class AddCartButton extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return AddCartButtonState();
  }
}
class AddCartButtonState extends State<AddCartButton>{
  TextEditingController myController = TextEditingController();
  bool isValid = false;
  @override
  Widget build(BuildContext context) {
    void _addToCart(){
      showDialog(
        context: context,
        builder: (BuildContext context) {
          return AlertDialog(
            title: Text("QUANTITY"),
            content: Column(
              children: <Widget>[
                TextFormField(
                    controller: myController,
                    decoration: new InputDecoration(labelText: "quantity"),
                    keyboardType: TextInputType.numberWithOptions(decimal: true),
                    inputFormatters: <TextInputFormatter>[],
                    autovalidate: true,
                    validator: (value) {
                      if (value.isEmpty) {
                        isValid  = false;
                        return "the quantity cannot be empty";
                      } else if (double.tryParse(value) == null) {
                        isValid = false;
                        return "the quantity must be valid number";
                      } else {
                        isValid = true;
                        return null;
                      }
                    }
                )
              ],
            ),
            actions: <Widget>[
              FlatButton(
                  disabledTextColor: Colors.grey,
                  child: Text("add"),
                  onPressed: isValid ? () { print("is valid"); }: null
              )
            ],
          );
        },
      );
    }
  }
}