Swift Best Practices

# 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

## SwiftLens MCP Integration (Claude Code)

This skill complements **SwiftLens MCP server** for semantic-level Swift code analysis.

**What SwiftLens Provides:**
- 15 tools for semantic Swift analysis using Apple's SourceKit-LSP
- Symbol lookup, cross-file references, type information
- Safe code modification and refactoring
- Compiler-grade understanding of Swift code structure

**What This Skill Provides:**
- Swift 6+ design patterns and best practices
- Concurrency strategies (async/await, actors, MainActor)
- API design guidelines and naming conventions
- Migration guidance (Swift 5 → Swift 6)

**Setup for Claude Code CLI:**

Create `.claude/mcps/swiftlens.json` in your Swift project:

```json
{
  "mcpServers": {
    "swiftlens": {
      "description": "SwiftLens MCP provides semantic Swift analysis via SourceKit-LSP",
      "command": "uvx",
      "args": ["swiftlens"]
    }
  }
}
```

**⚠️ Note**: This is **Claude Code** configuration (not Claude Desktop). See `references/swiftlens-mcp-claude-code.md` for complete setup guide, all 15 tools, index building, and usage examples.

**Workflow**: SwiftLens provides **runtime analysis** (what the code is doing), this skill provides **design expertise** (what the code should be doing).

## 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
```swift
// 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
}
```

#### MainActor for UI Code
```swift
// Apply at type level for consistent isolation
@MainActor
class ContentViewModel: ObservableObject {
    @Published var images: [UIImage] = []

    func fetchData() async throws {
        self.images = try await fetchImages()
    }
}

// Avoid MainActor.run when direct await works
await doMainActorStuff()  // Good
await MainActor.run { doMainActorStuff() }  // Unnecessary
```

#### Actor Isolation
```swift
actor DataCache {
    private var cache: [String: Data] = [:]

    func store(_ data: Data, forKey key: String) {
        cache[key] = data  // No await needed inside actor
    }

    nonisolated func cacheType() -> String {
        return "DataCache"  // No await needed - doesn't access isolated state
    }
}
```

### Common Pitfalls to Avoid

1. **Don't mark functions as `async` unnecessarily** - async calling convention has overhead
2. **Never use `DispatchSemaphore` with async/await** - risk of deadlock
3. **Don't create stateless actors** - use non-isolated async functions instead
4. **Avoid split isolation** - don't mix isolation domains within one type
5. **Check task cancellation** - long operations must check `Task.checkCancellation()`
6. **Don't assume async means background** - explicitly move work to background if needed
7. **Avoid excessive context switching** - group operations within same isolation domain

### API Design Quick Reference

#### Naming Conventions
- Types/protocols: `UpperCamelCase`
- Everything else: `lowerCamelCase`
- Protocols describing capabilities: `-able`, `-ible`, `-ing` suffixes (`Equatable`, `ProgressReporting`)
- Factory methods: Begin with `make` (`x.makeIterator()`)
- Mutating pairs: imperative vs past participle (`x.sort()` / `x.sorted()`)

#### Method Naming by Side Effects
- No side effects: Noun phrases (`x.distance(to: y)`)
- With side effects: Imperative verbs (`x.append(y)`, `x.sort()`)

#### Argument Labels
- Omit when arguments can't be distinguished: `min(number1, number2)`
- Value-preserving conversions omit first label: `Int64(someUInt32)`
- Prepositional phrases label at preposition: `x.removeBoxes(havingLength: 12)`
- Label all other arguments

### Swift 6 Breaking Changes

#### Must Explicitly Mark Types with @MainActor (SE-0401)
Property wrappers no longer infer actor isolation automatically.

```swift
@MainActor
struct LogInView: View {
    @StateObject private var model = ViewModel()
}
```

#### Global Variables Must Be Concurrency-Safe (SE-0412)
```swift
static let config = Config()  // Constant - OK
@MainActor static var state = State()  // Actor-isolated - OK
nonisolated(unsafe) var cache = [String: Data]()  // Unsafe - use with caution
```

#### Other Changes
- `@UIApplicationMain`/`@NSApplicationMain` deprecated (use `@main`)
- `any` required for existential types
- Import visibility requires explicit access control

### API Availability Patterns

```swift
// Basic availability
@available(macOS 15, iOS 18, *)
func modernAPI() { }

// Deprecation with message
@available(*, deprecated, message: "Use newMethod() instead")
func oldMethod() { }

// Renaming with auto-fix
@available(*, unavailable, renamed: "newMethod")
func oldMethod() { }

// Runtime checking
if #available(iOS 18, *) {
    // iOS 18+ code
}

// Inverted checking (Swift 5.6+)
if #unavailable(iOS 18, *) {
    // iOS 17 and lower
}
```

**Key differences:**
- `deprecated` - Warning, allows usage
- `obsoleted` - Error from specific version
- `unavailable` - Error, completely prevents usage

## How to Use This Skill

### When Writing Code

1. Apply naming conventions following role-based, clarity-first principles
2. Use appropriate isolation (`@MainActor` for UI, actors for mutable state)
3. Implement async/await patterns correctly with proper cancellation handling
4. Follow Swift 6 concurrency model - trust compiler's flow analysis
5. Document public APIs with clear, concise summaries

### When Reviewing Code

1. Check for concurrency safety violations
2. Verify proper actor isolation and Sendable conformance
3. Ensure async functions handle cancellation appropriately
4. Validate API naming follows Swift guidelines
5. Confirm availability annotations are correct for target platforms

### Code Quality Standards

- Minimise comments - code should be self-documenting where possible
- Avoid over-engineering and unnecessary abstractions
- Use meaningful variable names based on role, not type
- Follow established project architecture and patterns
- Prefer `count(where:)` over `filter().count`
- Use `InlineArray` for fixed-size, performance-critical data
- Trust compiler's concurrency flow analysis - avoid unnecessary `Sendable` conformances

## Resources

### references/

Detailed reference material to load when in-depth information is needed:

- **swiftlens-mcp-claude-code.md** - SwiftLens MCP server setup for Claude Code CLI, 15 semantic analysis tools, index building, usage examples, and integration workflows
- **api-design.md** - Complete API design conventions, documentation standards, parameter guidelines, and naming patterns
- **concurrency.md** - Detailed async/await patterns, actor best practices, common pitfalls, performance considerations, and thread safety patterns
- **swift6-features.md** - New language features in Swift 6/6.2, breaking changes, migration strategies, and modern patterns
- **availability-patterns.md** - Comprehensive `@available` attribute usage, deprecation strategies, and platform version management

Load these references when detailed information is needed beyond the core guidelines provided above.

## Platform Requirements

- Swift 6.0+ compiler for Swift 6 features
- Swift 6.2+ for InlineArray and enhanced concurrency features
- macOS 15.7+ with appropriate SDK
- iOS 18+ for latest platform features
- Use `#available` for runtime platform detection
- Use `@available` for API availability marking