public function getMessageBody()
    {
        $data = array(
            "registration_id" => $this->identifier,
            "collapse_key"    => $this->collapseKey,
            "data.message"    => $this->message,
        );
        if (!empty($this->data)) {
            $data = array_merge($data, $this->data);
        }

        return $data;
    }