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gh-Dimillian-Skills/swiftui-performance-audit/references/understanding-improving-swiftui-performance.md
Thomas Ricouard 91eb0d324d Add SwiftUI performance audit guide and references 
Introduced SKILL.md with a comprehensive workflow for auditing and improving SwiftUI runtime performance, including code review, profiling, diagnosis, and remediation steps. Added summarized Apple and WWDC resources under the references directory to support performance analysis and best practices.
2025-12-30 17:21:36 +01:00

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Understanding and Improving SwiftUI Performance (Summary)

Context: Apple guidance on diagnosing SwiftUI performance with Instruments and applying design patterns to reduce long or frequent updates.

Core concepts

  • SwiftUI is declarative; view updates are driven by state, environment, and observable data dependencies.
  • View bodies must compute quickly to meet frame deadlines; slow or frequent updates lead to hitches.
  • Instruments is the primary tool to find long-running updates and excessive update frequency.

Instruments workflow

  1. Profile via Product > Profile.
  2. Choose the SwiftUI template and record.
  3. Exercise the target interaction.
  4. Stop recording and inspect the SwiftUI track + Time Profiler.

SwiftUI timeline lanes

  • Update Groups: overview of time SwiftUI spends calculating updates.
  • Long View Body Updates: orange >500us, red >1000us.
  • Long Platform View Updates: AppKit/UIKit hosting in SwiftUI.
  • Other Long Updates: geometry/text/layout and other SwiftUI work.
  • Hitches: frame misses where UI wasn’t ready in time.

Diagnose long view body updates

  • Expand the SwiftUI track; inspect module-specific subtracks.
  • Set Inspection Range and correlate with Time Profiler.
  • Use call tree or flame graph to identify expensive frames.
  • Repeat the update to gather enough samples for analysis.
  • Filter to a specific update (Show Calls Made by MySwiftUIView.body).

Diagnose frequent updates

  • Use Update Groups to find long active groups without long updates.
  • Set inspection range on the group and analyze update counts.
  • Use Cause graph ("Show Causes") to see what triggers updates.
  • Compare causes with expected data flow; prioritize the highest-frequency causes.

Remediation patterns

  • Move expensive work out of body and cache results.
  • Use Observable() macro to scope dependencies to properties actually read.
  • Avoid broad dependencies that fan out updates to many views.
  • Reduce layout churn; isolate state-dependent subtrees from layout readers.
  • Avoid storing closures that capture parent state; precompute child views.
  • Gate frequent updates (e.g., geometry changes) by thresholds.

Verification

  • Re-record after changes to confirm reduced update counts and fewer hitches.