53 lines
2.2 KiB
Markdown
53 lines
2.2 KiB
Markdown
# Understanding and Improving SwiftUI Performance (Summary)
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Context: Apple guidance on diagnosing SwiftUI performance with Instruments and applying design patterns to reduce long or frequent updates.
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## Core concepts
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- SwiftUI is declarative; view updates are driven by state, environment, and observable data dependencies.
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- View bodies must compute quickly to meet frame deadlines; slow or frequent updates lead to hitches.
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- Instruments is the primary tool to find long-running updates and excessive update frequency.
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## Instruments workflow
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1. Profile via Product > Profile.
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2. Choose the SwiftUI template and record.
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3. Exercise the target interaction.
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4. Stop recording and inspect the SwiftUI track + Time Profiler.
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## SwiftUI timeline lanes
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- Update Groups: overview of time SwiftUI spends calculating updates.
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- Long View Body Updates: orange >500us, red >1000us.
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- Long Platform View Updates: AppKit/UIKit hosting in SwiftUI.
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- Other Long Updates: geometry/text/layout and other SwiftUI work.
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- Hitches: frame misses where UI wasn’t ready in time.
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## Diagnose long view body updates
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- Expand the SwiftUI track; inspect module-specific subtracks.
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- Set Inspection Range and correlate with Time Profiler.
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- Use call tree or flame graph to identify expensive frames.
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- Repeat the update to gather enough samples for analysis.
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- Filter to a specific update (Show Calls Made by `MySwiftUIView.body`).
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## Diagnose frequent updates
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- Use Update Groups to find long active groups without long updates.
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- Set inspection range on the group and analyze update counts.
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- Use Cause graph ("Show Causes") to see what triggers updates.
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- Compare causes with expected data flow; prioritize the highest-frequency causes.
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## Remediation patterns
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- Move expensive work out of `body` and cache results.
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- Use `Observable()` macro to scope dependencies to properties actually read.
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- Avoid broad dependencies that fan out updates to many views.
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- Reduce layout churn; isolate state-dependent subtrees from layout readers.
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- Avoid storing closures that capture parent state; precompute child views.
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- Gate frequent updates (e.g., geometry changes) by thresholds.
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## Verification
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- Re-record after changes to confirm reduced update counts and fewer hitches.
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