Working with URLSession

There are many considerations for the networking stack in your application. Given the breadth of these considerations, we didn't want a one-size-fits-all route for doing networking in Deferred. For instance, using URLSessionDelegate disables the convenience callback methods like URLSession.dataTask(with:completionHandler:) that would be used by a built-in solution to Deferred.

That said, the new Task module to Deferred provides a great opportunity for tying together the two parts.

Wrapping Completion Handlers

extension NSURLSession {

    /// Returns the data from the contents of a URL, based on `request`, as a
    /// future.
    ///
    /// The session bypasses delegate methods for result delivery. Delegate
    /// methods for handling authentication challenges are still called.
    ///
    /// - parameter request: Object that provides the URL, body data, and so on.
    /// - parameter configure: An optional callback for setting properties on
    ///   the URL task.
    func beginDataTask(with request: NSURLRequest) rethrows -> Task<(NSData, NSURLResponse)> {
        let deferred = Deferred<Task<(NSData, NSURLResponse)>.Result>()
        let task = dataTaskWithRequest(request) { (data, response, error) in
            if let error = error {
                deferred.fail(error)
            } else if let data = data, let response = response {
                deferred.succeed((data, response))
            } else {
                deferred.fail(NSURLError.BadServerResponse)
            }
        }
        defer { task.resume() }
        return Task(deferred, cancellation: task.cancel)
    }

}

Alternatives Considered

This approach notably doesn't include progress reporting or any other advanced use of URLSessionTask.

URLSessionTask instances can be observed in isolation, without their completion callbacks, using Key-Value Observing.

URLSessionDelegate

A vital lesson for maintaining a class that implements URLSessionDelegate is that URLSession instances are self-contained. The taskIdentifier property is unique for tasks in a session, and thus are useful for tracking constituent tasks.

A full-blown solution would be integrated into a network manager class that conforms to URLSessionDelegate. Consider the pseudocode below.

final class MyURLClient: NSObject, URLSessionTaskDelegate {

    private final class TaskState: NSObject {
        ...
    }

    private let urlSession: URLSession
    private var tasks = [Int: TaskState]()

        func URLSession(session: URLSession, didBecomeInvalidWithError error: NSError?) {
        tasks.removeAll()
    }

    func URLSession(session: URLSession, task: URLSessionTask, didCompleteWithError error: NSError?) {
        guard let state = tasks.removeValue(forKey: task.taskIdentifier) else { return }

        // use `state`
    }

    func URLSession(session: URLSession, dataTask task: NSURLSessionDataTask, didReceive response: URLResponse, completionHandler: URLSessionResponseDisposition -> Void) {
        state[task.taskIdentifier] = TaskState(...)

        completionHandler(.Allow)
    }


}