Networking with URLSession & async/await

Fetch real data from the web. You'll call URLSession.shared.data(from:) with async/await , decode JSON into Swift types with Codable , check HTTP status codes, and handle errors cleanly.

Learn Networking with URLSession & async/await in our free Swift course — a beginner-friendly interactive lesson with worked examples, a practice exercise…

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

• • Fetch data with URLSession.shared.data(from:)
• • Decode JSON into models using Codable and JSONDecoder
• • Read HTTP status codes via HTTPURLResponse
• • Treat 2xx as success and reject other ranges
• • Handle URLError and DecodingError separately
• • Write clean async throws fetching functions

📊 The Networking Pieces

Piece

Role

URL

Where to fetch from

data(from:)

Async fetch, returns (Data, URLResponse)

HTTPURLResponse

Exposes the statusCode

Codable

Model that maps to JSON

JSONDecoder

Turns Data into your model

1️⃣ Fetch and Decode

The core flow is three lines: build a URL , await the data, and decode it into a Codable model whose properties match the JSON keys.

Your turn. Fill in the protocol, the await keyword, and the decoder type.

2️⃣ Check the Status Code

A 404 or 500 still returns Data rather than throwing, so always cast to HTTPURLResponse and confirm the code is in the 200..<300 range before trusting the body.

3️⃣ Handle Errors

Networking can fail in two ways: the request itself ( URLError ) or the parsing ( DecodingError ). Catch them separately so you can show the right message.

Now you try. Fill in the error-handling keywords.

Pro Tips

• 💡 Always check the status code. A non-2xx response still returns bytes — don't assume success.
• 💡 Match JSON keys exactly, or use CodingKeys / keyDecodingStrategy to bridge naming.
• 💡 Keep fetching off the main thread. await frees the thread automatically.
• 💡 Surface errors to the user, distinguishing "no connection" from "bad data".

Common Errors (and the fix)

• "keyNotFound" DecodingError — a JSON key doesn't match your property; add CodingKeys or fix the name.
• "typeMismatch" DecodingError — e.g. a number arrived as a String; align the property type.
• Treating a 404 as success — you skipped the status check; cast to HTTPURLResponse .
• "Call can throw but is not marked with 'try'" — add try before await on the request and the decode.

📋 Quick Reference

Step

Code

Result

Build URL

URL(string: "…")!

an endpoint

Fetch

try await …data(from:url)

(Data,Response)

Status

resp as? HTTPURLResponse

statusCode

Decode

JSONDecoder().decode(T.self,…)

a model

Errors

do { try … } catch { … }

handled

Frequently Asked Questions

Mini-Challenge: Fetch and Validate

Write a full fetch that checks the status code and decodes a Todo, then report whether it's completed.

🎉 Lesson Complete!

• ✅ URLSession.shared.data(from:) fetches with async/await
• ✅ It returns (Data, URLResponse) and can throw
• ✅ Decode with JSONDecoder into a Codable model
• ✅ Cast to HTTPURLResponse and check the 2xx range
• ✅ Handle URLError and DecodingError separately
• ✅ Next lesson: Persistence with Core Data

Practice quiz

Continue this course

• Previous: Advanced Concurrency
• Next: Persistence with Core Data