Swift Patterns

Files: skills/swift-actor-persistence/SKILL.md + skills/swift-protocol-di-testing/SKILL.md

-------|-----------| | Actor (not class + lock) | Compiler-enforced thread safety | | In-memory cache + file persistence | Fast reads, durable writes | | Dictionary keyed by ID | O(1) lookups | | Generic over Codable & Identifiable | Reusable across model types | | Atomic file writes | Prevents corruption on crash |

Best Practices

• Use Sendable types for data crossing actor boundaries
• Minimal public API — only domain operations
• Load synchronously in init for simplicity
• Don't use nonisolated to bypass actor isolation

Part 2: Protocol-Based Dependency Injection

Pattern

1. Define small, focused protocols — one per external concern
2. Create production implementations — real file system, network, etc.
3. Create mock implementations — with configurable error properties
4. Inject via default parameters — production uses defaults; tests inject mocks
5. Write tests with Swift Testing — @Test, #expect(throws:)

Key Principles

• Single Responsibility: One protocol per concern
• Sendable conformance: Required for actor boundaries
• Only mock boundaries: Mock external dependencies, not internal types
• Error simulation: Design mocks with configurable error properties

Anti-Patterns

• God protocols covering all external access
• Mocking internal types with no external dependencies
• #if DEBUG instead of proper DI
• Over-engineering: no protocol needed if no external dependencies

Remember: Swift's actor model and protocol system provide compile-time safety guarantees for concurrency and testability.