int _n = 0; //counter variable

  void add(setState) {
    setState(() {
      _n++;
    });
  }

  void minus(setState) {
    setState(() {
      if (_n != 0) _n--;
    });
  }

  void confirmBox() {
    showDialog(
        context: context,
        builder: (BuildContext context) {
          return StatefulBuilder(builder: (context, StateSetter setState) {
            return Container(
              child: Scaffold(
                body: Column(
                  children: [
                    Center(
                      child: Column(
                        children: [
                          FloatingActionButton(
                            onPressed: () => add(setState),
                            child: Icon(
                              Icons.add,
                              color: Colors.black,
                            ),
                            backgroundColor: Colors.white,
                          ),
                          Text("$_n", //_n value is not updating yet
                              style: TextStyle(fontSize: 60.0)),
                          FloatingActionButton(
                            onPressed: () => minus(setState),
                            child: Icon(
                                const IconData(0xe15b,
                                    fontFamily: 'MaterialIcons'),
                                color: Colors.black),
                            backgroundColor: Colors.white,
                          ),
                        ],
                      ),
                    ),
                  ],
                ),
              ),
            );
          });
        });
  }