Issue #3183032: Use checkbox for single class option

Closes #3183032

src/Plugin/Field/FieldWidget/LinkClassFieldWidget.php

@@ -122,7 +122,7 @@ class LinkClassFieldWidget extends LinkWidget {
   protected function getModeOptions($key = NULL) {
     $options = [
       'force_class' => $this->t('Class are automatically added'),
-      'select_class' => $this->t('Let users select a class from a list'),
+      'select_class' => $this->t('Let users select a class'),
       'manual' => $this->t('Users can set a class manually'),
     ];
 
@@ -157,13 +157,26 @@ class LinkClassFieldWidget extends LinkWidget {
       case 'select_class':
         /** @var \Drupal\link\LinkItemInterface $item */
         $classes_available = $this->getSelectOptions($this->getSetting('link_class_select'));
-        $default_value = !empty($options['attributes']['class']) ? $options['attributes']['class'] : '';
-        $element['options']['attributes']['class'] = [
-          '#type' => 'select',
-          '#title' => $this->t('Select a style'),
-          '#options' => ['' => $this->t('- None -')] + $classes_available,
-          '#default_value' => $default_value,
-        ];
+        if (count($classes_available) === 1) {
+          $single_class = key($classes_available);
+          $default_classes = !empty($options['attributes']['class']) ? $options['attributes']['class'] : '';
+          $default_value = in_array($single_class, explode(' ', $default_classes), TRUE);
+          $element['options']['attributes']['class'] = [
+            '#type' => 'checkbox',
+            '#title' => $classes_available[$single_class],
+            '#default_value' => $default_value,
+            '#return_value' => $single_class,
+          ];
+        }
+        else {
+          $default_value = !empty($options['attributes']['class']) ? $options['attributes']['class'] : '';
+          $element['options']['attributes']['class'] = [
+            '#type' => 'select',
+            '#title' => $this->t('Select a style'),
+            '#options' => ['' => $this->t('- None -')] + $classes_available,
+            '#default_value' => $default_value,
+          ];
+        }
         break;
 
       case 'force_class':
@@ -178,6 +191,19 @@ class LinkClassFieldWidget extends LinkWidget {
     return $element;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function massageFormValues(array $values, array $form, FormStateInterface $form_state) {
+    foreach ($values as &$value) {
+      // Filter out empty (and invalid) classes.
+      if (array_key_exists('class', $value['options']['attributes']) && empty($value['options']['attributes']['class'])) {
+        unset($value['options']['attributes']['class']);
+      }
+    }
+    return parent::massageFormValues($values, $form, $form_state);
+  }
+
   /**
    * Convert textarea lines into an array.
    *