如何在Flutter中自定义SliverAppBar中的flexibleSpace属性？

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: CustomScrollView(
          slivers: [
            SliverAppBar(
              expandedHeight: 200,
              flexibleSpace: MyFlexibleSpaceBar(),
            ),
            SliverList(
              delegate: SliverChildBuilderDelegate(
                (context, index) => ListTile(
                  title: Text('Item $index'),
                ),
                childCount: 20,
              ),
            ),
          ],
        ),
      ),
    );
  }
}

class MyFlexibleSpaceBar extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return FlexibleSpaceBar(
      background: Container(
        decoration: BoxDecoration(
          gradient: LinearGradient(
            colors: [Colors.blue, Colors.purple],
            begin: Alignment.topLeft,
            end: Alignment.bottomRight,
          ),
        ),
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Text(
              'Custom Flexible Space',
              style: TextStyle(
                fontSize: 24,
                color: Colors.white,
              ),
            ),
            SizedBox(height: 10),
            Text(
              'Add your own widgets here',
              style: TextStyle(
                fontSize: 18,
                color: Colors.white,
              ),
            ),
          ],
        ),
      ),
    );
  }
}