class LinkItem

class LinkItem extends FieldItemBase implements LinkItemInterface {
    
    /**
     * {@inheritdoc}
     */
    public static function defaultFieldSettings() {
        return [
            'title' => DRUPAL_OPTIONAL,
            'link_type' => LinkItemInterface::LINK_GENERIC,
        ] + parent::defaultFieldSettings();
    }
    
    /**
     * {@inheritdoc}
     */
    public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
        $properties['uri'] = DataDefinition::create('uri')->setLabel(new TranslatableMarkup('URI'));
        $properties['title'] = DataDefinition::create('string')->setLabel(new TranslatableMarkup('Link text'));
        $properties['options'] = MapDataDefinition::create()->setLabel(new TranslatableMarkup('Options'));
        return $properties;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function schema(FieldStorageDefinitionInterface $field_definition) {
        return [
            'columns' => [
                'uri' => [
                    'description' => 'The URI of the link.',
                    'type' => 'varchar',
                    'length' => 2048,
                ],
                'title' => [
                    'description' => 'The link text.',
                    'type' => 'varchar',
                    'length' => 255,
                ],
                'options' => [
                    'description' => 'Serialized array of options for the link.',
                    'type' => 'blob',
                    'size' => 'big',
                    'serialize' => TRUE,
                ],
            ],
            'indexes' => [
                'uri' => [
                    [
                        'uri',
                        30,
                    ],
                ],
            ],
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function fieldSettingsForm(array $form, FormStateInterface $form_state) {
        $element = [];
        $element['link_type'] = [
            '#type' => 'radios',
            '#title' => $this->t('Allowed link type'),
            '#default_value' => $this->getSetting('link_type'),
            '#options' => [
                static::LINK_INTERNAL => $this->t('Internal links only'),
                static::LINK_EXTERNAL => $this->t('External links only'),
                static::LINK_GENERIC => $this->t('Both internal and external links'),
            ],
        ];
        $element['title'] = [
            '#type' => 'radios',
            '#title' => $this->t('Allow link text'),
            '#default_value' => $this->getSetting('title'),
            '#options' => [
                DRUPAL_DISABLED => $this->t('Disabled'),
                DRUPAL_OPTIONAL => $this->t('Optional'),
                DRUPAL_REQUIRED => $this->t('Required'),
            ],
        ];
        return $element;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function generateSampleValue(FieldDefinitionInterface $field_definition) {
        $random = new Random();
        if ($field_definition->getItemDefinition()
            ->getSetting('link_type') & LinkItemInterface::LINK_EXTERNAL) {
            // Set of possible top-level domains.
            $tlds = [
                'com',
                'net',
                'gov',
                'org',
                'edu',
                'biz',
                'info',
            ];
            // Set random length for the domain name.
            $domain_length = mt_rand(7, 15);
            switch ($field_definition->getSetting('title')) {
                case DRUPAL_DISABLED:
                    $values['title'] = '';
                    break;
                case DRUPAL_REQUIRED:
                    $values['title'] = $random->sentences(4);
                    break;
                case DRUPAL_OPTIONAL:
                    // In case of optional title, randomize its generation.
                    $values['title'] = mt_rand(0, 1) ? $random->sentences(4) : '';
                    break;
            }
            $values['uri'] = 'https://www.' . $random->word($domain_length) . '.' . $tlds[mt_rand(0, count($tlds) - 1)];
        }
        else {
            $values['uri'] = 'base:' . $random->name(mt_rand(1, 64));
        }
        return $values;
    }
    
    /**
     * {@inheritdoc}
     */
    public function isEmpty() {
        $value = $this->get('uri')
            ->getValue();
        return $value === NULL || $value === '';
    }
    
    /**
     * {@inheritdoc}
     */
    public function isExternal() {
        return $this->getUrl()
            ->isExternal();
    }
    
    /**
     * {@inheritdoc}
     */
    public static function mainPropertyName() {
        return 'uri';
    }
    
    /**
     * {@inheritdoc}
     */
    public function getUrl() {
        return Url::fromUri($this->uri, (array) $this->options);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getTitle() : ?string {
        return $this->title ?: NULL;
    }
    
    /**
     * {@inheritdoc}
     */
    public function setValue($values, $notify = TRUE) {
        // Treat the values as property value of the main property, if no array is
        // given.
        if (isset($values) && !is_array($values)) {
            $values = [
                static::mainPropertyName() => $values,
            ];
        }
        if (isset($values)) {
            $values += [
                'options' => [],
            ];
        }
        parent::setValue($values, $notify);
    }

}