ios-swiftuilisted

# iOS SwiftUI Expert Skill ## Core Rules 1. **Use @Observable (iOS 17+) over ObservableObject** — fine-grained property tracking, better performance, simpler syntax. 2. **Use @State to own @Observable instances**, NOT @StateObject. `@StateObject` is the pre-iOS 17 pattern. 3. **Use NavigationStack** (not NavigationView) with value-based `NavigationLink` + `.navigationDestination`. 4. **Never nest NavigationStack inside NavigationStack** — causes double navigation bars and broken behavior. 5. **Use LazyVStack/LazyHStack inside ScrollView** for large collections. Use `List` for very large datasets (cell reuse + prefetching). 6. **Keep `body` pure** — no data processing, network calls, or side effects. Use `.task` modifier for async work. 7. **Avoid `AnyView`** — it destroys view identity and kills diffing performance. Use `@ViewBuilder` or `Group` instead. 8. **Preserve view identity** — use ternary operators (`condition ? viewA : viewB`), not `if/else` that changes the view type in ways that break animations. 9. **Break large views into small subviews** — SwiftUI re-evaluates `body` often; smaller views = smaller re-evaluation scope. 10. **Use `.equatable()`** on expensive views to skip unnecessary re-renders. --- ## Decision Tables ### State Management — What to Use When | Scenario | iOS 17+ | Pre-iOS 17 | |---|---|---| | Simple value owned by view | `@State` | `@State` | | Pass value down for read/write | `@Binding` | `@Binding` | | Reference-type model owned by view |