Code Splitting

ClojureScript lets you break down a larger application into modules that can be dynamically loaded into your host environment at runtime.

This feature is covered in the Code Splitting Guide. I’m going to discuss how to use Figwheel with a build where you are utilizing code splitting.

Skip :main and :output-to

The first thing to note is that both the :main and the top-level :output-to compile options have no meaning and are ignored when you configure a code splitting build with :modules.

Previously, we have said that to use the --build Figwheel flag requires declaring a :main at the very least. This doesn’t apply when you configure a code splitting build.

Figwheel Code Splitting Guide

This requires figwheel-main version 0.1.5 or greater.

This is going to be a brief version of the official Code Splitting Guide with adjustments for working with Figwheel. You should read and understand that document first.

Make a Simple Project

Create a project folder:

mkdir -p hello-modules
cd hello-modules
mkdir src

Create a deps.edn file to work with Figwheel.

touch deps.edn

Edit this script to look like the following:

{:deps {com.bhauman/figwheel-main {:mvn/version "0.2.18"}}
 :paths ["src" "resources" "target"]}

The Sources

Create the foo.core namespace:

mkdir -p src/foo
touch src/foo/core.cljs

Edit this file to look like the following:

(ns ^:figwheel-load foo.core
  (:require [goog.dom :as gdom]
            [ :as events]
            [cljs.loader :as loader])
  (:import [ EventType]))

(println "I'm foo!")

(defn listen-to-button []
  (let [app (gdom/getElement "app")
        button (gdom/createDom
                (gdom/createTextNode "Load Bar!"))]
    (gdom/removeChildren app)
    (gdom/append app button)
    (events/listen button EventType.CLICK
                   (fn [e]
                     (loader/load :bar
                                  (fn []
                                    ((resolve 'bar.core/woz))))))))

(defonce init (listen-to-button))

(loader/set-loaded! :foo)

I altered the original example to make it reloadable and so that it works with the DOM on the Figwheel default dev page. I also added ^:figwheel-load metadata to the namespace so that I can easily load the file into my environment by saving it.

Create the bar.core namespace:

mkdir -p src/bar
touch src/bar/core.cljs
(ns bar.core
  (:require [cljs.loader :as loader]))

(println "I'm bar!")

(defn woz []
  (println "WOZ!"))

(loader/set-loaded! :bar)

Note I removed the (enable-console-print!) from the original examples. Figwheel already enables printing to the console by default. See the :client-print-to option.

Configure your build file

Let’s make our Figwheel build file support a :modules based build.

In a dev.cljs.edn file place the following:

^{:watch-dirs ["src"]}
{:modules {:foo {:entries #{foo.core}}
           :bar {:entries #{bar.core}}}}

This is all the compile configuration that’s needed for Figwheel to start building and hot reloading a build that is using code splitting.

The :watch-dirs config option is required when working with :modules.

Note that Figwheel will supply the :output-to config parameter for each of your modules if one doesn’t exist already.

If you do want to supply your own :output-to parameters to each module you will want to heed the advice that is true for the top level :output-to option. It needs to be on a path that the client can load and thus should normally be on the classpath under a public directory.

If you want to choose your :output-to you will probably also want to configure your top level :output-dir and :asset-path options as well.

You can learn more about the configuration params that Figwheel provides by using the -pc option with the above minimal configuration.

For example if you run:

clojure -m figwheel.main -pc -b dev -r

It will show compile configuration to be this:

---------------------- Compiler options ----------------------
  {:entries #{foo.core},
   :output-to "target/public/cljs-out/dev-foo.js"},
  {:entries #{bar.core},
   :output-to "target/public/cljs-out/dev-bar.js"}},
 :output-dir "target/public/cljs-out/dev",
 :asset-path "/cljs-out/dev",
 :aot-cache false,
 ([figwheel.repl :refer-macros [conns focus]]
   [stop-builds start-builds build-once reset clean status]]
  [cljs.pprint :refer [pprint] :refer-macros [pp]]
  [cljs.repl :refer-macros [source doc find-doc apropos dir pst]])}

The main difference here and an ordinary build is that there is neither a :main option nor a top-level :output-to option.

Build and start a REPL

Note that we haven’t created a host HTML page for our build yet. The Figwheel default dev page should work and host a REPL for your :modules build.

To begin just start the build as usual:

clojure -m figwheel.main -b dev -r

You will see the default dev page open up and a REPL start.

It is important to note that neither of the modules has been loaded yet. You can require (or use :figwheel-load metadata) to load them in to the client environment and start working on them in the normal hot reloaded workflow.

Since we already included ^:figwheel-load metadata on the foo.core namespace you can make a small whitespace change to the src/foo/core.cljs file and you will see a Load Bar! button replace all the current content of the page. Open the development console and press the button so you can see the dynamic loading at work.

Host page

We’re going to make a Host page now. Almost every thing that we covered on the Host Page doc still applies here. The only difference is that we need to load a cljs_base.js file along with any modules we want to initialize the page with.

Now make a resources/public/index.html file:

    <div id="app"></div>
    <!-- include the cljs_base.js target file -->
    <script src="target/cljs-out/dev/cljs_base.js" type="text/javascript"></script>
    <!-- You will normally want to include at least one module otherwise nothing will happen -->		 
    <script src="target/cljs-out/dev-foo.js" type="text/javascript"></script>

Note the name of the default :output-to file for the :foo module is [build-id]-[module-name].js.

If you customize the :output-to’s for the modules you will need to alter the paths on your host page accordingly.

Build the Project

Now if we return to the shell and build the project again:

clojure -m figwheel.main -b dev -r

We will see the page with our button on it and get a working REPL. We are now in a normal Figwheel hot reloaded workflow.

Release Compile

The release compile is the same as other advanced builds:

clojure -m figwheel.main -O advanced -bo dev

The advanced compile still outputs a cljs_base.js file and a compressed output file for each module so your HTML host page will still work just fine.