add YAML validation to widget

src/Plugin/Field/FieldWidget/YamlEditorWidget.php

@@ -10,6 +10,8 @@
 use Drupal\Core\Field\FieldItemListInterface;
 use Drupal\Core\Field\Plugin\Field\FieldWidget\StringTextareaWidget;
 use Drupal\Core\Form\FormStateInterface;
+use Symfony\Component\Yaml\Exception\ParseException;
+use Symfony\Component\Yaml\Yaml;
 
 /**
  * Plugin implementation of the 'yaml_editor_widget' widget.
@@ -28,18 +30,33 @@ class YamlEditorWidget extends StringTextareaWidget {
    * {@inheritdoc}
    */
   public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
-    $element['value'] = $element + [
-      '#type' => 'textarea',
-      '#default_value' => $items[$delta]->value,
-      '#rows' => $this->getSetting('rows'),
-      '#placeholder' => $this->getSetting('placeholder'),
-      '#attributes' => [
-        'data-yaml-editor' => 'true',
-        'class' => ['js-text-full', 'text-full'],
-      ],
-    ];
+    $element = parent::formElement($items, $delta, $element, $form, $form_state);
+    $element['value']['#attributes']['data-yaml-editor'] = 'true';
+    $element['#element_validate'][] = [get_class($this), 'validateElement'];
 
     return $element;
   }
 
+  /**
+   * Form validation handler for widget elements.
+   *
+   * @param array $element
+   *   The form element.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The form state.
+   */
+  public static function validateElement(array $element, FormStateInterface $form_state) {
+    if (empty($element['value']['#value'])) {
+      return;
+    }
+    try {
+      Yaml::parse($element['value']['#value']);
+    } catch (ParseException $exception) {
+      $form_state->setError($element, t("@name field contains malformed YAML string:<br>@error", [
+        '@name' => $element['#title'],
+        '@error' => $exception->getMessage(),
+      ]));
+    }
+  }
+
 }