CWebApplication runController中语句疑问 [ 求助交流 ]

public function runController($route)
{
    if(($ca=$this->createController($route))!==null)
    {
        list($controller,$actionID)=$ca;
        $oldController=$this->_controller;
        $this->_controller=$controller;
        $controller->init();
        $controller->run($actionID);
        $this->_controller=$oldController;
    }
    else
        throw new CHttpException(404,Yii::t('yii','Unable to resolve the request "{route}".',
			array('{route}'=>$route===''?$this->defaultController:$route)));
}

其中代码的 runController 中的

$oldController=$this->_controller;
$this->_controller=$controller;
$this->_controller=$oldController;

的用意什么。 欢迎讨论