A Turbolinks 5 drop-in for when permanent partials can't be lazy loaded (no-script requirement).

base.rb

# A turbolinks 5 drop-in for when permanent partials can't be lazy loaded (no-script requirement).
#
# Whenever a static partial is really heavy, it is undesirable to render it on every/many request(s), so this render drop-in only renders the partial's content if the `Turbolinks-Referrer` header is *not* present.
#
#   Usage:
#   ```ruby
# render partial: 'navbar', permanent: true
# # => <div id="turbolinks_navbar" data-turbolinks-permanent="true">[Always rendered]</div> # Behaviour as described in the RailsConf '15 keynote, where HTML might be lazy loaded via JS
#
# render partial: 'navbar', permanent: 'navbar'
# # => <div id="navbar" data-turbolinks-permanent="true">[Always rendered]</div>
#
# render partial: 'navbar', permanent: {id: 'unrelated_id'}
# # => <div id="unrelated_id" data-turbolinks-permanent="true">[Always rendered]</div>
#
# render partial: 'navbar', permanent: {turbolinks_render: false}
# # => <div id="turbolinks_navbar" data-turbolinks-permanent="true">[Never rendered (server side)]</div>
#
# render partial: 'navbar', permanent: {prerender: true, turbolinks_render: false}
# # => <div id="turbolinks_navbar" data-turbolinks-permanent="true">[Rendered on only on non TL requests]</div>
# ```
module ActionView
  class Base

    # Turbolinks that doesn't rerenders permanent partials on turbolink requests.
    # @overload turbolinks_render(options)
    #   @param [Hash] options
    #   @option options [String] :permanent The ID of the partial
    #   @option options [Hash] :permanent An options hash
    #     @option permanent [Hash] :id The id of the wrapper element
    #     @option permanent [Hash] :tag (:div) The element type of the wrapper tag
    def turbolinks_render(*args, &block)
      options = normalize_tl_options(*args)

      return normal_render(*args, &block) unless options

      if tl_should_render_for(options)
        inner_content = normal_render *args, &block
      end

      if options.fetch(:use_wrapper, true)
        content_tag options.fetch(:tag, :div),
                    inner_content,
                    options_for_renderer(options)
      else
        inner_content
      end
    end
    alias_method :normal_render, :render
    alias_method :render, :turbolinks_render


    private

    def normalize_tl_options(*args)
      render_options = args.first
      options = render_options.is_a?(Hash) && render_options.fetch(:permanent, nil)
      return nil unless options.present?

      if options.is_a?(Hash)
        options.presence
      else
        {id: partial_tl_id(*args)}
      end
    end

    # Defines the options that are passed to the turbolinks wrapper container
    def options_for_renderer(options)
      {
        id: options.fetch(:id),
        data: {turbolinks_permanent: true}
      }.merge(options.fetch(:html_options, {}))
    end

    # Generates the partial's wrapper id based on
    # @return [String] id The id passed as an option, or one based on the partial name
    def partial_tl_id(*args)
      permanent = args.first.fetch(:permanent, nil)
      return nil if permanent.blank?
      if permanent.is_a?(String)
        permanent
      elsif permanent.is_a?(Hash) && permanent.fetch(:id).present?
        permanent.fetch(:id)
      elsif permanent && args.first.fetch(:partial).present?
        "turbolinks_#{args.first.fetch(:partial)}"
      end
    end

    # Determines whether Turbolinks should render the partial's body
    def tl_should_render_for(options)
      if request.headers['Turbolinks-Referrer'].present?
        options.fetch(:turbolinks_render, true)
      else
        options.fetch(:prerender, true)
      end
    end

  end
end