Saving data between widgets in Flutter

hi i am making an app for restaurant but i need some help i have a listtilel and i would like to save those values in memory or firebase anywhere i would like to get which topics selected how should i do that basicly i meant i want to get choosen datas from user and i would like to add it orders page if you have any suggestions please let me know thanks

   import 'package:flutter/material.dart';
  import 'package:resat/BurgerListView/data/toppics_model.dart';
  import 'data/toppics.dart';

 class decor extends StatefulWidget {
  @override
  MyAppState createState() {
  return new MyAppState();
   }
   }


 class MyAppState extends State<decor> {

 List ketchup = [
'No',
'light',
'regular',
'extra',
   ];
       List calori = ['0 Cal', '1 Cal', '2 Cal', '3 Cal'];
       var i = 2;


    final List<topics> _topics = categories;

    @override
   Widget build(BuildContext context) {
// TODO: implement build

return Container(

    margin: EdgeInsets.all(6), //space between other listtiles
    decoration: BoxDecoration(

        border: Border.all(color: Colors.grey),
        borderRadius: BorderRadius.all(Radius.circular(8))),

    child: ListTile(

        title: Row(children: [
          Expanded(
              child:
              Text("Ketchup\n" + calori[i], textAlign: TextAlign.center,),),
          Expanded(
            child: (InkWell(
              child: Text("-", textAlign: TextAlign.center),
              onTap: () {
                setState(() {
                  if (i <= 0){
                    i = i;
                  }else
                    i--;
                  print(i);
                });
              },
            )),
          ),
          Expanded(child: Text(ketchup[i], textAlign: TextAlign.center)),
          Expanded(
            child: (InkWell(
              child: Text("+", textAlign: TextAlign.center),
              onTap: () {
                setState(() {
                  if (i > 2) {
                    i = i;
                  } else
                    i++;
                  print(i);
                });
              },
            )),
          ),

        ]),

        leading: Icon(Icons.pie_chart))
);
  }
 }