PSR-14: Event Dispatcher

Also Known As

TL;DR

Explanation

PSR-14 separates event dispatch (EventDispatcherInterface::dispatch($event)) from listener registration (ListenerProviderInterface::getListenersForEvent($event)). Any object can be an event; stoppable events implement StoppableEventInterface. This decouples producers from consumers and allows mixing listeners from different libraries. Implementations include Symfony EventDispatcher and league/event. The standard enables plugin architectures where third-party code attaches listeners without modifying the dispatching class — following Open/Closed Principle. Keep events as simple data-carrying objects; avoid placing behaviour in event classes.

Common Misconception

Why It Matters

Common Mistakes

• Not stopping propagation when an event has been fully handled — subsequent listeners process already-handled events.
• Listeners that throw exceptions without considering impact on other listeners in the chain.
• Not making event objects immutable — listeners that modify event data create hidden coupling.
• Using PSR-14 for high-throughput events — the overhead is fine for business events, not for thousands of events per second.

Code Examples

// Tightly coupled event dispatch without PSR-14:
class OrderService {
    public function place(Order $o): void {
        $this->save($o);
        $this->mailer->send($o);     // Direct call — coupled
        $this->inventory->reserve($o); // Direct call — coupled
    }
}
// PSR-14: dispatch(new OrderPlaced($o)) — listeners handle independently

// PSR-14 Event Dispatcher
use Psr\EventDispatcher\EventDispatcherInterface;
use Psr\EventDispatcher\ListenerProviderInterface;

// Define an event
class OrderPlaced {
    public function __construct(
        public readonly Order \$order,
        public readonly \DateTimeImmutable \$placedAt = new \DateTimeImmutable(),
    ) {}
}

// Dispatch after successful order
\$dispatcher->dispatch(new OrderPlaced(\$order));

// Register a listener
\$provider->listen(OrderPlaced::class, function(OrderPlaced \$event): void {
    \$mailer->sendConfirmation(\$event->order);
});

// PSR-14 compatible: Symfony EventDispatcher, League/Event, tightenco/collect

References

• ↗ https://www.php-fig.org/psr/psr-14/

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