插件派生提供了一种简单的方式来扩展单个插件，以使它能表现为多个插件。这是通过创建一个单独的类来完成的，这个类可以在插件定义中进行引用。

通常派生类在src/Plugin/Derivative目录下。

core/modules/system/src/Plugin/Block/SystemMenuBlock.php

如： 在core/modules/system/src/Plugin/Block/SystemMenuBlock.php通过注解定义一个派生类。

<?php

namespace Drupal\system\Plugin\Block;

use Drupal\Core\Block\BlockBase;
use Drupal\Core\Cache\Cache;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Menu\MenuActiveTrailInterface;
use Drupal\Core\Menu\MenuLinkTreeInterface;
use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * Provides a generic Menu block.
 *
 * @Block(
 *   id = "system_menu_block",
 *   admin_label = @Translation("Menu"),
 *   category = @Translation("Menus"),
 *   deriver = "Drupal\system\Plugin\Derivative\SystemMenuBlock"
 * )
 */
class SystemMenuBlock extends BlockBase implements ContainerFactoryPluginInterface {

  /**
   * The menu link tree service.
   *
   * @var \Drupal\Core\Menu\MenuLinkTreeInterface
   */
  protected $menuTree;

  /**
   * The active menu trail service.
   *
   * @var \Drupal\Core\Menu\MenuActiveTrailInterface
   */
  protected $menuActiveTrail;

  /**
   * Constructs a new SystemMenuBlock.
   *
   * @param array $configuration
   *   A configuration array containing information about the plugin instance.
   * @param string $plugin_id
   *   The plugin_id for the plugin instance.
   * @param array $plugin_definition
   *   The plugin implementation definition.
   * @param \Drupal\Core\Menu\MenuLinkTreeInterface $menu_tree
   *   The menu tree service.
   * @param \Drupal\Core\Menu\MenuActiveTrailInterface $menu_active_trail
   *   The active menu trail service.
   */
  public function __construct(array $configuration, $plugin_id, $plugin_definition, MenuLinkTreeInterface $menu_tree, MenuActiveTrailInterface $menu_active_trail) {
    parent::__construct($configuration, $plugin_id, $plugin_definition);
    $this->menuTree = $menu_tree;
    $this->menuActiveTrail = $menu_active_trail;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static(
      $configuration,
      $plugin_id,
      $plugin_definition,
      $container->get('menu.link_tree'),
      $container->get('menu.active_trail')
    );
  }

  /**
   * {@inheritdoc}
   */
  public function blockForm($form, FormStateInterface $form_state) {
    $config = $this->configuration;

    $defaults = $this->defaultConfiguration();
    $form['menu_levels'] = array(
      '#type' => 'details',
      '#title' => $this->t('Menu levels'),
      // Open if not set to defaults.
      '#open' => $defaults['level'] !== $config['level'] || $defaults['depth'] !== $config['depth'],
      '#process' => [[get_class(), 'processMenuLevelParents']],
    );

    $options = range(0, $this->menuTree->maxDepth());
    unset($options[0]);

    $form['menu_levels']['level'] = array(
      '#type' => 'select',
      '#title' => $this->t('Initial menu level'),
      '#default_value' => $config['level'],
      '#options' => $options,
      '#description' => $this->t('The menu will only be visible if the menu item for the current page is at or below the selected starting level. Select level 1 to always keep this menu visible.'),
      '#required' => TRUE,
    );

    $options[0] = $this->t('Unlimited');

    $form['menu_levels']['depth'] = array(
      '#type' => 'select',
      '#title' => $this->t('Maximum number of menu levels to display'),
      '#default_value' => $config['depth'],
      '#options' => $options,
      '#description' => $this->t('The maximum number of menu levels to show, starting from the initial menu level. For example: with an initial level 2 and a maximum number of 3, menu levels 2, 3 and 4 can be displayed.'),
      '#required' => TRUE,
    );

    return $form;
  }

  /**
   * Form API callback: Processes the menu_levels field element.
   *
   * Adjusts the #parents of menu_levels to save its children at the top level.
   */
  public static function processMenuLevelParents(&$element, FormStateInterface $form_state, &$complete_form) {
    array_pop($element['#parents']);
    return $element;
  }

  /**
   * {@inheritdoc}
   */
  public function blockSubmit($form, FormStateInterface $form_state) {
    $this->configuration['level'] = $form_state->getValue('level');
    $this->configuration['depth'] = $form_state->getValue('depth');
  }

  /**
   * {@inheritdoc}
   */
  public function build() {
    $menu_name = $this->getDerivativeId();
    $parameters = $this->menuTree->getCurrentRouteMenuTreeParameters($menu_name);

    // Adjust the menu tree parameters based on the block's configuration.
    $level = $this->configuration['level'];
    $depth = $this->configuration['depth'];
    $parameters->setMinDepth($level);
    // When the depth is configured to zero, there is no depth limit. When depth
    // is non-zero, it indicates the number of levels that must be displayed.
    // Hence this is a relative depth that we must convert to an actual
    // (absolute) depth, that may never exceed the maximum depth.
    if ($depth > 0) {
      $parameters->setMaxDepth(min($level + $depth - 1, $this->menuTree->maxDepth()));
    }

    $tree = $this->menuTree->load($menu_name, $parameters);
    $manipulators = array(
      array('callable' => 'menu.default_tree_manipulators:checkAccess'),
      array('callable' => 'menu.default_tree_manipulators:generateIndexAndSort'),
    );
    $tree = $this->menuTree->transform($tree, $manipulators);
    return $this->menuTree->build($tree);
  }

  /**
   * {@inheritdoc}
   */
  public function defaultConfiguration() {
    return [
      'level' => 1,
      'depth' => 0,
    ];
  }

  /**
   * {@inheritdoc}
   */
  public function getCacheTags() {
    // Even when the menu block renders to the empty string for a user, we want
    // the cache tag for this menu to be set: whenever the menu is changed, this
    // menu block must also be re-rendered for that user, because maybe a menu
    // link that is accessible for that user has been added.
    $cache_tags = parent::getCacheTags();
    $cache_tags[] = 'config:system.menu.' . $this->getDerivativeId();
    return $cache_tags;
  }

  /**
   * {@inheritdoc}
   */
  public function getCacheContexts() {
    // ::build() uses MenuLinkTreeInterface::getCurrentRouteMenuTreeParameters()
    // to generate menu tree parameters, and those take the active menu trail
    // into account. Therefore, we must vary the rendered menu by the active
    // trail of the rendered menu.
    // Additional cache contexts, e.g. those that determine link text or
    // accessibility of a menu, will be bubbled automatically.
    $menu_name = $this->getDerivativeId();
    return Cache::mergeContexts(parent::getCacheContexts(), ['route.menu_active_trails:' . $menu_name]);
  }

}

/core/modules/system/src/Plugin/Derivative/SystemMenuBlock.php

<?php

namespace Drupal\system\Plugin\Derivative;

use Drupal\Component\Plugin\Derivative\DeriverBase;
use Drupal\Core\Entity\EntityStorageInterface;
use Drupal\Core\Plugin\Discovery\ContainerDeriverInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * Provides block plugin definitions for custom menus.
 *
 * @see \Drupal\system\Plugin\Block\SystemMenuBlock
 */
class SystemMenuBlock extends DeriverBase implements ContainerDeriverInterface {

  /**
   * The menu storage.
   *
   * @var \Drupal\Core\Entity\EntityStorageInterface
   */
  protected $menuStorage;

  /**
   * Constructs new SystemMenuBlock.
   *
   * @param \Drupal\Core\Entity\EntityStorageInterface $menu_storage
   *   The menu storage.
   */
  public function __construct(EntityStorageInterface $menu_storage) {
    $this->menuStorage = $menu_storage;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, $base_plugin_id) {
    return new static(
      $container->get('entity.manager')->getStorage('menu')
    );
  }

  /**
   * {@inheritdoc}
   */
  public function getDerivativeDefinitions($base_plugin_definition) {
    foreach ($this->menuStorage->loadMultiple() as $menu => $entity) {
      $this->derivatives[$menu] = $base_plugin_definition;
      $this->derivatives[$menu]['admin_label'] = $entity->label();
      $this->derivatives[$menu]['config_dependencies']['config'] = array($entity->getConfigDependencyName());
    }
    return $this->derivatives;
  }

}

改变插件类型

通常实现一个hook来改变插件的类型。

namespace Drupal\block\Plugin\Type;

use Drupal\Component\Plugin\PluginType;
use Drupal\Core\Plugin\Discovery\HookDiscovery;
use Drupal\Component\Plugin\Factory\DefaultFactory;

class BlockPluginType extends PluginType {
  public function __construct() {
    $this->discovery = new HookDiscovery('block_info');
    $this->factory = new DefaultFactory($this);
  }
}

这样我们就可以通过hook_block_info来改变插件。

也可以通过派生来改变，代码如下：

namespace Drupal\block\Plugin\Type;
use Drupal\Component\Plugin\Discovery\DerivativeDiscoveryDecorator;

use Drupal\Component\Plugin\PluginType;
use Drupal\Core\Plugin\Discovery\HookDiscovery;
use Drupal\Component\Plugin\Factory\DefaultFactory;

class BlockPluginType extends PluginType {
  public function __construct() {
    $this->discovery = new DerivativeDiscoveryDecorator(new HookDiscovery('block_info'));
    $this->factory = new DefaultFactory($this);
  }
}