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Domain Events for PHP Entities

Adds support for Domain Events to plain old PHP entities. This package is inspired by and based on the Gist and blog posts by Benjamin Eberlei:

Requirements

PHP 7+
bcmath extension

Installation

Install using composer, or checkout / pull the files from github.com.

composer install somnambulist/domain-events
for Doctrine, register the Doctrine\DomainEventPublisher to your EntityManagers
for Symfony, register the Symfony\DomainEventPublisher
for Laravel, register the Laravel\DomainEventPublisher
implement Entities / objects that can raise events and flush / dispatch them
implement your own event publisher

Raising Events

To raise an event, decide which actions should result in a domain event. These should coincide with state changes in the domain objects and the events should originate from your main entities (aggregate roots).

For example: you may want to raise an event when a new User entity is created or that a role was added to the user.

This does necessitate some changes to how you typically work with entities and Doctrine in that you should remove setters and nullable constructor arguments. Instead you will need to manage changes to you entity through specific methods, for example:

completeOrder()
updatePermissions()
revokePermissions()
publishStory()

Internally, after updating the entity state, call: $this->raise(new NameOfEvent([])) and pass any specific parameters into the event that you want to make available to the listener. This could be the old vs new or the entire entity reference, it is entirely up to you.

public function __construct($id, $name, $another, $createdAt)
{
    $this->id        = $id;
    $this->name      = $name;
    $this->another   = $another;
    $this->createdAt = $createdAt;
    $this->raise(new MyEntityCreatedEvent(['id' => $id, 'name' => $name, 'another' => $another]));
}

Generally it is better to not raise events in the constructor but instead to use named constructors for primary object creation:

private function __construct($id, $name, $another, $createdAt)
{
    $this->id        = $id;
    $this->name      = $name;
    $this->another   = $another;
    $this->createdAt = $createdAt;
    $this->raise(new MyEntityCreatedEvent(['id' => $id, 'name' => $name, 'another' => $another]));
}

public static function create($id, $name, $another)
{
    $entity = new static($id, $name, $another, new DateTime());
    $entity->raise(new MyEntityCreatedEvent(['id' => $id, 'name' => $name, 'another' => $another]));
    
    return $entity;
}

Firing Domain Events

Depending on your framework / library choice, several integrations are provided.

Doctrine Integration

This implementation includes a Doctrine subscriber that will listen for entities that implement the RaisesDomainEvent interface and then ensures that releaseAndResetEvents() is called.

Note: it is not required to use the DomainEventPublisher subscriber. You can implement your own event dispatcher, use another dispatcher entirely (the frameworks) and then manually trigger the domain events by flushing the changes and then manually calling releaseAndResetEvents and dispatching the events.

If you do this note that the aggregate root class and primary identifier (if used) will not be set automatically. You will need to update your code to set these if you intend to use them.

To use the included listener, add it to your list of event subscribers in the Doctrine configuration. This is per entity manager.

Note: to use listeners with domain events that rely on Doctrine repositories it is necessary to defer loading those subscribers until after Doctrine has been resolved.

DomainEvent instances are proxied through a Doctrine EventArgs compatible layer that preserves the original event and name.

Symfony Integration

A basic Symfony EventDispatcher bridge is included. You can use this with the Symfony framework to register objects that raise events and then trigger the publishing at any time by calling Symfony\DomainEventPublisher::dispatch(). All registered entities will have the domain events collected, ordered and dispatched via the main Symfony EventDispatcher.

The domain events are proxied via a Symfony compatible EventProxy that allows the event name to be automatically converted to a Symfony dot name (entity.created). This preserves the original Event, name and properties.

To listen for the events, create a listener and register it with the main event dispatcher for the events matching the dot notation.

Laravel Integration

Laravels EventDispatcher does not have a typed event meaning you don't need any special translation of events. An integration is provided to make registering event emitting objects, similar to the Symfony integration.

class SomeServiceClass
{
    protected $publisher;
    
    public function __construct($publisher)
    {
        $this->publisher = $publisher;
    }

    public function doSomethingComplicated()
    {
        $object = new SomeDomainObject(); // or fetch from wherever
        $this->publisher->publishEventsFrom($object);
        
        $object->callSomeMethod();
        $object->callSomeOtherMethod();
        $object->doSomethingElse();
        
        $this->publisher->dispatch();
    }
}

This will order and dispatch the events via the standard Laravel Event Dispatcher.

To listen for the events, bind your listeners in the usual way for a Laravel project however: be sure to use the Event::class.

Symfony / Laravel Automatic Dispatch

If using the standalone publisher, you can create an onTerminate middleware / kernel listener that fires all domain events at the end of the request.

Remember that you will need to register the objects that will raise events before hand.

Creating a Domain Event Listener

Listeners can have their own dependencies (constructor is not defined), and are called after the onFlush Unit of Work event. The listener can perform any post processing as necessary, even triggering more events.

The listener should add methods that are named:

onNameOfTheEvent
without "event" suffixed
method will receive the Domain event instance
Domain event will have the class and id of the aggregate available

The example from the unit test:

class EventListener
{    
    public function onMyEntityCreated(MyEntityCreatedEvent $event)
    {
        printf(
            "New item created with id: %s, name: %s, another: %s",
            $event->getProperty('id'),
            $event->getProperty('name'),
            $event->getProperty('another')
        );
    }
}

The unit test shows how it can be implemented.

Be sure to read the posts by Benjamin Eberlei mentioned earlier and check out his Assertion library for low dependency entity validation.

somnambulist / domain-events

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