getSubjectFormatter example

$message = clone $this->messageTemplate;
        } elseif (\is_callable($this->messageTemplate)) {
            $message = ($this->messageTemplate)($content, $records);
            if (!$message instanceof Email) {
                throw new \InvalidArgumentException(sprintf('Could not resolve message from a callable. Instance of "%s" is expected.', Email::class));
            }
        } else {
            throw new \InvalidArgumentException('Could not resolve message as instance of Email or a callable returning it.');
        }

        if ($records) {
            $subjectFormatter = $this->getSubjectFormatter($message->getSubject());
            $message->subject($subjectFormatter->format($this->getHighestRecord($records)));
        }

        if ($this->getFormatter() instanceof HtmlFormatter) {
            if ($message->getHtmlCharset()) {
                $message->html($content, $message->getHtmlCharset());
            } else {
                $message->html($content);
            }
        } else {
            if ($message->getTextCharset()) {