isValid method
Implementation
bool isValid(String ccNum) {
int qtd = initialValue.toString().length;
if (ccNum.length < qtd) {
return false;
}
int? ccInt = int.tryParse(ccNum.substring(0, qtd));
if (ccInt == null) {
return false;
}
return finalValue == null
? ccInt == initialValue
: ccInt >= initialValue && ccInt <= finalValue!;
}