为什么EventSubscriber在symfony 2框架中不起作用

Question

我在Symfony 2框架中遇到eventSubscribers的问题。我写了这段代码：

src/MyBundle/UserBundle/EventListener/UsernameProfileEditExternalListener.php

class UsernameProfileEditExternalListener implements EventSubscriberInterface {

protected $userExternalForumRepository;
protected $userExternalDatesRepository;

public function __construct(UserExternalForumRepository $userExternalForumRepository, UserExternalDatesRepository $userExternalDatesRepository) {
    $this->userExternalForumRepository = $userExternalForumRepository;
    $this->userExternalDatesRepository = $userExternalDatesiRepository;
}

public static function getSubscribedEvents() {
    return array(
        'mybundle.user.change_username' => 'onEditUsername',
    );
}

public function onEditUsername(FormEvent $event) {
    die('It\'s working!');
}

我将此代码添加到文件src/MyBundle/UserBundle/Resources/config/services.yml：

username_profile_edit_listener:
    class: MyBundle\UserBundle\EventListener\UsernameProfileEditExternalListener
    arguments: [@user_external_forum_repository, @user_external_dates_repository]
    tags:
        - { name: kernel.event_subscriber, event: mybundle.user.change_username, method: onEditUsername }  

在此文件中src\AppBundle\Controller\Backend\UserController.php我发送此事件：

$dispatcher = $this->get('event_dispatcher');


$event = new FormEvent($usernameForm, $request);
$dispatcher->dispatch('mybundle.user.change_username', $event);

但它不起作用。我现在完全不知道为什么。我将非常感谢你的建议。我是Symfony的初学者。 services.yml文件可以缓存吗？