Symfony, How to generate Asset URL in a Twig Extension class?

I didn't want to deal with the Dependency Injection Container. This is what I did:

use Twig_Environment as Environment;

class MyTwigExtension extends \Twig_Extension
{
    protected $twig;
    protected $assetFunction;

    public function initRuntime(Environment $twig)
    {
        $this->twig = $twig;
    }

    protected function asset($asset)
    {
        if (empty($this->assetFunction)) {
             $this->assetFunction = $this->twig->getFunction('asset')->getCallable();
        }
        return call_user_func($this->assetFunction, $asset);
    }

I've looked at Twig_Extension class code, and found this initRuntime method there, to be overriden in our custom Extension class. It receives the Twig_Environment as an argument! This object has a getFunction method, which returns a Twig_Function instance. We only need to pass the function name (asset, in our case).

The Twig_Function object has a getCallable method, so we finally can have a callable asset function.

I've gone a bit further creating an asset method for my own extension class. Anywhere else on it, I can simply call $this->asset() and obtain the same result as {{ asset() }} in the templates.

EDIT: The getFunction call at initRuntime throws a scope exception when clearing the cache. So I moved it to the custom asset method. It works fine.

Here's a simple and clean way for Symfony 2.8:

services.yml:

app.twig_extension:
    class: Path\To\AcmeExtension
    arguments:
        assets: "@templating.helper.assets"

In the TWIG extension:

use Symfony\Bundle\FrameworkBundle\Templating\Helper\AssetsHelper;

class AcmeExtension
{
    protected $assets;

    public function __construct(AssetsHelper $assets)
    {
        $this->assets = $assets;
    }
}

Then you can use it in any function of the extension like this:

$this->assets->getUrl('myurl');

In Symfony 5.3 that worked for me: (just do what the assets extension does and inject Packages)

use Symfony\Component\Asset\Packages;
use Twig\Extension\AbstractExtension;
use Twig\Extension\ExtensionInterface;

class AppExtension extends AbstractExtension implements ExtensionInterface
{
    public function __construct(Packages $packages)
    {
        $this->packages = $packages;
    }

    // ... your other methods

    private function asset($path, $packageName = null)
    {
        return $this->packages->getUrl($path, $packageName);
    }
}

In Symfony 2.8 that works for me:

# services.yml
services:
    app.twig_extension:
        class: AppBundle\Twig\AppTwigExtension
        public: false
        arguments:
            - @templating.helper.assets
        tags:
            - { name: twig.extension }

AppTwigExtension class:

namespace AppBundle\Twig;

use Symfony\Bundle\FrameworkBundle\Templating\Helper\AssetsHelper;

/**
 * Class AppTwigExtension
 * @package AppBundle\Twig
 */
class AppTwigExtension extends \Twig_Extension
{
    const IMG_PATH = 'bundles/app/images/';

    private $assetsHelper;

    public function __construct(AssetsHelper $assetsHelper)
    {
        $this->assetsHelper = $assetsHelper;
    }

    public function getFilters()
    {
        return array(
            new \Twig_SimpleFilter('img', array($this, 'imagePathFilter'))
        );
    }

    /**
     * Get image path relatively to host
     * Usage in Twig template: {{ 'my_image.png'|img }} - equal to
     * {{ asset('bundles/app/images/my_image.png') }} in Twig template:
     *
     * @param string $imageName (e.g. my_image.png)
     * @return string
     */
    public function imagePathFilter($imageName)
    {
        return $this->assetsHelper->getUrl(self::IMG_PATH . $imageName);
    }

    public function getName()
    {
        return 'app_twig_extension';
    }
}