Swift & SwiftUI Expert

Swift & SwiftUI Expert

Expert-level guidance for Swift and SwiftUI development with modern patterns, iOS 17+/macOS 14+ best practices, and production-ready code generation.

Core Principles

When writing Swift/SwiftUI code, ALWAYS follow these principles:

1. Modern State Management (iOS 17+)

Use @Observable macro instead of ObservableObject:

// MODERN (iOS 17+)
@Observable
class UserViewModel {
    var name: String = ""
    var isLoggedIn: Bool = false
}

// View owns the model
struct ContentView: View {
    @State private var viewModel = UserViewModel()
    var body: some View {
        Text(viewModel.name)
    }
}

// Child view (read-only) - NO wrapper needed
struct ProfileView: View {
    var viewModel: UserViewModel  // Just pass it
    var body: some View {
        Text(viewModel.name)
    }
}

// Child view (needs binding)
struct EditView: View {
    @Bindable var viewModel: UserViewModel
    var body: some View {
        TextField("Name", text: $viewModel.name)
    }
}

2. Property Wrapper Decision Tree

Use Case	Property Wrapper
Local view state (value types)	@State
Two-way binding to parent state	@Binding
Observable model (view owns)	@State + @Observable class
Observable model (read-only child)	No wrapper (just pass object)
Observable model (needs binding)	@Bindable
Shared app-wide state	@Environment
User defaults	@AppStorage
Scene storage	@SceneStorage
Focus state	@FocusState

3. Architecture Patterns

For most apps: Modern MVVM with @Observable For large apps with teams: TCA (Composable Architecture) For simple views: No ViewModel needed

See references/architecture.md for detailed patterns.

Anti-Patterns to AVOID

State Management

// WRONG: @ObservedObject with initialization
@ObservedObject var viewModel = ViewModel()  // Will crash!

// RIGHT: Use @StateObject or @State
@StateObject private var viewModel = ViewModel()  // Legacy
@State private var viewModel = ViewModel()  // Modern (iOS 17+)

// WRONG: @State with reference types
@State var user: User  // Where User is a class

// RIGHT: @State only for value types
@State var userName: String
@State private var viewModel = ViewModel()  // Only with @Observable

// WRONG: Clearing data on error
func refresh() async {
    self.items = []  // Data disappears if error occurs!
    do {
        self.items = try await fetchItems()
    } catch {
        // Items now empty forever
    }
}

// RIGHT: Preserve existing data
func refresh() async {
    do {
        self.items = try await fetchItems()
    } catch {
        // Keep existing items, show error toast
    }
}

Performance

// WRONG: Heavy computation in body
var body: some View {
    let sorted = items.sorted { ... }.filter { ... }  // Runs every render!
    List(sorted) { ... }
}

// RIGHT: Memoize or compute elsewhere
var sortedItems: [Item] {
    items.sorted { ... }.filter { ... }
}  // Or use @State for expensive computations

// WRONG: Using id modifier on lazy content
LazyVStack {
    ForEach(items) { item in
        ItemView(item: item)
            .id(item.id)  // Breaks lazy loading!
    }
}

// RIGHT: Let ForEach handle identity
LazyVStack {
    ForEach(items) { item in
        ItemView(item: item)  // ForEach uses Identifiable
    }
}

Navigation

// WRONG: Using deprecated NavigationView
NavigationView {
    List { ... }
}

// RIGHT: Use NavigationStack (iOS 16+)
NavigationStack {
    List { ... }
}

Code Generation Guidelines

When generating Swift/SwiftUI code:

1. Target iOS 17+/macOS 14+ unless user specifies otherwise
2. Use @Observable instead of ObservableObject/@Published
3. Use NavigationStack instead of NavigationView
4. Use .task instead of .onAppear for async work
5. Use Swift concurrency (async/await) instead of Combine for new code
6. Make @State private always
7. Prefer value types (structs, enums) over classes
8. Use optionals correctly - never force unwrap without safety
9. Handle errors explicitly - use do-catch, not try?
10. Add accessibility - labels, hints, traits

Reference Documents

For detailed guidance, consult:

• references/state-management.md - Property wrappers, @Observable, data flow
• references/architecture.md - MVVM, TCA, Clean Architecture patterns
• references/navigation.md - NavigationStack, programmatic navigation, deep links
• references/performance.md - Lazy views, List optimization, render cycles
• references/concurrency.md - async/await, actors, MainActor, Task
• references/testing.md - Unit tests, ViewInspector, UI tests
• references/accessibility.md - VoiceOver, Dynamic Type, accessibility modifiers
• references/swiftdata.md - @Model, @Query, persistence patterns
• references/anti-patterns.md - Common mistakes and how to fix them

Examples

Basic View with State

struct CounterView: View {
    @State private var count = 0

    var body: some View {
        VStack(spacing: 20) {
            Text("Count: \(count)")
                .font(.largeTitle)

            Button("Increment") {
                count += 1
            }
            .buttonStyle(.borderedProminent)
        }
        .accessibilityElement(children: .combine)
        .accessibilityLabel("Counter at \(count)")
        .accessibilityHint("Double tap the button to increment")
    }
}

ViewModel with @Observable

@Observable
class TaskListViewModel {
    var tasks: [Task] = []
    var isLoading = false
    var errorMessage: String?

    @MainActor
    func loadTasks() async {
        isLoading = true
        defer { isLoading = false }

        do {
            tasks = try await TaskService.shared.fetchTasks()
        } catch {
            errorMessage = error.localizedDescription
        }
    }
}

struct TaskListView: View {
    @State private var viewModel = TaskListViewModel()

    var body: some View {
        NavigationStack {
            Group {
                if viewModel.isLoading {
                    ProgressView()
                } else {
                    List(viewModel.tasks) { task in
                        TaskRow(task: task)
                    }
                }
            }
            .navigationTitle("Tasks")
        }
        .task {
            await viewModel.loadTasks()
        }
    }
}

Navigation with Type-Safe Routes

enum Route: Hashable {
    case detail(Item)
    case settings
    case profile(userId: String)
}

struct ContentView: View {
    @State private var path = NavigationPath()

    var body: some View {
        NavigationStack(path: $path) {
            List(items) { item in
                Button(item.name) {
                    path.append(Route.detail(item))
                }
            }
            .navigationDestination(for: Route.self) { route in
                switch route {
                case .detail(let item):
                    DetailView(item: item)
                case .settings:
                    SettingsView()
                case .profile(let userId):
                    ProfileView(userId: userId)
                }
            }
        }
    }

    // Programmatic navigation
    func navigateToSettings() {
        path.append(Route.settings)
    }

    func popToRoot() {
        path.removeLast(path.count)
    }
}

Usage

This skill activates automatically when working with Swift/SwiftUI files or when the user mentions iOS/macOS/SwiftUI development.

Explicit invocation:

/swift-swiftui help me refactor this view to use @Observable
/swift-swiftui create a settings screen with navigation
/swift-swiftui fix state management in this component