SwiftUI Performance Debugging

When This Skill Activates

• Reports slow or janky SwiftUI views
• Sees excessive view re-renders or body re-evaluations
• Asks about

  Self._printChanges()

  or view debugging
• Has scrolling performance issues with lists or grids
• Asks why a view keeps updating when nothing changed
• Mentions

  @Observable

  or

  ObservableObject

  performance differences
• Wants to understand SwiftUI view identity or diffing
• Uses

  AnyView

  and asks about performance implications
• Has a hang or stutter traced to SwiftUI rendering

Decision Tree

What SwiftUI performance problem are you seeing?
|
+- Views re-render when they should not
|  +- Read body-reevaluation.md
|     +- Self._printChanges() to identify which property changed
|     +- @Observable vs ObservableObject observation differences
|     +- Splitting views to narrow observation scope
|
+- Scrolling is slow / choppy (lists, grids)
|  +- Read lazy-loading.md
|     +- VStack vs LazyVStack, ForEach without lazy container
|     +- List prefetching, grid cell reuse
|
+- Views lose state unexpectedly / animate when they should not
|  +- Read view-identity.md
|     +- Structural vs explicit identity
|     +- .id() misuse, conditional view branching
|
+- Known pitfall (AnyView, DateFormatter in body, etc.)
|  +- Read common-pitfalls.md
|     +- AnyView type erasure, object creation in body
|     +- Over-observation, expensive computations
|
+- General "my SwiftUI app is slow" (unknown cause)
|  +- Start with body-reevaluation.md, then common-pitfalls.md
|  +- Use Instruments SwiftUI template (see Debugging Tools below)

API Availability

Self._printChanges()

@Observable

@ObservableObject

LazyVStack

LazyHStack

LazyVGrid

LazyHGrid

.id()

os_signpost

Top 5 Mistakes -- Quick Reference

ForEach

VStack

ScrollView

LazyVStack

VStack

AnyView

@ViewBuilder

Group

AnyView

body

DateFormatter()

NumberFormatter()

static let

@State

@Observable

.id()

Debugging Tools

Self._printChanges()

var body: some View {
    let _ = Self._printChanges()
    // ... view content
}

ViewName: @self, @identity, _propertyName changed.

Instruments SwiftUI Template

1. Xcode > Product > Profile (Cmd+I)
2. Choose SwiftUI template (includes View Body, View Properties, Core Animation Commits)
3. Record, reproduce the slow interaction, stop
4. View Body lane shows which views had their body evaluated and how often
5. View Properties lane shows which properties changed

os_signpost for Custom Measurement

import os

private let perfLog = OSLog(subsystem: "com.app.perf", category: "SwiftUI")

var body: some View {
    let _ = os_signpost(.event, log: perfLog, name: "MyView.body")
    // ... view content
}

Review Checklist

View Identity

• No unstable

  .id()

  values (random, Date(), array index on mutable arrays)
• Conditional branches (

  if

  /

  else

  ) do not cause unnecessary view destruction

• ForEach

  uses stable, unique identifiers from the model

Body Re-evaluation

• Views observe only the properties they actually use

• @Observable

  classes preferred over

  ObservableObject

  (iOS 17+)
• No unnecessary

  @State

  changes that trigger body re-evaluation
• Large views split into smaller subviews to narrow observation scope

Lazy Loading

• Large collections use

  LazyVStack

  /

  LazyHStack

  , not

  VStack

  /

  HStack

• List

  or lazy stack used for 50+ items
• No

  .frame(maxHeight: .infinity)

  on children inside lazy containers (defeats laziness)

Common Pitfalls

• No

  AnyView

  type erasure (use

  @ViewBuilder

  or

  Group

  )
• No object allocation in

  body

  (

  DateFormatter

  ,

  NSPredicate

  , view models)
• Expensive computations moved to background with

  task { }

  or

  Task.detached

• Images use

  AsyncImage

  or

  .resizable()

  with proper sizing, not raw

  UIImage

  decoding in body

Reference Files

.id()

_printChanges()

@Observable

ObservableObject

List

ForEach

AnyView