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Testing Events & Hooks

When to Use

Verifying event subscribers work, hooks fire correctly, alter hooks modify data as expected.

Decision

Type Test approach Test type
Event subscriber Dispatch event, assert subscriber ran Kernel
Hook implementation Trigger hook, verify side effect Kernel or Browser
Alter hook Call API, verify altered result Kernel

Pattern

Testing event subscriber (Kernel): 

namespace Drupal\Tests\my_module\Kernel;

use Drupal\KernelTests\KernelTestBase;
use Symfony\Component\EventDispatcher\EventDispatcherInterface;

class EventSubscriberTest extends KernelTestBase {

  protected static $modules = ['my_module'];

  public function testSubscriberExecuted(): void {
    $dispatcher = $this->container->get('event_dispatcher');
    $event = new MyCustomEvent(['data' => 'original']);

    $dispatcher->dispatch($event, MyCustomEvent::EVENT_NAME);

    $this->assertEquals('modified', $event->getData());
  }
}

Testing hook implementation via test module: 

// In my_module_test.module
function my_module_test_node_presave($node) {
  \Drupal::state()->set('hook_fired', TRUE);
}

// In test
public function testHookFires(): void {
  $node = Node::create(['type' => 'page', 'title' => 'Test']);
  $node->save();

  $this->assertTrue(\Drupal::state()->get('hook_fired'));
}

Testing alter hook: 

public function testFormAlter(): void {
  $form_builder = $this->container->get('form_builder');
  $form = $form_builder->getForm('Drupal\node\NodeForm', Node::create(['type' => 'page']));

  // My module adds a custom field via hook_form_alter
  $this->assertArrayHasKey('my_custom_field', $form);
}

Reference: /core/tests/Drupal/KernelTests/Core/

Common Mistakes

  • Testing core hooks instead of custom implementations -- wastes time
  • Not using test modules to isolate hook testing -- real module conflicts with test expectations
  • Forgetting to enable test module in $modules -- hook doesn't fire
  • Testing event subscriber in Unit test -- no event dispatcher available (use Kernel)
  • Not verifying hook execution order when multiple modules implement same hook -- race conditions

See Also