How to use URLSession's Progress

[piercifani]

Maybe this is more of a Progress question instead of Deferred's, but here it is anyways:

Is there a way to pass the progress of a URLSessionTask as the progress to create a Task?

Executing the following code launches an exception:

extension URLSession {
    public func fetchData(with urlRequest: URLRequest) -> Task<Data> {
        let deferred = Deferred<Task<Data>.Result>()
        let task = self.dataTask(with: urlRequest) { (data, response, error) in
             ...
        }
        task.resume()
        if #available(iOS 11.0, *) {
            return Task(deferred, progress: task.progress)
        } else {
            return Task(deferred, cancellation: { [weak task] in
                task?.cancel()
            })
        }
    }

2018-05-09 13:08:35.030826+0200 xctest[74737:3872356] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[NSProgress addChild:withPendingUnitCount:]: NSProgress 0x7fbc49824650 was already the child of another progress 0x7fbc49915bc0'

[zwaldowski]

Hm, that’s exactly how we’re doing it in several projects. I’ll look into it, thanks as always! Can you provide any additional information about how you’re calling the code you pasted, like through andThen?

[piercifani]

Sure! here's how the Tasks are chained.

This PR doesn't use the URLSession's progress for the Task creation because it was crashing with that exception. If you want to reproduce it locally, you could clone the repo and hook the task.progress here and then Command+U

[zwaldowski]

Adding a test case for this, seems legitimate.

[piercifani]

Thanks a lot @zwaldowski !!!