How to use Drupal 8 Service and Dependency Injection

Category:

Every object which is using the Drupal\Core\DependencyInjection\ContainerInjectionInterface (e.g. controllers, forms and blocks), must implement the create() factory method to pass the dependencies into the class constructor. Controllers, forms and blocks are not defined via the *.services.yml file.

Example How to inject current_user service in custom form:

<?php

namespace Drupal\mymodule\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Session\AccountInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * Class MyModuleForm
 * @package Drupal\mymodule\Form
 */
class MyModuleForm extends FormBase {

  /**
   * @var AccountInterface $account
   */
  protected $account;

  /**
   * Class constructor.
   */
  public function __construct(AccountInterface $account) {
    $this->account = $account;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    // Instantiates this form class.
    return new static(
    // Load the service required to construct this class.
      $container->get('current_user')
    );
  }

  /**
   * {@inheritdoc}.
   */
  public function getFormId() {
    return 'mymodule_form';
  }

  /**
   * {@inheritdoc}.
   */
  public function buildForm(array $form, FormStateInterface $form_state) {

    // Get current user data.
    $displayName = $this->account->getDisplayName();

    // ...
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    // ...
  }
}

Example how do I use dependency injection in Controller:

namespace Drupal\mymodule\Controller;

use Drupal\Core\Controller\ControllerBase;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Drupal\Core\Extension\ModuleHandlerInterface;

class MyModuleController extends ControllerBase
{

  /**
   * The module handler service.
   *
   * @var \Drupal\Core\Extension\ModuleHandlerInterface
   */
  protected $moduleHandler;

  /**
   * Constructs a MyController object
   *
   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
   *   The module handler service.
   */
  public function __construct(ModuleHandlerInterface $module_handler)
  {
    $this->moduleHandler = $module_handler;
  }

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