interface WidgetInterface

interface WidgetInterface extends WidgetBaseInterface {
    
    /**
     * Returns a form to configure settings for the widget.
     *
     * Invoked from \Drupal\field_ui\Form\EntityDisplayFormBase to allow
     * administrators to configure the widget. 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 definition for the widget settings.
     */
    public function settingsForm(array $form, FormStateInterface $form_state);
    
    /**
     * Returns a short summary for the current widget settings.
     *
     * If an empty result is returned, a UI can still be provided to display
     * a settings form in case the widget has configurable settings.
     *
     * @return array
     *   A short summary of the widget settings.
     */
    public function settingsSummary();
    
    /**
     * Returns the form for a single field widget.
     *
     * Field widget form elements should be based on the passed-in $element, which
     * contains the base form element properties derived from the field
     * configuration.
     *
     * The BaseWidget methods will set the weight, field name and delta values for
     * each form element. If there are multiple values for this field, the
     * formElement() method will be called as many times as needed.
     *
     * Other modules may alter the form element provided by this function using
     * hook_field_widget_form_alter() or
     * hook_field_widget_WIDGET_TYPE_form_alter().
     *
     * The FAPI element callbacks (such as #process, #element_validate,
     * #value_callback, etc.) used by the widget do not have access to the
     * original $field_definition passed to the widget's constructor. Therefore,
     * if any information is needed from that definition by those callbacks, the
     * widget implementing this method, or a
     * hook_field_widget[_WIDGET_TYPE]_form_alter() implementation, must extract
     * the needed properties from the field definition and set them as ad-hoc
     * $element['#custom'] properties, for later use by its element callbacks.
     *
     * @param \Drupal\Core\Field\FieldItemListInterface $items
     *   Array of default values for this field.
     * @param int $delta
     *   The order of this item in the array of sub-elements (0, 1, 2, etc.).
     * @param array $element
     *   A form element array containing basic properties for the widget:
     *   - #field_parents: The 'parents' space for the field in the form. Most
     *       widgets can simply overlook this property. This identifies the
     *       location where the field values are placed within
     *       $form_state->getValues(), and is used to access processing
     *       information for the field through the getWidgetState() and
     *       setWidgetState() methods.
     *   - #title: The sanitized element label for the field, ready for output.
     *   - #description: The sanitized element description for the field, ready
     *     for output.
     *   - #required: A Boolean indicating whether the element value is required;
     *     for required multiple value fields, only the first widget's values are
     *     required.
     *   - #delta: The order of this item in the array of sub-elements; see $delta
     *     above.
     * @param array $form
     *   The form structure where widgets are being attached to. This might be a
     *   full form structure, or a sub-element of a larger form.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     *
     * @return array
     *   The form elements for a single widget for this field.
     *
     * @see hook_field_widget_form_alter()
     * @see hook_field_widget_WIDGET_TYPE_form_alter()
     */
    public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state);
    
    /**
     * Assigns a field-level validation error to the right widget sub-element.
     *
     * Depending on the widget's internal structure, a field-level validation
     * error needs to be flagged on the right sub-element.
     *
     * @param array $element
     *   An array containing the form element for the widget, as generated by
     *   formElement().
     * @param \Symfony\Component\Validator\ConstraintViolationInterface $violation
     *   A constraint violation reported during the validation phase.
     * @param array $form
     *   The form structure where field elements are attached to. This might be a
     *   full form structure, or a sub-element of a larger form.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     *
     * @return array|bool
     *   The element on which the error should be flagged, or FALSE to completely
     *   ignore the violation (use with care!).
     */
    public function errorElement(array $element, ConstraintViolationInterface $violation, array $form, FormStateInterface $form_state);
    
    /**
     * Massages the form values into the format expected for field values.
     *
     * @param array $values
     *   The submitted form values produced by the widget.
     *   - If the widget does not manage multiple values itself, the array holds
     *     the values generated by the multiple copies of the $element generated
     *     by the formElement() method, keyed by delta.
     *   - If the widget manages multiple values, the array holds the values
     *     of the form element generated by the formElement() method.
     * @param array $form
     *   The form structure where field elements are attached to. This might be a
     *   full form structure, or a sub-element of a larger form.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The form state.
     *
     * @return array
     *   An array of field values, keyed by delta.
     */
    public function massageFormValues(array $values, array $form, FormStateInterface $form_state);
    
    /**
     * Returns if the widget 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 widget can be used, FALSE otherwise.
     */
    public static function isApplicable(FieldDefinitionInterface $field_definition);

}