module Turbolinks
  # Intercepts calls to _compute_redirect_to_location (used by redirect_to) for two purposes.
  #
  # 1. Corrects the behavior of redirect_to with the :back option by using the X-XHR-Referer
  # request header instead of the standard Referer request header.
  #
  # 2. Stores the return value (the redirect target url) to persist through to the redirect 
  # request, where it will be used to set the X-XHR-Redirected-To response header.  The 
  # Turbolinks script will detect the header and use replaceState to reflect the redirected
  # url. 
  module XHRHeaders
    extend ActiveSupport::Concern

    def _compute_redirect_to_location(*args)
      options, request = _normalize_redirect_params(args)

      store_for_turbolinks begin
        if options == :back && request.headers["X-XHR-Referer"]
          super(*[(request if args.length == 2), request.headers["X-XHR-Referer"]].compact)
        else
          super(*args)
        end
      end
    end

    private
      def store_for_turbolinks(url)
        session[:_turbolinks_redirect_to] = url if request.headers["X-XHR-Referer"]
        url
      end

      def set_xhr_redirected_to
        if session[:_turbolinks_redirect_to]
          response.headers['X-XHR-Redirected-To'] = session.delete :_turbolinks_redirect_to
        end
      end

      # Ensure backwards compatibility
      # Rails < 4.2:  _compute_redirect_to_location(options)
      # Rails >= 4.2: _compute_redirect_to_location(request, options)
      def _normalize_redirect_params(args)
        options, req = args.reverse
        [options, req || request]
      end
  end
end
