Flutter dropdownButton setState on change在选择后不更新下拉文本以显示所选项目

import 'package:flutter/material.dart';

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

class _MyDropdownButtonState extends State<MyDropdownButton> {
  String _selectedItem;

  @override
  Widget build(BuildContext context) {
    return DropdownButton(
      value: _selectedItem,
      items: [
        DropdownMenuItem(
          value: 'Item 1',
          child: Text('Item 1'),
        ),
        DropdownMenuItem(
          value: 'Item 2',
          child: Text('Item 2'),
        ),
        DropdownMenuItem(
          value: 'Item 3',
          child: Text('Item 3'),
        ),
      ],
      onChanged: (value) {
        setState(() {
          _selectedItem = value;
        });
      },
      hint: Text('Select an item'),
    );
  }
}