How to Publish a Domain Event to the Event Stream

1. Create a Domain Event and Dispatch it from the Aggregate

// domain/Event/ItemBonusAdded.ts
class ItemBonusAdded {
  constructor(
    public readonly itemId: number,
    public readonly bonusId: number,
  ) {}
}

// domain/Entity/Item.ts
class Item extends AggregateRoot {
  public addItemBonusId(bonusId: number) {
    // Validate
    if (bonusId < 0) {
      throw new BonusIdMustBePositiveException();
    }

    // Update state
    this.bonusIds.push(bonusId);

    // Raise a new domain event
    this.record(new ItemBonusAdded(this.id, bonusId));
  }
}

2. Create a Saga to Listen for the Domain Event

// infrastructure/IntegrationEvents/Sagas/ItemDomainEvents.ts
class ItemDomainEvents {
  @Saga()
  public created$ = (events$: Observable<ItemCreated>): Observable<never> => {
    return events$.pipe(
      ofType(ItemCreated),
      switchMap((domainEvent) => {
        const eventMessage = this.createItemEvent(ItemEventNames.ItemCreated, domainEvent);

        return forkJoin([
          this.eventStream.emit(this.topic, domainEvent, eventMessage),
          this.messageStream.emit(this.topic, domainEvent, eventMessage),
        ]);
      }),
      switchMap(() => EMPTY),
    );
  };

  @Saga()
  public bonusAdded$ = (events$: Observable<ItemBonusAdded>): Observable<never> => {
    return events$.pipe(
      ofType(ItemBonusAdded),
      switchMap((domainEvent) => {
        const eventMessage = this.createItemEvent(ItemEventNames.ItemUpdated, domainEvent);

        return forkJoin([
          this.eventStream.emit(this.topic, domainEvent, eventMessage),
          this.messageStream.emit(this.topic, domainEvent, eventMessage),
        ]);
      }),
      switchMap(() => EMPTY),
    );
  };

  public constructor(
    private readonly eventStream: EventStream,
    private readonly messageStream: MessageStream,
  ) {}

  private createItemEvent(name: ItemEventNames, domainEvent: ItemCreated | ItemUpdated | ItemBonusAdded): ItemEvents {
    return new ItemEventV2(name, domainEvent.id, {
      id: domainEvent.id,
      gameType: domainEvent.gameType,
      warcraftId: domainEvent.warcraftId,
      names: domainEvent.names,
      // ...
    });
  }
}

3. Configure the Event Types

// @tsm/types/app-item
// DomainEvents/ItemEvents.ts
export type ItemEvents = ItemEventV1 | ItemEventV2;

export enum ItemEventNames {
  ItemCreated = 'ItemCreated',
  ItemUpdated = 'ItemUpdated',
}

/**
 * Version 1
 */
export interface ItemEventV1Payload {
  id: string;
  gameType: GameType;
  warcraftId: number;
  names: Locales;
  // ...
}
export class ItemEventV1 extends Event<ItemEventNames, 1, ItemEventV1Payload>(1) {}

/**
 * Version 2
 */
export interface ItemEventV2Payload {
  id: string;
  gameType: GameType;
  warcraftId: number;
  names: Locales;
  iconImage: string; // new Field
  // ...
}
export class ItemEventV2 extends Event<ItemEventNames, 2, ItemEventV1Payload>(2) {}