Is it true that there is no way at all to subclass URLSessionDataTask, and if so is there a way to add an associated property?

I've always wanted to do something like

class URLSessionDataTask_Plus: URLSessionDataTask {
    
    var obs: NSKeyValueObservation? = nil
    // store the progress observer there 
}

which would be very convenient. I don't know how to do it. If instead you do something like

///URLSessionDataTask, plus more!
class URLSessionDataTask_Plus {
    
    var task: URLSessionDataTask? = nil
    var obs: NSKeyValueObservation? = nil
}

there's really no advantage, as you anyway have to still maintain that variable externally (or maintain that whole object externally).

Progress observations are a real nuisance to hang on to as you never know how many you'll need or where.

Is there some way to achieve this concept? ->

class URLSessionDataTask_Plus: URLSessionDataTask {
    
    var obs: NSKeyValueObservation? = nil
    // store the progress observer there 
}

(I note that the same applies even if you use the futuristic async calls .. https://stackoverflow.com/a/78804162/294884 )

Solution

FTR, I've been trying out simply adding an objc_getAssociatedObject.

public var ass_observer: NSKeyValueObservation?

it does seem to work well
as far as I can determine, I cough think it has no leaks

Outline:

extension URLSession {
    
    ///Timed data task. Basically a super-data-task which autonomously and
    ///invisibly does progress, timers, and any other features we need at this
    ///low level.
    static func tDataTask(
        with request: URLRequest,
        completionHandler: @escaping @Sendable (Data?, URLResponse?, (any Error)?) -> Void
    ) {
        var _firstbyte = true // (NB is workaround for poor behavior of .progress(
        
        LoggingEtc("Waiting for first byte")

        let t = URLSession.shared.dataTask(with: request) { (d, r, e) in
            LoggingEtc("All data arrived")
            completionHandler(d, r, e)
        }

        // Setup logging, timers, etc
        // Use t.taskIdentifier as needed for uniqueness
        
        t.ass_observer = t.progress.observe(\.fractionCompleted) { progress, _ in
            if progress.fractionCompleted < 0.02 { return }
            if _firstbyte {
                LoggingEtc("First byte arrived")
                _firstbyte = false
            }
            ProgressEtc()
        }
        
        t.resume()
    }
}

and ...

fileprivate var _key_to_ass_observer: UInt8 = 0
extension URLSessionDataTask {
    public var ass_observer: NSKeyValueObservation? {
        get {
            return objc_getAssociatedObject(self, &_key_to_ass_observer) as? NSKeyValueObservation ?? nil
        }
        set(newValue) {
            objc_setAssociatedObject(self, &_key_to_ass_observer, newValue, .OBJC_ASSOCIATION_RETAIN)
        }
    }
}

Call thus from your API singleton - fire and forget, nothing to retain, etc.

URLSession.tDataTask(with: request) { (data, response, error) in
    
    .. 1, error handling
    
    .. 2, parsing
    
    .. 3, update databases, convert bitcoin, whatever
    
    .. 4, report to whoever called you as relevant
    DispatchQueue.main.async { completionOnMain?(200, []) }
}