Writing re-frame events that do not change app-db

Question

There are certain events that do not result in app-db changing. They only change the dom, e.g: init a custom scroll, getting the selected text, etc. How should I deal with them in re-frame, since the event handler requires to return a new app-db? I am getting around by returning the existing db, but it does not seem right. Is there a better way to do it? A few of my handlers look like this:

 (re-frame/reg-event-db
    :init-link-viewer
    (fn [db [_ highlights]]
      (utils/load-highlights highlights)
      (utils/init-selection)
      db))

I think you could look at https://github.com/Day8/re-frame/blob/2ccd95c397927c877fd184b038e4c754221a502d/docs/Effects.md — Michiel Borkent, Oct 19 '16 at 08:17
Thanks. This was useful. I should have looked it before posting the question. — Chin365, Oct 19 '16 at 13:13

Daniel Compton · Accepted Answer · 2016-10-19T21:08:26.443

7

You can use the reg-event-fx function to register an effect handler which returns an effects map (as opposed to reg-event-db which only returns db). Your effects map can be empty, and doesn't need to return a db. See Effects for more info on this.

You could rewrite your event as:

(reg-event-fx
  :init-link-viewer
  (fn [db [_ highlights]]
    (utils/load-highlights highlights)
    (utils/init-selection)
    {}))

However you may want to take this further, and return your side effects as data. This means that your event handlers are easily testable, and decouples the event from it's side effects. This will mean that you need to write and register effects handlers as well. This could look something like:

(reg-event-fx
  :init-link-viewer
  (fn [db [_ highlights]]
    {:load-highlights highlights
     :init-selection true}))

edited Oct 19 '16 at 21:08

answered Oct 19 '16 at 20:55

Daniel Compton

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If I return data from the handler, can I ensure that `load-highlights` is called first then `init-selection`? I am aware of the fact that event handlers do not ensure any order of effects. – Chin365 Oct 20 '16 at 06:05
Your second sentence answers your first :) You would need to do this with callbacks or some other mechanism. – Daniel Compton Oct 20 '16 at 19:25
I think the `db` in the `reg-event-fx` functions above should be `cofx` but all works as described because it's not used. – Phil Cooper Jun 29 '21 at 15:39

Writing re-frame events that do not change app-db

1 Answers1