D8 Dependency Injection (DI)

For an explanation on what DI is and why it's a good idea to use it, refer to the Drupal docs.

When you're using DI, you're asking the Service Container, which Drupal borrows from Symfony, to pass the correct object into your class so that you can interact with it in some way. See drupal docs for a full list of services provided by Drupal core.

The main thing to keep in mind is that any time you are writing a class in Drupal 8, you should be accessing external services through the service container (SC). If you use \Drupal::someMethod() within a class, that is a red flag as it's an opportunity to use DI instead.

Blocks, Controllers, Forms

Controllers (extending ControllerBase), blocks (extending BlockBase) and forms (extending FormBase) have special access to the SC. They can access it directly via their create()method. The example below gets access to the database and request_stack services, and then passes those into the __construct() method so that they can be used later on in the class. These services could also be accessed statically, \Drupal::database(), \Drupal::service('database'), \Drupal::request(), \Drupal::service('request_stack'), but since we're writing a class that's not the recommended approach.

  protected $db;
  protected $requestStack;

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('database'),
      $container->get('request_stack')
    );
  }

  /**
   * {@inheritdoc}
   */
  public function __construct(Connection $db, RequestStack $requestStack) {
    $this->db = $db;
    $this->requestStack = $requestStack;
  }

In the other methods of our class, we can now access our services like this: $this->db->someMethod() or $this->requestStack->someMethod().

Roll your own service

If you're writing a class with functionality that will be (or could be) used multiple other places in the application, consider making it a service. For instance, if you're interacting with a 3rd party API, you may want to write a class to make common interactions easier (connect, fetch, etc).

In this case, refer to drupal docs for creating a service. The part of the .services.yml relevant to this article comes in the arguments line:

arguments: ['@database', '@request_stack']

This instructs Drupal to reach into the SC and pass the database and request_stack services into your class. This replaces the create() method in the example above. You can then add your services to your constructor like this:

  protected $db;
  protected $requestStack;

  /**
   * {@inheritdoc}
   */
  public function __construct(Connection $db, RequestStack $requestStack) {
    $this->db = $db;
    $this->requestStack = $requestStack;
  }

Now the other methods of your class can reference these services like this: $this->db->someMethod() or $this->requestStack->someMethod().