drupal/core/modules/link/link.module

<?php

/**
 * @file
 * Defines simple link field types.
 */

use Drupal\Core\Entity\EntityInterface;

/**
 * Implements hook_help().
 */
function link_help($path, $arg) {
  switch ($path) {
    case 'admin/help#link':
      $output = '';
      $output .= '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('The Link module defines a simple link field type for the Field module. Links are external URLs, can have an optional link text for each link, and they can be formatted when displayed. See the <a href="@field-help">Field module help page</a> for more information about fields.', array('@field-help' => url('admin/help/field'))) . '</p>';
      return $output;
  }
}

/**
 * Implements hook_field_info().
 */
function link_field_info() {
  $types['link'] = array(
    'label' => t('Link'),
    'description' => t('Stores a URL string, optional varchar link text, and optional blob of attributes to assemble a link.'),
    'instance_settings' => array(
      'title' => DRUPAL_OPTIONAL,
    ),
    'default_widget' => 'link_default',
    'default_formatter' => 'link',
    'field item class' => '\Drupal\link\Type\LinkItem',
  );
  return $types;
}

/**
 * Implements hook_field_instance_settings_form().
 */
function link_field_instance_settings_form($field, $instance) {
  $form['title'] = array(
    '#type' => 'radios',
    '#title' => t('Allow link text'),
    '#default_value' => isset($instance['settings']['title']) ? $instance['settings']['title'] : DRUPAL_OPTIONAL,
    '#options' => array(
      DRUPAL_DISABLED => t('Disabled'),
      DRUPAL_OPTIONAL => t('Optional'),
      DRUPAL_REQUIRED => t('Required'),
    ),
  );
  return $form;
}

/**
 * Implements hook_field_load().
 */
function link_field_load($entity_type, $entities, $field, $instances, $langcode, &$items, $age) {
  foreach ($entities as $id => $entity) {
    foreach ($items[$id] as $delta => &$item) {
      // Unserialize the attributes into an array. The value stored in the
      // field data should either be NULL or a non-empty serialized array.
      if (empty($item['attributes'])) {
        $item['attributes'] = array();
      }
      else {
        $item['attributes'] = unserialize($item['attributes']);
      }
    }
  }
}

/**
 * Implements hook_field_is_empty().
 */
function link_field_is_empty($item, $field_type) {
  return !isset($item['url']) || $item['url'] === '';
}

/**
 * Implements hook_field_presave().
 */
function link_field_presave(EntityInterface $entity, $field, $instance, $langcode, &$items) {
  foreach ($items as $delta => &$item) {
    // Trim any spaces around the URL and link text.
    $item['url'] = trim($item['url']);
    $item['title'] = trim($item['title']);

    // Serialize the attributes array.
    $item['attributes'] = !empty($item['attributes']) ? serialize($item['attributes']) : NULL;
  }
}

/**
 * Implements hook_theme().
 */
function link_theme() {
  return array(
    'link_formatter_link_separate' => array(
      'variables' => array('title' => NULL, 'url_title' => NULL, 'href' => NULL, 'options' => array()),
    ),
  );
}

/**
 * Formats a link as separate link title and URL elements.
 */
function theme_link_formatter_link_separate($vars) {
  $output = '';
  $output .= '<div class="link-item">';
  if (!empty($vars['title'])) {
    $output .= '<div class="link-title">' . check_plain($vars['title']) . '</div>';
  }
  $output .= '<div class="link-url">' . l($vars['url_title'], $vars['href'], $vars['options']) . '</div>';
  $output .= '</div>';
  return $output;
}