Registering custom effect handlers in re-om

You can register custom coeffect handlers using re-om's register-fx! function.

register-fx! receives two parameters, the first one is the effect identifier and the second one is the effect handler.

An effect handler is a function that performs a side-effect described by an effect. Remember that effects are descriptions of side-effects, so effect handlers are interpreters of those descriptions that will know how to perform the described side-effects. The effect handler receives only one parameter: the description of the side-effect to be performed, i. e., the effect.

Example:

(re-om/register-fx!
  :om/state
  (fn [[owner mutations]]
    (doseq [[kw value] mutations]
      (om/set-state! owner kw value))

In this example we're registering a :om/state effect which can be used to mutate values of the local state of an Om control using the keywords those values are associated to. This effect allows you to change an Om control local state from pure event handlers as shown in the following code snippet:

(re-om/register-event-handler!
  ::update-state-on-click
  [(re-om/inject-cofx :om/state owner [:selected :expanded])]
  (fn [{{:keys [selected expanded]} :om/state} [owner]]
    {:om/state [owner
                {:selected (not selected)
                 :expanded (not expanded)}]}))

In this example, the event handler for the ::update-state-on-click event returns an effects map containing the om/state effect which describes a mutation of the values associated to the :selected and :expanded keywords in the state of an Om control referenced by the owner parameter.

Notice how the value associated to the om/state effect in the effects map is a vector whose elements are the owner and a map that associates the keywords that will change in the control's state to their new values.

Notice also that the previous values of those two keywords were retrieved using a custom coeffect also identified by :om/state, If you want to know how to register custom coeffect handlers, have a look at Registering custom coeffect handlers in re-om.