For a long time the drupal_set_message() function has been a mainstay of Drupal development. You can use this function to print a message or an error response to a user. This is useful to do as it can be done almost anywhere before the final page rendering functions and will be printed into the same area of a Drupal page.
This has recently been deprecated in Drupal 8.5.0 and so I was looking around on how to do the same thing in a new way. Drupal now has a messenger service, powered by the class Drupal\Core\Messenger\Messenger. This can be included into your code using the following shortcut.
$messenger = \Drupal::messenger();
This can be used to print messages to the user. For example, to print a standard message with a blue background do the following.
$messenger->addMessage('Message printed to a user.');
This can be done in one line like this.
\Drupal::messenger()->addMessage('Message printed to a user.');
There are four different types of message available, each of which is printed with other messages of the same type and on a different coloured background.
// Green status message. $messenger->addStatus('Print a status to the user.'); // Blue message. $messenger->addMessage('Message printed to a user.'); // Yellow warning message. $messenger->addWarning('A warning about something!'); // Red error message. $messenger->addError('An error happened!');
If you are in a Drupal 8 Form's submitForm() handler then you can also get access to the messenger service through the form object.
If you are still using drupal_set_message() then I recommend that you swap to the messenger service soon as it will be removed in future versions of Drupal.