Quick and dirty Javascript KVO

KVO.js

/* Extends the Class Object Written by Resig. http://ejohn.org/blog/simple-javascript-inheritance/ */
Class.prototype.addObserver = function(/** String */ key, /** Object */ observer, /** Function */ method) 
{	
	if (key !== null && key !== undefined && 
		observer !== null && observer !== undefined && 
		method !== null && method !== undefined) 
		{
		eval("this._" + key + " = this." + key);
		
		this.__defineGetter__(key, function() 
		{
			return eval("this._" + key);
		});	
		this.__defineSetter__(key, function(value) 
		{
			eval("this._" + key + " = value");
			method.apply(observer);
		});	
	}
};