class UserLoginBlock

class UserLoginBlock extends BlockBase implements ContainerFactoryPluginInterface, TrustedCallbackInterface {
  use RedirectDestinationTrait;
  
  /**
   * The route match.
   *
   * @var \Drupal\Core\Routing\RouteMatchInterface
   */
  protected $routeMatch;
  
  /**
   * Constructs a new UserLoginBlock instance.
   *
   * @param array $configuration
   *   The plugin configuration, i.e. an array with configuration values keyed
   *   by configuration option name. The special key 'context' may be used to
   *   initialize the defined contexts by setting it to an array of context
   *   values keyed by context names.
   * @param string $plugin_id
   *   The plugin_id for the plugin instance.
   * @param mixed $plugin_definition
   *   The plugin implementation definition.
   * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
   *   The route match.
   */
  public function __construct(array $configuration, $plugin_id, $plugin_definition, RouteMatchInterface $route_match) {
    parent::__construct($configuration, $plugin_id, $plugin_definition);
    $this->routeMatch = $route_match;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static($configuration, $plugin_id, $plugin_definition, $container->get('current_route_match'));
  }
  
  /**
   * {@inheritdoc}
   */
  protected function blockAccess(AccountInterface $account) {
    $route_name = $this->routeMatch
      ->getRouteName();
    if ($account->isAnonymous() && !in_array($route_name, [
      'user.login',
      'user.logout',
    ])) {
      return AccessResult::allowed()->addCacheContexts([
        'route.name',
        'user.roles:anonymous',
      ]);
    }
    return AccessResult::forbidden();
  }
  
  /**
   * {@inheritdoc}
   */
  public function build() {
    $form = \Drupal::formBuilder()->getForm('Drupal\\user\\Form\\UserLoginForm');
    unset($form['name']['#attributes']['autofocus']);
    // When unsetting field descriptions, also unset aria-describedby attributes
    // to avoid introducing an accessibility bug.
    // @todo Do this automatically in https://www.drupal.org/node/2547063.
    unset($form['name']['#description']);
    unset($form['name']['#attributes']['aria-describedby']);
    unset($form['pass']['#description']);
    unset($form['pass']['#attributes']['aria-describedby']);
    $form['name']['#size'] = 15;
    $form['pass']['#size'] = 15;
    // Instead of setting an actual action URL, we set the placeholder, which
    // will be replaced at the very last moment. This ensures forms with
    // dynamically generated action URLs don't have poor cacheability.
    // Use the proper API to generate the placeholder, when we have one. See
    // https://www.drupal.org/node/2562341. The placeholder uses a fixed string
    // that is
    // Crypt::hashBase64('\Drupal\user\Plugin\Block\UserLoginBlock::build');
    // This is based on the implementation in
    // \Drupal\Core\Form\FormBuilder::prepareForm(), but the user login block
    // requires different behavior for the destination query argument.
    $placeholder = 'form_action_p_4r8ITd22yaUvXM6SzwrSe9rnQWe48hz9k1Sxto3pBvE';
    $form['#attached']['placeholders'][$placeholder] = [
      '#lazy_builder' => [
        '\\Drupal\\user\\Plugin\\Block\\UserLoginBlock::renderPlaceholderFormAction',
        [],
      ],
    ];
    $form['#action'] = $placeholder;
    // Build action links.
    $items = [];
    if (\Drupal::config('user.settings')->get('register') != UserInterface::REGISTER_ADMINISTRATORS_ONLY) {
      $items['create_account'] = [
        '#type' => 'link',
        '#title' => $this->t('Create new account'),
        '#url' => Url::fromRoute('user.register', [], [
          'attributes' => [
            'title' => $this->t('Create a new user account.'),
            'class' => [
              'create-account-link',
            ],
          ],
        ]),
      ];
    }
    $items['request_password'] = [
      '#type' => 'link',
      '#title' => $this->t('Reset your password'),
      '#url' => Url::fromRoute('user.pass', [], [
        'attributes' => [
          'title' => $this->t('Send password reset instructions via email.'),
          'class' => [
            'request-password-link',
          ],
        ],
      ]),
    ];
    return [
      'user_login_form' => $form,
      'user_links' => [
        '#theme' => 'item_list',
        '#items' => $items,
      ],
    ];
  }
  
  /**
   * #lazy_builder callback; renders a form action URL including destination.
   *
   * @return array
   *   A renderable array representing the form action.
   *
   * @see \Drupal\Core\Form\FormBuilder::renderPlaceholderFormAction()
   */
  public static function renderPlaceholderFormAction() {
    return [
      '#type' => 'markup',
      '#markup' => UrlHelper::filterBadProtocol(Url::fromRoute('<current>', [], [
        'query' => \Drupal::destination()->getAsArray(),
        'external' => FALSE,
      ])
        ->toString()),
      '#cache' => [
        'contexts' => [
          'url.path',
          'url.query_args',
        ],
      ],
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public static function trustedCallbacks() {
    return [
      'renderPlaceholderFormAction',
    ];
  }

}