build_observer

# File actionpack/lib/action_view/helpers/prototype_helper.rb, line 704
      def build_observer(klass, name, options = {})
        if options[:with] && !options[:with].include?("=")
          options[:with] = "'#{options[:with]}=' + value"
        else
          options[:with] ||= 'value' if options[:update]
        end

        callback = options[:function] || remote_function(options)
        javascript  = "new #{klass}('#{name}', "
        javascript << "#{options[:frequency]}, " if options[:frequency]
        javascript << "function(element, value) {"
        javascript << "#{callback}}"
        javascript << ", '#{options[:on]}'" if options[:on]
        javascript << ")"
        javascript_tag(javascript)
      end