class TelephoneLinkFormatter

class TelephoneLinkFormatter extends FormatterBase {
    
    /**
     * {@inheritdoc}
     */
    public static function defaultSettings() {
        return [
            'title' => '',
        ] + parent::defaultSettings();
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsForm(array $form, FormStateInterface $form_state) {
        $elements['title'] = [
            '#type' => 'textfield',
            '#title' => t('Title to replace basic numeric telephone number display'),
            '#default_value' => $this->getSetting('title'),
        ];
        return $elements;
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsSummary() {
        $summary = [];
        $settings = $this->getSettings();
        if (!empty($settings['title'])) {
            $summary[] = t('Link using text: @title', [
                '@title' => $settings['title'],
            ]);
        }
        else {
            $summary[] = t('Link using provided telephone number.');
        }
        return $summary;
    }
    
    /**
     * {@inheritdoc}
     */
    public function viewElements(FieldItemListInterface $items, $langcode) {
        $element = [];
        $title_setting = $this->getSetting('title');
        foreach ($items as $delta => $item) {
            // If the telephone number is 5 or less digits, parse_url() will think
            // it's a port number rather than a phone number which causes the link
            // formatter to throw an InvalidArgumentException. Avoid this by inserting
            // a dash (-) after the first digit - RFC 3966 defines the dash as a
            // visual separator character and so will be removed before the phone
            // number is used. See https://bugs.php.net/bug.php?id=70588 for more.
            // While the bug states this only applies to numbers <= 65535, a 5 digit
            // number greater than 65535 will cause parse_url() to return FALSE so
            // we need the work around on any 5 digit (or less) number.
            // First we strip whitespace so we're counting actual digits.
            $phone_number = preg_replace('/\\s+/', '', $item->value);
            if (strlen($phone_number) <= 5) {
                $phone_number = substr_replace($phone_number, '-', 1, 0);
            }
            // Render each element as link.
            $element[$delta] = [
                '#type' => 'link',
                // Use custom title if available, otherwise use the telephone number
                // itself as title.
'#title' => $title_setting ?: $item->value,
                // Prepend 'tel:' to the telephone number.
'#url' => Url::fromUri('tel:' . rawurlencode($phone_number)),
                '#options' => [
                    'external' => TRUE,
                ],
            ];
            if (!empty($item->_attributes)) {
                $element[$delta]['#options'] += [
                    'attributes' => [],
                ];
                $element[$delta]['#options']['attributes'] += $item->_attributes;
                // Unset field item attributes since they have been included in the
                // formatter output and should not be rendered in the field template.
                unset($item->_attributes);
            }
        }
        return $element;
    }

}