Creating a plugin that can be defined in themes

In general, plugins should not be defined in a custom theme but in a custom module. There are some cases where plugins make sense in a Custom Theme, for example, the BreakPoints plugin and the Layout Plugin.

How to create a plugin that can be defined inside a theme

Make sure your plugin provides a YML Discovery method. Which is the only one that you can use in a theme.

In your module.services.yml file,make sure you pass the theme handler. Example (from the layout_plugin module):

services:
  plugin.manager.layout_plugin:
    class: Drupal\layout_plugin\Plugin\Layout\LayoutPluginManager
    arguments: ['@container.namespaces', '@cache.discovery', '@module_handler', '@theme_handler']

In your Plugin Manager file (src/Plugin/Layout/MyPluginManager.php):

• Add the ThemeHandler attribute to your plugin.
  

    /**
     * The theme handler.
     *
     * @var \Drupal\Core\Extension\ThemeHandlerInterface
     */
    protected $themeHandler;
  
  

• In your _construct function, make sure you assign the ThemeHandler
  

    $this->themeHandler = $theme_handler;
  
  

• add $this->moduleHandler->getModuleDirectories() + $this->themeHandler->getThemeDirectories() in getDiscovery() function
  

    /**
     * {@inheritdoc}
     */
    protected function getDiscovery() {
      ...
      $this->discovery = new YamlDiscovery(..., $this->moduleHandler->getModuleDirectories() + $this->themeHandler->getThemeDirectories());
      ...
    }
  
  

• redefine the providerExists method like this:
  

    /**
     * {@inheritdoc}
     */
    protected function providerExists($provider) {
      return $this->moduleHandler->moduleExists($provider) || $this->themeHandler->themeExists($provider);
    }
  
  

For good examples take a look at both the BreakPoints module and the Layout Plugin module.