The onBeforeValidate handler is used for when an editor saves a piece of content, but before LiveWhale validates the data, you can perform your own custom validation.

(Note, the $id variable will only be populated when saving an existing piece of content. When creating new content, it will be blank.)

To add a validation check, look to the $_LW->POST array for the submitted values, and add error messages to the $_LW->REGISTERED_MESSAGES['failure'] array as needed.

<?php
    $_LW->REGISTERED_APPS['my_app']=[
       'title'=>'My App',
       'handlers'=>['onBeforeValidate'],
    ];

    class LiveWhaleApplicationMyApp {

      public function onBeforeValidate($data_type, $id) {
      global $_LW;

      // if saving an event from a backend editor
      if ($data_type=='events' && ($_LW->page=='events_edit' || $_LW->page=='events_sub_edit')) { 
        if (empty($_LW->_POST['images'])) { // require an image
          $_LW->REGISTERED_MESSAGES['failure'][]='You must attach an image to your event.';
        };
      };

      }

   }
?>