drupal/core/vendor/Symfony/Component/EventDispatcher

EventDispatcher Component

EventDispatcher implements a lightweight version of the Observer design pattern.

use Symfony\Component\EventDispatcher\EventDispatcher;
use Symfony\Component\EventDispatcher\Event;

$dispatcher = new EventDispatcher();

$dispatcher->addListener('event_name', function (Event $event) {
    // ...
});

$dispatcher->dispatch('event_name');

Resources

Unit tests:

https://github.com/symfony/symfony/tree/master/tests/Symfony/Tests/Component/EventDispatcher