Filters are now annotated plugins instead of an info hook and callbacks

Previously, a module providing a filter would implement
hook_filter_info(), and then define one of several callbacks:

• settings callback
• prepare callback
• process callback
• tips callback

Instead of an info hook and several callbacks, filters have been converted to annotated plugins.

The callback functions become methods.
The data from the info hooks are now in the annotation.
Filters use the annotation class of Drupal\filter\Annotation\Filter, and implement Drupal\filter\Plugin\FilterInterface, filters have to be in the Plugin/Filter namespace.

To implement a filter in a module create a class in {module}/src/Plugin/Filter/{FilterName}.php and extend FilterBase (which implements FilterInterface).

Drupal 7

function filter_filter_info() {
  $filters['filter_html'] = array(
    'title' => t('Limit allowed HTML tags'),
    'process callback' => '_filter_html',
    'settings callback' => '_filter_html_settings',
    'default settings' => array(
      'allowed_html' => '<a> <em> <strong> <cite> <blockquote> <code> <ul> <ol> <li> <dl> <dt> <dd>',
      'filter_html_help' => 1,
      'filter_html_nofollow' => 0,
    ),
    'tips callback' => '_filter_html_tips',
    'weight' => -10,
  );
  return $filters;
}

/**
 * Settings callback.
 */
function _filter_html_settings($form, &$form_state, $filter, $format, $defaults) {
    // ...
}

/**
 * Process callback.
 */
function _filter_html($text, $filter) {
  // ...
}

/**
 * Tips callback.
 */
function _filter_html_tips($filter, $format, $long = FALSE) {
  // ...
}

Drupal 8

/**
 * @file
 * Contains \Drupal\filter\Plugin\Filter\FilterHtml.
 */

namespace Drupal\filter\Plugin\Filter;

use Drupal\filter\Annotation\Filter;
use Drupal\Core\Annotation\Translation;
use Drupal\filter\Plugin\FilterBase;

/**
 * Provides a filter to limit allowed HTML tags.
 *
 * @Filter(
 *   id = "filter_html",
 *   title = @Translation("Limit allowed HTML tags"),
 *   type = Drupal\filter\Plugin\FilterInterface::TYPE_HTML_RESTRICTOR,
 *   settings = {
 *     "allowed_html" = "<a> <em> <strong> <cite> <blockquote> <code> <ul> <ol> <li> <dl> <dt> <dd> <h4> <h5> <h6>",
 *     "filter_html_help" = 1,
 *     "filter_html_nofollow" = 0
 *   },
 *   weight = -10
 * )
 */
class FilterHtml extends FilterBase {

  public function settingsForm(array $form, array &$form_state) {
    // ...
  }

  public function process($text, $langcode, $cache, $cache_id) {
    // ...
  }

  public function tips($long = FALSE) {
    // ...
  }

}