Multiple Selects - Support required fields

multiple_selects__support_required_fields.patch

diff --git a/src/Plugin/Field/FieldWidget/OptionsMultipleSelectWidget.php b/src/Plugin/Field/FieldWidget/OptionsMultipleSelectWidget.php
index d635943..f5a21f0 100755
--- a/src/Plugin/Field/FieldWidget/OptionsMultipleSelectWidget.php
+++ b/src/Plugin/Field/FieldWidget/OptionsMultipleSelectWidget.php
@@ -31,10 +31,22 @@ class OptionsMultipleSelectWidget extends OptionsSelectWidget implements WidgetI
   /**
    * {@inheritdoc}
    */
+  public function form(FieldItemListInterface $items, array &$form, FormStateInterface $form_state, $get_delta = NULL) {
+    $element = parent::form($items, $form, $form_state, $get_delta);
+    $element['widget']['#element_validate'][] = array(get_class($this), 'validateMultipleElements');
+    $element['widget']['#column'] = $this->column;
+    return $element;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
   public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
-    $element = parent::formElement($items, $delta, $element, $form, $form_state);
+    $element['#required'] = FALSE;
+    $option_element = parent::formElement($items, $delta, $element, $form, $form_state);
+    $option_element['#multiple'] = FALSE;
 
-    $element[$this->column] = $element;
+    $element[$this->column] = $option_element;
     $element[$this->column]['#default_value'] = empty($items[$delta]->{$this->column}) ? '_none' : $items[$delta]->{$this->column};
     $element[$this->column]['#multiple'] = FALSE;
     unset($element['#type']);
@@ -87,15 +99,22 @@ class OptionsMultipleSelectWidget extends OptionsSelectWidget implements WidgetI
     }
   }
 
+  public static function validateMultipleElements(array $element, FormStateInterface $form_state) {
+    if ($element['#required'] == TRUE) {
+      foreach (Element::children($element) as $key) {
+        if (is_integer($key) && $element[$key][$element['#column']]['#value'] != '_none') {
+          return;
+        }
+      }
+      $form_state->setError($element[0], t('@name field is required.', array('@name' => $element['#title'])));
+    }
+  }
+
   /**
    * {@inheritdoc}
    */
   public static function validateElement(array $element, FormStateInterface $form_state) {
-    if ($element['#required'] && $element['#value'] == '_none') {
-      $form_state->setError($element, t('@name field is required.', array('@name' => $element['#title'])));
-    }
-
-    if ($element['#value'] === '_none') {
+    if (isset($element['#value']) && $element['#value'] === '_none') {
       $form_state->setValueForElement($element, NULL);
     }
   }