如何在listview.builder完成列表视图构建之前显示进度指示器

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('ListView with Progress Indicator')),
        body: MyListView(),
      ),
    );
  }
}

class MyListView extends StatefulWidget {
  @override
  _MyListViewState createState() => _MyListViewState();
}

class _MyListViewState extends State<MyListView> {
  bool _isLoading = true;
  List<String> _items = [];

  @override
  void initState() {
    super.initState();
    _loadData();
  }

  void _loadData() async {
    // Simulate a delay
    await Future.delayed(Duration(seconds: 3));
    setState(() {
      _isLoading = false;
      _items = List<String>.generate(20, (i) => 'Item $i');
    });
  }

  @override
  Widget build(BuildContext context) {
    if (_isLoading) {
      return Center(child: CircularProgressIndicator());
    } else {
      return ListView.builder(
        itemCount: _items.length,
        itemBuilder: (context, index) {
          return ListTile(title: Text(_items[index]));
        },
      );
    }
  }
}