zend-framework，从另一个动作助手中调用动作助手

我正在编写一个动作助手，我需要从该助手中调用另一个动作助手.但我不知道如何.在示例代码中:

i am writing an action helper and i need to call another action helper from within that helper. but i dont know how. here in the sample code:

class Common_Controller_Action_Helper_SAMPLE extends Zend_Controller_Action_Helper_Abstract
{
    protected $_view;
    public function __construct(Zend_View_Interface $view = null, array $options = array())
    {
        $this->_view = $view;
    }

    public function preDispatch()
    {
        $flashMessenger = $this->_helper->FlashMessenger; // IT IS NULL
    }
}