typescript - Angular 6 - Passing messages via service to from component to message component

Question

I am trying out how to pass a message from app.component.ts to be displayed in messageComponent.ts

on app.component.html I have added <app-messagecomponent></app-messagecomponent>

Add the moment it's just showing nothing.

I also have a method in a service:

message.service.ts

message(message) {
    return message;
}

So what I want to do to pass a message from other components via the message service so it's displaying in the app-messagecomponent.html

For example from app.component.ts:

sendMessageToService() {
    this.myservice.message('my Message to be displayed in the messageComponent');
}

How can I do this?

Answer 1

In this case to pass a message from one component to another component using a service , you can create a global message bus or event bus (publish/subscribe pattern).

For this we need the Subject (to emit values using .next() method) and Observable (to listen to the messages using .subscribe() ) from Rxjs which is now an essential part of angular 6. (For this example I am using Rxjs 6 along with rxjs-compat package)

Here we will be sending a message using MessageService class which is declared as @Injectable to inject as a dependency in component. The message will be emitted on a button click from app.component.html . The same message will be retrieved in message.component.ts to show it in the html template message.component.html. We will include the selector for MessageComponent which is <app-messagecomponent></app-messagecomponent> in the app.component.html.

Here is the complete code below

message.service.ts

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

@Injectable()
export class MessageService {
    private subject = new Subject<any>();

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

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

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

app.component.ts

import { Component } from '@angular/core';
import { MessageService } from './message.service';

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: [ './app.component.css' ]
})
export class AppComponent  {
  name = 'Angular 6';

  constructor(private service:MessageService){}

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

  clearMessage():void{
    this.service.clearMessage();
  }
}

app.component.html

<button (click)="sendMessage()">Click here to test message</button> <br><br>
<app-messagecomponent></app-messagecomponent>

message.component.ts

import { Component, OnDestroy } from '@angular/core';
import { Subscription } from 'rxjs';
import { MessageService } from './message.service';

@Component({
    selector: 'app-messagecomponent',
    templateUrl: 'message.component.html'
})

export class MessageComponent implements OnDestroy {
    message: any = {};
    subscription: Subscription;

    constructor(private messageService: MessageService) {
        // subscribe to app component messages
        this.subscription = this.messageService.getMessage().subscribe(message => { this.message = message; });
    }

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

message.component.html

<p>The incoming message :  </p>  <br>
{{message?.text }}

Here I have used Elvis operator in case the message is undefined .

Here is a working demo : https://stackblitz.com/edit/rxjs-snaghl

Let me know if you are looking for something like this.