feat: store event operators in EventBus allowing publish sync

[lucas-gregoire]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features) => I will work on that after

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

Issue Number: Discord discussion

What is the new behavior?

The EventBus now exposes a new property named eventOperators which is an array of event handlers functions in order to be used / subscribed somewhere else than from the EventBus.
This an easy way I found, without introducing breaking changes, to open the event bus to be able to call event handlers synchronously.
Ex:

export class SyncPublisher<EventBase extends IEvent> implements IEventPublisher<EventBase> {
  private eventOperators: EventOperator<EventBase>[] = [];

  async publish<T extends EventBase>(event: T) {
    const event$ = of(event);

    await lastValueFrom(
      merge(
        event$.pipe(map(() => undefined)),
        ...this.eventOperators.map((operator) => event$.pipe(operator)),
      ),
    );
  }

  setEventOperators<T extends EventBase>(eventOperators: EventOperator<T>[]) {
    this.eventOperators = eventOperators;
  }
}

@Module({
  imports: [
    CqrsModule.forRoot({
      eventPublisher: new SyncPublisher(),
    }),
  ],
  // [...]
})
export class AppModule implements OnModuleInit {
  constructor(private readonly eventBus: EventBus) {}

  onModuleInit() {
    (this.eventBus.publisher as SyncPublisher<IEvent>).setEventOperators(
      this.eventBus.eventOperators,
    );
  }
}

Does this PR introduce a breaking change?

• Yes
• No

Other information

[lucas-gregoire]

Awaiting asynchronous event publisher => end method not called

[lucas-gregoire]

Awaiting synchronous event publisher => end method called