--- /dev/null
+/**
+ * Implements hook_field_widget_form_alter().
+ */
+function {{ machine_name }}_field_widget_form_alter(&$element, \Drupal\Core\Form\FormStateInterface $form_state, $context) {
+ // Add a css class to widget form elements for all fields of type mytype.
+ $field_definition = $context['items']->getFieldDefinition();
+ if ($field_definition->getType() == 'mytype') {
+ // Be sure not to overwrite existing attributes.
+ $element['#attributes']['class'][] = 'myclass';
+ }
+}