<?php
function ajax_dropdown_search_form_views_exposed_form_alter(&$form, &$form_state, $form_id) {

  if(!empty($form_state['values'])) {
    $form_state['input'] = array_merge($form_state['input'],$form_state['values']);
  }
  if (($form['#id'] == 'views-exposed-form-product-listing-page') || ($form['#id'] == 'views-exposed-form-product-listing-sidebar-page')) {

      /**
       * Build brand dropdown.
       */
      $first_options = array('All'=>'Select Your Brand') + ajax_dropdown_search_entity_query_printer_brand_dropdown();
      // Set default to variable.
      $value_dropdown_first = isset($form_state['input']['field_printer_brand_tid']) ? $form_state['input']['field_printer_brand_tid'] : null;

      $form['field_printer_brand_tid'] = array(
        '#type' => 'select',
        '#options' => $first_options,
        '#default_value' => $value_dropdown_first,
        '#required' => TRUE,
        '#validated' => TRUE,
        '#title' => 'Brand Name',
        '#ajax' => array(
          'event' => 'change',
          'callback' => 'first_ajax_callback',
          'wrapper' => 'second_dropdown',
         ),
      );

      /**
       * Build device type dropdown.
       */
      $second_dropdown = array('All' => 'Select Device Type') + ajax_dropdown_search_get_printer_type_dropdown($value_dropdown_first);
      // Set default to variable.
      $value_dropdown_second = isset($form_state['input']['field_printer_type_tid']) ? $form_state['input']['field_printer_type_tid'] : null;

      $form['field_printer_type_tid'] = array(
        '#type' => 'select',
        '#validated' => TRUE,
        '#required' => TRUE,
        '#DANGEROUS_SKIP_CHECK' => TRUE,
        '#title' => 'Device Type',
        '#prefix' => '<div id="second_dropdown">',
        '#suffix' => '</div>',
        '#options' => $second_dropdown,
        '#default_value' => $value_dropdown_second,
        '#ajax' => array(
          'event' => 'change',
          'callback' => 'second_ajax_callback',
          'wrapper' => 'third_dropdown',
        ),
      );

      // Build printer model dropdown.
      $form['field_printer_model_value'] = array(
        '#type' => 'select',
        '#validated' => TRUE,
        '#required' => TRUE,
        '#DANGEROUS_SKIP_CHECK' => TRUE,
        '#title' => 'Printer Model',
        '#prefix' => '<div id="third_dropdown">',
        '#suffix' => '</div>',
        '#options' => array('All'=>'Select Printer Model') + ajax_dropdown_search_get_printer_dropdown($value_dropdown_first,$value_dropdown_second),
        '#default_value' => isset($form_state['input']['field_printer_model_value']) ? $form_state['input']['field_printer_model_value'] : null,
      );
  }

  return $form;
}

/**
 * [first_ajax_callback description]
 * @param  [type] $form       [description]
 * @param  [type] $form_state [description]
 * @return [type]             [description]
 */
function first_ajax_callback(&$form, &$form_state) {
  $commands = array();
  $form['field_printer_model_value']['#options'] = array('All' => 'Select Printer Model');

  $commands[] = ajax_command_replace("#second_dropdown", drupal_render($form['field_printer_type_tid']));
  $commands[] = ajax_command_replace("#third_dropdown", drupal_render($form['field_printer_model_value']));

  return array('#type' => 'ajax', '#commands' => $commands);
}

/**
 * [second_ajax_callback description]
 * @param  [type] $form       [description]
 * @param  [type] $form_state [description]
 * @return [type]             [description]
 */
function second_ajax_callback(&$form, &$form_state) {
  $commands = array();
  $commands[] = ajax_command_replace("#third_dropdown", drupal_render($form['field_printer_model_value']));

  return array('#type' => 'ajax', '#commands' => $commands);
}

/**
 * Query taxonomy terms printer brands.
 * @return array string taxonomy names
 */
function ajax_dropdown_search_entity_query_printer_brand_dropdown() {
  $taxonomy_query = new EntityFieldQuery;
  $taxonomy_query->entityCondition('entity_type', 'taxonomy_term')
    ->entityCondition('bundle', 'printer_brand')
    ->propertyOrderBy('weight');
  $taxonomy_terms = $taxonomy_query->execute();
  $printer_brand = array();
  foreach ($taxonomy_terms['taxonomy_term'] as $term) {
    $term_load = taxonomy_term_load($term->tid);
    $printer_brand[$term_load->tid] = $term_load->name;
  }

  return $printer_brand;
}

/**
 * Queries and loads nodes of content type printer model.
 * Filter by brand tid.
 * @param  integer  $brand    taxonomy term id
 * @return mixed    printer   model nodes
 */
function ajax_dropdown_search_printer_model_query_filter_by_brand($brand) {
  $query = new EntityFieldQuery();
  $query->entityCondition('entity_type', 'node')
  ->entityCondition('bundle', 'printer_model')
  ->propertyCondition('status', 1)
  ->fieldCondition('field_printer_brand', 'tid', $brand, '=')
  ->propertyOrderBy('title', 'ASC');
  $result = $query->execute();

  if (isset($result['node'])) {
    $array_nid = array_keys($result['node']);
    $nodes = node_load_multiple($array_nid);
  }

  return $nodes;
}

/**
 * Load the type by the printer type taxonomy term id.
 * @param  integer  $brand  taxonomy term id
 * @return array    string  Device type
 */
function ajax_dropdown_search_get_printer_type_dropdown($brand) {
  // Call array of node load objects of content type printer model.
  $printer_models = ajax_dropdown_search_printer_model_query_filter_by_brand($brand);
  $second_dropdown = array();
  foreach($printer_models as $model) {
    $second_dropdown[$model->field_printer_type['und'][0]['tid']] = taxonomy_term_load($model->field_printer_type['und'][0]['tid'])->field_friendly_name['und'][0]['safe_value'];
  }

  return $second_dropdown;
}

/**
 * Queries and loads nodes of content type printer model.
 * Filter by brand and type tid.
 * @param  integer  $brand    taxonomy term id
 * @param  integer  $type     taxonomy term id
 * @return mixed    printer   model nodes
 */
function ajax_dropdown_search_printer_model_query_filter_by_brand_and_type($brand, $type) {
  $query = new EntityFieldQuery();
  $query->entityCondition('entity_type', 'node')
  ->entityCondition('bundle', 'printer_model')
  ->propertyCondition('status', 1)
  ->fieldCondition('field_printer_brand', 'tid', $brand, '=')
  ->fieldCondition('field_printer_type', 'tid', $type, '=')
  ->propertyOrderBy('title', 'ASC');
  $result = $query->execute();

  if (isset($result['node'])) {
    $array_nid = array_keys($result['node']);
    $nodes = node_load_multiple($array_nid);
  }

  return $nodes;
}

/**
 * Load printer based on brand and type values.
 * @param  integer  $brand  taxonomy term id
 * @param  integer  $type   taxonomy term id
 * @return array    string  model name
 */
function ajax_dropdown_search_get_printer_dropdown($brand, $type) {
  $printer_models = ajax_dropdown_search_printer_model_query_filter_by_brand_and_type($brand, $type);
  $printers = array();
  foreach($printer_models as $model) {
    if($brand == 'All' || $type == 'All') {
      break;
    }
    elseif($model->field_printer_brand['und'][0]['tid'] == $brand && $model->field_printer_type['und'][0]['tid']) {
      $printers[$model->field_printer_model['und'][0]['safe_value']] = $model->field_printer_model['und'][0]['safe_value'];
    }
  }

  return $printers;
}