dispatch-sync errors, even when called from an effect handler

[bendlas]

This should be a valid pattern:

(re-frame/reg-fx
  :fx-1
  (fn [_] (re-frame/dispatch-sync [:ev-1])))

but with re-frame 0.10.3-rc2, it gives me

You can't call dispatch-sync within an event handler.

ref #42

[danielcompton]

Is this a new error in that version of re-frame, or has it existed in the past? I'm still waking up from my coffee, but I'm not sure why that would be a valid pattern? An fx is always going to be run in the context of an event being handled, so dispatch-sync isn't safe there.

[bendlas]

I just ran into it and haven't tested it on older versions. I presume it has been an error case before.
The reason I'd consider it a valid pattern derives from the documentation (https://github.com/Day8/re-frame/wiki/FAQ#2-why-cant-i-call-dispatch-sync-in-an-event-handler). The reason it gives is, that the database updates from the event handler, would override the ones from dispatch-sync.

reg-fx doesn't have that problem: It's for capturing the side-effect "off-shoots" from the event loop, after database updates have already been applied (I think, and even if they haven't, dispatch-sync from an effect handler could be run against the intermediate db). So one could reasonably expect for effects to be able to run dispatch-sync.

Does that make sense?

[bendlas]

OK, i see. The issue is, that this would require ordering the :db effect before other effects, which isn't something that re-frame does, right?

FWIW, for my use case, (js/setTimeout #(re-frame/dispatch-sync ...) 0) works, because I just need it to break a .pushState loop. I'll leave it to you, whether this should be supported by re-frame.

[danielcompton]

Thanks, will probably close it for now, btw there is also ^:flush-dom metadata you can attach to your event which may be helpful.