Overrides \Drupal\Core\Entity\EntityRenderController::alterBuild().
Overrides EntityRenderController::alterBuild
protected function alterBuild(array &$build, EntityInterface $entity, EntityDisplay $display, $view_mode, $langcode = NULL) {
parent::alterBuild($build, $entity, $display, $view_mode, $langcode);
// Add contextual links for this custom block.
if (!empty($entity->id->value) && $view_mode == 'full') {
$build['#contextual_links']['custom_block'] = array(
'block',
array(
$entity
->id(),
),
);
}
}