Subject와 BehaviorSubject

import { Subject } from 'rxjs';

const events$ = new Subject<string>();
events$.subscribe(v => console.log('A:', v));

events$.next('hello'); // A: hello
events$.subscribe(v => console.log('B:', v));
events$.next('world'); // A: world, B: world

import { BehaviorSubject } from 'rxjs';

const count$ = new BehaviorSubject<number>(0);
count$.subscribe(v => console.log('A:', v)); // A: 0
count$.next(5);
count$.subscribe(v => console.log('B:', v)); // B: 5 (gets latest)

const state$ = new BehaviorSubject({ loggedIn: false });
console.log(state$.value); // { loggedIn: false }
state$.next({ loggedIn: true });
console.log(state$.getValue()); // { loggedIn: true }

class CounterService {
  private count$ = new BehaviorSubject(0);
  readonly value$ = this.count$.asObservable();
  increment() { this.count$.next(this.count$.value + 1); }
}

private data$ = new Subject<number>();
readonly stream$ = this.data$.asObservable();
// consumers: stream$.subscribe(...) but no stream$.next()

import { ReplaySubject } from 'rxjs';

const recent$ = new ReplaySubject<number>(2); // buffer last 2
recent$.next(1); recent$.next(2); recent$.next(3);
recent$.subscribe(v => console.log(v)); // 2, 3

const s$ = new Subject<number>();
s$.subscribe(v => console.log(v));
s$.next(1);
s$.complete();
s$.next(2); // ignored