我试图在树枝上使用自定义的PHP功能，我做错了什么？

I tried to use my custom php function in twig. When I type {{current_url}} in my page.html.twig nothing happens. I also tried {{kint(current_url)}}, it outputs NULL This is my code

<?php
namespace Drupal
extprev\plugin\block\customtwigoutput;


use Drupal\Core\Template\TwigExtension;
use  Drupal
extprev
extprevmodule\path;
use \Symfony\Component\HttpFoundation\Request;


class AppExtension extends \Twig_Extension {

public function url() {
   $current_path = \Drupal::service('path.current')->getPath();
   return $current_path;
}


  public function getFilters() {
     return array(
       'current_url' => new \Twig_Function_Function(array('Drupal
extprev\plugin\block\customtwigoutput\AppExtension', 'url')),
     );
   }
}

What am I doing wrong?

Did you add services.yml to it? This is a sample;

twig.url:
    class: AppBundle\Twig\Extension\AppExtension
    tags:
        - { twig.extension }

Maybe you should try with a twig function instead :

In your extension :

public function getFunctions()
{
    return array(
        new \Twig_SimpleFunction('current_url', array($this, 'url')),
    );
}

In your view

 {{ current_url() }}