swift-protocol-di-testing

Compare original and translation side by side

🇺🇸

Original

English

🇨🇳

Translation

Chinese

Swift Protocol-Based Dependency Injection for Testing

面向测试的Swift基于协议的依赖注入

Patterns for making Swift code testable by abstracting external dependencies (file system, network, iCloud) behind small, focused protocols. Enables deterministic tests without I/O.

通过将外部依赖（文件系统、网络、iCloud）抽象到小型聚焦协议之后，让Swift代码具备可测试性的模式。无需I/O操作即可实现确定性测试。

When to Activate

适用场景

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)

编写访问文件系统、网络或外部API的Swift代码
需要在不触发真实故障的情况下测试错误处理流程
构建可跨环境（应用、测试、SwiftUI预览）运行的模块
使用Swift并发（actor、Sendable）设计可测试架构

Core Pattern

核心模式

1. Define Small, Focused Protocols

1. 定义小型聚焦协议

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?
}

每个协议仅处理一个外部关注点。

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

2. 创建默认（生产环境）实现

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)
    }
}

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

3. 创建用于测试的模拟实现

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
    }
}

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

4. 使用默认参数注入依赖

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...
    }
}

生产环境代码使用默认实现；测试环境注入模拟实现。

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

5. 使用Swift Testing编写测试用例

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()
    }
}

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

最佳实践

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

单一职责：每个协议应只处理一个关注点——不要创建包含大量方法的“上帝协议”
符合Sendable协议：当协议跨actor边界使用时必须满足此要求
默认参数：让生产环境代码默认使用真实实现；仅测试环境需要指定模拟实现
错误模拟：为模拟实现设计可配置的错误属性，用于测试故障流程
仅模拟边界依赖：只模拟外部依赖（文件系统、网络、API），不要模拟内部类型

Anti-Patterns to Avoid

需避免的反模式

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

创建一个覆盖所有外部访问的大型单一协议
模拟无外部依赖的内部类型
使用
```
#if DEBUG
```
条件编译而非规范的依赖注入
与actor配合使用时忘记遵循Sendable协议
过度设计：如果某个类型没有外部依赖，则不需要为其定义协议

When to Use

使用场景

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

任何涉及文件系统、网络或外部API的Swift代码
测试在真实环境中难以触发的错误处理流程
构建需要在应用、测试及SwiftUI预览环境中运行的模块
使用Swift并发（actor、结构化并发）且需要可测试架构的应用