Plugins used in entities with plugin collections can react when the entities' dependencies are removed

When a configurtion entity implementing EntityWithPluginCollectionInterface is reacting to dependency removal, it iterates through all the plugins in the plugin collection to check whether any plugin implements Drupal\Core\Plugin\RemovableDependentPluginInterface, and if so, invokes the plugin's onCollectionDependencyRemoval() method, so that the plugin can react to the dependency removal as well. Implementations of onCollectionDependencyRemoval() can be used to update the configuration of the plugin itself and notify the entity to recalculate its dependencies and/or remove the plugin from entity's plugin collection. This can prevent the entity from being unnecessarily deleted on dependency removal if the change to the plugin configuration or the removal of the plugin results in the entity no longer having the dependencies.

onCollectionDependencyRemoval() returns a Drupal\Core\Plugin\RemovableDependentPluginReturn enum that has one of three values:

• RemovableDependentPluginReturn::Changed if the configuration of the plugin instance has changed
• RemovableDependentPluginReturn::Remove if the plugin instance should be removed from the plugin collection
• RemovableDependentPluginReturn::Unchanged if the configuration of the plugin instance has not changed.

For example, the Drupal\image\Entity\ImageStyle configuration entity type has a collection of Drupal\image\ImageEffectInterface plugins. Image effect plugin classes that extend Drupal\image\ImageEffectBase have this onCollectionDependencyRemoval() implementation that removes the plugin from the entity's collection if the module providing the plugin is uninstalled:

  public function onCollectionDependencyRemoval(array $dependencies): RemovableDependentPluginReturn {
    // If the module that provides the image effect plugin is uninstalled,
    // the plugin instance should be removed from the collection.
    return in_array($this->getPluginDefinition()['provider'], $dependencies['module'] ?? []) ? RemovableDependentPluginReturn::Remove : RemovableDependentPluginReturn::Unchanged;
  }