examples-3.x-dev/modules/events_example/events_example.services.yml

# Subscribing to an event requires you to create a new service tagged with the
# 'event_subscriber' tag. This tells the service container, and by proxy the
# event dispatcher service, that the class registered here can be queried to get
# a list of events that it would like to be notified about.
#
# For more on defining and tagging services see
# https://api.drupal.org/api/drupal/core%21core.api.php/group/container/8.2.x
services:

  # Give your service a unique name; the convention is to prefix service names
  # with the name of the module that implements them.
  events_example_subscriber:

    # Point to the class that will contain your implementation of
    # \Symfony\Component\EventDispatcher\EventSubscriberInterface
    class: Drupal\events_example\EventSubscriber\EventsExampleSubscriber
    tags:
      - {name: event_subscriber}