Observer

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  myService: Ember.inject.service('my-service'),
  appName: 'Ember Twiddle',
  init() {
    this.get('myService');
  }
});

services.my-service.js

import Ember from 'ember';

export default Ember.Service.extend({
    init() {
        this._super(...arguments);
      	console.log(123, this);
        this.set('foo', 'bar');
        this.set('bar', 'foo');
        this.addObserver('foo', this , 'fooDidChange');
    },

    fooDidChange(sender, key, value, rev) {
        console.log(sender, key);
        // your code
        console.log('1');
    },});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
<br>
<br>

twiddle.json

{
  "version": "0.14.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "2.18.2",
    "ember-template-compiler": "2.18.2",
    "ember-testing": "2.18.2"
  },
  "addons": {
    "ember-data": "2.18.2"
  }
}