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swiftui-performance-audit/skill.md

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+---
+name: swiftui-performance-audit
+description: Audit and improve SwiftUI runtime performance from code review and architecture. Use for requests to diagnose slow rendering, janky scrolling, high CPU/memory usage, excessive view updates, or layout thrash in SwiftUI apps, and to provide guidance for user-run Instruments profiling when code review alone is insufficient.
+---
+
+# SwiftUI Performance Audit
+
+## Overview
+
+Audit SwiftUI view performance end-to-end, from instrumentation and baselining to root-cause analysis and concrete remediation steps.
+
+## Workflow Decision Tree
+
+- If the user provides code, start with "Code-First Review."
+- If the user only describes symptoms, ask for minimal code/context, then do "Code-First Review."
+- If code review is inconclusive, go to "Guide the User to Profile" and ask for a trace or screenshots.
+
+## 1. Code-First Review
+
+Collect:
+- Target view/feature code.
+- Data flow: state, environment, observable models.
+- Symptoms and reproduction steps.
+
+Focus on:
+- View invalidation storms from broad state changes.
+- Unstable identity in lists (`id` churn, `UUID()` per render).
+- Heavy work in `body` (formatting, sorting, image decoding).
+- Layout thrash (deep stacks, `GeometryReader`, preference chains).
+- Large images without downsampling or resizing.
+- Over-animated hierarchies (implicit animations on large trees).
+
+Provide:
+- Likely root causes with code references.
+- Suggested fixes and refactors.
+- If needed, a minimal repro or instrumentation suggestion.
+
+## 2. Guide the User to Profile
+
+Explain how to collect data with Instruments:
+- Use the SwiftUI template in Instruments (Release build).
+- Reproduce the exact interaction (scroll, navigation, animation).
+- Capture SwiftUI timeline and Time Profiler.
+- Export or screenshot the relevant lanes and the call tree.
+
+Ask for:
+- Trace export or screenshots of SwiftUI lanes + Time Profiler call tree.
+- Device/OS/build configuration.
+
+## 3. Analyze and Diagnose
+
+Prioritize likely SwiftUI culprits:
+- View invalidation storms from broad state changes.
+- Unstable identity in lists (`id` churn, `UUID()` per render).
+- Heavy work in `body` (formatting, sorting, image decoding).
+- Layout thrash (deep stacks, `GeometryReader`, preference chains).
+- Large images without downsampling or resizing.
+- Over-animated hierarchies (implicit animations on large trees).
+
+Summarize findings with evidence from traces/logs.
+
+## 4. Remediate
+
+Apply targeted fixes:
+- Narrow state scope (`@State`/`@Observable` closer to leaf views).
+- Stabilize identities for `ForEach` and lists.
+- Move heavy work out of `body` (precompute, cache, `@State`).
+- Use `equatable()` or value wrappers for expensive subtrees.
+- Downsample images before rendering.
+- Reduce layout complexity or use fixed sizing where possible.
+
+## Common Code Smells (and Fixes)
+
+Look for these patterns during code review.
+
+### Expensive formatters in `body`
+
+```swift
+var body: some View {
+    let number = NumberFormatter() // slow allocation
+    let measure = MeasurementFormatter() // slow allocation
+    Text(measure.string(from: .init(value: meters, unit: .meters)))
+}
+```
+
+Prefer cached formatters in a model or a dedicated helper:
+
+```swift
+final class DistanceFormatter {
+    static let shared = DistanceFormatter()
+    let number = NumberFormatter()
+    let measure = MeasurementFormatter()
+}
+```
+
+### Computed properties that do heavy work
+
+```swift
+var filtered: [Item] {
+    items.filter { $0.isEnabled } // runs on every body eval
+}
+```
+
+Prefer precompute or cache on change:
+
+```swift
+@State private var filtered: [Item] = []
+// update filtered when inputs change
+```
+
+### Sorting/filtering in `body` or `ForEach`
+
+```swift
+List {
+    ForEach(items.sorted(by: sortRule)) { item in
+        Row(item)
+    }
+}
+```
+
+Prefer sort once before view updates:
+
+```swift
+let sortedItems = items.sorted(by: sortRule)
+```
+
+### Inline filtering in `ForEach`
+
+```swift
+ForEach(items.filter { $0.isEnabled }) { item in
+    Row(item)
+}
+```
+
+Prefer a prefiltered collection with stable identity.
+
+### Unstable identity
+
+```swift
+ForEach(items, id: \.self) { item in
+    Row(item)
+}
+```
+
+Avoid `id: \.self` for non-stable values; use a stable ID.
+
+### Image decoding on the main thread
+
+```swift
+Image(uiImage: UIImage(data: data)!)
+```
+
+Prefer decode/downsample off the main thread and store the result.
+
+### Broad dependencies in observable models
+
+```swift
+@Observable class Model {
+    var items: [Item] = []
+}
+
+var body: some View {
+    Row(isFavorite: model.items.contains(item))
+}
+```
+
+Prefer granular view models or per-item state to reduce update fan-out.
+
+## 5. Verify
+
+Ask the user to re-run the same capture and compare with baseline metrics.
+Summarize the delta (CPU, frame drops, memory peak) if provided.
+
+## Outputs
+
+Provide:
+- A short metrics table (before/after if available).
+- Top issues (ordered by impact).
+- Proposed fixes with estimated effort.
+
+## References
+
+Add Apple documentation and WWDC resources under `references/` as they are supplied by the user.
+- Optimizing SwiftUI performance with Instruments: `references/optimizing-swiftui-performance-instruments.md`
+- Understanding and improving SwiftUI performance: `references/understanding-improving-swiftui-performance.md`
+- Understanding hangs in your app: `references/understanding-hangs-in-your-app.md`
+- Demystify SwiftUI performance (WWDC23): `references/demystify-swiftui-performance-wwdc23.md`