When is the callback updateProperty triggered for a customized widget ?

Hi , 

For a customized widget, sometimes the callback function updateProperty is triggered as my expected, but sometimes not.  

I found the following situations that updateProperty will NOT be triggered : 

1. afterRender is empty
2. Div doesn't has nested div/span inside it . for example,  <div class="widget-content widget-helloWorld"><div class="DisplayText"></div></div> works,  but <div class="widget-content widget-helloWorld DisplayText"></div> doesn't work
3. call this.setProperty function in afterRender won't be helpful in the above cases

I'm really confused about the behavior of triggering updateProperty , could someone give me a detailed description for updateProperty ? 

=============afterRender====================

this.afterRender = function () {
   //this.setProperty("BarTitle", 'test');
   valueElem = this.jqElement.find(".DisplayText");
   valueElem.text(this.getProperty("BarTitle"));
};

Regards,

Sean