interface FormatterInterface

interface FormatterInterface extends PluginSettingsInterface {
    
    /**
     * Returns a form to configure settings for the formatter.
     *
     * Invoked from \Drupal\field_ui\Form\EntityDisplayFormBase to allow
     * administrators to configure the formatter. The field_ui module takes care
     * of handling submitted form values.
     *
     * @param array $form
     *   The form where the settings form is being included in.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     *
     * @return array
     *   The form elements for the formatter settings.
     */
    public function settingsForm(array $form, FormStateInterface $form_state);
    
    /**
     * Returns a short summary for the current formatter settings.
     *
     * If an empty result is returned, a UI can still be provided to display
     * a settings form in case the formatter has configurable settings.
     *
     * @return string[]
     *   A short summary of the formatter settings.
     */
    public function settingsSummary();
    
    /**
     * Allows formatters to load information for field values being displayed.
     *
     * This should be used when a formatter needs to load additional information
     * from the database in order to render a field, for example a reference
     * field that displays properties of the referenced entities such as name or
     * type.
     *
     * This method operates on multiple entities. The $entities_items parameter
     * is an array keyed by entity ID. For performance reasons, information for
     * all involved entities should be loaded in a single query where possible.
     *
     * Changes or additions to field values are done by directly altering the
     * items.
     *
     * @param \Drupal\Core\Field\FieldItemListInterface[] $entities_items
     *   An array with the field values from the multiple entities being rendered.
     */
    public function prepareView(array $entities_items);
    
    /**
     * Builds a renderable array for a fully themed field.
     *
     * @param \Drupal\Core\Field\FieldItemListInterface $items
     *   The field values to be rendered.
     * @param string $langcode
     *   (optional) The language that should be used to render the field. Defaults
     *   to the current content language.
     *
     * @return array
     *   A renderable array for a themed field with its label and all its values.
     */
    public function view(FieldItemListInterface $items, $langcode = NULL);
    
    /**
     * Builds a renderable array for a field value.
     *
     * @param \Drupal\Core\Field\FieldItemListInterface $items
     *   The field values to be rendered.
     * @param string $langcode
     *   The language that should be used to render the field.
     *
     * @return array
     *   A renderable array for $items, as an array of child elements keyed by
     *   consecutive numeric indexes starting from 0.
     */
    public function viewElements(FieldItemListInterface $items, $langcode);
    
    /**
     * Returns if the formatter can be used for the provided field.
     *
     * @param \Drupal\Core\Field\FieldDefinitionInterface $field_definition
     *   The field definition that should be checked.
     *
     * @return bool
     *   TRUE if the formatter can be used, FALSE otherwise.
     */
    public static function isApplicable(FieldDefinitionInterface $field_definition);

}