Angular 4 pass data between 2 not related components

This is a case where you want to use a shared service, as your components are structured as siblings and grandchildren. Here’s an example from a video I created a video about sharing data between components that solves this exact problem.

Start by creating a BehaviorSubject in the service

import { Injectable } from '@angular/core';
import { BehaviorSubject } from 'rxjs/BehaviorSubject';
@Injectable()
export class DataService {

  private messageSource = new BehaviorSubject("default message");
  currentMessage = this.messageSource.asObservable();

  constructor() { }

  changeMessage(message: string) {
    this.messageSource.next(message)
  }

}

Then inject this service into each component and subscribe to the observable.

import { Component, OnInit } from '@angular/core';
import { DataService } from "../data.service";
@Component({
  selector: 'app-parent',
  template: `
    {{message}}
  `,
  styleUrls: ['./sibling.component.css']
})
export class ParentComponent implements OnInit {

  message:string;

  constructor(private data: DataService) { }

  ngOnInit() {
    this.data.currentMessage.subscribe(message => this.message = message)
  }

}

You can change the value from either component and the value will be updated, even if you don’t have the parent/child relationship.

import { Component, OnInit } from '@angular/core';
import { DataService } from "../data.service";
@Component({
  selector: 'app-sibling',
  template: `
    {{message}}
    <button (click)="newMessage()">New Message</button>
  `,
  styleUrls: ['./sibling.component.css']
})
export class SiblingComponent implements OnInit {

  message:string;

  constructor(private data: DataService) { }

  ngOnInit() {
    this.data.currentMessage.subscribe(message => this.message = message)
  }

  newMessage() {
    this.data.changeMessage("Hello from Sibling")
  }

}