private static let sharedQueue = DispatchQueue(label: "my-label", qos: .background)

private static let sharedURLSession: URLSession = {
    var queue = OperationQueue()
    queue.maxConcurrentOperationCount = 1
    //queue.qualityOfService = .background
    queue.underlyingQueue = sharedQueue
    return URLSession(configuration: .ephemeral, delegate: nil, delegateQueue: queue)
}()

let dataTask = sharedURLSession.dataTask(with: request) { [weak self] (dataOpt, response, error) in
    do {
        assert(Thread.current.qualityOfService == .background, "This should run in background QoS thread!")
        ...
    } catch {
        ...
    }
}