What is subject and BehaviorSubject in Angular?

In Angular, a Subject is a class that represents a value that changes over time. It’s part of the RxJS library, which is used for reactive programming in Angular. A Subject is an observer and a publisher at the same time, meaning it can both receive and emit values.

BehaviorSubject is a subclass of Subject that has a concept of “current value”. Unlike a regular Subject, a BehaviorSubject stores the latest value that has been emitted, and this value can be retrieved by any new subscriber. This means that when a new subscriber subscribes to a BehaviorSubject, it will immediately receive the latest value emitted by the subject, without having to wait for a new value to be emitted.

Here’s an example of using a BehaviorSubject in Angular to manage a shared state between multiple components:

import { Injectable } from '@angular/core';
import { BehaviorSubject } from 'rxjs';

@Injectable({
  providedIn: 'root'
})
export class DataService {
  private dataSource = new BehaviorSubject<string>('default value');
  currentData = this.dataSource.asObservable();

  updateData(data: string) {
    this.dataSource.next(data);
  }
}

In this example, the DataService defines a BehaviorSubject named dataSource and a public property named currentData that exposes the subject as an observable. The updateData method is used to emit a new value to the dataSource subject.

Multiple components can subscribe to the currentData observable to receive updates to the shared state, and the latest value will always be available to new subscribers through the BehaviorSubject‘s current value.