/**
* Sets the dom tree and the css selector to compare against
*
* @param mixed $dom Dom tree to search into.
* @param mixed $selector Css selector to match element.
* @param string $message Customised message on failure.
*/
public function __construct($dom, $selector, $message = '%s')
{
parent::__construct($message);
$this->dom = $dom;
$this->selector = $selector;
$css_selector = new CssSelector($this->dom);
$this->value = $css_selector->getTexts($this->selector);
}