--- /dev/null
+<?php
+
+namespace Drupal\Core\Entity\EntityReferenceSelection;
+
+use Drupal\Component\Plugin\ConfigurablePluginInterface;
+use Drupal\Component\Utility\NestedArray;
+use Drupal\Core\Database\Query\SelectInterface;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Plugin\PluginBase;
+
+/**
+ * Provides a base class for configurable selection handlers.
+ */
+abstract class SelectionPluginBase extends PluginBase implements SelectionInterface, ConfigurablePluginInterface {
+
+ /**
+ * Constructs a new selection object.
+ *
+ * @param array $configuration
+ * A configuration array containing information about the plugin instance.
+ * @param string $plugin_id
+ * The plugin_id for the plugin instance.
+ * @param mixed $plugin_definition
+ * The plugin implementation definition.
+ */
+ public function __construct(array $configuration, $plugin_id, $plugin_definition) {
+ parent::__construct($configuration, $plugin_id, $plugin_definition);
+ $this->setConfiguration($configuration);
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function defaultConfiguration() {
+ return [
+ 'target_type' => NULL,
+ // @todo Remove this key in Drupal 9.0.x.
+ 'handler' => $this->getPluginId(),
+ 'entity' => NULL,
+ ];
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function getConfiguration() {
+ return $this->configuration;
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function setConfiguration(array $configuration) {
+ // Resolve backward compatibility level configurations, if any.
+ $this->resolveBackwardCompatibilityConfiguration($configuration);
+
+ // Merge in defaults.
+ $this->configuration = NestedArray::mergeDeep(
+ $this->defaultConfiguration(),
+ $configuration
+ );
+
+ // Ensure a backward compatibility level configuration.
+ $this->ensureBackwardCompatibilityConfiguration();
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function calculateDependencies() {
+ return [];
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
+ return $form;
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function validateConfigurationForm(array &$form, FormStateInterface $form_state) {}
+
+ /**
+ * {@inheritdoc}
+ */
+ public function submitConfigurationForm(array &$form, FormStateInterface $form_state) {}
+
+ /**
+ * {@inheritdoc}
+ */
+ public function entityQueryAlter(SelectInterface $query) {}
+
+ /**
+ * Moves the backward compatibility level configurations in the right place.
+ *
+ * In order to keep backward compatibility, we copy all settings, except
+ * 'target_type', 'handler' and 'entity' under 'handler_settings', following
+ * the structure from the field config. If the plugin was instantiated using
+ * the 'handler_settings' level, those values will be used. In case of
+ * conflict, the root level settings will take precedence. The backward
+ * compatibility aware configuration will have the next structure:
+ * - target_type
+ * - handler (will be removed in Drupal 9.0.x, it's the plugin id)
+ * - entity
+ * - setting_1
+ * - setting_2
+ * ...
+ * - setting_N
+ * - handler_settings: (will be removed in Drupal 9.0.x)
+ * - setting_1
+ * - setting_2
+ * ...
+ * - setting_N
+ *
+ * @param array $configuration
+ * The configuration array to be altered.
+ *
+ * @deprecated Scheduled for removal in Drupal 9.0.x.
+ *
+ * @see https://www.drupal.org/node/2870971
+ */
+ protected function resolveBackwardCompatibilityConfiguration(array &$configuration) {
+ if (isset($this->defaultConfiguration()['handler_settings'])) {
+ throw new \InvalidArgumentException("{$this->getPluginDefinition()['class']}::defaultConfiguration() should not contain a 'handler_settings' key. All settings should be placed in the root level.");
+ }
+
+ // Extract the BC level from the passed configuration, if any.
+ if (array_key_exists('handler_settings', $configuration)) {
+ if (!is_array($configuration['handler_settings'])) {
+ throw new \InvalidArgumentException("The setting 'handler_settings' is reserved and cannot be used.");
+ }
+ @trigger_error("Providing settings under 'handler_settings' is deprecated and will be removed before 9.0.0. Move the settings in the root of the configuration array. See https://www.drupal.org/node/2870971.", E_USER_DEPRECATED);
+
+ // Settings passed in the root level take precedence over BC settings.
+ $configuration += $configuration['handler_settings'];
+ unset($configuration['handler_settings']);
+ }
+ }
+
+ /**
+ * Ensures a backward compatibility level configuration.
+ *
+ * @deprecated Scheduled for removal in Drupal 9.0.x.
+ *
+ * @see https://www.drupal.org/node/2870971
+ */
+ protected function ensureBackwardCompatibilityConfiguration() {
+ $keys = ['handler', 'target_type', 'entity', 'handler_settings'];
+ // Synchronize back 'handler_settings'.
+ foreach ($this->configuration as $key => $value) {
+ // Filter out keys that belong strictly to the root level.
+ if (!in_array($key, $keys, TRUE)) {
+ $this->configuration['handler_settings'][$key] = $value;
+ }
+ }
+ }
+
+}