curl to Swift

What this tool is used for

• Converting an API's curl example into URLSession code for an app.
• Reproducing a captured request inside an existing Swift client.
• Working out where App Transport Security will object before the app is built.
• Producing a snippet for a playground or a small tool.
• Choosing between the async and completion-handler forms.

Frequently Asked Questions

Why does my http:// URL fail on iOS?

App Transport Security blocks cleartext HTTP by default. curl has no such restriction. You must use https, or add an NSAppTransportSecurity exception in Info.plist — which App Review scrutinises, so treat it as a last resort.

How do I use async/await?

let (data, response) = try await URLSession.shared.data(for: request). The older completion-handler dataTask(with:) form still works and is required below iOS 15, but you must remember to call .resume() — a task that is never resumed silently does nothing.

How do I read the status code?

Cast the URLResponse: guard let http = response as? HTTPURLResponse. URLSession does NOT throw on a 404 — only on transport errors — so without this check a failed request looks successful.

How do I send a JSON body?

request.httpBody = try JSONEncoder().encode(payload) and set request.setValue("application/json", forHTTPHeaderField: "Content-Type"). Also set request.httpMethod = "POST" — the default is GET, and a GET with a body is silently dropped.

How do I set a timeout?

request.timeoutInterval for a single request, or timeoutIntervalForRequest and timeoutIntervalForResource on a URLSessionConfiguration. The default request timeout is 60 seconds.

What runs on the main actor?

URLSession's async methods return on a background context, so any UI update afterwards must hop to the main actor — marking the method `@MainActor` or wrapping the update. Swift concurrency makes this a compile-time error rather than a runtime glitch.

How are background transfers handled?

With a background `URLSessionConfiguration`, which hands the transfer to the system so it continues after the app is suspended. It requires the delegate API rather than async/await, and the app is relaunched to receive the completion.

Common errors and gotchas

• Forgetting App Transport Security blocks plain HTTP by default, so a working curl fails in the app.
• Setting a body on a GET, which URLSession may drop silently.
• Assuming a non-2xx surfaces as an error, when you must cast the response and check the status.
• Using a shared session for requests that need different caching or cookie behaviour.
• Retaining a completion handler in a way that creates a cycle.