performance-audit

name	performance-audit
description	Audit SwiftUI view performance end-to-end. Use when debugging SwiftUI slowness, frame drops, scroll jank, or when asked for a performance audit, profiling help, or optimization.
argument-hint	[file or view name]

SwiftUI Performance Audit

Target: $ARGUMENTS

Workflow Decision Tree

• User provides code → Start with Code-First Review.
• User only describes symptoms → Ask for minimal code/context, then Code-First Review.
• Code review inconclusive → Guide the User to Profile; ask for 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 (same list as Code-First Review). 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.

---

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.

---

Common Code Smells (and Fixes)

Expensive formatters in body

var body: some View {
    let number = NumberFormatter()      // slow allocation
    let measure = MeasurementFormatter() // slow allocation
    Text(measure.string(from: .init(value: meters, unit: .meters)))
}

Fix: Cache formatters in a model or dedicated helper (e.g. static let shared with reused instances).

Computed properties that do heavy work

var filtered: [Item] {
    items.filter { $0.isEnabled }  // runs on every body eval
}

Fix: Precompute or cache on change (e.g. @State private var filtered: [Item]; update when inputs change).

Sorting/filtering in body or ForEach

List {
    ForEach(items.sorted(by: sortRule)) { item in Row(item) }
}

Fix: Sort once before view updates: let sortedItems = items.sorted(by: sortRule) (e.g. in model or view model).

Inline filtering in ForEach

ForEach(items.filter { $0.isEnabled }) { item in Row(item) }

Fix: Use a prefiltered collection with stable identity.

Unstable identity

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

Fix: Avoid id: \.self for non-stable values; use a stable ID (e.g. id: \.id on a type with stable id).

Image decoding on the main thread

Image(uiImage: UIImage(data: data)!)

Fix: Decode/downsample off the main thread and store the result; use a cached/downsampled image in the view.

Broad dependencies in observable models

@Observable class Model {
    var items: [Item] = []
}
var body: some View {
    Row(isFavorite: model.items.contains(item))  // broad dependency
}

Fix: Prefer granular view models or per-item state to reduce update fan-out.

---

Outputs

Provide:

• A short metrics table (before/after if available).
• Top issues (ordered by impact).
• Proposed fixes with estimated effort.

---

References

• references/optimizing-swiftui-performance-instruments.md
• references/understanding-improving-swiftui-performance.md
• references/understanding-hangs-in-your-app.md
• references/demystify-swiftui-performance-wwdc23.md