如何取消从 Symfony2 中的“\Swift_Events_SendListener”事件中发送的电子邮件?

iol*_*leo 1 email events symfony

按照本指南,我设置了一个事件侦听器:

<?php

class EmailListener implements \Swift_Events_SendListener
{
    public $beforeSendEvt = null;
    public $sendEvt = null;

    public function beforeSendPerformed(\Swift_Events_SendEvent $evt)
    {
        $this->beforeSendEvt = $evt;
    }

    public function sendPerformed(\Swift_Events_SendEvent $evt)
    {
        $this->sendEvt = $evt;
    }
}
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现在我想用beforeSendPerformed方法取消这封电子邮件。我怎么做?