Firestore 获取长度数据在颤振流中消失

Question

I make a chat using firebase firestore.

So. I tried express not read count in the Chat list.

But, Initially the number appears, but it changes to null data.

I don't know Why data chage null data?

class ChatLength extends StatelessWidget {
  const ChatLength({super.key, required this.docId, required this.uid});

  final String docId;
  final String uid;

  @override
  Widget build(BuildContext context) {
    return StreamBuilder(
      stream: FirebaseFirestore.instance
          .collection('message_read')
          .doc(docId)
          .collection('message')
          .where('userId', isNotEqualTo: uid)
          .where('read', isEqualTo: false)
          .snapshots(),
      builder: (context, snapshot) {
        print(snapshot.data?.size);
        if (snapshot.data?.size != null) {
          return Container(
            child: Text('${snapshot.data?.size}'),
          );
        } else {
          return Container();
        }
      },
    );
  }
}

===== Debug console ====
flutter: 1 
flutter: null    //  Data changes to null immediately

1. I'm trying get a snapshot data from firestore.

2. And I put the imported data into the text.

3. But Data changes to null immediately.

Answer 1

Since you use snapshots() the code is not just reading the data once, but then continues listening for changes to the data. So the two values you see mean that the snapshots() stream was triggered twice, the first time with a single message - and the second time without any messages.

My educated guess is that your code changes the read value of the document after it gets it, since it has now displayed that message to the user. But doing so means the document no longer meets the criteria of your query, so the snapshots stream gets a new event without that message in it.

Consider using a different mechanism for the query, for example I usually use a timestamp to determine what messages to show. Step by step:

• Ensure each message document has a timestamp field.
• For each user store (either in the database or in local storage of the app) when they last started the app.
• Then request from the database the messages since they last started the app.
• Make sure to start the query before you update the timestamp value, otherwise you'll never get any results.