Understanding Subject and BehaviorSubject in Angular

In Angular, RxJS (Reactive Extensions for JavaScript) plays a crucial role in managing asynchronous operations and data streams. Among the many types of Observables that RxJS provides, Subject and BehaviorSubject are two powerful utilities that developers often use. This blog post will delve into what Subject and BehaviorSubject are, their differences, and practical use cases in Angular applications.

What is a Subject?

A Subject in RxJS is a special type of Observable that allows values to be multicasted to multiple Observers. In simpler terms, a Subject can emit data to multiple subscribers, making it a useful tool for scenarios where you want to share a single source of data among many parts of your application.

Key Characteristics of Subject

1. Multicasting: Multiple observers can subscribe to a Subject, and each observer will receive the same data emitted by the Subject.
2. Emit Data Manually: Unlike regular Observables, which emit data based on a predefined sequence or event, Subjects can emit data manually using methods like next(), error(), and complete().
3. No Initial Value: A Subject does not hold an initial value. Subscribers only receive data emitted after they have subscribed.

Example Usage of Subject:

import { Component } from '@angular/core';
import { Subject } from 'rxjs';

@Component({
  selector: 'app-subject-example',
  template: `<button (click)="sendMessage()">Send Message</button>`,
})
export class SubjectExampleComponent {
  private subject = new Subject<string>();

  constructor() {
    this.subject.subscribe(message => console.log(`Subscriber 1: ${message}`));
    this.subject.subscribe(message => console.log(`Subscriber 2: ${message}`));
  }

  sendMessage() {
    this.subject.next('Hello from Subject');
  }
}

In this example, when the button is clicked, the Subject emits a message. Both subscribers receive the message simultaneously.

What is a BehaviorSubject?

A BehaviorSubject is a specialized version of a Subject that requires an initial value and emits its current value to new subscribers. This feature makes BehaviorSubject particularly useful for representing “state” or “current value” scenarios.

Key Characteristics of BehaviorSubject

1. Initial Value: A BehaviorSubject must be initialized with a value. This initial value is emitted immediately to any new subscribers.
2. Current Value: It always holds the latest value emitted, which can be accessed using value property.
3. Emit Data Manually: Like Subject, BehaviorSubject can also emit data manually using methods like next(), error(), and complete().

Example Usage of BehaviorSubject:

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

@Component({
  selector: 'app-behaviorsubject-example',
  template: `
    <div>{{ currentMessage }}</div>
    <button (click)="sendMessage()">Send New Message</button>
  `,
})
export class BehaviorSubjectExampleComponent {
  private behaviorSubject = new BehaviorSubject<string>('Initial Message');
  currentMessage: string;

  constructor() {
    this.behaviorSubject.subscribe(message => (this.currentMessage = message));
  }

  sendMessage() {
    this.behaviorSubject.next('Hello from BehaviorSubject');
  }
}

In this example, the BehaviorSubject is initialized with an “Initial Message”. When the component is instantiated, the current message is displayed. Clicking the button updates the message, and the new value is emitted to the subscribers.

Differences Between Subject and BehaviorSubject

1. Initial Value: Subject does not require an initial value, whereas BehaviorSubject does.
2. Current Value: BehaviorSubject holds and provides access to the current value, while Subject does not maintain any state.
3. Emission to New Subscribers: Subject only emits new values to subscribers after they have subscribed. In contrast, BehaviorSubject emits the latest value (or the initial value if no new value has been emitted) to any new subscribers.

When to Use Subject and BehaviorSubject

• Use Subject: When you need a multicast Observable that does not need to maintain or emit an initial value. Typical use cases include event emitters or when you need to broadcast to multiple subscribers without requiring a current state.
• Use BehaviorSubject: When you need an Observable that maintains and emits the current value. Common scenarios include state management in Angular services, where the current state needs to be shared and updated across different parts of the application.

Conclusion

Understanding the differences between Subject and BehaviorSubject and knowing when to use each can significantly enhance your ability to manage data streams in Angular applications. Subject is ideal for broadcasting events, while BehaviorSubject is perfect for state management scenarios where the current value needs to be shared and updated. By leveraging these tools effectively, you can build more responsive and maintainable Angular applications.

Don’t forget to checkout: Understand BehaviorSubject in Angular with Real Time Example