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',
        ];
    }

}