class Listener4Assist implements ListenerAssist {

    public synchronized void taskEnd(DownloadTask task, EndCause cause,
                                     @Nullable Exception realCause) {
        Listener4Model model = modelHandler.removeOrCreate(task, task.getInfo());
        if (assistExtend != null
                && assistExtend.dispatchTaskEnd(task, cause, realCause, model)) {
            return;
        }

        if (callback != null) callback.taskEnd(task, cause, realCause, model);
    }

      Listener4Assist(ListenerModelHandler<T> handler);
    public  Listener4Assist(ListenerModelHandler.ModelCreator<T> creator);

}