Common Drupal 8 preprocess and theme suggestion applications.

MY_D8_THEME.theme.php

<?php

/**
 * @file
 * Preprocess and suggestions for a Drupal sub-theme.
 *
 */

use Drupal\block\Entity\Block;

/**
 * Implements hook_preprocess().
 *
 * Add a global base path variables to all twig templates and javascript.
 */
function MY_D8_THEME_preprocess(&$variables, $hook) {
  $variables['base_path'] = base_path();
  if (isset($variables['directory'])) {
    $variables['theme_path'] = base_path() . $variables['directory'];
    if (isset($variables['#attached'])) {
      $variables['#attached']['drupalSettings']['path']['themePath'] = $variables['theme_path'];
    }
  }
}

/**
 * Implements template_preprocess_html().
 *
 * Add classes to the body.
 */
function MY_D8_THEME_preprocess_html(&$variables) {
  // Add a class.
  $variables['attributes']['class'][] = 'my-new-class';
}


/**
 * Implements hook_preprocess_node().
 *
 * Add variables to node template.
 *  Author picture and name.
 */
function MY_D8_THEME_preprocess_node(&$variables) {
  $variables['author_name'] = $variables['node']->getOwner()->getDisplayName();
  $variables['author_picture'] = user_view($variables['node']->getOwner(), 'picture');
}


/**
 * Implements hook_preprocess_block().
 */
function MY_D8_THEME_preprocess_block(&$variables) {
  // Add region to block content to use in
  if (isset($variables['elements']['#id'])) {
    $block = Block::load($variables['elements']['#id']);
    $region = $block->getRegion();
    $variables['content']['#attributes']['region'] = $region;
  }
}

/**
 * Implements hook_preprocess_page_title().
 *
 * Add entity information inline to the title if unpublished and moderation
 * state if enabled.
 */
function MY_D8_THEME_preprocess_page_title(&$variables, $hook) {
  // Load the entity from current route.
  foreach (\Drupal::routeMatch()->getParameters() as $entity) {
    // Get all classes of current entity to filter for content entity.
    if (is_object($entity)) {
      $classes = class_parents($entity);
      if (isset($classes['Drupal\Core\Entity\ContentEntityBase'])) {
        // Add an Unpublished suffix to the title for any content entity
        if (method_exists($entity, 'isPublished')) {
          if (!$entity->isPublished()) {

            // Set the page title suffix.
            $variables['title_suffix'][] = [
              '#markup' => t('Unpublished'),
              // Example with Bootstrap classes.
              '#prefix' => '<span class="label label-danger">',
              '#suffix' => '</span>',
            ];
          }
        }

        // Check content moderation if enable.
        $moduleHandler = \Drupal::service('module_handler');
        if ($moduleHandler->moduleExists('content_moderation')){
          // Add a moderation state label.
          if (isset($entity->moderation_state)) {
            if ($current_state = $entity->moderation_state->value) {
              $moderation_info = \Drupal::service('content_moderation.moderation_information');
              $workflow = $moderation_info->getWorkflowForEntity($entity);

              // Add moderation state label to the title.
              $variables['title_suffix'][] = [
                '#markup' => $workflow->getTypePlugin()->getState($current_state)->label(),
                // Example with Bootstrap classes.
                '#prefix' => '<span class="label label-info">',
                '#suffix' => '</span>',
              ];
            }
          }
        }
      }
    }
  }
}

/**
 * Implements hook_preprocess_comment().
 *
 * Add variables to comment template:
 *  user picture, author name, comment depth and changed date.
 */
function MY_D8_THEME_preprocess_comment(&$variables) {
  $variables['author_raw'] = $variables['comment']->getAuthorName();
  $variables['author_picture'] = user_view($variables['comment']->getOwner(), 'picture');
  $variables['comment_depth'] = count(explode('.', $variables['comment']->getThread()));
  if ($variables['comment']->getChangedTime() == $variables['comment']->getCreatedTime()) {
    $variables['changed_short'] = NULL;
  }
  else {
    $variables['changed_short'] = format_date($variables['comment']->getChangedTime(), 'short');
  }
}

/**
 * Implements hook_theme_suggestions_HOOK_alter().
 *
 * Add custom page theme suggestion based on node type.
 */
function MY_D8_THEME_theme_suggestions_page_alter(array &$suggestions, array $variables) {
  $status = \Drupal::requestStack()->getCurrentRequest()->attributes->get('exception');
  $requestAttributes = Drupal::request()->attributes;

  // Suggestion by node type.
  if ($node = $requestAttributes->get('node')) {
    array_splice($suggestions, 1, 0, 'page__node__' . $node->getType());
  }
}

/**
 * Implements hook_theme_suggestions_HOOK_alter().
 *
 * Add custom block theme suggestion based on block type and display (view mode).
 */
function MY_D8_THEME_theme_suggestions_block_alter(array &$suggestions, array $variables) {
  // View mode suggestion for custom blocks.
  if (isset($variables['elements']['#configuration']['view_mode'])) {
    $view_mode = $variables['elements']['#configuration']['view_mode'];
    $suggestions[] = 'block__' . $view_mode;
  }

  // Region suggestion for blocks in panels.
  if (isset($variables['elements']['#configuration']['region'])) {
    $region = $variables['elements']['#configuration']['region'];
    $suggestions[] = 'block__' . $region;
    if (isset($variables['elements']['#configuration']['provider'])) {
      $provider = $variables['elements']['#configuration']['provider'];
      $suggestions[] = 'block__' . $region . '__' . $provider;
    }
  }
  // Region suggestion for blocks in Drupal.
  if (isset($variables['elements']['#id'])) {
    if ($block = Block::load($variables["elements"]["#id"])) {
      $region = $block->getRegion();
      $suggestions[] = 'block__' . $region;
      $suggestions[] = 'block__' . $region . '__' . $variables['elements']['#base_plugin_id'];
      $suggestions[] = 'block__' . $region . '__' . $variables['elements']['#id'];
      $suggestions[] = 'block__' . $region . '__' . $variables['elements']['#base_plugin_id'] . '__' . $variables['elements']['#id'];
    }
  }

  // Custom Blocks (Bundles and view mode).
  if ($variables['elements']['#base_plugin_id'] === 'block_content'
    && isset($variables['elements']['content']['#block_content'])) {
    // Bundle type.
    $bundle = $variables['elements']['content']['#block_content']->bundle();
    $suggestions[] = 'block__' . $region . '__' . $bundle;
    if ($view_mode = $variables['elements']['content']['#view_mode']) {
      $suggestions[] = 'block__' . $region . '__' . $bundle . '__' . $view_mode;
      $suggestions[] = 'block__' . $bundle . '__' . $view_mode;
    }
    $suggestions[] = 'block__' . $bundle;
  }
}

/**
 * Implements hook_theme_suggestions_HOOK_alter()
 *
 * Add menu theme suggestion based on region.
 *
 * @NOTE: You need to add the region to the block content arrtibute 
 *   (see hook_preprocess_block() above).
 */
function MY_D8_THEME_theme_suggestions_menu_alter(array &$suggestions, array $variables) {
  if (isset($variables['attributes']['region'])) {
    $region = $variables['attributes']['region'];
    $suggestions[] = 'menu__' . $region;
  }
}

/**
 * Implements hook_theme_suggestions_field_alter().
 *
 * Add field theme suggestion based on display (view mode).
 */
function MY_D8_THEME_theme_suggestions_field_alter(array &$suggestions, array $variables) {
  $name = $variables['element']['#field_name'];

  // View mode.
  if ($view_mode = $variables['element']['#view_mode']) {
    $suggestions[] = 'field__' . $view_mode;
    $suggestions[] = 'field__' . $view_mode . '__' . $name;
  }
}

/**
 * Implements hook_theme_suggestions_user_alter().
 *
 * Add user theme suggestion based on display (view mode).
 */
function MY_D8_THEME_theme_suggestions_user_alter(array &$suggestions, array $variables) {
  if ($view_mode = $variables['elements']['#view_mode']) {
    $suggestions[] = 'user__' . $view_mode;
  }
}

/**
 * Implements hook_theme_suggestions_views_exposed_form_alter().
 */
function MY_D8_THEME_theme_suggestions_views_exposed_form_alter(array &$suggestions, array $variables) {
  if (isset($variables['form']['#theme'])) {
    // Add all views exposed theme function except base one.
    array_pop($variables['form']['#theme']);
    $suggestions = $variables['form']['#theme'] + $suggestions;
  }
}

/**
 * Implements hook_page_attachments_alter().
 */
function MY_D8_THEME_page_attachments_alter(array &$page) {
  _MY_D8_THEME_page_attachments_add_viewport_metatag($page);
}

/**
 * Add 'viewport-fit=cover' to the viewport meta-tag.
 *
 * The main reason to do this is to fully support non-rectangular screens, like
 * on the iPhone X, XR and XS.
 *
 * @param array $page
 *   The page array to alter.
 *
 * @see https://webkit.org/blog/7929/designing-websites-for-iphone-x/
 */
function _MY_D8_THEME_attachments_add_viewport_metatag(array &$page): void {
  $viewport = [
    '#type' => 'html_tag',
    '#tag' => 'meta',
    '#attributes' => [
      'name' => 'viewport',
      'content' => 'initial-scale=1.0, viewport-fit=cover',
    ],
  ];

  $page['#attached']['html_head'][] = [$viewport, 'viewport'];
}