import 'package:flutter/material.dart';class MenusDemo extends StatefulWidget { @override _MenusDemoState createState() => new _MenusDemoState();}class _MenusDemoState extends State<MenusDemo> { String _bodyStr = '顯示菜單的點擊'; @override Widget build(BuildContext context) {  return new Scaffold(   appBar: new AppBar(    title: new Text('菜單演示'),    actions: <Widget> [     new PopupMenuButton<String>(      onSelected: (String value) {       setState(() {        _bodyStr = value;       });      },      itemBuilder: (BuildContext context) => <PopupMenuItem<String>>[       new PopupMenuItem<String>(        value: '選項一的值',        child: new Text('選項一')       ),       new PopupMenuItem<String>(        value: '選項二的值',        child: new Text('選項二')       )      ]     )    ]   ),   body: new Center(    child: new Text(_bodyStr)   )  ); }}void main() { runApp(new MaterialApp(  title: 'Flutter教程',  home: new MenusDemo(), ));}