如何在动作助手中获得$ acl-> isAllowed？

I'm trying to create a action helper for creating a menu. But I don't know how why it doesn't know the function isAllowed. I get the following error message:

Call to a member function isAllowed() on a non-object

<?php
class Zend_Controller_Action_Helper_Menu extends Zend_Controller_Action_Helper_Abstract
{
    private $_acl;
    public function createSubMenu($request,$identity){
        $controller = $request->getControllerName();
        $action = $request->getActionName();
        $identity = $identity->user_role;
        $access = $this->hasAccess($identity,$action,$controller);

        $return;

        $return .= "<ul>";
            $return .= "<li><a href=''>".$identity."</a></li>";
        $return .= "</ul>";

        return $access;
    }

    private function hasAccess($role, $action, $controller)
    {
        $this->_acl = new Zend_Acl();
        if (!$this->_acl) {
            $this->_acl = Zend_Controller_Front::getInstance()->getPlugin('Acl');
        }
        return $this->_acl->isAllowed($role, $controller, $action);
    }
}

I'm no Zend expert, but:

• $this->_acl = new Zend_Acl(); - this will create new ACL object without any roles set, so it is useless (maybe you wanted to do this other way around - first get plugin, and if there isn't one create new?)
• ->getPlugin('Acl'); - You should use full plugin class name

I usually create and initiate ACL object in plugin (on preDispatch) and save it in Zend_Registry so i can later use Zend_Registry::get('Zend_Acl')->isAllowed($role, $resource, $privilege);