observer test

components.my-component.js

import Ember from 'ember';
var {observer, on} = Ember;

export default Ember.Component.extend({
  item: null,
  text: "not updated",
  update: observer('item', function(){
  	this.set('text', 'update');
  }),
  update2: on('didInitAttrs', observer('item', function(){
    this.set('text', 'update 2');
})),
  update3: observer('attrs.item', function(){
    this.set('text', 'update 3');
})
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName:'Ember Twiddle'
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
{{my-component item="a"}}
<br>
<br>

templates.components.my-component.hbs

{{text}}

twiddle.json

{
  "version": "0.4.8",
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/1.13.8/ember.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/1.13.9/ember-data.js",
    "ember-template-compiler": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/1.13.8/ember-template-compiler.js"
  }
}