networking

Networking (URLSession + async/await)

Build a correct, Sendable networking layer with structured concurrency. No third-party library needed for most apps. Concurrency/isolation rules follow ios-standards.

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The core call

let (data, response) = try await URLSession.shared.data(for: request)
guard let http = response as? HTTPURLResponse else { throw APIError.nonHTTP }
guard 200..<300 ~= http.statusCode else { throw APIError.status(http.statusCode, data) }
let value = try decoder.decode(T.self, from: data)

Three things people skip and regret:

1. Cast to HTTPURLResponse and check the status code — URLSession does not throw on 4xx/5xx; you get a normal response with an error body.
2. Decode errors are not network errors — keep them distinct so you can log the payload.
3. Build URLs with URLComponents (proper percent-encoding of query items), never string concatenation.

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A typed, Sendable client

struct Endpoint<Response: Decodable> {
    var path: String
    var method = "GET"
    var query: [URLQueryItem] = []
    var body: Data? = nil
}

actor APIClient {
    private let base: URL
    private let session: URLSession
    private let decoder: JSONDecoder

    init(base: URL, session: URLSession = .shared) {
        self.base = base
        self.session = session
        decoder = JSONDecoder()
        decoder.keyDecodingStrategy = .convertFromSnakeCase
        decoder.dateDecodingStrategy = .iso8601
    }

    func send<R>(_ endpoint: Endpoint<R>) async throws -> R {
        var comps = URLComponents(url: base.appending(path: endpoint.path),
                                  resolvingAgainstBaseURL: false)!
        if !endpoint.query.isEmpty { comps.queryItems = endpoint.query }
        var req = URLRequest(url: comps.url!)
        req.httpMethod = endpoint.method
        req.httpBody = endpoint.body
        if endpoint.body != nil { req.setValue("application/json", forHTTPHeaderField: "Content-Type") }

        let (data, response) = try await session.data(for: req)
        guard let http = response as? HTTPURLResponse else { throw APIError.nonHTTP }
        guard 200..<300 ~= http.statusCode else { throw APIError.status(http.statusCode, data) }
        do { return try decoder.decode(R.self, from: data) }
        catch { throw APIError.decoding(error) }
    }
}

enum APIError: Error { case nonHTTP, status(Int, Data), decoding(Error), offline }