How to dispatch a ResponseEvent so that a cookie is added after login?

Question

I want to add a session cookie when a user logs in.

The previous implementation I had, was setting the cookie in hook_user_login with this snippet:

$response = new Response();
$response->headers->setCookie($plain_cookie);
$response->send();

But this was causing randomly the following exception:

RuntimeException: Failed to start the session because headers have already been sent

After some research, I found out that a way to handle it is via an EventSubscriber. https://www.drupal.org/docs/develop/creating-modules/subscribe-to-and-dispatch-events

However, I have not managed to dispatch a ResponseEvent in order to add the cookie to the response header, aka. the cookie is not being added after login. I have tried event low priorities in order for the event to be exeuted at the end, but again it does not work.

function custom_user_login($account) {
  $response = new Response();
  $event = new MyResponseEvent($response);

  $event_dispatcher = \Drupal::service('event_dispatcher');
  $event_dispatcher->dispatch($event, MyResponseEvent::EVENT_NAME);
}

And in my EventSubscriber:

public function onUserLogin(MyResponseEvent $event) {
  $response = $event->getResponse();
  $response->headers->setCookie($plain_cookie);
  $event->setResponse($response);
}

How could I do it?

Answer 1

Finally, since I could not find a way to create an EventSubscriber for the login event, I returned to my initial implementation and tried to fix the problem with the $response->send() command.

It seems that if we just want to modify the response headers (e.g. add some cookies) in the context of a hook where a response is already in place, the proper function to use, instead of $response->send() is:

 $response->sendHeaders();

This works fine, setting the cookies properly to the response and does not trigger the fatal error:

Headers already sent.