Have you ever wanted to preview your new Drupal theme in a production environment without making it the default yet?

I did when I was working on my redesign of dri.es earlier in the year. I wanted the ability to add ?preview to the end of any URL on dri.es and have that URL render in my upcoming theme.

It allowed me to easily preview my new design with a few friends and ask for their feedback. I would send them a quick message like this: Hi Matt, check out an early preview of my site's upcoming redesign: https://dri.es?preview. Please let me know what you think!.

Because I use Drupal for my site, I created a custom Drupal 8 module to add this functionality.

Like all Drupal modules, my module has a *.info.yml file. The purpose of the *.info.yml file is to let Drupal know about the existence of my module and to share some basic information about the module. My theme preview module is called Previewer so it has a *.info.yml file called previewer.info.yml:

name: Previewerdescription: Allows previewing of a theme by adding <code>?preview to URLs.
package: Custom
type: module
core: 8.x

The module has only one PHP class, Previewer, that implements Drupal's ThemeNegotiatorInterface interface:

<?php namespace Drupal\previewer\Theme;use Drupal\Core\Routing\RouteMatchInterface;use Drupal\Core\Theme\ThemeNegotiatorInterface;class Previewer implements ThemeNegotiatorInterface {  /**   * The function applies() determines if it wants to set the    * active theme. If the ?preview query string is part of the   * URL, return TRUE to denote that Previewer wants to set   * the theme. determineActiveTheme() will be called to   * ask for the theme's name.   */  public function applies(RouteMatchInterface $route_match) {    if (isset($_GET['preview'])) {      return TRUE;    }    return FALSE;  }  /**   * The function determineActiveTheme() is responsible   * for returning the name of the theme that is to be used.   */  public function determineActiveTheme(RouteMatchInterface $route_match) {    return 'dries'; // Yes, the name of my theme is 'dries'.  }}?>

The function applies() checks if ?preview is set as part of the current URL. If so, applies() returns TRUE to tell Drupal that it would like to specify what theme to use. If Previewer is allowed to specify the theme, its determineActiveTheme() function will be called. determineActiveTheme() returns the name of the theme. Drupal uses the specified theme to render the current page request.

For this to work, we have to tell Drupal about our theme negotiator class Previewer. This is done by registering it a service in previewer.services.yml:

services:  theme.negotiator.previewer:    class: Drupal\previewer\Theme\Previewer    tags:      - { name: theme_negotiator, priority: 10 }

previewer.services.yml tells Drupal to call our class Drupal\previewer\Theme\Previewer when it has to decide what theme to load.

A service is a common concept in Drupal (inherited from Symfony). Many of Drupal's features are separated into a service. Each service does just one job. Structuring your application around a set of independent and reusable service classes is an object-oriented programming best-practice. To some it might feel unnecessarily complex, but it actually promotes reusable, configurable and decoupled code.

Note that Drupal 8 adheres to PSR-4 namespaces and autoloading. This means that files must be named in specific ways and placed in specific directories in order to be recognized and loaded. Here is what my directory structure looks like:

$ tree previewerpreviewer├── previewer.info.yml├── previewer.services.yml└── src    └── Theme        └── Previewer.php

And that's it!