How do I instantiate a custom class with dependency injection?

Question

I am relatively fresh to the DI party and am struggling to get my head around how exactly to use Dependency Injection. I understand that I can pass requirements as part of a service, but what about in my own class?

Say I have the below class where I want the language manager service to be injected:

class Foo implements ContainerInjectionInterface {

  protected $languageManager; 

  public function __construct(LanguageManagerInterface $languageManager) {
    $this->languageManager = $languageManager;
  }

  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('language_manager')
    );
  }
}

When I call new Foo() I am required to pass an instance of LanguageManagerInterface. That makes sense in terms of a normal constructor method, but that would require me to instantiate the class like the below:

$languageManager = \Drupal::service('language_manager');
$foo = new Foo($languageManager)

That just doesn't feel right to me.

Is there something I am missing, or would this class always have to be a service to take advantage of dependency injection?

Answer 1

A class implementing ContainerInjectionInterface is instantiated by calling the factory method create().

See Drupal\Core\DependencyInjection\ClassResolver

/**
 * Implements the class resolver interface supporting class names and services.
 */
class ClassResolver implements ClassResolverInterface, ContainerAwareInterface {
  use DependencySerializationTrait;
  use ContainerAwareTrait;

  /**
   * {@inheritdoc}
   */
  public function getInstanceFromDefinition($definition) {
    if ($this->container->has($definition)) {
      $instance = $this->container->get($definition);
    }
    else {
      if (!class_exists($definition)) {
        throw new \InvalidArgumentException(sprintf('Class "%s" does not exist.', $definition));
      }

      if (is_subclass_of($definition, 'Drupal\Core\DependencyInjection\ContainerInjectionInterface')) {
        $instance = $definition::create($this->container);
      }
      else {
        $instance = new $definition();
      }
    }

    if ($instance instanceof ContainerAwareInterface) {
      $instance->setContainer($this->container);
    }

    return $instance;
  }

}

Answer 2

In my humble opinion, the factory design pattern is the most appropriate way to instantiate an object (of a custom class with dependency injection).

From D.O.'s documentation:

Instantiation

Creating classes directly is discouraged. Instead, use a factory function that creates the appropriate object and returns it. This provides two benefits:

• It provides a layer of indirection, as the function may be written to return a different object (with the same interface) in different circumstances as appropriate.
• PHP does not allow class constructors to be chained, but does allow the return value from a function or method to be chained.

A code example from a D.O. core issue ([PP-1] Add a factory method to create FileStorage instances) shows how one would go about doing that in Drupal. So, for your example that would be something like this:

Service Definition

foo_factory:
  class: Drupal\your_module\FooFactory  
  arguments: ['@language_manager']

and the related instantiation code:

$fooFactory = \Drupal::service('foo_factory');
$foo = $fooFactory->create();

Answer 3

The definition of the class in question is correct. All you need is correctly call the object method. To do that - drupal has special mechanism - class resolver. Here is how to initialize your object correctly:

use Foo;

\Drupal::classResolver(Foo::class)->yourClassMethod($some_arguments_from_hook_or_preprocess);

Read more about class resolver