Pass variable to void function in flutter

I want to pass parameter from my dialouge box(Void function) to another void function but getting error

can't be assigned to the parameter type () void flutter

and setState also not working.

Please check my code here:

First function

  void _quantity(BuildContext context, productId,quantity){

    setState(() {
      productId = productId;
      quantity = quantity;
      _quantityController.text = '$quantity';
    });
    var alert = new AlertDialog(
     actions: <Widget>[
        FlatButton(
          child: Text("Save"),
          onPressed: _addtoCart(context, productId)
        )
      ],
    );

    showDialog(context: context,builder: (context) => alert);

  }

Second Function:

void _addtoCart(BuildContext context, productId) {
    print("Quantity: $quantity");
    print("productId: $productId");
    print("data: $data");
  }

Please check screenshot here