swift-protocol-di-testing

Topics in the source

• Swift Protocol-Based Dependency Injection for Testing
• When to Activate
• Core Pattern
• 1. Define Small, Focused Protocols
• 2. Create Default (Production) Implementations
• 3. Create Mock Implementations for Testing
• 4. Inject Dependencies with Default Parameters
• 5. Write Tests with Swift Testing

Patterns for making Swift code testable by abstracting external dependencies (file system, network, iCloud) behind small, focused protocols. Enables deterministic tests without I/O. Writing Swift code that accesses file system, network, or external APIs Need to test error handling paths without triggering real failures Building modules that work across environments (app, test, SwiftUI preview) Designing testable architecture with Swift concurrency (actors, Sendable) Each protocol handles exactly one external concern. Production code uses defaults; tests inject mocks. Single Responsibility: Each protocol should handle one concern — don't create "god protocols" with many methods Sendable conformance: Required when protocols are used across actor boundaries Default parameters: Let production code use real implementations by default; only tests need to specify mocks Error simulation: Design mocks with configurable error properties for testing failure paths Only mock boundaries: Mock external dependencies (file system, network, APIs), not internal types Creating a single large protocol that covers all external access Mocking internal types that have no external dependencies Using #if DE

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