Allow subscribe to be safely called from any context

[dannyfreeman]

The warn-when-not-reactive warnings are no longer valid if we bypass
the subscription cache when not in a reagent component, so they've been
removed.

This should address issue #753

This change probably requires some documentation cleanup

[mbertheau]

I love this. It seems to be an easy way – API-wise – to make values from the subscription calculation available in event and effect handlers.

What seems a bit problematic, or at least questionable (in the literal sense), is: what db state is used? In event handlers and possibly effect handlers you work with data passed into the function. subscribe uses a potentially different version of the app-db.

It seems to me that the API that one would want in that situation is (get-subscription-value db [:subscription parameters]), i.e. you pass in the db that should be used.

[dannyfreeman]

There is only one db state in existence at a time, it will not be changed out from under running code in a single threaded environment. If you call a subscription in an event then the db value will be the same as the db being used in the event.

If we want something like get-subscription-value then a subscription can just be extracted into a function. Writing a generic utility for this would involve re-implementing subscriptions without de-referencing the global app-db. I've done something similar with re-frame to allow atoms other than app-db to be used. It was a lot of work. That is not something i want to waste my time on again because the re-frame maintainers don't seem interested in large patches like that.

[mbertheau]

There is only one db state in existence at a time, it will not be changed out from under running code in a single threaded environment. If you call a subscription in an event then the db value will be the same as the db being used in the event.

Yeah. What I mean is that you might want to get the result of a calculation captured in a subscription based on the db that you're currently building a new version of in the event handler.

Some pseudo code demonstrating the mechanics I have in mind, not an actual use case:

(reg-sub :count-foo [db] (-> db :foos count))

(reg-event-handler :button-clicked [db new-foo]
  (let [db (-> db (update :foos conj new-foo)]
    (if (> 10 (get-sub-value db :count-foo))
      (assoc db :big? true)
      db)))

If we want something like get-subscription-value then a subscription can just be extracted into a function.

Well, not "just" because you have do extract all input subscriptions as well and compose everything together, and that might not be trivial, and/or impose significant boilerplate code when all subscriptions are written in a way that allows for that.

Writing a generic utility for this would involve re-implementing subscriptions without de-referencing the global app-db. I've done something similar with re-frame to allow atoms other than app-db to be used. It was a lot of work. That is not something i want to waste my time on again because the re-frame maintainers don't seem interested in large patches like that.

Sounds like a challenge! :)