ios-performance

iOS Performance Optimization Skill

Core Rules

1. Use [weak self] in escaping closures by default — use [unowned self] only when the closure's lifetime is strictly shorter than the captured object (e.g., parent owns child, child's closure references parent).
2. Non-escaping closures do NOT need [weak self] — map, filter, forEach, reduce, compactMap, sorted(by:) are non-escaping. The closure executes synchronously and releases captures immediately.
3. Delegates must be weak var — the delegate protocol must conform to AnyObject (or be marked @objc). Strong delegates create retain cycles between owner and delegate.
4. Use @Observable over ObservableObject (iOS 17+) — @Observable tracks property access per-view, so only views reading a changed property re-evaluate. ObservableObject with @Published invalidates ALL observing views on ANY published property change.
5. Break SwiftUI views into small subviews — each subview localizes its state dependency, so changes only re-evaluate the subview, not the entire parent body.
6. Use LazyVStack/LazyHStack inside ScrollView for large collections — VStack evaluates ALL children upfront. Use List for very large datasets (10,000+) since it reuses cells.
7. Use .task modifier instead of .onAppear + Task {} — .task automatically cancels when the view disappears. Use .task(id:) to restart when a dependency changes.
8. Profile with Instruments BEFORE optimizing — measure first, optimize second. Never guess where the bottleneck is.
9. Never block the main thread — all file I/O, network calls, JSON decoding of large payloads, image processing, and database queries must run off the main thread.
10. Reuse expensive objects — URLSession, DateFormatter, JSONDecoder, NumberFormatter, NSRegularExpression are expensive to create. Use shared instances or caches.

Performance Targets