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Hook and Event Reuse

When to Use

When you need to respond to system events (entity operations, form alterations, request processing) without duplicating logic across similar hooks or event subscribers.

Hooks vs Events (Drupal 11+)

Mechanism Use When Example
Hook classes (attributes) Standard Drupal operations (form alter, entity presave, theme) #[Hook('form_alter')], #[Hook('entity_presave')]
Event subscribers Symfony events, custom events KernelEvents::REQUEST, custom domain events
Legacy .module hooks Legacy code (being phased out) hook_form_alter(), hook_preprocess_HOOK()

Decision

If you need... Use... Why
Alter forms Hook class with #[Hook('form_alter')] Modern, OOP, supports DI and autowiring
React to entity save Hook class with #[Hook('entity_presave')] Standard Drupal pattern
Custom application event Event dispatcher + subscribers Decoupled, reusable across modules
Shared hook logic across forms Base hook class or service method Avoid duplicating logic in each hook

Pattern

// Modern hook class (Drupal 11+)
namespace Drupal\mymodule\Hook;

use Drupal\Core\Hook\Attribute\Hook;
use Drupal\Core\Form\FormStateInterface;

class MyModuleHooks {

  public function __construct(
    protected MyService $myService,
  ) {}

  #[Hook('form_alter')]
  public function formAlter(array &$form, FormStateInterface $form_state, string $form_id): void {
    // Shared validation logic via service
    if (str_starts_with($form_id, 'node_')) {
      $this->myService->addCustomValidation($form, $form_state);
    }
  }

  #[Hook('entity_presave')]
  public function entityPresave($entity): void {
    if ($entity->getEntityTypeId() === 'node') {
      // Shared presave logic
      $this->myService->processNode($entity);
    }
  }
}

// Extract shared logic to service
class MyService {

  public function addCustomValidation(array &$form, FormStateInterface $form_state): void {
    // Logic used by multiple form_alter implementations
    $form['#validate'][] = [$this, 'validateCustomField'];
  }

  public function processNode($node): void {
    // Logic used by multiple entity hooks
    if (!$node->get('field_custom')->isEmpty()) {
      // Process custom field
    }
  }
}

Reference: core/lib/Drupal/Core/Hook/, core/modules/system/src/Hook/SystemHooks.php, core/lib/Drupal/Component/EventDispatcher/

Event Subscribers (When Needed)

namespace Drupal\mymodule\EventSubscriber;

use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpKernel\KernelEvents;

class MyEventSubscriber implements EventSubscriberInterface {

  public static function getSubscribedEvents() {
    return [
      KernelEvents::REQUEST => ['onRequest', 100],
    ];
  }

  public function onRequest($event): void {
    // Event logic
  }
}

Common Mistakes

  • Duplicating logic across similar hooks — Extract to service method, call from multiple hooks
  • Using event subscribers for standard Drupal operations — Use hook classes (modern) for form_alter, entity hooks, theme hooks
  • Complex logic directly in hook — Keep hooks thin, delegate to services
  • Not checking entity type/bundle in generic hooks — Always filter in entity_presave, entity_insert, etc.
  • Multiple event subscribers doing similar things — Consolidate into one subscriber or extract shared logic to service

See Also