DomainEvent

[CamilYed]

In fact, some classes that implement the OrderDomainEvent interface aren't exactly domain events.

  public static record ItemQuantityChangeFailedBecauseAlreadyPaid(
    OrderId orderId,
    VinylId vinylId
  ) implements OrderDomainEvent {
  }

  public static record ItemQuantityChangeFailedBecauseNotExists(
    OrderId orderId,
    VinylId notExistingItem
  ) implements OrderDomainEvent {
  }

  public static record OrderPayFailedBecauseAlreadyPaid(
    OrderId orderId,
    Instant when
  ) implements OrderDomainEvent {
  }

  public static record OrderPayFailedBecauseDifferentAmounts(
    OrderId orderId,
    Instant when,
    Money difference
  ) implements OrderDomainEvent {
  }


  public OrderDomainEvent pay(Money amount, Delivery delivery) {
    if (unpaid) {
      final var toPay = orderLines.totalCost().add(delivery.cost());
      if (amount.equalTo(toPay)) {
        this.delivery = delivery;
        unpaid = false;
        return orderPaidSuccessfully();
      } else {
        return amountToBePaidIsDifferent(amount.subtract(toPay));
      }
    } else return orderPayFailedBecauseAlreadyPaid();
  }

  public OrderDomainEvent changeItemQuantity(VinylId product, QuantityChange change) {
    if (unpaid) {
      final var orderLine = orderLines.findLineToUpdate(product);
      if (orderLine.isPresent()) {
        final var lineToUpdate = orderLine.get();
        final var updatedLine = lineToUpdate.changeQuantity(change);
        orderLines.add(updatedLine);
        return itemQuantityChanged(product, updatedLine.quantity());
      }
      return itemDoesNotExists(product);
    }
    return itemQuantityChangeFailedBecauseAlreadyPaid(product);
  }

Maybe aggregate should return some object called Result, or throw some exceptions...