How can I emit an object of changes as a subject and subscribe to only one key changing?

Question

I have a sidebar, and when you make changes to it I need to subscribe to those changes and react to them so we are using a Subject.

The thing is we don't know the best architecture for a Subject, should it be a single string emitted or an object?

If we emit an object of all the changes as key:value pairs, how do we subscribe to just one of those changes elsewhere instead of reacting to the entire object?

I want to void polluting my code with a Subject emitter for every change or is that the "best practice"

Example of current implementation

Emitter

/**
 * A function that emits an RXJS event to any observers subscribed to this subjet
 * when the user changes the calendar in the sidebar.
 * @return {Observable} An Observable that, whenever subscribed, will execute the
 * specified function.
 * @static false
 */
public emitCalendar = (calendar: any): void => {
    this.selectedCalendar = calendar;
    this.onChangeLeftPane$.next({ calendar: calendar });
};

And then what is the best way to subscribe just to

onChangeLeftPane$.calendar

Answer 1

Setup a separate Observable to listen to change with filter operator

    this.onChangeCalendar=this.onChangeLeftPane$.asObservable()
.pipe(filter(obj=>Objects.keys(obj).includes('calendar'))
this.onChangeLeftPane$.next({ calendar: calendar });

Answer 2

You can use pluck to only get the calendar from your object and then if necessary add filter (so that you don't emit undefined if calendar is missing) and/or distinctUntilChanged (so that you don't emit the same value twice if it didn't change).

import { pluck, filter, distinctUntilChanged } from 'rxjs/operators';

onChangeLeftPane$.pipe(
  pluck('calendar'), // extract the property 'calendar' from the object
  filter(Boolean), // don't emit if the property 'calendar' was missing from the object
  distinctUntilChanged(), // only emit if this calendar is distinct from the previous one  
)

https://stackblitz.com/edit/typescript-eoyqi9