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Applying Property Based Testing to User Interfaces
Traditional user interface development methodologies present incredible challenges for automated testing. Mutable object graphs coupled to mutable view graphs coupled to unmanaged programmatic mutation are a never-ending siege upon test automation. As a result it is not uncommon that Quality Assurance (QA) teams must enumerate limited paths into a vast state space they cannot hope to cover. It should thus come as little surprise that end users encounter so many simple bugs in their interactions.
The sad state of affairs is further exacerbated when a product must target multiple platforms. Automated UI testing frameworks between platforms vary in their capability (and certainly in their APIs) requiring an incredible amount of effort duplication. And such testing frameworks cannot fix the broken foundations and thus still require a considerable amount of human intervention. This level of human intervention is rarely something many smaller teams can even afford.
By correcting the foundations we can provide radically enhanced automated testing of user interfaces.
In Om Next you do not construct user interfaces on quicksand - there are no mutable objects graphs and using local mutation is discouraged for anything beyond platform specific transient styling and animations. Thus every critical state of an Om Next application is a complete and consistent snapshot. Not only that, React's approach to rendering means that the data should closely correspond to the shape of the UI. This correspondence is further strengthened when embracing a component co-located query model - the UI data tree must precisely match the component tree. And because mutations are reified it is a simple matter to fabricate a series of mutation expressions to test some invariant.
This means we can confidently test UI state transitions without rendering anything at all.
The implications are enormous for applications that intend to leverage React Native - shared test automation can now cover the web, iOS, and Android.
As we shall see Om Next is ready for this kind of test automation without doing anything more than being constructed on simple, good foundations.
We will see how Om Next's architecture when paired with property based testing can dramatically increase our confidence in the correctness of user interface code with minimal effort. We will use test.check a robust and featureful implementation of the property based testing methodology. Even if you are not familiar with property based testing, the benefits will become immediately clear.
We assume you are now familiar with the previous tutorial setups. We will not cover that material.
However your project.clj should look like the following:
(defproject om-tutorial "0.1.0-SNAPSHOT"
:description "FIXME: write description"
:url "http://example.com/FIXME"
:license {:name "Eclipse Public License"
:url "http://www.eclipse.org/legal/epl-v10.html"}
:dependencies [[org.clojure/clojure "1.7.0"]
[org.clojure/clojurescript "1.7.170"]
[org.clojure/test.check "0.8.2"]
[org.omcljs/om "1.0.0-alpha21"]
[figwheel-sidecar "0.5.0-SNAPSHOT" :scope "test"]])We will build the state and state transition model for a simple application. This simple application has a few concepts, people, friends, and two friending operations - add and remove.
Your ns form should look like the following:
(ns om-tutorial.core
(:require [clojure.test.check :as tc]
[clojure.test.check.generators :as gen]
[clojure.test.check.properties :as prop :include-macros true]
[om.next :as om :refer-macros [defui]]
[om.dom :as dom]))We import the various bits of test.check that we'll need and we only
need defui from om.next.
(enable-console-print!)
(def init-data
{:people [{:id 0 :name "Bob" :friends []}
{:id 1 :name "Laura" :friends []}
{:id 2 :name "Mary" :friends []}]})
(defui Friend
static om/Ident
(ident [this props]
[:person/by-id (:id props)])
static om/IQuery
(query [this]
[:id :name]))
(defui Person
static om/Ident
(ident [this props]
[:person/by-id (:id props)])
static om/IQuery
(query [this]
[:id :name {:friends (om/get-query Friend)}]))
(defui People
static om/IQuery
(query [this]
[{:people (om/get-query Person)}]))We declare some simple data and the defui bits we'll need to
normalize this information.
(defmulti read om/dispatch)
(defmethod read :people
[{:keys [state query] :as env} key _]
(let [st @state]
{:value (om/db->tree query (get st key) st)}))We use a new helper, om.next/db->tree to avoid having to manually
join in references.
Let's define our mutations. First let's do friend/add:
(defmulti mutate om/dispatch)
(defn add-friend [state id friend]
(letfn [(add* [friends ref]
(cond-> friends
(not (some #{ref} friends)) (conj ref)))]
(-> state
(update-in [:person/by-id id :friends]
add* [:person/by-id friend]))))
(defmethod mutate 'friend/add
[{:keys [state] :as env} key {:keys [id friend] :as params}]
{:action
(fn [] (swap! state add-friend id friend))})There are two bugs in add-friend. The first is that it permits self
friending, the second, it does not change the friend list of the other
person. Don't worry our property based tests will catch this.
Lets write the friend/remove mutation:
(defn remove-friend [state id friend]
(letfn [(remove* [friends ref]
(cond->> friends
(some #{ref} friends) (into [] (remove #{ref}))))]
(-> state
(update-in [:person/by-id id :friends]
remove* [:person/by-id friend]))))
(defmethod mutate 'friend/remove
[{:keys [state] :as env} key {:keys [id friend] :as params}]
{:action (fn [] (swap! state remove-friend id friend))})There's only one bug in this one, it's one of the same bugs in the add friends code.
We now define some useful top levels:
(def app-state
(atom (om/tree->db People init-data true)))
(def parser (om/parser {:read read :mutate mutate}))The first thing that we need to be able to do is generate random transactions. A huge part of property based testing is that by writing generators property based testing can give us something important back in return - shrinking. That is property based testing can find the failure and then automatically shrink it into a minimal case. We don't have time to cover every aspect of building generators but it should be clear what the following accomplishes:
(def gen-tx-add-remove
(gen/vector
(gen/fmap seq
(gen/tuple
(gen/elements '[friend/add friend/remove])
(gen/fmap (fn [[n m]] {:id n :friend m})
(gen/tuple
(gen/elements [0 1 2])
(gen/elements [0 1 2])))))))Using a REPL try the following:
(gen/sample gen-tx-add-remove 10)
;; ([]
;; []
;; [(friend/add {:id 1, :friend 1}) (friend/remove {:id 1, :friend 2})]
;; [(friend/add {:id 0, :friend 0})
;; (friend/remove {:id 0, :friend 1})
;; (friend/add {:id 0, :friend 1})]
;; [(friend/add {:id 0, :friend 2})
;; (friend/add {:id 2, :friend 2})
;; (friend/remove {:id 2, :friend 1})
;; (friend/add {:id 2, :friend 1})])As you can see all gen-tx-add-remove does is generate random
transactions containing friend adds and removes.
We'll focus on testing the UI data tree. This allows us to test the normalized model since the UI data tree is completely derived from it, but it also gives us confidence that the user will never see something unexpected. If it's not in the UI data tree it won't be present in a stateless view derived from it.
The other reason to focus on the UI data tree is that this is what the user actually interacts against. They click buttons, move mice, type keys, and gesture to generate a stream of transactions that transition the normalized state that we then denormalize yet again to present feedback to the user.
The user is the unknown actor in our system and they will inevitably produce some unexpected series of transactions that could put the UI into a corrupted state. So we want to model the user as process and test this.
This bears repeating.
If you want to increase confidence in the correctness of user interface code model the user.
Anything less than this is not testing anything of importance.
Here's our code to test that self friending should not be possible:
(defn self-friended? [{:keys [id friends]}]
(boolean (some #{id} (map :id friends))))
(defn prop-no-self-friending []
(prop/for-all [tx gen-tx-add-remove]
(let [parser (om/parser {:read read :mutate mutate})
state (atom (om/tree->db People init-data true))]
(parser {:state state} tx)
(let [ui (parser {:state state} (om/get-query People))]
(not (some self-friended? (:people ui)))))))If you've gone through all the previous tutorials this test should be
obvious. We are using test.check to generate random transactions and we
then want to know if anyone in the UI data tree has themselves in
the friend list.
Let's try the following at the REPL:
(tc/quick-check 100 (prop-no-self-friending))
;; ... elided ...
;; {:result false, :seed 1445791557331, :failing-size 1, :num-tests 2,
;; :fail [[(friend/add {:id 0, :friend 0})]],
;; :shrunk {:total-nodes-visited 1, :depth 0, :result false,
;; :smallest [[(friend/add {:id 0, :friend 0})]]}}We quickly determine that the property does not hold and we get the smallest transaction that proves the property does not hold.
Let's fix our friend adding mutation:
(defn add-friend [state id friend]
(letfn [(add* [friends ref]
(cond-> friends
(not (some #{ref} friends)) (conj ref)))]
(if-not (= id friend) ;; FIXED
(-> state
(update-in [:person/by-id id :friends]
add* [:person/by-id friend]))
state)))Rerun the test. We should see no failures now.
We now want to know with confidence that regardless of how many friends are added and removed that friend lists always remain consistent. That is A is a friend of B if and only if B is a friend of A.
(defn friends-consistent? [people]
(let [indexed (zipmap (map :id people) people)]
(letfn [(consistent? [[id {:keys [friends]}]]
(let [xs (map (comp :friends indexed :id) friends)]
(every? #(some #{id} (map :id %)) xs)))]
(every? consistent? indexed))))
(defn prop-friend-consistency []
(prop/for-all [tx gen-tx-add-remove]
(let [parser (om/parser {:read read :mutate mutate})
state (atom (om/tree->db People init-data true))]
(parser {:state state} tx)
(let [ui (parser {:state state} (om/get-query People))]
(friends-consistent? (:people ui))))))Again our test should be pretty easy to understand. We're just checking that in our UI data tree that it's not possible for anyone to ever be in a situation where a user would see inconsistent friend lists.
Let's run the following test:
(tc/quick-check 100 (prop-friend-consistency))
;; ... elided ...
;; {:result false, :seed 1445791900621, :failing-size 3, :num-tests 4,
;; :fail [[(friend/add {:id 0, :friend 1})]],
;; :shrunk {:total-nodes-visited 2, :depth 0, :result false,
;; :smallest [[(friend/add {:id 0, :friend 1})]]}}Oops looks like we forget to do the corresponding operation in add friend. Let's fix that:
(defn add-friend [state id friend]
(letfn [(add* [friends ref]
(cond-> friends
(not (some #{ref} friends)) (conj ref)))]
(if-not (= id friend)
(-> state
(update-in [:person/by-id id :friends]
add* [:person/by-id friend])
(update-in [:person/by-id friend :friends] ;; FIXED
add* [:person/by-id id]))
state)))Let's try again:
(tc/quick-check 100 (prop-friend-consistency))
;; ... elided ...
;; {:result false, :seed 1445792089722, :failing-size 4, :num-tests 5,
;; :fail [[(friend/add {:id 0, :friend 1})
;; (friend/remove {:id 1, :friend 0})
;; (friend/remove {:id 1, :friend 0})]],
;; :shrunk {:total-nodes-visited 7, :depth 1, :result false,
;; :smallest [[(friend/add {:id 0, :friend 1}) (friend/remove {:id 1, :friend 0})]]}}Hrm it appears there's still something wrong. The minimal example gives us a hint that it's probably something with friend removal. And if we examine friend removal we'll see that we are in fact missing a corresponding remove.
Let's fix that:
(defn remove-friend [state id friend]
(letfn [(remove* [friends ref]
(cond->> friends
(some #{ref} friends) (into [] (remove #{ref}))))]
(-> state
(update-in [:person/by-id id :friends]
remove* [:person/by-id friend])
(update-in [:person/by-id friend :friends] ;; FIXED
remove* [:person/by-id id]))))Let's try again:
(tc/quick-check 100 (prop-friend-consistency))This should succeed.
The fundamental problem with automating testing of user interfaces reduces to obstacles around modeling the user. By removing all such obstacles Om Next makes it possible to deliver confidence without actually providing any specific features around testability.
Model the user.
The entire correct final source code follows:
(ns om-tutorial.core
(:require [goog.dom :as gdom]
[clojure.test.check :as tc]
[clojure.test.check.generators :as gen]
[clojure.test.check.properties :as prop]
[om.next :as om :refer-macros [defui]]
[om.dom :as dom]))
(enable-console-print!)
(def init-data
{:people [{:id 0 :name "Bob" :friends []}
{:id 1 :name "Laura" :friends []}
{:id 2 :name "Mary" :friends []}]})
(defui Friend
static om/Ident
(ident [this props]
[:person/by-id (:id props)])
static om/IQuery
(query [this]
[:id :name]))
(defui Person
static om/Ident
(ident [this props]
[:person/by-id (:id props)])
static om/IQuery
(query [this]
[:id :name {:friends (om/get-query Friend)}]))
(defui People
static om/IQuery
(query [this]
[{:people (om/get-query Person)}]))
(defmulti read om/dispatch)
(defmethod read :people
[{:keys [state query] :as env} key _]
(let [st @state]
{:value (om/db->tree query (get st key) st)}))
(defmulti mutate om/dispatch)
(defn add-friend [state id friend]
(letfn [(add* [friends ref]
(cond-> friends
(not (some #{ref} friends)) (conj ref)))]
(if-not (= id friend)
(-> state
(update-in [:person/by-id id :friends]
add* [:person/by-id friend])
(update-in [:person/by-id friend :friends]
add* [:person/by-id id]))
state)))
(defmethod mutate 'friend/add
[{:keys [state] :as env} key {:keys [id friend] :as params}]
{:action
(fn [] (swap! state add-friend id friend))})
(defn remove-friend [state id friend]
(letfn [(remove* [friends ref]
(cond->> friends
(some #{ref} friends) (into [] (remove #{ref}))))]
(if-not (= id friend)
(-> state
(update-in [:person/by-id id :friends]
remove* [:person/by-id friend])
(update-in [:person/by-id friend :friends]
remove* [:person/by-id id]))
state)))
(defmethod mutate 'friend/remove
[{:keys [state] :as env} key {:keys [id friend] :as params}]
{:action (fn [] (swap! state remove-friend id friend))})
(def app-state
(atom (om/tree->db People init-data true)))
(def parser (om/parser {:read read :mutate mutate}))
(def gen-tx-add-remove
(gen/vector
(gen/fmap seq
(gen/tuple
(gen/elements '[friend/add friend/remove])
(gen/fmap (fn [[n m]] {:id n :friend m})
(gen/tuple
(gen/elements [0 1 2])
(gen/elements [0 1 2])))))))
(defn self-friended? [{:keys [id friends]}]
(boolean (some #{id} (map :id friends))))
(defn prop-no-self-friending []
(prop/for-all [tx gen-tx-add-remove]
(let [parser (om/parser {:read read :mutate mutate})
state (atom (om/tree->db People init-data true))]
(parser {:state state} tx)
(let [ui (parser {:state state} (om/get-query People))]
(not (some self-friended? (:people ui)))))))
(defn friends-consistent? [people]
(let [indexed (zipmap (map :id people) people)]
(letfn [(consistent? [[id {:keys [friends]}]]
(let [xs (map (comp :friends indexed :id) friends)]
(every? #(some #{id} (map :id %)) xs)))]
(every? consistent? indexed))))
(defn prop-friend-consistency []
(prop/for-all [tx gen-tx-add-remove]
(let [parser (om/parser {:read read :mutate mutate})
state (atom (om/tree->db People init-data true))]
(parser {:state state} tx)
(let [ui (parser {:state state} (om/get-query People))]
(friends-consistent? (:people ui))))))