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About 10% of the time, event handlers need to cause side effects.

This tutorial explains how side effects are actioned, how you can create your own side effects, and how you can make side effects a noop in event replays.

Where Effects Come From

When an event handler is registered via reg-event-fx, it must return effects.

Like this:

(reg-event-fx              ;; -fx registration, not -db registration
  (fn [cofx [_ a]]        ;; 1st argument is coeffects, instead of db
    {:db       (assoc (:db cofx) :flag  a)
     :dispatch [:do-something-else 3]}))   ;; return effects

-fx handlers return a description of the side-effects required, and that description is a map.

The Effects Map

An effects map contains instructions.

Each key/value pair in the map is one instruction - the key uniquely identifies the particular side effect required, and the value for that key provides further data. The type of value depends on the specific side-effect.

Here's the two instructions from the example above:

{:db       (assoc db :flag  a)         ;; side effect on app-db
 :dispatch [:do-something-else 3]}     ;; dispatch this event

The :db key instructs that "app-db" should be reset! to the value supplied.

And the :dispatch key instructs that an event should be dispatched. The value is the vector to dispatch.

There are many other possible effects, like for example :dispatch-later, dispatch-n, :set-local-store, etc.

And so on. And so on. Which brings us to a problem.

Infinite Effects

While re-frame supplies a number of built-in effect handlers, the set of possible effects is open ended.

What if you use PostgreSQL and want an effect which issues mutating queries? Or what if you want to send logs to Logentries or metrics to DataDog. Or write values to windows.location. And what happens if your database is X, Y or Z?

The list of effects is long and varied, with everyone needing to use a different combination.

So effect handling has to be extensible. You need a way to define your own side effects.

Extensible Side Effects

re-frame provides a function reg-fx through which you can register your own Effect Handlers.

Use it like this:

(reg-fx         ;; <-- registration function
   :butterfly   ;;  <1>
   (fn [value]  ;;  <2>

<1> the key for the effect. When later an effects map contains the key :butterfly, the function we are registering will be used to action it.

<2> the function which actions the side effect. Later, it will be called with one argument - the value in the effects map, for this key.

So, if an event handler returned these two effects:

{:dispatch   [:save-maiden 42]
 :butterfly  "Flapping"}         ;; butterfly effect, but no chaos !!

Then the function we registered for :butterfly would be called to handle that effect. And it would be called with the parameter "Flapping".

So, terminology:

  • :butterfly is an "effect key"
  • and the function registered is an "effect handler".

So re-frame has both event handlers and effect handlers and they are different, despite them both starting with e and ending in t!!

Writing An Effect Handler

A word of advice - make them as simple as possible, and then simplify them further. You don't want them containing any fancy logic.

Why? Well, because they are all side-effecty they will be a pain to test rigorously. And the combination of fancy logic and limited testing always ends in tears. If not now, later.

A second word of advice - when you create an effect handler, you also have to design (and document!) the structure of the value expected.

When you do, realise that you are designing a nano DSL for value and try to make that design simple too. If you resist being terse and smart, and instead, favor slightly verbose and obvious, your future self will thank you. Create as little cognitive overhead as possible for the eventual readers of your effectful code.

This advice coming from the guy who named effects fx ... Oh, the hypocrisy.

In my defence, here's the built-in effect handler for :db:

  (fn [value]
    (reset! re-frame.db/app-db value)))

So, yeah, simple ... and, because of it, I can almost guarantee there's no bug in ... bang, crash, smoke, flames.

Note: the return value of an effect handler is ignored.

:db Not Always Needed

An effects map does not need to include the effect key :db.

It is perfectly valid for an event handler to not change app-db.

In fact, it is perfectly valid for an event handler to return an effects map of {}. Slightly puzzling, but not a problem.

What Makes This Work?

A silently inserted interceptor.

Whenever you register an event handler via either reg-event-db or reg-event-fx, an interceptor, cunningly named do-fx, is inserted at the beginning of the chain.

Example: if your event handler registration looked like this:

  [debug (path :right)]     ;; <-- two interceptors, apparently
  (fn [cofx _]
     {})                    ;; <-- imagine returned effects here

While it might look like you have registered with 2 interceptors, reg-event-fx will make it 3:

[do-fx debug (path :right)]

It silently inserts do-fx at the front, and this is a good thing.

The placement of do-fx at the beginning of the interceptor chain means its :after function would be the final act when the chain is executed (forwards and then backwards, as described in the Interceptor Tutorial).

In this final act, the :after function extracts :effects from context and simply iterates across the key/value pairs it contains, calling the registered "effect handlers" for each.

For the record, the FISA Court requires that we deny all claims that do-fx is secretly injected NSA surveillance-ware.
We also note that you've been particularly sloppy with your personal grooming today, including that you forgot to clean your teeth. Again.

If ever you want to take control of the way effect handling is done, create your own alternative to reg-event-fx and, in it, inject your own version of the do-fx interceptor at the front of the interceptor chain. It is only a few lines of code.

Order Of Effects?

There isn't one.

do-fx does not currently provide you with control over the order in which side effects occur. The :db side effect might happen before :dispatch, or not. You can't rely on it.

Note: if you feel you need ordering, then please open an issue and explain the usecase. The current absence of good usecases is the reason ordering isn't implemented. So give us a usercase and we'll revisit, maybe.

Further Note: if later ordering was needed, it might be handled via metadata on :effects. Also, perhaps by allowing reg-fx to optionally take two functions:

  • an effects pre-process fn <-- new. Takes :effects returns :effects
  • the effects handler (as already described above).

Anyway, these are all just possibilities. But not needed or implemented yet.

Effects With No Data

Some effects have no associated data:

  (fn [coeffect _]
     {:exit-fullscreen nil}))    ;;   <--- no data, use a nil

In these cases, although it looks odd, just supply nil as the value for this key.

The associated effect handler would look like:

  (fn [_]             ;; we don't bother with that nil value
     (.exitFullscreen js/document)))

Testing And Noops

When you are running tests or replaying events, it is sometimes useful to stub out effects.

This is easily done - you simply register a noop effect handler.

Want to stub out the :dispatch effect? Do this:

  (fn [_] ))    ;; a noop

If your test does alter registered effect handlers, and you are using cljs.test, then you can use a fixture to restore all effect handlers at the end of your test:

(defn fixture-re-frame
  (let [restore-re-frame (atom nil)]
    {:before #(reset! restore-re-frame (re-frame.core/make-restore-fn))
     :after  #(@restore-re-frame)}))

(use-fixtures :each (fixture-re-frame))

re-frame.core/make-restore-fn creates a checkpoint for re-frame state (including registered handlers) to which you can return.

Existing Effect Handlers

re-frame's built-in effect handlers, like dispatch-n and dispatch-later, are detailed the API document.

And please review the External-Resources document for a list of 3rd party Effect Handlers.


The 4 Point Summary in note form:

  1. Event handlers should only return a description of required effects
  2. They return a map like {:effect1 value1 :effect2 value2}
  3. Keys of this map can refer to builtin effect handlers (see below) or custom ones
  4. We use reg-fx to register our own effect handlers, built-in ones are already registered

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