ios-networking

name

ios-networking

description

Build, review, or improve networking code in iOS/macOS apps using URLSession with async/await, structured concurrency, and modern Swift patterns. Use when working with REST APIs, downloading files, uploading data, WebSocket connections, pagination, retry logic, request middleware, caching, background transfers, or network reachability monitoring. Also use when handling HTTP requests, API clients, network error handling, or data fetching in Swift apps.

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

URLError Code	Meaning	Action
.notConnectedToInternet	Device offline	Show offline UI, queue for retry
.networkConnectionLost	Connection dropped mid-request	Retry with backoff
.timedOut	Server did not respond in time	Retry once, then show error
.cancelled	Task was cancelled	No action needed; do not show error
.cannotFindHost	DNS failure	Check URL, show error
.secureConnectionFailed	TLS handshake failed	Check cert pinning, ATS config
.userAuthenticationRequired	Authentication required to access a resource	Trigger auth flow

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 .sleep(for: initialDelay  (pow(, (attempt))))
            }
        }
    }
     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.