Protocol-based dependency injection for testable Swift code — mock file system, network, and external APIs using focused protocols and Swift Testing. Use when Swift code needs testing and file system, network, or external APIs must be mocked.

Swift Protocol Based Dependency Injection for 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. When to Activate 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) Core Pattern 1. Define Small, Focused Protocols Each protocol handles exactly one external concern. 2. Create Default (Production) Implementations 3. Create Mock Implementations for Testing 4. Inject Dependencies with Default Parameters Production code uses defaults; tests inject mocks. 5. Write Tests with Swift Testing Best Practices 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 Anti Patterns to Avoid Creating a single large protocol that covers all external access Mocking internal types that have no external dependencies Using if DEBUG conditionals instead of proper dependency injection Forgetting Sendable conformance when used with actors Over engineering: if a type has no external dependencies, it doesn't need a protocol When to Use Any Swift code that touches file system, network, or external APIs Testing error handling paths that are hard to trigger in real environments Building modules that need to work in app, test, and SwiftUI preview contexts Apps using Swift concurrency (actors, structured concurrency) that need testable architecture

swift-protocol-di-testing