Projecting to the outbox

Rather than manually appending to the outbox, it is very common that what you want in the outbox is based on state changes done in the event log. The easiest thing will then be to create an outbox projection that projects from the event log and puts it in the outbox when there are changes.

With the approach to private events defined here we can imagine two events that represents the ability to deposit and withdraw money to/from an account.

[EventType("adaab3e5-f797-4335-80d4-06758773f7e1")]
public record DepositToDebitAccountPerformed(double Amount);

[EventType("507a71d9-862f-4615-b8e8-2427d9568373")]
public record WithdrawalFromDebitAccountPerformed(double Amount);

With a public event that communicates the balance, as defined here We can then drop in an implementation of IOutboxProjections from the Aksio.Cratis.Events.Outbox namespace:

using Aksio.Cratis.Events.Outbox;

public class DebitAccountsOutboxProjections : IOutboxProjections
{
    public ProjectionId Identifier => "e53ae165-1658-486c-b03c-7c6041428851";

    public void Define(IOutboxProjectionsBuilder builder) => builder
        .For<AccountBalance>(p => p
            .From<DepositToDebitAccountPerformed>(_ => _
                .Add(m => m.Balance).With(e => e.Amount))
            .From<WithdrawalFromDebitAccountPerformed>(_ => _
                .Subtract(m => m.Balance).With(e => e.Amount)));
}