Swift Architecture

Swift App Architecture Patterns

MVVM with SwiftUI

// Model — pure data, no UI dependencies
struct Article: Identifiable, Codable {
    let id: UUID
    let title: String
    let body: String
    let publishedAt: Date
}

// ViewModel — transforms model for display, owns async operations
@MainActor
final class ArticleListViewModel: ObservableObject {
    @Published private(set) var articles: [Article] = []
    @Published private(set) var isLoading = false
    @Published var errorMessage: String?

    private let repository: any ArticleRepository

    init(repository: any ArticleRepository) {
        self.repository = repository
    }

    func load() async {
        isLoading = true
        defer { isLoading = false }
        do {
            articles = try await repository.fetchAll()
        } catch {
            errorMessage = error.localizedDescription
        }
    }
}

// View — purely declarative, no business logic
struct ArticleListView: View {
    @StateObject private var vm: ArticleListViewModel

    init(repository: any ArticleRepository) {
        _vm = StateObject(wrappedValue: ArticleListViewModel(repository: repository))
    }

    var body: some View {
        Group {
            if vm.isLoading { ProgressView() }
            else { List(vm.articles) { ArticleRow(article: $0) } }
        }
        .task { await vm.load() }
        .alert("Error", isPresented: .constant(vm.errorMessage != nil)) { }
              message: { Text(vm.errorMessage ?? "") }
    }
}

Repository Pattern

protocol ArticleRepository {
    func fetchAll() async throws -> [Article]
    func fetch(id: UUID) async throws -> Article?
    func save(_ article: Article) async throws
}

struct RemoteArticleRepository: ArticleRepository {
    let client: APIClient

    func fetchAll() async throws -> [Article] {
        try await client.get("/articles", as: [Article].self)
    }

    func fetch(id: UUID) async throws -> Article? {
        try await client.get("/articles/\(id)", as: Article.self)
    }

    func save(_ article: Article) async throws {
        try await client.post("/articles", body: article) as Article
    }
}

// In-memory implementation for tests/previews
final class InMemoryArticleRepository: ArticleRepository {
    var articles: [Article] = []
    func fetchAll() async throws -> [Article] { articles }
    func fetch(id: UUID) async throws -> Article? { articles.first { $0.id == id } }
    func save(_ article: Article) async throws { articles.append(article) }
}

Dependency Injection Container

// Using point-free/swift-dependencies style
import Dependencies

extension DependencyValues {
    var articleRepository: any ArticleRepository {
        get { self[ArticleRepositoryKey.self] }
        set { self[ArticleRepositoryKey.self] = newValue }
    }
}

private enum ArticleRepositoryKey: DependencyKey {
    static let liveValue: any ArticleRepository = RemoteArticleRepository(client: .live)
    static let testValue: any ArticleRepository = InMemoryArticleRepository()
    static let previewValue: any ArticleRepository = InMemoryArticleRepository(articles: .preview)
}

// Usage in ViewModel
@MainActor
final class ArticleListViewModel: ObservableObject {
    @Dependency(\.articleRepository) var repository
}

Clean Architecture Layers

Presentation (SwiftUI views + ViewModels)
      ↓
Domain (Use Cases / Interactors)
      ↓
Data (Repositories → Remote + Local)

// Domain layer — use case
struct FetchArticlesUseCase {
    let remote: any ArticleRepository
    let local: any ArticleRepository

    func execute() async throws -> [Article] {
        do {
            let articles = try await remote.fetchAll()
            for article in articles { try await local.save(article) }
            return articles
        } catch {
            // Fallback to local cache
            return try await local.fetchAll()
        }
    }
}

Modular Feature Design

// Each feature is a standalone SPM module
// Feature/ArticleFeature/Sources/ArticleListView.swift
public struct ArticleListView: View {
    public init(store: ArticleStore) { self.store = store }
    private let store: ArticleStore
    // ...
}

// App target wires features together
import ArticleFeature
import ProfileFeature

@main
struct MyApp: App {
    var body: some Scene {
        WindowGroup {
            TabView {
                ArticleListView(store: .live)
                ProfileView(store: .live)
            }
        }
    }
}

Common Anti-Patterns

• Fat ViewControllers / fat Views — move business logic to ViewModel or UseCase
• Views depending on networking — always go through ViewModel → Repository
• Singleton abuse — use DI instead; singletons make testing hard
• Circular module dependencies — extract shared types to a Core module
• Tight coupling between features — communicate via protocols or shared events, not direct imports