swift-development

Swift Best Practices Skill

Overview

Apply modern Swift development best practices focusing on Swift 6+ features, concurrency safety, API design principles, and code quality guidelines for iOS and macOS projects targeting macOS 15.7+.

When to Use This Skill

Use this skill when:

• Writing new Swift code for iOS or macOS applications
• Reviewing Swift code for correctness, safety, and style
• Implementing Swift concurrency features (async/await, actors, MainActor)
• Designing Swift APIs and public interfaces
• Migrating code from Swift 5 to Swift 6
• Addressing concurrency warnings, data race issues, or compiler errors related to Sendable/isolation
• Working with modern Swift language features introduced in Swift 6 and 6.2

Core Guidelines

Fundamental Principles

1. Clarity at point of use is paramount - evaluate designs by examining use cases, not just declarations
2. Clarity over brevity - compact code comes from the type system, not minimal characters
3. Write documentation for every public declaration - if you can't describe functionality simply, the API may be poorly designed
4. Name by role, not type - var greeting = "Hello" not var string = "Hello"
5. Favour elegance through simplicity - avoid over-engineering unless complexity genuinely warrants it

Swift 6 Concurrency Model

Swift 6 enables complete concurrency checking by default with region-based isolation (SE-0414). The compiler now proves code safety, eliminating many false positives whilst catching real concurrency issues at compile time.

Critical understanding:

• Async ≠ background - async functions can suspend but don't automatically run on background threads
• Actors protect mutable shared state through automatic synchronisation
• @MainActor ensures UI-related code executes on the main thread
• Global actor-isolated types are automatically Sendable

Essential Patterns

Async/Await

// Parallel execution with async let
func fetchData() async -> (String, Int) {
    async let stringData = fetchString()
    async let intData = fetchInt()
    return await (stringData, intData)
}

// Always check cancellation in long-running operations
func process(_ items: [Item]) async throws -> [Result] {
    var results: [Result] = []
    for item in items {
        try Task.checkCancellation()
        results.append(await process(item))
    }
    return results
}