Flutter -添加新小部件时，列中的StatefulWidgets不会更改

import 'package:flutter/material.dart';

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  List<String> items = ['Item 1', 'Item 2', 'Item 3'];

  void addItem() {
    setState(() {
      items.add('New Item');
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('My Widget'),
      ),
      body: Column(
        children: [
          RaisedButton(
            child: Text('Add Item'),
            onPressed: addItem,
          ),
          ListView.builder(
            shrinkWrap: true,
            itemCount: items.length,
            itemBuilder: (context, index) {
              return ListTile(
                title: Text(items[index]),
              );
            },
          ),
        ],
      ),
    );
  }
}