Flutter text form field with validation does not work when deleting a very last character

Question

I have a form with validation. Everything works correctly when adding text. When deleting text, everything is also correct. But if I remove the very last character then the validation event doesn't fire. Why might this be happening? I need to validate the form for empty input, but I cannot do this, because when the last character is removed, the validation event does not fire.

class CalcWidget extends StatefulWidget {
const CalcWidget({
Key? key,
required this.nomenclature,
}) : super(key: key);

final Nomenclature nomenclature;

 @override
_CalcWidgetState createState() => _CalcWidgetState();
}

class _CalcWidgetState extends State<CalcWidget> {


final _formKey = GlobalKey<FormState>();
final _controller = TextEditingController();

@override
Widget build(BuildContext context) {
return Container(
  width: MediaQuery.of(context).size.width,
  child: SingleChildScrollView(
    child: Form(
        key: _formKey,
        child: Padding(
          padding: const EdgeInsets.only(top: 15, left: 10, right: 15),
          child: Column(
            children: [
              TextFormField(
                onChanged: doMath,
                // keyboardType: TextInputType.number,
                inputFormatters: [
                  FilteringTextInputFormatter.allow(RegExp('[0-9.]+')),
                ],
                keyboardType:
                    TextInputType.numberWithOptions(decimal: true),
                controller: _controller,
                decoration: InputDecoration(
                  filled: true,
                ),
                validator: (value) {
              
                  if (value == null || value.isEmpty) {
                    setState(() {
                      _sumValue = "";
                      _discountValue = "—";
                    });
                    return "Empty input";
                  }
                  if (num.parse(value) >
                      num.parse(widget.nomenclature.a011)) {
                    setState(() {
                      _sumValue = "";
                      _discountValue = "—";
                    });

                    return "Error quantity";
                  }

                  return null;
                },
              ),
              Padding(
                padding: const EdgeInsets.only(top: 8),
                child: ListTile(
                  title: Text(_priceName),
                  trailing: Text(_priceValue),
                ),
              ),
              Padding(
                padding: const EdgeInsets.only(top: 8),
                child: ListTile(
                  title: Text("Discount"),
                  trailing: Text(_discountValue),
                ),
              ),
              Padding(
                padding: const EdgeInsets.only(top: 2),
                child: ListTile(
                  title: Text("Sum"),
                  trailing: Text(_sumValue),
                ),
              ),
            ],
          ),
        )),
  ),
);
}

Answer 1

Try adding: autovalidateMode: AutovalidateMode.always to your TextFormField.

(autovalidateMode: AutovalidateMode.onUserInteraction might work too)