Extending Volt: adding tests

Managed to figure it out myself by looking at the Volt compiler source. The example below is implemented as a Twig extension so PhpStorm can actually autocomplete the test (given that the Symfony plugin is enabled for your project).

<?php

namespace MyApp\Volt\Test;

use Phalcon\Mvc\View\Engine\Volt\Compiler;

class RouteTest extends \Twig_Extension  {
    /** @var Compiler */
    private $compiler;

    /**
     * We require the Volt compiler to be passed through as we need
     * to compile the left part of our test expression.
     *
     * @param Compiler $compiler
     */
    public function __construct(Compiler $compiler) {
        $this->compiler = $compiler;
    }

    /**
     * @param array $expression
     * @return null|string
     */
    public function resolveExpression($expression) {
        // Only respond to "is" expressions
        if ($expression['type'] != 311) return null;
        // Only respond if the right part is PHVOLT_T_IDENTIFIER
        if ($expression['right']['type'] != 265) return null;
        // Only respond if the test is "route"
        if ($expression['right']['value'] != 'route') return null;
        // Compile the left expression
        $left = $this->compiler->expression($expression['left']);
        // Return the PHP code of our test
        return sprintf('%s == $this->router->getMatchedRoute()->getName()', $left);
    }

    /**
     * Returns the name of the extension.
     * Required to be implemented by Twig_Extension.
     *
     * @return string The extension name
     */
    public function getName()
    {
        return 'RouteTest';
    }

    /**
     * Method to let our Twig extension (and PhpStorm when using the Symfony plugin) know
     * which tests we implement.
     * @return array
     */
    public function getTests()
    {
        return [new \Twig_SimpleTest('route', [$this, 'routeTest'])];
    }

    /**
     * Dummy function used when defining our test in the getTests method.
     */
    public function routeTest() {
    }
}