你可以计算广告指数和项目指数来实现你想要的。下面是一个简化的例子，其中有两个项目之间的广告。

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        useMaterial3: true,
      ),
      home: const MyHomePage(),
      debugShowCheckedModeBanner: false,
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key});

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          backgroundColor: Theme.of(context).colorScheme.inversePrimary,
          title: const Text('Demo'),
        ),
        //actual code starts here
        body: const _ListWithAds());
  }
}

class _ListWithAds extends StatelessWidget {
  const _ListWithAds();

  List<int> get items => Iterable.generate(100, (i) => i).toList();

  @override
  Widget build(BuildContext context) {
    var length = (items.length * 1.5).floor();

    // if you don't want last item to be an Ad, uncomment below line
    // if (length % 3 == 0) {
    //   length--;
    // }

    return ListView.builder(
        itemCount: length,
        itemBuilder: (BuildContext context, index) {
          if (index % 3 == 2) {
            return const ListTile(
              title: Text('Ad'),
            );
          } else {
            final newIndex = (index * 2 / 3).ceil();
            return ListTile(
              title: Text('Item #$newIndex'),
            );
          }
        });
  }
}

输出：