re-frame/docs/Coeffects.md

Coeffects

This tutorial explains coeffects.

It explains what they are, how they can be "injected", and how to manage them in tests.

What Are They?

coeffects are the data resources that an event handler needs to perform its computation.

Because the majority of event handlers only need db and event, there's a specific registration function, called reg-event-db, which delivers ONLY these two coeffects as arguments to an event handler, making this common case easy to program.

But sometimes an event handler needs other data inputs to perform its computation. Things like a random number, or a GUID, or the current datetime. Perhaps it needs access to a DataScript connection.

An Example

This handler obtains data directly from LocalStore:

(reg-event-db
   :load-defaults
   (fn [db _]
     (let [val (js->clj (.getItem js/localStorage "defaults-key"))]  ;; <-- Problem
       (assoc db :defaults val))))

This works, but there's a cost.

Because it has directly accessed LocalStore, this event handler is not pure, and impure functions cause well-documented paper cuts.

How We Want It

Our goal in this tutorial will be to rewrite this event handler so that it only uses data from arguments. This will take a few steps.

The first is that we switch to using reg-event-fx (instead of reg-event-db).

Event handlers registered via reg-event-fx are slightly different to those registered via reg-event-db. -fx handlers get two arguments, but the first is not db. Instead it is an argument which we will call cofx (that's a nice distinct name which will aid communication).

Previous tutorials showed there's a :db key in cofx. We now want cofx to have other keys and values, like this:

(reg-event-fx                     ;; note: -fx
   :load-defaults
   (fn [cofx event]                 ;; cofx means coeffects
     (let [val (:local-store cofx)  ;; <-- get data from cofx
           db  (:db cofx)]          ;; <-- more data from cofx
       {:db (assoc db :defaults val))})) ;; returns an effect

Notice how cofx magically contains a :local-store key with the right value. Nice! But how do we make this magic happen?

Abracadabra

Each time an event handler is executed, a brand new context (map) is created, and within that context is a :coeffects key which is a further map (initially empty).

That pristine context value (containing a pristine :coeffects map) is threaded through a chain of Interceptors before it finally reaches our event handler, which sits on the end of the chain, itself wrapped up in an interceptor. We know this story well from a previous tutorial.

So, all members of the Interceptor chain have the opportunity to assoc into :coeffects within their :before function, cumulatively building up what it holds. Later, our event handler, which sits on the end of the chain, magically finds just the right data (like a value for the key :local-store) in its first cofx argument. So, it is the event handler's Interceptors which put it there.

Which Interceptors?

If Interceptors put data in :coeffects, then we'll need to add the right ones when we register our event handler.

Something like this (this handler is the same as before, except for one detail):

(reg-event-fx
   :load-defaults
   [ (inject-cofx :local-store "defaults-key") ]     ;; <-- this is new
   (fn [cofx event]
     (let [val (:local-store cofx)
           db  (:db cofx)]
       {:db (assoc db :defaults val))}))

Look at that - my event handler has a new Interceptor! It is injecting the right key/value pair (:local-store) into context's :coeffects, which itself then goes on to be the first argument to our event handler (cofx).

inject-cofx

inject-cofx is part of the re-frame API.

It is a function which returns an Interceptor whose :before function loads a key/value pair into a context's :coeffects map.

inject-cofx takes either one or two arguments. The first is always the id of the coeffect required (called a cofx-id). The 2nd is an optional addition value.

So, in the case above, the cofx-id was :local-store and the additional value was "defaults-key" which was presumably the LocalStore key.

More inject-cofx

Here's some other usage examples:

• (inject-cofx :random-int 10)
• (inject-cofx :guid)
• (inject-cofx :now)

I could create an event handler which has access to 3 coeffects:

(reg-event-fx
    :some-id
    [(inject-cofx :random-int 10) (inject-cofx :now)  (inject-cofx :local-store "blah")]  ;; 3
    (fn [cofx _]
       ... in here I can access cofx's keys :now :local-store and :random-int))

But that's probably just greedy, and not very useful.

And so, to the final piece in the puzzle: how does inject-cofx know what to do when it is given :now or :local-store? Each cofx-id requires a different action.

Meet reg-cofx

This function is also part of the re-frame API.

It allows you to associate a cofx-id (like :now or :local-store) with a handler function that injects the right key/value pair.

The function you register will be passed two arguments:

• a :coeffects map (to which it should add a key/value pair), and
• optionally, the additional value supplied to inject-cofx

and it is expected to return a modified :coeffects map.

Example Of reg-cofx

Above, we wrote an event handler that wanted :now data to be available. Here is how a handler could be registered for :now:

(reg-cofx               ;; registration function
   :now                 ;; what cofx-id are we registering
   (fn [coeffects _]    ;; second parameter not used in this case
      (assoc coeffects :now (js.Date.))))   ;; add :now key, with value

The outcome is:

1. because that cofx handler above is now registered for :now, I can
2. add an Interceptor to an event handler which
3. looks like (inject-cofx :now)
4. which means within that event handler I can access a :now value from cofx

As a result, my event handler is pure.

Another Example Of reg-cofx

This:

(reg-cofx               ;; new registration function
   :local-store
   (fn [coeffects local-store-key]
      (assoc coeffects
             :local-store
             (js->clj (.getItem js/localStorage local-store-key)))))

With these two registrations in place, I could now use both (inject-cofx :now) and (inject-cofx :local-store "blah") in an event handler's interceptor chain.

To put this another way: I can't use (inject-cofx :blah) UNLESS I have previously used reg-cofx to register a handler for :blah. Otherwise inject-cofx doesn't know how to inject a :blah.

Secret Interceptors

In a previous tutorial we learned that reg-events-db and reg-events-fx add Interceptors to the front of any chain during registration. We found they inserted an Interceptor called do-fx.

I can now reveal that they also add (inject-cofx :db) at the front of each chain.

Guess what that injects into the :coeffects of every event handler? This is how :db is always available to event handlers.

Okay, so that was the last surprise. Now you know everything.

If ever you wanted to use DataScript, instead of an atom-containing-a-map like app-db, you'd replace reg-event-db and reg-event-fx with your own registration functions and have them auto insert the DataScript connection.

Testing

During testing, you may want to stub out certain coeffects.

You may, for example, want to test that an event handler works using a specific now.

In your test, you'd mock out the cofx handler:

(reg-cofx
   :now
   (fn [coeffects _]
      (assoc coeffects :now (js/Date. 2016 1 1)))   ;; now was then

If your test does alter registered coeffect handlers, and you are using cljs.test, then you can use a fixture to restore all coeffects 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.

The 5 Point Summary

In note form:

1. Event handlers should only source data from their arguments
2. We want to "inject" required data into the first, cofx argument
3. We use the (inject-cofx :key) interceptor in registration of the event handler
4. It will look up the registered cofx handler for that :key to do the injection
5. We must have previously registered a cofx handler via reg-cofx

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