Async load with URLSession

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Goal

You will write PostsLoader — the object that fetches posts over the network using URLSession and async/await. After this page the view model from page 02 can call loader.fetch() and get back a [Post] or throw an error.

Prerequisites

• 02 The view model

The loader

Add a new file PostsLoader.swift:

import Foundation

struct PostsLoader {
    // Public, keyless sample API — no auth, no secrets.
    private let url = URL(string: "https://jsonplaceholder.typicode.com/posts")!

    func fetch() async throws -> [Post] {
        let (data, response) = try await URLSession.shared.data(from: url)
        guard let http = response as? HTTPURLResponse, http.statusCode == 200 else {
            throw URLError(.badServerResponse)
        }
        return try JSONDecoder().decode([Post].self, from: data)
    }
}

What each line does

URLSession.shared.data(from:) is the async variant of the classic completion- handler API. It suspends the current task while the request is in flight and resumes with (Data, URLResponse) when the response arrives. The try await unwraps both the network error (if the request itself fails) and the suspension point.

The guard checks the HTTP status code. A 200 means the server returned the resource. Anything else — 404, 500, redirect loops — throws URLError(.badServerResponse). The caller (PostsViewModel.load()) catches this and transitions to .failed.

JSONDecoder().decode([Post].self, from: data) maps the JSON array to [Post]. It works because Post is Decodable and its property names match the JSON keys the API returns:

[
  { "id": 1, "userId": 1, "title": "...", "body": "..." },
  ...
]

JSONDecoder ignores keys it has no matching property for (userId here), so the struct only needs to declare the properties it cares about.

No API key required

jsonplaceholder.typicode.com is a free, public, read-only fake REST API. No registration, no token, no rate-limit header. This keeps the sample free of credentials.

Rule for real projects: never hard-code API keys, tokens, or credentials in source files. Store secrets in environment variables, Xcode configuration files, or a secrets manager — never in a file committed to version control.

Run a quick smoke test in a playground (optional)

If you want to verify the loader independently before wiring it to the UI, paste this into an Xcode Playground:

import Foundation

// (paste Post and PostsLoader here)

Task {
    let posts = try await PostsLoader().fetch()
    print("Loaded \(posts.count) posts")
    print(posts[0].title)
}

Expected output: Loaded 100 posts followed by the first post title.

Next → 04 Bind the view