on-value callback for on-value subscriptions

[pbzdyl]

I found it useful to have ability to specify my own on-value callback in on-value subscriptions so I can plug in my event handler to sync some Firebase path with app-db. This way I can have an easy access to some data in other event handlers that otherwise don't have access to data from subscriptions directly.

[deg]

I don't quite see why this is needed.

Can you give me an example where this works better than just subscribing to the :on-value from somewhere and dispatching the event from there?

[pbzdyl]

I have one example in my sample app.

How would I dispatch an event whenever value in the subscription is changed?

[deg]

Ah, now I understand what you are saying. Basically, this is a special-case workaround of a larger problem of how to get the result of a subscription in an event.

It looks like this has been actively discussed for the past few months. See:
day8/re-frame#255. The discussion is summarized nicely in https://github.com/Day8/re-frame/blob/master/docs/FAQs/UseASubscriptionInAnEventHandler.md and the short answer is to use inject-sub from re-frame-utils.

While your solution is elegant, it will introduce syntax into re-frame-firebase that may need to be replaced by a more general solution soon. I'm not comfortable doing this now.

I don't yet have enough experience using this library, and I'd like to keep it lean, until I understand more about how we use it.

But, we can leave this PR open. I'm happy to revisit if you find an example that needs this and can't be solved in the more general way.

[pbzdyl]

I have migrated to the solution with injecting subscription and it works thus I am closing this PR. Thank you!