THiddenField displays a hidden input field on a Web page.
The value of the input field can be accessed via {@link getValue Value} property.
If upon postback the value is changed, a {@link onValueChanged OnValueChanged}
event will be raised.
/**
* Ensure that the ID attribute is rendered and registers the javascript code
* for initializing the active control.
*/
protected function addAttributesToRender($writer)
{
parent::addAttributesToRender($writer);
$writer->addAttribute('id', $this->getClientID());
$this->getActiveControl()->registerCallbackClientScript($this->getClientClassName(), $this->getPostBackOptions());
}