Joomla 3.X custom field which implements chained dropdown controls with ajax autocomplete capabilities.

basicformfieldlist.php

<?php
/**
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 * @author     Pep Lainez <jlainezs@cloudconceptes.com>
 * @copyright  Copyright (C) 2018 CloudConceptes
 * @license    GNU/GPL https://www.gnu.org/licenses/gpl-3.0.html
 */
defined('JPATH_PLATFORM') or die;

JFormHelper::loadFieldClass('list');

/**
 * Base class.
 * Strictly not needed, but it allows to focus important tasks in descendants.
 */
abstract class BasicFormFieldList extends JFormFieldList
{
	protected $placeholder = false;
	protected $style;

	/**
	 * Gets the data
	 *
	 * @return mixed
	 *
	 * @since version
	 */
	protected function getDbOptions()
	{
		return [];
	}

	/**
	 * Gets the options
	 *
	 * @see JFormFieldList::getOptions()
	 *
	 * @return array
	 */
	protected function getOptions()
	{
		$options = parent::getOptions();
		$dbOptions = $this->getDbOptions();

		if (count($dbOptions))
		{
			foreach ($dbOptions as $dbOption)
			{
				$tmp = JHtml::_('select.option', $dbOption->id, $dbOption->title);
				$options[] = $tmp;
			}
		}

		return $options;
	}

	/**
	 * Method to attach a JForm object to the field.
	 *
	 * @param   \SimpleXMLElement  $element  The SimpleXMLElement object representing the `<field>` tag for the form field object.
	 * @param   mixed              $value    The form field value to validate.
	 * @param   string             $group    The field name group control value. This acts as as an array container for the field.
	 *                                       For example if the field has name="foo" and the group value is set to "bar" then the
	 *                                       full field name would end up being "bar[foo]".
	 *
	 * @return  boolean  True on success.
	 *
	 * @since   11.1
	 */
	public function setup(\SimpleXMLElement $element, $value, $group = null)
	{
		$this->style = (isset($element['style']) ? (string) $element['style'] : '');

		return parent::setup($element, $value, $group); // TODO: Change the autogenerated stub
	}


	/**
	 * Method to get the field input markup.
	 *
	 * @return  string  The field input markup.
	 *
	 * @since   11.1
	 */
	protected function getInput()
	{
		$attr = '';

		// Initialize some field attributes.
		$attr .= !empty($this->class) ? ' class="' . $this->class . '"' : '';
		$attr .= $this->disabled ? ' disabled' : '';
		$attr .= !empty($this->size) ? ' size="' . $this->size . '"' : '';
		$attr .= $this->multiple ? ' multiple' : '';
		$attr .= $this->required ? ' required aria-required="true"' : '';
		$attr .= $this->autofocus ? ' autofocus' : '';
		$attr .= !empty($this->style) ? ' style="' . $this->style . '"' : '';

		if ($this->getAttribute('placeholder'))
		{
			$attr .= ' data-placeholder = "' . JText::_($this->getAttribute('placeholder')) . '"';
		}

		// Initialize JavaScript field attributes.
		$attr .= $this->onchange ? ' onchange="' . $this->onchange . '"' : '';

		// Get the field options.
		$options = $this->getOptions();

		return JHtml::_('select.genericlist', $options, $this->name, trim($attr), 'value', 'text', $this->value, $this->id);
	}
}

chainedlist.php

<?php
/**
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 * @author     Pep Lainez <jlainezs@cloudconceptes.com>
 * @copyright  Copyright (C) 2018 CloudConceptes
 * @license    GNU/GPL https://www.gnu.org/licenses/gpl-3.0.html
 */
defined('JPATH_PLATFORM') or die;

use \Joomla\CMS\Factory;

require_once __DIR__ . '/basicformfieldlist.php';

/**
 * Form Field class for chaining drop downs
 *
 * @since       1.0.1
 */
class FormFieldChainedList extends BasicFormFieldList
{
	protected $placeholder = false;
	protected $watch = '';
	protected $option = '';
	protected $task  = '';
	protected $format;

	/**
	 * Configures the control
	 *
	 * @param SimpleXMLElement $element
	 * @param mixed            $value
	 * @param null             $group
	 *
	 * @return bool
	 *
	 * @since 1.0.1
	 * @throws Exception
	 */
	public function setup(\SimpleXMLElement $element, $value, $group = null)
	{
		$input = Factory::getApplication()->input;
		$this->watch  = ((isset($element['watch']) && !empty($element['watch'])) ? (string) $element['watch'] : '');
		$this->option = (isset($element['option']) ? (string) $element['option'] : $input->get('option'));
		$this->task   = (isset($element['task']) ? (string) $element['task'] : '');
		$this->format = (isset($element['format']) ? (string) $element['format'] : '');

		return parent::setup($element, $value, $group);
	}

	private static $commonScriptsLoaded = false;

	protected function getInput()
	{
		$watch_at = '#' . $this->watch;
		$me = '#' . $this->id;

		if (!self::$commonScriptsLoaded)
		{
			self::$commonScriptsLoaded = true;
			Factory::getDocument()->addScriptDeclaration('
				function chainedlist_updateValues(who, data, fullUpdate)
				{
					jQuery(who).children("option:not(:selected)").remove();
					jQuery(who).prop("selectedIndex", -1);
					for (i=0; i < data.length; i++){
						let option = jQuery("<option>").prop("value", data[i].id).html(data[i].title);
						jQuery(who).append(option);
					}
					let ChosenInputValue = jQuery(who + "_chzn input").val();
					jQuery(who).trigger("liszt:updated");
					jQuery(who + "_chzn input").val(ChosenInputValue);
					jQuery(who + "_chzn").removeClass("chzn-container-single-nosearch");
					jQuery(who + "_chzn input").prop("readonly", false);
				}
				
				function chainedlist_getItems(me, selected_value, option, task, initial_value, like, format, token){
					jQuery.ajax({
						url: "index.php?" + token + "=1",
						data:{
								option        : option,
								task          : task,
								parent_value  : selected_value,
								initial_value : initial_value,
								like          : like,
								format        : format,
							},
						method: "get",
						dataType: "json"						
					})
					.done(function(data, textStatus, xhqr){
							chainedlist_updateValues(me, data);
					});					
				}
				
				function prepareUpdateFor(who){
					jQuery(who).on("change", function(){
						jQuery(document).trigger("update_chained", who);
					});
				}
			');
		}

		$script = '
			jQuery(document).ready(function(){
				let token = "' . JSession::getFormToken() . '";';

		if (!empty($this->watch))
		{
			$script .= '		
				prepareUpdateFor("' . $watch_at . '");				
				
				jQuery(document).on("update_chained", function(e, sender){
					let selected_value = jQuery("' . $watch_at . '").val();
					if (sender !== "' . $watch_at . '") return;
					chainedlist_getItems(
								"' . $me . '",
								selected_value,
								"' . $this->option . '",
								"' . $this->task . '",
								' . (int) $this->value . ',
								"",
								"json",
								token
						);
				});';
		}
		else
		{
			$script .= 'let selected_value = 0;';
		}

		$script .= '
		        let timeOut'.$this->id.' = null;
				jQuery("' . $me . '_chzn input").on("keyup", function(e){
					let like = jQuery(this).val();
					let selected_value = jQuery("' . $watch_at . '").val();
                    clearTimeout(timeOut'.$this->id.');
                    timeOut'.$this->id.' = setTimeout(function(){
					chainedlist_getItems(
								"' . $me . '",
								selected_value,
								"'. $this->option .'",
								"' . $this->task . '",
								' . (int) $this->value . ',
								like,
								"json",
								token
						);
						}, 350);
				});
				jQuery("' . $me . '").chosen({disable_search_threshold : 0}).trigger("liszt:updated");
				jQuery("' . $me . '_chzn").removeClass("chzn-container-single-nosearch");
				jQuery("' . $me . '_chzn input").prop("readonly", false);

			});';
		Factory::getDocument()->addScriptDeclaration($script);

		return parent::getInput();
	}
}

myeditform.xml

<?xml version="1.0" encoding="utf-8"?>
<form>
    <fieldset name="COM_MM_DETAILS" addfieldpath="administrator/components/com_yourcomponent/models/fields">
        <field name="country_id" type="country"
               label="COM_MM_COUNTRY_LABEL"
               description="COM_MM_PROVINCE_COUNTRY_TITLE_DESCR"
               option="com_yourcomponent"
               task="the_controller.ajaxsearch"
               format="json"
               class="inputbox input-medium" required="true">
        </field>

        <field name="province_id" type="province"
               watch="jform_country_id"
               label="COM_MM_PROVINCE_TITLE_LABEL"
               description="COM_MM_CITY_PROVINCE_TITLE_DESCR"
               option="com_yourcomponent"
               task="anothercontroller.ajaxsearch"
               format="json" />

        <field name="city_id" type="city"
               watch="jform_province_id"
               label="COM_MM_CITY_TITLE_LABEL"
               description="COM_MM_CITY_PROVINCE_TITLE_DESCR"
               option="com_yourcomponent"
               task="evenanothercomponent.ajaxsearch"
               format="json" />
   </fielfset>
</form>