Flutter StatefulWidget获取下拉按钮的值

import 'package:flutter/material.dart';

class MyDropdownButton extends StatefulWidget {
  @override
  _MyDropdownButtonState createState() => _MyDropdownButtonState();
}

class _MyDropdownButtonState extends State<MyDropdownButton> {
  String selectedValue;

  @override
  Widget build(BuildContext context) {
    return DropdownButton(
      value: selectedValue,
      items: [
        DropdownMenuItem(
          value: 'Option 1',
          child: Text('Option 1'),
        ),
        DropdownMenuItem(
          value: 'Option 2',
          child: Text('Option 2'),
        ),
        DropdownMenuItem(
          value: 'Option 3',
          child: Text('Option 3'),
        ),
      ],
      onChanged: (value) {
        setState(() {
          selectedValue = value;
        });
      },
    );
  }
}