forms - Taxonomy list dependent on choice from another taxonomy list, drupal 8 -

i have taxonomy option list make choose toyota.

1. i want second taxonomy option list models of toyota (eg. corolla, hilux etc...).
2. when choose benz second list contains c-class, ml, etc...

i have created entity vehicle google examples on xampp localhost, windows 10.

in vehicle form i'm able populate first list. second appears empty.

here code. please help:

public function buildform(array $form, formstateinterface $form_state, $params = null) {   $options = array();    $tax = "make";     $terms = \drupal::entitymanager()->getstorage('taxonomy_term')->loadtree($tax, $parent = 0, $max_depth = null, $load_entities = false);   foreach ($terms $term) {     $options[] = $term->name;   }     $form['make'] = array(     '#type' => 'select',     '#title' => t('make'),     'weight' => 0,     '#options' => $options,     '#ajax' => array(       'callback' => [$this, 'changeoptionsajax'],       'wrapper' => 'model_wrapper',     ),   );    $form['model'] = array(     '#type' => 'select',     '#title' => t('model'),     'weight' => 1,     '#options' => $this->getoptions($form_state),     '#prefix' => '<div id="model_wrapper">',     '#suffix' => '</div>',   );    return $form; }  public function getoptions(formstateinterface $form_state) {   $options = array();   if ($form_state->getvalue('make') == "benz") {      $tax="benz";   }   elseif ($form_state->getvalue('make') == "bmw") {     $tax="bmw";   }    elseif ($form_state->getvalue('make') == "toyota") {     $tax="toyota";   }    else {     $tax="title";     // title taxonomy list i'm using default if make not found   }    $terms = \drupal::entitymanager()->getstorage('taxonomy_term')->loadtree($tax, $parent = 0, $max_depth = null, $load_entities = false);   foreach ($terms $term) {     $options[] = $term->name;   }     return $options; }  public function changeoptionsajax(array &$form, formstateinterface $form_state) {   return $form['model']; } 

here give working sample based on example vehiculesform.php:

i took liberty rename variable better readability.

<?php  namespace drupal\example\form;  use drupal\core\form\formbase; use drupal\core\form\formstateinterface; use symfony\component\dependencyinjection\containerinterface; use drupal\core\entity\entitytypemanagerinterface;  /**  * vehiculesform.  */ class vehiculesform extends formbase {   /**    * term storage.    *    * @var \drupal\taxonomy\termstorageinterface    */   protected $termstorage;    /**    * {@inheritdoc}    */   public function __construct(entitytypemanagerinterface $entity) {     $this->termstorage = $entity->getstorage('taxonomy_term');   }    /**    * {@inheritdoc}    */   public static function create(containerinterface $container) {     // instantiates form class.     return new static(     // load service required construct class.     $container->get('entity_type.manager')     );   }    /**    * {@inheritdoc}    */   public function getformid() {     return 'vehicules_form';   }    /**    * {@inheritdoc}    */   public function buildform(array $form, formstateinterface $form_state, $params = null) {     $brands = $this->termstorage->loadtree('make', 0, null, true);     $options = [];     if ($brands) {       foreach ($brands $brand) {         $options[$brand->getname()] = $brand->getname();       }     }     $form['brand'] = array(       '#type'    => 'select',       '#title'   => $this->t('brand'),       '#options' => $options,       '#ajax'    => array(         'callback' => [$this, 'selectmodelsajax'],         'wrapper'  => 'model_wrapper',       ),     );      $form['model'] = array(       '#type'      => 'select',       '#title'     => $this->t('model'),       '#options'   => ['_none' => $this->t('- select brand before -')],       '#prefix'    => '<div id="model_wrapper">',       '#suffix'    => '</div>',       '#validated' => true,     );      $form['actions']['submit'] = [       '#type'  => 'submit',       '#value' => $this->t('send'),     ];      return $form;   }    /**    * {@inheritdoc}    */   public function validateform(array &$form, formstateinterface $form_state) {   }    /**    * {@inheritdoc}    */   public function submitform(array &$form, formstateinterface $form_state) {   }    /**    * called via ajax populate model field according brand.    *    * @param array $form    *   associative array containing structure of form.    * @param \drupal\core\form\formstateinterface $form_state    *   current state of form.    *    * @return array    *   form model field structure.    */   public function selectmodelsajax(array &$form, formstateinterface $form_state) {     $options = [];      $vocabulary = 'title';     switch ($form_state->getvalue('brand')) {       case 'benz':         $vocabulary = 'benz';         break;       case 'bmw':         $vocabulary = 'bmw';         break;       case 'toyota':         $vocabulary = 'toyota';         break;     }      $models = $this->termstorage->loadtree($vocabulary, 0, null, true);     if ($models) {       foreach ($models $model) {         $options[$model->id()] = $model->getname();       }     }     $form['model']['#options'] = $options;      return $form['model'];   } } 

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also, suggest make improvments on code such:

• don't use switch link taxonomies reference fields.
• add validation ensure security (check don't spoof field example) !!
• don't use brandname id. avoid $options[$brand->getname()] = $brand->getname(); , use $options[$brand->id()] = $brand->getname();.

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hope !