Instructions on how to update a Drupal 7 filter module to Drupal 8 (to go in a Drupal.org handbook page)

filter-d7-to-d8.html

<ol>
 <li>create a file called <code>filter.services.yml</code> file, with the following format:
   <code><pre>
services:
  plugin.manager.filter:
    class: Drupal\modulename\Filter\FilterName
    arguments: ['@container.namespaces']
</pre></code>
Replace <code>modulename</code> with the module's machine name, and <code>FilterName</code> with a unique name for the filter.</li>
<li>create a folder called <code>lib/Drupal/modulename/Plugin</code> (replacing <code>modulename</code> with the module's machine name). In Mac and Unix environments, in the module's folder, you can use the following shell command:
  <code>mkdir -p lib/Drupal/modulename/Plugin</code>
</li>
<li>in the <code>lib/Drupal/modulename/Plugin</code> folder, create a <code>FilterName.php</code> file, replcacing <code>FilterName</code> with the unique name you chose in step 1.</li>
<li>Start the file with the following code (replace <code>FilterName</code> with the unique name):

<code><pre>
/**
 * @file
 * Contains \Drupal\pirate\Plugin\Filter\FilterName.
 */

namespace Drupal\pirate\Plugin\Filter;

use Drupal\filter\Annotation\Filter;
use Drupal\Core\Annotation\Translation;
use Drupal\filter\Plugin\FilterBase;
</pre></code>

Use annotations for the filter's information and default settings. Here's the example in the Pirate module. This goes directly above the class declaration for the filter:

<code><pre>
/**
 * Provides a filter to limit allowed HTML tags.
 *
 * @Filter(
 *   id = "filter_pirate",
 *   module = "pirate",
 *   title = @Translation("Ah, Squiddy! I got nothing against ye. I just heard there was gold in yer belly. Ha ha har, ha ha ha har!"),
 *   type = FILTER_TYPE_TRANSFORM_IRREVERSIBLE,
 *   settings = {
 *     "pirate_display_tip" = 0,
 *   },
 *   weight = -10
 * )
 */
</pre></code>

Notice the format of the annotation, especially the settings property.

</li>
<li>Declare your class.

<code><pre>
class FilterPirate extends FilterBase {
...
}
</pre></code>

Inside the class, declare and ... the following methods:

<code><pre>
public function settingsForm(array $form, array &$form_state) { 
...
}
</pre></code>

This returns a form using the Forms API.

<code><pre>
/** 
 * {@inheritdoc}
 */
public function process($text, $langcode, $cache, $cache_id) {
...
}
</pre></code>

You might remember this as the "process callback" property in <code>hook_filter_info()</code> from such versions of Drupal as 7 or the process operation in such versions of Drupal as 4.6, 4.7, 5 and 6.

<code><pre>
/**
 * {@inheritdoc}
 */
public function tips($long = FALSE) {
...
}
</pre></code>

You might remember this as <code>hook_filter_tips()</code> from Drupal 6 and before and the "tips callback" property in <code>hook_filter_info</code> in Drupal 7.</li>
</ol>