Flutter 页面绘制完毕第一帧

class EmptyPageState extends State<EmptyPage> with AutoDisposeMixin{  
 @override  
 void initState() {  
 super.initState();  
 WidgetsBinding.instance.addPostFrameCallback((mag) {  

 });  
 }  
  
 @override  
 Widget build(BuildContext context) {  
 return Container();  
 }