Drupal - How can I programmatically change the active theme?
In Drupal 8, you use theme negotiators, which essentially are services using a specific tag. See the theme negotiators implemented by Drupal, to understand exactly how they work; the example given in the change record is not updated.
user.services.yml
theme.negotiator.admin_theme:
class: Drupal\user\Theme\AdminNegotiator
arguments: ['@current_user', '@config.factory', '@entity_type.manager', '@router.admin_context']
tags:
- { name: theme_negotiator, priority: -40 }
AdminNegotiator.php
<?php
namespace Drupal\user\Theme;
use Drupal\Core\Config\ConfigFactoryInterface;
use Drupal\Core\DependencyInjection\DeprecatedServicePropertyTrait;
use Drupal\Core\Entity\EntityTypeManagerInterface;
use Drupal\Core\Routing\AdminContext;
use Drupal\Core\Routing\RouteMatchInterface;
use Drupal\Core\Session\AccountInterface;
use Drupal\Core\Theme\ThemeNegotiatorInterface;
/**
* Sets the active theme on admin pages.
*/
class AdminNegotiator implements ThemeNegotiatorInterface {
use DeprecatedServicePropertyTrait;
/**
* {@inheritdoc}
*/
protected $deprecatedProperties = ['entityManager' => 'entity.manager'];
/**
* The current user.
*
* @var \Drupal\Core\Session\AccountInterface
*/
protected $user;
/**
* The config factory.
*
* @var \Drupal\Core\Config\ConfigFactoryInterface
*/
protected $configFactory;
/**
* The entity type manager.
*
* @var \Drupal\Core\Entity\EntityTypeManagerInterface
*/
protected $entityTypeManager;
/**
* The route admin context to determine whether a route is an admin one.
*
* @var \Drupal\Core\Routing\AdminContext
*/
protected $adminContext;
/**
* Creates a new AdminNegotiator instance.
*
* @param \Drupal\Core\Session\AccountInterface $user
* The current user.
* @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
* The config factory.
* @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
* The entity type manager.
* @param \Drupal\Core\Routing\AdminContext $admin_context
* The route admin context to determine whether the route is an admin one.
*/
public function __construct(AccountInterface $user, ConfigFactoryInterface $config_factory, EntityTypeManagerInterface $entity_type_manager, AdminContext $admin_context) {
$this->user = $user;
$this->configFactory = $config_factory;
$this->entityTypeManager = $entity_type_manager;
$this->adminContext = $admin_context;
}
/**
* {@inheritdoc}
*/
public function applies(RouteMatchInterface $route_match) {
return ($this->entityTypeManager->hasHandler('user_role', 'storage') && $this->user->hasPermission('view the administration theme') && $this->adminContext->isAdminRoute($route_match->getRouteObject()));
}
/**
* {@inheritdoc}
*/
public function determineActiveTheme(RouteMatchInterface $route_match) {
return $this->configFactory->get('system.theme')->get('admin');
}
}
The code is pretty easy to understand: The applies()
method returns TRUE
when the current route is the one for which your module wants to change the theme; the determineActiveTheme()
method returns the theme machine name of the theme to apply.
See also ThemeNegotiator::determineActiveTheme() should not require a RouteMatch to be passed in for a possible change on the arguments received from the methods used by theme negotiators; if that patch is applied, you will need to change your theme negotiator code too.
Another solution to this question is using the Theme Switcher module. it's compatible with Drupal 8 and 9 that developed by themeKey module developer.
Theme Switcher module allows you to create theme-switching rules which allow automatic selection of a theme based on Drupal 8 Conditions system. Because of this it can also be easily extended to support additional custom conditions exposed by other modules. In Drupal 8 conditions are no longer defined through a hook and they are plugins now.
With this module you can apply a specific theme to a unique node, use different themes based on the page language, use contrib conditions like a domain to apply one theme or another. The best of all is that you can do al of the above at once!