Agent skill

swift-protocol-di-testing

Protocol-based dependency injection for testable Swift code — mock file system, network, and external APIs using focused protocols and Swift Testing.

View SKILL.md on GitHub Repository
Stars 132,726
Forks 19,206

Install this agent skill to your Project

npx add-skill https://github.com/affaan-m/everything-claude-code/tree/main/skills/swift-protocol-di-testing

SKILL.md

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(forUbiquityContainerIdentifier: nil)
    }
}

public struct DefaultFileAccessor: FileAccessorProviding {
    public init() {}

    public func read(from url: URL) throws -> Data {
        try Data(contentsOf: url)
    }

    public func write(_ data: Data, to url: URL) throws {
        try data.write(to: url, options: .atomic)
    }

    public func fileExists(at url: URL) -> Bool {
        FileManager.default.fileExists(atPath: url.path)
    }
}

3. Create Mock Implementations for Testing

swift
public final class MockFileAccessor: FileAccessorProviding, @unchecked Sendable {
    public var files: [URL: Data] = [:]
    public var readError: Error?
    public var writeError: Error?

    public init() {}

    public func read(from url: URL) throws -> Data {
        if let error = readError { throw error }
        guard let data = files[url] else {
            throw CocoaError(.fileReadNoSuchFile)
        }
        return data
    }

    public func write(_ data: Data, to url: URL) throws {
        if let error = writeError { throw error }
        files[url] = data
    }

    public func fileExists(at url: URL) -> Bool {
        files[url] != nil
    }
}

4. Inject Dependencies with Default Parameters

Production code uses defaults; tests inject mocks.

swift
public actor SyncManager {
    private let fileSystem: FileSystemProviding
    private let fileAccessor: FileAccessorProviding

    public init(
        fileSystem: FileSystemProviding = DefaultFileSystemProvider(),
        fileAccessor: FileAccessorProviding = DefaultFileAccessor()
    ) {
        self.fileSystem = fileSystem
        self.fileAccessor = fileAccessor
    }

    public func sync() async throws {
        guard let containerURL = fileSystem.containerURL(for: .sync) else {
            throw SyncError.containerNotAvailable
        }
        let data = try fileAccessor.read(
            from: containerURL.appendingPathComponent("data.json")
        )
        // Process data...
    }
}

5. Write Tests with Swift Testing

swift
import Testing

@Test("Sync manager handles missing container")
func testMissingContainer() async {
    let mockFileSystem = MockFileSystemProvider(containerURL: nil)
    let manager = SyncManager(fileSystem: mockFileSystem)

    await #expect(throws: SyncError.containerNotAvailable) {
        try await manager.sync()
    }
}

@Test("Sync manager reads data correctly")
func testReadData() async throws {
    let mockFileAccessor = MockFileAccessor()
    mockFileAccessor.files[testURL] = testData

    let manager = SyncManager(fileAccessor: mockFileAccessor)
    let result = try await manager.loadData()

    #expect(result == expectedData)
}

@Test("Sync manager handles read errors gracefully")
func testReadError() async {
    let mockFileAccessor = MockFileAccessor()
    mockFileAccessor.readError = CocoaError(.fileReadCorruptFile)

    let manager = SyncManager(fileAccessor: mockFileAccessor)

    await #expect(throws: SyncError.self) {
        try await manager.sync()
    }
}

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

Recommended Agent Skills

Expand your agent's capabilities with these related and highly-rated skills.

affaan-m/everything-claude-code

python-testing

Python testing best practices using pytest including fixtures, parametrization, mocking, coverage analysis, async testing, and test organization. Use when writing or improving Python tests.

132,726 19,206
Explore
affaan-m/everything-claude-code

golang-patterns

Go-specific design patterns and best practices including functional options, small interfaces, dependency injection, concurrency patterns, error handling, and package organization. Use when working with Go code to apply idiomatic Go patterns.

132,726 19,206
Explore
affaan-m/everything-claude-code

e2e-testing

Playwright E2E testing patterns, Page Object Model, configuration, CI/CD integration, artifact management, and flaky test strategies.

132,726 19,206
Explore
affaan-m/everything-claude-code

agentic-engineering

Operate as an agentic engineer using eval-first execution, decomposition, and cost-aware model routing. Use when AI agents perform most implementation work and humans enforce quality and risk controls.

132,726 19,206
Explore
affaan-m/everything-claude-code

api-design

REST API design patterns including resource naming, status codes, pagination, filtering, error responses, versioning, and rate limiting for production APIs.

132,726 19,206
Explore
affaan-m/everything-claude-code

python-patterns

Python-specific design patterns and best practices including protocols, dataclasses, context managers, decorators, async/await, type hints, and package organization. Use when working with Python code to apply Pythonic patterns.

132,726 19,206
Explore

Didn't find tool you were looking for?

Be as detailed as possible for better results