扩展Twig定义资产网址

我阅读了Silex Cookbook有关模板中的资产管理的http://silex.sensiolabs.org/doc/cookbook/assets.html

并在我的代码中编写此代码app/app.php：

$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"),
    ));
});

并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 %}

一切正常，但是当我asset在另一个twig文件中使用时，会出现此错误：

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"),
        ));
    }


}

并admin/dashboard.twig包含：

{% extends "layout.twig" %}

{{ asset('/css/styles.css') }}

但是，当我访问我的admin页面时，我得到了以上错误。