minLength validation only when not empty

[awhitford]

I only want the minLength validation when a value is specified; an empty field is fine.

It seems that this is arguably redundant:

            validators: [
              FormBuilderValidators.required(),
              FormBuilderValidators.minLength(4),
            ],

I thought I could simply remove the required() validator, but minLength(4) still complains that the empty field is not at least 4 characters.

Maybe the minLength validator could have an optional flag to say whether it should apply to empty fields? (That way, backwards compatibility can be maintained.)

[awhitford]

My workaround is to do this:

              validators: [
                (val) => val.isNotEmpty
                    ? FormBuilderValidators.minLength(4).call(val)
                    : null,
              ],

This is not awful, but I'm open to a more elegant solution.

[danvick]

I only want the minLength validation when a value is specified; an empty field is fine.

It seems that this is arguably redundant:

            validators: [
              FormBuilderValidators.required(),
              FormBuilderValidators.minLength(4),
            ],

I thought I could simply remove the required() validator, but minLength(4) still complains that the empty field is not at least 4 characters.

Maybe the minLength validator could have an optional flag to say whether it should apply to empty fields? (That way, backwards compatibility can be maintained.)

This actually sounds like an elegant suggestion, I'll consider having it added.

[awhitford]

When the field is truly required, adding FormBuilderValidators.required(), provides a better user interface experience. It is better that it says, "field must not be empty", rather than just a "field must be at least 4 characters."

However, when the field is not required, adding the flexibility to FormBuilderValidators.minLength would be nice. Maybe something like:

validators: [
              FormBuilderValidators.minLength(4, allowEmpty: true),
            ],

[awhitford]

I just tried using this and it did not work:

    FormBuilderValidators.minLength(4, allowEmpty: true),

Alas, I get a "Value must have a length greater than or equal to 4." message when the field is blank.

The implementation is presently:

static FormFieldValidator minLength(
  num minLength, {
  bool allowEmpty = false,
  String errorText,
}) {
  return (valueCandidate) {
    if (allowEmpty && (valueCandidate == null || valueCandidate?.length == 0))
      return errorText ??
          "Value must have a length greater than or equal to $minLength";
    if (valueCandidate != null && valueCandidate.length < minLength) {
      return errorText ??
          "Value must have a length greater than or equal to $minLength";
    }
    return null;
  };
}

May I recommend this alternative:

static FormFieldValidator minLength(
  num minLength, {
  bool allowEmpty = false,
  String errorText,
}) {
  return (valueCandidate) {
    final valueLength = valueCandidate?.length ?? 0;
    if (valueLength < minLength && (!allowEmpty || valueLength > 0)) {
      return errorText ??
          "Value must have a length greater than or equal to $minLength";
    }
    return null;
  };
}

[danvick]

HI @awhitford,
I now see the mistake I made.

Could you be kind enough to submit a PR for this?

[awhitford]

Note that I see that you added allowEmpty to the maxLength validator too, but that is not necessary because an empty value will not trigger a max length validation.

[awhitford]

PR has been submitted. However, please note that I was unable to completely validate it because the build is broken:

../lib/src/fields/form_builder_phone_field.dart:181:11: Error: No named parameter with the name 'visualDensity'.

I assume that this is work in progress.

[awhitford]

Confirmed that this now works great with version 3.10.0. Thank you!