<?php

    new my_acf_extension();

    class my_acf_extension
    {
        public function __construct()
        {
            // state field on city
            add_action('acf/load_field/key=field_579376941cecc', array($this, 'load_state_field_choices'));
            // state field on post
            add_action('acf/load_field/key=field_579376f522130', array($this, 'load_state_field_choices'));
            // city field on post
            add_action('acf/load_field/key=field_5793770922131', array($this, 'load_city_field_choices'));
            // ajax action for loading city choices
            add_action('wp_ajax_load_city_field_choices', array($this, 'ajax_load_city_field_choices'));
            // enqueue js extension for acf
            // do this when ACF in enqueuing scripts
            add_action('acf/input/admin_enqueue_scripts', array($this, 'enqueue_script'));
        } // end public function __construct

        public function load_state_field_choices($field)
        {
            // this funciton dynamically loads the state select field choices
            // from the state post type

            // I only want to do this on Posts and the city CPT
            global $post;
            if (!$post ||
                !isset($post->ID) ||
                (get_post_type($post->ID) != 'post' && get_post_type($post->ID) != 'city')) {
                return $field;
            }

            // get states
            $args = array(
                'post_type' => 'state',
                'posts_per_page' => -1,
                'orderby' => 'title',
                'order' => 'ASC'
            );
            $query = new WP_Query($args);
            $choices = array('' => '-- State --');
            if (count($query->posts)) {
                // populate choices
                foreach ($query->posts as $state) {
                    $choices[$state->ID] = $state->post_title;
                }
            }
            $field['choices'] = $choices;
            return $field;
        } // end public function load_state_field_choices

        public function load_city_field_choices($field)
        {
            // this function dynamically loads city field choices
            // based on the currently saved state

            // I only want to do this on Posts
            global $post;
            if (!$post ||
                !isset($post->ID) ||
                get_post_type($post->ID) != 'post') {
                return $field;
            }
            // get the state post id
            // I generally use get_post_meta() instead of get_field()
            // when building functionality, but get_field() could be
            // subsitited here
            $state = intval(get_post_meta($post->ID, 'state', true));
            $cities = $this->get_cities($state);
            $field['choices'] = $cities;
            return $field;
        } // end public funciton load_city_field_choices

        public function enqueue_script()
        {
            // enqueue acf extenstion

            // only enqueue the script on the post page where it needs to run
            /* *** THIS IS IMPORTANT
                   ACF uses the same scripts as well as the same field identification
                   markup (the data-key attribute) if the ACF field group editor
                   because of this, if you load and run your custom javascript on
                   the field group editor page it can have unintended side effects
                   on this page. It is important to alway make sure you're only
                   loading scripts where you need them.
            */
            global $post;
            if (!$post ||
                !isset($post->ID) ||
                get_post_type($post->ID) != 'post') {
                return;
            }

            $handle = 'my-acf-extension';

            // I'm using this method to set the src because
            // I don't know where this file will be located
            // you should alter this to use the correct fundtions
            // to set the src value to point to the javascript file
            $src = '/'.str_replace(ABSPATH, '', dirname(__FILE__)).'/my-acf-extension.js';
            // make this script dependent on acf-input
            $depends = array('acf-input');

            wp_enqueue_script($handle, $src, $depends);
        } // end public function enqueue_script

        public function ajax_load_city_field_choices()
        {
            // this funtion is called by AJAX to load cities
            // based on state selecteion

            // we can use the acf nonce to verify
            if (!wp_verify_nonce($_POST['nonce'], 'acf_nonce')) {
                die();
            }
            $state = 0;
            if (isset($_POST['state'])) {
                $state = intval($_POST['state']);
            }
            $cities = $this->get_cities($state);
            $choices = array();
            foreach ($cities as $value => $label) {
                $choices[] = array('value' => $value, 'label' => $label);
            }
            echo json_encode($choices);
            exit;
        } // end public function ajax_load_city_field_choices

        private function get_cities($post_id)
        {
            // $post_id is post ID of state post
            // get all cities related to the state
            $args = array(
                'post_type' => 'city',
                'posts_per_page' => -1,
                'orderby' => 'title',
                'order' => 'ASC',
                'meta_query' => array(
                    array(
                        'key' => 'state',
                        'value' => $post_id          // [if acf supports only one value]
                      //  'value' => '"'.$post_id.'"',  // [if acf supports multiple values]
                      //  'compare' => 'LIKE'           // [if acf supports multiple values]
                    )
                )
            );
            $query = new WP_Query($args);
            $choices = array('' => '-- City --');
            if (count($query->posts)) {
                // populate choices
                foreach ($query->posts as $post) {
                    $choices[$post->ID] = $post->post_title;
                }
            }
            return $choices;
        } // end private function get_cities
    } // end class my_acf_extension