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, \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;
    }

}