cocos creator重写GLoader

在ccc里边重写了gloder.但是会报错Simulator: E/jswrapper (271): ERROR:
Uncaught TypeError: Cannot set property 'spriteFrame' of undefined, location: F:/client/assets/scripts/Lib/fairygui.js:0:0
 
这是我自己写的gloder类

@ccclass
export default class HttpLoader extends fgui.GLoader {
protected loadExternal(): void{
if (this.url.lastIndexOf("ui://") == -1)
{
this.loadimg(this,this.url)
// console.log("开始加载"+this.url)
}
// super.loadExternal();
// console.log("开始加载" + this.url)
}
public constructor()
{
super();
}
private loadimg(container, url)
{
// cc.loader.load(url,this.loadedImg)
cc.loader.load({ url: url, type: 'jpg' }, this.loadedImg);
}
private loadedImg(err, texture)
{
var sprite = new cc.SpriteFrame(texture);
// console.log("开始加载图片完成" + JSON.stringify(sprite));
// console.log("开始加载图片完成" + JSON.stringify(texture));
// this.texture = sprite;
// this.onExternalLoadSuccess(sprite);
super.onExternalLoadSuccess(sprite);
// onExternalLoadSuccess(sprite);
}
public onExternalLoadSuccess(texture: cc.SpriteFrame): void{
console.log("onExternalLoadSuccess");
}
protected onExternalLoadFailed(): void {
console.log("onExternalLoadFailed");
}
}