Event Dispatcher

Synchronous publish-subscribe within a single request. Decouples code that produces events from code that reacts to them.

Intermediate

Dispatch an event object: `$dispatcher->dispatch(new UserRegistered($user))`.
Listeners react: register via `#[AsEventListener(event: UserRegistered::class)]` on a class or method.
Subscribers implement `EventSubscriberInterface::getSubscribedEvents()` — handy for grouping multiple events.
Priority controls listener order; higher fires first. Listeners can call `$event->stopPropagation()`.
Symfony itself uses the dispatcher for kernel events (request, response, exception, terminate).
Event dispatcher is SYNCHRONOUS — for async, use Symfony Messenger instead.

final class UserRegistered
{
    public function __construct(public readonly User $user) {}
}

#[AsEventListener]
final class SendWelcomeEmail
{
    public function __invoke(UserRegistered $event): void { /* ... */ }
}

Common gotchas

A throwing listener aborts the dispatch chain. Wrap in try/catch if a failure should not block other listeners.
Don't use events for plain function calls. They shine when multiple unrelated bits of code need to react to the same fact.