reagent/demo/reagentdemo/intro.cljs

(ns reagentdemo.intro
  (:require [reagent.core :as r]
            [reagent.dom :as rdom]
            [reagentdemo.syntax :as s]
            [reagentdemo.common :as common :refer [demo-component]]
            [simpleexample.core :as simple]
            [todomvc.core :as todo]))

(defn simple-component []
  [:div
   [:p "I am a component!"]
   [:p.someclass
    "I have " [:strong "bold"]
    [:span {:style {:color "red"}} " and red "] "text."]])

(defn simple-parent []
  [:div
   [:p "I include simple-component."]
   [simple-component]])

(defn hello-component [name]
  [:p "Hello, " name "!"])

(defn say-hello []
  [hello-component "world"])

(defn lister [items]
  [:ul
   (for [item items]
     ^{:key item} [:li "Item " item])])

(defn lister-user []
  [:div
   "Here is a list:"
   [lister (range 3)]])

(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)}]])

(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]]])))

(defn timer-component []
  (let [seconds-elapsed (r/atom 0)]
    (fn []
      (js/setTimeout #(swap! seconds-elapsed inc) 1000)
      [:div
       "Seconds Elapsed: " @seconds-elapsed])))

(defn render-simple []
  (rdom/render
    [simple-component]
    (.-body js/document)))

(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]]]))

(def ns-src (s/syntaxed "(ns example
  (:require [reagent.core :as r]))"))


(defn intro []
  (let [github {:href "https://github.com/reagent-project/reagent"}
        clojurescript {:href "https://github.com/clojure/clojurescript"}
        react {:href "https://reactjs.org/"}
        hiccup {:href "https://github.com/weavejester/hiccup"}
        react-keys {:href "https://reactjs.org/docs/lists-and-keys.html#keys"}]
    [:div.demo-text

     [:h2 "Introduction to Reagent"]

     [:p [:a github "Reagent"] " provides a minimalistic interface
     between " [:a clojurescript "ClojureScript"] " and " [:a
     react "React"] ". It allows you to define efficient React
     components using nothing but plain ClojureScript functions and
     data, that describe your UI using a " [:a hiccup "Hiccup"] "-like
     syntax."]

     [:p "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."]

     [:p "A very basic Reagent component may look something like this: "]
     [demo-component {:comp simple-component
                      :src (s/src-of [:simple-component])}]

     [:p "You can build new components using other components as
     building blocks. Like this:"]
     [demo-component {:comp simple-parent
                      :src (s/src-of [:simple-parent])}]

     [:p "Data is passed to child components using plain old Clojure
     data types. Like this:"]

     [demo-component {:comp say-hello
                      :src (s/src-of [:hello-component :say-hello])}]

     [:p [:strong "Note: "]
      "In the example above, " [:code "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."]

     [:p "Here is another example that shows items in a "
     [:code "seq"] ":" ]

     [demo-component {:comp lister-user
                      :src (s/src-of [:lister :lister-user])}]

     [:p [:strong "Note: "]
     "The " [:code "^{: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 "
     [:code ":key"] " item in the first argument to a component (if it
     is a map). See React’s " [:a react-keys "documentation"] "
     for more info."]]))

(defn managing-state []
  [:div.demo-text
   [:h2 "Managing state in Reagent"]

   [:p "The easiest way to manage state in Reagent is to use Reagent’s
   own version of " [:code "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 " [:code "atom"]" is automagically
   re-rendered when its value changes."]

   [:p "Let’s demonstrate that with a simple example:"]
   [demo-component {:comp counting-component
                    :src [:pre
                          ns-src
                          (s/src-of [:click-count :counting-component])]}]

   [:p "Sometimes you may want to maintain state locally in a
   component. That is easy to do with an " [:code "atom"] " as well."]

   [:p "Here is an example of that, where we call "
    [:code "setTimeout"] " every time the component is rendered to
   update a counter:"]

   [demo-component {:comp timer-component
                    :src (s/src-of [:timer-component])}]

   [:p "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."]

   [:p "This allows you to perform some setup of newly created
   components without resorting to React’s lifecycle events."]

   [:p "By simply passing an "[:code "atom"]" around you can share
   state management between components, like this:"]

   [demo-component {:comp shared-state
                    :src [:pre
                          ns-src
                          (s/src-of [:atom-input :shared-state])]}]

   [:p [:strong "Note: "] "Component functions can be called with any
   arguments – as long as they are immutable. You "[:em "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."]])

(defn essential-api []
  [:div.demo-text
   [:h2 "Essential API"]

   [:p "Reagent supports most of React’s API, but there is really only
   one entry-point that is necessary for most applications: "
    [:code "reagent.dom/render"] "."]

   [:p "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:"]

   [demo-component {:src [:pre
                          ns-src
                          (s/src-of [:simple-component :render-simple])]}]])

(defn performance []
  [:div.demo-text
   [:h2 "Performance"]

   [:p "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."]

   [:p "Mounted components are only re-rendered when their parameters
   have changed. The change could come from a deref’ed "
   [:code "atom"] ", the arguments passed to the component or
   component state."]

   [:p "All of these are checked for changes with "
   [:code "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 " [:code ":div"] ", " [:code ":p"] ", etc."]

   [:p "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."]

   [:p "There are a couple of situations that you might have to care
   about, though. If you give Reagent a big " [:code "seq"] " of
   components to render, you might have to supply all of them with a
   unique " [:code ":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."]

   [:p "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."]

   [:p "Incidentally, this page also uses another React trick: the
   entire page is pre-rendered using Node, and "
   [:code "reagent.dom.server/render-to-string"] ". When it is loaded
   into the browser, React automatically attaches event-handlers to
   the already present DOM tree."]])

(defn bmi-demo []
  [:div.demo-text
   [:h2 "Putting it all together"]

   [:p "Here is a slightly less contrived example: a simple BMI
   calculator."]

   [:p "Data is kept in a single " [:code "reagent.core/atom"] ": a map
   with height, weight and BMI as keys."]

   [demo-component {:comp bmi-component
                    :src [:pre
                          ns-src
                          (s/src-of [:calc-bmi :bmi-data :slider
                                     :bmi-component])]}]])

(defn complete-simple-demo []
  [:div.demo-text
   [:h2 "Complete demo"]

   [:p "Reagent comes with a couple of complete examples, with
   Leiningen project files and everything. Here’s one of them in
   action:"]

   [demo-component {:comp simple/simple-example
                    :complete true
                    :src (s/src-of nil "simpleexample/core.cljs")}]])

(defn todomvc-demo []
  [:div.demo-text
   [:h2 "Todomvc"]

   [:p "The obligatory todo list looks roughly like this in
   Reagent (cheating a little bit by skipping routing and
   persistence):"]

   [demo-component {:comp todo/todo-app
                    :complete true
                    :src (s/src-of nil "todomvc/core.cljs")}]])

(defn main []
  (let [show-all (r/atom false)
        head "Reagent: Minimalistic React for ClojureScript"]
    (js/setTimeout #(reset! show-all true) 500)
    (fn []
      [:div.reagent-demo
       [:h1 head]
       [intro]
       [managing-state]
       [essential-api]
       [bmi-demo]
       [performance]
       ;; Show heavy examples on load, to make html file smaller
       (when @show-all [complete-simple-demo])
       (when @show-all [todomvc-demo])])))