Firestore DocumentSnapshot returns null-CodePudding

I am making an app that returns a listview showing data from firestore data.

Please look at my code , especially the variable att_topic.

StreamBuilder<DocumentSnapshot<Map<String, dynamic>>>(
                  stream: FirebaseFirestore.instance.collection('userinfo').doc(docsdata.id).snapshots(),
                  builder: (BuildContext context,
                  AsyncSnapshot<DocumentSnapshot> snapshot) {
                  if (!snapshot.hasData) {
                    return Text("Loading");
                    }
                  var data_snap = snapshot.data! as DocumentSnapshot<Map<String,dynamic>>;
                  var att_topic = data_snap['attended_topic'] ;

                  return
                    Column(
                      children: [
                        Text(data_snap['via']),
                        Text(data_snap['myposts'].toString()),
                        Container(
                          height: MediaQuery.of(context).size.height*0.4,// ListView for attended topics
                          child:ListView.builder(
                                  padding: const EdgeInsets.symmetric(vertical: 3),
                                  itemCount: att_topic.length,
                                  itemBuilder: (context, index) {


                                    print(att_topic);
                                    print(att_topic.length);
                                    print(att_topic[index]);
                                    print('^^^^^^^^');
                                  return Text(att_topic[index].toString());
                                  }
                                  )
                        ),

In this code, print(att_topic); print(att_topic.length);print(att_topic[index]); print('^^^^^^^^'); gives me :

I/flutter (15223): {topic_id_220914143250_aSjL7LsloHM6ALWRFCEyke6HQje2: {attended: false, read: true, vote_left: false, vote_right: false}, topic_id_220914135806_aSjL7LsloHM6ALWRFCEyke6HQje2: {attended: true, read: true, vote_left: false, vote_right: true}, topic_id_220914141354_aSjL7LsloHM6ALWRFCEyke6HQje2: {attended: true, read: true, vote_left: false, vote_right: true}}
I/flutter (15223): 3
I/flutter (15223): null
I/flutter (15223): ^^^^^^^^

It seems that att_topic as template of Map<Map<String,dynamic>>> and definetely has length. So I called att_topic[index] for print test and ListView but it only returns null.

Can you please why this happens, and how to solve ?

CodePudding user response：

I think att_topic is not a list. Hence number-based indexing doesn't work on that. It is a Map of Maps.

{
  topic_id_220914143250_aSjL7LsloHM6ALWRFCEyke6HQje2: {
    attended: false,
    read: true,
    vote_left: false,
    vote_right: false
  },
  topic_id_220914135806_aSjL7LsloHM6ALWRFCEyke6HQje2: {
    attended: true,
    read: true,
    vote_left: false,
    vote_right: true
  },
  topic_id_220914141354_aSjL7LsloHM6ALWRFCEyke6HQje2: {
    attended: true,
    read: true,
    vote_left: false,
    vote_right: true
  }
}

Map-of-Maps can be accessed like this:

void main() {
  final data = {
    "topic_id_220914143250_aSjL7LsloHM6ALWRFCEyke6HQje2": {
      "attended": false,
      "read": true,
      "vote_left": false,
      "vote_right": false
    },
  };
  print(data.length);
  print(data.keys.toList()[0]);
}

So you can access individual item by using keys getter. There are other ways to do this as well like by using values getter.

Text(att_topic.keys.toList()[index].toString())

Text(att_topic.values.toList()[index].toString())