What problem does it solve? Swift code that directly touches the file system, network, or external APIs is hard to test deterministically. This Skill provides patterns for abstracting those dependencies behind small, focused protocols so tests can run without real I/O and can simulate failure paths on demand. ## Core Features & Use Cases - Focused Protocol Design: Define single-responsibility protocols such as FileSystemProviding, FileAccessorProviding, and BookmarkStorageProviding, each covering one external concern. - Mock Implementations: Build mocks with configurable error properties to test error handling paths that are difficult to trigger in real environments. - Default Parameter Injection: Production code uses real implementations by default while tests inject mocks, with Sendable conformance for actor-based concurrency. - Use Case: When building an actor-based SyncManager that reads from an iCloud container, inject a MockFileAccessor to verify that missing containers and corrupt files throw the expected errors using Swift Testing's #expect macros. ## Quick Start Show me how to make my Swift SyncManager testable by abstracting its file system access behind protocols with mock implementations for Swift Testing.
