization	references/localization.md
Deprecated API lookup	references/latest-apis.md
Handling soft-deprecated APIs	references/soft-deprecation.md
Previews	references/previews.md
Instruments trace analysis	references/trace-analysis.md
Instruments trace recording	references/trace-recording.md

Correctness Checklist

These are hard rules -- violations are always bugs:

• [ ] @State properties are private
• [ ] @Binding only where a child modifies parent state
• [ ] Changing parent-owned inputs are not stored as @State/@StateObject; intentional state seeds are documented as one-time
• [ ] @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/\.offset; id outlives the view and isn't derived from mutable content)
• [ ] Constant number of views per ForEach element; List rows are unary
• [ ] No closures stored in custom @Environment/@FocusedValue keys
• [ ] Custom @Entry default values are stable (no Model()/Date()/UUID() expressions)
• [ ] SwiftUI display scale comes from @Environment(\.displayScale), not global screen state
• [ ] Safe-area content does not double-apply GeometryProxy.safeAreaInsets
• [ ] .animation(_:value:) always includes the value parameter
• [ ] @FocusState properties are private
• [ ] No redundant @FocusState writes inside tap gesture handlers on .focusable() views
• [ ] Version-specific APIs are gated with #available and have sensible fallbacks
• [ ] import Charts present in files using chart types
• [ ] Previews use self-contained mock data; no dependency on live services or network