source_maps.rb

# Steps to implement:
#   1. Ensure you're using >= coffeescript-1.6.1
#
#   2. Put this file in an initializer.
#
#   3. Edit an environment's config to turn on source maps:
#
#      `config.assets.sourcemaps = true`
#
#   4. Add /public/sourcemaps to your .gitignore
#
#   5. Turn on source maps in Google Chrome:
#      http://www.html5rocks.com/en/tutorials/developertools/sourcemaps/#toc-realworld

require 'tilt/template'

module Tilt
  class CoffeeScriptTemplate < Template
    def evaluate(scope, locals, &block)
      if Rails.configuration.assets.sourcemaps
        @output ||= begin
          root_filename = scope.pathname.basename.to_s.split('.').first

          # Get a relative path of our original Coffeescript file
          rel_path = if scope.pathname.to_s.start_with?(Bundler.bundle_path.to_s)
            Pathname('bundler').join(scope.pathname.relative_path_from(Bundler.bundle_path)).dirname
          else
            scope.pathname.relative_path_from(Rails.root).dirname
          end

          map_files_path    = Rails.root.join('public','sourcemaps', rel_path)
          map_file_path     = map_files_path.join("#{root_filename}.map")
          coffee_file_path  = map_files_path.join("#{root_filename}.coffee")

          FileUtils.mkdir_p(map_files_path) unless File.directory?(map_files_path)

          compiled = CoffeeScript.compile(data, options.merge(pathname: scope.pathname, sourceMap:true, filename:"#{root_filename}.coffee"))

          # Put the original .coffee file and its map file in /public/sourcemaps/... where the browser can get it
          coffee_file_path.open('w') {|f| f.puts data }
          map_file_path.open('w')    {|f| f.puts compiled["v3SourceMap"]}

          # Add the header declaration to the js file, telling browser where to find the mappings files
          "//@ sourceMappingURL=/#{map_file_path.relative_path_from(Rails.root.join('public'))}\n\n#{compiled['js']}"
        end
      else
        @output ||= CoffeeScript.compile(data, options)
      end
    end
  end
end