Swift 6 Development Skill

Instructions

Follow this guide when working on non-UI Swift code, business logic, algorithms, or the data layer. Your goal is to write high-performance, thread-safe, and expressive Swift code.

What This Skill Does

• Concurrent Programming: Write safe concurrent code using Actors, async/await, Task, and TaskGroup.
• Type Safety: Define Sendable types and use Typed Throws for precise error handling.
• Testing: Write modern unit tests using the Swift Testing framework.
• Data Processing: Write Codable models and encapsulate JSON parsing and network requests.
• Macros: Use or create macros to reduce boilerplate.

When to Use This Skill

• When writing ViewModel, Service, or Repository layer code.
• When defining data models (structs, enums).
• When implementing algorithms or utility functions.
• When writing unit tests.
• When fixing concurrency warnings or data races.

Examples

Example 1: Defining a Safe Actor Service

/// A thread-safe user data service
actor UserService {
    private var cache: [String: User] = [:]
    
    // Use typed throws to specify the error type explicitly
    func fetchUser(id: String) async throws(NetworkError) -> User {
        if let cached = cache[id] {
            return cached
        }
        
        // Simulate network request
        let user = try await NetworkClient.shared.get("/users/\(id)", as: User.self)
        cache[id] = user
        return user
    }
}

Example 2: Testing with Swift Testing

import Testing
@testable import MyApp

@Test("User parsing should succeed")
func userParsing() async throws {
    let json = """
    { "id": "1", "name": "Alice" }
    """.data(using: .utf8)!
    
    let user = try JSONDecoder().decode(User.self, from: json)
    
    #expect(user.name == "Alice")
    #expect(user.id == "1")
}

Best Practices

• Conform to Sendable: Any type passed between concurrency domains must conform to the

  Sendable

  protocol.
• Avoid Global Mutable State: Avoid global variables unless protected by an Actor.
• Implicitly Unwrapped Optionals: Strictly forbidden (the force unwrap operator), except where unavoidable in initializers (very rare).
• Use

  let

  over

  var

  : Default to constants; use variables only when necessary.
• Error Handling: Prefer

  throws

  and

  do-catch

  over returning

  Optional

  or

  Result

  types (throws is more natural in async contexts).

Notes

• Always pay attention to "Strict Concurrency Checking" compiler warnings.
• When handling closures across Actor boundaries, be mindful of

  [weak self]

  or

  [unowned self]

  (though usually not needed in

  Task

  unless breaking a reference cycle).