当修改对象的属性时,能否执行其它语句(有例子)?解决思路

当修改对象的属性时,能否执行其它语句(有例子)?
下面的代码运行良好
Perl code

function A(text,value)
{
    var num=0;
    this.GetNum=function(){return num;};
    this.AddNum=function(){++num;}

    this.Text=text;
    this.Value=value;
}

   var obj=new A("Hi","World");
   var num;
   
   obj.AddNum();
   num=obj.GetNum();//此时num应为1;

   obj.AddNum();
   num=obj.GetNum();//此时num应为2;



怎样写JavaScript,达到下面这样的效果:
Perl code

    var obj=new A("Hi","World");
   
    obj.Text="Hello";//这一句执行完后num自动加一(用 = 赋值时能执行其它语句)
    num=obj.GetNum();//此时num应为1;



------解决方案--------------------
可以参考
http://articles.sitepoint.com/print/javascript-objects
的代码

onpropertychange
------解决方案--------------------
用属性不好监控,改成调用方法吧
JScript code

    function A(text,value)
    {
        var num=0;
        this.GetNum=function(){return num;};
        this.AddNum=function(){++num;}
        this.SetText = function(txt){  this.Text = txt;  this.AddNum();};

        this.Text=text;
        this.Value=value;
    }

    var obj=new A("Hi","World");
   
    obj.SetText("Hello");//这一句执行完后num自动加一(用 = 赋值时能执行其它语句)
    num=obj.GetNum();//此时num应为1;