Magento dev tricks and tips

1/ To debug email sending code:

For example, when placing an order in FrontEnd; the order immediately gets marked as processed; and the quote is cleared. In order to debug sending email:

in file `vendor/magento/module-checkout/Controller/Onepage/Success.php`, disable this code: `$session->clearQuote();`

2/ Store object in memory per session

There is a deprecated technique still being used in Magento: use registry to store objects in memory.
Example:

$this->registry->unregister('current_creditmemo');
$this->registry->register('current_creditmemo', $creditmemo);