URLSession Delegate Callback Order After Cancellation or Timeout

Callback Order and Behavior

In a delegate-based URLSessionDataTask, urlSession(_:task:didCompleteWithError:) is the terminal callback of the task lifecycle. It is guaranteed to be the final method called for a given task, occurring after all urlSession(_:task:didReceive:) callbacks have finished executing.

Delegate-Based Cancellation

When a task is cancelled via task.cancel(), the following sequence occurs:

• Buffered Data: Any data already received by the underlying CFNetwork stack but not yet delivered to the delegate may still be dispatched via didReceive data callbacks.
• Terminal Signal: Once all pending data delivery is complete, the session invokes didCompleteWithError: with a URLError.cancelled code.

The completion callback will not be invoked before final data callbacks; it serves as the definitive signal that no further data will be delivered for that task.

Async/Await Behavior (Swift 5.5+)

The behavior of the async data(for:) method differs fundamentally from the delegate pattern because it is designed as an atomic operation rather than a stream.

• All-or-Nothing: data(for:) does not return partial data. It either returns the complete Data object upon successful completion or throws an error.
• Cancellation: If the surrounding Swift Task is cancelled while data(for:) is awaiting a response, the method will throw URLError.cancelled immediately. It will not return any partially received data.

Verification Steps

To verify this behavior in your specific environment (assuming iOS 15+ or macOS 12+), you can use the following scoped tests:

1. Delegate Test: Implement URLSessionDataDelegate and add print statements with timestamps to both didReceive data and didCompleteWithError. Call task.cancel() during a large download to confirm the completion handler always prints last.
2. Async Test: Wrap let (data, response) = try await session.data(for: request) in a Task, call task.cancel() after a short delay, and verify that the catch block captures the cancellation error without any data being returned.

Diagnostic Note: Are you using a URLSessionConfiguration.background session? Background sessions handle delegate callbacks differently (often via the app delegate) and may exhibit different timing regarding when the system wakes the app to deliver the final completion error.