class LinkWidget

class LinkWidget extends WidgetBase {
    
    /**
     * {@inheritdoc}
     */
    public static function defaultSettings() {
        return [
            'placeholder_url' => '',
            'placeholder_title' => '',
        ] + parent::defaultSettings();
    }
    
    /**
     * Gets the URI without the 'internal:' or 'entity:' scheme.
     *
     * The following two forms of URIs are transformed:
     * - 'entity:' URIs: to entity autocomplete ("label (entity id)") strings;
     * - 'internal:' URIs: the scheme is stripped.
     *
     * This method is the inverse of ::getUserEnteredStringAsUri().
     *
     * @param string $uri
     *   The URI to get the displayable string for.
     *
     * @return string
     *
     * @see static::getUserEnteredStringAsUri()
     */
    protected static function getUriAsDisplayableString($uri) {
        $scheme = parse_url($uri, PHP_URL_SCHEME);
        // By default, the displayable string is the URI.
        $displayable_string = $uri;
        // A different displayable string may be chosen in case of the 'internal:'
        // or 'entity:' built-in schemes.
        if ($scheme === 'internal') {
            $uri_reference = explode(':', $uri, 2)[1];
            // @todo '<front>' is valid input for BC reasons, may be removed by
            //   https://www.drupal.org/node/2421941
            $path = parse_url($uri, PHP_URL_PATH);
            if ($path === '/') {
                $uri_reference = '<front>' . substr($uri_reference, 1);
            }
            $displayable_string = $uri_reference;
        }
        elseif ($scheme === 'entity') {
            [
                $entity_type,
                $entity_id,
            ] = explode('/', substr($uri, 7), 2);
            // Show the 'entity:' URI as the entity autocomplete would.
            // @todo Support entity types other than 'node'. Will be fixed in
            //   https://www.drupal.org/node/2423093.
            if ($entity_type == 'node' && ($entity = \Drupal::entityTypeManager()->getStorage($entity_type)
                ->load($entity_id))) {
                $displayable_string = EntityAutocomplete::getEntityLabels([
                    $entity,
                ]);
            }
        }
        elseif ($scheme === 'route') {
            $displayable_string = ltrim($displayable_string, 'route:');
        }
        return $displayable_string;
    }
    
    /**
     * Gets the user-entered string as a URI.
     *
     * The following two forms of input are mapped to URIs:
     * - entity autocomplete ("label (entity id)") strings: to 'entity:' URIs;
     * - strings without a detectable scheme: to 'internal:' URIs.
     *
     * This method is the inverse of ::getUriAsDisplayableString().
     *
     * @param string $string
     *   The user-entered string.
     *
     * @return string
     *   The URI, if a non-empty $uri was passed.
     *
     * @see static::getUriAsDisplayableString()
     */
    protected static function getUserEnteredStringAsUri($string) {
        // By default, assume the entered string is a URI.
        $uri = trim($string);
        // Detect entity autocomplete string, map to 'entity:' URI.
        $entity_id = EntityAutocomplete::extractEntityIdFromAutocompleteInput($string);
        if ($entity_id !== NULL) {
            // @todo Support entity types other than 'node'. Will be fixed in
            //   https://www.drupal.org/node/2423093.
            $uri = 'entity:node/' . $entity_id;
        }
        elseif (in_array($string, [
            '<nolink>',
            '<none>',
            '<button>',
        ], TRUE)) {
            $uri = 'route:' . $string;
        }
        elseif (!empty($string) && parse_url($string, PHP_URL_SCHEME) === NULL) {
            // @todo '<front>' is valid input for BC reasons, may be removed by
            //   https://www.drupal.org/node/2421941
            // - '<front>' -> '/'
            // - '<front>#foo' -> '/#foo'
            if (strpos($string, '<front>') === 0) {
                $string = '/' . substr($string, strlen('<front>'));
            }
            $uri = 'internal:' . $string;
        }
        return $uri;
    }
    
    /**
     * Form element validation handler for the 'uri' element.
     *
     * Disallows saving inaccessible or untrusted URLs.
     */
    public static function validateUriElement($element, FormStateInterface $form_state, $form) {
        $uri = static::getUserEnteredStringAsUri($element['#value']);
        $form_state->setValueForElement($element, $uri);
        // If getUserEnteredStringAsUri() mapped the entered value to an 'internal:'
        // URI , ensure the raw value begins with '/', '?' or '#'.
        // @todo '<front>' is valid input for BC reasons, may be removed by
        //   https://www.drupal.org/node/2421941
        if (parse_url($uri, PHP_URL_SCHEME) === 'internal' && !in_array($element['#value'][0], [
            '/',
            '?',
            '#',
        ], TRUE) && substr($element['#value'], 0, 7) !== '<front>') {
            $form_state->setError($element, new TranslatableMarkup('Manually entered paths should start with one of the following characters: / ? #'));
            return;
        }
    }
    
    /**
     * Form element validation handler for the 'title' element.
     *
     * Conditionally requires the link title if a URL value was filled in.
     */
    public static function validateTitleElement(&$element, FormStateInterface $form_state, $form) {
        if ($element['uri']['#value'] !== '' && $element['title']['#value'] === '') {
            // We expect the field name placeholder value to be wrapped in $this->t() here,
            // so it won't be escaped again as it's already marked safe.
            $form_state->setError($element['title'], new TranslatableMarkup('@title field is required if there is @uri input.', [
                '@title' => $element['title']['#title'],
                '@uri' => $element['uri']['#title'],
            ]));
        }
    }
    
    /**
     * Form element validation handler for the 'title' element.
     *
     * Requires the URL value if a link title was filled in.
     */
    public static function validateTitleNoLink(&$element, FormStateInterface $form_state, $form) {
        if ($element['uri']['#value'] === '' && $element['title']['#value'] !== '') {
            $form_state->setError($element['uri'], new TranslatableMarkup('The @uri field is required when the @title field is specified.', [
                '@title' => $element['title']['#title'],
                '@uri' => $element['uri']['#title'],
            ]));
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
        
        /** @var \Drupal\link\LinkItemInterface $item */
        $item = $items[$delta];
        $element['uri'] = [
            '#type' => 'url',
            '#title' => $this->t('URL'),
            '#placeholder' => $this->getSetting('placeholder_url'),
            // The current field value could have been entered by a different user.
            // However, if it is inaccessible to the current user, do not display it
            // to them.
'#default_value' => !$item->isEmpty() && (\Drupal::currentUser()->hasPermission('link to any page') || $item->getUrl()
                ->access()) ? static::getUriAsDisplayableString($item->uri) : NULL,
            '#element_validate' => [
                [
                    static::class,
                    'validateUriElement',
                ],
            ],
            '#maxlength' => 2048,
            '#required' => $element['#required'],
            '#link_type' => $this->getFieldSetting('link_type'),
        ];
        // If the field is configured to support internal links, it cannot use the
        // 'url' form element and we have to do the validation ourselves.
        if ($this->supportsInternalLinks()) {
            $element['uri']['#type'] = 'entity_autocomplete';
            // @todo The user should be able to select an entity type. Will be fixed
            //   in https://www.drupal.org/node/2423093.
            $element['uri']['#target_type'] = 'node';
            // Disable autocompletion when the first character is '/', '#' or '?'.
            $element['uri']['#attributes']['data-autocomplete-first-character-blacklist'] = '/#?';
            // The link widget is doing its own processing in
            // static::getUriAsDisplayableString().
            $element['uri']['#process_default_value'] = FALSE;
        }
        // If the field is configured to allow only internal links, add a useful
        // element prefix and description.
        if (!$this->supportsExternalLinks()) {
            $element['uri']['#field_prefix'] = rtrim(Url::fromRoute('<front>', [], [
                'absolute' => TRUE,
            ])->toString(), '/');
            $element['uri']['#description'] = $this->t('This must be an internal path such as %add-node. You can also start typing the title of a piece of content to select it. Enter %front to link to the front page. Enter %nolink to display link text only. Enter %button to display keyboard-accessible link text only.', [
                '%add-node' => '/node/add',
                '%front' => '<front>',
                '%nolink' => '<nolink>',
                '%button' => '<button>',
            ]);
        }
        elseif ($this->supportsExternalLinks() && $this->supportsInternalLinks()) {
            $element['uri']['#description'] = $this->t('Start typing the title of a piece of content to select it. You can also enter an internal path such as %add-node or an external URL such as %url. Enter %front to link to the front page. Enter %nolink to display link text only. Enter %button to display keyboard-accessible link text only.', [
                '%front' => '<front>',
                '%add-node' => '/node/add',
                '%url' => 'http://example.com',
                '%nolink' => '<nolink>',
                '%button' => '<button>',
            ]);
        }
        elseif ($this->supportsExternalLinks() && !$this->supportsInternalLinks()) {
            $element['uri']['#description'] = $this->t('This must be an external URL such as %url.', [
                '%url' => 'http://example.com',
            ]);
        }
        // Make uri required on the front-end when title filled-in.
        if (!$this->isDefaultValueWidget($form_state) && $this->getFieldSetting('title') !== DRUPAL_DISABLED && !$element['uri']['#required']) {
            $parents = $element['#field_parents'];
            $parents[] = $this->fieldDefinition
                ->getName();
            $selector = $root = array_shift($parents);
            if ($parents) {
                $selector = $root . '[' . implode('][', $parents) . ']';
            }
            $element['uri']['#states']['required'] = [
                ':input[name="' . $selector . '[' . $delta . '][title]"]' => [
                    'filled' => TRUE,
                ],
            ];
        }
        $element['title'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Link text'),
            '#placeholder' => $this->getSetting('placeholder_title'),
            '#default_value' => $items[$delta]->title ?? NULL,
            '#maxlength' => 255,
            '#access' => $this->getFieldSetting('title') != DRUPAL_DISABLED,
            '#required' => $this->getFieldSetting('title') === DRUPAL_REQUIRED && $element['#required'],
        ];
        // Post-process the title field to make it conditionally required if URL is
        // non-empty. Omit the validation on the field edit form, since the field
        // settings cannot be saved otherwise.
        //
        // Validate that title field is filled out (regardless of uri) when it is a
        // required field.
        if (!$this->isDefaultValueWidget($form_state) && $this->getFieldSetting('title') === DRUPAL_REQUIRED) {
            $element['#element_validate'][] = [
                static::class,
                'validateTitleElement',
            ];
            $element['#element_validate'][] = [
                static::class,
                'validateTitleNoLink',
            ];
            if (!$element['title']['#required']) {
                // Make title required on the front-end when URI filled-in.
                $parents = $element['#field_parents'];
                $parents[] = $this->fieldDefinition
                    ->getName();
                $selector = $root = array_shift($parents);
                if ($parents) {
                    $selector = $root . '[' . implode('][', $parents) . ']';
                }
                $element['title']['#states']['required'] = [
                    ':input[name="' . $selector . '[' . $delta . '][uri]"]' => [
                        'filled' => TRUE,
                    ],
                ];
            }
        }
        // Ensure that a URI is always entered when an optional title field is
        // submitted.
        if (!$this->isDefaultValueWidget($form_state) && $this->getFieldSetting('title') == DRUPAL_OPTIONAL) {
            $element['#element_validate'][] = [
                static::class,
                'validateTitleNoLink',
            ];
        }
        // Exposing the attributes array in the widget is left for alternate and more
        // advanced field widgets.
        $element['attributes'] = [
            '#type' => 'value',
            '#tree' => TRUE,
            '#value' => !empty($items[$delta]->options['attributes']) ? $items[$delta]->options['attributes'] : [],
            '#attributes' => [
                'class' => [
                    'link-field-widget-attributes',
                ],
            ],
        ];
        // If cardinality is 1, ensure a proper label is output for the field.
        if ($this->fieldDefinition
            ->getFieldStorageDefinition()
            ->getCardinality() == 1) {
            // If the link title is disabled, use the field definition label as the
            // title of the 'uri' element.
            if ($this->getFieldSetting('title') == DRUPAL_DISABLED) {
                $element['uri']['#title'] = $element['#title'];
                // By default the field description is added to the title field. Since
                // the title field is disabled, we add the description, if given, to the
                // uri element instead.
                if (!empty($element['#description'])) {
                    if (empty($element['uri']['#description'])) {
                        $element['uri']['#description'] = $element['#description'];
                    }
                    else {
                        // If we have the description of the type of field together with
                        // the user provided description, we want to make a distinction
                        // between "core help text" and "user entered help text". To make
                        // this distinction more clear, we put them in an unordered list.
                        $element['uri']['#description'] = [
                            '#theme' => 'item_list',
                            '#items' => [
                                // Assume the user-specified description has the most relevance,
                                // so place it first.
$element['#description'],
                                $element['uri']['#description'],
                            ],
                        ];
                    }
                }
            }
            else {
                $element += [
                    '#type' => 'fieldset',
                ];
            }
        }
        return $element;
    }
    
    /**
     * Indicates enabled support for link to routes.
     *
     * @return bool
     *   Returns TRUE if the LinkItem field is configured to support links to
     *   routes, otherwise FALSE.
     */
    protected function supportsInternalLinks() {
        $link_type = $this->getFieldSetting('link_type');
        return (bool) ($link_type & LinkItemInterface::LINK_INTERNAL);
    }
    
    /**
     * Indicates enabled support for link to external URLs.
     *
     * @return bool
     *   Returns TRUE if the LinkItem field is configured to support links to
     *   external URLs, otherwise FALSE.
     */
    protected function supportsExternalLinks() {
        $link_type = $this->getFieldSetting('link_type');
        return (bool) ($link_type & LinkItemInterface::LINK_EXTERNAL);
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsForm(array $form, FormStateInterface $form_state) {
        $elements = parent::settingsForm($form, $form_state);
        $elements['placeholder_url'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Placeholder for URL'),
            '#default_value' => $this->getSetting('placeholder_url'),
            '#description' => $this->t('Text that will be shown inside the field until a value is entered. This hint is usually a sample value or a brief description of the expected format.'),
        ];
        $elements['placeholder_title'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Placeholder for link text'),
            '#default_value' => $this->getSetting('placeholder_title'),
            '#description' => $this->t('Text that will be shown inside the field until a value is entered. This hint is usually a sample value or a brief description of the expected format.'),
            '#states' => [
                'invisible' => [
                    ':input[name="instance[settings][title]"]' => [
                        'value' => DRUPAL_DISABLED,
                    ],
                ],
            ],
        ];
        return $elements;
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsSummary() {
        $summary = [];
        $placeholder_title = $this->getSetting('placeholder_title');
        $placeholder_url = $this->getSetting('placeholder_url');
        if (empty($placeholder_title) && empty($placeholder_url)) {
            $summary[] = $this->t('No placeholders');
        }
        else {
            if (!empty($placeholder_title)) {
                $summary[] = $this->t('Title placeholder: @placeholder_title', [
                    '@placeholder_title' => $placeholder_title,
                ]);
            }
            if (!empty($placeholder_url)) {
                $summary[] = $this->t('URL placeholder: @placeholder_url', [
                    '@placeholder_url' => $placeholder_url,
                ]);
            }
        }
        return $summary;
    }
    
    /**
     * {@inheritdoc}
     */
    public function massageFormValues(array $values, array $form, FormStateInterface $form_state) {
        foreach ($values as &$value) {
            $value['uri'] = static::getUserEnteredStringAsUri($value['uri']);
            $value += [
                'options' => [],
            ];
        }
        return $values;
    }
    
    /**
     * {@inheritdoc}
     *
     * Override the '%uri' message parameter, to ensure that 'internal:' URIs
     * show a validation error message that doesn't mention that scheme.
     */
    public function flagErrors(FieldItemListInterface $items, ConstraintViolationListInterface $violations, array $form, FormStateInterface $form_state) {
        
        /** @var \Symfony\Component\Validator\ConstraintViolationInterface $violation */
        foreach ($violations as $offset => $violation) {
            $parameters = $violation->getParameters();
            if (isset($parameters['@uri'])) {
                $parameters['@uri'] = static::getUriAsDisplayableString($parameters['@uri']);
                $violations->set($offset, new ConstraintViolation($this->t($violation->getMessageTemplate(), $parameters), $violation->getMessageTemplate(), $parameters, $violation->getRoot(), $violation->getPropertyPath(), $violation->getInvalidValue(), $violation->getPlural(), $violation->getCode()));
            }
        }
        parent::flagErrors($items, $violations, $form, $form_state);
    }

}