class for dropdown taxonomy parents & children

dropdown-tax.php

<?php 
class dropdown_taxonomy_class {
    public $tax = "adresati";

    public function the_tax_hierarchy( $tax, $parent = '', $args = array( 'hide_empty' => false ) ) {
			// taxonomy name or slug
        $defaults = array(
            'parent'     => $parent,
            'hide_empty' => false
        );
        $merge_args = wp_parse_args( $args, $defaults );
        // get all direct decendants of the $parent
        $terms = get_terms( $this->tax, $merge_args );
        // prepare a new array.  these are the children of $parent
        // we'll ultimately copy all the $terms into this new array, but only after they
        // find their own children
        $children = array();
        // go through all the direct decendants of $parent, and gather their children
        foreach ( $terms as $term ) {
            // recurse to get the direct decendants of "this" term
            $term->children = $this->the_tax_hierarchy( $this->tax, $term->term_id );
            // add the term to our new array
            $children[ $term->term_id ] = $term;
        }

        // send the results back to the caller
        return $children;
    }


public function custom_select_fields() {
	//This gets top layer terms only.  This is done by setting parent to 0.
    $categories = $this->the_tax_hierarchy( $this->tax );
		$query_obj = get_queried_object();
    $parent_category = $query_obj->parent_category;
    $child_category = $query_obj->child_category;

    $parent_has_children = ! empty( $parent_category ) && $categories[ $parent_category ] && ! empty( $categories[ $parent_category ]->children );

    // Creative way to use wp_localize_script which creates a JS variable from array
    // You should actually change this to load your JavaScript file and move JS below to that file
    wp_register_script( 'custom_select_fields', '' );
    wp_localize_script( 'custom_select_fields', 'slcustom_categories', $categories );
    wp_enqueue_script( 'custom_select_fields' );
    ?>
    <h3 id="temppp">Select a parent tax</h3>
    <table class="form-table">
        <tbody>
            <tr>
                <th>
                    Parent
                </th>
                <td>
			 <div uk-form-custom="target: > * > span:first-child">
                    <select name="parent_category" id="parent_category">
                    <?php
                        foreach( (array) $categories as $term_id => $cat ){
                            ?>
                            <option value="<?php echo esc_attr( $term_id ) ?>"<?php echo selected( $parent_category, $term_id ); ?>><?php echo $cat->name; ?></option>

                            <?php
                        }
                    ?>
                    </select>
										<button class="uk-button uk-button-default" type="button" tabindex="-1">
												<span></span>
												<span uk-icon="icon: pencil"></span>
										</button>
			</div>
                </td>
            </tr>
            <tr id="child_category_row" style="<?php if( ! $parent_has_children ){ echo 'display: none;'; }?>">
                <th>
                    Child
                </th>
                <td>

                    <select class="uk-select" name="child_category" id="child_category">
                        <?php
                            if( $parent_has_children ){
                                foreach( (array) $categories[$parent_category]->children as $c_term_id => $child ){
                                    ?>
                                    <option value="<?php echo esc_attr( $c_term_id ) ?>"<?php echo selected( $child_category, $c_term_id ); ?>>
																			<?php echo $child->name; ?>
																		</option>
                                    <?php
                                }
                            }
                        ?>
                    </select>
                </td>
            </tr>
        </tbody>
    </table>
    <?php
}

}
?>

<script>

// js you can move it to your main script file
	
jQuery( function($){
    // slcustom_categories var should be available here
    $('#parent_category').change( function(e){
        var child_cat_select = $( '#child_category' );
        var term_id = $(this).val();

        console.log( 'Parent Category Changed', term_id );

        // Remove any existing
        child_cat_select.find( 'option' ).remove();

        // Loop through children and add to children dropdown
        if( slcustom_categories && slcustom_categories[ term_id ] && slcustom_categories[ term_id ]['children'] ){
            console.log( 'Adding Children', slcustom_categories[ term_id ]['children'] );
            $.each( slcustom_categories[term_id]['children'], function( i, v ){
                console.log( 'Adding Child: ', v );
                child_cat_select.append( '<option value="' + v['term_id'] + '">' + v[ 'name' ] + '</option>');
            });

            // Show if child cats
            $( '#child_category_row' ).show();
        } else {
            // Hide if no child cats
            $( '#child_category_row' ).hide();
        }
    });

    // Trigger change on initial page load to load child categories
    $('#parent_category').change();
});
</script>

and then we use in taxonomy-adresati.php
<?php // the header
get_header();

// get the class of dropdown taxonomy
get_template_part( 'dropdown-tax' );

// create a new instance for the class
$custom = new dropdown_taxonomy_class();

// turn on the function of fields
$custom->custom_select_fields();

// our normal footer
get_footer(); ?>