Reagent: Minimalistic React for ClojureScript
Introduction to Reagent
Reagent provides a minimalistic interface between ClojureScript and React. It allows you to define efficient React components using nothing but plain ClojureScript functions and data, that describe your UI using a Hiccup-like syntax.
The goal of Reagent is to make it possible to define arbitrarily complex UIs using just a couple of basic concepts, and to be fast enough by default that you rarely have to think about performance.
A very basic Reagent component may look something like this:
Source
(defn simple-component [] [:div [:p "I am a component!"] [:p.someclass "I have " [:strong "bold"] [:span {:style {:color "red"}} " and red "] "text."]])
You can build new components using other components as building blocks. Like this:
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(defn simple-parent [] [:div [:p "I include simple-component."] [simple-component]])
Data is passed to child components using plain old Clojure data types. Like this:
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(defn hello-component [name] [:p "Hello, " name "!"]) (defn say-hello [] [hello-component "world"])
Note: In the example above, hello-component
might just
as well have been called as a normal Clojure function instead of
as a Reagent component, i.e with parenthesis instead of square
brackets. The only difference would have been performance, since
”real” Reagent components are only re-rendered when their data
have changed. More advanced components though (see below) must
be called with square brackets.
Here is another example that shows items in a seq
:
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(defn lister [items] [:ul (for [item items] ^{:key item} [:li "Item " item])]) (defn lister-user [] [:div "Here is a list:" [lister (range 3)]])
Note: The ^{:key item}
part above isn’t really necessary
in this simple example, but attaching a unique key to every item
in a dynamically generated list of components is good practice,
and helps React to improve performance for large lists. The key
can be given either (as in this example) as meta-data, or as a :key
item in the first argument to a component (if it
is a map). See React’s documentation
for more info.
Managing state in Reagent
The easiest way to manage state in Reagent is to use Reagent’s
own version of atom
. It works exactly like the one in
clojure.core, except that it keeps track of every time it is
deref’ed. Any component that uses an atom
is automagically
re-rendered when its value changes.
Let’s demonstrate that with a simple example:
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(ns example (:require [reagent.core :as r]))(def click-count (r/atom 0)) (defn counting-component [] [:div "The atom " [:code "click-count"] " has value: " @click-count ". " [:input {:type "button" :value "Click me!" :on-click #(swap! click-count inc)}]])
Sometimes you may want to maintain state locally in a
component. That is easy to do with an atom
as well.
Here is an example of that, where we call setTimeout
every time the component is rendered to
update a counter:
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(defn timer-component [] (let [seconds-elapsed (r/atom 0)] (fn [] (js/setTimeout #(swap! seconds-elapsed inc) 1000) [:div "Seconds Elapsed: " @seconds-elapsed])))
The previous example also uses another feature of Reagent: a component function can return another function, that is used to do the actual rendering. This function is called with the same arguments as the first one.
This allows you to perform some setup of newly created components without resorting to React’s lifecycle events.
By simply passing an atom
around you can share
state management between components, like this:
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(ns example (:require [reagent.core :as r]))(defn atom-input [value] [:input {:type "text" :value @value :on-change #(reset! value (-> % .-target .-value))}]) (defn shared-state [] (let [val (r/atom "foo")] (fn [] [:div [:p "The value is now: " @val] [:p "Change it here: " [atom-input val]]])))
Note: Component functions can be called with any arguments – as long as they are immutable. You could use mutable objects as well, but then you have to make sure that the component is updated when your data changes. Reagent assumes by default that two objects are equal if they are the same object.
Essential API
Reagent supports most of React’s API, but there is really only
one entry-point that is necessary for most applications: reagent.dom/render
.
It takes two arguments: a component, and a DOM node. For example, splashing the very first example all over the page would look like this:
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(ns example (:require [reagent.dom :as rdom]))(defn simple-component [] [:div [:p "I am a component!"] [:p.someclass "I have " [:strong "bold"] [:span {:style {:color "red"}} " and red "] "text."]]) (defn render-simple [] (rdom/render [simple-component] (.-body js/document)))
Putting it all together
Here is a slightly less contrived example: a simple BMI calculator.
Data is kept in a single reagent.core/atom
: a map
with height, weight and BMI as keys.
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(ns example (:require [reagent.core :as r]))(defn calc-bmi [{:keys [height weight bmi] :as data}] (let [h (/ height 100)] (if (nil? bmi) (assoc data :bmi (/ weight (* h h))) (assoc data :weight (* bmi h h))))) (def bmi-data (r/atom (calc-bmi {:height 180 :weight 80}))) (defn slider [param value min max invalidates] [:input {:type "range" :value value :min min :max max :style {:width "100%"} :on-change (fn [e] (let [new-value (js/parseInt (.. e -target -value))] (swap! bmi-data (fn [data] (-> data (assoc param new-value) (dissoc invalidates) calc-bmi)))))}]) (defn bmi-component [] (let [{:keys [weight height bmi]} @bmi-data [color diagnose] (cond (< bmi 18.5) ["orange" "underweight"] (< bmi 25) ["inherit" "normal"] (< bmi 30) ["orange" "overweight"] :else ["red" "obese"])] [:div [:h3 "BMI calculator"] [:div "Height: " (int height) "cm" [slider :height height 100 220 :bmi]] [:div "Weight: " (int weight) "kg" [slider :weight weight 30 150 :bmi]] [:div "BMI: " (int bmi) " " [:span {:style {:color color}} diagnose] [slider :bmi bmi 10 50 :weight]]]))
Performance
React itself is very fast, and so is Reagent. In fact, Reagent will be even faster than plain React a lot of the time, thanks to optimizations made possible by ClojureScript.
Mounted components are only re-rendered when their parameters
have changed. The change could come from a deref’ed atom
, the arguments passed to the component or
component state.
All of these are checked for changes with identical?
which is basically only a pointer
comparison, so the overhead is very low. Maps passed as arguments
to components are compared the same way: they are considered equal
if all their entries are identical. This also applies to built-in
React components like :div
, :p
, etc.
All this means that you simply won’t have to care about performance most of the time. Just define your UI however you like – it will be fast enough.
There are a couple of situations that you might have to care
about, though. If you give Reagent a big seq
of
components to render, you might have to supply all of them with a
unique :key
attribute to speed up rendering (see
above). Also note that anonymous functions are not, in general,
equal to each other even if they represent the same code and
closure.
But again, in general you should just trust that React and Reagent will be fast enough. This very page is composed of a single Reagent component with thousands of child components (every single parenthesis etc in the code examples is a separate component), and yet the page can be updated many times every second without taxing the browser the slightest.
Incidentally, this page also uses another React trick: the
entire page is pre-rendered using Node, and reagent.dom.server/render-to-string
. When it is loaded
into the browser, React automatically attaches event-handlers to
the already present DOM tree.