ios-api-client-foundation

iOS API Client Foundation

RULES — Follow these with no exceptions

1. One singleton: APIClient.shared. Never instantiate APIClient() in a view or view model — inject the shared instance or mock it in tests.
2. Use async/await throughout — no closure-based callback variants, no Combine Publisher returns. Every method is async throws.
3. Typed envelopes only. Every endpoint returns DataResponse<T>, PaginatedResponse<T>, or EmptyResponse. Raw JSON dictionaries are a code smell.
4. JSONEncoder uses .convertToSnakeCase and JSONDecoder uses .convertFromSnakeCase. Phoenix/Rails/Django all expect snake_case on the wire; camelCase on wire is a bug.
5. Bearer token injected automatically from KeychainService.getToken() on every authenticated request. The caller never touches Authorization headers.
6. 401 is handled centrally — the client calls AppState.handleUnauthorized() on every 401, which logs the user out. Do not add per-call 401 handling.
7. Base URL comes from Configuration.apiBaseURL, which reads from Info.plist (fed by xcconfig). Never hardcode "http://..." in a service method.
8. Multipart uploads use upload(path:imageData:filename:). Do not hand-roll boundary handling in feature code.

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When to invoke

• Creating or editing APIClient.swift or Services/Auth*.swift.
• Adding a new endpoint method (typically as an extension APIClient).
• Debugging a 401 loop or a decode error.
• Porting a web-first backend API to iOS.

The envelopes

struct DataResponse<T: Decodable>: Decodable { let data: T }
struct PaginatedResponse<T: Decodable>: Decodable {
    let data: [T]
    let cursor: String?
}
struct EmptyResponse: Decodable {}

Every endpoint returns one of these. Example:

// GET /posts/123 → { "data": { "id": 123, "title": "..." } }
let response: DataResponse<Post> = try await APIClient.shared.get(path: "/posts/123")
let post = response.data

The canonical method signatures

@MainActor
final class APIClient: APIClientProtocol {
    static let shared = APIClient()

    func get<T: Decodable>(path: String) async throws -> T
    func post<T: Decodable, B: Encodable>(path: String, body: B) async throws -> T
    func post<T: Decodable>(path: String) async throws -> T
    func put<T: Decodable, B: Encodable>(path: String, body: B) async throws -> T
    func delete<T: Decodable>(path: String) async throws -> T
    func upload<T: Decodable>(path: String, imageData: Data, filename: String, mimeType: String = "image/jpeg") async throws -> T
}

APIClientProtocol (see templates/APIClientProtocol.swift) is the seam ViewModels inject for testability; templates/MockAPIClient.swift is the matching test double.

Adding an endpoint

Put new endpoints in a feature-specific extension file, not in the body of APIClient:

// Services/APIClient+Posts.swift
extension APIClient {
    func fetchFeed(cursor: String? = nil) async throws -> PaginatedResponse<Post> {
        let query = cursor.map { "?cursor=\($0)" } ?? ""
        return try await get(path: "/feed\(query)")
    }

    func toggleLike(postId: Int, liked: Bool) async throws -> EmptyResponse {
        if liked {
            return try await delete(path: "/posts/\(postId)/like")
        } else {
            return try await post(path: "/posts/\(postId)/like")
        }
    }
}

Encoder / decoder config

private let encoder: JSONEncoder = {
    let e = JSONEncoder()
    e.keyEncodingStrategy = .convertToSnakeCase
    e.dateEncodingStrategy = .iso8601
    return e
}()

private let decoder: JSONDecoder = {
    let d = JSONDecoder()
    d.keyDecodingStrategy = .convertFromSnakeCase
    d.dateDecodingStrategy = .iso8601
    return d
}()

401 handling

// Inside the shared request dispatch:
if httpResponse.statusCode == 401 {
    await AppState.shared.handleUnauthorized()
    throw APIError.unauthorized
}

AppState.handleUnauthorized() clears the keychain token and flips isAuthenticated = false. SwiftUI then routes to the login screen automatically.

Common pitfalls

Forgetting convertToSnakeCase on the encoder

Symptom: Phoenix replies with {"errors": {"first_name": ["can't be blank"]}} even though you sent {"firstName": "Joe"}.

Fix: encoder.keyEncodingStrategy = .convertToSnakeCase. Same for the decoder.

Hardcoding the base URL

Symptom: Works on simulator, fails on device — or ships prod pointing at localhost.

Fix: Read from Configuration.apiBaseURL, which pulls from Info.plist, which is fed by Config.xcconfig (so Debug and Release can differ).

Calling APIClient() in a test

Symptom: Tests hit the real network.

Fix: APIClient.shared is the only instance. For tests, inject APIClientProtocol into view models and use MockAPIClient (both provided as templates).

Manual Authorization header in feature code

Symptom: A view model pulls the token from Keychain and sets the header on a one-off URLRequest.

Fix: Move that call through APIClient. Bearer injection is centralized — feature code should never touch auth headers.

Template reference

See <plugin-root>/templates/APIClient.swift for the full canonical implementation including envelopes, encoder/decoder, bearer injection, 401 handling, and multipart upload. Copy it into Services/APIClient.swift on day 1.