How to access the current value of an RxJS Subject or Observable in Angular?

Accessing Current Value of RxJS Subject or Observable

In Angular services, it's common to use RxJS Subjects and Observables to manage state. However, these constructs do not naturally provide a way to retrieve their current value outside of subscription. This article explores how to access the current value in such scenarios.

Consider the following Angular service that utilizes a Subject to manage a boolean property isLoggedIn:

<code class="typescript">import {Subject} from 'rxjs/Subject';
import {Injectable} from 'angular2/core';

@Injectable()
export class SessionStorage {
  private _isLoggedInSource = new Subject<boolean>();
  isLoggedIn = this._isLoggedInSource.asObservable();
  ...
}</code>

In some cases, you may need to access the current value of isLoggedIn without subscribing to it. This is not possible with a regular Subject, as it only emits values to its subscribers and does not store its current state.

Solution: Using BehaviorSubject

To overcome this limitation, consider switching to BehaviorSubject, a specialized RxJS type that maintains an internal buffer to store the latest emitted value.

<code class="typescript">import {BehaviorSubject} from 'rxjs/BehaviorSubject';

@Injectable()
export class SessionStorage {
  private _isLoggedInSource = new BehaviorSubject<boolean>(false);
  isLoggedIn = this._isLoggedInSource.asObservable();
  ...
}</code>

Compared to Subject, BehaviorSubject has two primary benefits:

1. When new subscribers start listening, it immediately emits the latest value it has stored.
2. It exposes a getValue() method that provides programmatic access to the current value.

Example with BehaviorSubject:

Using BehaviorSubject, you can access the current value of isLoggedIn even without subscribing:

<code class="typescript">const sessionStorage = new SessionStorage();
const isLoggedInCurrentValue = sessionStorage._isLoggedInSource.getValue();
console.log(isLoggedInCurrentValue); // True or False</code>

In summary, if you need to access the current value of an RxJS Subject or Observable, consider switching to BehaviorSubject, which provides both immediate emission for new subscribers and a getValue() method for direct retrieval.

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