beforeObservers

application.controller.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName:'Ember Twiddle',
  
  propWillChange(prop) {
    //your new before observer
    alert(`propWillChange ran with ${prop}`);
  },

  propDidChange: Ember.observer('prop', function() {

    let prop = this.get('prop');

    if (this._oldProp !== prop) {
      this.propWillChange(this._oldProp);
      this._oldProp = prop;
    }

    alert(`propDidChange ran with ${prop}`);

  }),
                                
  actions: {
    changeProp(value){
      this.set('prop', value);
    }
  }
});

application.template.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
prop: {{prop}}
<br>
<br>
<button onclick={{action "changeProp" 1}}>Change prop to 1</button>
<button onclick={{action "changeProp" 2}}>Change prop to 2</button>
<br>

twiddle.json

{
  "version": "0.5.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.2.0/ember.debug.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/2.2.0/ember-data.js",
    "ember-template-compiler": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.2.0/ember-template-compiler.js"
  }
}