class BaseFieldOverride

class BaseFieldOverride extends FieldConfigBase {
    
    /**
     * The base field definition.
     *
     * @var \Drupal\Core\Field\BaseFieldDefinition
     */
    protected $baseFieldDefinition;
    
    /**
     * The original override.
     */
    public BaseFieldOverride $original;
    
    /**
     * Creates a base field override object.
     *
     * @param \Drupal\Core\Field\BaseFieldDefinition $base_field_definition
     *   The base field definition to override.
     * @param string $bundle
     *   The bundle to which the override applies.
     *
     * @return \Drupal\Core\Field\Entity\BaseFieldOverride
     *   A new base field override object.
     */
    public static function createFromBaseFieldDefinition(BaseFieldDefinition $base_field_definition, $bundle) {
        $values = $base_field_definition->toArray();
        $values['bundle'] = $bundle;
        $values['baseFieldDefinition'] = $base_field_definition;
        return \Drupal::entityTypeManager()->getStorage('base_field_override')
            ->create($values);
    }
    
    /**
     * Constructs a BaseFieldOverride object.
     *
     * In most cases, base field override entities are created via
     * BaseFieldOverride::createFromBaseFieldDefinition($definition, 'bundle')
     *
     * @param array $values
     *   An array of base field bundle override properties, keyed by property
     *   name. The field to override is specified by referring to an existing
     *   field with:
     *   - field_name: The field name.
     *   - entity_type: The entity type.
     *   Additionally, a 'bundle' property is required to indicate the entity
     *   bundle to which the bundle field override is attached to. Other array
     *   elements will be used to set the corresponding properties on the class;
     *   see the class property documentation for details.
     * @param string $entity_type
     *   (optional) The type of the entity to create. Defaults to
     *   'base_field_override'.
     *
     * @throws \Drupal\Core\Field\FieldException
     *   Exception thrown if $values does not contain a field_name, entity_type or
     *   bundle value.
     */
    public function __construct(array $values, $entity_type = 'base_field_override') {
        if (empty($values['field_name'])) {
            throw new FieldException('Attempt to create a base field bundle override of a field without a field_name');
        }
        if (empty($values['entity_type'])) {
            throw new FieldException("Attempt to create a base field bundle override of field {$values['field_name']} without an entity_type");
        }
        if (empty($values['bundle'])) {
            throw new FieldException("Attempt to create a base field bundle override of field {$values['field_name']} without a bundle");
        }
        parent::__construct($values, $entity_type);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFieldStorageDefinition() {
        return $this->getBaseFieldDefinition()
            ->getFieldStorageDefinition();
    }
    
    /**
     * {@inheritdoc}
     */
    public function isDisplayConfigurable($context) {
        return $this->getBaseFieldDefinition()
            ->isDisplayConfigurable($context);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getDisplayOptions($display_context) {
        return $this->getBaseFieldDefinition()
            ->getDisplayOptions($display_context);
    }
    
    /**
     * {@inheritdoc}
     */
    public function isReadOnly() {
        return $this->getBaseFieldDefinition()
            ->isReadOnly();
    }
    
    /**
     * {@inheritdoc}
     */
    public function isComputed() {
        return $this->getBaseFieldDefinition()
            ->isComputed();
    }
    
    /**
     * {@inheritdoc}
     */
    public function isInternal() : bool {
        return $this->getBaseFieldDefinition()
            ->isInternal();
    }
    
    /**
     * {@inheritdoc}
     */
    public function getClass() {
        return $this->getBaseFieldDefinition()
            ->getClass();
    }
    
    /**
     * {@inheritdoc}
     */
    public function getUniqueIdentifier() {
        return $this->getBaseFieldDefinition()
            ->getUniqueIdentifier();
    }
    
    /**
     * Gets the base field definition.
     *
     * @return \Drupal\Core\Field\BaseFieldDefinition
     */
    protected function getBaseFieldDefinition() {
        if (!isset($this->baseFieldDefinition)) {
            $fields = \Drupal::service('entity_field.manager')->getBaseFieldDefinitions($this->entity_type);
            $this->baseFieldDefinition = $fields[$this->getName()];
        }
        return $this->baseFieldDefinition;
    }
    
    /**
     * {@inheritdoc}
     *
     * @throws \Drupal\Core\Field\FieldException
     *   If the bundle is being changed.
     */
    public function preSave(EntityStorageInterface $storage) {
        // Filter out unknown settings and make sure all settings are present, so
        // that a complete field definition is passed to the various hooks and
        // written to config.
        $field_type_manager = \Drupal::service('plugin.manager.field.field_type');
        $default_settings = $field_type_manager->getDefaultFieldSettings($this->getType());
        $this->settings = array_intersect_key($this->settings, $default_settings) + $default_settings;
        // Call the parent's presave method to perform validate and calculate
        // dependencies.
        parent::preSave($storage);
        if ($this->isNew()) {
            // @todo This assumes that the previous definition isn't some
            //   non-config-based override, but that might not be the case:
            //   https://www.drupal.org/node/2321071.
            $previous_definition = $this->getBaseFieldDefinition();
        }
        else {
            // Some updates are always disallowed.
            if ($this->entity_type != $this->original->entity_type) {
                throw new FieldException("Cannot change the entity_type of an existing base field bundle override (entity type:{$this->entity_type}, bundle:{$this->original->bundle}, field name: {$this->field_name})");
            }
            if ($this->bundle != $this->original->bundle) {
                throw new FieldException("Cannot change the bundle of an existing base field bundle override (entity type:{$this->entity_type}, bundle:{$this->original->bundle}, field name: {$this->field_name})");
            }
            $previous_definition = $this->original;
        }
        // Notify the entity storage.
        $this->entityTypeManager()
            ->getStorage($this->getTargetEntityTypeId())
            ->onFieldDefinitionUpdate($this, $previous_definition);
    }
    
    /**
     * {@inheritdoc}
     */
    public static function postDelete(EntityStorageInterface $storage, array $field_overrides) {
        $entity_type_manager = \Drupal::entityTypeManager();
        // Clear the cache upfront, to refresh the results of getBundles().
        \Drupal::service('entity_field.manager')->clearCachedFieldDefinitions();
        
        /** @var \Drupal\Core\Field\Entity\BaseFieldOverride $field_override */
        foreach ($field_overrides as $field_override) {
            // Inform the system that the field definition is being updated back to
            // its non-overridden state.
            // @todo This assumes that there isn't a non-config-based override that
            //   we're returning to, but that might not be the case:
            //   https://www.drupal.org/node/2321071.
            $entity_type_manager->getStorage($field_override->getTargetEntityTypeId())
                ->onFieldDefinitionUpdate($field_override->getBaseFieldDefinition(), $field_override);
        }
    }
    
    /**
     * Loads a base field bundle override config entity.
     *
     * @param string $entity_type_id
     *   ID of the entity type.
     * @param string $bundle
     *   Bundle name.
     * @param string $field_name
     *   Name of the field.
     *
     * @return \Drupal\Core\Field\FieldConfigInterface|null
     *   The base field bundle override config entity if one exists for the
     *   provided field name, otherwise NULL.
     */
    public static function loadByName($entity_type_id, $bundle, $field_name) {
        return \Drupal::entityTypeManager()->getStorage('base_field_override')
            ->load($entity_type_id . '.' . $bundle . '.' . $field_name);
    }
    
    /**
     * Implements the magic __sleep() method.
     */
    public function __sleep() {
        // Only serialize necessary properties, excluding those that can be
        // recalculated.
        unset($this->baseFieldDefinition);
        return parent::__sleep();
    }

}