swift-expert

SKILL.md

Swift Expert

Core Workflow

Architecture Analysis - Identify platform targets, dependencies, design patterns

Design Protocols - Create protocol-first APIs with associated types

Implement - Write type-safe code with async/await and value semantics

Optimize - Profile with Instruments, ensure thread safety

Test - Write comprehensive tests with XCTest and async patterns

Validation checkpoints: After step 3, run swift build to verify compilation. After step 4, run swift build -warnings-as-errors to surface actor isolation and Sendable warnings. After step 5, run swift test and confirm all async tests pass.

Reference Guide

Load detailed guidance based on context:

Topic
Reference
Load When

SwiftUI
references/swiftui-patterns.md
Building views, state management, modifiers

Concurrency
references/async-concurrency.md
async/await, actors, structured concurrency

Protocols
references/protocol-oriented.md
Protocol design, generics, type erasure

Memory
references/memory-performance.md
ARC, weak/unowned, performance optimization

Testing
references/testing-patterns.md
XCTest, async tests, mocking strategies

Code Patterns

async/await — Correct vs. Incorrect

// ✅ 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)
        }
    }
}

SwiftUI State Management

// ✅ 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+
}

Protocol-Oriented Architecture

// ✅ 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") }
}

Actor for Thread Safety

// ✅ 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]
    }
}

Constraints

MUST DO

Use type hints and inference appropriately

Follow Swift API Design Guidelines

Use async/await for asynchronous operations (see pattern above)

Ensure Sendable compliance for concurrency

Use value types (struct/enum) by default

Document APIs with markup comments (/// …)

Use property wrappers for cross-cutting concerns

Profile with Instruments before optimizing

MUST NOT DO

Use force unwrapping (!) without justification

Create retain cycles in closures

Mix synchronous and asynchronous code improperly

Ignore actor isolation warnings

Use implicitly unwrapped optionals unnecessarily

Skip error handling

Use Objective-C patterns when Swift alternatives exist

Hardcode platform-specific values

Output Templates

When implementing Swift features, provide:

Protocol definitions and type aliases

Model types (structs/classes with value semantics)

View implementations (SwiftUI) or view controllers

Tests demonstrating usage

Brief explanation of architectural decisions

Documentation