如何从RaisedButton打开dropDownButton

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: Text('DropDownButton Example')),
        body: Center(
          child: DropDownButtonExample(),
        ),
      ),
    );
  }
}

class DropDownButtonExample extends StatefulWidget {
  @override
  _DropDownButtonExampleState createState() => _DropDownButtonExampleState();
}

class _DropDownButtonExampleState extends State<DropDownButtonExample> {
  String _selectedValue;
  bool _isDropdownOpen = false;

  @override
  Widget build(BuildContext context) {
    return Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: [
        RaisedButton(
          onPressed: () {
            setState(() {
              _isDropdownOpen = !_isDropdownOpen;
            });
          },
          child: Text('Open Dropdown'),
        ),
        if (_isDropdownOpen)
          DropdownButton<String>(
            value: _selectedValue,
            onChanged: (String newValue) {
              setState(() {
                _selectedValue = newValue;
                _isDropdownOpen = false;
              });
            },
            items: <String>['Option 1', 'Option 2', 'Option 3'].map<DropdownMenuItem<String>>((String value) {
              return DropdownMenuItem<String>(
                value: value,
                child: Text(value),
              );
            }).toList(),
          ),
      ],
    );
  }
}