Swift Protocol Di Testing

---
name: swift-protocol-di-testing
description: "Use when protocol-based dependency injection for testable Swift code — mock file system, network, and external APIs using focused protocols and Swift Testing. Triggers on \"swift-protocol-di-testing\", \"swift protocol di testing\", \"testing\"."
metadata:
  origin: ECC
---

# 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)
    }