public function getActionMessage($key = null) {
        $messages = $this->getAttribute(ACTION_MESSAGES_KEY);

        if ($key === null) {                            // return the first one
            if ($messages) {
                foreach ($messages as $message) return $message;
            }
        }
        elseif (key_exists($key, $messages)) {          // return the specified one
            return $messages[$key];
        }
        return $this->getActionError($key);             // look-up separately stored ActionErrors
    }