protected function registerCustomFunctions()
    {
        $functionList = $this->functionGenerator->getFunctionList();

        foreach ($functionList as $function) {
            if (isset($function['name']) && isset($function['callable'])) {
                $twigFunction = new Twig_SimpleFunction($function['name'], $function['callable']);
                $this->engine->addFunction($twigFunction);
            }
        }
    }