Issue #3082478 by artem_sylchuk, wombatbuddy, Arno2Mars, muaz91: Rules...

<?php

namespace Drupal\private_message\Plugin\RulesAction;

use Drupal\rules\Core\RulesActionBase;

use Drupal\Core\Plugin\ContainerFactoryPluginInterface;

use Symfony\Component\DependencyInjection\ContainerInterface;

use Drupal\user\UserInterface;

use Drupal\private_message\Entity\PrivateMessage;

/**

 * Provides "Send private message" rules action.

 *

 * @RulesAction(

 *   id = "private_message_send_message",

 *   label = @Translation("Send private message"),

 *   category = @Translation("System"),

 *   context_definitions = {

 *     "author" = @ContextDefinition("entity:user",

 *       label = @Translation("From"),

 *       description = @Translation("The author of the message.")

 *     ),

 *     "recipient" = @ContextDefinition("entity:user",

 *       label = @Translation("To"),

 *       description = @Translation("The recipient of the message.")

 *     ),

 *     "message" = @ContextDefinition("string",

 *       label = @Translation("Message"),

 *       description = @Translation("The message.")

 *     ),

 *   }

 * )

 */

class SendPrivateMessage extends RulesActionBase implements ContainerFactoryPluginInterface {

  /**

   * The Private Message service.

   *

   * @var \Drupal\private_message\Service\PrivateMessageService

   */

  protected $privateMessageService;

  /**

   * {@inheritdoc}

   */

  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {

    $instance = new static($configuration, $plugin_id, $plugin_definition);

    $instance->privateMessageService = $container->get('private_message.service');

    return $instance;

  }

  /**

   * Send a private message.

   *

   * @param \Drupal\user\UserInterface $author

   *   The author of the message.

   * @param \Drupal\user\UserInterface $recipient

   *   The recipient of the message.

   * @param string $message

   *   The text of the message.

   */

  protected function doExecute(UserInterface $author, UserInterface $recipient, $message) {

    $members = [$author, $recipient];

    // Create a thread if one does not exist.

    $private_message_thread = $this->privateMessageService->getThreadForMembers($members);

    // Add a Message to the thread.

    $private_message = PrivateMessage::create();

    $private_message->set('owner', $author);

    $private_message->set('message', $message);

    $private_message->save();

    $private_message_thread->addMessage($private_message)->save();

  }

}