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.