How do I inject a dependency into a @FieldType plugin?

An example for a field formatter plugin with dependency injections. This works differently from services. First parameter injected is the container to get all services needed.

Drupal\Core\Field\Plugin\Field\FieldFormatter

class StringFormatter extends FormatterBase implements ContainerFactoryPluginInterface { /** * The entity type manager. * * @var \Drupal\Core\Entity\EntityTypeManagerInterface */ protected $entityTypeManager; /** * Constructs a StringFormatter instance. * * @param string $plugin_id * The plugin_id for the formatter. * @param mixed $plugin_definition * The plugin implementation definition. * @param \Drupal\Core\Field\FieldDefinitionInterface $field_definition * The definition of the field to which the formatter is associated. * @param array $settings * The formatter settings. * @param string $label * The formatter label display setting. * @param string $view_mode * The view mode. * @param array $third_party_settings * Any third party settings settings. * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager * The entity type manager. */ public function __construct($plugin_id, $plugin_definition, FieldDefinitionInterface $field_definition, array $settings, $label, $view_mode, array $third_party_settings, EntityTypeManagerInterface $entity_type_manager) { parent::__construct($plugin_id, $plugin_definition, $field_definition, $settings, $label, $view_mode, $third_party_settings); $this->entityTypeManager = $entity_type_manager; } /** * {@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['label'], $configuration['view_mode'], $configuration['third_party_settings'], $container->get('entity_type.manager') ); } 

The accepted answer works for the field widget and field formatter classes. The actual @fieldType definition class constructor requires this signature:

public function __construct( \Drupal\Core\TypedData\ComplexDataDefinitionInterface $definition, $name = NULL, \Drupal\Core\TypedData\TypedDataInterface $parent = NULL ) 

This works for Drupal 8, 9 and 10. For an example, refer to the FieldItemBase class.