I made this thread last week: http://support.advancedcustomfields.com/forums/topic/field-validation-on-saving-a-post/
Since I have not received an answer, I just went ahead and bought ACF Pro to check out the validate_value Filter. The documentation says “This filter allows you to perform custom validation on a field’s value and either allow or prevent the post from being saved.” To me it seems like the validation is only executed on publishing as well, am I missing something here? Also, for the validation I need a radio box to not have any default value, which doesn’t seem to be possible anymore, or is it?
Thanks in advance!
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As for you question, the radio button is not a type of input which can allow a null value. It was a mistake for it to ever allow such behavior.
The acf/validate_value filter can be used on any value to return a customized validation result which is performed on submission of the post and displayed via AJAX.
Does that help?