Dependency Injection

When to Use

Media source plugin needs services like HTTP client, logger, cache, file system, or custom services.

Decision

Service	Use When	Container Service ID
http_client	Fetching remote resources, API calls	`http_client`
logger.factory	Error logging, debugging	`logger.factory` → get('yourmodule')
cache.default	Caching API responses, metadata	`cache.default`
file_system	Thumbnail downloads, file operations	`file_system`
token	Directory paths with tokens	`token`
Custom service	Module-specific logic	`yourmodule.service_name`

Pattern

Service injection in plugin (15 lines):

use Drupal\Core\Cache\CacheBackendInterface;
use GuzzleHttp\ClientInterface;
use Psr\Log\LoggerInterface;

class CustomApi extends MediaSourceBase {
  protected ClientInterface $httpClient;
  protected LoggerInterface $logger;
  protected CacheBackendInterface $cache;

  public function __construct(
    array $configuration, $plugin_id, $plugin_definition,
    EntityTypeManagerInterface $entity_type_manager,
    EntityFieldManagerInterface $entity_field_manager,
    FieldTypePluginManagerInterface $field_type_manager,
    ConfigFactoryInterface $config_factory,
    ClientInterface $http_client,
    LoggerInterface $logger,
    CacheBackendInterface $cache,
  ) {
    parent::__construct($configuration, $plugin_id, $plugin_definition,
      $entity_type_manager, $entity_field_manager, $field_type_manager, $config_factory);
    $this->httpClient = $http_client;
    $this->logger = $logger;
    $this->cache = $cache;
  }

  public static function create(ContainerInterface $container, array $configuration,
    $plugin_id, $plugin_definition): static {
    return new static(
      $configuration, $plugin_id, $plugin_definition,
      $container->get('entity_type.manager'),
      $container->get('entity_field.manager'),
      $container->get('plugin.manager.field.field_type'),
      $container->get('config.factory'),
      $container->get('http_client'),
      $container->get('logger.factory')->get('yourmodule'),
      $container->get('cache.default'),
    );
  }
}

Common Mistakes

Using \Drupal::service() in plugins → Breaks testability, violates DI principles
Not calling parent constructor → Missing base services, plugin breaks
Forgetting create() method → Services not injected, constructor fails
Wrong parent constructor signature → Parameter order mismatches cause errors
Injecting unnecessary services → Adds complexity, slows instantiation

WHY: - DI over static calls: \Drupal::service() creates global dependencies; DI enables unit testing with mocked services - Parent constructor: MediaSourceBase requires 4 core services; not calling parent means these aren't initialized - create() is factory: Plugins instantiated via static factory; forgetting this means services never injected

Dependency Injection

When to Use

Decision

Pattern

Common Mistakes

See Also