IOS Networking

iOS Networking

Use URLSession with async/await and structured concurrency for ordinary HTTP, REST, uploads, downloads, and streaming. Use Network.framework for lower-level protocols and delegate/task APIs for durable background transfers.

Contents

• Core URLSession async/await
• API Client Architecture
• Error Handling
• Pagination
• Network Reachability
• Configuring URLSession
• App Transport Security (ATS)
• Common Mistakes
• Review Checklist
• References

Core URLSession async/await

URLSession gained native async/await overloads in iOS 15. Prefer these for foreground data, upload, download, and streaming work. Background URLSession transfers are the main exception: they still use task/delegate APIs so the system can deliver events after suspension or relaunch.

Validate networking policy locally with URLProtocol fixtures for valid 2xx, malformed 2xx, one-time 401 refresh, bounded 429/5xx retry, timeout/offline, cancellation, and nonretryable 4xx. Inspect headers, status, and error classification; fix the policy and rerun. Retry only safe/idempotent or explicitly replayable requests, and never loop token refresh.

Data Requests

// Basic GET
let (data, response) = try await URLSession.shared.data(from: url)

// With a configured URLRequest
var request = URLRequest(url: url)
request.httpMethod = "POST"
request.setValue("application/json", forHTTPHeaderField: "Content-Type")
request.httpBody = try JSONEncoder().encode(payload)
request.timeoutInterval = 30
request.cachePolicy = .reloadIgnoringLocalCacheData

let (data, response) = try await URLSession.shared.data(for: request)

Response Validation

Always validate the HTTP status code before decoding. URLSession does not throw for 4xx/5xx responses -- it only throws for transport-level failures.

guard let httpResponse = response as? HTTPURLResponse else {
    throw NetworkError.invalidResponse
}

guard (200..<300).contains(httpResponse.statusCode) else {
    throw NetworkError.httpError(
        statusCode: httpResponse.statusCode,
        data: data
    )
}

JSON Decoding with Codable

func fetch<T: Decodable>(_ type: T.Type, from url: URL) async throws -> T {
    let (data, response) = try await URLSession.shared.data(from: url)

    guard let httpResponse = response as? HTTPURLResponse,
          (200..<300).contains(httpResponse.statusCode) else {
        throw NetworkError.invalidResponse
    }

    let decoder = JSONDecoder()
    decoder.dateDecodingStrategy = .iso8601
    decoder.keyDecodingStrategy = .convertFromSnakeCase
    return try decoder.decode(T.self, from: data)
}

Downloads and Uploads

Use download(for:) for large files -- it streams to disk instead of loading the entire payload into memory.

// Download to a temporary file
let (localURL, response) = try await URLSession.shared.download(for: request)

// Move or copy the returned temporary file promptly.
let destination = documentsDirectory.appendingPathComponent("file.zip")
try FileManager.default.moveItem(at: localURL, to: destination)

For delegate-based URLSessionDownloadDelegate, move or open the temporary file before urlSession(_:downloadTask:didFinishDownloadingTo:) returns.

Background sessions are delegate-driven transfer queues. Use task creation APIs such as downloadTask(with:) and file-backed uploadTask(with:fromFile:), then handle URLSessionDelegate / task delegate callbacks. Do not use async convenience APIs such as data(for:), download(for:), or upload(for:) as the durable background-session pattern.

// Upload data
let (data, response) = try await URLSession.shared.upload(for: request, from: bodyData)

// Upload from file
let (data, response) = try await URLSession.shared.upload(for: request, fromFile: fileURL)

Streaming with AsyncBytes

Use bytes(for:) for streaming responses, progress tracking, or line-delimited data (e.g., server-sent events).

let (bytes, response) = try await URLSession.shared.bytes(for: request)

for try await line in bytes.lines {
    // Process each line as it arrives (e.g., SSE stream)
    handleEvent(line)
}

API Client Architecture

Protocol-Based Client

Define a protocol for testability. This lets you swap implementations in tests without mocking URLSession directly.

protocol APIClientProtocol: Sendable {
    func fetch<T: Decodable & Sendable>(
        _ type: T.Type,
        endpoint: Endpoint
    ) async throws -> T

    func send<T: Decodable & Sendable>(
        _ type: T.Type,
        endpoint: Endpoint,
        body: some Encodable & Sendable
    ) async throws -> T
}

struct Endpoint: Sendable {
    let path: String
    var method: String = "GET"
    var queryItems: [URLQueryItem] = []
    var headers: [String: String] = [:]

    func url(relativeTo baseURL: URL) -> URL {
        guard let components = URLComponents(
            url: baseURL.appendingPathComponent(path),
            resolvingAgainstBaseURL: true
        ) else {
            preconditionFailure("Invalid URL components for path: \(path)")
        }
        var mutableComponents = components
        if !queryItems.isEmpty {
            mutableComponents.queryItems = queryItems
        }
        guard let url = mutableComponents.url else {
            preconditionFailure("Failed to construct URL from components")
        }
        return url
    }
}

The client accepts a baseURL, optional custom URLSession, JSONDecoder, and an array of RequestMiddleware interceptors. Each method builds a URLRequest from the endpoint, applies middleware, executes the request, validates the status code, and decodes the result. See references/urlsession-patterns.md for the complete APIClient implementation with convenience methods, request builder, and test setup.

Production clients should receive an injected, configured URLSession instead of calling URLSession.shared internally. Configure URLSessionConfiguration with request/resource timeouts, cache policy or URLCache, waitsForConnectivity, data-cost policy, and delegates when authentication challenges, redirects, metrics, pinning, or background transfer handling matter.

Lightweight Closure-Based Client

For apps using the MV pattern, use closure-based clients for testability and SwiftUI preview support. See references/lightweight-clients.md for the full pattern (struct of async closures, injected via init).

Request Middleware / Interceptors

Middleware transforms requests before they are sent. Use this for authentication, logging, analytics headers, and similar cross-cutting concerns.

protocol RequestMiddleware: Sendable {
    func prepare(_ request: URLRequest) async throws -> URLRequest
}

struct AuthMiddleware: RequestMiddleware {
    let tokenProvider: @Sendable () async throws -> String

    func prepare(_ request: URLRequest) async throws -> URLRequest {
        var request = request
        let token = try await tokenProvider()
        request.setValue("Bearer \(token)", forHTTPHeaderField: "Authorization")
        return request
    }
}

Token Refresh Flow

Handle 401 responses by refreshing the token and retrying once.

func fetchWithTokenRefresh<T: Decodable & Sendable>(
    _ type: T.Type,
    endpoint: Endpoint,
    tokenStore: TokenStore
) async throws -> T {
    do {
        return try await fetch(type, endpoint: endpoint)
    } catch NetworkError.httpError(statusCode: 401, _) {
        try await tokenStore.refreshToken()
        return try await fetch(type, endpoint: endpoint)
    }
}

Error Handling

Structured Error Types

enum NetworkError: Error, Sendable {
    case invalidResponse
    case httpError(statusCode: Int, data: Data)
    case decodingFailed(Error)
    case noConnection
    case timedOut
    case cancelled

    /// Map a URLError to a typed NetworkError
    static func from(_ urlError: URLError) -> NetworkError {
        switch urlError.code {
        case .notConnectedToInternet, .networkConnectionLost:
            return .noConnection
        case .timedOut:
            return .timedOut
        case .cancelled:
            return .cancelled
        default:
            return .httpError(statusCode: -1, data: Data())
        }
    }
}

Key URLError Cases

Decoding Server Error Bodies

struct APIErrorResponse: Decodable, Sendable {
    let code: String
    let message: String
}

func decodeAPIError(from data: Data) -> APIErrorResponse? {
    try? JSONDecoder().decode(APIErrorResponse.self, from: data)
}

// Usage in catch block
catch NetworkError.httpError(let statusCode, let data) {
    if let apiError = decodeAPIError(from: data) {
        showError("Server error: \(apiError.message)")
    } else {
        showError("HTTP \(statusCode)")
    }
}

Retry with Exponential Backoff

Use structured concurrency for retries. Respect task cancellation between attempts. Skip retries for cancellation and 4xx client errors (except 429).

func withRetry<T: Sendable>(
    maxAttempts: Int = 3,
    initialDelay: Duration = .seconds(1),
    operation: @Sendable () async throws -> T
) async throws -> T {
    var lastError: Error?
    for attempt in 0..<maxAttempts {
        do {
            return try await operation()
        } catch {
            lastError = error
            if error is CancellationError { throw error }
            if case NetworkError.httpError(let code, _) = error,
               (400..<500).contains(code), code != 429 { throw error }
            if attempt < maxAttempts - 1 {
                try await Task.sleep(for: initialDelay * Int(pow(2.0, Double(attempt))))
            }
        }
    }
    throw lastError!
}

Pagination

Build cursor-based or offset-based pagination with AsyncSequence. Always check Task.isCancelled between pages. See references/urlsession-patterns.md for complete CursorPaginator and offset-based implementations.

Network Reachability

Use NWPathMonitor from the Network framework -- not third-party Reachability libraries. On current OS targets it conforms to AsyncSequence; wrap pathUpdateHandler only for compatibility or custom projections.

import Network

func observeNetworkStatus() async {
    let monitor = NWPathMonitor()

    for await path in monitor {
        handle(path.status)
    }
}

Check path.isExpensive (cellular) and path.isConstrained (Low Data Mode) to adapt behavior (reduce image quality, skip prefetching).

Use Network.framework for low-level TCP, UDP, listeners, Bonjour, path monitoring, or WebSocket protocol work -- not ordinary REST APIs. For iOS 26 NetworkConnection<QUIC>, openStream(...) and inboundStreams(...) are async throwing APIs; see references/network-framework.md#quic-multiplexed-streams.

Configuring URLSession

Inject a configured session when production code needs timeouts, caching, connectivity waiting, data-cost policy, authentication challenges, redirects, metrics, or background delegates. Use URLSession.shared only for simple one-off work. See URLSession patterns for the full configuration and test setup.

App Transport Security (ATS)

ATS makes HTTPS the URL Loading System default. Do not enable blanket arbitrary loads; use the narrowest justified domain/local-network exception. Configure TLS explicitly for Network.framework. Keep deep trust and SPKI pinning design in swift-security.

Common Mistakes

DON'T: Force-unwrap URL(string:) with dynamic input. DO: Use URL(string:) with proper error handling. Force-unwrap is acceptable only for compile-time-constant strings.

DON'T: Decode JSON on the main thread for large payloads. DO: Keep decoding on the calling context of the URLSession call, which is off-main by default. Only hop to @MainActor to update UI state.

DON'T: Ignore cancellation in long-running network tasks. DO: Check Task.isCancelled or call try Task.checkCancellation() in loops (pagination, streaming, retry). Use .task in SwiftUI for automatic cancellation.

DON'T: Use Alamofire or Moya when URLSession async/await handles the need. DO: Use URLSession directly. With async/await, the ergonomic gap that justified third-party libraries no longer exists. Reserve third-party libraries for genuinely missing features (e.g., image caching).

DON'T: Mock URLSession directly in tests. DO: Use URLProtocol subclass for transport-level mocking, or use protocol-based clients that accept a test double.

DON'T: Fire network requests from body or view initializers. DO: Use .task or .task(id:) to trigger network calls.

Review Checklist

• Foreground transfers use async/await; background sessions use delegate/task APIs
• Error handling covers URLError cases (.notConnectedToInternet, .timedOut, .cancelled)
• Requests are cancellable (respect Task cancellation via .task modifier or stored Task references)
• Authentication tokens injected via middleware, not hardcoded
• Response HTTP status codes validated before decoding
• Large downloads use download(for:) not data(for:)
• Network calls happen off @MainActor (only UI updates on main)
• URLSession configured with appropriate timeouts and caching
• Production clients inject configured sessions instead of using URLSession.shared
• Background transfers use task/delegate APIs, not async convenience APIs
• Retry logic excludes cancellation and 4xx client errors
• Pagination checks Task.isCancelled between pages
• Sensitive tokens stored in Keychain (not UserDefaults or plain files)
• No force-unwrapped URLs from dynamic input
• Server error responses decoded and surfaced to users
• Network.framework code configures TLS/trust explicitly and keeps deep pinning work in swift-security
• NetworkConnection<QUIC> stream APIs are treated as async throwing
• Ensure network response model types conform to Sendable; use @MainActor for UI-updating completion paths

References

• See references/urlsession-patterns.md for complete API client implementation, multipart uploads, download progress, URLProtocol mocking, retry/backoff, certificate pinning, request logging, and pagination implementations.
• See references/background-websocket.md for background URLSession configuration, background downloads/uploads, WebSocket patterns with structured concurrency, and reconnection strategies.
• See references/lightweight-clients.md for the lightweight closure-based client pattern (struct of async closures, injected via init for testability and preview support).
• See references/network-framework.md for Network.framework (NWConnection, NWListener, NWBrowser, NWPathMonitor) and low-level TCP/UDP/WebSocket patterns.
• See references/file-storage-patterns.md for file system directory selection, FileProtectionType, backup exclusion, and storage pressure handling.