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

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:

final class DistanceFormatter {
    static let shared = DistanceFormatter()
    let number = NumberFormatter()
    let measure = MeasurementFormatter()
}

Computed properties that do heavy work

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

Prefer precompute or cache on change:

@State private var filtered: [Item] = []
// update filtered when inputs change

Sorting/filtering in body or ForEach

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

Prefer sort once before view updates:

let sortedItems = items.sorted(by: sortRule)

Inline filtering in ForEach

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

Prefer a prefiltered collection with stable identity.

Unstable identity

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

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

Prefer decode/downsample off the main thread and store the result.

Broad dependencies in observable models

@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