async def _wait(self, entity_type, entity_id, action, predicate=None):
        """
        Block the calling routine until a given action has happened to the
        given entity

        :param entity_type: The entity's type.
        :param entity_id: The entity's id.
        :param action: the type of action (e.g., 'add', 'change', or 'remove')
        :param predicate: optional callable that must take as an
            argument a delta, and must return a boolean, indicating
            whether the delta contains the specific action we're looking
            for. For example, you might check to see whether a 'change'
            has a 'completed' status. See the _Observer class for details.

        """
        q = asyncio.Queue(loop=self._connector.loop)

        async def callback(delta, old, new, model):
            await q.put(delta.get_id())

        self.add_observer(callback, entity_type, action, entity_id, predicate)
        entity_id = await q.get()
        # object might not be in the entity_map if we were waiting for a
        # 'remove' action
        return self.state._live_entity_map(entity_type).get(entity_id)