flutter dynamic expansionTile

Reacting to you comment and edit of the question I took the liberty to write a working example. Feel free to edit or comment. I hope, this is what you wanted to achieve.

import 'dart:async';

import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;
import 'dart:convert';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'ExpansionTile Test',
      home: new MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  Future<http.Response> _responseFuture;

  @override
  void initState() {
    super.initState();
    _responseFuture = http.get('http://174.138.61.246:8080/support/dc/1');
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text('ExpansionTile Test'),
      ),
      body: new FutureBuilder(
        future: _responseFuture,
        builder: (BuildContext context, AsyncSnapshot<http.Response> response) {
          if (!response.hasData) {
            return const Center(
              child: const Text('Loading...'),
            );
          } else if (response.data.statusCode != 200) {
            return const Center(
              child: const Text('Error loading data'),
            );
          } else {
            List<dynamic> json = JSON.decode(response.data.body);
            return new MyExpansionTileList(json);
          }
        },
      ),
    );
  }
}

class MyExpansionTileList extends StatelessWidget {
  final List<dynamic> elementList;

  MyExpansionTileList(this.elementList);

  List<Widget> _getChildren() {
    List<Widget> children = [];
    elementList.forEach((element) {
      children.add(
        new MyExpansionTile(element['did'], element['dname']),
      );
    });
    return children;
  }

  @override
  Widget build(BuildContext context) {
    return new ListView(
      children: _getChildren(),
    );
  }
}

class MyExpansionTile extends StatefulWidget {
  final int did;
  final String name;
  MyExpansionTile(this.did, this.name);
  @override
  State createState() => new MyExpansionTileState();
}

class MyExpansionTileState extends State<MyExpansionTile> {
  PageStorageKey _key;
  Future<http.Response> _responseFuture;

  @override
  void initState() {
    super.initState();
    _responseFuture =
        http.get('http://174.138.61.246:8080/support/dcreasons/${widget.did}');
  }

  @override
  Widget build(BuildContext context) {
    _key = new PageStorageKey('${widget.did}');
    return new ExpansionTile(
      key: _key,
      title: new Text(widget.name),
      children: <Widget>[
        new FutureBuilder(
          future: _responseFuture,
          builder:
              (BuildContext context, AsyncSnapshot<http.Response> response) {
            if (!response.hasData) {
              return const Center(
                child: const Text('Loading...'),
              );
            } else if (response.data.statusCode != 200) {
              return const Center(
                child: const Text('Error loading data'),
              );
            } else {
              List<dynamic> json = JSON.decode(response.data.body);
              List<Widget> reasonList = [];
              json.forEach((element) {
                reasonList.add(new ListTile(
                  dense: true,
                  title: new Text(element['reason']),
                ));
              });
              return new Column(children: reasonList);
            }
          },
        )
      ],
    );
  }
}

Following Rainer Wittmann approach, I modified it to fit my needs and implemented for Cloud Firestore, but instead of futures I used streams.

My basic structure of Cloud Firestore is:

Collection projects

• name

• Collection surveys:

  • surveyName

Solution:

class ProjectList extends StatelessWidget {
  ProjectList({this.firestore});

  final Firestore firestore;

  @override
  Widget build(BuildContext context) {
    return StreamBuilder<QuerySnapshot>(
      stream: firestore.collection('projects').snapshots(),
      builder: (BuildContext context, AsyncSnapshot<QuerySnapshot> snapshot) {
        if (!snapshot.hasData) return const Text('Loading...');
        //final int projectsCount = snapshot.data.documents.length;
        List<DocumentSnapshot> documents = snapshot.data.documents;
        return ExpansionTileList(
          firestore: firestore,
          documents: documents,
        );
      },
    );
  }
}

class ExpansionTileList extends StatelessWidget {
  final List<DocumentSnapshot> documents;
  final Firestore firestore;

  ExpansionTileList({this.documents, this.firestore});

  List<Widget> _getChildren() {
    List<Widget> children = [];
    documents.forEach((doc) {
      children.add(
        ProjectsExpansionTile(
          name: doc['name'],
          projectKey: doc.documentID,
          firestore: firestore,
        ),
      );
    });
    return children;
  }

  @override
  Widget build(BuildContext context) {
    return ListView(
      children: _getChildren(),
    );
  }
}

class ProjectsExpansionTile extends StatelessWidget {
  ProjectsExpansionTile({this.projectKey, this.name, this.firestore});

  final String projectKey;
  final String name;
  final Firestore firestore;

  @override
  Widget build(BuildContext context) {
    PageStorageKey _projectKey = PageStorageKey('$projectKey');

    return ExpansionTile(
      key: _projectKey,
      title: Text(
        name,
        style: TextStyle(fontSize: 28.0),
      ),
      children: <Widget>[
        StreamBuilder(
            stream: firestore
                .collection('projects')
                .document(projectKey)
                .collection('surveys')

                .snapshots(),
            builder:
                (BuildContext context, AsyncSnapshot<QuerySnapshot> snapshot) {
              if (!snapshot.hasData) return const Text('Loading...');
              //final int surveysCount = snapshot.data.documents.length;
              List<DocumentSnapshot> documents = snapshot.data.documents;

              List<Widget> surveysList = [];
              documents.forEach((doc) {
                PageStorageKey _surveyKey =
                    new PageStorageKey('${doc.documentID}');

                surveysList.add(ListTile(
                  key: _surveyKey,
                  title: Text(doc['surveyName']),
                ));
              });
              return Column(children: surveysList);
            })
      ],
    );
  }
}

Hope this helps to those lost in nested collections in cloud firestore.

Happy coding!