Flutter: Items in StreamBuilder(Using firebase realtime database) are sorted randomly

Question

I'm creating a simple application with Firebase Realtime database where a user inputs a text and it gets added to a list of chats.

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.indigo,
      ),
      home: MyHomePage(),
    );
  }
}

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

class _MyHomePageState extends State<MyHomePage> {
  var _firebaseRef = FirebaseDatabase().reference().child('chats');
  TextEditingController _txtCtrl = TextEditingController();

  @override
  Widget build(BuildContext context) {
    var comments = _firebaseRef.orderByChild('time').limitToLast(10);
    return Scaffold(
      body: Container(
        child: SafeArea(
          child: Column(
            children: <Widget>[
              Container(
                  child: Row(children: <Widget>[
                Expanded(child: TextField(controller: _txtCtrl)),
                SizedBox(
                    width: 80,
                    child: OutlineButton(
                        child: Text("Add"),
                        onPressed: () {
                          sendMessage();
                        }))
              ])),
              StreamBuilder(
                stream: comments.onValue,
                builder: (context, snap) {
                  if (snap.hasData &&
                      !snap.hasError &&
                      snap.data.snapshot.value != null) {
                    Map data = snap.data.snapshot.value;
                    List item = [];

                    data.forEach(
                        (index, data) => item.add({"key": index, ...data}));

                    return Expanded(
                      child: ListView.builder(
                        itemCount: item.length,
                        itemBuilder: (context, index) {
                          return ListTile(
                            title: Text(item[index]['message']),
                          );
                        },
                      ),
                    );
                  } else
                    return Center(child: Text("No data"));
                },
              ),
            ],
          ),
        ),
      ),
    );
  }

  sendMessage() {
    _firebaseRef.push().set({
      "message": _txtCtrl.text,
      'time': DateTime.now().millisecondsSinceEpoch
    });
  }


}

It stores and retrieves data perfectly. But when I try adding data, the new items are placed at random points in the list.

For example, in the image below, the last item I placed into the list was 'Nine'. But it was put in the center of the list:

I've tried sorting the list by timestamps, but it did nothing.

What could be causing this issue? And how can I fix it?

Solution

When you call snap.data.snapshot.value; the data in the snapshot (which is ordered) is converted to a Map<String, Object> which isn't ordered. To maintain the order, you'll want to listen to onChild... instead.

Note that FlutterFire has a convenient firebase_list library that handles most of the heavy lifting of onChild... for you.

Also see:

• Flutter Firebase Database wrong timestamp order
• Flutter sort Firebase snapshot by timestamp
• Flutter: Firebase Real-Time database orderByChild has no impact on query result