Taxonomy Term Vocabulary Exists Validation

gistfile1.txt

<?php

namespace Drupal\taxonomy\Plugin\Validation\Constraint;

use Symfony\Component\Validator\Constraint;

/**
 * Checks that the submitted value is a existing vocabulary.
 *
 * @Constraint(
 *   id = "VocabularyExists",
 *   label = @Translation("Vocabulary Exists", context = "Validation"),
 *   type = {"entity"}
 * )
 */
class VocabularyExists extends Constraint {
  // The message that will be shown if the vocabulary does not exist.
  public $message = '%vocabulary does not exist';
}

gistfile2.txt

<?php

namespace Drupal\taxonomy\Plugin\Validation\Constraint;

use Symfony\Component\Validator\Constraint;
use Symfony\Component\Validator\ConstraintValidator;

/**
 * Validates the VocabularyExists constraint.
 */
class VocabularyExistsValidator extends ConstraintValidator {

  /**
   * {@inheritdoc}
   */
  public function validate($items, Constraint $constraint) {
    foreach ($items as $item) {
      // Check if the supplied vocabulary exists.
      if (!$this->exists($item->value)) {
        $this->context->addViolation($constraint->message, ['%value' => $item->value]);
      }
    }
  }

  /**
   * Vocabulary exists?
   *
   * @param string $value
   */
  private function exists($value) {
    return FALSE;
  }

}

Term.php

/**
   * {@inheritdoc}
   */
  public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
    /** @var \Drupal\Core\Field\BaseFieldDefinition[] $fields */
    $fields = parent::baseFieldDefinitions($entity_type);

    $fields['tid']->setLabel(t('Term ID'))
      ->setDescription(t('The term ID.'));

    $fields['uuid']->setDescription(t('The term UUID.'));

    $fields['vid']->setLabel(t('Vocabulary'))
      ->setDescription(t('The vocabulary to which the term is assigned.'))
      ->addPropertyConstraints('value', [
        'VocabularyExists'
      ]);