Why is my listener not returning any data?-CodePudding

I'm trying to keep everything modular and I stumbled onto a problem. I want to keep all code related to Firebase Realtime Database and Firebase Cloud Storage in my message_dao widget, so I have my receivedChats widget calling a message_dao function that activates the listener. However, it looks like receivedChats is receiving an empty list because when I print out the chats variable it's empty.

message_dao

Future<List> activateChatListener() async { 

    var msg;

    _receivedChatsStream = _messagesRef.onValue.listen((event) {

      event.snapshot.children.forEach((snapshot) {
        dataList.add(snapshot.value);

      });

      //Some other code that returns a future

    });

    return dataList;
  }

receivedChats

class _ReceivedChatsState extends State<ReceivedChats> {
  var chats = [];

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

Future<void> loadChats() async {

    var dataList = await messageDao.activateChatListener();

    setState(() {
      chats = dataList;
    });

    print('chats: '   chats.toString()) //This shows an empty list

  }

//...other code...
}

CodePudding user response：

You are returning the listbefore the api has finished fetching data

Try this instead

Future<List> activateChatListener() async { 

    var msg;

    _receivedChatsStream = _messagesRef.onValue.listen((event) {

      event.snapshot.children.forEach((snapshot) {
        dataList.add(snapshot.value);

      });

      //Some other code that returns a future
     return dataList;
    });

    
  }

CodePudding user response：

Because your loadChats will be called once. Instead of storing your data into dataList array, you can directly use that snapshot through StreamBuilder

Example:

class NoteList extends StatelessWidget {
  final db = FirebaseFirestore.instance;
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Notes"),
        centerTitle: true,
      ),
      body: StreamBuilder<QuerySnapshot>(
        stream: db.collection('notes').snapshots(),
        builder: (context, snapshot) {
          if (!snapshot.hasData) {
            return Center(
              child: CircularProgressIndicator(),
            );
          } else
            return ListView(
              children: snapshot.data.docs.map((doc) {
                return Card(
                  child: ListTile(
                    title: Text(doc.data()['title']),
                  ),
                );
              }).toList(),
            );
        },
      ),
    );
  }
}

Here is the full example that you can refer and you can solve your problem.