I am trying to create a simple form with a single field. I have a custom constraint created for the validation of a custom field.
I would like to add the same constraint to the form element, instead of duplicating the code from the constraint class in the form validation handler.
namespace Drupal\mymodule\Form;
use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;
/**
 * Provides a My Module form.
 */
class ExampleForm extends FormBase {
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'mymodule_example';
  }
  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $form['message'] = [
      '#type' => 'textarea',
      '#title' => $this->t('Message'),
      '#required' => TRUE,
    ];
    $form['actions'] = [
      '#type' => 'actions',
    ];
    $form['actions']['submit'] = [
      '#type' => 'submit',
      '#value' => $this->t('Send'),
    ];
    return $form;
  }
  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {
    // Check for the possibility if ->addConstraint() can be used here
    if (mb_strlen($form_state->getValue('message')) < 10) {
      $form_state->setErrorByName('name', $this->t('Message should be at least 10 characters.'));
    }
  }
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $this->messenger()->addStatus($this->t('The message has been sent.'));
    $form_state->setRedirect('<front>');
  }
}
Is it possible?