Swift Modern Architecture Skill

Build iOS apps using Swift 6 and iOS 18+ best practices. This skill ensures code uses modern patterns: SwiftData (not Core Data), Observation framework (not Combine), Swift concurrency (not GCD), and current SwiftUI APIs.

Core Principles

1. Swift 6 Concurrency First

Always use Swift concurrency (async/await , actor , @MainActor ) instead of GCD or completion handlers. Use structured concurrency (TaskGroup , async let ) over unstructured tasks.

2. Observation Framework Over Combine

Use @Observable macro for state management instead of ObservableObject with @Published . The Observation framework is more efficient and has cleaner syntax.

3. SwiftData Over Core Data

For new projects, always use SwiftData with @Model and @Query . SwiftData provides simpler APIs while maintaining Core Data's power.

4. Modern SwiftUI APIs

Use NavigationStack (not NavigationView ), @Entry for environment values, .task modifier for async work, and built-in components like ContentUnavailableView .

5. Type Safety

Use enums instead of strings for identifiers, typed throws for specific errors, and proper Sendable conformance for thread safety.

6. Value Types When Possible

Prefer structs and enums over classes unless reference semantics are required. Use actor for thread-safe shared mutable state.

When to Use This Skill

Activate this skill when:

• Writing Swift or iOS application code

• Designing application architecture

• Reviewing or modernizing existing Swift code

• Setting up SwiftUI views, view models, or data models

• Implementing networking, persistence, or business logic

• Working with async operations or concurrency

Architecture Pattern: MVVM with Observation

View Model Structure

@Observable final class ViewModel { // Private dependencies private let service: ServiceProtocol

// Public readable state
private(set) var data: [Item] = []
private(set) var isLoading = false
private(set) var error: Error?

// User input state (use @Bindable in view)
var searchText = ""
var selectedFilter: Filter = .all

init(service: ServiceProtocol) {
    self.service = service
}

// Public actions
func loadData() async {
    isLoading = true
    defer { isLoading = false }
    
    do {
        data = try await service.fetchData()
    } catch {
        self.error = error
    }
}

}

View Structure

struct ContentView: View { @Bindable var viewModel: ViewModel

var body: some View {
    content
        .task { await viewModel.loadData() }
}

@ViewBuilder
private var content: some View {
    if viewModel.isLoading {
        ProgressView()
    } else {
        List(viewModel.data) { item in
            ItemRow(item: item)
        }
    }
}

}

SwiftData Quick Reference

Model Definition

import SwiftData

@Model final class Item { var name: String var createdAt: Date @Relationship(deleteRule: .cascade) var children: [ChildItem]

init(name: String) {
    self.name = name
    self.createdAt = Date()
    self.children = []
}

}

Querying Data

// In SwiftUI view @Query(sort: \Item.createdAt, order: .reverse) private var items: [Item]

// With filter @Query(filter: #Predicate<Item> { $0.isComplete }) private var completedItems: [Item]

// With dynamic predicate @Query private var items: [Item]

init(searchText: String) { let predicate = #Predicate<Item> { item in searchText.isEmpty || item.name.contains(searchText) } _items = Query(filter: predicate) }

Model Context Operations

@Environment(.modelContext) private var modelContext

func addItem() { let item = Item(name: "New") modelContext.insert(item) try? modelContext.save() }

func deleteItem(_ item: Item) { modelContext.delete(item) try? modelContext.save() }

API Client Pattern

Create an actor for thread-safe API operations:

actor APIClient { private let session: URLSession private let decoder: JSONDecoder

init(session: URLSession = .shared) {
    self.session = session
    self.decoder = JSONDecoder()
}

func fetch<T: Decodable>(_ endpoint: Endpoint) async throws -> T {
    let (data, response) = try await session.data(for: endpoint.urlRequest)
    
    guard let httpResponse = response as? HTTPURLResponse,
          (200...299).contains(httpResponse.statusCode) else {
        throw APIError.invalidResponse
    }
    
    return try decoder.decode(T.self, from: data)
}

}

Navigation Pattern

Use type-safe navigation with NavigationStack :

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

var body: some View {
    NavigationStack(path: $path) {
        RootView()
            .navigationDestination(for: Item.self) { item in
                ItemDetailView(item: item)
            }
            .navigationDestination(for: User.self) { user in
                UserProfileView(user: user)
            }
    }
}

}

Testing with Swift Testing

Use the modern Swift Testing framework instead of XCTest:

import Testing

@Test("View model loads data successfully") func dataLoading() async throws { let viewModel = ViewModel(service: MockService()) await viewModel.loadData() #expect(viewModel.data.isEmpty == false) }

@Test("Validation fails with invalid input", arguments: [ "invalid-email", "missing@", "@domain.com" ]) func emailValidation(invalidEmail: String) throws { #expect(throws: ValidationError.self) { try validateEmail(invalidEmail) } }

Common Modernization Checks

Before writing code, verify you're using:

• ✅ @Observable NOT ObservableObject

• ✅ @Query NOT @FetchRequest

• ✅ NavigationStack NOT NavigationView

• ✅ async/await NOT completion handlers

• ✅ @MainActor NOT DispatchQueue.main.async

• ✅ actor NOT serial DispatchQueue

• ✅ SwiftData.ModelContext NOT NSManagedObjectContext

• ✅ Swift Testing @Test NOT XCTest

• ✅ Typed throws(ErrorType) when appropriate

Bundled Resources

References

Load when you need detailed guidance:

modern-patterns.md - Comprehensive patterns for Swift 6/iOS 18+

• Load when: Implementing any feature, especially concurrency, data persistence, or API calls

anti-patterns.md - What NOT to do and why

• Load when: Reviewing code, modernizing legacy patterns, or unsure about approach

examples.md - Complete working implementations

• Load when: Starting new features (Todo app, Weather app, Auth flow examples)

Usage Pattern

• Read the relevant reference file before implementing complex features

• Check anti-patterns when reviewing existing code

• Reference complete examples when starting new app components

Quick Decision Tree

Need state management? → Use @Observable for view models → Use @State for simple view-local state → Use @Environment for dependency injection

Need data persistence? → Use SwiftData with @Model and @Query

→ Never use Core Data for new code

Need async operations? → Use async/await and structured concurrency → Mark UI-bound code with @MainActor

→ Use actor for thread-safe shared state

Need navigation? → Use NavigationStack with NavigationPath

→ Type-safe destinations with .navigationDestination(for:)

Need API calls? → Create an actor with async throws methods → Use URLSession.data(from:) with async/await

Error Prevention

This skill actively prevents these outdated patterns:

• Core Data (NSManagedObject , @FetchRequest )

• Combine (ObservableObject , @Published , .sink )

• GCD (DispatchQueue , DispatchGroup )

• Old SwiftUI (NavigationView , NavigationLink(destination:) )

• Manual threading (Thread , NSOperationQueue )

• Completion handlers when async/await is available

• XCTest when Swift Testing is more appropriate

When encountering these patterns in existing code, suggest modern alternatives from the references.