urlSession(_:dataTask:didReceive:) not called when using completion handler-based dataTask(w

Description: I'm noticing that when using the completion handler variant of URLSession.dataTask(with:), the delegate method urlSession(_:dataTask:didReceive:) is not called—even though a delegate is set when creating the session.

Here's a minimal reproducible example:

✅ Case where delegate method is called:

class CustomSessionDelegate: NSObject, URLSessionDataDelegate {
    func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive data: Data) {
        print("✅ Delegate method called: Data received")
    }
}

let delegate = CustomSessionDelegate()
let session = URLSession(configuration: .default, delegate: delegate, delegateQueue: nil)

let request = URLRequest(url: URL(string: "https://httpbin.org/get")!)
let task = session.dataTask(with: request) // ✅ No completion handler
task.resume()

In this case, the delegate method didReceive is called as expected.

❌ Case where delegate method is NOT called:

class CustomSessionDelegate: NSObject, URLSessionDataDelegate {
    func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive data: Data) {
        print("❌ Delegate method NOT called")
    }
}

let delegate = CustomSessionDelegate()
let session = URLSession(configuration: .default, delegate: delegate, delegateQueue: nil)

let request = URLRequest(url: URL(string: "https://httpbin.org/get")!)
let task = session.dataTask(with: request) { data, response, error in
    print("Completion handler called")
}
task.resume()

Here, the completion handler is executed, but the delegate method didReceive is never called.

Notes: I’ve verified this behavior on iOS 16, 17, and 18.

Other delegate methods such as urlSession(_:task:didFinishCollecting:) do get called with the completion handler API.

This happens regardless of whether swizzling or instrumentation is involved — the issue is reproducible even with direct method implementations.

Questions:

1. Is this the expected behavior (i.e., delegate methods like didReceive are skipped when a completion handler is used)?
2. If yes, is there any official documentation that explains this?
3. Is there a recommended way to ensure delegate methods are invoked, even when using completion handler APIs?

Thanks in advance!