class WidgetBase

abstract class WidgetBase extends PluginSettingsBase implements WidgetInterface, ContainerFactoryPluginInterface {
    use AllowedTagsXssTrait;
    
    /**
     * The field definition.
     *
     * @var \Drupal\Core\Field\FieldDefinitionInterface
     */
    protected $fieldDefinition;
    
    /**
     * The widget settings.
     *
     * @var array
     */
    protected $settings;
    
    /**
     * Constructs a WidgetBase object.
     *
     * @param string $plugin_id
     *   The plugin_id for the widget.
     * @param mixed $plugin_definition
     *   The plugin implementation definition.
     * @param \Drupal\Core\Field\FieldDefinitionInterface $field_definition
     *   The definition of the field to which the widget is associated.
     * @param array $settings
     *   The widget settings.
     * @param array $third_party_settings
     *   Any third party settings.
     */
    public function __construct($plugin_id, $plugin_definition, FieldDefinitionInterface $field_definition, array $settings, array $third_party_settings) {
        parent::__construct([], $plugin_id, $plugin_definition);
        $this->fieldDefinition = $field_definition;
        $this->settings = $settings;
        $this->thirdPartySettings = $third_party_settings;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
        return new static($plugin_id, $plugin_definition, $configuration['field_definition'], $configuration['settings'], $configuration['third_party_settings']);
    }
    
    /**
     * {@inheritdoc}
     */
    public function form(FieldItemListInterface $items, array &$form, FormStateInterface $form_state, $get_delta = NULL) {
        $field_name = $this->fieldDefinition
            ->getName();
        $parents = $form['#parents'];
        // Store field information in $form_state.
        if (!static::getWidgetState($parents, $field_name, $form_state)) {
            $field_state = [
                'items_count' => count($items),
                'array_parents' => [],
            ];
            static::setWidgetState($parents, $field_name, $form_state, $field_state);
        }
        // Collect widget elements.
        $elements = [];
        // If the widget is handling multiple values (e.g Options), or if we are
        // displaying an individual element, just get a single form element and make
        // it the $delta value.
        if ($this->handlesMultipleValues() || isset($get_delta)) {
            $delta = isset($get_delta) ? $get_delta : 0;
            $element = [
                '#title' => $this->fieldDefinition
                    ->getLabel(),
                '#description' => FieldFilteredMarkup::create(\Drupal::token()->replace($this->fieldDefinition
                    ->getDescription())),
            ];
            $element = $this->formSingleElement($items, $delta, $element, $form, $form_state);
            if ($element) {
                if (isset($get_delta)) {
                    // If we are processing a specific delta value for a field where the
                    // field module handles multiples, set the delta in the result.
                    $elements[$delta] = $element;
                }
                else {
                    // For fields that handle their own processing, we cannot make
                    // assumptions about how the field is structured, just merge in the
                    // returned element.
                    $elements = $element;
                }
            }
        }
        else {
            $elements = $this->formMultipleElements($items, $form, $form_state);
        }
        // Allow modules to alter the field multi-value widget form element.
        // This hook can also be used for single-value fields.
        $context = [
            'form' => $form,
            'widget' => $this,
            'items' => $items,
            'default' => $this->isDefaultValueWidget($form_state),
        ];
        \Drupal::moduleHandler()->alter([
            'field_widget_multivalue_form',
            'field_widget_multivalue_' . $this->getPluginId() . '_form',
        ], $elements, $form_state, $context);
        // Populate the 'array_parents' information in $form_state->get('field')
        // after the form is built, so that we catch changes in the form structure
        // performed in alter() hooks.
        $elements['#after_build'][] = [
            get_class($this),
            'afterBuild',
        ];
        $elements['#field_name'] = $field_name;
        $elements['#field_parents'] = $parents;
        // Enforce the structure of submitted values.
        $elements['#parents'] = array_merge($parents, [
            $field_name,
        ]);
        // Most widgets need their internal structure preserved in submitted values.
        $elements += [
            '#tree' => TRUE,
        ];
        return [
            // Aid in theming of widgets by rendering a classified container.
'#type' => 'container',
            // Assign a different parent, to keep the main id for the widget itself.
'#parents' => array_merge($parents, [
                $field_name . '_wrapper',
            ]),
            '#attributes' => [
                'class' => [
                    'field--type-' . Html::getClass($this->fieldDefinition
                        ->getType()),
                    'field--name-' . Html::getClass($field_name),
                    'field--widget-' . Html::getClass($this->getPluginId()),
                ],
            ],
            'widget' => $elements,
        ];
    }
    
    /**
     * Special handling to create form elements for multiple values.
     *
     * Handles generic features for multiple fields:
     * - number of widgets
     * - AHAH-'add more' button
     * - table display and drag-n-drop value reordering
     */
    protected function formMultipleElements(FieldItemListInterface $items, array &$form, FormStateInterface $form_state) {
        $field_name = $this->fieldDefinition
            ->getName();
        $cardinality = $this->fieldDefinition
            ->getFieldStorageDefinition()
            ->getCardinality();
        $parents = $form['#parents'];
        // Determine the number of widgets to display.
        switch ($cardinality) {
            case FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED:
                $field_state = static::getWidgetState($parents, $field_name, $form_state);
                $max = $field_state['items_count'];
                $is_multiple = TRUE;
                break;
            default:
                $max = $cardinality - 1;
                $is_multiple = $cardinality > 1;
                break;
        }
        $title = $this->fieldDefinition
            ->getLabel();
        $description = FieldFilteredMarkup::create(\Drupal::token()->replace($this->fieldDefinition
            ->getDescription()));
        $elements = [];
        for ($delta = 0; $delta <= $max; $delta++) {
            // Add a new empty item if it doesn't exist yet at this delta.
            if (!isset($items[$delta])) {
                $items->appendItem();
            }
            // For multiple fields, title and description are handled by the wrapping
            // table.
            if ($is_multiple) {
                $element = [
                    '#title' => $this->t('@title (value @number)', [
                        '@title' => $title,
                        '@number' => $delta + 1,
                    ]),
                    '#title_display' => 'invisible',
                    '#description' => '',
                ];
            }
            else {
                $element = [
                    '#title' => $title,
                    '#title_display' => 'before',
                    '#description' => $description,
                ];
            }
            $element = $this->formSingleElement($items, $delta, $element, $form, $form_state);
            if ($element) {
                // Input field for the delta (drag-n-drop reordering).
                if ($is_multiple) {
                    // We name the element '_weight' to avoid clashing with elements
                    // defined by widget.
                    $element['_weight'] = [
                        '#type' => 'weight',
                        '#title' => $this->t('Weight for row @number', [
                            '@number' => $delta + 1,
                        ]),
                        '#title_display' => 'invisible',
                        // Note: this 'delta' is the FAPI #type 'weight' element's property.
'#delta' => $max,
                        '#default_value' => $items[$delta]->_weight ?: $delta,
                        '#weight' => 100,
                    ];
                }
                $elements[$delta] = $element;
            }
        }
        if ($elements) {
            $elements += [
                '#theme' => 'field_multiple_value_form',
                '#field_name' => $field_name,
                '#cardinality' => $cardinality,
                '#cardinality_multiple' => $this->fieldDefinition
                    ->getFieldStorageDefinition()
                    ->isMultiple(),
                '#required' => $this->fieldDefinition
                    ->isRequired(),
                '#title' => $title,
                '#description' => $description,
                '#max_delta' => $max,
            ];
            // Add 'add more' button, if not working with a programmed form.
            if ($cardinality == FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED && !$form_state->isProgrammed()) {
                $id_prefix = implode('-', array_merge($parents, [
                    $field_name,
                ]));
                $wrapper_id = Html::getUniqueId($id_prefix . '-add-more-wrapper');
                $elements['#prefix'] = '<div id="' . $wrapper_id . '">';
                $elements['#suffix'] = '</div>';
                $elements['add_more'] = [
                    '#type' => 'submit',
                    '#name' => strtr($id_prefix, '-', '_') . '_add_more',
                    '#value' => t('Add another item'),
                    '#attributes' => [
                        'class' => [
                            'field-add-more-submit',
                        ],
                    ],
                    '#limit_validation_errors' => [
                        array_merge($parents, [
                            $field_name,
                        ]),
                    ],
                    '#submit' => [
                        [
                            get_class($this),
                            'addMoreSubmit',
                        ],
                    ],
                    '#ajax' => [
                        'callback' => [
                            get_class($this),
                            'addMoreAjax',
                        ],
                        'wrapper' => $wrapper_id,
                        'effect' => 'fade',
                    ],
                ];
            }
        }
        return $elements;
    }
    
    /**
     * After-build handler for field elements in a form.
     *
     * This stores the final location of the field within the form structure so
     * that flagErrors() can assign validation errors to the right form element.
     */
    public static function afterBuild(array $element, FormStateInterface $form_state) {
        $parents = $element['#field_parents'];
        $field_name = $element['#field_name'];
        $field_state = static::getWidgetState($parents, $field_name, $form_state);
        $field_state['array_parents'] = $element['#array_parents'];
        static::setWidgetState($parents, $field_name, $form_state, $field_state);
        return $element;
    }
    
    /**
     * Submission handler for the "Add another item" button.
     */
    public static function addMoreSubmit(array $form, FormStateInterface $form_state) {
        $button = $form_state->getTriggeringElement();
        // Go one level up in the form, to the widgets container.
        $element = NestedArray::getValue($form, array_slice($button['#array_parents'], 0, -1));
        $field_name = $element['#field_name'];
        $parents = $element['#field_parents'];
        // Increment the items count.
        $field_state = static::getWidgetState($parents, $field_name, $form_state);
        $field_state['items_count']++;
        static::setWidgetState($parents, $field_name, $form_state, $field_state);
        $form_state->setRebuild();
    }
    
    /**
     * Ajax callback for the "Add another item" button.
     *
     * This returns the new page content to replace the page content made obsolete
     * by the form submission.
     */
    public static function addMoreAjax(array $form, FormStateInterface $form_state) {
        $button = $form_state->getTriggeringElement();
        // Go one level up in the form, to the widgets container.
        $element = NestedArray::getValue($form, array_slice($button['#array_parents'], 0, -1));
        // Ensure the widget allows adding additional items.
        if ($element['#cardinality'] != FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED) {
            return;
        }
        // Add a DIV around the delta receiving the Ajax effect.
        $delta = $element['#max_delta'];
        $element[$delta]['#prefix'] = '<div class="ajax-new-content">' . (isset($element[$delta]['#prefix']) ? $element[$delta]['#prefix'] : '');
        $element[$delta]['#suffix'] = (isset($element[$delta]['#suffix']) ? $element[$delta]['#suffix'] : '') . '</div>';
        return $element;
    }
    
    /**
     * Generates the form element for a single copy of the widget.
     */
    protected function formSingleElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
        $element += [
            '#field_parents' => $form['#parents'],
            // Only the first widget should be required.
'#required' => $delta == 0 && $this->fieldDefinition
                ->isRequired(),
            '#delta' => $delta,
            '#weight' => $delta,
        ];
        $element = $this->formElement($items, $delta, $element, $form, $form_state);
        if ($element) {
            // Allow modules to alter the field widget form element.
            $context = [
                'form' => $form,
                'widget' => $this,
                'items' => $items,
                'delta' => $delta,
                'default' => $this->isDefaultValueWidget($form_state),
            ];
            \Drupal::moduleHandler()->alter([
                'field_widget_form',
                'field_widget_' . $this->getPluginId() . '_form',
            ], $element, $form_state, $context);
        }
        return $element;
    }
    
    /**
     * {@inheritdoc}
     */
    public function extractFormValues(FieldItemListInterface $items, array $form, FormStateInterface $form_state) {
        $field_name = $this->fieldDefinition
            ->getName();
        // Extract the values from $form_state->getValues().
        $path = array_merge($form['#parents'], [
            $field_name,
        ]);
        $key_exists = NULL;
        $values = NestedArray::getValue($form_state->getValues(), $path, $key_exists);
        if ($key_exists) {
            // Account for drag-and-drop reordering if needed.
            if (!$this->handlesMultipleValues()) {
                // Remove the 'value' of the 'add more' button.
                unset($values['add_more']);
                // The original delta, before drag-and-drop reordering, is needed to
                // route errors to the correct form element.
                foreach ($values as $delta => &$value) {
                    $value['_original_delta'] = $delta;
                }
                usort($values, function ($a, $b) {
                    return SortArray::sortByKeyInt($a, $b, '_weight');
                });
            }
            // Let the widget massage the submitted values.
            $values = $this->massageFormValues($values, $form, $form_state);
            // Assign the values and remove the empty ones.
            $items->setValue($values);
            $items->filterEmptyItems();
            // Put delta mapping in $form_state, so that flagErrors() can use it.
            $field_state = static::getWidgetState($form['#parents'], $field_name, $form_state);
            foreach ($items as $delta => $item) {
                $field_state['original_deltas'][$delta] = isset($item->_original_delta) ? $item->_original_delta : $delta;
                unset($item->_original_delta, $item->_weight);
            }
            static::setWidgetState($form['#parents'], $field_name, $form_state, $field_state);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function flagErrors(FieldItemListInterface $items, ConstraintViolationListInterface $violations, array $form, FormStateInterface $form_state) {
        $field_name = $this->fieldDefinition
            ->getName();
        $field_state = static::getWidgetState($form['#parents'], $field_name, $form_state);
        if ($violations->count()) {
            // Locate the correct element in the form.
            $element = NestedArray::getValue($form_state->getCompleteForm(), $field_state['array_parents']);
            // Do not report entity-level validation errors if Form API errors have
            // already been reported for the field.
            // @todo Field validation should not be run on fields with FAPI errors to
            //   begin with. See https://www.drupal.org/node/2070429.
            $element_path = implode('][', $element['#parents']);
            if ($reported_errors = $form_state->getErrors()) {
                foreach (array_keys($reported_errors) as $error_path) {
                    if (strpos($error_path, $element_path) === 0) {
                        return;
                    }
                }
            }
            // Only set errors if the element is visible.
            if (Element::isVisibleElement($element)) {
                $handles_multiple = $this->handlesMultipleValues();
                $violations_by_delta = $item_list_violations = [];
                foreach ($violations as $violation) {
                    // Separate violations by delta.
                    $property_path = explode('.', $violation->getPropertyPath());
                    $delta = array_shift($property_path);
                    if (is_numeric($delta)) {
                        $violations_by_delta[$delta][] = $violation;
                    }
                    else {
                        $item_list_violations[] = $violation;
                    }
                    $violation->arrayPropertyPath = $property_path;
                }
                
                /** @var \Symfony\Component\Validator\ConstraintViolationInterface[] $delta_violations */
                foreach ($violations_by_delta as $delta => $delta_violations) {
                    // Pass violations to the main element if this is a multiple-value
                    // widget.
                    if ($handles_multiple) {
                        $delta_element = $element;
                    }
                    else {
                        $original_delta = $field_state['original_deltas'][$delta];
                        $delta_element = $element[$original_delta];
                    }
                    foreach ($delta_violations as $violation) {
                        // @todo: Pass $violation->arrayPropertyPath as property path.
                        $error_element = $this->errorElement($delta_element, $violation, $form, $form_state);
                        if ($error_element !== FALSE) {
                            $form_state->setError($error_element, $violation->getMessage());
                        }
                    }
                }
                
                /** @var \Symfony\Component\Validator\ConstraintViolationInterface[] $item_list_violations */
                // Pass violations to the main element without going through
                // errorElement() if the violations are at the ItemList level.
                foreach ($item_list_violations as $violation) {
                    $form_state->setError($element, $violation->getMessage());
                }
            }
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public static function getWidgetState(array $parents, $field_name, FormStateInterface $form_state) {
        return NestedArray::getValue($form_state->getStorage(), static::getWidgetStateParents($parents, $field_name));
    }
    
    /**
     * {@inheritdoc}
     */
    public static function setWidgetState(array $parents, $field_name, FormStateInterface $form_state, array $field_state) {
        NestedArray::setValue($form_state->getStorage(), static::getWidgetStateParents($parents, $field_name), $field_state);
    }
    
    /**
     * Returns the location of processing information within $form_state.
     *
     * @param array $parents
     *   The array of #parents where the widget lives in the form.
     * @param string $field_name
     *   The field name.
     *
     * @return array
     *   The location of processing information within $form_state.
     */
    protected static function getWidgetStateParents(array $parents, $field_name) {
        // Field processing data is placed at
        // $form_state->get(['field_storage', '#parents', ...$parents..., '#fields', $field_name]),
        // to avoid clashes between field names and $parents parts.
        return array_merge([
            'field_storage',
            '#parents',
        ], $parents, [
            '#fields',
            $field_name,
        ]);
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsForm(array $form, FormStateInterface $form_state) {
        return [];
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsSummary() {
        return [];
    }
    
    /**
     * {@inheritdoc}
     */
    public function errorElement(array $element, ConstraintViolationInterface $error, array $form, FormStateInterface $form_state) {
        return $element;
    }
    
    /**
     * {@inheritdoc}
     */
    public function massageFormValues(array $values, array $form, FormStateInterface $form_state) {
        return $values;
    }
    
    /**
     * Returns the array of field settings.
     *
     * @return array
     *   The array of settings.
     */
    protected function getFieldSettings() {
        return $this->fieldDefinition
            ->getSettings();
    }
    
    /**
     * Returns the value of a field setting.
     *
     * @param string $setting_name
     *   The setting name.
     *
     * @return mixed
     *   The setting value.
     */
    protected function getFieldSetting($setting_name) {
        return $this->fieldDefinition
            ->getSetting($setting_name);
    }
    
    /**
     * Returns whether the widget handles multiple values.
     *
     * @return bool
     *   TRUE if a single copy of formElement() can handle multiple field values,
     *   FALSE if multiple values require separate copies of formElement().
     */
    protected function handlesMultipleValues() {
        $definition = $this->getPluginDefinition();
        return $definition['multiple_values'];
    }
    
    /**
     * {@inheritdoc}
     */
    public static function isApplicable(FieldDefinitionInterface $field_definition) {
        // By default, widgets are available for all fields.
        return TRUE;
    }
    
    /**
     * Returns whether the widget used for default value form.
     *
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     *
     * @return bool
     *   TRUE if a widget used to input default value, FALSE otherwise.
     */
    protected function isDefaultValueWidget(FormStateInterface $form_state) {
        return (bool) $form_state->get('default_value_widget');
    }
    
    /**
     * Returns the filtered field description.
     *
     * @return \Drupal\Core\Field\FieldFilteredMarkup
     *   The filtered field description, with tokens replaced.
     */
    protected function getFilteredDescription() {
        return FieldFilteredMarkup::create(\Drupal::token()->replace($this->fieldDefinition
            ->getDescription()));
    }

}