nested topic example for SO https://stackoverflow.com/q/70823362/10976714

nested_topics_example.dart

import 'package:flutter/material.dart';

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



class NestedTopicsExample extends StatelessWidget {
  const NestedTopicsExample({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatelessWidget {
  final String title;
  const MyHomePage({Key? key, required this.title}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(title),
      ),
      body: const Center(child: NestedTopicWidget()),
    );
  }
}

class NestedTopicWidget extends StatelessWidget {
  const NestedTopicWidget({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return ListView.builder(
      itemCount: topics.length,
      itemBuilder: (context, index) {
        return TopicWidget(topic: topics[index]);
      },
    );
  }
}

class TopicWidget extends StatelessWidget {
  final Topic topic;
  const TopicWidget({Key? key, required this.topic}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return ExpansionTile(
      title: Text(topic.text),
      children: List.generate(topic.subTopics.length, (index) => TopicWidget(topic: topic.subTopics[index])),
    );
  }
}

final topics = <Topic>[
  // topic 1
  Topic(
    text: 'Topic 1',
    subTopics: [
      Topic(text: 'Sub Topic 1'),
      Topic(
        text: 'Sub Topic 2',
        subTopics: [
          Topic(
            text: 'sub sub topic 1',
          ),
        ],
      ),
      Topic(text: 'Sub Topic 3'),
    ],
  ),
  // topic 2
  Topic(
    text: 'Topic 2',
    subTopics: [
      Topic(
        text: 'Sub Topic 1',
      ),
      Topic(
        text: 'Sub Topic 2',
      ),
    ],
  ),
  // topic 3
  Topic(
    text: 'Topic 3',
  )
];

class Topic {
  final String text;
  final List<Topic> subTopics = [];
  Topic({required this.text, List<Topic> subTopics = const []}) {
    this.subTopics.addAll(subTopics);
  }
}

List<String> l = [
  "Topic 1",
  " Sub Topic 1",
  " Sub Topic 2",
  "  SubSub Topic 1",
  " Sub Topic 3",
  "Topic 2",
  " Sub Topic 1",
  " Sub Topic 2",
  "Topic 3",
];