public function body() {
        $event = [];
        if (!empty($this->replyToken)) {
            $event['replyToken'] = $this->replyToken;
        }
        $event['type'] = $this->eventType();
        $event['timestamp'] = time();
        $event['source'] = $this->source;
        $event['message'] = $this->message;
        $result = ['events' => [$event]];
        return json_encode($result);
    }