Why doesn't the AJAX callback call the class contructor?

Question

I've got a Form class, the constructor of which sets a private attribute for the Model. When the form is rendered, everything seems to work as expected. The last form element is a select which, based upon the selected value, adds another textfield to the form if needed. However, the problem is, when the ajax is called for the first time, everything is OK and the additional field is added. When I try to select a different value though, I get this error as if the constructor did not run thus not setting the $model attribute of the class.

js_zfg1OcYBPiNuiGY77hX8zdIjztYiO7UAz7wbokItO2I.js:2515 Uncaught AjaxError: An AJAX HTTP error occurred. HTTP Result Code: 200
Debugging information follows.
Path: /admin/asphira/contacts/add?ajax_form=1&_wrapper_format=drupal_ajax
StatusText: OK
ResponseText: Error: Call to a member function getKeyedList() on null in
Drupal\asphira\Form\ContactForm->buildForm() (line 112 of C:\xampp\htdocs\reality\modules\asphira\src\Form\ContactForm.php).

Here is the form class.

namespace Drupal\asphira\Form;

use Drupal\asphira\Model\ContactTypeModel;
use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Locale\CountryManager;

class ContactForm extends FormBase {

  private $model;

  public function __construct() {
    $this->model = new ContactTypeModel();
  }

  public function getFormId() {
    return 'contact_form';
  }

  public function buildForm(array $form, FormStateInterface $form_state) {
    $form['name'] = array(
      '#title' => t('Name'),
      '#type' => 'textfield',
      '#required' => TRUE,
    );

    $form['surname'] = array(
      '#title' => t('Surname'),
      '#type' => 'textfield',
      '#required' => TRUE,
    );

    $form['nee_surname'] = array(
      '#title' => t('Née surname'),
      '#type' => 'textfield',
      '#required' => TRUE,
    );

    $form['street'] = array(
      '#title' => t('Street'),
      '#type' => 'textfield',
      '#required' => TRUE,
    );

    $form['street_no'] = array(
      '#title' => t('Street no.'),
      '#type' => 'textfield',
      '#required' => TRUE,
    );

    $form['zip_code'] = array(
      '#title' => t('ZIP code'),
      '#type' => 'textfield',
      '#required' => TRUE,
    );

    $form['city'] = array(
      '#title' => t('City'),
      '#type' => 'textfield',
      '#required' => TRUE,
    );

    $form['country'] = array(
      '#title' => t('Country'),
      '#type' => 'select',
      '#options' => CountryManager::getStandardList(),
      '#required' => TRUE,
    );

    $form['phone'] = array(
      '#title' => t('Telephone'),
      '#type' => 'textfield',
    );

    $form['email'] = array(
      '#title' => t('Email'),
      '#type' => 'email',
      '#required' => TRUE,
    );

    $form['contact_type_container'] = array(
      '#type' => 'container',
      '#attributes' => array(
        'id' => 'contact-type-container',
      ),
    );

    $contact_types = $this->model->getKeyedList('t_contact_type',
      'contact_type_id', 'title');

    $form['contact_type_container']['contact_type'] = array(
      '#title' => t('Contact type'),
      '#type' => 'select',
      '#options' => $contact_types,
      '#required' => TRUE,
      '#ajax' => array(
        'callback' => array($this, 'ajaxSelectContactType'),
        'wrapper' => 'contact-type-container',
        'progress' => array(
          'type' => 'throbber',
          'message' => $this->t('Working...'),
        ),
      )
    );

    $form['actions'] = array('#type' => 'actions');
    $form['actions']['submit'] = array(
      '#type' => 'submit',
      '#value' => t('Save'),
    );

    return $form;
  }

  public function submitForm(array &$form, FormStateInterface $form_state) {
    // TODO: Implement submitForm() method.
  }

  public function ajaxSelectContactType(array &$form, FormStateInterface   $form_state) {
    $contact_type = $form_state->getValue('contact_type');
    $needs_specification = $this->model->needsFurtherSpeification($contact_type);

    if ($needs_specification) {
      $form['contact_type_container']['contact_type_specification'] = array(
        '#title' => t('Specification'),
        '#type' => 'textfield',
        '#description' => '',
        '#required' => TRUE,
      );
    }

    return $form['contact_type_container'];
  }

}

Do you have any idea how is it possible that the constructor gets passed by without calling?

Answer 1

Your problem is actually something different entirely.

It is your use of private. Don't use it, use protected. Drupal has a special logic to prevent services from being serialized but it doesn't work properly on private properties, as they are invisible to the trait that is used for that.

Also, like in a similar question recently, sounds like you're re-inventing the wheel a bit. Drupal uses entities for storage of most things, instead of making your own model classes, define it as a content or config entity, which will give you a ton of things, like storage and standard entity form classes that have $this->entity. For content entities, you can also easily use base fields and widget/formatter definitions to automatically build the form and output, at least to a certain degree.

The documentation is still very much in progress, but see https://www.drupal.org/developing/api/entity and some of the sub-points, also the examples module. there are also plenty of tutorials.

Answer 2

The code is missing the most important method a class that derives from FormBase should implement: create(). That is the static method that instantiate any class derived from FormBase. Take ActionAdminManageForm::create() as example.

public static function create(ContainerInterface $container) {
  return new static(
  $container->get('plugin.manager.action')
  );
}

Its constructor is then the following one.

public function __construct(ActionManager $manager) {
  $this->manager = $manager;
}