58. Introduction to Subjects (Multicasting)

Standard Observables are unicast (or 'cold'): each subscriber gets its own independent execution of the Observable logic (e.g., each HTTP subscriber triggers a new request).

Subjects are a special type of Observable that are multicast (or 'hot'): they allow multiple Observers to subscribe to the same execution. A Subject is both an Observer (it can receive values via next()) and an Observable (it can be subscribed to).

Basic Subject (Subject<T>)

A basic Subject is a stream that doesn't hold state. It only emits values to subscribers that are currently listening after the value is emitted.

Use Case: Event emitters in services for non-related component communication (replacing Angular's EventEmitter in services).

typescript import { Subject } from 'rxjs';

const statusUpdate$ = new Subject();

// Subscriber A starts listening statusUpdate$.subscribe(message => console.log('Sub A:', message));

// The stream emits a value statusUpdate$.next('System Initialized'); // Output: Sub A: System Initialized

// Subscriber B starts listening later statusUpdate$.subscribe(message => console.log('Sub B:', message));

statusUpdate$.next('User logged in'); /* Output: Sub A: User logged in Sub B: User logged in */

statusUpdate$.complete(); // Terminates the stream for all subscribers

Crucially: If Subscriber B had subscribed after 'System Initialized' was emitted, it would have missed that message. The next lesson introduces Subjects that store state.