feat: add swiftui-patterns skill

Add comprehensive SwiftUI skill covering @Observable state management,
view composition, type-safe NavigationStack routing, performance
optimization with lazy containers, and modern preview patterns.

skills/swiftui-patterns/SKILL.md

@@ -0,0 +1,257 @@
+---
+name: swiftui-patterns
+description: SwiftUI architecture patterns, state management with @Observable, view composition, navigation, performance optimization, and modern iOS/macOS UI best practices.
+---
+
+# SwiftUI Patterns
+
+Modern SwiftUI patterns for building declarative, performant user interfaces on Apple platforms. Covers the Observation framework, view composition, type-safe navigation, and performance optimization.
+
+## When to Activate
+
+- Building SwiftUI views and managing state (`@State`, `@Observable`, `@Binding`)
+- Designing navigation flows with `NavigationStack`
+- Structuring view models and data flow
+- Optimizing rendering performance for lists and complex layouts
+- Working with environment values and dependency injection in SwiftUI
+
+## State Management
+
+### Property Wrapper Selection
+
+Choose the simplest wrapper that fits:
+
+| Wrapper | Use Case |
+|---------|----------|
+| `@State` | View-local value types (toggles, form fields, sheet presentation) |
+| `@Binding` | Two-way reference to parent's `@State` |
+| `@Observable` class + `@State` | Owned model with multiple properties |
+| `@Observable` class (no wrapper) | Read-only reference passed from parent |
+| `@Bindable` | Two-way binding to an `@Observable` property |
+| `@Environment` | Shared dependencies injected via `.environment()` |
+
+### @Observable ViewModel
+
+Use `@Observable` (not `ObservableObject`) — it tracks property-level changes so SwiftUI only re-renders views that read the changed property:
+
+```swift
+@Observable
+final class ItemListViewModel {
+    private(set) var items: [Item] = []
+    private(set) var isLoading = false
+    var searchText = ""
+
+    private let repository: any ItemRepository
+
+    init(repository: any ItemRepository = DefaultItemRepository()) {
+        self.repository = repository
+    }
+
+    func load() async {
+        isLoading = true
+        defer { isLoading = false }
+        items = (try? await repository.fetchAll()) ?? []
+    }
+}
+```
+
+### View Consuming the ViewModel
+
+```swift
+struct ItemListView: View {
+    @State private var viewModel = ItemListViewModel()
+
+    var body: some View {
+        List(viewModel.items) { item in
+            ItemRow(item: item)
+        }
+        .searchable(text: $viewModel.searchText)
+        .overlay { if viewModel.isLoading { ProgressView() } }
+        .task { await viewModel.load() }
+    }
+}
+```
+
+### Environment Injection
+
+Replace `@EnvironmentObject` with `@Environment`:
+
+```swift
+// Inject
+ContentView()
+    .environment(authManager)
+
+// Consume
+struct ProfileView: View {
+    @Environment(AuthManager.self) private var auth
+
+    var body: some View {
+        Text(auth.currentUser?.name ?? "Guest")
+    }
+}
+```
+
+## View Composition
+
+### Extract Subviews to Limit Invalidation
+
+Break views into small, focused structs. When state changes, only the subview reading that state re-renders:
+
+```swift
+struct OrderView: View {
+    @State private var viewModel = OrderViewModel()
+
+    var body: some View {
+        VStack {
+            OrderHeader(title: viewModel.title)
+            OrderItemList(items: viewModel.items)
+            OrderTotal(total: viewModel.total)
+        }
+    }
+}
+```
+
+### ViewModifier for Reusable Styling
+
+```swift
+struct CardModifier: ViewModifier {
+    func body(content: Content) -> some View {
+        content
+            .padding()
+            .background(.regularMaterial)
+            .clipShape(RoundedRectangle(cornerRadius: 12))
+    }
+}
+
+extension View {
+    func cardStyle() -> some View {
+        modifier(CardModifier())
+    }
+}
+```
+
+## Navigation
+
+### Type-Safe NavigationStack
+
+Use `NavigationStack` with `NavigationPath` for programmatic, type-safe routing:
+
+```swift
+@Observable
+final class Router {
+    var path = NavigationPath()
+
+    func navigate(to destination: Destination) {
+        path.append(destination)
+    }
+
+    func popToRoot() {
+        path = NavigationPath()
+    }
+}
+
+enum Destination: Hashable {
+    case detail(Item.ID)
+    case settings
+    case profile(User.ID)
+}
+
+struct RootView: View {
+    @State private var router = Router()
+
+    var body: some View {
+        NavigationStack(path: $router.path) {
+            HomeView()
+                .navigationDestination(for: Destination.self) { dest in
+                    switch dest {
+                    case .detail(let id): ItemDetailView(itemID: id)
+                    case .settings: SettingsView()
+                    case .profile(let id): ProfileView(userID: id)
+                    }
+                }
+        }
+        .environment(router)
+    }
+}
+```
+
+## Performance
+
+### Use Lazy Containers for Large Collections
+
+`LazyVStack` and `LazyHStack` create views only when visible:
+
+```swift
+ScrollView {
+    LazyVStack(spacing: 8) {
+        ForEach(items) { item in
+            ItemRow(item: item)
+        }
+    }
+}
+```
+
+### Stable Identifiers
+
+Always use stable, unique IDs in `ForEach` — avoid using array indices:
+
+```swift
+// Use Identifiable conformance or explicit id
+ForEach(items, id: \.stableID) { item in
+    ItemRow(item: item)
+}
+```
+
+### Avoid Expensive Work in body
+
+- Never perform I/O, network calls, or heavy computation inside `body`
+- Use `.task {}` for async work — it cancels automatically when the view disappears
+- Use `.sensoryFeedback()` and `.geometryGroup()` sparingly in scroll views
+- Minimize `.shadow()`, `.blur()`, and `.mask()` in lists — they trigger offscreen rendering
+
+### Equatable Conformance
+
+For views with expensive bodies, conform to `Equatable` to skip unnecessary re-renders:
+
+```swift
+struct ExpensiveChartView: View, Equatable {
+    let dataPoints: [DataPoint]
+
+    static func == (lhs: Self, rhs: Self) -> Bool {
+        lhs.dataPoints.count == rhs.dataPoints.count
+    }
+
+    var body: some View {
+        // Complex chart rendering
+    }
+}
+```
+
+## Previews
+
+Use `#Preview` macro with inline mock data for fast iteration:
+
+```swift
+#Preview("Empty state") {
+    ItemListView()
+        .environment(ItemListViewModel(repository: EmptyMockRepository()))
+}
+
+#Preview("Loaded") {
+    ItemListView()
+        .environment(ItemListViewModel(repository: PopulatedMockRepository()))
+}
+```
+
+## Anti-Patterns to Avoid
+
+- Using `ObservableObject` / `@Published` / `@StateObject` / `@EnvironmentObject` in new code — migrate to `@Observable`
+- Putting async work directly in `body` or `init` — use `.task {}` or explicit load methods
+- Creating view models as `@State` inside child views that don't own the data — pass from parent instead
+- Using `AnyView` type erasure — prefer `@ViewBuilder` or `Group` for conditional views
+- Ignoring `Sendable` requirements when passing data to/from actors
+
+## References
+
+See skill: `swift-actor-persistence` for actor-based persistence patterns.
+See skill: `swift-protocol-di-testing` for protocol-based DI and testing with Swift Testing.