class LanguageItem

class LanguageItem extends FieldItemBase implements OptionsProviderInterface {
    
    /**
     * {@inheritdoc}
     */
    public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
        $properties['value'] = DataDefinition::create('string')->setLabel(t('Language code'))
            ->setRequired(TRUE);
        $properties['language'] = DataReferenceDefinition::create('language')->setLabel(t('Language object'))
            ->setDescription(t('The referenced language'))
            ->setComputed(TRUE)
            ->setReadOnly(FALSE);
        return $properties;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function schema(FieldStorageDefinitionInterface $field_definition) {
        return [
            'columns' => [
                'value' => [
                    'type' => 'varchar_ascii',
                    'length' => 12,
                ],
            ],
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function setValue($values, $notify = TRUE) {
        // Treat the values as property value of the language property, if no array
        // is given as this handles language codes and objects.
        if (isset($values) && !is_array($values)) {
            $this->set('language', $values, $notify);
        }
        else {
            // Make sure that the 'language' property gets set as 'value'.
            if (isset($values['value']) && !isset($values['language'])) {
                $values['language'] = $values['value'];
            }
            parent::setValue($values, $notify);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function applyDefaultValue($notify = TRUE) {
        // Default to the site's default language. When language module is enabled,
        // this behavior is configurable, see language_field_info_alter().
        $this->setValue([
            'value' => \Drupal::languageManager()->getDefaultLanguage()
                ->getId(),
        ], $notify);
        return $this;
    }
    
    /**
     * {@inheritdoc}
     */
    public function onChange($property_name, $notify = TRUE) {
        // Make sure that the value and the language property stay in sync.
        if ($property_name == 'value') {
            $this->writePropertyValue('language', $this->value);
        }
        elseif ($property_name == 'language') {
            $this->writePropertyValue('value', $this->get('language')
                ->getTargetIdentifier());
        }
        parent::onChange($property_name, $notify);
    }
    
    /**
     * {@inheritdoc}
     */
    public static function generateSampleValue(FieldDefinitionInterface $field_definition) {
        // Defer to the callback in the item definition as it can be overridden.
        $constraint = $field_definition->getItemDefinition()
            ->getConstraint('ComplexData');
        if (isset($constraint['value']['AllowedValues']['callback'])) {
            $languages = call_user_func($constraint['value']['AllowedValues']['callback']);
        }
        else {
            $languages = array_keys(\Drupal::languageManager()->getLanguages(LanguageInterface::STATE_ALL));
        }
        $values['value'] = $languages[array_rand($languages)];
        return $values;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getPossibleValues(?AccountInterface $account = NULL) {
        return array_keys(\Drupal::languageManager()->getLanguages(LanguageInterface::STATE_ALL));
    }
    
    /**
     * {@inheritdoc}
     */
    public function getPossibleOptions(?AccountInterface $account = NULL) {
        $languages = \Drupal::languageManager()->getLanguages(LanguageInterface::STATE_ALL);
        return array_map(function (LanguageInterface $language) {
            return $language->getName();
        }, $languages);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getSettableValues(?AccountInterface $account = NULL) {
        return $this->getPossibleValues($account);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getSettableOptions(?AccountInterface $account = NULL) {
        return $this->getPossibleValues($account);
    }

}