dynamic_less_css.rb

# Example Rails Usage:
# config.middleware.use Rack::LessCss, "/stylesheets", File.join(RAILS_ROOT, "public", "stylesheets")
#
# public/stylesheets/foo.less is now used to render /stylesheets/foo.css
#
require 'less'
module Rack
  class LessCss
    File = ::File
    def initialize(app, css_path, css_dir)
      @app, @css_path, @css_dir = app, css_path, css_dir
    end

    def generate_content_for(request_path)
      if request_path =~ %r{^#{@css_path.chomp("/")}/(.+)\.css$}
        if File.exists?(path = File.join(@css_dir, "#{$1}.css"))
          File.read(path)
        elsif File.exists?(path = File.join(@css_dir, "#{$1}.less"))
          Less::Engine.new(File.read(path)).to_css
        end
      end
    end

    def call(env)
      begin
        rv = generate_content_for(env["REQUEST_URI"] || env["PATH_INFO"])
      rescue => e
        return [500, {"Content-Type" => "text/plain"}, [e.class.name, e.message, e.backtrace.join("\n")]]
      end

      if rv
        [200, {"Content-Type" => "text/css", "Cache-Control" => "max-age=3600, public"}, [rv]]
      else
        @app.call(env)
      end
    end
  end
end