sharing data using service angular 6

1import { Component, Input } from '@angular/core';
2//Child-Component
3@Component({
4  selector: 'app-child',
5  template: `
6      Say {{ message }}
7  `,
8  styleUrls: ['./child.component.css']
9})
10export class ChildComponent {
11
12  @Input() childMessage: string;
13
14  constructor() { }
15
16}
17======================================================================
18import { Component } from '@angular/core'; 
19//Parent-Component data transfer to Child-Component
20@Component({
21  selector: 'app-parent',
22  template: `
23    <app-child [childMessage]="parentMessage"></app-child>
24  `,
25  styleUrls: ['./parent.component.css']
26})
27export class ParentComponent{
28  parentMessage = "message from parent"
29  constructor() { }
30}

1import { Injectable } from '@angular/core';
2import { BehaviorSubject } from 'rxjs';
3
4@Injectable()
5export class DataService {
6
7  private messageSource = new BehaviorSubject('default message');
8  currentMessage = this.messageSource.asObservable();
9
10  constructor() { }
11
12  changeMessage(message: string) {
13    this.messageSource.next(message)
14  }
15
16}
17