drupal_set_message() and drupal_get_messages() replaced by Messenger service

The procedural functions drupal_set_message() and drupal_get_messages() have been deprecated and replaced with a dedicated Messenger service.

You should now inject this service as a dependency for your own services and controllers:

my_module.services.yml:

services:
  my_module.service:
    class: \Drupal\my_module\MyModuleService
    arguments: ['@messenger']

namespace Drupal\my_module;

use Drupal\Core\Messenger\MessengerInterface;

class MyModuleService {

  /**
   * The Messenger service.
   *
   * @var \Drupal\Core\Messenger\MessengerInterface
   */
  protected $messenger;

  /**
   * MyModuleService constructor.
   *
   * @param \Drupal\Core\Messenger\MessengerInterface $messenger
   *   The messenger service.
   */
  public function __construct(MessengerInterface $messenger) {
    $this->messenger = $messenger;
  }

  /**
   * Before Example.
   */
  public function beforeExample() {
    // Add message (defaults to "status" message type).
    drupal_set_message('Hello world');

    // Add specific type of message.
    drupal_set_message('Hello world', 'custom');
    drupal_set_message('Hello world', 'error');
    drupal_set_message('Hello world', 'status');
    drupal_set_message('Hello world', 'warning');

    // Retrieve and remove specific type of message.
    $errors = drupal_get_messages('error');

    // Retrieve specific type of message, without removing it.
    $errors = drupal_get_messages('error', FALSE);

    // Retrieve and remove all messages.
    $messages = drupal_get_messages();

    // Retrieve all messages, without removing them.
    $messages = drupal_get_messages(NULL, FALSE); // or
    $messages = drupal_set_message();
  }

  /**
   * After Example.
   */
  public function afterExample() {
    // Add message (defaults to "status" message type).
    $this->messenger->addMessage('Hello world');

    // Add specific type of message.
    $this->messenger->addMessage('Hello world', 'custom');
    $this->messenger->addError('Hello world');
    $this->messenger->addStatus('Hello world');
    $this->messenger->addWarning('Hello world');

    // Retrieve and remove specific type of message.
    $this->messenger->deleteByType('error');

    // Retrieve specific type of message, without removing it.
    $this->messenger->messagesByType('error');

    // Retrieve and remove all messages.
    $messages = $this->messenger->deleteAll();

    // Retrieve all messages, without removing them.
    $messages = $this->messenger->all();
  }

}

We did add some easier way (Drupal\Core\Messenger\MessengerTrait) for custom code:

use Drupal\Core\Messenger\MessengerTrait;
class MyModuleService {
  use MessengerTrait;

  public function traitUsageExample() {
    // Add message (defaults to "status" message type).
    $this->messenger()->addMessage('Hello world');

    // Add specific type of message.
    $this->messenger()->addMessage('Hello world', 'custom');
    $this->messenger()->addError('Hello world');
    $this->messenger()->addStatus('Hello world');
    $this->messenger()->addWarning('Hello world');

    // Retrieve and remove specific type of message.
    $this->messenger()->deleteByType('error');

    // Retrieve specific type of message, without removing it.
    $this->messenger()->messagesByType('error');

    // Retrieve and remove all messages.
    $messages = $this->messenger()->deleteAll();

    // Retrieve all messages, without removing them.
    $messages = $this->messenger()->all();
  }
}

Note, a few common base classes now have this trait added, so you may not need to redefine it in your implementations if you extend from them:

Drupal/Core/Block/BlockBase
Drupal/Core/Form/ConfigFormBase
Drupal/Core/Plugin/PluginBase

Procedural code

In rare occasions, you may need to use the Messenger service in procedural functions (hooks, alters, callbacks/static method callbacks, etc.) where the dependency injected Messenger service is not accessible (e.g. no $this->messenger). In these instances, the \Drupal::messenger() static helper method is available for you to use. This helper method should only be used in places where the service cannot be injected and should never be used as a way to create a new service that requires this service.

e.g.

\Drupal::messenger()->addError("migration failed");

Differences from drupal_get_messages($type)

drupal_get_messages($type)

returned an nested array structure keyed by message type:

['error' => ['error message 1', 'error message 2']]

MessengerInterface::messagesByType($type)

just returns an array of messages in the format:

['error message 1', 'error message 2']