Overriding the default handlers for any entity in Drupal 8

In Drupal 8 Entities declare handlers which takes care of different functions like Access, Storage , List building, View building, form etc.

Let’s take an example of Webform Entity. Below are the handlers declared in Webform Entity annotation.

*   handlers = {
*     "storage" = "\Drupal\webform\WebformEntityStorage",
*     "view_builder" = "Drupal\webform\WebformEntityViewBuilder",
*     "list_builder" = "Drupal\webform\WebformEntityListBuilder",
*     "access" = "Drupal\webform\WebformEntityAccessControlHandler",
*     "form" = {
*       "add" = "Drupal\webform\WebformEntityAddForm",
*       "duplicate" = "Drupal\webform\WebformEntityAddForm",
*       "delete" = "Drupal\webform\WebformEntityDeleteForm",
*       "edit" = "Drupal\webform\WebformEntityElementsForm",
*       "export" = "Drupal\webform\WebformEntityExportForm",
*       "settings" = "Drupal\webform\EntitySettings\WebformEntitySettingsGeneralForm",
*       "settings_form" = "Drupal\webform\EntitySettings\WebformEntitySettingsFormForm",
*       "settings_submissions" = "Drupal\webform\EntitySettings\WebformEntitySettingsSubmissionsForm",
*       "settings_confirmation" = "Drupal\webform\EntitySettings\WebformEntitySettingsConfirmationForm",
*       "settings_assets" = "Drupal\webform\EntitySettings\WebformEntitySettingsAssetsForm",
*       "settings_access" = "Drupal\webform\EntitySettings\WebformEntitySettingsAccessForm",
*       "handlers" = "Drupal\webform\WebformEntityHandlersForm",
*       "variants" = "Drupal\webform\WebformEntityVariantsForm",
*     }
*   },

Now we want to override form handler for webform entity. We can use hook_entity_type_alter to alter the class for handler.

Create mymodule.module file add this code:

/**
 * @param array $entity_types
 */
function mymodule_entity_type_alter(array &$entity_types)
{
  if (isset($entity_types['webform'])) {
    $webform_entity_type = $entity_types['webform'];
    $handlers = $webform_entity_type->getHandlerClasses();
    $handlers['form']['settings_mymodule'] = 'Drupal\mymodule\EntitySettings\CustomFormHandler';
    $webform_entity_type->setHandlerClass('form', $handlers['form']);
  }
}

Now you can create your own Form handler at mymodule/src/EntitySettings/CustomFormHandler, You can either extend the WebformEntitySettingsBaseForm class or make what you want, in our case I extend WebformEntitySettingsBaseForm to add my own page settings . 

And don't forget to add your own route like this:

entity.webform.mymodule:
  path: '/admin/structure/webform/manage/{webform}/settings/mymodule'
  defaults:
    _entity_form: 'webform.settings_mymodule'

Don't forget to rebuild your caches after this. To do this you can use this Drush command: drush cr if you don’t currently use Drush, I highly recommend using it, or the Drupal Console.