Register a default producer for events to use.
        :param producer: the default producer to to dispatch events on.

def register_producer(cls, producer):
        """
        Register a default producer for events to use.
        :param producer: the default producer to to dispatch events on.
        """
        log.info('@Registry.register_producer `{}`'
                 .format(producer.__class__.__name__))
        cls._producer = (cls._producer or producer)