02-DomainEvent.php

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01-IdentifiesAggregate.php 02-DomainEvent.php 03-RecordsEvents.php 04-ProtectsInvariants.php 05-ProtectsMoreInvariants.php 06-IsEventSourced.php 07-EventStore.php 08-AggregateRepository.php index.markdown

02-DomainEvent.php
¶
```
<?php
```
¶

Before coding your Domain Model, discover the Domain Events that you need. A Domain Event is something that happened in the past, and that is of interest to the business. They are first-class citizens of your model, and in an Event Sourced system, they are the most important elements. So work closely with your Domain Expert to figure out what is and is not important to the business.
```
namespace Buttercup\Protects\Tests;
use Buttercup\Protects\DomainEvent;
use Buttercup\Protects\Tests\Misc\ProductId;
```

For the sake of the example, we imagine a simple e-commerce system. We’re very interested in what people put in their shopping basket, and what they take out. Because every shopping session starts with a new basket, we’re interested in that event as well.

final class BasketWasPickedUp implements DomainEvent
{
    private $basketId;

    public function __construct(BasketId $basketId)
    {
        $this->basketId = $basketId;
    }

    public function getAggregateId()
    {
        return $this->basketId;
    }
}

¶

You’ve already noticed some interesting concepts:
- The name of the event is in the past tense. BasketWasPickedUp fits nicely into our Ubiquitous Language. We could have called it something bland like BasketWasCreated, but I want to illustrate the use of language. BasketPickedUp, without the ‘was’, would’ve worked as well. I prefer to make sentences though.
- We implement DomainEvent.
- The getAggregateId() method returns IdentifiesAggregate. We’ll get to Aggregates soon.
```
final class ProductWasAddedToBasket implements DomainEvent
{
    private $basketId;
    private $productId;
    private $productName;
```

Notice how all the properties go in the constructor.

    public function __construct(BasketId $basketId, ProductId $productId, $productName)
    {
        $this->basketId = $basketId;
        $this->productName = $productName;
        $this->productId = $productId;
    }

    public function getAggregateId()
    {
        return $this->basketId;
    }

¶

We only deal with ProductId. We haven’t got a Product, strictly speaking we don’t even known whether we’ll ever need it, and what it would look like. All we know is that in this context, we’re only interested in the id…
```
    public function getProductId()
    {
        return $this->productId;
    }
```

… and in the name. We’ll get to that.

    public function getProductName()
    {
        return $this->productName;
    }
}

Notice that Domain Events are immutable. Once they have been initialized, there’s no way to change them, there are No setters. That makes perfect sense: History can’t be altered!

final class ProductWasRemovedFromBasket implements DomainEvent
{
    private $basketId;
    private $productId;

    public function __construct(BasketId $basketId, ProductId $productId)
    {
        $this->basketId = $basketId;
        $this->productId = $productId;
    }

    public function getAggregateId()
    {
        return $this->basketId;
    }

    public function getProductId()
    {
        return $this->productId;
    }
}

Let’s test one of our Domain Events. Did we mention this documentation doubles as test suite?

$event = new ProductWasAddedToBasket(new BasketId('BAS1'), new ProductId('PRO1'), "The Princess Bride");
it('should equal another instance with the same value',
    $event->getAggregateId()->equals(new BasketId('BAS1')));
it("should expose a ProductId",
    $event->getProductId()->equals(new ProductId('PRO1')));
it("should expose a productName",
    $event->getProductName() == "The Princess Bride");