class BlockRepository

class BlockRepository implements BlockRepositoryInterface {

  /**
   * The block storage.
   *
   * @var \Drupal\Core\Entity\EntityStorageInterface
   */
  protected $blockStorage;

  /**
   * The theme manager.
   *
   * @var \Drupal\Core\Theme\ThemeManagerInterface
   */
  protected $themeManager;

  /**
   * The context handler.
   *
   * @var \Drupal\Core\Plugin\Context\ContextHandlerInterface
   */
  protected $contextHandler;

  /**
   * Constructs a new BlockRepository.
   *
   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
   *   The entity type manager service.
   * @param \Drupal\Core\Theme\ThemeManagerInterface $theme_manager
   *   The theme manager.
   * @param \Drupal\Core\Plugin\Context\ContextHandlerInterface $context_handler
   *   The plugin context handler.
   */
  public function __construct(EntityTypeManagerInterface $entity_type_manager, ThemeManagerInterface $theme_manager, ContextHandlerInterface $context_handler) {
    $this->blockStorage = $entity_type_manager
      ->getStorage('block');
    $this->themeManager = $theme_manager;
    $this->contextHandler = $context_handler;
  }

  /**
   * {@inheritdoc}
   */
  public function getVisibleBlocksPerRegion(array &$cacheable_metadata = []) {
    $active_theme = $this->themeManager
      ->getActiveTheme();

    // Build an array of the region names in the right order.
    $empty = array_fill_keys($active_theme
      ->getRegions(), []);
    $full = [];
    foreach ($this->blockStorage
      ->loadByProperties([
      'theme' => $active_theme
        ->getName(),
    ]) as $block_id => $block) {

      /** @var \Drupal\block\BlockInterface $block */
      $access = $block
        ->access('view', NULL, TRUE);
      $region = $block
        ->getRegion();
      if (!isset($cacheable_metadata[$region])) {
        $cacheable_metadata[$region] = CacheableMetadata::createFromObject($access);
      }
      else {
        $cacheable_metadata[$region] = $cacheable_metadata[$region]
          ->merge(CacheableMetadata::createFromObject($access));
      }

      // Set the contexts on the block before checking access.
      if ($access
        ->isAllowed()) {
        $full[$region][$block_id] = $block;
      }
    }

    // Merge it with the actual values to maintain the region ordering.
    $assignments = array_intersect_key(array_merge($empty, $full), $empty);
    foreach ($assignments as &$assignment) {
      uasort($assignment, 'Drupal\\block\\Entity\\Block::sort');
    }
    return $assignments;
  }

  /**
   * {@inheritdoc}
   */
  public function getUniqueMachineName(string $suggestion, string $theme = NULL) : string {
    if ($theme) {
      $suggestion = $theme . '_' . $suggestion;
    }

    // Get all the block machine names that begin with the suggested string.
    $query = $this->blockStorage
      ->getQuery();
    $query
      ->accessCheck(FALSE);
    $query
      ->condition('id', $suggestion, 'CONTAINS');
    $block_ids = $query
      ->execute();
    $block_ids = array_map(function ($block_id) {
      $parts = explode('.', $block_id);
      return end($parts);
    }, $block_ids);

    // Iterate through potential IDs until we get a new one. E.g.
    // For example, 'plugin', 'plugin_2', 'plugin_3', etc.
    $count = 1;
    $machine_default = $suggestion;
    while (in_array($machine_default, $block_ids)) {
      $machine_default = $suggestion . '_' . ++$count;
    }
    return $machine_default;
  }

}