How to subscribe on variable changes?

Question

I have declared the following variable:

public filter: IFilterWeekScheduleClassShort = {};

How to subscribe on changes(listen object filter)?

I tried this:

private filter: IFilterWeekScheduleClassShort = {};
private filterChanges: BehaviorSubject<IFilterWeekScheduleClassShort> = new BehaviorSubject(this.filter);

this.filterChanges.subscribe((model) => {
    console.log(model);
});

Is it okay?

My final code:

public _filter: IFilterWeekScheduleClassShort = {};
private filterChanges: BehaviorSubject<IFilterWeekScheduleClassShort> = new BehaviorSubject(this._filter);

  this.filter = {a: 1, b: 2}

  set filter(value: any) {
    this._filter = value;
    this.filterChanges.next(this._filter);
  }

  get filter() {
    return this.filterChanges.asObservable();
  }

Subscribe on:

this.filter.subscribe(model => {
   console.log(model);
});

Changes I make from form:

[(ngModel)]="_filter.teacherId"

Answer 1

Make a setter

private _filter: IFilterWeekScheduleClassShort = {};
set filter(value) {
  this.doSomethingOnVariableChange(value);
  this._filter = value;
}

Now you can use it like this

this.filter = 'x';

And your function doSomethingOnVariableChange will be called with 'x' .

With an observable :

private _filter: IFilterWeekScheduleClassShort = {};
private _filter$ = new BehaviorSubject(this._filter);
set filter(value) {
  this._filter = value;
  this._filter$.next(this._filter);
}
get filter() {
  return this._filter$.asObservable();
}

Answer 2

I think the closes that what you want to get is using scan() :

const filterChanges$ = new Subject()
const defaultFilter = {}

const filter$ = filterChanges$
  .pipe(
    scan((acc, mergeFilter) => {
      return {
        ...acc,
        ...mergeFilter,
      }
    }, defaultFilter)
  )

filter$.subscribe(console.log);

filterChanges$.next({ name: 'abc' })
filterChanges$.next({ num: 42 })
filterChanges$.next({ name: 'xyz' })

Live demo: https://stackblitz.com/edit/rxjs6-demo-ngqkzv?file=index.ts

This will output:

{name: "abc"}
{name: "abc", num: 42}
{name: "xyz", num: 42}

Some time ago I made a small wrapper around window.Proxy called rxjs-observable-object that lets you wrap any object and then listen to changes on it ( https://github.com/martinsik/rxjs-ds#object ):

import { map } from 'rxjs/operators';
import { ObservableObject } from 'rxjs-observable-object';

const defaultFilter = {}
const { proxy, events } = new ObservableObject(defaultFilter);

const filter$ = events.onSet
  .pipe(
    map(({ target }) => target)
  )

filter$.subscribe(console.log);

proxy['name'] = 'abc';
proxy['num'] = 42;
proxy['name'] = 'xyz';

Live demo: https://stackblitz.com/edit/rxjs6-demo-hojjkk?file=index.ts

However, rxjs-observable-object is a little outdated now and requires rxjs-comapt package installed as well.

The output for both examples is the same.