How to create event listener that inject the view data in symfony2?

You should consider creating a global variable or twig extension to make categories available in your templates, you can't do that by using events (since the template is parsed inside the controller, not before/after it)

This approach, although valid and commonly used in some frameworks, is not very common in Symfony as it suits more MVC than HMVC architecture.

I would suggest you a different one with the same result:

Instead of adding parameter to every controller return, render another controller which returns just a subview of what you're trying to show. Simple example:

 // article/index.html.twig

 <div class="category-bar">{{ render(controller('MyVendorMyBundle:CategoryController:bar')) }}</div>

 <div class="article-list">
     {% for article in articles %>
         {# Print article here #}
     {% endfor %}
 </div>

// CategoryController

class CategoryController extends Controller
{
    /**
     * @Template
     */
    public function barAction()
    {
        return ['categories' => $this->fetchCategoriesSomehow()];
    }
}

So when you render your article list action, twig will fire a subrequest to render categories bar above it.

Furthermore, if you don't like making subrequests, nothing stops you from creating a twig extension service which would fetch categories and render template for you.

In most cases I would go with @Wouter J's suggestion and create a twig extension or a global variable.

However, what you want to do is actually possible (regardless if that's the right solution or not).

The @Template annotation has a vars attribute, which lets you to specify which atttributes from the request should be passed to the template:

/**
 * @ParamConverter("post", class="SensioBlogBundle:Post")
 * @Template("SensioBlogBundle:Post:show.html.twig", vars={"post"})
 */
public function showAction()
{
}

Note, that request attributes can be set by you:

$request->attributes->set('categories', []);

So, you could implement a listener which would set the categories attribute on the request and than configure the vars on the @Template annotation:

/**
 * @Template("SensioBlogBundle:Post:show.html.twig", vars={"categories"})
 */
public function showAction(Post $post)
{
}

Have a look at the TemplateListener from the SensioFrameworkExtraBundle for more insight. The listener defines template vars on kernel.controller and uses them to render the view on kernel.view.

You could avoid defining vars on the annotation if your listener was registered after the TemplateListener::onController(). It would have to add categories to the _template_vars request attribute.

Use Twig extension to create function that will return list of available categories

<?php

class CategoriesExtension extends \Twig_Extension
{
    public function getFunctions()
    {
        return [
            new \Twig_SimpleFunction('getCategories', [$this, 'getCategoriesList'])
        ];
    }

    /**
     * @return null|string
     */
    public function getCategoriesList()
    {
        return CategoryQuery::create()->find();
    }

    /**
     * Returns the name of the extension.
     *
     * @return string The extension name
     */
    public function getName()
    {
        return 'list_categories';
    }

}

You can pass parameter to function if You would like do some conditions on query.

The trick is to get the twig service in your listener and then use addGlobal to add your categories

namespace Cerad\Bundle\CoreBundle\EventListener;

use Symfony\Component\DependencyInjection\ContainerAware;

use Symfony\Component\HttpKernel\HttpKernel;
use Symfony\Component\HttpKernel\KernelEvents;
use Symfony\Component\HttpKernel\Event\FilterControllerEvent;

use Symfony\Component\EventDispatcher\EventSubscriberInterface;

class MyEventListener extends ContainerAware implements EventSubscriberInterface
{
public static function getSubscribedEvents()
{
    return array(
        KernelEvents::CONTROLLER => array(
            array('doCategories', -1100),
    );
}
public function doCategories(FilterControllerEvent $eventx)
{
    // Query your categories
    $categories = array('cat1','cat2');

    // Make them available to all twig templates
    $twig = $this->container->get('twig');
    $twig->addGlobal('categories',$categories);
}

# services.yml
cerad_core__my__event_listener:
    class:  '%cerad_core__my__event_listener__class%'
    calls:
        - [setContainer, ['@service_container']]
    tags:
        - { name: kernel.event_subscriber }