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Intro to RxJS in Angular: Observables, Subjects, and BehaviorSubjects

Skyler De Francesca
Full Stack Developer

The foundation of Angular is built upon the RxJS library. You may not need extensive knowledge of this library to write an Angular App, but understanding some key features will make your life a lot easier. The three items which you will come across in your Angular application are Subjects, BehaviorSubjects, and Observables. It is imperative to understand their uses as you begin to learn Angular.   

Observables

The Observable is the core type of the RxJS library. Its primary use is to be “listened” to or “observed” for future events. Listening to these events is done via calling the subscribe() function of the observable where you can access the value that is being emitted. There is no way to invoke the event or value change using an observable alone, so you can kind of think of it as a “read-only” type. Due to this, it is usually best practice to expose Observables in cases where you do not want other parts of the application to invoke events, i.e., you just want them to listen for changes.   

e.g., the HTTP client uses an Observable as the way to expose the event of an HTTP request.  

/**  

 * get function returns an Observable that will emit an event  

 * when the response is received  

 */  

this.httpClient.get<Inventory[]>('https://sample-api/items').subscribe(items => {  

  // Do something with response  

});  

Subjects

Subjects are a type of Observable. However, unlike an Observable, Subjects can emit events/values to its subscribers using the next() function. Therefore, you can publish changes (using next()) to a Subject and listen for changes (using subscribe()). You can also cast Subjects to an Observable for instances where you want to conceal the Subject-like behavior and only expose the ability to subscribe to changes.  

The way to differentiate a Subject from a BehaviorSubject is:  

a) Subjects have no initial value 

b) Subscribers will only be notified and receive events/values after the subscription is made – i.e., Subscribers will not receive the last emitted value upon subscription.  

e.g., say we want to use Subjects to notify subscribers when an event has occurred: 

const subject = new Subject();  

   

subject.next('event 0');  

   

subject.subscribe(event => console.log(event));  

   

subject.next('event 1');  

subject.next('event 2');  

subject.next('event 3');  

   

/**  

 * Expected output:  

 * event 1  

 * event 2  

 * event 3  

 */  

Since event 0 was emitted before the subscription was made, the subscriber will not receive that value. If the use case for a subject requires the Subject to emit that initial value, a BehaviorSubject would be a better choice.  

BehaviorSubjects 

BehaviorSubjects are a type of Subject that:  

a) Has an initial value 

b) Subscribers will receive the last emitted value upon subscription.  

Using the same code as we used for the Subject, but using a BehaviorSubject instead, we will see now that the ‘event 0’ will be emitted. Also, notice how we must add an initial value (‘event -1’) upon creation of the BehaviorSubject. 

const behaviorSubject = new BehaviorSubject('event -1');  

   

behaviorSubject.next('event 0');  

   

behaviorSubject.subscribe(event => console.log(event));  

   

behaviorSubject.next('event 1');  

behaviorSubject.next('event 2');  

behaviorSubject.next('event 3');  

   

/**  

 * Expected output:  

 * event 0  

 * event 1  

 * event 2  

 * event 3  

 */  

When to Use Observables vs Subjects vs BehaviorSubjects 

Subjects are great for when you want to emit an event where the state of the event is not important, i.e. it is not important for the subscribers of the event to know about previous values emitted.  

BehaviorSubjects are the opposite, they are useful if there is a current “state” of the event that you want all subscribers to be able to access.  

Observables are useful for exposing Subjects/BehaviorSubjects to other parts of the application while also concealing the ability to emit values/changes.  

Example uses of Observables, Subjects, and BehaviorSubjects 

Say that we have a login service that is used for authenticating a user and storing their profile data. Below is a simple implementation of this login service that leverages Observables, Subjects, and BehaviorSubjects in a somewhat realistic way. 

interface UserProfile {  

  username: string;  

  email: string;  

}  

   

@Injectable({  

  providedIn: 'root'  

})  

export class LoginService {  

   

  constructor(private httpClient: HttpClient) { }  

   

  /**  

   * This subject is used to emit the event of a  

   * successful (true) or unsuccessful (false)  

   * login attempt.  

   */  

  private _loginSuccess$ = new Subject<boolean>();  

   

  /**  

   * This BehaviourSubject is used to hold and emit the data of the  

   * logged in user, or undefined if the user is not logged in  

   *   

   */  

  private _userProfile$ = new BehaviorSubject<UserProfile | undefined>(undefined);  

   

  /**  

   * Expose _loginSuccess$ as observable to conceal   

   * subject like behaviour.  

   */  

  public get loginSuccess(): Observable<boolean> {  

    return this._loginSuccess$;  

  }  

   

  /**  

   * Expose _userProfile$ as Observable to conceal  

   * subject like behavior.  

   */  

  public get userProfile(): Observable<UserProfile | undefined> {  

    return this._userProfile$;  

  }  

   

  public login(username: string) {  

    // Call fake login api and get response  

    this.httpClient.post<UserProfile>('https://fake-api/login', { username }).subscribe({     

      // next will be triggered if http request is successful     

      next: (userData) => {  

        // invoke _loginSuccess$ subject to emit true  

        this._loginSuccess$.next(true);  

        // invoke _userProfile$ behaviourSubject to emit and store response data  

        this._userProfile$.next(userData)  

      },  

      // if login is unsuccessful, invoke _loginSuccess$ subject to emit false  

      error: () => this._loginSuccess$.next(false)  

    });  

  }  

}  

Firstly, we are using a Subject to emit the event of a successful or unsuccessful login attempt. Here we do not care for the value to be stateful which is why using a Subject makes the most sense.   

 private _loginSuccess$ = new Subject<boolean>();  

In the login function, we are calling the next() function of this Subject when we get a response (true) or error (false) from the login API. This will notify subscribers when there is a successful or unsuccessful login attempt.  

...subscribe({   

      next: (userData) => {  

        this._loginSuccess$.next(true);  

      },   

      error: () => this._loginSuccess$.next(false)  

    });  

Next, we have a BehaviorSubject that holds the value of the logged-in user’s profile. Since we want future subscribers to have access to the latest value, using a BehaviorSubject makes sense.  

  private _userProfile$ = new BehaviorSubject<UserProfile | undefined>(undefined);  

We are calling the next() function to set the value of this BehaviorSubject when the HTTP request to login is successful.   

...subscribe({     

      next: (userData) => {  

        this._userProfile$.next(userData)  

      }  

    });  

Lastly, we are exposing both the _loginSuccess$ Subject and the _userProfile$ BehaviorSubject via getter functions that are of type Observable. This allows us to only expose the subscribe/unsubscribe capabilities of the Subjects.  

public get loginSuccess(): Observable<boolean> {  

  return this._loginSuccess$;  

}  

   

public get userProfile(): Observable<UserProfile | undefined> {  

  return this._userProfile$;  

}  

This login service example should give you a pretty good idea of the use cases for using Observables, Subjects, and BehaviorSubjects. It might also help to see how a client would interact with this service to better understand the use cases. Below is an example of a component using this service: 

@Component({ 

  selector: 'app-login', 

  templateUrl: './login.component.html', 

  styleUrls: ['./login.component.css'] 

}) 

export class LoginComponent implements OnInit, OnDestroy { 

  

  constructor(private loginService: LoginService) { } 

  

  // Store profile observable which we can display after login 

  profile = this.loginService.userProfile; 

  

  // Can be used to display loading animation 

  loading = false; 

  

  // Displays success/error message for login 

  loginMessage = ''; 

  

  // Holds array of subscriptions made during component lifetime. 

  subscriptions: Subscription[] = [] 

  

  ngOnInit() { 

    // Add subscription to subscriptions array 

    this.subscriptions.push( 

      // listen for login success/error 

      this.loginService.loginSuccess.subscribe(success => { 

        this.loading = false; 

        this.loginMessage = success ? 'Login was successful' : 'Error logging in'; 

      }) 

    ); 

  } 

  

  login(username: string) { 

    this.loading = true; 

    // Calls login function in service to trigger login 

    this.loginService.login(username) 

  } 

  

  ngOnDestroy(): void { 

    // When component is destroyed, it is important to clean up subscriptions 

    this.subscriptions.forEach(sub => sub.unsubscribe()) 

  } 

} 

Firstly, we hold a reference to the user profile Observable that our service exposes. 

// Store profile observable which we can display after login 

  profile = this.loginService.userProfile; 

Note that we are not using this profile variable anywhere else in the code above. Say we want to display the profile in the HTML of the component, we can do that by using the async pipe. The async pipe will subscribe to the observable behind the scenes, eliminating the need to subscribe manually in the component.  

<div class="profile"> 

  <div> 

    {{(profile | async)?.email}} 

  </div> 

  <div> 

    {{(profile | async)?.username}} 

  </div> 

</div> 

Next, we have a subscription array that is used to hold subscriptions made to Observables that persist longer than the lifetime of the component. This will allow us to unsubscribe from these Observables when the component is destroyed (in the ngOnDestroy()). It is important to unsubscribe from these subscriptions to avoid a memory leak. Note that the async pipe will automatically unsubscribe when the component is destroyed, so no need to handle the cleanup of the profile

  // Holds array of subscriptions made during component lifetime. 

  subscriptions: Subscription[] = [] 

 

  ngOnDestroy(): void { 

    // When component is destroyed, it is important to clean up subscriptions 

    this.subscriptions.forEach(sub => sub.unsubscribe()) 

  } 

In the ngOnInit, we subscribe to the login success Observable (that really is a Subject behind the scenes) and react to any events it emits. Here we can handle a boolean used for a loading animation and/or display success/error messages if the login succeeded/failed. We are adding the subscription to the subscriptions array so that it can be unsubscribed from when the component is destroyed. 

  ngOnInit() { 

    // Add subscription to subscriptions array 

    this.subscriptions.push( 

      // listen for login success/error 

      this.loginService.loginSuccess.subscribe(success => { 

        this.loading = false; 

        this.loginMessage = success ? 'Login was successful' : 'Error logging in'; 

      }) 

    ); 

  } 

Lastly, we have a login function that triggers the login HTTP request in the service. This function can be triggered by a user action, such as clicking a “submit” button in the component’s login form.  

  login(username: string) { 

    this.loading = true; 

    // Calls login function in service to trigger login 

    this.loginService.login(username) 

  } 

Together, the LoginService and LoginComponent show a practical, end-to-end use of these powerful RxJS types within your application.  

Conclusion

RxJS is a very powerful and extensive library that you will use a lot when building Angular applications. The high-level explanations and examples in this blog only scratch the surface of what this library has to offer. Understanding what Observables, Subjects, and BehaviorSubjects are, and how they can be used in an Angular application, is a good first step in becoming proficient in RxJS and Angular.  


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About The Author

Hello, I am Skyler De Francesca, a Senior Developer at Indellient. I love to build just about anything, but my expertise lies in the realm of web development. I enjoy learning new skills, technologies, and concepts and applying them to solve real-world problems.