如何在颤动中将下拉选择项设置为TextFormField

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('Dropdown TextFormField'),
        ),
        body: MyForm(),
      ),
    );
  }
}

class MyForm extends StatefulWidget {
  @override
  _MyFormState createState() => _MyFormState();
}

class _MyFormState extends State<MyForm> {
  String _selectedItem;

  final _formKey = GlobalKey<FormState>();

  @override
  Widget build(BuildContext context) {
    return Form(
      key: _formKey,
      child: Column(
        children: [
          TextFormField(
            decoration: InputDecoration(
              labelText: 'TextFormField',
            ),
            validator: (value) {
              if (value.isEmpty) {
                return 'Please enter some text';
              }
              return null;
            },
          ),
          DropdownButtonFormField<String>(
            value: _selectedItem,
            onChanged: (String newValue) {
              setState(() {
                _selectedItem = newValue;
              });
            },
            items: <String>['Option 1', 'Option 2', 'Option 3']
                .map<DropdownMenuItem<String>>((String value) {
              return DropdownMenuItem<String>(
                value: value,
                child: Text(value),
              );
            }).toList(),
            decoration: InputDecoration(
              labelText: 'DropdownFormField',
            ),
          ),
          RaisedButton(
            onPressed: () {
              if (_formKey.currentState.validate()) {
                // 执行提交操作
              }
            },
            child: Text('Submit'),
          ),
        ],
      ),
    );
  }
}