Flutter, when cleaning list that already got added to list, the values are gone

Issue

So i need to sort the elements by their date, so for an example if there are entries and 2 of them have the same date the result will be 3 in the result list.

  Future<List<List<MoodData>>> moodData() async {
    var result = await database
        .ref()
        .child("users/")
        .child(user!.uid)
        .child("moodData")
        .once();
    List<MoodData> x = [];
    List<List<MoodData>> resultdata = [];

    result.snapshot.children.forEach((element) {
      maxID = int.parse(element.key.toString());
      print(element.child("date").value);
      if (x.length != 2) {
        x.add(MoodData(
            id: int.parse(element.key.toString()),
            date: element.child("date").value.toString(),
            moodValue: double.parse(element.child("y_value").value.toString()),
            text: element.child("text").value.toString()));
      } else {
        resultdata.add(x);
        x.clear();
      }
    });
    print(resultdata);
    return resultdata;
  }

The problem is when I am then cleaning the list that got added to the resultdata the resultdata only has empty lists

Solution

When you adding x to resultdata it not produces the copy of x, x just becomes an element of resultdata. Then you have 2 options for accessing x data:

• Using given name x
• Get it from resultdata by index

So when you call x.clear() after resultdata.add(x) it's the same as calling resultdata.last.clear().

The right solution is adding a copy of x([...x]) to resultdata:

resultdata.add([...x]);
x.clear();

Answered By - Sergey Lobanov