Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/modules/options/tests/options_test/options_test.info.yml

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+name: 'Options test'
+type: module
+description: 'Support module for the Options module tests.'
+core: 8.x
+package: Testing
+version: VERSION

core/modules/options/tests/options_test/options_test.module

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+<?php
+
+/**
+ * @file
+ * Helper module for the List module tests.
+ */
+
+use Drupal\Core\Entity\FieldableEntityInterface;
+use Drupal\Core\Field\FieldStorageDefinitionInterface;
+
+/**
+ * Implements callback_allowed_values_function().
+ *
+ * @see options_allowed_values().
+ */
+function options_test_allowed_values_callback(FieldStorageDefinitionInterface $definition, FieldableEntityInterface $entity = NULL) {
+  $values = array(
+    'Group 1' => array(
+      0 => 'Zero',
+    ),
+    1 => 'One',
+    'Group 2' => array(
+      2 => 'Some <script>dangerous</script> & unescaped <strong>markup</strong>',
+    ),
+    'More <script>dangerous</script> markup' => array(
+      3 => 'Three',
+    ),
+  );
+
+  return $values;
+}
+
+/**
+ * Implements callback_allowed_values_function().
+ *
+ * @todo This function violates the recommendation in options_allowed_values()
+ *   to return a list of all possible values in any context when $items is
+ *   NULL. Since this is not yet used for testing Views integration, that is
+ *   alright for now. Fix this in https://www.drupal.org/node/2012130.
+ *
+ * @see options_allowed_values().
+ */
+function options_test_dynamic_values_callback(FieldStorageDefinitionInterface $definition, FieldableEntityInterface $entity = NULL, &$cacheable = NULL) {
+  $values = array();
+  if (isset($entity)) {
+    $cacheable = FALSE;
+    $values = array(
+      $entity->label(),
+      $entity->url(),
+      $entity->uuid(),
+      $entity->bundle(),
+    );
+  }
+  // We need the values of the entity as keys.
+  return array_combine($values, $values);
+}