title: Angular2 - subscribe to Service variable changes
body: <p>I have an authentication service that makes the authenticated variable equal to true or false. </p>
  
  <pre><code>checkAuthentication(){
    this._authService.getAuthentication()
      .subscribe(value =&gt; this.authenticated = value);
  }
  </code></pre>
  
  <p>How do I execute a function when <code>this.authenticated</code> has changed value? ngOnChanges is not picking up the change. </p>