Example Callback function for Ultimate Member dropdown field

example.php

function getCities() {
//get the value from the 'parent' field, sent via the AJAX post.
  $choice = $_POST['parent_option'];

  //Depending on the value of $choice, return a different array.
  switch($choice) {
    case "France":
      $cities = [
        "Paris" =>"Paris",
        "Marseille" => "Marseille",
        "Lyon" => "Lyon"
      ];
      break;
    case "Spain":
      $cities = [
        "Madrid"=>"Madrid",
        "Barcelona"=>"Barcelona"
      ];
      break;
    default:
      //code to do something if other options are not selected (throw an error, or set $cities to a default array)
      $cities = ["no city"];
      
  }
return $cities;

}

In case it helps anyone else coming here to figure this out:

1. The parent and child fields must be “Dropdown” field types; not just any field can provide a parent value

2. If you need to populate a dropdown based on a field that is not a type that supports being a "parent option", you’ll need to write some custom js to capture the input value and assign it to a dummy/hidden single-option dropdown that can be used as the parent - e.g.:

$('#user_email').on( "focusout", function() {
	var domain = $(this).val().replace(/.*@/, "");
	$('select[name="email_domain"]').append($('<option></option>').attr('selected', 'selected').attr('val', domain).text(domain));
	$('select[name="email_domain"]').trigger('change'); // a change to this element is what triggers the ajax call
});

3. The callback to populate the options list will then be able to access the Email Domain parent value - e.g. (in the callback):



$domain = $_POST['parent_option'];


4. 400 error on the POST? I also had to add a kludgy “nopriv” action into my own functions file (based on the auth version in the plugin code) because I was using this in the registration form, when the user is not yet logged in - e.g.:

if(function_exists('UM')) {
    add_action('wp_ajax_nopriv_um_select_options', array(UM()->form(), 'ajax_select_options'));
}