rxjs - do something when observable starts executing

you could wrap the http.post observable in a Observable.create, and execute the logic you want there:

const wrapped$: Observable<any> = Observable.create((observer: 
    Observer<any>) => {
      showLoader();
       http.post('...').subscribe(
       value => observer.next(value),
       error => observer.error(error),
       () => observer.complete()
  )
});

wrapped$.subscribe() // and subscribe to this source instead

The defer observable factory function is most likely what you are looking for:

import { defer } from 'rxjs';

const post = defer(() => {
  this.showLoader();
  return this.http.post('someUrl', resource).pipe(
    catchError(),
    finalize(() => this.hideLoader())
  );
});
post.subscribe();

A bit prettier way to do the same as in previous suggestions:

const withLoader$ = of(null).pipe(
  tap(() => showLoader()),
  switchMap(() => this.http.post('someUrl', resource)),
  finalize(() => this.hideLoader())
);

Advanced sample bellow:

apply(observable: Observable<any>): Observable<any> {
  return of(null).pipe(
    map(() => this.show()),
    switchMap((pbKey: string) => observable.pipe(
      finalize(() => this.hide(pbKey))
    ))
  );
}