gistfile1.php

<?php

/**
 * @file
 * Definition of Drupal\Core\Plugin\Discovery\AnnotatedClassDiscovery.
 */

namespace Drupal\Core\Plugin\Discovery;

use Drupal\Component\Plugin\Discovery\AnnotatedClassDiscovery as ComponentAnnotatedClassDiscovery;
use Drupal\Core\Extension\ModuleHandlerInterface;

/**
 * Defines a discovery mechanism to find annotated plugins in PSR-0 namespaces.
 */
class AnnotatedClassDiscovery extends ComponentAnnotatedClassDiscovery {

  /**
   * Module handler.
   *
   * @var \Drupal\Core\Extension\ModuleHandlerInterface
   */
  protected $moduleHandler;

  /**
   * Constructs an AnnotatedClassDiscovery object.
   *
   * @param string $owner
   *   The plugin type owner.
   * @param string $type
   *   The plugin type name.
   * @param Drupal\Core\Extension\ModuleHandlerInterfac $module_handler
   *   Module handler
   * @param array $plugin_namespaces
   *   An array of paths keyed by it's corresponding namespaces.
   */
  function __construct($owner, $type, ModuleHandlerInterface $module_handler, array $plugin_namespaces = array()) {
    $this->owner = $owner;
    $this->type = $type;
    $this->moduleHandler = $module_handler;
    $annotation_namespaces = array(
      'Drupal\Component\Annotation' => DRUPAL_ROOT . '/core/lib',
      'Drupal\Core\Annotation' => DRUPAL_ROOT . '/core/lib',
    );
    parent::__construct($plugin_namespaces, $annotation_namespaces, 'Drupal\Core\Annotation\Plugin');
  }

  /**
   * Overrides Drupal\Component\Plugin\Discovery\AnnotatedClassDiscovery::getPluginNamespaces().
   *
   * This is overridden rather than set in the constructor, because Drupal
   * modules can be enabled (and therefore, namespaces registered) during the
   * lifetime of a plugin manager.
   */
  protected function getPluginNamespaces() {
    $plugin_namespaces = parent::getPluginNamespaces();
    $plugin_namespaces += array(
      "Drupal\\Component\\Plugin\\{$this->owner}\\{$this->type}" => array(DRUPAL_ROOT . '/core/lib'),
      "Drupal\\Core\\Plugin\\{$this->owner}\\{$this->type}" => array(DRUPAL_ROOT . '/core/lib'),
    );
    foreach ($this->moduleHandler->getModuleList() as $filename => $module) {
      $plugin_namespaces["Drupal\\$module\\Plugin\\{$this->owner}\\{$this->type}"] = array(dirname($filename) . '/lib');
    }
    return $plugin_namespaces;
  }

}

This is overridden rather than set in the constructor, because Drupal
modules can be enabled (and therefore, namespaces registered) during the
lifetime of a plugin manager.

When modules get enabled or disabled, we're invoking DrupalKernel::updateModules()... Perhaps we should/need to trigger an event there?

Recalculating the list on every invocation looks bad to me.