class SwitchUserBlock

class SwitchUserBlock extends BlockBase implements ContainerFactoryPluginInterface {
    use RedirectDestinationTrait;
    
    /**
     * The FormBuilder object.
     *
     * @var \Drupal\Core\Form\FormBuilderInterface
     */
    protected $formBuilder;
    
    /**
     * The Current User object.
     *
     * @var \Drupal\Core\Session\AccountInterface
     */
    protected $currentUser;
    
    /**
     * The user storage.
     *
     * @var \Drupal\Core\Entity\EntityStorageInterface
     */
    protected $userStorage;
    
    /**
     * Constructs a new SwitchUserBlock object.
     *
     * @param array $configuration
     *   A configuration array containing information about the plugin instance.
     * @param string $plugin_id
     *   The plugin_id for the plugin instance.
     * @param mixed $plugin_definition
     *   The plugin implementation definition.
     * @param \Drupal\Core\Session\AccountInterface $current_user
     *   Current user.
     * @param \Drupal\Core\Entity\EntityStorageInterface $user_storage
     *   The user storage.
     * @param \Drupal\Core\Form\FormBuilderInterface $form_builder
     *   The form builder service.
     */
    public function __construct(array $configuration, $plugin_id, $plugin_definition, AccountInterface $current_user, EntityStorageInterface $user_storage, FormBuilderInterface $form_builder) {
        parent::__construct($configuration, $plugin_id, $plugin_definition);
        $this->formBuilder = $form_builder;
        $this->currentUser = $current_user;
        $this->userStorage = $user_storage;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
        return new static($configuration, $plugin_id, $plugin_definition, $container->get('current_user'), $container->get('entity_type.manager')
            ->getStorage('user'), $container->get('form_builder'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function defaultConfiguration() {
        return [
            'list_size' => 12,
            'include_anon' => FALSE,
            'show_form' => TRUE,
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function blockAccess(AccountInterface $account) {
        return AccessResult::allowedIfHasPermission($account, 'switch users');
    }
    
    /**
     * {@inheritdoc}
     */
    public function blockForm($form, FormStateInterface $form_state) {
        $anonymous = new AnonymousUserSession();
        $form['list_size'] = [
            '#type' => 'number',
            '#title' => $this->t('Number of users to display in the list'),
            '#default_value' => $this->configuration['list_size'],
            '#min' => 1,
            '#max' => 50,
        ];
        $form['include_anon'] = [
            '#type' => 'checkbox',
            '#title' => $this->t('Include %anonymous', [
                '%anonymous' => $anonymous->getDisplayName(),
            ]),
            '#default_value' => $this->configuration['include_anon'],
        ];
        $form['show_form'] = [
            '#type' => 'checkbox',
            '#title' => $this->t('Allow entering any user name'),
            '#default_value' => $this->configuration['show_form'],
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function blockSubmit($form, FormStateInterface $form_state) {
        $this->configuration['list_size'] = $form_state->getValue('list_size');
        $this->configuration['include_anon'] = $form_state->getValue('include_anon');
        $this->configuration['show_form'] = $form_state->getValue('show_form');
    }
    
    /**
     * {@inheritdoc}
     */
    public function getCacheMaxAge() {
        return 0;
    }
    
    /**
     * {@inheritdoc}
     */
    public function build() {
        $build = [];
        if ($accounts = $this->getUsers()) {
            $build['devel_links'] = $this->buildUserList($accounts);
            if ($this->configuration['show_form']) {
                $build['devel_form'] = $this->formBuilder
                    ->getForm('\\Drupal\\devel\\Form\\SwitchUserForm');
            }
        }
        return $build;
    }
    
    /**
     * Provides the list of accounts that can be used for the user switch.
     *
     * Inactive users are omitted from all of the following db selects. Users
     * with 'switch users' permission and anonymous user if include_anon property
     * is set to TRUE, are prioritized.
     *
     * @return \Drupal\Core\Session\AccountInterface[]
     *   List of accounts to be used for the switch.
     */
    protected function getUsers() {
        $list_size = $this->configuration['list_size'];
        $include_anonymous = $this->configuration['include_anon'];
        $list_size = $include_anonymous ? $list_size - 1 : $list_size;
        // Users with 'switch users' permission are prioritized so
        // we try to load first users with this permission.
        $query = $this->userStorage
            ->getQuery()
            ->condition('uid', 0, '>')
            ->condition('status', 0, '>')
            ->sort('access', 'DESC')
            ->range(0, $list_size);
        $roles = user_roles(TRUE, 'switch users');
        if (!empty($roles) && !isset($roles[Role::AUTHENTICATED_ID])) {
            $query->condition('roles', array_keys($roles), 'IN');
        }
        $user_ids = $query->execute();
        // If we don't have enough users with 'switch users' permission, add
        // uids until we hit $list_size.
        if (count($user_ids) < $list_size) {
            $query = $this->userStorage
                ->getQuery()
                ->condition('uid', 0, '>')
                ->condition('status', 0, '>')
                ->sort('access', 'DESC')
                ->range(0, $list_size);
            // Excludes the prioritized user ids only if the previous query return
            // some records.
            if (!empty($user_ids)) {
                $query->condition('uid', array_keys($user_ids), 'NOT IN');
                $query->range(0, $list_size - count($user_ids));
            }
            $user_ids += $query->execute();
        }
        
        /** @var \Drupal\Core\Session\AccountInterface[] $accounts */
        $accounts = $this->userStorage
            ->loadMultiple($user_ids);
        if ($include_anonymous) {
            $anonymous = new AnonymousUserSession();
            $accounts[$anonymous->id()] = $anonymous;
        }
        uasort($accounts, 'static::sortUserList');
        return $accounts;
    }
    
    /**
     * Builds the user listing as renderable array.
     *
     * @param \Drupal\core\Session\AccountInterface[] $accounts
     *   The accounts to be rendered in the list.
     *
     * @return array
     *   A renderable array.
     */
    protected function buildUserList(array $accounts) {
        $links = [];
        foreach ($accounts as $account) {
            $links[$account->id()] = [
                'title' => $account->getDisplayName(),
                'url' => Url::fromRoute('devel.switch', [
                    'name' => $account->getAccountName(),
                ]),
                'query' => $this->getDestinationArray(),
                'attributes' => [
                    'title' => $account->hasPermission('switch users') ? $this->t('This user can switch back.') : $this->t('Caution: this user will be unable to switch back.'),
                ],
            ];
            if ($account->isAnonymous()) {
                $links[$account->id()]['url'] = Url::fromRoute('user.logout');
            }
            if ($this->currentUser
                ->id() === $account->id()) {
                $links[$account->id()]['title'] = new FormattableMarkup('<strong>%user</strong>', [
                    '%user' => $account->getDisplayName(),
                ]);
            }
        }
        return [
            '#theme' => 'links',
            '#links' => $links,
            '#attached' => [
                'library' => [
                    'devel/devel',
                ],
            ],
        ];
    }
    
    /**
     * Helper callback for uasort() to sort accounts by last access.
     *
     * @param \Drupal\Core\Session\AccountInterface $a
     *   First account.
     * @param \Drupal\Core\Session\AccountInterface $b
     *   Second account.
     *
     * @return int
     *   Result of comparing the last access times:
     *   - -1 if $a was more recently accessed
     *   -  0 if last access times compare equal
     *   -  1 if $b was more recently accessed
     */
    public static function sortUserList(AccountInterface $a, AccountInterface $b) {
        $a_access = (int) $a->getLastAccessedTime();
        $b_access = (int) $b->getLastAccessedTime();
        if ($a_access === $b_access) {
            return 0;
        }
        // User never access to site.
        if ($a_access === 0) {
            return 1;
        }
        return $a_access > $b_access ? -1 : 1;
    }

}