Render Twig in hook_mail()

Question

I want to render twig when sending email (if it is possible) or include a template.

My code is like so:

/**
 * Implements hook_mail().
 */
function reg_email_mail($key, &$message, $params) {
  $options = array(
    'langcode' => $message['langcode'],
  );

  switch ($key) {
    case 'new_user':
      $message['from'] = \Drupal::config('system.site')->get('mail');
      $message['subject'] = t('User created: @title', array('@title' => $params['title']), $options);
      $message['body'][] = \Drupal\Core\Mail\MailFormatHelper::htmlToText($params['message']);
      break;
  }
}

I'd like to change $params['message'] or part of this with twig. Is it possible?

Answer 1

$body_data = [
  '#theme' => 'something',
  '#somearg1' => 'somevalue1',
  '#somearg2' => 'somevalue2',
];
$body_content = \Drupal::service('renderer')->render($body_data);
$message['body'] = explode("\n", $body_content);

Answer 2

It's possible, but if you want to keep the HTML output from Twig, you need to use a mail plugin that doesn't replace the HTML tags with other strings, or implement it and let Drupal use that plugin instead of the default one.

The code you are showing already replaces HTML tags with other strings, since it's calling \Drupal\Core\Mail\MailFormatHelper::htmlToText(), which as you see, replaces HTML tags with other strings. That is also the reason why you see ======== instead of <hi> and </h1>.
Even if you would remove that method call, the default implementation of the mail plugin uses the following code, in PhpMail::format().

  // Join the body array into one string.
  $message['body'] = implode("\n\n", $message['body']);

  // Convert any HTML to plain-text.
  $message['body'] = MailFormatHelper::htmlToText($message['body']);
  // Wrap the mail body for sending.
  $message['body'] = MailFormatHelper::wrapMail($message['body']); 

It doesn't check the headers set for the email message; it always converts HTML to plain text.