Build Robust iOS Networking Layers

This skill is expert in iOS networking layer architecture and implementation, specializing in robust, scalable, maintainable networking solutions using URLSession, modern Swift concurrency, and industry best practices. Its core architecture is protocol-oriented: a NetworkServiceProtocol defining generic async request and upload methods, and an Endpoint protocol defining baseURL, path, method, headers, parameters, and body. It configures dedicated URLSession instances with explicit timeout and connectivity-waiting settings, implements the core request method with async/await - building the URLRequest, validating the HTTP status code range, and decoding with ISO8601 date strategy - and defines a comprehensive NetworkError enum (invalidURL, invalidResponse, noData, serverError with code and body, decodingError, networkUnavailable, timeout, unauthorized) each with a LocalizedError description.

Use this skill when building or refactoring an iOS app's networking layer to be protocol-based, testable, and built on modern Swift concurrency rather than completion-handler callbacks. It covers token-based authentication with automatic refresh-and-retry on a 401 (using a DispatchSemaphore to prevent concurrent refresh races), concrete Endpoint enum implementations mapping cases to paths/methods/bodies, a MockNetworkService conforming to the same protocol for unit testing, request caching via URLRequest.CachePolicy, and concurrent request batching using withThrowingTaskGroup to fan out multiple endpoint calls and collect results. It is not a guide for third-party networking libraries like Alamofire - it's built specifically around native URLSession and Swift concurrency (async/await, task groups).

class AuthenticatedNetworkService: NetworkService {
    private var accessToken: String?
    private var refreshToken: String?
    private let tokenRefreshSemaphore = DispatchSemaphore(value: 1)
    
    override func request<T: Codable>(_ endpoint: Endpoint, responseType: T.Type) async throws -> T {
        do {
            return try await performAuthenticatedRequest(endpoint, responseType: responseType)
        } catch NetworkError.unauthorized {
            try await refreshTokenIfNeeded()
            return try await performAuthenticatedRequest(endpoint, responseType: responseType)
        }
    }
    
    private func refreshTokenIfNeeded() async throws {
        tokenRefreshSemaphore.wait()
        defer { tokenRefreshSemaphore.signal() }
        
        guard let refreshToken = refreshToken else {
            throw NetworkError.unauthorized
        }
        
        let refreshEndpoint = AuthEndpoint.refresh(token: refreshToken)
        let response: TokenResponse = try await super.request(refreshEndpoint, responseType: TokenResponse.self)
        
        self.accessToken = response.accessToken
        self.refreshToken = response.refreshToken
    }
}

Given an API surface, the skill produces a NetworkService conforming to NetworkServiceProtocol, an AuthenticatedNetworkService subclass like the one above handling token refresh transparently, concrete Endpoint enum cases per API resource, a MockNetworkService for tests, cached and batched request extension methods, and a typed NetworkError enum surfaced through every failure path.

iOS engineers building or modernizing an app's networking layer who want protocol-oriented, testable code built on native URLSession and Swift concurrency rather than a third-party dependency or completion-handler callbacks. It suits teams that need production concerns handled explicitly - certificate pinning, exponential backoff retries, background sessions for large transfers, network reachability monitoring, and logging without exposing sensitive data - alongside the core request/response plumbing.