ios-networking

# iOS Networking

Modern networking patterns for iOS 26+ using URLSession with async/await and
structured concurrency. All examples target Swift 6.3. No third-party
dependencies required -- URLSession covers the vast majority of networking
needs.

## Contents

- [Core URLSession async/await](#core-urlsession-asyncawait)
- [API Client Architecture](#api-client-architecture)
- [Error Handling](#error-handling)
- [Pagination](#pagination)
- [Network Reachability](#network-reachability)
- [Configuring URLSession](#configuring-urlsession)
- [App Transport Security (ATS)](#app-transport-security-ats)
- [Common Mistakes](#common-mistakes)
- [Review Checklist](#review-checklist)
- [References](#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.

### Data Requests

```swift
// 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.

```swift
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

```swift
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.

```swift
// 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.

```swift
// 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).

```swift
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.

```swift
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
}
```

```swift
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](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`,
`