ios-performance-profiler

Performance Profiler

View Identity & Diffing

• Stable, persistent identity (Identifiable with stable id, avoid random UUIDs in rows)
• Prefer value types + structural identity over class reference identity when possible
• Avoid .id() unless absolutely necessary (use stable keys instead)

Body Computation & Redraws

• No heavy work (network, decoding, sorting, filtering) inside body
• Expensive derived state computed once and stored (.onAppear, ViewModel, or memoized computed property)
• @State, @Bindable, @Observable properties are narrowly scoped
• Child views extracted when parent state changes frequently
• Prefer @Observable (Observation framework) over @ObservableObject / @Published for new code — finer-grained invalidation, less boilerplate

Lists & Large Collections

• Use List / LazyVStack / LazyHGrid instead of ScrollView + ForEach + VStack
• Rows are lightweight; avoid @StateObject / @ObservedObject inside row bodies
• Use stable Identifiable conformance or explicit id: key path

Concurrency & Main Actor (Swift 6 era)

• @MainActor used appropriately on view-bound types (avoid overuse → thread hops)
• Nonisolated properties / computed vars when safe
• Actors used for model / service layers; avoid @MainActor on pure data models
• No data races when using @Observable types across concurrency domains
• Prefer .task over .onAppear for async work (automatic cancellation on view disappear, structured concurrency)

Environment & State Propagation

• Avoid injecting large @Observable objects via @Environment when only a small slice of state is needed — extract a child view that reads only what it needs
• Minimize over-subscription from EnvironmentObject — split into focused, smaller observable types if necessary

Memory & Retain Cycles

• [weak self] in async / escaping closures that capture self
• @StateObject for view-owned view models (not in List rows)
• Remote images use proper caching (AsyncImage with cache, or Kingfisher / Nuke)
• Downscale images to display size before rendering (.resizable() + .frame() alone doesn't reduce memory)
• Use preparingThumbnail(of:) for large images
• Avoid long-lived strong references in @Observable / @ObservableObject

Layout & Geometry

• Minimize GeometryReader usage inside scroll views (causes layout thrashing)
• Avoid PreferenceKey updates that trigger parent re-layouts in tight loops
• Use drawingGroup() for complex vector paths / heavy Canvas content

Animations & Transitions

• Scope animations narrowly (.animation(…, value: …)) instead of global .animation()
• Use .matchedGeometryEffect only when necessary (expensive)
• Test with reduced motion enabled

Navigation & Sheets

• Use lazy destination resolution in NavigationStack (avoid pre-building destination views)
• Ensure .sheet / .fullScreenCover content isn't computed until presentation
• Avoid deep view hierarchies in destinations — flatten or split into smaller views

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Quick Wins

Issue	Fix / Pattern	Severity
Parent redraws child unnecessarily	Extract stable child view / use Equatable conformance	🟡
Heavy logic in body	Move to ViewModel / .task / computed property	🔴
Unstable ForEach IDs	Use stable id: \.self or persistent model ID	🔴
Retain cycle	\[weak self\] in closures	🔴
@StateObject recreated in List row	Move ownership to parent / use @Observable value type	🔴
Broad / unnecessary animations	.animation(nil) or value-specific .animation(…, value:)	🟡
Frequent main-actor hops	Use nonisolated properties, actors for model layer	🟡
GeometryReader in scroll views	Move geometry reads outside scroll or cache values	🟡
Large images not downscaled	preparingThumbnail(of:) or resize before display	🟡
Over-subscribed @Environment	Extract child view reading only needed state	🟡
Eager sheet / destination init	Lazy destination resolution in NavigationStack	🟡
Still using @ObservableObject	Migrate to @Observable (Observation framework)	🟢
Complex vector / Canvas paths	Use drawingGroup() for off-screen rendering	🟢

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Debug Helpers (Xcode 16+)

// In any View body — logs when/why view redraws
let _ = Self._printChanges()

// os_signpost for custom performance intervals
import os.signpost

let log = OSLog(subsystem: "com.app.performance", category: "ViewLoading")
let signpostID = OSSignpostID(log: log)
os_signpost(.begin, log: log, name: "LoadData", signpostID: signpostID)
// ... work ...
os_signpost(.end, log: log, name: "LoadData", signpostID: signpostID)

Instruments & Tools

• SwiftUI template + Animation instrument in Instruments
• SwiftUI View Body instrument (Xcode 16+) — tracks body evaluations per view
• SwiftUI Performance HUD — ⌥⌘P in Simulator
• MetricKit / MXMetricManager — production performance & diagnostic data
• Memory Graph Debugger — catch retain cycles and leaks at runtime

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Severity Legend

🔴 Critical — Visible lag, dropped frames, memory growth, crashes

🟡 Moderate — Noticeable jank on low-end devices or large datasets

🟢 Minor — Optimization opportunity / future-proofs for larger scale