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 share 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 new design: 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. The module is probably too simple to share on Drupal.org so I figured I’d start with sharing it on my blog instead.

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: Previewer
description: Allows previewing of a theme by adding '?preview' to URLs.
package: Custom
type: module
core: 8.x

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

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.

Next, 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: DrupalpreviewerThemePreviewer
    tags:
      - { name: theme_negotiator, priority: 10 }

previewer.services.yml tells Drupal to call our class DrupalpreviewerThemePreviewer 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 complex, but it actually promotes reusable 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 previewer
previewer
├── previewer.info.yml
├── previewer.services.yml
└── src
    └── Theme
        └── Previewer.php

And that’s it!