isValid method
Implementation
bool isValid(NsgDataItem item) {
if (isEmpty) return true;
var r = logicalOperator == NsgLogicalOperator.and;
for (var param in paramList) {
if (logicalOperator == NsgLogicalOperator.and) {
r &= param.isValid(item);
} else {
r |= param.isValid(item);
}
}
return r;
}