Swift Protocol-Based Dependency Injection for Testing

Patterns for making Swift code testable by abstracting external dependencies behind small, focused protocols.

When to Activate

• Making Swift code testable without real I/O operations
• Abstracting file system, network, or database access behind protocols
• Writing deterministic tests with configurable mock behavior
• Supporting multiple environments (app, tests, previews)

Core Pattern -- Protocol Design

Each protocol handles exactly one external concern: