Group by Taxonomy Term Depth

Views was moved into core in D8, so I'm moving the issue over to its issue queue.

In our case we wanted to group the terms by their root ancestors, so we did in the following manner: Add to the view fields the parent field and, configure for it a field formatter plugin that always returns the root ancestor label, then group the results by the parent label.

- Term R1
  - Term A.1
    - Term A.1.1
- Term R2
   - Term B.1
      - Term B.1.1

Will result:

Term R1
- Term A1
- Term A1.1

Term R2
- Term B.1
- Term B.1.1

1. Create a field formatter view plugin that always returns the root parent label

namespace Drupal\judicial\Plugin\Field\FieldFormatter;

use Drupal\Core\Url;
use Drupal\Core\Field\FieldDefinitionInterface;
use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Field\Plugin\Field\FieldFormatter\EntityReferenceFormatterBase;
use Drupal\taxonomy\TermInterface;

/**
 * Plugin implementation of the 'entity reference taxonomy term RSS' formatter.
 *
 * @FieldFormatter(
 *   id = "entity_reference_root_term_name",
 *   label = @Translation("Root term name"),
 *   description = @Translation("Display label of the root term for this term ancestry."),
 *   field_types = {
 *     "entity_reference"
 *   }
 * )
 */
class RootTaxonomyTermNameFieldFormatter extends EntityReferenceFormatterBase {

  /**
   * {@inheritdoc}
   */
  public function viewElements(FieldItemListInterface $items, $langcode) {
    /** @var \Drupal\taxonomy\Entity\Term $item */
    $item = $items->getEntity();
    $ret = '';
    if ($item instanceof TermInterface) {
      $ancestors = \Drupal::service('entity_type.manager')->getStorage("taxonomy_term")->loadAllParents($item->id());
      /** @var \Drupal\taxonomy\Entity\Term $root */
      $root = end($ancestors);
      $ret = $root->getName();
    }
    return [['#markup' => $ret]];
  }

  /**
   * {@inheritdoc}
   */
  public static function isApplicable(FieldDefinitionInterface $field_definition) {
    // This formatter is only available for taxonomy terms.
    return $field_definition->getFieldStorageDefinition()->getSetting('target_type') == 'taxonomy_term';
  }
}

2. Add name and parent field to the list of fields

3. Set the plugin as field formatter for the parents field

4. Configure grouping by the parent field (which returns always to root label)

5. Save the view and check the listing