Validation with hook_form_alter()

Question

In Drupal 8, for the Comment entity, I want to validate an integer field (stored as field_nieuwe_waarde_value). In each comment posted for a node, it needs to be greater than the value for the previous comment.

The code doesn't give any errors, but the validation doesn't work.

use Drupal\Core\Form\FormStateInterface;

       /**
        * Implements hook_form_alter().
        */

       function waarde_validatie_form_alter(&$form, $form_state, $form_id) {
         if ($form_id == 'comment_waarde_van_dit_voorwerp__form'){
           $form['#validate'][] = 'start_drupal_waarde_voorwerp_form_validate';
         }  
       }

       /**
        * Validation form for the waarde_voorwerp form.
        *
        * Check if the value is > then the last submitted value
        */
       function start_drupal_waarde_voorwerp_form_validate($form, FormStateInterface &$form_state) {
         // Look for the just submitted value.
         $nieuwste_waarde = $form_state->getValue('field_nieuwe_waarde'); 
         // Look for all early submitted values.
         $alle_waarden = $form_state->getUserInput('field_nieuwe_waarde');
         // Search for the last submitted value.
         $laatste_bod = end($alle_waarden); 
         if ($nieuwste_waarde <= $laatste_waarde) {
           $form_state->setErrorByName( t('Gelieve hoger dan'), $laatste_waarde, t('in te stellen'));
         } 
       }

Answer 1

Adding a validation handler in hook_form_alter() is the Drupal 7 method of adding validation for a field. It will still work in Drupal 8, however the preferred method of adding validation to Field API Field in Drupal 8 is to add a Constraint to the field. Constraints will be applied when the Node is created, no matter the means of creation. Adding a validation handler to the form only validates the field when the node is created using the Form API - IE through a browser. If a node is created by any other means - IE through an API or programatically, the validation is not applied. Constraints are applied no matter the method of creating the node.

Documentation on how to add a constraint can be seen here: https://www.drupal.org/docs/8/api/entity-validation-api/providing-a-custom-validation-constraint

See Method 2 - adding constraints to fields added to an entity you haven't defined, or fields added through the UI

Answer 2

Based in the second comment "The function is working and validating. Its printing after validating, but it is not getting the values from getUserInput", we can conclude that the problem is about the getUserInput returned values. This is the only answer we can give to the question.

Maybe the given form fields, used as arguments, are not set? We cannot check that for you unless you put here also the form definition.

PS: $form_state->setErrorByName() receives only 2 arguments. But validation should still work even with these wrong arguments.