Build email before cancelling the send

Included a proof of concept patch that works in my particular situation.

It's a quick fix, and I've not looked into any side effects of the patch.
Maybe someone with more experience in the symfony_mailer module could point out any obvious flaws.

my particular use case is I want a preview of the mail as close to what the user will actually receive

True this could be useful, however it seems like a fairly unusual requirement. And sending the email is slower with the patch, which is bad for the majority case. If $send is FALSE, the Drupal Core MailManager will execute format() but not mail() which has an fairly undefined result, but probably not very close to the final email.

Here's something you could try. Instead of setting $send to FALSE, enable the NullTransport, which you can do using a hook.

Maybe someone with more experience in the symfony_mailer module could point out any obvious flaws.

It looks like it would only work for LegacyEmailBuilder and not for e.g. UserEmailBuilder.

Or even simpler: throw a SkipMailException late in the PHASE_POST_RENDER phase.

I don't think the patch would have any performance impact for sending mails (it would though for not sending mails with $send FALSE).

I hadn't thought about using the NullTransporter, thank you for the idea, we should be able to implement a non intrusive module with that approach.

It might be an unusual requirement, but it's a feature we have provided clients, to give them peace of mind before sending out emails.

I don't think the patch would have any performance impact for sending mails (it would though for not sending mails with $send FALSE).

I agree, and still I don't want to make skip sending slower as it could be a common case for some sites.

I propose the way this module should work is:

1. SkipMailException cancels immediately that it is thrown for performance
2. NullTransport builds the email then doesn't send it for inspection
3. For MailManagerReplacement::mail($send = FALSE) we choose performance, and sites that want inspection can easily alter with a hook

Hopefully this gives you what your client needs without causing any disadvantage for other sites.

I'll share a quick overview of our implementation in case it has value for anyone ending in this issue.

We ended up implementing a skipsend in the $params argument, and then having a PHASE_POST_RENDER hook (I think this could be any phase hook) set the the transport DSN to a

NullTransport</mail>.
The hook is as follows:

<?php
function hook_mailer_post_render($email) {
  if ($legacy_message = $email->getParam('legacy_message')) {
    if (!empty($legacy_message['params']['skipsend'])) {
      $email->setTransportDsn('null://default');
    }
  }
}
?>

This assumes there's a <code>NullTransport

transporter named null for it to work. And in our preview controller we're calling the mail with the extra parameter but with $send TRUE (so MailManagerReplacement::mail() and LegacyEmailBuilder::buil() wont end the process to early) :

// ...
$params['__skipsend__'] = TRUE;
$result = $mailManager->mail($module, $key, $to, $langcode, $params);
// ...

Thank you for your input on the issue, and all your work on the module!