PopupMenuButton控件即弹出菜单控件，点击控件会出现菜单。

import 'package:flutter/material.dart';
class MenusDemo extends StatefulWidget {
  @override
  _MenusDemoState createState() => new _MenusDemoState();
}
class _MenusDemoState extends State<MenusDemo> {
  String _bodyStr = '显示菜单的点击';
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text('菜单演示'),actions: <Widget> [
          new PopupMenuButton<String>(
            onSelected: (String value) {
              setState(() {
                _bodyStr = value;
              });
            },itemBuilder: (BuildContext context) => <PopupMenuItem<String>>[
              new PopupMenuItem<String>(
                value: '选项一的值',child: new Text('选项一')
              ),new PopupMenuItem<String>(
                value: '选项二的值',child: new Text('选项二')
              )
            ]
          )
        ]
      ),body: new Center(
        child: new Text(_bodyStr)
      )
    );
  }
}
void main() {
  runApp(new MaterialApp(
    title: 'Flutter教程',home: new MenusDemo(),));
}