Swift Expert Skill for Claude

// ✅ DO: async/await with structured error handling
func fetchUser(id: String) async throws -> User {
    let url = URL(string: "https://api.example.com/users/\(id)")!
    let (data, _) = try await URLSession.shared.data(from: url)
    return try JSONDecoder().decode(User.self, from: data)
}

// ❌ DON'T: mixing completion handlers with async context
func fetchUser(id: String) async throws -> User {
    return try await withCheckedThrowingContinuation { continuation in
        // Avoid wrapping existing async APIs this way when a native async version exists
        legacyFetch(id: id) { result in
            continuation.resume(with: result)
        }
    }
}

// ✅ DO: use @Observable (Swift 5.9+) for view models
@Observable
final class CounterViewModel {
    var count = 0
    func increment() { count += 1 }
}

struct CounterView: View {
    @State private var vm = CounterViewModel()

    var body: some View {
        VStack {
            Text("\(vm.count)")
            Button("Increment", action: vm.increment)
        }
    }
}

// ❌ DON'T: reach for ObservableObject/Published when @Observable suffices
class LegacyViewModel: ObservableObject {
    @Published var count = 0  // Unnecessary boilerplate in Swift 5.9+
}

// ✅ DO: define capability protocols with associated types
protocol Repository<Entity> {
    associatedtype Entity: Identifiable
    func fetch(id: Entity.ID) async throws -> Entity
    func save(_ entity: Entity) async throws
}

struct UserRepository: Repository {
    typealias Entity = User
    func fetch(id: UUID) async throws -> User { /* … */ }
    func save(_ user: User) async throws { /* … */ }
}

// ❌ DON'T: use classes as base types when a protocol fits
class BaseRepository {  // Avoid class inheritance for shared behavior
    func fetch(id: UUID) async throws -> Any { fatalError("Override required") }
}

// ✅ DO: isolate mutable shared state in an actor
actor ImageCache {
    private var cache: [URL: UIImage] = [:]

    func image(for url: URL) -> UIImage? { cache[url] }
    func store(_ image: UIImage, for url: URL) { cache[url] = image }
}

// ❌ DON'T: use a class with manual locking
class UnsafeImageCache {
    private var cache: [URL: UIImage] = [:]
    private let lock = NSLock()  // Error-prone; prefer actor isolation
    func image(for url: URL) -> UIImage? {
        lock.lock(); defer { lock.unlock() }
        return cache[url]
    }
}