Introduction
# SwiftUI Performance Audit
_Attribution: copied from @Dimillian’s `Dimillian/Skills` (2025-12-31)._
## 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`