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Swift Protocol DI Testing

Your AI writes testable Swift code by abstracting file system, network, and API dependencies behind focused protocols. It manages mocks for these external systems and supports Swift concurrency patterns for reliable unit tests.

affaan-m on GitHub

Curated by PromptsEdge from a public repo · MIT license. All credit goes to the author.

What is this skill?

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 use

  • 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

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