This is an introduction to testing re-frame apps. It walks you through some choices.
For any re-frame app, there's three things to test:
-
Event Handlers - most of your testing focus will be here because this is where most of the logic lives
-
Subscription Handlers - often not a lot to test here. Only Layer 3 subscriptions need testing.
-
View functions - I don't tend to write tests for views. There, I said it. Hey! It is mean to look at someone with that level of disapproval, while shaking your head. I have my reasons ...
In my experience with the re-frame architecture, View Functions tend to be an unlikely source of bugs. And every line of code you write is like a ball & chain you must forevermore drag about, so I dislike maintaining tests which don't deliver good bang for buck.
And, yes, in theory there's also Effect Handlers (Domino 3) to test,
but you'll hardly ever write one, and, anyway, each one is different, so
I've got no good general insight to offer you for them. They will be ignored
in this tutorial.
Let's establish some terminology to aid the further explanations in this tutorial. Every unittest has 3 steps:
- setup initial conditions
- execute the thing-under-test
- verify that the thing-under-test did the right thing
Event Handlers are pure functions which should make them easy to test, right?
First, create a named event handler using defn like this:
(defn select-triangle
[db [_ triangle-id]]
... return a modified version of db)You'd register this handler in a separate step:
(re-frame.core/reg-event-db ;; this is a "-db" event handler, not "-fx"
:select-triangle
[some-interceptors]
select-triangle) ;; <--- defn above. don't use an annonomous fnThis arrangement means the event handler function
select-triangle is readily available to be unittested.
To test select-triangle, a unittest must pass in values for the two arguments
db and v. And, so, our setup would have to construct both values.
But how to create a useful db value?
db is a map of a certain structure, so one way would be to simply assoc values
into a map at certain paths to simulate a real-world db value or, even easier, just use
a map literal, like this:
;; a test
(let [
;; setup - create db and event
db {:some 42 :thing "hello"} ; a literal
event [:select-triangle :other :event :args]
;; execute
result-db (select-triange db event)]
;; validate that result-db is correct)
(is ...)This certainly works in theory, but in practice,
unless we are careful, constructing the db
value in setup could:
- be manual and time-consuming
- tie tests to the internal structure of
app-db
The setup of every test could end up relying on the internal structure
of app-db and any change in that structure (which is inevitable over time)
would result in a lot re-work in the tests. That's too fragile.
So, this approach doesn't quite work.
In re-frame,
Eventsare central. They are the "language of the system". They provide the eloquence.
The db value (stored in app-db) is the cumulative result
of many event handlers running.
We can use this idea. In setup, instead of manually trying to create that db value, we could
"build up" a db value by threading db through many event handlers
which cumulatively create the required initial state. Tests then need
know nothing about the internal structure of that db.
Like this:
(let [
;; setup - cummulatively build up db
db (-> {} ;; empty db
(initialise-db [:initialise-db]) ;; each event handler expects db and event
(clear-panel [:clear-panel])
(draw-triangle [:draw-triangle 1 2 3]))
event [:select-triange :other :stuff]
;; now execute the event handler under test
db' (select-triange db event)]
;; validate that db' is correct
(is ...)
This approach works so long as all the event handlers are
of the -db kind, but the threading gets a little messy when some event
handlers are of the -fx kind which take a coeffect argument and
return effects, instead of a db value.
So, this approach is quite workable in some cases, but can get messy in the general case.
There is further variation which is quite general but not as pure.
During test setup we could literally just dispatch the events
which would put app-db into the right state.
Except, we'd have to use dispatch-sync rather dispatch to
force immediate handling of events, rather than queuing.
;; setup - cummulatively build up db
(dispatch-sync [:initialise-db])
(dispatch-sync [:clear-panel])
(dispatch-sync [:draw-triangle 1 2 3])
;; execute
(dispatch-sync [:select-triange :other :stuff])
;; validate that the value in 'app-db' is correct
;; perhaps with subscriptionsNotes:
- we use
dispatch-syncbecausedispatchis async (event is handled not now, but soon) - Not pure. We are choosing to mutate the global
app-db. But having said that, there's something about this approach which is remarkably pragmatic. - the setup is now very natural. The associated handlers can be either
-dbor-fx - if the handlers have effects other than just updating app-db, we might need to stub out XXX
- How do we look at the results ????
If this method appeals to you, you should ABSOLUTELY review the utilities in this helper library: re-frame-test.
In summary, event handlers should be easy to test because they are pure functions. The interesting
part is the unittest "setup" where we need to establish an initial value for db.
Here's a Subscription Handler from the todomvc example:
(reg-sub
:visible-todos
;; signal function
(fn [query-v _]
[(subscribe [:todos])
(subscribe [:showing])])
;; computation function
(fn [[todos showing] _] ;; that 1st parameter is a 2-vector of values
(let [filter-fn (case showing
:active (complement :done)
:done :done
:all identity)]
(filter filter-fn todos))))How do we test this?
First, we could split the computation function from its registration, like this:
(defn visible-todos
[[todos showing] _]
(let [filter-fn (case showing
:active (complement :done)
:done :done
:all identity)]
(filter filter-fn todos)))
(reg-sub
:visible-todos
(fn [query-v _]
[(subscribe [:todos])
(subscribe [:showing])])
visible-todos) ;; <--- computation function used hereThat makes visible-todos available for direct unit testing. But, as we experienced
with Event Handlers, the challenge is around constructing db values (first parameter)
in a way which doesn't become fragile.
Components/views are more tricky and there are a few options.
But remember my ugly secret - I don't tend to write tests for my views.
But here's how, theoretically, I'd write tests if I wasn't me ...
If a View Function is Form-1, then it is fairly easy to test.
A trivial example:
(defn greet
[name]
[:div "Hello " name])
(greet "Wiki")
;;=> [:div "Hello " "Wiki"]So, here, testing involves passing values into the function and checking the data structure returned for correctness.
What's returned is hiccup, of course. So how do you test hiccup for correctness?
hiccup is just a clojure data structure - vectors containing keywords, and maps, and other vectors, etc. Perhaps you'd use https://github.com/nathanmarz/specter to declaratively check on the presence of certain values and structures? Or do it more manually.
But what if the View Function has a subscription?
(defn my-view
[something]
(let [val (subscribe [:query-id])] ;; <-- reactive subscription
[:div .... using @val in here]))The use of subscribe makes the function impure (it obtains data from places other than its args).
A testing plan might be:
- setup
app-dbwith some values in the right places (via dispatch of events?) - call
my-view(with a parameter) which will return hiccup - check the hiccup structure for correctness.
Continuing on, in a second phase you could then:
- change the value in
app-db(which will cause the subscription to fire) - call view functions again (hiccup returned)
- check the new hiccup for correctness.
Which is all possible, if a little messy.
There is a pragmatic method available to handle the impurity: use with-redefs
to stub out subscribe. Like this:
(defn subscription-stub [x]
(atom
(case x
[:query-id] 42)))
(deftest some-test
(with-redefs [re-frame/subscribe (subscription-stub)]
(testing "some some view which does a subscribe"
..... call the view function and the hiccup output)))For more integration level testing, you can use with-mounted-component
from the reagent-template
to render the component in the browser and validate the generated DOM.
Or ... there is another option: you can structure in the first place for pure view functions.
The trick here is to create an outer and inner component. The outer sources the data (via a subscription), and passes it onto the inner as props (parameters).
As a result, the inner component, which does the testable work, is pure and easily tested. The outer is impure but trivial.
To get a more concrete idea, I'll direct you to another page in the re-frame docs
which has nothing to do with testing, but it does use this simple-outer-subscribe-with-complicated-inner-render
pattern for a different purpose:
Using Stateful JS Components
Note: this technique could be made simple and almost invisible via the use of macros.
This pattern has been independently discovered by many. For example, here it is called the Container/Component pattern.
https://juxt.pro/blog/posts/cljs-apps.html
Event handlers will be your primary focus when testing. Remember to review the utilities in re-frame-test.