interface FieldItemInterface

interface FieldItemInterface extends ComplexDataInterface {
    
    /**
     * Defines field item properties.
     *
     * Properties that are required to constitute a valid, non-empty item should
     * be denoted with \Drupal\Core\TypedData\DataDefinition::setRequired().
     *
     * @return \Drupal\Core\TypedData\DataDefinitionInterface[]
     *   An array of property definitions of contained properties, keyed by
     *   property name.
     *
     * @see \Drupal\Core\Field\BaseFieldDefinition
     */
    public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition);
    
    /**
     * Returns the name of the main property, if any.
     *
     * Some field items consist mainly of one main property, e.g. the value of a
     * text field or the target_id of an entity reference. If the field item has
     * no main property, the method returns NULL.
     *
     * @return string|null
     *   The name of the value property, or NULL if there is none.
     *
     * @see \Drupal\Core\Field\BaseFieldDefinition
     */
    public static function mainPropertyName();
    
    /**
     * Returns the schema for the field.
     *
     * This method is static because the field schema information is needed on
     * creation of the field. FieldItemInterface objects instantiated at that
     * time are not reliable as field settings might be missing.
     *
     * Computed fields having no schema should return an empty array.
     *
     * @param \Drupal\Core\Field\FieldStorageDefinitionInterface $field_definition
     *   The field definition.
     *
     * @return array
     *   An empty array if there is no schema, or an associative array with the
     *   following key/value pairs:
     *   - columns: An array of Schema API column specifications, keyed by column
     *     name. The columns need to be a subset of the properties defined in
     *     propertyDefinitions(). The 'not null' property is ignored if present,
     *     as it is determined automatically by the storage controller depending
     *     on the table layout and the property definitions. It is recommended to
     *     avoid having the column definitions depend on field settings when
     *     possible. No assumptions should be made on how storage engines
     *     internally use the original column name to structure their storage.
     *   - unique keys: (optional) An array of Schema API unique key definitions.
     *     Only columns that appear in the 'columns' array are allowed.
     *   - indexes: (optional) An array of Schema API index definitions. Only
     *     columns that appear in the 'columns' array are allowed. Those indexes
     *     will be used as default indexes. Field definitions can specify
     *     additional indexes or, at their own risk, modify the default indexes
     *     specified by the field-type module. Some storage engines might not
     *     support indexes.
     *   - foreign keys: (optional) An array of Schema API foreign key
     *     definitions. Note, however, that the field data is not necessarily
     *     stored in SQL. Also, the possible usage is limited, as you cannot
     *     specify another field as related, only existing SQL tables,
     *     such as {taxonomy_term_data}.
     */
    public static function schema(FieldStorageDefinitionInterface $field_definition);
    
    /**
     * Gets the entity that field belongs to.
     *
     * @return \Drupal\Core\Entity\FieldableEntityInterface
     *   The entity object.
     */
    public function getEntity();
    
    /**
     * Gets the langcode of the field values held in the object.
     *
     * @return string
     *   The langcode.
     */
    public function getLangcode();
    
    /**
     * Gets the field definition.
     *
     * @return \Drupal\Core\Field\FieldDefinitionInterface
     *   The field definition.
     */
    public function getFieldDefinition();
    
    /**
     * Magic method: Gets a property value.
     *
     * @param string $property_name
     *   The name of the property to get; e.g., 'title' or 'name'.
     *
     * @return mixed
     *   The property value.
     *
     * @throws \InvalidArgumentException
     *   If a not existing property is accessed.
     */
    public function __get($property_name);
    
    /**
     * Magic method: Sets a property value.
     *
     * @param string $property_name
     *   The name of the property to set; e.g., 'title' or 'name'.
     * @param mixed $value
     *   The value to set, or NULL to unset the property. Optionally, a typed
     *   data object implementing Drupal\Core\TypedData\TypedDataInterface may be
     *   passed instead of a plain value.
     *
     * @throws \InvalidArgumentException
     *   If a not existing property is set.
     */
    public function __set($property_name, $value);
    
    /**
     * Magic method: Determines whether a property is set.
     *
     * @param string $property_name
     *   The name of the property to get; e.g., 'title' or 'name'.
     *
     * @return bool
     *   Returns TRUE if the property exists and is set, FALSE otherwise.
     */
    public function __isset($property_name);
    
    /**
     * Magic method: Unsets a property.
     *
     * @param string $property_name
     *   The name of the property to get; e.g., 'title' or 'name'.
     */
    public function __unset($property_name);
    
    /**
     * Returns a renderable array for a single field item.
     *
     * @param string|array $display_options
     *   Can be either the name of a view mode, or an array of display settings.
     *   See EntityViewBuilderInterface::viewField() for more information.
     *
     * @return array
     *   A renderable array for the field item.
     *
     * @see \Drupal\Core\Entity\EntityViewBuilderInterface::viewField()
     * @see \Drupal\Core\Entity\EntityViewBuilderInterface::viewFieldItem()
     * @see \Drupal\Core\Field\FieldItemListInterface::view()
     */
    public function view($display_options = []);
    
    /**
     * Defines custom presave behavior for field values.
     *
     * This method is called during the process of saving an entity, just before
     * values are written into storage. When storing a new entity, its identifier
     * will not be available yet. This should be used to massage item property
     * values or perform any other operation that needs to happen before values
     * are stored. For instance this is the proper phase to auto-create a new
     * entity for an entity reference field item, because this way it will be
     * possible to store the referenced entity identifier.
     */
    public function preSave();
    
    /**
     * Defines custom post-save behavior for field values.
     *
     * This method is called during the process of saving an entity, just after
     * values are written into storage. This is useful mostly when the business
     * logic to be implemented always requires the entity identifier, even when
     * storing a new entity. For instance, when implementing circular entity
     * references, the referenced entity will be created on pre-save with a dummy
     * value for the referring entity identifier, which will be updated with the
     * actual one on post-save.
     *
     * In the rare cases where item properties depend on the entity identifier,
     * massaging logic will have to be implemented on post-save and returning TRUE
     * will allow them to be rewritten to the storage with the updated values.
     *
     * @param bool $update
     *   Specifies whether the entity is being updated or created.
     *
     * @return bool
     *   Whether field items should be rewritten to the storage as a consequence
     *   of the logic implemented by the custom behavior.
     */
    public function postSave($update);
    
    /**
     * Defines custom delete behavior for field values.
     *
     * This method is called during the process of deleting an entity, just before
     * values are deleted from storage.
     */
    public function delete();
    
    /**
     * Defines custom revision delete behavior for field values.
     *
     * This method is called from during the process of deleting an entity
     * revision, just before the field values are deleted from storage. It is only
     * called for entity types that support revisioning.
     */
    public function deleteRevision();
    
    /**
     * Generates placeholder field values.
     *
     * Useful when populating site with placeholder content during site building
     * or profiling.
     *
     * @param \Drupal\Core\Field\FieldDefinitionInterface $field_definition
     *   The field definition.
     *
     * @return array
     *   An associative array of values.
     */
    public static function generateSampleValue(FieldDefinitionInterface $field_definition);
    
    /**
     * Defines the storage-level settings for this plugin.
     *
     * @return array
     *   A list of default settings, keyed by the setting name.
     */
    public static function defaultStorageSettings();
    
    /**
     * Defines the field-level settings for this plugin.
     *
     * @return array
     *   A list of default settings, keyed by the setting name.
     */
    public static function defaultFieldSettings();
    
    /**
     * Returns a settings array that can be stored as a configuration value.
     *
     * For all use cases where field settings are stored and managed as
     * configuration, this method is used to map from the field type's
     * representation of its settings to a representation compatible with
     * deployable configuration. This includes:
     * - Array keys at any depth must not contain a ".".
     * - Ideally, array keys at any depth are either numeric or can be enumerated
     *   as a "mapping" within the configuration schema. While not strictly
     *   required, this simplifies configuration translation UIs, configuration
     *   migrations between Drupal versions, and other use cases.
     * - To support configuration deployments, references to content entities
     *   must use UUIDs rather than local IDs.
     *
     * An example of a conversion between representations might be an
     * "allowed_values" setting that's structured by the field type as a
     * \Drupal\Core\TypedData\OptionsProviderInterface::getPossibleOptions()
     * result (i.e., values as keys and labels as values). For such a use case,
     * in order to comply with the above, this method could convert that
     * representation to a numerically indexed array whose values are sub-arrays
     * with the schema definable keys of "value" and "label".
     *
     * @param array $settings
     *   The field's settings in the field type's canonical representation.
     *
     * @return array
     *   An array (either the unmodified $settings or a modified representation)
     *   that is suitable for storing as a deployable configuration value.
     *
     * @see \Drupal\Core\Config\Config::set()
     */
    public static function storageSettingsToConfigData(array $settings);
    
    /**
     * Returns a settings array in the field type's canonical representation.
     *
     * This function does the inverse of static::storageSettingsToConfigData(). It's
     * called when loading a field's settings from a configuration object.
     *
     * @param array $settings
     *   The field's settings, as it is stored within a configuration object.
     *
     * @return array
     *   The settings, in the representation expected by the field type and code
     *   that interacts with it.
     *
     * @see \Drupal\Core\Field\FieldItemInterface::storageSettingsToConfigData()
     */
    public static function storageSettingsFromConfigData(array $settings);
    
    /**
     * Returns a settings array that can be stored as a configuration value.
     *
     * Same as static::storageSettingsToConfigData(), but for the field's settings.
     *
     * @param array $settings
     *   The field's settings in the field type's canonical representation.
     *
     * @return array
     *   An array (either the unmodified $settings or a modified representation)
     *   that is suitable for storing as a deployable configuration value.
     *
     * @see \Drupal\Core\Field\FieldItemInterface::storageSettingsToConfigData()
     */
    public static function fieldSettingsToConfigData(array $settings);
    
    /**
     * Returns a settings array in the field type's canonical representation.
     *
     * This function does the inverse of static::fieldSettingsToConfigData().
     * It's called when loading a field's settings from a configuration
     * object.
     *
     * @param array $settings
     *   The field's settings, as it is stored within a configuration
     *   object.
     *
     * @return array
     *   The field settings, in the representation expected by the field type
     *   and code that interacts with it.
     *
     * @see \Drupal\Core\Field\FieldItemInterface::fieldSettingsToConfigData()
     */
    public static function fieldSettingsFromConfigData(array $settings);
    
    /**
     * Returns a form for the storage-level settings.
     *
     * Invoked from \Drupal\field_ui\Form\FieldStorageConfigEditForm to allow
     * administrators to configure storage-level settings.
     *
     * Field storage might reject settings changes that affect the field
     * storage schema if the storage already has data. When the $has_data
     * parameter is TRUE, the form should not allow changing the settings that
     * take part in the schema() method. It is recommended to set #access to
     * FALSE on the corresponding elements.
     *
     * @param array $form
     *   The form where the settings form is being included in.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The form state of the (entire) configuration form.
     * @param bool $has_data
     *   TRUE if the field already has data, FALSE if not.
     *
     * @return array
     *   The form definition for the field settings.
     */
    public function storageSettingsForm(array &$form, FormStateInterface $form_state, $has_data);
    
    /**
     * Returns a form for the field-level settings.
     *
     * Invoked from \Drupal\field_ui\Form\FieldConfigEditForm to allow
     * administrators to configure field-level settings.
     *
     * @param array $form
     *   The form where the settings form is being included in.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The form state of the (entire) configuration form.
     *
     * @return array
     *   The form definition for the field settings.
     */
    public function fieldSettingsForm(array $form, FormStateInterface $form_state);
    
    /**
     * Calculates dependencies for field items.
     *
     * Dependencies are saved in the field configuration entity and are used to
     * determine configuration synchronization order. For example, if the field
     * type's default value is a content entity, this method should return an
     * array of dependencies listing the content entities.
     *
     * @param \Drupal\Core\Field\FieldDefinitionInterface $field_definition
     *   The field definition.
     *
     * @return array
     *   An array of dependencies grouped by type (config, content, module,
     *   theme). For example:
     *   @code
     *   array(
     *     'config' => array('user.role.anonymous', 'user.role.authenticated'),
     *     'content' => array('node:article:f0a189e6-55fb-47fb-8005-5bef81c44d6d'),
     *     'module' => array('node', 'user'),
     *     'theme' => array('seven'),
     *   );
     *   @endcode
     *
     * @see \Drupal\Core\Config\Entity\ConfigDependencyManager
     * @see \Drupal\Core\Config\Entity\ConfigEntityInterface::getConfigDependencyName()
     */
    public static function calculateDependencies(FieldDefinitionInterface $field_definition);
    
    /**
     * Calculates dependencies for field items on the storage level.
     *
     * Dependencies are saved in the field storage configuration entity and are
     * used to determine configuration synchronization order. For example, if the
     * field type storage depends on a particular entity type, this method should
     * return an array of dependencies listing the module that provides the entity
     * type.
     *
     * Dependencies returned from this method are stored in field storage
     * configuration and are always considered hard dependencies. If the
     * dependency is removed the field storage configuration must be deleted.
     *
     * @param \Drupal\Core\Field\FieldStorageDefinitionInterface $field_storage_definition
     *   The field storage definition.
     *
     * @return array
     *   An array of dependencies grouped by type (config, content, module,
     *   theme). For example:
     *   @code
     *   [
     *     'config' => ['user.role.anonymous', 'user.role.authenticated'],
     *     'content' => ['node:article:f0a189e6-55fb-47fb-8005-5bef81c44d6d'],
     *     'module' => ['node', 'user'],
     *     'theme' => ['seven'],
     *   ];
     *   @endcode
     *
     * @see \Drupal\Core\Config\Entity\ConfigDependencyManager
     * @see \Drupal\Core\Config\Entity\ConfigEntityInterface::getConfigDependencyName()
     */
    public static function calculateStorageDependencies(FieldStorageDefinitionInterface $field_storage_definition);
    
    /**
     * Informs the plugin that a dependency of the field will be deleted.
     *
     * @param \Drupal\Core\Field\FieldDefinitionInterface $field_definition
     *   The field definition.
     * @param array $dependencies
     *   An array of dependencies that will be deleted keyed by dependency type.
     *   Dependency types are, for example, entity, module and theme.
     *
     * @return bool
     *   TRUE if the field definition has been changed as a result, FALSE if not.
     *
     * @see \Drupal\Core\Config\ConfigEntityInterface::onDependencyRemoval()
     */
    public static function onDependencyRemoval(FieldDefinitionInterface $field_definition, array $dependencies);

}