Networking with URLSession

URLSession (from Foundation) makes HTTP requests. Combined with Codable and async/await, fetching data from an API takes only a few lines.

async and await get a full treatment in Level 3. For now: a network call takes time, await marks the point where your code waits for it without freezing the program, and functions that wait are marked async.

A GET request

import Foundation
#if canImport(FoundationNetworking)
import FoundationNetworking   // needed for URLSession on Linux
#endif

struct Repo: Decodable {
    let fullName: String
    let stargazersCount: Int
}

enum APIError: Error {
    case badStatus(Int)
}

func fetchRepo(_ name: String) async throws -> Repo {
    let url = URL(string: "https://api.github.com/repos/\(name)")!
    let (data, response) = try await URLSession.shared.data(from: url)

    if let http = response as? HTTPURLResponse, !(200..<300).contains(http.statusCode) {
        throw APIError.badStatus(http.statusCode)
    }

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

do {
    let repo = try await fetchRepo("swiftlang/swift")
    print("\(repo.fullName) has \(repo.stargazersCount) stars")
} catch {
    print("Request failed: \(error)")
}

swiftlang/swift has 68734 stars

(The number will be different when you run it.)

• data(from:) returns the body and the response. It throws for network failures — no connection, timeout, invalid host.
• A 404 or 500 is not a thrown error — the request succeeded at the network level. Always check statusCode yourself.
• Top-level code in a script can use await directly.

POST with JSON and headers

var request = URLRequest(url: URL(string: "https://httpbin.org/post")!)
request.httpMethod = "POST"
request.setValue("application/json", forHTTPHeaderField: "Content-Type")
request.timeoutInterval = 10
request.httpBody = try JSONEncoder().encode(["title": "Hello"])

let (_, response) = try await URLSession.shared.data(for: request)
print((response as? HTTPURLResponse)?.statusCode ?? 0)

200

Apps and HTTPS

Apple platforms require HTTPS for network requests by default (App Transport Security). Keep it that way — plain http:// sends data unencrypted.

Common mistakes

• Treating any response as success without checking the status code.
• Putting API keys in app source code. Anything shipped inside an app can be extracted; keep secrets on your server.