Render Sass stylesheets dynamically

If - for whatever reason - you have to render stylesheets dynamically, the following snippet might be of help. It emulates what "sprockets" would to when precompiling your assets, and give your stylesheets access to all the regular bells and whistles (like asset_path, proper @imports etc):

Copyclass DynamicStylesheetsController < ApplicationController

    def show
      logical_path = RELATIVE_PATH_TO_YOUR_TEMPLATE
      path = File.join(Rails.root, logical_path)
      template = Sass::Rails::SassTemplate.new(path)
      environment = YourApp::Application.assets
      context = environment.context_class.new(environment, logical_path, Pathname.new(path))
      output = template.render(context)
      render :text => output, :content_type => 'text/css'
    end
    
end

Injecting information

I needed to inject some color values into my stylesheets, and did not want to run everything through ERB first (which is also hard, because of @import statements etc). So I did basically this:

1. Before the template.render, insert:

   Copycontext.singleton_class.send(:define_method, :theme_colors) do
     HASH_OF_COLOR_VALUES # e.g. { 'primary' => [100, 250, 10] }
   end
   

2. In an initializer, add

   Copymodule Sass::Script::Functions
     def color(color_name)
       assert_type color_name, :String
       context = @options[:custom][:resolver].context
       Sass::Script::Color.new(context.theme_colors[color_name.to_s])
     end
     declare :color, :args => [:string]
   end
   

3. In your sass template, use something like

   Copy$PRIMARY_COLOR = color(primary)
   

Performance

Sass is not especially fast (and it usually does not have to be). For my medium-sized stylesheet, rendering took about 0.5 seconds. It's probably still a good idea to

• use some form of action caching or page caching
• make sure you set proper expiration and cache headers

Caveat

This solution is probably somewhat brittle, since the methods used are not exactly part of a public API. If you know of a cleaner way to do this, please drop me a line.