Working with APIs

By the end of this lesson you'll fetch real data from the internet with URLSession , turn JSON into Swift types with Codable , handle failures cleanly with do/try/catch , and show the result in a SwiftUI view — the skill that turns a static app into a live one.

Learn Working with APIs in our free Swift course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick reference.

Part of the free Swift course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

What You'll Learn in This Lesson

1️⃣ Codable — Turning JSON Into Swift Types

APIs speak JSON — plain text like {' "id": 1, "title": "Hello" '} . To use it, you describe its shape with a model (a struct ) and mark it Codable . That one word lets JSONDecoder read the text and fill in your struct automatically — every property name must match a JSON key, with the matching type. Read this worked example and run it; it needs no network.

2️⃣ URLSession + async/await — Fetching Over the Network

URLSession is Swift's built-in networking tool. The modern way to use it is async/await : you write try await URLSession.shared.data(from: url) and the line pauses until the bytes arrive, then carries on — reading top-to-bottom like ordinary code, with no nested callbacks. URL(string:) returns an Optional, so unwrap it safely, and remember URLSession only throws for transport errors — you check the HTTP status yourself.

Your turn. Below, the JSON and the calls are written for you — fill in the four ___ blanks marked with // 👉 to make the model and the decode work.

3️⃣ Error Handling — do / try / catch

Networking fails all the time — no signal, a typo in a URL, a server hiccup. Swift makes that explicit: a function that can fail is marked throws , you call it with try , and you wrap the call in do {' … '} catch {' … '} so a failure becomes a message instead of a crash. Inside catch , the thing that went wrong is available as error .

Now you try. This function is missing the three concurrency/error keywords. Fill in the three ___ blanks so it suspends, fetches, and handles failure.

4️⃣ Using the Data in SwiftUI with .task

A view that shows live data stores it in @State and loads it when it appears. The .task {' … '} modifier runs async code as the view shows up and cancels it automatically if the view disappears — so prefer it over .onAppear for loading. Code inside .task already runs on the main actor, so assigning to @State there updates the UI safely.

Sequential await calls wait for each other. When requests are independent, async let starts them all at once so the screen loads faster:

For snake_case APIs, skip hand-written CodingKeys with one line: decoder.keyDecodingStrategy = .convertFromSnakeCase maps first_name to firstName for the whole response.

📋 Quick Reference — Networking & JSON

No blanks this time — just a brief and an outline. Put together everything from this lesson: a Codable model, an async throws fetch, and a do/catch . Build it in Xcode and check your output against the expected line in the comments.

This was the final Swift lesson. Here's what you can now do:

Where to go next: wire this into a real app idea. Persist data with SwiftData or Core Data, add pull-to-refresh and loading states, structure logic into an MVVM view model, then publish to TestFlight . Practise by building a weather, news, or GitHub-search app against a free public API — you now have every piece you need.

Practice quiz

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