Skill: DataSource

Guide for creating DataSources following the Repository pattern.

Before Creating — Clarify Requirements

CRITICAL: Do NOT assume the API schema or operations. Ask the user before writing any code:

1. Operations: Which operations does the contract need and with what parameters? (e.g., paginated list, single item by ID, both, with filters, etc.)
2. Entity fields: What fields does the DTO need? For GraphQL, the user can provide the query directly — derive DTOs from its response shape. Otherwise, ask for the fields or API documentation. Never fetch external API docs on your own.

Only proceed to implementation after the user confirms these details.

DataSource Types

Templates

Each reference contains full templates: contract, implementation, DTO, mock, fixtures, and tests.

• REST API: See [remote-rest.md](references/remote-rest.md)
• GraphQL API: See [remote-graphql.md](references/remote-graphql.md)
• Memory cache: See [local-memory.md](references/local-memory.md)
• UserDefaults: See [local-userdefaults.md](references/local-userdefaults.md)

File Structure

Features/{Feature}/
├── Sources/
│   └── Data/
│       ├── DataSources/
│       │   ├── Remote/
│       │   │   ├── {Name}RemoteDataSourceContract.swift
│       │   │   └── {Name}RESTDataSource.swift (or {Name}GraphQLDataSource.swift)
│       │   └── Local/
│       │       ├── {Name}LocalDataSourceContract.swift
│       │       ├── {Name}MemoryDataSource.swift
│       │       └── {Name}UserDefaultsDataSource.swift    # Optional: UserDefaults
│       └── DTOs/
│           └── {Name}DTO.swift
└── Tests/
    ├── Unit/Data/
    │   └── DataSources/
    │       ├── Remote/
    │       │   └── {Name}RESTDataSourceTests.swift (or {Name}GraphQLDataSourceTests.swift)
    │       └── Local/
    │           └── {Name}MemoryDataSourceTests.swift (or {Name}UserDefaultsDataSourceTests.swift)
    ├── Shared/Fixtures/
    │   ├── {name}.json
    │   └── {name}s_response.json
    └── Shared/Mocks/

Key Principles

Contracts

• Internal visibility, separate file from implementation
• Remote contracts: nonisolated protocol: Sendable with @concurrent on async methods. Local contracts (Memory, UserDefaults): : Actor
• Transport-agnostic: same contract for REST or GraphQL
• Remote DataSource implementations: nonisolated struct with @concurrent on async methods
• Transport clients (HTTPClientContract, GraphQLClientContract) also use @concurrent for off-MainActor execution
• ChallengeNetworking uses nonisolated default isolation — networking types don't need nonisolated annotations
• DataSources only work with DTOs — parameters and return types must be DTOs, never domain objects
• Remote: @concurrent async throws. Local (Memory, UserDefaults): methods are actor-isolated (implicitly async from caller)

Sendable vs Actor contracts: Use : Actor when the DataSource has its own mutable state to protect (Memory, UserDefaults, ImageDiskCacheContract). Use : Sendable with nonisolated methods only for stateless wrappers around thread-safe APIs (e.g., FileSystem wrapping FileManager). See /concurrency skill "Actor Reentrancy" section for when and why to choose : Sendable.

DTOs (Data Transfer Objects)

"A Data Transfer Object is one of those objects our mothers told us never to write. It's often little more than a bunch of fields and the getters and setters for them."
— Martin Fowler, PoEAA

DTOs are intentionally anemic — they exist purely to transfer data between systems.

Rules:

• Decodable, Equatable
• Internal visibility, properties match JSON keys
• NO behavior — only data (anemic by design)
• NO toDomain() methods — mapping belongs in the Repository
• REST IDs are Int, GraphQL IDs are String

Error Mapping

DataSources catch transport errors and map them to APIError:

• REST: HTTPError → APIError via HTTPErrorMapper
• GraphQL: GraphQLError → APIError via GraphQLErrorMapper

Repositories and upper layers only see APIError, never transport-specific errors.

Visibility Summary

Checklists

RemoteDataSource (REST)

• Create DTO
• Create Contract in Remote/ with async throws
• Create RESTDataSource in Remote/ with HTTPErrorMapper
• Create Mock in Tests/Shared/Mocks/
• Create JSON fixtures in Tests/Shared/Fixtures/
• Create tests

RemoteDataSource (GraphQL)

• Create DTO (IDs are String)
• Create Contract in Remote/ with async throws
• Create GraphQLDataSource in Remote/ with GraphQLErrorMapper
• Create Mock in Tests/Shared/Mocks/
• Create JSON fixtures in Tests/Shared/Fixtures/
• Create tests

MemoryDataSource

• Create Contract in Local/ with : Actor
• Create actor Implementation in Local/
• Create actor Mock with setter methods and call tracking
• Create tests (use await for all mock reads/writes)

LocalDataSource (UserDefaults)

• Create Contract in Local/ with : Actor
• Create actor Implementation in Local/ with private let userDefaults
• Create actor Mock with setter methods and call tracking
• Create async tests using custom UserDefaults suite (nonisolated(unsafe) on test property)