swift-protocol-di-testing

# 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. ```swift // File system access public protocol FileSystemProviding: Sendable { func containerURL(for purpose: Purpose) -> URL? } // File read/write operations public protocol FileAccessorProviding: Sendable { func read(from url: URL) throws -> Data func write(_ data: Data, to url: URL) throws func fileExists(at url: URL) -> Bool } // Bookmark storage (e.g., for sandboxed apps) public protocol BookmarkStorageProviding: Sendable { func saveBookmark(_ data: Data, for key: String) throws func loadBookmark(for key: String) throws -> Data? } ``` ### 2. Create Default (Production) Implementations ```swift public struct DefaultFileSystemProvider: FileSystemProviding { public init() {} public func containerURL(for purpose: Purpose) -> URL? { FileManager.default.url(forUbiquityContaine...