What is the equivalent of "onCancel" callback of BehaviorSubject of RxDart but in RxJs?

[MiniSuperDev]

https://pub.dev/documentation/rxdart/latest/rx/BehaviorSubject/BehaviorSubject.html

In RxDart when you instantiated a BehaviorSubject, you can pass onCancel callback, that is called every time all listeners was unsubscribed.
How can i have this callback in RxJs?

[voliva]

Not sure, but I don't think there's a built-in solution. But would this work?

const subject = new BehaviorSubject();
const observable$ = subject.pipe(
  finalize(() => console.log('onCancel')),
  share()
);

Everyone consuming observable$ will share a subscription to the subject. Once that subscription is closed because all of them unsubscribed, you'll get "onCancel" logged.

[MiniSuperDev]

Thank you @voliva , it works, although now I have to create an extra observable for each subject.
That is the only way?
I leave you a minimum example of what I want to do with your suggestion, in case you have any suggestions in the code.
The idea is to just subscribe to an external data source (i.e. web socket), and unsubscribe when all listeners are unsubscribed.

import { timer, finalize, share, Subject, Observable, map } from 'rxjs';

class Repository {
  private subjectById: Map<string, [Subject<string>, Observable<string>]> =
    new Map();

  getNumbers$(id: string): Observable<string> {
    if (!this.subjectById.has(id)) {
      console.log(`subscribing to remote data source with id: ${id}`);
      const subject = new Subject<string>();

      const remoteDataSource = timer(0, 100).pipe(map((x) => id));
      const remoteSub = remoteDataSource.subscribe((x) => subject.next(x));
      const sharedObservable = subject.pipe(
        finalize(() => {
          console.log(
            `on cancel: so unsubscribing from remote data source with id: ${id}`
          );
          remoteSub.unsubscribe();
          this.subjectById.delete(id);
          subject.complete();
        }),
        share()
      );
      this.subjectById.set(id, [subject, sharedObservable]);
    }

    return this.subjectById.get(id)![1];
  }
}

async function main() {
  const repository = new Repository();

  const sub1 = repository
    .getNumbers$('1')
    .subscribe((id) => console.log(`sub1: ${id}`));
  const sub2 = repository
    .getNumbers$('1')
    .subscribe((id) => console.log(`sub2: ${id}`));

  await sleep(300);

  sub1.unsubscribe();
  sub2.unsubscribe();

  const sub3 = repository
    .getNumbers$('1')
    .subscribe((id) => console.log(`sub3: ${id}`));

  await sleep(300);

  sub3.unsubscribe();

  await sleep(300);
}

main();

function sleep(ms: number) {
  return new Promise((resolve) => {
    setTimeout(resolve, ms);
  });
}

subscribing to remote data source with id: 1
sub1: 1
sub2: 1
sub1: 1
sub2: 1
sub1: 1
sub2: 1
on cancel: so unsubscribing from remote data source with id: 1
subscribing to remote data source with id: 1
sub3: 1
sub3: 1
sub3: 1
on cancel: so unsubscribing from remote data source with id: 1