Creating and grouping value objects about php-ddd HOT 4 OPEN

inteface ContractType
{
    public function createContract(): EmploymentContract;
}

final class FixedContractType implements ContractType
{
    public function createContract(): EmploymentContract 
    {
        return new FixedContract($this->startDate, $this->endDate)
    }
}

final class PermanentContractType implements ContractType
{
    public function createContract(): EmploymentContract 
    {
        return new PermanentContract($this->startDate)
    }
}

final class FixedContract implements EmploymentContract extends AggregateRoot {}
final class PermanentContract implement EmploymentContract extends AggregateRoot {}

$whateverContract = $sharedContractTypeRepository->ofId($employmentContractId);
$whateverContract->modify($modifyContractCommand);

final class EmploymentContract extends AggregateRoot
{
    protected function onFixedContractWasSigned(FixedContractWasSigned $event): void
    {
        // ...
    }

    protected function onPermanentContractWasSigned(PermanentContractWasSigned $event): void
    {
        // ...
    }

    public static function signed(
        EmploymentContractId $anId,
        ContractType $aContractType
    ) {
        $aggregate = ...;
        $aggregate->applyEvents($aContractType->onSigning($anId));
    }
}

interface ContractType
{
    /**
     * @param EmploymentContractId $id
     * @return DomainEvent[] Events relative to the contract type
     */
    public function onSigning(EmploymentContractId $id): array;
}

final class FixedContractType implements ContractType
{
    private $startDate;
    private $endDate;

    public function __construct(\DateTimeInterface $startDate, \DateTimeInterface $endDate)
    {
        $this->startDate = $startDate;
        $this->endDate = $endDate;
    }

    public function onSigning(EmploymentContractId $id): array
    {
        return [new FixedContractWasSigned($id, $this->startDate, $this->endDate)];
    }
}

final class PermanentContractType implements ContractType
{
    private $startDate;

    public function __construct(\DateTimeInterface $startDate)
    {
        $this->startDate = $startDate;
    }

    public function onSigning(EmploymentContractId $id): array
    {
        return [new PermanentContractWasSigned($id, $this->startDate)];
    }
}

$repos = new EmploymentContractEventStore();
$fixedContract = EmploymentContract::signed(
    $fixedContractId = new EmploymentContractId(),
    new FixedContractType(
        new DateTimeImmutable('2000-01-01'),
        new DateTimeImmutable('2000-12-31')
    )
);
$repos->save($fixedContract);
$repos->getContract($fixedContractId); // Configured as fixed

$permanentContract = EmploymentContract::signed(
    $permanentContractId = new EmploymentContractId(),
    new PermanentContractType(new DateTimeImmutable('2000-01-01'))
);
$repos->save($permanentContract);
$repos->getContract($permanentContractId); // Configured as permanent