Well long break,Let me guess It over 3 month since I wrote my last post. I’ve been kind of  busy lately not manage to dedicate much time to thing that I do.

Rails 3.1.0 is been in picture for a quite a while a now and major feature that been included in Rails 3.1.0 is asset pipeline and pre-processor.

For those is who aren’t aware of Rails 3.1.0 asset pipeline and pre-processor there is a very good article on rails guide and recently Ryan Bates introduce a screencast on Rails 3.1.0 asset pipeline.

I’m not here to talk about asset pipeline you can get it all if you just googled it.

So what I’m here upto then ? 🙂

Well ever since I was made aware of what asset pipeline  and pre-processor more importantly pre-processor ,it always trilled me of using a custom pre-processor .

As you all know that Rails 3.1.0.rc1 is bundle with coffee-script and sass support. In your asset folder you may find js and css file that look something like this “*.js.coffee” , “*.css.scss“. Now you can add a chain of pre-processor that just look like this “*.js.coffee.erb” , “*.css.scss.erb”

The post is all about chaining your own pre-processor just as above something like “*.js.coffee.mustache”

Mustache (Mustache is taken just for example here you can use your own) because I been in love with it ever since I explored it . So how to accomplish this .

Stay with me as I will show how ?

Firstly you need to be aware of Sprockets and Tilt library

First the code

config/initializers/mustache_template.rb

require "mustache"
require "tilt"

class MustacheTemplate < Tilt::Template
  def prepare; end

  def evaluate(context, locals, &block)
    @data = ::Mustache.render @data ,:mustache_variable => "World" ,:name => "Viren Negi"
  end

end

Rails.application.assets.register_preprocessor 'application/javascript', MustacheTemplate

That all it take to define a custom preprocessor Before I explain the above code let me show what exactly my preprocessed file look like .

app/assets/javascripts/user.js.coffee.mustache

# Here is the custom mustache and coffee-script code

a =  "Hello {{ mustache_variable }} from {{ name }}";

if a
  alert a
else
  alert "Sorry LOL"
  alert "The co-processor is not working"

well as you would have guess I received and alert prompt on UI stating “Hello World from Viren Negi”

Well that quite neat 🙂 .

Let me describe what happening under the hood.

Rails.application.assets is Sprocket::Environment instance as you would guess register_preprocessor is Sprocket method to register the preproccessor .

a closer look at the Sprocket ” register_preprocessor “ method

sprockets/lib/sprockets/processing.rb

 def register_preprocessor(mime_type, klass, &block)
      expire_index!

      if block_given?
        name  = klass.to_s
        klass = Class.new(Processor) do
          @name      = name
          @processor = block
        end
      end

      @preprocessors[mime_type].push(klass)
    end

As you guess till now that the method except 2 argument and block (if given).

For all those dump guys like me who still haven’t figure it out.
What it basically does is “it push the class to mime_type” in our case (mime_type = “application/js” and klass = MustacheTemplate)

what it internally does when processing the template is some Tilt magic.

The ruby code

def prepare ;   end

def evaluate(context,locals,&block)

...

...

end

defined above are basically the Tilt method which is overridden in MustacheTemplate to process the template .

That it that how you add your own custom pre-processor .

Hope that it help you in same way or other.

Thanks

Viren Negi