How to do dependency injection on complex classes like when extending WidgetBase

Question

Since the Drupal sniffer whines at you if you call \Drupal::service() in your code, I have been trying to figure out how to use dependency injection on a widget I’ve been working on. Here’s what I’ve currently arrived at:

<?php

use Drupal\Core\Field\FieldDefinitionInterface;
use Drupal\Core\Field\WidgetBase;
use Drupal\xois\SicsService;
use Symfony\Component\DependencyInjection\ContainerInterface;

class XoisSicSelect extends WidgetBase {

  /**
  * SICs service.
  *
  * @var \Drupal\xois\SicsService
  */
  protected $sicsService;

  /**
  * {@inheritdoc}
  */
  public function __construct($plugin_id, $plugin_definition, FieldDefinitionInterface $field_definition, array $settings, array $third_party_settings, SicsService $sicsService) {
    parent::__construct($plugin_id, $plugin_definition, $field_definition, $settings, $third_party_settings);
    $this->sicsService = $sicsService;
  }

  /**
  * {@inheritdoc}
  */
  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('xois.sics')
    );
  }

}

Pretty much the standard DI pattern, no? Only difference is that WidgetBase takes a whole load of construct parameters already.
Problem is, it doesn’t work. When run, it gives this delightful error:

The website encountered an unexpected error. Please try again later.
Recoverable fatal error: Argument 6 passed to Drupal\xois\Plugin\Field\FieldWidget\XoisSicSelect::__construct() must be an instance of Drupal\xois\SicsService, none given, called in /var/www/docroot/core/lib/Drupal/Core/Field/WidgetPluginManager.php on line 125 and defined in Drupal\xois\Plugin\Field\FieldWidget\XoisSicSelect->__construct() (line 35 of modules/xois/src/Plugin/Field/FieldWidget/XoisSicSelect.php). 

Anyone got an idea on how to accomplish this?

Answer 1

You will need to implement ContainerFactoryPluginInterface in order to use dependency injection in your class.

Include use Drupal\Core\Plugin\ContainerFactoryPluginInterface; and implement the interface so it looks like:

class XoisSicSelect extends WidgetBase implements ContainerFactoryPluginInterface {

    ....

}

You will also need to make sure your __construct() and create() methods contain the correct parameters.

If you look at ContainerFactoryPluginInterface.php, you will see the parameters the create() method takes in:

public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition);

Thus, your final code should look like:

use Drupal\Core\Field\FieldDefinitionInterface;
use Drupal\Core\Field\WidgetBase;
use Drupal\xois\SicsService;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Drupal\Core\Plugin\ContainerFactoryPluginInterface;

class XoisSicSelect extends WidgetBase implements ContainerFactoryPluginInterface {
  /**
   * SICs service.
   *
   * @var \Drupal\xois\SicsService
   */
  protected $sicsService;

  /**
   * {@inheritdoc}
   */
  public function __construct($plugin_id, $plugin_definition, FieldDefinitionInterface $field_definition, array $settings, array $third_party_settings, SicsService $sicsService) {
    parent::__construct($plugin_id, $plugin_definition, $field_definition, $settings, $third_party_settings);
    $this->sicsService = $sicsService;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static(
      $plugin_id,
      $plugin_definition,
      $configuration['field_definition'],
      $configuration['settings'],
      $configuration['third_party_settings'],
      $container->get('xois.sics')
    );
  }
}

Answer 2

This was a difficult one for myself as well. How I figured out to resolve it was to go to the documentation page for the class (ex WidgetBase), and look for the __construct() method (ex WidgetBase::__construct()). Click on that, and you'll see a section for "X methods override CLASSNAME::__construct()". For example, DateRangeDefaultWidget::__construct overrides WidgetBase::__construct(). Click on the override, and you'll an example of what the __construct() override should look like:

public function __construct($plugin_id, $plugin_definition, FieldDefinitionInterface $field_definition, array $settings, array $third_party_settings, EntityStorageInterface $date_storage) {
  parent::__construct($plugin_id, $plugin_definition, $field_definition, $settings, $third_party_settings);

  $this->dateStorage = $date_storage;
}

And if you look at the class page (ex DateRangeDefaultWidget), you can find what the create() method should look like: DateRangeDefaultWidget::create):

public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
  return new static(
  $plugin_id, 
  $plugin_definition, 
  $configuration['field_definition'], 
  $configuration['settings'], 
  $configuration['third_party_settings'], 
  $container->get('entity_type.manager')->getStorage('date_format')
  );
}

So it takes a bit of detective work, but once you figure out the method of detecting, it's actually fairly straightforward for any classes.

Answer 3

You need to copy the create() method and add your argument, not replace it.

There's no magic involved, it's just a plain static PHP method that contructs an object, including all its arguments.