drupal/core/modules/block_content/block_content.module

<?php

/**
 * @file
 * Allows the creation of content blocks through the user interface.
 */

use Drupal\Core\Url;
use Drupal\Core\Routing\RouteMatchInterface;
use Drupal\field\Entity\FieldConfig;
use Drupal\field\Entity\FieldStorageConfig;
use Drupal\Core\Database\Query\SelectInterface;
use Drupal\Core\Database\Query\AlterableInterface;
use Drupal\Core\Database\Query\ConditionInterface;

/**
 * Implements hook_help().
 */
function block_content_help($route_name, RouteMatchInterface $route_match) {
  switch ($route_name) {
    case 'help.page.block_content':
      $field_ui = \Drupal::moduleHandler()->moduleExists('field_ui') ? Url::fromRoute('help.page', ['name' => 'field_ui'])->toString() : '#';
      $output = '';
      $output .= '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('The Block Content module allows you to create and manage custom <em>block types</em> and <em>content-containing blocks</em>. For more information, see the <a href=":online-help">online documentation for the Block Content module</a>.', [':online-help' => 'https://www.drupal.org/documentation/modules/block_content']) . '</p>';
      $output .= '<h3>' . t('Uses') . '</h3>';
      $output .= '<dl>';
      $output .= '<dt>' . t('Creating and managing block types') . '</dt>';
      $output .= '<dd>' . t('Users with the <em>Administer blocks</em> permission can create and edit block types with fields and display settings, from the <a href=":types">Block types</a> page under the Structure menu. For more information about managing fields and display settings, see the <a href=":field-ui">Field UI module help</a> and <a href=":field">Field module help</a>.', [':types' => Url::fromRoute('entity.block_content_type.collection')->toString(), ':field-ui' => $field_ui, ':field' => Url::fromRoute('help.page', ['name' => 'field'])->toString()]) . '</dd>';
      $output .= '<dt>' . t('Creating content blocks') . '</dt>';
      $output .= '<dd>' . t('Users with the <em>Administer blocks</em> permission can create, edit, and delete content blocks of each defined block type, from the <a href=":block-library">Content blocks page</a>. After creating a block, place it in a region from the <a href=":blocks">Block layout page</a>, just like blocks provided by other modules.', [':blocks' => Url::fromRoute('block.admin_display')->toString(), ':block-library' => Url::fromRoute('entity.block_content.collection')->toString()]) . '</dd>';
      $output .= '</dl>';
      return $output;
  }
}

/**
 * Implements hook_theme().
 */
function block_content_theme($existing, $type, $theme, $path) {
  return [
    'block_content_add_list' => [
      'variables' => ['content' => NULL],
      'file' => 'block_content.pages.inc',
    ],
  ];
}

/**
 * Implements hook_entity_type_alter().
 */
function block_content_entity_type_alter(array &$entity_types) {
  /** @var \Drupal\Core\Entity\EntityTypeInterface[] $entity_types */
  // Add a translation handler for fields if the language module is enabled.
  if (\Drupal::moduleHandler()->moduleExists('language')) {
    $translation = $entity_types['block_content']->get('translation');
    $translation['block_content'] = TRUE;
    $entity_types['block_content']->set('translation', $translation);
  }

  // Swap out the default EntityChanged constraint with a custom one with
  // different logic for inline blocks.
  $constraints = $entity_types['block_content']->getConstraints();
  unset($constraints['EntityChanged']);
  $constraints['BlockContentEntityChanged'] = NULL;
  $entity_types['block_content']->setConstraints($constraints);
}

/**
 * Adds the default body field to a block type.
 *
 * @param string $block_type_id
 *   Id of the block type.
 * @param string $label
 *   (optional) The label for the body instance. Defaults to 'Body'
 *
 * @return \Drupal\field\Entity\FieldConfig
 *   A Body field object.
 */
function block_content_add_body_field($block_type_id, $label = 'Body') {
  // Add or remove the body field, as needed.
  $field = FieldConfig::loadByName('block_content', $block_type_id, 'body');
  if (empty($field)) {
    $field = FieldConfig::create([
      'field_storage' => FieldStorageConfig::loadByName('block_content', 'body'),
      'bundle' => $block_type_id,
      'label' => $label,
      'settings' => [
        'display_summary' => FALSE,
        'allowed_formats' => [],
      ],
    ]);
    $field->save();

    /** @var \Drupal\Core\Entity\EntityDisplayRepositoryInterface $display_repository */
    $display_repository = \Drupal::service('entity_display.repository');

    // Assign widget settings for the default form mode.
    $display_repository->getFormDisplay('block_content', $block_type_id)
      ->setComponent('body', [
        'type' => 'text_textarea_with_summary',
      ])
      ->save();

    // Assign display settings for default view mode.
    $display_repository->getViewDisplay('block_content', $block_type_id)
      ->setComponent('body', [
        'label' => 'hidden',
        'type' => 'text_default',
      ])
      ->save();
  }

  return $field;
}

/**
 * Implements hook_query_TAG_alter().
 *
 * Alters any 'entity_reference' query where the entity type is
 * 'block_content' and the query has the tag 'block_content_access'.
 *
 * These queries should only return reusable blocks unless a condition on
 * 'reusable' is explicitly set.
 *
 * Block_content entities that are not reusable should by default not be
 * selectable as entity reference values. A module can still create an instance
 * of \Drupal\Core\Entity\EntityReferenceSelection\SelectionInterface
 * that will allow selection of non-reusable blocks by explicitly setting
 * a condition on the 'reusable' field.
 *
 * @see \Drupal\block_content\BlockContentAccessControlHandler
 */
function block_content_query_entity_reference_alter(AlterableInterface $query) {
  if ($query instanceof SelectInterface && $query->getMetaData('entity_type') === 'block_content' && $query->hasTag('block_content_access')) {
    $data_table = \Drupal::entityTypeManager()->getDefinition('block_content')->getDataTable();
    if (array_key_exists($data_table, $query->getTables()) && !_block_content_has_reusable_condition($query->conditions(), $query->getTables())) {
      $query->condition("$data_table.reusable", TRUE);
    }
  }
}

/**
 * Utility function to find nested conditions using the reusable field.
 *
 * @todo Replace this function with a call to the API in
 *   https://www.drupal.org/project/drupal/issues/2984930
 *
 * @param array $condition
 *   The condition or condition group to check.
 * @param array $tables
 *   The tables from the related select query.
 *
 * @see \Drupal\Core\Database\Query\SelectInterface::getTables
 *
 * @return bool
 *   Whether the conditions contain any condition using the reusable field.
 */
function _block_content_has_reusable_condition(array $condition, array $tables) {
  // If this is a condition group call this function recursively for each nested
  // condition until a condition is found that return TRUE.
  if (isset($condition['#conjunction'])) {
    foreach (array_filter($condition, 'is_array') as $nested_condition) {
      if (_block_content_has_reusable_condition($nested_condition, $tables)) {
        return TRUE;
      }
    }
    return FALSE;
  }
  if (isset($condition['field'])) {
    $field = $condition['field'];
    if (is_object($field) && $field instanceof ConditionInterface) {
      return _block_content_has_reusable_condition($field->conditions(), $tables);
    }
    $field_parts = explode('.', $field);
    $data_table = \Drupal::entityTypeManager()->getDefinition('block_content')->getDataTable();
    foreach ($tables as $table) {
      if ($table['table'] === $data_table && $field_parts[0] === $table['alias'] && $field_parts[1] === 'reusable') {
        return TRUE;
      }
    }

  }
  return FALSE;
}