Hm, I'm not sold on it but it's your call. (It's not just a required callback, but a required async callback, which isn't a familiar pattern.) So that means it would be called before the state is set and have to update state?

Another thing I'm thinking is that maybe these callbacks should be named something different (prefixed with an underscore?) or we should register a wrapper object instead of the component itself because if we define them in the mixin, users can't add them to their own specs.