function Class1()
{
  var oldValue='';
  var name='xu';
  var id='1';
  this.setName=function(n)
  {
   oldValue=name;
   name=n;
   this.propertyChange('name',oldValue,n);
 }
 this.setID=function(n)
 {
   oldValue=id;
   id=n;
   this.propertyChange('id',oldValue,n);
 }
 this.display=function()
 {
   alert(name+'\'s id is :'+id);
 }
}
Class1.prototype={
 propertyChange:function(propertyName,oldValue,newValue)
 {
   alert(propertyName+'\'s value changed from '+oldValue+' to '+newValue); 
 }
};
var c=new Class1();
c.setName('xu22');
c.setID('5');
c.display();

if (!Object.prototype.watch)
{
   Object.prototype.watch = function (prop, handler) 
   {
     var oldval = this[prop], newval = oldval,
     getter = function ()
     {
       return newval;
     },
     setter = function (val) 
     {
       oldval = newval;
       return newval = handler.call(this, prop, oldval, val);
     };
     if (delete this[prop])
     { 
       if (Object.defineProperty) // ECMAScript 5
       {      
         Object.defineProperty(this, prop, {get: getter,set: setter});
       }
       else if (Object.prototype.__defineGetter__ && Object.prototype.__defineSetter__) 
       {
         Object.prototype.__defineGetter__.call(this, prop, getter);
         Object.prototype.__defineSetter__.call(this, prop, setter);
       }
     }
   };
}
if (!Object.prototype.unwatch)
{
   Object.prototype.unwatch = function (prop) 
   {
     var val = this[prop];
     delete this[prop]; 
     this[prop] = val;
   };
}