module ActionController
  # When our views change, they should bubble up into HTTP cache freshness
  # and bust browser caches. So the template digest for the current action
  # is automatically included in the ETag.
  #
  # Enabled by default for apps that use Action View. Disable by setting
  #
  #   config.action_controller.etag_with_template_digest = false
  #
  # Override the template to digest by passing +:template+ to +fresh_when+
  # and +stale?+ calls. For example:
  #
  #   # We're going to render widgets/show, not posts/show
  #   fresh_when @post, template: 'widgets/show'
  #
  #   # We're not going to render a template, so omit it from the ETag.
  #   fresh_when @post, template: false
  #
  module EtagWithTemplateDigest
    extend ActiveSupport::Concern

    include ActionController::ConditionalGet

    included do
      class_attribute :etag_with_template_digest
      self.etag_with_template_digest = true

      ActiveSupport.on_load :action_view, yield: true do
        etag do |options|
          determine_template_etag(options) if etag_with_template_digest
        end
      end
    end

    private
    def determine_template_etag(options)
      if template = pick_template_for_etag(options)
        lookup_and_digest_template(template)
      end
    end

    def pick_template_for_etag(options)
      options.fetch(:template) { "#{controller_name}/#{action_name}" }
    end

    def lookup_and_digest_template(template)
      ActionView::Digestor.digest name: template, finder: lookup_context
    end
  end
end
