I keep list of hashtags and their appearance in realtime db.

And I want to make a service class with a function to retrieve hashtags as a List in descending order of appearance. So below is the preferred result.

List<String> expHashList = [개발자, 인사담당자, VC, 작가, 경영자];
List<String> topicList = [커리어, 채용, 이직, 리더십, 조직문화];

I tried below but it returns empty list([]).

class HashtagService {
  static final HashtagService _instance = HashtagService._internal();
  factory HashtagService() => _instance;

  late final DatabaseReference _database;

  HashtagService._internal() {
    _database = FirebaseDatabase.instance.reference().child('/hashtags');
  }

  List<String> getSuggestion(String type) {
    if (type == 'exp') {
      var query = _database.child('expHash').orderByValue().limitToLast(10);
      return FirebaseList(query: query).cast<String>();
    } else {
      var query = _database.child('topic').orderByValue().limitToLast(10);
      return FirebaseList(query: query).cast<String>();
    }
  }
}

This is second approach but I can't figure out type casting and sort compare function to work correctly. And this version returns Future<List> instead.

  Future<List<String>> getSuggestion() async {
    return await _database.child('expHash').get().then((snapshot) {
      Map<String, int> map = snapshot.value;   // type error -> <dynamic, dynamic>
      List<String> hashtagList = map.keys.toList();   
      hashtagList.sort((k1, k2) => map[k1]! > map[k2]! ? 1 : -1);   // is this right?
      return hashtagList;
    });
  }

What's the best way to get the result of orderByValue(or orderByKey or orderByChild) as a List or List?

EDIT(Answer)

List<String?> _resultList = [];

var query = _database.child('topic').orderByValue().limitToLast(10);
query.onChildAdded.listen((Event event) {
    _resultList.add(event.snapshot.key);
});
return await query.get().then((snapshot) {return _resultList;});

But this returns Future<List<String>> instead. So I had to use FutureBuilder to process output.

CodePudding user response：

In recent versions of the FlutterFire library for Firebase you should be able to use snapshot.value.values to get an ordered list of the result snapshots of a query.

If that is not working, the previous approach is either Flutter: Firebase Real-Time database orderByChild has no impact on query result or Flutter Firebase Database wrong timestamp order or