Rules Plugin Derivatives Policy

The Plug-in System of Drupal 8 provides a mechanism called Plugin Derivatives. This powerful concept allows a single plugin class to expose multiple plugins to the user interface.

In Rules, currently we use plugin derivates whenever dealing with entities: Instead of exposing a single "Create a new Entity" action, the user is presented with options like "Create a new Node", "Create a new Taxonomy Term" or "Create a new User".

According to our Derivatives policy, we want to have derivatives whenever dealing with entity type based Actions or Conditions. Other cases might make sense, feel free to discuss them in the policy issue.

A good reference implementation is the EntityCreate action:

The EntityCreate action doesn't contain the context annotations:

 * Provides a generic 'Create a new entity' action.
 * @Action(
 *   id = "rules_entity_create",
 *   deriver = "Drupal\rules\Plugin\Action\EntityCreateDeriver",
 * )
class EntityCreate extends RulesActionBase implements ContainerFactoryPluginInterface {

Instead it references the EntityCreateDeriver who based on all entity types, provides derivatives.

   * {@inheritdoc}
  public function getDerivativeDefinitions($base_plugin_definition) {
    foreach ($this->entityManager->getDefinitions() as $entity_type_id => $entity_type) {
      // Only allow content entities and ignore configuration entities.
      if (!$entity_type instanceof ContentEntityTypeInterface) {
      $this->derivatives["entity:$entity_type_id"] = [
        'label' => $this->t('Create a new @entity_type', ['@entity_type' => $entity_type->getLowercaseLabel()]),
        'category' => $entity_type->getLabel(),
        'entity_type_id' => $entity_type_id,
        'context' => [],
        'provides' => [
          'entity' => new ContextDefinition("entity:$entity_type_id", $entity_type->getLabel()),
      ] + $base_plugin_definition;
      // Add a required context for the bundle key, and optional contexts for
      // other required base fields. This matches the storage create() behavior,
      // where only the bundle requirement is enforced.
      $bundle_key = $entity_type->getKey('bundle');
      $base_field_definitions = $this->entityManager->getBaseFieldDefinitions($entity_type_id);
      foreach ($base_field_definitions as $field_name => $definition) {
        if ($field_name != $bundle_key && !$definition->isRequired()) {
        $required = ($field_name == $bundle_key);
        $multiple = ($definition->getCardinality() === 1) ? FALSE : TRUE;
        $this->derivatives["entity:$entity_type_id"]['context'][$field_name] = new ContextDefinition(
          $definition->getType(), $definition->getLabel(), $required, $multiple, $definition->getDescription()
    return $this->derivatives;

Also see the related ticket.