class AttributeClassDiscovery

class AttributeClassDiscovery extends ComponentAttributeClassDiscovery {
  
  /**
   * A suffix to append to each PSR-4 directory associated with a base namespace.
   *
   * This suffix is used to form the directories where plugins are found.
   *
   * @var string
   */
  protected $directorySuffix = '';
  
  /**
   * A suffix to append to each base namespace.
   *
   * This suffix is used to obtain the namespaces where plugins are found.
   *
   * @var string
   */
  protected $namespaceSuffix = '';
  
  /**
   * Constructs an AttributeClassDiscovery object.
   *
   * @param string $subdir
   *   Either the plugin's subdirectory, for example 'Plugin/views/filter', or
   *   empty string if plugins are located at the top level of the namespace.
   * @param \Traversable $rootNamespacesIterator
   *   An object that implements \Traversable which contains the root paths
   *   keyed by the corresponding namespace to look for plugin implementations.
   *   If $subdir is not an empty string, it will be appended to each namespace.
   * @param string $pluginDefinitionAttributeName
   *   (optional) The name of the attribute that contains the plugin definition.
   *   Defaults to 'Drupal\Component\Plugin\Attribute\Plugin'.
   */
  public function __construct(string $subdir, protected \Traversable $rootNamespacesIterator, string $pluginDefinitionAttributeName = 'Drupal\\Component\\Plugin\\Attribute\\Plugin') {
    if ($subdir) {
      // Prepend a directory separator to $subdir,
      // if it does not already have one.
      if ('/' !== $subdir[0]) {
        $subdir = '/' . $subdir;
      }
      $this->directorySuffix = $subdir;
      $this->namespaceSuffix = str_replace('/', '\\', $subdir);
    }
    parent::__construct([], $pluginDefinitionAttributeName);
  }
  
  /**
   * {@inheritdoc}
   */
  protected function prepareAttributeDefinition(AttributeInterface $attribute, string $class) : void {
    parent::prepareAttributeDefinition($attribute, $class);
    if (!$attribute->getProvider()) {
      $attribute->setProvider($this->getProviderFromNamespace($class));
    }
  }
  
  /**
   * Extracts the provider name from a Drupal namespace.
   *
   * @param string $namespace
   *   The namespace to extract the provider from.
   *
   * @return string|null
   *   The matching provider name, or NULL otherwise.
   */
  protected function getProviderFromNamespace(string $namespace) : ?string {
    preg_match('|^Drupal\\\\(?<provider>[\\w]+)\\\\|', $namespace, $matches);
    if (isset($matches['provider'])) {
      return mb_strtolower($matches['provider']);
    }
    return NULL;
  }
  
  /**
   * {@inheritdoc}
   */
  protected function getPluginNamespaces() : array {
    $plugin_namespaces = [];
    if ($this->namespaceSuffix) {
      foreach ($this->rootNamespacesIterator as $namespace => $dirs) {
        // Append the namespace suffix to the base namespace, to obtain the
        // plugin namespace; for example, 'Drupal\views' may become
        // 'Drupal\views\Plugin\Block'.
        $namespace .= $this->namespaceSuffix;
        foreach ((array) $dirs as $dir) {
          // Append the directory suffix to the PSR-4 base directory, to obtain
          // the directory where plugins are found. For example,
          // DRUPAL_ROOT . '/core/modules/views/src' may become
          // DRUPAL_ROOT . '/core/modules/views/src/Plugin/Block'.
          $plugin_namespaces[$namespace][] = $dir . $this->directorySuffix;
        }
      }
    }
    else {
      // Both the namespace suffix and the directory suffix are empty,
      // so the plugin namespaces and directories are the same as the base
      // directories.
      foreach ($this->rootNamespacesIterator as $namespace => $dirs) {
        $plugin_namespaces[$namespace] = (array) $dirs;
      }
    }
    return $plugin_namespaces;
  }

}