--- /dev/null
+<?php
+
+namespace Drupal\entity_test\Plugin\Field\FieldType;
+
+use Drupal\Core\Field\FieldStorageDefinitionInterface;
+use Drupal\Core\Field\Plugin\Field\FieldType\StringItem;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\Core\TypedData\DataDefinition;
+use Drupal\entity_test\TypedData\ComputedString;
+
+/**
+ * Defines the 'Internal Property' entity test field type.
+ *
+ * @FieldType(
+ * id = "internal_property_test",
+ * label = @Translation("Internal Property (test)"),
+ * description = @Translation("A field containing one string, from which two strings are computed (one internal, one not)."),
+ * category = @Translation("Test"),
+ * default_widget = "string_textfield",
+ * default_formatter = "string"
+ * )
+ */
+class InternalPropertyTestFieldItem extends StringItem {
+
+ /**
+ * {@inheritdoc}
+ */
+ public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
+ $properties = parent::propertyDefinitions($field_definition);
+
+ // Add a computed property that is non-internal.
+ $properties['non_internal_value'] = DataDefinition::create('string')
+ ->setLabel(new TranslatableMarkup('Computed string, non-internal property'))
+ ->setComputed(TRUE)
+ ->setClass(ComputedString::class)
+ ->setInternal(FALSE);
+ // Add a computed property that is internal.
+ $properties['internal_value'] = DataDefinition::create('string')
+ ->setLabel(new TranslatableMarkup('Computed string, internal property'))
+ ->setComputed(TRUE)
+ ->setClass(ComputedString::class);
+ return $properties;
+ }
+
+}