Angular 9 - Communicating Between Components with Observable & Subject

Built with Angular 9.1.3 and RxJS 6.5.5

Other versions available:

• Angular: Angular 14, 10, 8, 7, 6, 2/5
• React: React Hooks + RxJS, React + RxJS
• Vue: Vue.js + RxJS
• ASP.NET Core: Blazor WebAssembly

This is a quick tutorial to show how you can send messages between components in Angular 9 and RxJS.

The way to communicate between components is to use an Observable and a Subject (which is a type of observable), I won't go too much into the details about how observables work here since it's a big subject, but in a nutshell there are two methods that we're interested in: Observable.subscribe() and Subject.next().

Observable.subscribe()

The observable subscribe method is used by angular components to subscribe to messages that are sent to an observable.

Subject.next()

The subject next method is used to send messages to an observable which are then sent to all angular components that are subscribers (a.k.a. observers) of that observable.

Angular 9 Component Communication Example

Here's a simple example that shows messages being sent from a home component to the root app component via a message service using observables. Styling of the example is all done with Bootstrap 4.4 css.

(See on StackBlitz at https://stackblitz.com/edit/angular-9-communicating-between-components)

Angular 9 Message Service

With the message service you can subscribe to new messages in any component with onMessage() method, send messages from any component with the sendMessage(message: string) method, and clear messages from any component with the clearMessages() method.

The clearMessages() method actually just sends an empty message by calling this.subject.next() without any arguments, the logic to clear the messages when an empty message is received is in the app component below.

import { Injectable } from '@angular/core';
import { Observable, Subject } from 'rxjs';

@Injectable({ providedIn: 'root' })
export class MessageService {
    private subject = new Subject<any>();

    sendMessage(message: string) {
        this.subject.next({ text: message });
    }

    clearMessages() {
        this.subject.next();
    }

    onMessage(): Observable<any> {
        return this.subject.asObservable();
    }
}

App Component that Receives Messages

The app component uses the message service to subscribe to new messages and push them into the messages array which is displayed in the app component template. If an empty message is received then the messages array is cleared which automatically removes the messages from the UI.

import { Component, OnDestroy } from '@angular/core';
import { Subscription } from 'rxjs';

import { MessageService } from './_services/index';

@Component({ selector: 'app', templateUrl: 'app.component.html' })
export class AppComponent implements OnDestroy {
    messages: any[] = [];
    subscription: Subscription;

    constructor(private messageService: MessageService) {
        // subscribe to home component messages
        this.subscription = this.messageService.onMessage().subscribe(message => {
          if (message) {
            this.messages.push(message);
          } else {
            // clear messages when empty message received
            this.messages = [];
          }
        });
    }

    ngOnDestroy() {
        // unsubscribe to ensure no memory leaks
        this.subscription.unsubscribe();
    }
}

Home Component that Sends Messages

The home component uses the message service to send messages that are displayed by the app component.

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

import { MessageService } from '../_services/index';

@Component({ templateUrl: 'home.component.html' })
export class HomeComponent {
    constructor(private messageService: MessageService) { }

    sendMessage(): void {
        // send message to subscribers via observable subject
        this.messageService.sendMessage('Message from Home Component to App Component!');
    }

    clearMessages(): void {
        // clear messages
        this.messageService.clearMessages();
    }
}