manifest/web/modules/custom/yufu_admin/yufu_admin.module

<?php

/**
 * @file
 * Primary module hooks for Yufu Admin module.
 */

use Drupal\Core\Entity\EntityInterface;
use Drupal\Core\Url;
use Drupal\node\NodeInterface;

/**
 * Implements hook_entity_operation().
 */
function yufu_admin_entity_operation(EntityInterface $entity) {
  $operations = [];
  $entityType = $entity->getEntityType();
  // @todo Only for entity node - should we add other entities as well?
  if ($entityType->id() === 'node' && \Drupal::currentUser()->hasPermission('use jsonapi operation link')) {
    // Build the url.
    $url = Url::fromRoute(sprintf('jsonapi.%s--%s.individual', $entityType->id(), $entity->bundle()),
      ['entity' => $entity->uuid()]
    );
    $operations['view-jsonapi-output'] = [
      'title' => t('jsonapi Output'),
      'weight' => 50,
      'url' => $url,
    ];
  }
  return $operations;
}

/**
 * Implements hook_mail().
 */
function yufu_admin_mail($key, &$message, $params) {
  $options = ['langcode' => $message['langcode']];

  switch ($key) {
    case 'concept_drafted':
      $message['from'] = \Drupal::config('system.site')->get('mail');
      $message['subject'] = t('YUFU: New concept draft: @title', ['@title' => $params['node_title']], $options);
      $message['body'][] = $params['message'];
      break;
    case 'concept_approved':
      $message['from'] = \Drupal::config('system.site')->get('mail');
      $message['subject'] = t('YUFU: Your concept (changes) were approved: @title', ['@title' => $params['node_title']], $options);
      $message['body'][] = $params['message'];
      break;
    case 'concept_rejected':
      $message['from'] = \Drupal::config('system.site')->get('mail');
      $message['subject'] = t('YUFU: Your concept (changes) were rejected: @title', ['@title' => $params['node_title']], $options);
      $message['body'][] = $params['message'];
  }
}

/**
 * Implements hook_ENTITY_TYPE_presave().
 */
function yufu_admin_node_presave(EntityInterface $entity) {
  if ($entity instanceOf NodeInterface && $entity->bundle() == 'concept') {
    // Get transition: concept_drafted, concept_approved, concept_rejected.
    if ($entity->isNew()) {
      $transition = 'concept_drafted';
    }
    else {
      $transition = _yufu_admin_get_node_transition($entity);
    }
    switch ($transition) {
      case 'concept_drafted':
        // Send email to editors.
        _yufu_admin_concept_drafted_mail($entity);
        break;
      case 'concept_approved':
        // Send email to revision creator (user).
        break;
      case 'concept_rejectd':
        // Send email to revision creator (user).
        // This transition happens on revision delete.
        // Curently it is dead.
        break;
    }
  }
}

/**
 * Compare original node moderation state with current state.
 *
 * @param \Drupal\node\NodeInterface $node
 *   Node to check transition on.
 *
 * @return string|null
 *   Name of the transition.
 */
function _yufu_admin_get_node_transition(NodeInterface $node) {
  $original = $node->original;
  $moderation_state = $node->moderation_state->value;
  $previous_state = $original->moderation_state->value;
  if ($moderation_state == 'draft' && $previous_state == 'published') {
    return 'concept_drafted';
  }
  else if ($moderation_state == 'published' && $previous_state == 'draft') {
    return 'concept_approved';
  }
  else if ($moderation_state == 'published' && $previous_state == 'published') {
    return 'stay_published';
  }
  else if ($moderation_state == 'draft' && $previous_state == 'draft') {
    return 'stay_draft';
  }
  return NULL;
}

function _yufu_admin_concept_drafted_mail(NodeInterface $node) {
  $mailManager = \Drupal::service('plugin.manager.mail');
  $module = 'yufu_admin';
  $key = 'concept_drafted';
  // To all editors.
  $to = _yufu_admin_get_all_editors_emails();
  $params['message'] = t('New concept created: @title - @url', [
    'title' => $node->getTitle(),
    'url' => $node->toUrl()->toString(),
  ]);
  $params['node_title'] = $entity->label();
  $langcode = \Drupal::currentUser()->getPreferredLangcode();
  $send = true;

  $result = $mailManager->mail($module, $key, $to, $langcode, $params, NULL, $send);
  if ($result['result'] !== true) {
    drupal_set_message(t('There was a problem sending your message and it was not sent.'), 'error');
  }
  else {
    drupal_set_message(t('Your message has been sent.'));
  }
}

function _yufu_admin_get_all_editors_emails() {
  $user_storage = \Drupal::entityTypeManager()->getStorage('user');
  $ids = $user_storage->getQuery()
    ->condition('status', 1)
    ->condition('roles', 'administrator')
    ->execute();
  $editors = $user_storage->loadMultiple($ids);
  $emails = [];
  foreach ($editors as $editor) {
    /* @var \Drupal\user\UserInterface $user */
    $emails[] = $editor->getEmail();
  }
  return $emails;
}