@classmethod
def subprocess_handler(cls, package, sleep_time, timeout, retry):
    """Handler function for subprocesses.
        :param package: task package.
        :param sleep_time: max sleep time after each download.
        :param timeout: timeout.
        :param retry: retry count.
        :rtype: MultiprocessingSpider.packages.ResultPackage or its subclass or None.
        """
    raise NotImplementedError
