Demystify SwiftUI Performance (WWDC23) (Summary)

Context: WWDC23 session on building a mental model for SwiftUI performance and triaging hangs/hitches.

Performance loop

• Measure -> Identify -> Optimize -> Re-measure.
• Focus on concrete symptoms (slow navigation, broken animations, spinning cursor).

Dependencies and updates

• Views form a dependency graph; dynamic properties are a frequent source of updates.
• Use Self._printChanges() in debug only to inspect extra dependencies.
• Eliminate unnecessary dependencies by extracting views or narrowing state.
• Consider @Observable for more granular property tracking.

Common causes of slow updates

• Expensive view bodies (string interpolation, filtering, formatting).
• Dynamic property instantiation and state initialization in body.
• Slow identity resolution in lists/tables.
• Hidden work: bundle lookups, heap allocations, repeated string construction.

Avoid slow initialization in view bodies

• Don’t create heavy models synchronously in view bodies.
• Use .task to fetch async data and keep init lightweight.

Lists and tables identity rules

• Stable identity is critical for performance and animation.
• Ensure a constant number of views per element in ForEach.
• Avoid inline filtering in ForEach; pre-filter and cache collections.
• Avoid AnyView in list rows; it hides identity and increases cost.
• Flatten nested ForEach when possible to reduce overhead.

Table specifics

• TableRow resolves to a single row; row count must be constant.
• Prefer the streamlined Table initializer to enforce constant rows.
• Use explicit IDs for back deployment when needed.

Debugging aids

• Use Instruments for hangs and hitches.
• Use _printChanges to validate dependency assumptions during debug.