class ContentEntityType

class ContentEntityType extends EntityType implements ContentEntityTypeInterface {
    
    /**
     * An array of entity revision metadata keys.
     *
     * @var array
     */
    protected $revision_metadata_keys = [];
    
    /**
     * The required revision metadata keys.
     *
     * @var array
     */
    protected $requiredRevisionMetadataKeys = [];
    
    /**
     * {@inheritdoc}
     */
    public function __construct($definition) {
        parent::__construct($definition);
        $this->handlers += [
            'storage' => 'Drupal\\Core\\Entity\\Sql\\SqlContentEntityStorage',
            'view_builder' => 'Drupal\\Core\\Entity\\EntityViewBuilder',
        ];
        // Only new instances should provide the required revision metadata keys.
        // The cached instances should return only what already has been stored
        // under the property $revision_metadata_keys. The BC layer in
        // ::getRevisionMetadataKeys() has to detect if the revision metadata keys
        // have been provided by the entity type annotation, therefore we add keys
        // to the property $requiredRevisionMetadataKeys only if those keys aren't
        // set in the entity type annotation.
        if (!isset($this->revision_metadata_keys['revision_default'])) {
            $this->requiredRevisionMetadataKeys['revision_default'] = 'revision_default';
        }
        // Add the required revision metadata fields here instead in the getter
        // method, so that they are serialized as part of the object even if the
        // getter method doesn't get called. This allows the list to be further
        // extended. Only new instances of the class will contain the new list,
        // while the cached instances contain the previous version of the list.
        $this->revision_metadata_keys += $this->requiredRevisionMetadataKeys;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getConfigDependencyKey() {
        return 'content';
    }
    
    /**
     * {@inheritdoc}
     *
     * @throws \InvalidArgumentException
     *   If the provided class does not implement
     *   \Drupal\Core\Entity\ContentEntityStorageInterface.
     *
     * @see \Drupal\Core\Entity\ContentEntityStorageInterface
     */
    protected function checkStorageClass($class) {
        $required_interface = ContentEntityStorageInterface::class;
        if (!is_subclass_of($class, $required_interface)) {
            throw new \InvalidArgumentException("{$class} does not implement {$required_interface}");
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function getRevisionMetadataKeys($include_backwards_compatibility_field_names = TRUE) {
        // Provide backwards compatibility in case the revision metadata keys are
        // not defined in the entity annotation.
        if ((!$this->revision_metadata_keys || $this->revision_metadata_keys == $this->requiredRevisionMetadataKeys) && $include_backwards_compatibility_field_names) {
            $base_fields = \Drupal::service('entity_field.manager')->getBaseFieldDefinitions($this->id());
            if (isset($base_fields['revision_uid']) && ($revision_user = 'revision_uid') || isset($base_fields['revision_user']) && ($revision_user = 'revision_user')) {
                @trigger_error('The revision_user revision metadata key is not set for entity type: ' . $this->id . ' See: https://www.drupal.org/node/2831499', E_USER_DEPRECATED);
                $this->revision_metadata_keys['revision_user'] = $revision_user;
            }
            if (isset($base_fields['revision_timestamp']) && ($revision_timestamp = 'revision_timestamp') || isset($base_fields['revision_created']) && ($revision_timestamp = 'revision_created')) {
                @trigger_error('The revision_created revision metadata key is not set for entity type: ' . $this->id . ' See: https://www.drupal.org/node/2831499', E_USER_DEPRECATED);
                $this->revision_metadata_keys['revision_created'] = $revision_timestamp;
            }
            if (isset($base_fields['revision_log']) && ($revision_log = 'revision_log') || isset($base_fields['revision_log_message']) && ($revision_log = 'revision_log_message')) {
                @trigger_error('The revision_log_message revision metadata key is not set for entity type: ' . $this->id . ' See: https://www.drupal.org/node/2831499', E_USER_DEPRECATED);
                $this->revision_metadata_keys['revision_log_message'] = $revision_log;
            }
        }
        return $this->revision_metadata_keys;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getRevisionMetadataKey($key) {
        $keys = $this->getRevisionMetadataKeys();
        return isset($keys[$key]) ? $keys[$key] : FALSE;
    }
    
    /**
     * {@inheritdoc}
     */
    public function hasRevisionMetadataKey($key) {
        $keys = $this->getRevisionMetadataKeys();
        return isset($keys[$key]);
    }
    
    /**
     * {@inheritdoc}
     */
    public function setRevisionMetadataKey($key, $field_name) {
        if ($field_name !== NULL) {
            // Update the property holding the required revision metadata keys,
            // which is used by the BC layer for retrieving the revision metadata
            // keys.
            // @see \Drupal\Core\Entity\ContentEntityType::getRevisionMetadataKeys()
            $this->requiredRevisionMetadataKeys[$key] = $field_name;
            // Add the new revision metadata key.
            $this->revision_metadata_keys[$key] = $field_name;
        }
        else {
            unset($this->requiredRevisionMetadataKeys[$key], $this->revision_metadata_keys[$key]);
        }
        return $this;
    }

}