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Dependency Injection Patterns - DIP

When to Use

Inject all dependencies through constructor. Use autowiring in Drupal 10.2+ to reduce boilerplate. Use service decorators to extend existing services.

Decision

Pattern Use when How
Constructor injection Always for required dependencies Type-hint in __construct()
Autowiring Drupal 10.2+, own module services autowire: true in services.yml
Manual DI Need precise control, complex dependencies Explicit arguments in services.yml
Service decorators Extend existing service decorates: key in services.yml
Setter injection Optional dependencies (rare) Setter methods, avoid if possible

Pattern

GOOD: Constructor injection with autowiring

# mymodule.services.yml
services:
  _defaults:
    autoconfigure: true

  mymodule.node_publisher:
    class: Drupal\mymodule\NodePublisher
    autowire: true  # Auto-injects type-hinted dependencies

class NodePublisher {
  public function __construct(
    private EntityTypeManagerInterface $entityTypeManager,
    private CurrentUserInterface $currentUser,
    private LoggerInterface $logger,
  ) {}
}

GOOD: Service decorator (extending service without modification)

# mymodule.services.yml
services:
  mymodule.path_validator:
    class: Drupal\mymodule\CustomPathValidator
    decorates: path.validator
    arguments: ['@mymodule.path_validator.inner']
    decoration_priority: 5

class CustomPathValidator implements PathValidatorInterface {
  public function __construct(
    private PathValidatorInterface $innerValidator,  // Original service
  ) {}

  public function isValid($path) {
    // Add custom logic before/after original
    $result = $this->innerValidator->isValid($path);
    // Custom processing
    return $result;
  }
}

BAD: Static calls (violates DIP)

class NodePublisher {
  public function publish($nid) {
    // Static service calls
    $node = \Drupal::entityTypeManager()->getStorage('node')->load($nid);
    \Drupal::logger('mymodule')->notice('Published');
  }
}

Reference:

Common Mistakes

  • Using \Drupal:: static calls in services -- constructor injection. WHY: Static calls are global state; impossible to mock in tests, violates DIP
  • Not using autowiring in Drupal 10.2+ -- enable autowiring. WHY: Manual DI is 10+ lines of boilerplate per service; autowiring does it automatically
  • Setter injection for required dependencies -- constructor injection. WHY: Object in inconsistent state if setter not called; constructor ensures valid object
  • Not using decoration_priority when multiple decorators -- set priority. WHY: Unpredictable decorator order; conflicts between modules
  • Injecting too many services (>8) into one class -- refactor class, violates SRP. WHY: High dependency count indicates class doing too much

See Also