swiftui-performance-audit

Files (5)

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

SKILL.md (first lines)

# 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:

From steipete/agent-scripts, MIT.

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