Show a different view mode depending on role

Question

Using the Group module, when a user views a group, I want to use a different view mode depending on whether the user is a member of the group or not.
Conceptually, this is almost identical to showing a different view mode of a node depending on the user role.

I'd have thought it was easy, but I'm struggling. What I know:

• The route is entity.group.canonical

• I can modify the result of that route using alterRoutes() in a route subcriber

  protected function alterRoutes(RouteCollection $collection) {
    if ($route = $collection->get('entity.group.canonical')) {
      // Do something.
    }
  }
  

I'm not sure how to modify the route. Swapping in a whole new custom controller seems extreme.

Answer 1

Turns out, there's a hook for that.

function hook_entity_view_mode_alter(&$view_mode, Drupal\Core\Entity\EntityInterface $entity, $context) {

  // For nodes, change the view mode when it is teaser.
  if ($entity->getEntityTypeId() == 'node' && $view_mode == 'teaser') {
    $view_mode = 'my_custom_view_mode';
  }
}