38. Subject: a hot multicast stream

Subject — a stream you can "push" into

So far we've only subscribed to Observables. But sometimes you need to send values from the outside: events from a component, user actions, an EventEmitter. That's what a Subject is for.

A Subject is, at the same time:

• An Observable — you can subscribe to it.
• An Observer — it has a .next(value) method anyone can use to "push" values.

Multicast (one source — many subscribers)

Unlike a cold Observable, which runs its logic separately for each subscriber, a Subject acts like a "hub": every subscriber gets the very same values. That's what multicast means.

No memory

A plain Subject remembers nothing. A new subscriber hears only what happens after it subscribes. Past values are lost to it. This matters — in the next lessons we'll study BehaviorSubject and ReplaySubject, which fix exactly that.

const { Subject } = Rx;

const subject = new Subject();

subject.subscribe(value => console.log('Sub A: ' + value));

subject.next(1);

subject.subscribe(value => console.log('Sub B: ' + value));

subject.next(2);