rxjs - do something when observable starts executing

Question

Is there a way to attach a handler when an observable starts executing (someone called subscribe on it)?

like in angular:

    this.http.post('someUrl', resource).pipe(
       catchError(),
       finalize((() => this.hideLoader()),
       **executing(() => this.showLoader()) <------**
    )

Answer 1

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();