How do I add a view mode to a simple custom entity? I have created a super-simple drupal 8 custom entity.  We now need a custom view mode for this entity.  What do I need to add to my definition to make my entity appear as an option on the 'Add View Mode' page: /admin/structure/display-modes/view/add ?

My super-simple entity definition is

 * @ContentEntityType(
 *   id = "schedulecal",
 *   label = @Translation("ScheduleCalendar"),
 *   base_table = "scheduler_entity_calendars",
 *   entity_keys = {
 *     "id" = "id",
 *     "uuid" = "uuid",
 *   },
 * )


I can't find this info anywhere in doc.
You need a view builder class in the handler section:

 *   handlers = {
 *     "view_builder" = "Drupal\Core\Entity\EntityViewBuilder",


and a field UI route:

 *   field_ui_base_route = "my_entity.settings",


pointing at the config form for the settings of your entity type. If you don't have settings return at least a #markup element:

  public function buildForm(array $form, FormStateInterface $form_state) {
    $form['my_entity_settings']['#markup'] = 'Settings form for My Entity entities. Manage field settings here.';
    return $form;
  }