ECC/pi/core/skills/swiftui-patterns/SKILL.md

name, description

name	description
swiftui-patterns	SwiftUI architecture patterns, state management with @Observable, view composition, navigation, performance optimization, and modern iOS/macOS UI best practices. Use when building or reviewing SwiftUI views, @Observable state, navigation, or render performance.

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:

@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

struct ItemListView: View {
    @State private var viewModel: ItemListViewModel

    init(viewModel: ItemListViewModel = ItemListViewModel()) {
        _viewModel = State(initialValue: viewModel)
    }

    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:

// 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:

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

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:

@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:

ScrollView {
    LazyVStack(spacing: 8) {
        ForEach(items) { item in
            ItemRow(item: item)
        }
    }
}

Stable Identifiers

Always use stable, unique IDs in ForEach — avoid using array indices:

// 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:

struct ExpensiveChartView: View, Equatable {
    let dataPoints: [DataPoint] // DataPoint must conform to Equatable

    static func == (lhs: Self, rhs: Self) -> Bool {
        lhs.dataPoints == rhs.dataPoints
    }

    var body: some View {
        // Complex chart rendering
    }
}

Previews

Use #Preview macro with inline mock data for fast iteration:

#Preview("Empty state") {
    ItemListView(viewModel: ItemListViewModel(repository: EmptyMockRepository()))
}

#Preview("Loaded") {
    ItemListView(viewModel: 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.