[]请问如何使我的自定义类UIDropBox继承Laya.Sprite？

var UIDropBox = (function () {



    var Sprite = Laya.Sprite;

    var Text = Laya.Text;



    function UIDropBox() {

        UIDropBox.super(this);

        this.txt = this.getTextFeild();

        this.txt.y = 50;

        this.txt.text = 'aasassa';

        console.log(this.addChild);//undefined

        console.log(this._super);//undefined

        this.addChild(this.txt);

    }



    Laya.class(UIDropBox, "UIDropBox", Sprite);



    UIDropBox.prototype = {

        getTextFeild: function () {

            var text = new Text();

            text.overflow = Text.VISIBLE;

            text.color = "#FFFFFF";

            text.fontSize = 20;

            return text;

        }

    }



    return UIDropBox;



})();