message_notify.module

<?php

/**
 * @file
 * Message notify module.
 */

use Drupal\Core\Entity\Entity\EntityViewMode;

/**
 * Implements hook_mail().
 *
 * Set's the message subject and body as configured.
 */
function message_notify_mail($key, &$message, $params) {
  $message['subject'] = $params['mail_subject'];
  $message['body'][] = $params['mail_body'];
}

/**
 * Implements hook_entity_bundle_create().
 *
 * We cannot easily set the the visibilty of extra fields, so we set the
 * bundle settings upon creation of new message bundle.
 */
function message_notify_entity_bundle_create($entity_type_id, $bundle) {
  if ($entity_type_id != 'message') {
    return;
  }

  // Do nothing if these view modes are not yet available.
  if (!EntityViewMode::load('message.mail_body') || !EntityViewMode::load('message.mail_subject')) {
    return;
  }

  // If this bundle is being created from a .yml file, ignore.
  if (\Drupal::isConfigSyncing()) {
    return;
  }

  $storage = \Drupal::entityTypeManager()->getStorage('entity_view_display');
  /** @var \Drupal\Core\Entity\Display\EntityViewDisplayInterface $default */
  if (method_exists(\Drupal::service('entity_display.repository'), 'getViewDisplay')) {
    $default = \Drupal::service('entity_display.repository')->getViewDisplay($entity_type_id, $bundle);
  }
  else {
    // Pre-Drupal 8.8.
    $default = entity_get_display($entity_type_id, $bundle, 'default');
  }

  // Setup the subject/title display mode if it doesn't exist.
  if (!$storage->load($entity_type_id . '.' . $bundle . '.mail_subject')) {
    $display = $default->createCopy('mail_subject');
    $display->set('content', ['partial_0' => ['weight' => 0]]);
    $display->set('hidden', ['partial_1' => TRUE]);
    $display->save();
  }

  // Setup the body display if it doesn't exist.
  if (!$storage->load($entity_type_id . '.' . $bundle . '.mail_body')) {
    $display = $default->createCopy('mail_body');
    $display->set('content', ['partial_1' => ['weight' => 0]]);
    $display->set('hidden', ['partial_0' => TRUE]);
    $display->save();
  }
}