Drupal 8 EventSubscriber that sets the category field based on entity browser id

EventSubscriber.php

<?php

/**
 * @file
 * Contains \Drupal\image_library\EventSubscriber.
 */

namespace Drupal\image_library\EventSubscriber;

use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\EventDispatcher\Event;
use Drupal\entity_browser\Events\Events;
use Drupal\entity_browser\Events\EntitySelectionEvent;

/**
 * Class EventSubscriber.
 *
 * @package Drupal\image_library
 */
class EventSubscriber implements EventSubscriberInterface {

  private $browser_prefix = 'image_library_browser';
  private $vocabolary = 'image_categories';

  /**
   * {@inheritdoc}
   */
  static function getSubscribedEvents() {
    $events[Events::SELECTED][] = array('onSelected');
    return $events;
  }

  /**
   * Set image category to image fields based on entity browser id
   * uses the following convention for browser id to figure out target category
   * browser id: media_library_browser__{{term_name}}
   */
  public function onSelected(EntitySelectionEvent $event) {
    $browser = $event->getBrowserID();

    // Ignore if we are not dealing with a media library browser
    if (strpos($browser, $this->browser_prefix) === false) {
      return;
    }

    // Figure out target category from browser id
    $category  = str_replace($this->browser_prefix . '__', '', $browser);
    if (empty($category)) {
      continue;
    }

    // Only act on entities that has no image category set
    $uncategorized = array_filter($event->getEntities(), function ($entity) {
      return !isset($entity->field_category->entity);
    });

    // Loop through image entities and set categories
    foreach($uncategorized as $entity) {
      // Get target tid from category
      $terms = taxonomy_term_load_multiple_by_name($category, $this->vocabolary);
      if (empty($terms)) {
        continue;
      }

      // Get first matched term tid
      $tid = reset(array_keys($terms));

      // Save tid on entity
      $entity->field_category->target_id = $tid;
      $entity->save();
    }

  }

}