Event Bus and Observer Pattern in PHP: Implementation and Examples

Event Bus and Observer Pattern in PHP

In modern PHP web application development, there is often a need to flexibly respond to various events: sending an email after registration, logging user actions, updating the cache when data changes. The Observer pattern and its extension — Event Bus — are ideal for these purposes.

Observer Pattern: Basics

The Observer pattern is a behavioral design pattern that creates a subscription mechanism allowing one object to watch and react to events occurring in other objects. Main components:

• Subject — the object that generates events
• Observer — the object that reacts to events
• Event — information about the action that occurred

Event Bus: Evolution of Observer

Event Bus is a centralized bus that manages all events in an application. Unlike the classic Observer, where subscription is tightly coupled to the subject, Event Bus provides:

• Loose coupling between senders and receivers
• Ability to add/remove handlers at runtime
• Global access to the bus from any part of the application
• Simplified testing and debugging

Example Implementation of Event Bus in PHP

Let's create a simple yet functional implementation of Event Bus, compatible with PSR-14 (Event Dispatcher).

<?php// Event interfaceinterface EventInterface{    public function getName(): string;}

// Base eventclass UserRegisteredEvent implements EventInterface{    public function __construct(        private string $email,        private string $username    ) {}

    public function getName(): string    {        return 'user.registered';    }

    public function getEmail(): string    {        return $this->email;    }

    public function getUsername(): string    {        return $this->username;    }}

// Handler interfaceinterface EventHandlerInterface{    public function handle(EventInterface $event): void;}

// Event Busclass EventBus{    private array $listeners = [];

    public function addListener(string $eventName, EventHandlerInterface $handler): void    {        $this->listeners[$eventName][] = $handler;    }

    public function removeListener(string $eventName, EventHandlerInterface $handler): void    {        if (!isset($this->listeners[$eventName])) {            return;        }

        $this->listeners[$eventName] = array_filter(            $this->listeners[$eventName],            fn($h) => $h !== $handler        );    }

    public function dispatch(EventInterface $event): void    {        $eventName = $event->getName();

        if (!isset($this->listeners[$eventName])) {            return;        }

        foreach ($this->listeners[$eventName] as $handler) {            $handler->handle($event);        }    }}?>

Practical Usage

Now let's create handlers and demonstrate the bus in action:

<?php// Email sending handlerclass SendWelcomeEmailHandler implements EventHandlerInterface{    public function handle(EventInterface $event): void    {        if (!$event instanceof UserRegisteredEvent) {            return;        }

        echo "Sending welcome email to {$event->getEmail()}";        // mail($event->getEmail(), 'Welcome!', '...');    }}

// Logging handlerclass LogUserRegistrationHandler implements EventHandlerInterface{    public function handle(EventInterface $event): void    {        if (!$event instanceof UserRegisteredEvent) {            return;        }