Opal & Sprockets

Opal comes with built-in sprockets support, and provides a simple Opal::Server class to make it easy to get a rack server up and running for trying out opal. This server will automatically recompile ruby sources when they change, meaning you just need to refresh your page to autorun.

Getting setup

Add opal to a Gemfile:

source 'https://rubygems.org'

gem 'opal', '>= 0.6.0'

And install with bundle install.

We need a directory to hold our opal code, so create app/ and add a simple demo script to app/application.rb:

# app/application.rb
require 'opal'

puts "hello world"

Finally, we need a html page to run, so create index.html:

<!DOCTYPE html>
    <meta charset="utf-8">
    <title>opal server example</title>
    <script src="/assets/application.js"></script>

Using Opal::Server

Opal::Server can be run like any rack app, so just add a config.ru file:

# config.ru
require 'bundler'

run Opal::Server.new { |s|
  s.append_path 'app'

  s.main = 'application'

  s.index_path = 'index.html'

This rack app simply adds our app/ directory to opal load path, and sets our main file to application, which will be found inside app/.

Running the app

Run bundle exec rackup and visit the page http://localhost:9292 in any browser. Observe the console to see the printed statement.

You can just change app/application.rb and refresh the page to see any changes.

Using an existing sprockets instance

We only need to append Opal paths to the existing sprockets instance.

require 'sprockets'
environment = Sprockets::Environment.new

require 'opal'
Opal.paths.each do |path|
  environment.append_path path