def get_field_callback(self, field, event):
        # type: (str, str) -> Optional[Tuple[Callable, bool]]
        """
        Retrieves the registered method for the given event. Returns None if
        not found

        :param field: Name of the dependency field
        :param event: A component life cycle event
        :return: A 2-tuple containing the callback associated to the given
                 event and flag indicating if the callback must be called in
                 valid state only
        """
        try:
            return self.factory_context.field_callbacks[field][event]
        except KeyError:
            return None
Retrieves the registered method for the given event. Returns None if
        not found

        :param field: Name of the dependency field
        :param event: A component life cycle event
        :return: A 2-tuple containing the callback associated to the given
                 event and flag indicating if the callback must be called in
                 valid state only