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Hey everyone,

I'm encountering a strange issue with URLSession in my Swift app. I've implemented the URLSessionDelegate, specifically urlSession(_:dataTask:didReceive:) and urlSession(_:task:didCompleteWithError:), but they don't seem to be getting called at all when I start a data task.

Here's a simplified version of how I'm setting up the session and task:

let configuration = URLSessionConfiguration.default
                                    let session = URLSession(configuration: configuration, delegate: self, delegateQueue: nil)
                                    let dataTask = session.dataTask(with: url) { data, response, error in
                                        // This completion handler is called, but delegate methods are not.
                                        if let error = error {
                                            print("Completion handler error: \(error)")
                                        } else if let data = data {
                                            print("Completion handler received data.")
                                        }
                                    }
                                    dataTask.resume()

I'm using a background session, but I've tried default and ephemeral configurations as well. I've checked that self is correctly set as the delegate. What am I missing?

Hi Jane,

This is a common pitfall! If you provide a completion handler to dataTask(with:completionHandler:), it will often execute the completion handler *instead* of calling the delegate methods for data delivery, especially for simple tasks. The completion handler is essentially a shortcut that wraps the delegate calls.

Try using the initializer that *doesn't* take a completion handler:

let dataTask = session.dataTask(with: url)
                                    dataTask.resume()

And then rely solely on your delegate methods like urlSession(_:dataTask:didReceive:) and urlSession(_:task:didCompleteWithError:). Let me know if that resolves it!

Sam's right. Also, make sure your delegate object (self in your case) has a sufficient lifespan. If the delegate object is deallocated before the session finishes, you'll miss delegate callbacks. For background sessions, you need to be particularly careful about managing the lifecycle of your delegate.

Are you sure the task is actually completing? Sometimes errors can occur early, and the delegate methods might not be called if the task is invalidated or fails in a way that bypasses standard callbacks.

You both are lifesavers! Sam, removing the completion handler fixed it immediately. The delegate methods are now firing as expected. I was so focused on the delegate implementation itself that I overlooked the interaction with the task's completion handler.

Alex, thanks for the reminder about object lifecycles. I'm managing the session in a singleton, so deallocation shouldn't be an issue, but it's good practice to keep in mind.

Closing this thread as solved!