How to do initialization logic on an observable pipe once per subscription in rxjs

Question

So I've got this observable pipe where I need to do an operation once at the beginning of the subscription, just like you can use finalize() to do an operation once at the end of a subscription

So this is what I started with, unfortunately it will do the startup once per each next() call that is made towards the subject.

    const notificationSubject = new BehaviorSubject<Notification | undefined>(undefined);
    const notifications$ = this.notificationSubject.pipe(
      tap(() => startup()),
      filter(isValueDefined),
      finalize(() => shutdown())
    );

   notifications$.subscribe(noti => foo(noti));
   notifications$.subscribe(noti => bar(noti));

Then we got this variant:

    let isStartedUp = false;
    const internalStartup = () => {
      if(!isStartedUp){
        isStartedUp = true;
        startup();
      }
    }

    const notifications$ = notificationSubject.pipe(
      tap(() => internalStartup()),
      filter(isValueDefined),
      finalize(() => shutdown())
    );

   notifications$.subscribe(noti => foo(noti));
   notifications$.subscribe(noti => bar(noti));

... which does it's job however it does it a little too well as now the startup is only made once ever (and only on the first subscription) instead of once per subscription that is created.

I imagine there being something along the lines of this but I haven't found it.

const notifications$ = notificationSubject.pipe(
      initialize(() => startup()),
      finalize(() => shutdown())
    );

Answer 1

You can usedefer to execute some code on every subscribe.

export function initialize<T>(initializer: () => void): MonoTypeOperatorFunction<T> {
  return (source: Observable<T>) => defer(() => {
    initializer();
    return source;
  });
}

const notifications$ = notificationSubject.pipe(
  initialize(() => startup()),
  finalize(() => shutdown())
);