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In Drupal 8 is possible to use event subscriber that is the separate class that defines the callback that should be called on the defined event. See Event Systems Overview & How To Subscribe To and Dispatch Events

But I'm wondering how to define event listener that will trigger the method of the service on the desired event?

here Symfony references:

Events and Event Listeners

Built-in Symfony Events

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    You can't define an event listener in a service (that wouldn't make sense, what's guaranteeing that the service has even been instantiated when the event is raised?). But you can inject a service into the event listener, and call a method on it that way, which achieves the same thing – Clive Sep 19 '18 at 14:29
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    You could point an event listener to any php callable, but Drupal doesn't use event listeners, only subscribers. An event subscriber class is always a service, implementing getSubscribedEvents() to define which class method to trigger on the desired event. – 4k4 Sep 19 '18 at 16:45
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If I understood correctly, I think you are asking about implementing minimalistic single responsibility event listeners declared in the YML config file, like in this Java example:

class MyListener implements ActionListener {
  public void actionPerformed(ActionEvent e) {
    myTextArea.append('Hello from MyListener');
  }
}

That is not possible since this is this what the EventDispatcher actually does:

    public function addSubscriber(EventSubscriberInterface $subscriber)
    {
        foreach ($subscriber->getSubscribedEvents() as $eventName => $params) {
            if (is_string($params)) {
                $this->addListener($eventName, array($subscriber, $params));
            } elseif (is_string($params[0])) {
                $this->addListener($eventName, array($subscriber, $params[0]), isset($params[1]) ? $params[1] : 0);
            } else {
                foreach ($params as $listener) {
                    $this->addListener($eventName, array($subscriber, $listener[0]), isset($listener[1]) ? $listener[1] : 0);
                }
            }
        }
    }

You can still have your separate ActionListener's, but you would have to write some code to call them from your implementation of EventSubscriberInterface.

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  • I guess it is the correct answer. But also, it worth to mention that better implement EventSubscriberInterface in the service where the event callback will be placed and add the tag - { name: event_subscriber } in *.services.yml file. In the result we have a service and the event callback in the same place. – voleger Sep 20 '18 at 14:41
  • @voleger, since this is the correct answer, would you please mark it as the answer. Thank you! – Cesar Moore Sep 21 '18 at 13:20

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