Firebase 实时数据库按键排序

Question

I want to sort data in Firebase realtime database.

I am using timestamp as key when saving data and I want to sort data by timestamps. I used below code for this purpose.

  Widget buildList(ChatUser chatUser) {
return Flexible(
    child: StreamBuilder(
        stream: _service
            .getMessages(chatUser.uid!)
            .orderByKey()
            .onValue,
        builder: (context, snapshot) {
          List<ChatMessage> messageList = [];
          if (snapshot.hasData) {
            final myMessages = Map<dynamic, dynamic>.from(
                (snapshot.data as DatabaseEvent).snapshot.value
                    as Map<dynamic, dynamic>);

            myMessages.forEach((key, value) {
              final currentMessage = Map<String, dynamic>.from(value);
              final message = ChatMessage().fromJson(currentMessage);
              messageList.add(message);
            });

            if (messageList.isNotEmpty) {
              return ListView.builder(
                  padding: const EdgeInsets.all(10),
                  reverse: true,
                  itemCount: messageList.length,
                  controller: scrollController,
                  itemBuilder: (context, index) {
                    return buildItem(index, messageList[index], chatUser);
                  });
            } else {
              return const Center(
                child: Text('Henüz Mesaj yok.'),
              );
            }
          } else {
            return const Center(
              child: CircularProgressIndicator(
                color: Colors.red,
              ),
            );
          }
        }));

}

As a result, data does not come according to key values, it comes in different orders.

Any suggestions? Thanks.

Answer 1

Have you tried with ".orderByChild()"?

Answer 2

The problem is in how you process the results here:

  if (snapshot.hasData) {
    final myMessages = Map<dynamic, dynamic>.from(
        (snapshot.data as DatabaseEvent).snapshot.value
            as Map<dynamic, dynamic>);

    myMessages.forEach((key, value) {
      final currentMessage = Map<String, dynamic>.from(value);
      final message = ChatMessage().fromJson(currentMessage);
      messageList.add(message);
    });

The order of the keys inside a Map is by definition undefined. So when you call (snapshot.data as DatabaseEvent).snapshot.value as Map<dynamic, dynamic>) , you're actually dropping all ordering information that the database returns.

To process the results in the correct order, iterate over the children of the snapshot , and only then convert each child to a Map .

Answer 3

Complementing Frank , try to assign snapshot to a List of snapshots using List snapshotList = xxx.children.toList(); If you do something like snapshotList[i].value you will notice that the key is not present, the solution to get it back is to use the get .key .

You can see bellow an exemple how I did to solve the same problem in my project.

final List<DataSnapshot> snapshotList = snapshot.data.children.toList();

final List<Map> commentsList = [];

for (var i in snapshotList) {
  Map<String?, Map> comment = {i.key: i.value as Map};
  commentsList.add(comment);
}

In the code above, commentsList will get you a list of Maps ordered according to original database.

I hope this help. If anyone has a more straightforward solution, please, share with us.