Performance

Performance & Battery Audit

Scan for layout thrash, heavy animations, timers, and background tasks. Flag hotspots and suggest fixes.

When to use this skill

• Before App Store submission
• After adding complex animations or layouts
• When users report lag, jank, or battery drain
• When reviewing Views with frequent state changes

Quick Reference: Common Issues

Step 1: Layout Performance Audit

1.1 GeometryReader Misuse

# GeometryReader (expensive layout pass)
rg --type swift -g '!*Tests*' 'GeometryReader' YourApp/Views/ | head -15

Issues: GeometryReader inside ScrollView/List causes relayout on scroll.

Fix (iOS 17+):

// ❌ Expensive
GeometryReader { geo in content.frame(width: geo.size.width * 0.8) }

// ✅ Modern alternative
content.containerRelativeFrame(.horizontal) { width, _ in width * 0.8 }

1.2 Layout Invalidation in Body

# Inline expensive object creation
rg --type swift -g '!*Tests*' 'DateFormatter\(\)|NumberFormatter\(\)|JSONDecoder\(\)' YourApp/Views/

Fix: Move to static/cached properties.

Step 2: Animation Performance

2.1 Animation Scope

# Animations without value parameter (animate everything)
rg --type swift -g '!*Tests*' '\.animation\([^,)]+\)' YourApp/Views/ | grep -v 'value:' | head -15

Fix: Always scope animations:

.animation(.easeInOut, value: isExpanded)  // ✅ Only animates this change

2.2 Heavy/Repeating Animations

# Repeating animations (battery impact)
rg --type swift -g '!*Tests*' '\.repeatForever|\.repeat\(' YourApp/Views/

# Canvas/TimelineView (GPU intensive)
rg --type swift -g '!*Tests*' 'Canvas\s*\{|TimelineView' YourApp/Views/

watchOS: TimelineView(.periodic) is extremely battery-heavy. Prefer .animation schedule or pause when not visible.

2.3 Expensive Rendering Modifiers

# GPU-heavy modifiers
rg --type swift -g '!*Tests*' '\.drawingGroup\(|\.shadow\(radius:|\.blur\(|\.mask\s*\{|\.compositingGroup\(' YourApp/Views/ | head -20

Guidance:

• .drawingGroup() — Use only for complex overlapping content, not everywhere
• .shadow(radius:) — Expensive with large radii; cache or use images
• .blur() — Very expensive; consider pre-blurred assets
• .mask { } — Recalculates every frame; avoid in animations
• .compositingGroup() — Use sparingly for correct blending

Step 3: ForEach & List Efficiency

3.1 Unstable ForEach IDs

# ForEach with \.self on non-Hashable-stable data (causes full diff)
rg --type swift -g '!*Tests*' 'ForEach.*\\\.self' YourApp/Views/ | head -15

# ForEach without explicit id:
rg --type swift -g '!*Tests*' 'ForEach\([^)]+\)\s*\{' YourApp/Views/ | grep -v 'id:' | head -15

Issues:

• \.self on arrays that get reordered/modified causes full re-render
• Missing stable IDs cause diffing cost

Fix: Use stable, unique identifiers:

ForEach(items, id: \.stableID) { item in ... }

3.2 LazyVStack/LazyHStack Usage

# Check for lazy stacks in lists
rg --type swift -g '!*Tests*' 'LazyVStack|LazyHStack|LazyVGrid|LazyHGrid' YourApp/Views/

# Non-lazy stacks in ScrollView (loads all content)
rg --type swift -g '!*Tests*' -B2 'ScrollView' YourApp/Views/ | grep -A2 'VStack\|HStack' | head -20

Step 4: State Management

4.1 @State Arrays/Dictionaries

# Large state objects in Views
rg --type swift -g '!*Tests*' '@State\s+(private\s+)?var\s+\w+:\s*\[' YourApp/Views/ | head -15

Fix: Large collections should be in ViewModel/ObservableObject, not View @State.

4.2 @StateObject Initialization

# @StateObject in body (wrong - should be in init or property)
rg --type swift -g '!*Tests*' -B3 '@StateObject' YourApp/Views/ | head -20

Fix: Initialize @StateObject in property declaration or init, never in body:

// ✅ Correct
@StateObject private var viewModel = MyViewModel()

// ❌ Wrong - recreates on every render
var body: some View {
    let vm = MyViewModel() // BAD
}

Step 5: Timer & Background Task Audit

5.1 Timer Usage

# Timer instances
rg --type swift -g '!*Tests*' 'Timer\.|Timer\.publish|Timer\.scheduledTimer' YourApp/

Issues:

• Timers not invalidated when view disappears
• High-frequency timers (< 0.1s) for non-critical updates

5.2 Background Tasks

rg --type swift -g '!*Tests*' 'BGTaskScheduler|BGAppRefreshTask|BGProcessingTask' YourApp/

# Extended runtime sessions (Watch)
rg --type swift 'WKExtendedRuntimeSession' "YourApp Watch App/"

Step 6: Network Efficiency

# URLSession configuration
rg --type swift -g '!*Tests*' 'URLSession|URLSessionConfiguration' YourApp/

Best practices:

• Use isDiscretionary = true for non-urgent downloads
• Cache responses where appropriate

Step 7: Watch App Performance

WATCH_DIR="YourApp Watch App"

# Timers on Watch (battery critical)
rg --type swift 'Timer\.' "$WATCH_DIR/"

# TimelineView on Watch (use .animation not .periodic)
rg --type swift 'TimelineView.*\.periodic' "$WATCH_DIR/"

# Heavy animations on Watch
rg --type swift '\.animation\(|withAnimation' "$WATCH_DIR/Views/"

watchOS priority: Battery > Smoothness. TimelineView(.periodic) drains battery fast—use .animation schedule instead.

Step 8: Instruments Verification

For issues found, verify with Instruments:

1. Time Profiler — CPU hotspots
2. Core Animation — Frame drops, offscreen renders
3. Allocations — Memory leaks, spikes
4. Energy Log — Battery impact

Skip Patterns

• *Tests*/, *UITests*/
• #Preview { } blocks

Output Format