buildCurrentSelectionArea example

          // media wrapper when items are added. We set focus to the container           // because a media item does not necessarily have required fields and           // we do not want to set focus to the remove button automatically.           // @see ::updateFormCallback()           'tabindex' => '-1',
        ],
      ];
      foreach ($added_media as $delta => $media) {
        $form['media'][$delta] = $this->buildEntityFormElement($media, $form, $form_state, $delta);
      }

      $form['selection'] = $this->buildCurrentSelectionArea($form, $form_state);
      $form['actions'] = $this->buildActions($form, $form_state);
    }

    // Allow the current selection to be set in a hidden field so the selection     // can be passed between different states of the form. This field is filled     // via JavaScript so the default value should be empty.     // @see Drupal.behaviors.MediaLibraryItemSelection     $form['current_selection'] = [
      '#type' => 'hidden',
      '#default_value' => '',
      '#attributes' => [