validate method
Validate the value Return null if the value is valid Return the error message if the value is invalid Return null if the value is empty
Implementation
@override
String? validate() {
double? doubleValue = double.tryParse(_value ?? '');
if (_value == null ||
_value?.trim().isEmpty == true ||
doubleValue == null) {
return null;
} else if (doubleValue > max) {
return "$message ${max.toString()}";
} else {
return null;
}
}