扩展Twig以定义资产url

我阅读了用于管理模板中资产的Silex Cookbookhttp://silex.sensiolabs.org/doc/cookbook/assets.html

并在我的app/app.php:中编写此代码

$app['twig'] = $app->share($app->extend('twig', function ($twig, $app) {
    $twig->addFunction(new 'Twig_SimpleFunction('asset', function ($asset) {
        // implement whatever logic you need to determine the asset path
        return sprintf('http://assets.examples.com/%s', ltrim($asset, '/'));
    }));
    return $twig;
}));
$app->get('/', function () use ($app) {
    return $app['twig']->render('index.twig', array(
        'title' => "Hello World",
        'colors' => array("red", "green", "yellow"),
    ));
});

和index.twig包含：

{% extends "layout.twig" %}
{% block title %}
    {{ title }}
{% endblock %}
{% block content %}
    <h1>{{ title }}</h1>
    <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Animi quibusdam numquam laudantium eum asperiores non libero odio quae debitis beatae perferendis eius esse molestiae voluptatum vel inventore quasi. Quo sint sequi sunt amet sapiente tempora autem iusto praesentium rerum ducimus.</p>
    <ul>
        {% for color in colors %}
            <li>{{ color }}</li>        
        {% endfor %}
    </ul>
    {{ asset('/css/styles.css') }}
{% endblock %}

一切都很好，但当我在另一个twig文件中使用asset时，我会收到以下错误：

Twig_Error_Syntax in Parser.php line 370:
A template that extends another one cannot have a body in "admin/dashboard.twig" at line 3.

例如，我的AdminDashboard控制器包含：

<?php

namespace App'Controller'Admin;
use Silex'Application;
class AdminDashboard
{
    function __construct()
    {
        return "Dashboard";
    }
    function indexAction(Application $app)
    {
        return $app['twig']->render('admin/dashboard.twig', array(
            'title' => "Hello World",
            'colors' => array("red", "green", "yellow"),
        ));
    }

}

和admin/dashboard.twig包含：

{% extends "layout.twig" %}
{{ asset('/css/styles.css') }}

但当我访问我的admin页面时，我得到了上面的错误。