BehaviorSubject

A BehaviorSubject is a Subject that always holds a current value. It is seeded at construction and replays the latest value to each new subscriber immediately on subscription - so a value is always available, even before the first next.

Seeding a value

The constructor takes an initial value, or a factory invoked once to compute it.

import { BehaviorSubject } from '@remotex-labs/xobservable';

const direct = new BehaviorSubject<number>(0);
const lazy = new BehaviorSubject<string>(() => localStorage.getItem('state') ?? 'default');

A factory enables lazy or side-effecting initialization; it runs exactly once during construction.

Replaying to subscribers

Every new subscriber receives the current value right away, then any following emissions.

const count = new BehaviorSubject<number>(0);

count.subscribe((v) => console.log('A', v)); // A 0  (replayed immediately)
count.next(5);                               // A 5
count.subscribe((v) => console.log('B', v)); // B 5  (replayed immediately)

Reading the current value

The value getter returns the latest emitted value (or the initial value).

const state = new BehaviorSubject<number>(1);

state.value; // 1
state.next(2);
state.value; // 2

API

new BehaviorSubject<T>(initialValue)

Parameter	Description
initialValue	The initial value, or a factory () => T invoked once at construction.

get value(): T

The most recent value: the latest emitted value, or the initial value.

subscribe(...) and next(value)

Both override Subject: subscribe replays the current value after registering the observer, and next stores the value before broadcasting so later subscribers receive it. After completion, subscribe returns a no-op unsubscribed and next is ignored.

Like Subject and Observable, the unsubscribe function subscribe returns is idempotent and Disposable, so it can be bound with a using declaration - including the no-op returned after completion. See Automatic cleanup with using.