Events and hooks

In Drupal 8, there are two methods of responding to interacting with the system and other modules. The first is the normal hook system that is the same as previous versions of Drupal. The second is the event system using the Symfony EventDispatcher. The EventDispatcher system is still new to Drupal, so there aren't many places in Drupal that can use it and you'll find yourself using hooks for a lot of things still. You can see a list of the ones available at https://api.drupal.org/api/drupal/core!core.api.php/group/events/8. But the EventDispatcher is definitely the future of interacting with the Drupal system.

Using the EventDispatcher

An event listener is first and foremost just another type of...