def callbacks(cls, eventType=None):
        """
        Returns a list of callback methods that can be invoked whenever an event is processed.

        :return: {subclass of <Event>: <list>, ..}
        """
        key = '_{0}__callbacks'.format(cls.__name__)
        try:
            callbacks = getattr(cls, key)
        except AttributeError:
            callbacks = {}
            setattr(cls, key, callbacks)

        return callbacks.get(eventType, []) if eventType is not None else callbacks