How to add additional drupalSettings to all pages with an event subscriber

Question

I need to set the user timezone on all pages in the JS drupalSettings object, so that it can be accessed on any page. As drupalSettings.user is provided by core, I want to set drupalSettings.user.timezone, with the user timezone from the server.

While this can quickly and easily be done in hook_page_attachments(), I am trying to switch from using hooks, to using Event Subscribers, as Drupal hooks are the old paradigm, and event subscribers is the new paradigm.

Can someone explain how to add attached assets (a library, or drupalSettings) to all pages from an event subscriber?

Additional Context

As mentioned by 4uk4 in the comments, this event is not dispatched by Drupal core, but rather by Symfony underneath the hood. This means it is not a true 'Drupal' methodology, as core maintainers have not chosen to dispatch an event that is equivalent to hook_page_attachments(). So which is better? The answer is 'neither' at this point, as core is ambivalent as to whether it invokes a hook or dispatches an event. It does however seem that core is moving towards creating events that match existing hooks, as can be seen in this issue: https://www.drupal.org/project/drupal/issues/2551893. And here is a discussion that considers the differences between hooks and events: https://www.previousnext.com.au/blog/alter-or-dispatch-drupal-8-events-versus-alter-hooks

Answer 1

This can be done by acting upon the Symfony\Component\HttpKernel\KernelEvents::RESPONSE event. See the code below for an example (Note - you'll need to register the event subscriber, which is not shown here):

namespace Drupal\[MODULE]\EventSubscriber;

use Drupal\Core\Render\AttachmentsInterface;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpKernel\KernelEvents;

/**
 * Adds the user's timezone to the drupalSettings JS object on all pages.
 */
class SetUserTimezoneToDrupalSettings implements EventSubscriberInterface {

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() {
    return [
      // Respond to KernelEvents::RESPONSE.
      KernelEvents::RESPONSE => ['onResponse', 1000],
    ];
  }

  /**
   * Event handler for KernelEvents::RESPONSE.
   */
  public function onResponse(\Symfony\Component\HttpKernel\Event\ResponseEvent $event) {
    // Only act upon the master request and not sub-requests.
    if ($event->isMasterRequest()) {
      $response = $event->getResponse();
      // Only act if the response is one that is able to have attachments.
      if ($response instanceof AttachmentsInterface) {
        // Get any existing attachments.
        $attachments = $response->getAttachments();
        // Set the user's timezone to the drupalSettings array.
        $attachments['drupalSettings']['user']['timezone'] = date_default_timezone_get();
        // Set the updated array back on the object.
        $response->setAttachments($attachments);
      }
    }
  }

}