h2. Asset Pipeline

This guide will cover the ideology of the asset pipeline introduced in Rails 3.1.
By referring to this guide you will be able to:

* Properly organize your application assets
* Understand the benefits of the asset pipeline
* Adding a preprocessor to the pipeline
* Package assets with a gem

endprologue.

h3. What Is The Asset Pipeline?

The asset pipeline is a new feature introduced in Rails 3.1 using the "Sprockets":http://getsprockets.org/ engine. It allows developers to place design elements in +app/assets+ instead of +public+, there are many advantages to this. A big one is that they are now processed by Rails instead of your webserver, allowing you to use preprocessors like CoffeeScript or SCSS. Another advantage is that your CSS and JavaScript is compiled into one file by default, this allows users to cache all the CSS and JavaScript data so your pages render faster.

h4. Why Should I Use it?

Using the asset pipeline allows you to package JavaScript, CSS, or images with your Rails application, library, or plugin. It also makes it easy to create dynamic CSS with LESS and clean JavaScript with CoffeeScript to name a few of the popular preprocessors. The big difference is that they are now categorized in app/assets, treating them like first-class citizens instead of just throwing them into categorized folders in the public directory.

h3. How to Use the Asset Pipeline

h4. Asset Organization

h4. Default Files Loaded

h4. Directives

h4. Stacking Preprocessors

h3. Packaging Assets with Your Gems

You may find it useful to package certain assets with your gem. A simple example would be the "pjax_rails":https://github.com/rails/pjax_rails/ gem. This gem bundles the latest "PJAX":https://github.com/defunkt/jquery-pjax library and some helper methods. If you take a look at the source of pjax_rails, you'll see that it bundles the assets in +lib/assets+ just the same way as you would in +app/assets+. This allows pjax_rails to update the PJAX file without forcing you to copy it to your public folder like you had to pre Rails 3.1. But if you want the user to load your JavaScript files in their template, you will have to ask them to add a directive to do so. Also avoid any common names such as +form_check.js+ instead try using +mygem/form_check.js+ so it's clear where it's coming from. This will also make it unlikely that your users will create a file with the same name causing the asset pipeline to choose the user's file over yours.

h3. More on Sprockets

Sprockets is the engine that handles the asset pipeline in Rails 3.1 and above. Their official website is available at "http://getsprockets.org/":http://getsprockets.org/ and the source code is "available on github":https://github.com/sstephenson/sprockets.
