get Data from different Collections using Firebase and flutter and update the UI when data changes

i'm developing an app with flutter and i have some difficultes. i used the follow function with Streambuilder to get data from two collections but the UI doesn't update when data changes in firebase. here my function :

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  List<Post> listStream = [];
  AuthenticationService service = AuthenticationService();
  Stream<List<Post>> fetch() async* {
    User user = service.getCurrentUser();
    var customerDocument = await FirebaseFirestore.instance
        .collection("XXXX")
        .doc(user.uid)
        .get();

    List<String> abbonierten =
        List<String>.from(customerDocument["EEEE"]);
    for (int j = 0; j < abbonierten.length; j++) {
      var providerDocument = await FirebaseFirestore.instance
          .collection("WWWWW")
          .doc(abbonierten.elementAt(j))
          .get();
      List<Map> posts = List<Map>.from(providerDocument["posts"] ?? []);
      for (int item = 0; item < posts.length; item++) {
        Post post =
            Post.fromJson(providerDocument.data(), posts.elementAt(item));
        listStream.add(post);
      }
    }
    yield listStream;
  }
  StreamChatList streamChatList = StreamChatList();
  ChatService chatService = ChatService();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Kindacode.com'),
      ),
      body: Padding(
        padding: const EdgeInsets.all(30),
        child: StreamBuilder(
          stream: fetch(),
          builder: (context, snapshot) {
            if (snapshot.hasData) {
              List<Post> data = snapshot.data;
              return ListView.builder(
                  itemCount: data.length,
                  itemBuilder: (context, index) {
                    return Text(data.elementAt(index).title);
                  });
            }
            return Center(child: LinearProgressIndicator());
          },
        ),
      ),
    );
  }
}

or is there another method to achieve this task. can someome help me with it pls ?