isNegative method

FieldValidator<T, num> isNegative([
  1. MessageCallBack<num>? message
])

check if the value is negative

Implementation

FieldValidator<T, num> isNegative([MessageCallBack<num>? message]) {
  return next((messages, value) {
    if (!value.isNegative) {
      return message?.call(messages, value) ?? messages.isNegative;
    }
    return null;
  });
}