Agent skill

swift-expert

Builds iOS/macOS/watchOS/tvOS applications, implements SwiftUI views and state management, designs protocol-oriented architectures, handles async/await concurrency, implements actors for thread safety, and debugs Swift-specific issues. Use when building iOS/macOS applications with Swift 5.9+, SwiftUI, or async/await concurrency. Invoke for protocol-oriented programming, SwiftUI state management, actors, server-side Swift, UIKit integration, Combine, or Vapor.

View SKILL.md on GitHub Repository
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Install this agent skill to your Project

npx add-skill https://github.com/Jeffallan/claude-skills/tree/main/skills/swift-expert

Metadata

Additional technical details for this skill

role
specialist
scope
implementation
author
https://github.com/Jeffallan
domain
language
version
1.1.0
triggers
Swift, SwiftUI, iOS development, macOS development, async/await Swift, Combine, UIKit, Vapor
output format
code
related skills

SKILL.md

Swift Expert

Core Workflow

  1. Architecture Analysis - Identify platform targets, dependencies, design patterns
  2. Design Protocols - Create protocol-first APIs with associated types
  3. Implement - Write type-safe code with async/await and value semantics
  4. Optimize - Profile with Instruments, ensure thread safety
  5. 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

swift
// ✅ 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

swift
// ✅ 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

swift
// ✅ 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

swift
// ✅ 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:

  1. Protocol definitions and type aliases
  2. Model types (structs/classes with value semantics)
  3. View implementations (SwiftUI) or view controllers
  4. Tests demonstrating usage
  5. Brief explanation of architectural decisions

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