opal.rb

    class Server
      attr_accessor :headers

      def initialize debug_or_options = {}
        unless Hash === debug_or_options
          warn "passing a boolean to control debug is deprecated.\n"+
            "Please pass an Hash instead: Server.new(debug: true)"
          options = {:debug => debug_or_options}
        else
          options = debug_or_options
        end

        @headers     = {}
        @use_index   = true
        @public_root = nil
        @public_urls = ['/']
        @sprockets   = options.fetch(:sprockets, ::Sprockets::Environment.new)
        @debug       = options.fetch(:debug, true)
        @prefix      = options.fetch(:prefix, '/assets')

        Opal.paths.each { |p| @sprockets.append_path(p) }

        yield self if block_given?
        create_app
      end

      def headers
        @headers
      end

      def create_app
        server, sprockets, prefix, headers = self, @sprockets, self.prefix, @headers
        sprockets.logger.level ||= Logger::DEBUG
        source_map_enabled = self.source_map_enabled
        if source_map_enabled
          maps_prefix = SOURCE_MAPS_PREFIX_PATH
          maps_app = SourceMapServer.new(sprockets, maps_prefix)
          ::Opal::Sprockets::SourceMapHeaderPatch.inject!(maps_prefix)
        end

        @app = Rack::Builder.app do
          not_found = lambda { |env| [404, {}, []] }
          use Rack::Deflater
          use Rack::ShowExceptions
          use Index, server if server.use_index
          use Headers, server
          if source_map_enabled
            map(maps_prefix) do
              require 'rack/conditionalget'
              require 'rack/etag'
              use Rack::ConditionalGet
              use Rack::ETag
              run maps_app
            end
          end
          map(prefix) { run sprockets }
          run Rack::Static.new(not_found, root: server.public_root, urls: server.public_urls)
        end
      end

      class Headers
        def initialize(app, server)
          @app    = app
          @server = server
        end

        def call(env)
          status, headers, body = @app.call env

          if env['PATH_INFO'][@server.prefix]
            status, headers, body = @app.call env
            [status, headers.merge(@server.headers), [body]]
          else
            @app.call env
          end
        end
      end
    end