Checking selected object in checkbox list

I would like to identify the selected items from a list linked to the firebase and save them to another list in the firebase after clicking save. I tried to save each time the user selected an item, but it didn’t work very well. Does anyone have a suggestion or did something like that?

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:flutter/material.dart';

class CheckBoxList extends StatefulWidget {
  @override
  _CheckBoxListState createState() => _CheckBoxListState();
}

class _CheckBoxListState extends State<CheckBoxList> {

  List<bool> _itens = List();

  _loadingItens(){

    _itens = [];
    _itens.add(false);
    _itens.add(false);
    _itens.add(false);
    _itens.add(false);
    _itens.add(false);

  }

  Future getList() async {
    var firestore = Firestore.instance;

    QuerySnapshot qn = await firestore.collection("adicionais").getDocuments();

    return qn.documents;
  }

  @override
  void initState() {
    _loadingItens();
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("CheckBoxList"),
      ),
      body: Container(
        child: Column(
          children: <Widget>[
            Expanded(
                child: FutureBuilder(
                    future: getList(),
                    // ignore: missing_return
                    builder: (_, snapshot) {
                      if (snapshot.connectionState == ConnectionState.waiting) {
                        return ListView.builder(
                            padding: EdgeInsets.all(0.0),
                            itemCount: 4,
                            itemBuilder: (_, index) {
                              return ListBody(
                                children: <Widget>[Text("Loading...")],
                              );
                            });
                      } else {
                        return ListView.builder(
                            padding: EdgeInsets.all(0.0),
                            itemCount: snapshot.data.length,
                            itemBuilder: (_, index) {
                              String nome = snapshot.data[index].data["nome"];
                              String valor = snapshot.data[index].data["valor"];
                              return CheckboxListTile(
                                  title: Row(
                                    mainAxisAlignment: MainAxisAlignment.spaceBetween,
                                    children: <Widget>[
                                      Text(nome),
                                      Text("\$ $valor")
                                    ],
                                  ),
                                  value: _itens[index],
                                  onChanged: (value) {
                                    setState(() {
                                      _itens[index] = value;
                                    });
                                  });
                            });
                      }
                    }))
          ],
        ),
      ),
      bottomNavigationBar: Container(
        decoration: BoxDecoration(
            color: Colors.white,
            border: Border(
              top: BorderSide(
                color: Colors.grey.shade200,
                width: 1.0,
              ),
            )),
        child: Padding(
            padding: EdgeInsets.only(left:16, right:16, top: 16, bottom: 16),
            child: Row(
              mainAxisAlignment: MainAxisAlignment.center,
              children: <Widget>[
                Padding(
                  padding: EdgeInsets.only(),
                  child: GestureDetector(
                    child: Container(
                      width: (MediaQuery.of(context).size.width * 0.90),
                      height: 60,
                      decoration: BoxDecoration(
                          color: Color(0xff0269BA),
                          borderRadius: BorderRadius.all(Radius.circular(10))),
                      child: Row(
                        mainAxisAlignment: MainAxisAlignment.center,
                        children: <Widget>[
                          Padding(
                              padding: EdgeInsets.only(left: 8, right: 8),
                              child: Text("Save",
                                  style: TextStyle(
                                    color: Colors.white,
                                    fontSize: 18,
                                    fontWeight: FontWeight.w500,
                                  ))),
                        ],
                      ),
                    ),
                    onTap: () {
                    /**/
                    },
                  ),
                )
              ],
            )),
      ),
    );
  }
}