stackoverflow question#43012603

components.my-component.js

import Ember from 'ember';

export default Ember.Component.extend({
  
  // this will work like a charm
  computedTotalTask: Ember.computed.alias('model.length'),
  
  didReceiveAttrs(){
    // below will work on initial assignment; not later on because model is not updated
    // only elements are added or removed
    this._super(...arguments);
    this.set('totalTask', this.get('model.length'));
    console.log(this.get('model'));
  },
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  model: [0,1,2,3],
  isLengthZero: Ember.computed.equal('model.length', 0),
  
  actions: {
    onremove(){
      if (this.get('model').length >0) {
      	this.get('model').removeAt(0);
      }
    },
    
    onadd() {
      this.get('model').pushObject(this.get('model').length);
    }
  }
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{my-component model=model}}
<button onclick={{action 'onremove'}} disabled={{isLengthZero}}>Remove elemet from array</button>
<button onclick={{action 'onadd'}}>Add element to array</button>
<br>
<br>

templates.components.my-component.hbs

{{yield}}
Total task is {{totalTask}} (it does not work due to way didReceiveAttrs works)
<br>
Computed total tas is {{computedTotalTask}}

twiddle.json

{
  "version": "0.12.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.12.0",
    "ember-template-compiler": "2.12.0",
    "ember-testing": "2.12.0"
  },
  "addons": {
    "ember-data": "2.12.1"
  }
}