Dependency Injection

Overview

Dependency Injection can be accomplished by using the same strategy you would in Block Plugins.

Below is an example of a class with Dependency Injection in practice.

<?php

namespace Drupal\MY_THEME\Plugin\preprocessors;

use Drupal\Core\Config\ConfigFactoryInterface;
use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
use Drupal\preprocessors\PreprocessorPluginBase;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * Provide plugin to alter variables for nodes.
 */
final class NodePreprocessor extends PreprocessorPluginBase implements ContainerFactoryPluginInterface {

  /**
   * The ConfigFactory service.
   *
   * @var \Drupal\Core\Config\ConfigFactoryInterface
   */
  protected ConfigFactoryInterface $configFactory;

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) : static {
    $instance = new static($configuration, $plugin_id, $plugin_definition);
    $instance->configFactory = $container->get('config.factory');
    return $instance;
  }

  /**
   * Add personal tweaks to variables in this function.
   *
   * {@inheritdoc}
   */
  public function preprocess(array &$variables, string $hook, array $info) : void {
    $defaultTheme = $this->configFactory->get('system.theme')->get('default');
    $variables['defaultTheme'] = $defaultTheme;  
  }

}

To summarize:

1. Make sure your class implements the ContainerFactoryPluginInterface
2. Declare class properties for the services you would like to inject.
3. Implement the ::create() method in your class and appropriately return an instance with the injected services.
4. You may now access your services via your class properties.