How to Get the Current Value of an RxJS Subject or Observable in Angular?

Getting Current Value of RxJS Subject or Observable

In Angular applications, it's common to use subjects or observables for data sharing and communication. However, unlike variables, subjects and observables do not intrinsically hold a current value. When they emit a value, it's immediately passed to subscribers and the observable completes its task.

Scenario:

Suppose you have an isLoggedIn subject in an Angular service that represents the user's login status. You want another component to retrieve the current login status without subscribing to the isLoggedIn observable.

Solution:

To address this, RxJS offers a BehaviorSubject, which is specifically designed to provide a current value. Unlike subjects and observables, a BehaviorSubject remembers the last emitted value and immediately emits it to newly subscribed observers.

Implementation:

To utilize a BehaviorSubject, you can modify your SessionStorage service as follows:

@Injectable()
export class SessionStorage extends Storage {
  private _isLoggedInSource = new BehaviorSubject<boolean>(false);  // Initialize with a default value
  isLoggedIn = this._isLoggedInSource.asObservable();
  constructor() {
    super('session');
  }
  setIsLoggedIn(value: boolean) {
    this.setItem('_isLoggedIn', value, () => {
      this._isLoggedInSource.next(value);
    });
  }
}

In your component, you can retrieve the current login status using the getValue() method of the BehaviorSubject:

import {SessionStorage} from 'app/services/session-storage.service';
...
const sessionStorage = new SessionStorage();

const isLoggedIn = sessionStorage.isLoggedInSource.getValue();

By using BehaviorSubject, you have a mechanism to obtain the current value of your isLoggedIn subject without relying on subscriptions, providing greater flexibility in your application design and data handling.

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