swiftui-debugging

SwiftUI Performance Debugging

Systematic guide for diagnosing and fixing SwiftUI performance problems: unnecessary view re-evaluations, identity issues, expensive body computations, and lazy loading mistakes.

When This Skill Activates

Use this skill when the user:

• 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

The Performance Loop

Every investigation follows the same loop: symptom -> measure -> identify -> optimize -> RE-MEASURE. The last step is the one people skip -- an "optimization" that was never re-measured is a guess, and SwiftUI guesses are wrong often enough (a fix can shift cost elsewhere) that the loop is not optional. Never close a performance issue on the strength of the diff alone.

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)