Class _MyAppState extends State<MyApp> with WidgetsBindingObserver {
  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addObserver(this);//添加觀察者
  }

  @override
  void didChangeMetrics() {
    // TODO: implement didChangeMetrics
    super.didChangeMetrics();
    _updateTree(context as Element);
  }
  _updateTree(Element el) {
    el.markNeedsBuild();
    el.visitChildren(_updateTree);
  }
  @override
  void dispose() {
    super.dispose();
    WidgetsBinding.instance.removeObserver(this);//銷毀
  }