Swiftui Performance Audit

SwiftUI Performance Audit

Attribution: Sourced from steipete/agent-scripts by Peter Steinberger. Originally created by @Dimillian from Dimillian/Skills (2025-12-31).

When to Use

• Auditing SwiftUI view rendering, scrolling, or CPU/memory performance
• Identifying unnecessary view updates, body re-evaluations, or layout thrashing
• Guiding the user to profile with Instruments when code review is inconclusive

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).

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.

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).

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)

Expensive formatters in body

// ❌ Slow allocation on every render
var body: some View {
    let number = NumberFormatter()
    Text(number.string(from: 42)!)
}

// ✅ Cached formatter
final class Formatters {
    static let number = NumberFormatter()
}

Unstable identity in ForEach

// ❌ UUID() per render — destroys identity
ForEach(items, id: \.self) { item in Row(item) }

// ✅ Stable Identifiable ID
ForEach(items) { item in Row(item) }

Sorting/filtering in body

// ❌ Runs on every body eval
List {
    ForEach(items.sorted(by: sortRule)) { item in Row(item) }
}

// ✅ Sort once before view updates
let sortedItems = items.sorted(by: sortRule)

Broad dependencies in observable models

// ❌ Whole view tree re-renders on any items change
@Observable class Model { var items: [Item] = [] }
var body: some View { Row(isFavorite: model.items.contains(item)) }

// ✅ Granular view models or per-item state

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.