future_with_stream

import 'package:flutter/material.dart';
import 'package:firebase_core/firebase_core.dart';
import 'package:cloud_firestore/cloud_firestore.dart';

const Color darkBlue = Color.fromARGB(255, 18, 32, 47);

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  
  await Firebase.initializeApp(
    options: const FirebaseOptions(
      apiKey: "AIzaSyDOdBmVvI30Jg49oup4gsJMZMjTPhHOloQ",
      authDomain: "fullstack-labs.firebaseapp.com",
      databaseURL: "https://fullstack-labs.firebaseio.com",
      projectId: "fullstack-labs",
      storageBucket: "fullstack-labs.appspot.com",
      messagingSenderId: "251418261584",
      appId: "1:251418261584:web:2e01357a093d27476b7955"
    )
  );
  runApp(
    MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: Home(),
        ),
      ),
    );
  }
}

class Home extends StatefulWidget {
  const Home({Key? key}) : super(key: key);

  @override
  _HomeState createState() => _HomeState();
}

class _HomeState extends State<Home> {

  static String? userID = '101'; // FirebaseAuth.instance.currentUser?.uid
  static final userColeccion = FirebaseFirestore.instance.collection("users");

  var groupfav = ' ';
  Stream<QuerySnapshot>? taskGroup;

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

  @override
  Widget build(BuildContext context) {
    
    return Scaffold(
      appBar: AppBar(
        title: const Text("Home"),
        automaticallyImplyLeading: false,
      ),
      body: FutureBuilder( // the future builder fetches the initial data
        future: userColeccion.doc('$userID').get(),
        builder: (context, snapshot) {

          if (snapshot.hasData) {
            var docData = (snapshot.data as DocumentSnapshot).data() as Map<String, dynamic>;
            var groupfav = docData['groupfav'];


            // then once the 'groupfav' has arrived, 
            // start listening

            return Column(
              children: [
                Text('Tasks for $groupfav:'),
                Expanded(
                  child: StreamBuilder(
                    stream: FirebaseFirestore.instance
                    .collection("groups")
                    .doc(groupfav) // pass the obtained value
                    .collection("task")
                    .snapshots(),
                    builder: (context,taskSnapshot) {
                      
                      if (taskSnapshot.hasData) {
                        
                        var taskList = (taskSnapshot.data as QuerySnapshot<Map<String, dynamic>>).
                          docs.map((d) => Task.fromJson(d.data())).toList();
                      
                        return ListView.builder(
                          itemCount: taskList.length,
                          itemBuilder: (context, index) {
                            var task = taskList[index];
                            return Container(
                              padding: const EdgeInsets.all(20),
                              child: Column(
                                children: [
                                  Text(task.titulo!, style: const TextStyle(color: Colors.blue, fontSize: 30)),
                                  Text(task.contenido!)
                                ]
                              )
                            );
                          }
                        );
                      }
                      
                      return const Center(
                        child: CircularProgressIndicator()
                      );

                    }
                  ),
                )
              ]
            );
          }
          
          return const Center(
            child: CircularProgressIndicator()
          );
        }
      )
   );
  }
}

class Task {
  
  String? titulo;
  String? contenido;
  
  Task({ this.titulo, this.contenido });
  
  factory Task.fromJson(Map<String, dynamic> json) {
    return Task(
      titulo: json['titulo'],
      contenido: json['contenido']
    );
  }
}