ClojureScript Core.Async

Featuring WebSockets

I got it in my mind to build Cloj a lightweight web application written entirely in Clojure, and built on-top of http-kit. Things have been smooth sailing until getting into ClojureScript, I hadn't realized there was so much of a difference to Clojure.

Taking a step back

The First thing I did was implement Ajax with ClojureScript, getting a Json response back, and then updating the dom with some help from dommy. I highly recommend dommy, you get ClojureScript side templating and DOM manipulation. Better than just DOM manipulation with domina. I also used cljs-ajax which I'll eventually do away with, it's a great place to see how to wrap the google clojure ajax stuff.

Client Ajax ClojureScript:
(ns cloj.cljs.script
(:require [ajax.core :refer [GET POST]]
	[dommy.utils :as utils]
  	[dommy.core :as dommy]
	[cloj.js.util :refer [log]]
	[dommy.macros :only [node sel sel1]])
	[cljs.core.async.macros :refer [go]]))
(defn receive [event]
    (let [resp (js->clj event)]
    (dommy/append! (sel1 :#foo) [:li (get-in resp ["test"])])
    (set! (.-scrollTop (sel1 :#foo)) (.-scrollHeight (sel1 :#foo)))))

(defn error-handler [event]
    (log (str "Something went wrong: " event)))
(defn ping-server [e]
    (GET "/api" {:handler receive :error-handler error-handler})
    (.preventDefault e))
(defn ^:export init []
    (set! (.-onclick (sel1 :#ping)) ping-server))

On init, I bind the ping-server function to the button's (whose id is ping) on-click event. The ping-server function then makes an Ajax request to localhost:1337/api and returns it's result to either the receive function, or the error-hanlder function. In the receive function, I use the handy js->clj to convert from json to a clojure object. I can then extract values using (get-in response ["key"]).

The [:li (some-string-here)] becomes, thanks to dommy, an html node;


All of the scrollTop and scrollHeight stuff is to make the div I'm appending the content too, always scroll to the lastest appended item.

Introducing WebSockets

This is mostly taken from ddellacosta's project and was essential in finding what I wanted, and then adding it to my project. But, I promise, I still learned some things…

It's always useful for me when I can see what others have in their header;

(ns cloj.cljs.sockets
	[cljs.core.async :refer [chan <! >! put!]]
	[cljs.reader :as reader]
	[dommy.utils :as utils]
	[dommy.core :as dommy])
	[dommy.macros :only [node sel sel1]])
	[cljs.core.async.macros :refer [go]]))

First, create send and receive functions;

(def send (chan))
(def receive (chan))

Setting up the WebSocket;

(def ws-url "ws://localhost:1337/async")
(def ws (new js/WebSocket ws-url))

Now, for an event channel, where el is the dom element, and type is the event type;

(defn event-chan
	[c el type]
	(let [writer #(put! c %)]
	(dommy/listen! el type writer)
	{:chan c
 	 :unsubscribe #(dommy/unlisten! el type writer)}))

Now we set up functions that will handle the sending and receiving. Here we send a String "Test" to the server whenever our make-sender function is called. Notice we pass an element with id websocket to an event-chan function, with type of :click:

(defn make-sender []
	(event-chan send (sel1 :#websocket) :click)
	(while true
 		(let [evt  (<! send)]
   			(when (= (.-type evt) "click")
     		(.send ws "Test"))))))

You can also do slick things like this:

(.send ws {:name "Name" :last "Last"})

Different event types can send different things, of course.

We create our receiver function, let's call it make-receiver;

(defn make-receiver []
	(set! (.-onmessage ws) (fn [msg] (put! receive msg)))

Notice this calls an add-message function, which takes the message, grabs the raw data, reads it in, and then appends to the html element with id foo:

(defn add-message []
	(while true
 	(let [msg            (<! receive)
          raw-data       (.-data msg)
          data           (reader/read-string raw-data)]
   		 (dommy/append! (sel1 :#foo) [:li (str data)])
   		 (set! (.-scrollTop (sel1 :#foo)) (.-scrollHeight (sel1 :#foo)))))))

Finally, for when the page loads, we set up the functions;

(defn init! []

(defn ^:export init []
	(set! (.-onload js/window) init!))
Server side handling

Gotta love http-kit;

(:use [compojure.route :only [files resources not-found]]
	  [compojure.handler :only [site]] ; 
      [compojure.core :only [defroutes GET POST DELETE ANY context]]

Using compojure for routing;

(defroutes all-routes
	(GET "/async" [] async-handler)
	(resources "/js/" {:root "js"}))

It calls async-handler which just echos back;

(defn async-handler [request]
	(with-channel request channel
	(info "/async") ;; 
	(on-close channel (fn [status] (println "channel closed: " status)))
	(on-receive channel (fn [data] ;; echo it back
                      (send! channel data)
                      (info "Sent WebSocket Data.")))))

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