class UserSelection

class UserSelection extends DefaultSelection {
    
    /**
     * The database connection.
     *
     * @var \Drupal\Core\Database\Connection
     */
    protected $connection;
    
    /**
     * Constructs a new UserSelection 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\Entity\EntityTypeManagerInterface $entity_type_manager
     *   The entity type manager service.
     * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
     *   The module handler service.
     * @param \Drupal\Core\Session\AccountInterface $current_user
     *   The current user.
     * @param \Drupal\Core\Database\Connection $connection
     *   The database connection.
     * @param \Drupal\Core\Entity\EntityFieldManagerInterface $entity_field_manager
     *   The entity field manager.
     * @param \Drupal\Core\Entity\EntityTypeBundleInfoInterface $entity_type_bundle_info
     *   The entity type bundle info service.
     * @param \Drupal\Core\Entity\EntityRepositoryInterface $entity_repository
     *   The entity repository.
     */
    public function __construct(array $configuration, $plugin_id, $plugin_definition, EntityTypeManagerInterface $entity_type_manager, ModuleHandlerInterface $module_handler, AccountInterface $current_user, Connection $connection, EntityFieldManagerInterface $entity_field_manager = NULL, EntityTypeBundleInfoInterface $entity_type_bundle_info = NULL, EntityRepositoryInterface $entity_repository = NULL) {
        parent::__construct($configuration, $plugin_id, $plugin_definition, $entity_type_manager, $module_handler, $current_user, $entity_field_manager, $entity_type_bundle_info, $entity_repository);
        $this->connection = $connection;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
        return new static($configuration, $plugin_id, $plugin_definition, $container->get('entity_type.manager'), $container->get('module_handler'), $container->get('current_user'), $container->get('database'), $container->get('entity_field.manager'), $container->get('entity_type.bundle.info'), $container->get('entity.repository'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function defaultConfiguration() {
        return [
            'filter' => [
                'type' => '_none',
                'role' => NULL,
            ],
            'include_anonymous' => TRUE,
        ] + parent::defaultConfiguration();
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
        $configuration = $this->getConfiguration();
        $form['include_anonymous'] = [
            '#type' => 'checkbox',
            '#title' => $this->t('Include the anonymous user.'),
            '#default_value' => $configuration['include_anonymous'],
        ];
        // Add user specific filter options.
        $form['filter']['type'] = [
            '#type' => 'select',
            '#title' => $this->t('Filter by'),
            '#options' => [
                '_none' => $this->t('- None -'),
                'role' => $this->t('User role'),
            ],
            '#ajax' => TRUE,
            '#limit_validation_errors' => [],
            '#default_value' => $configuration['filter']['type'],
        ];
        $form['filter']['settings'] = [
            '#type' => 'container',
            '#attributes' => [
                'class' => [
                    'entity_reference-settings',
                ],
            ],
            '#process' => [
                [
                    '\\Drupal\\Core\\Field\\Plugin\\Field\\FieldType\\EntityReferenceItem',
                    'formProcessMergeParent',
                ],
            ],
        ];
        if ($configuration['filter']['type'] == 'role') {
            $form['filter']['settings']['role'] = [
                '#type' => 'checkboxes',
                '#title' => $this->t('Restrict to the selected roles'),
                '#required' => TRUE,
                '#options' => array_diff_key(user_role_names(TRUE), [
                    RoleInterface::AUTHENTICATED_ID => RoleInterface::AUTHENTICATED_ID,
                ]),
                '#default_value' => $configuration['filter']['role'],
            ];
        }
        $form += parent::buildConfigurationForm($form, $form_state);
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    protected function buildEntityQuery($match = NULL, $match_operator = 'CONTAINS') {
        $query = parent::buildEntityQuery($match, $match_operator);
        $configuration = $this->getConfiguration();
        // Filter out the Anonymous user if the selection handler is configured to
        // exclude it.
        if (!$configuration['include_anonymous']) {
            $query->condition('uid', 0, '<>');
        }
        // The user entity doesn't have a label column.
        if (isset($match)) {
            $query->condition('name', $match, $match_operator);
        }
        // Filter by role.
        if (!empty($configuration['filter']['role'])) {
            $query->condition('roles', $configuration['filter']['role'], 'IN');
        }
        // Adding the permission check is sadly insufficient for users: core
        // requires us to also know about the concept of 'blocked' and 'active'.
        if (!$this->currentUser
            ->hasPermission('administer users')) {
            $query->condition('status', 1);
        }
        return $query;
    }
    
    /**
     * {@inheritdoc}
     */
    public function createNewEntity($entity_type_id, $bundle, $label, $uid) {
        $user = parent::createNewEntity($entity_type_id, $bundle, $label, $uid);
        // In order to create a referenceable user, it needs to be active.
        if (!$this->currentUser
            ->hasPermission('administer users')) {
            
            /** @var \Drupal\user\UserInterface $user */
            $user->activate();
        }
        return $user;
    }
    
    /**
     * {@inheritdoc}
     */
    public function validateReferenceableNewEntities(array $entities) {
        $entities = parent::validateReferenceableNewEntities($entities);
        // Mirror the conditions checked in buildEntityQuery().
        if ($role = $this->getConfiguration()['filter']['role']) {
            $entities = array_filter($entities, function ($user) use ($role) {
                
                /** @var \Drupal\user\UserInterface $user */
                return !empty(array_intersect($user->getRoles(), $role));
            });
        }
        if (!$this->currentUser
            ->hasPermission('administer users')) {
            $entities = array_filter($entities, function ($user) {
                
                /** @var \Drupal\user\UserInterface $user */
                return $user->isActive();
            });
        }
        return $entities;
    }
    
    /**
     * {@inheritdoc}
     */
    public function entityQueryAlter(SelectInterface $query) {
        parent::entityQueryAlter($query);
        // Bail out early if we do not need to match the Anonymous user.
        if (!$this->getConfiguration()['include_anonymous']) {
            return;
        }
        if ($this->currentUser
            ->hasPermission('administer users')) {
            // In addition, if the user is administrator, we need to make sure to
            // match the anonymous user, that doesn't actually have a name in the
            // database.
            $conditions =& $query->conditions();
            foreach ($conditions as $key => $condition) {
                if ($key !== '#conjunction' && is_string($condition['field']) && $condition['field'] === 'users_field_data.name') {
                    // Remove the condition.
                    unset($conditions[$key]);
                    // Re-add the condition and a condition on uid = 0 so that we end up
                    // with a query in the form:
                    // WHERE (name LIKE :name) OR (:anonymous_name LIKE :name AND uid = 0)
                    $or = $this->connection
                        ->condition('OR');
                    $or->condition($condition['field'], $condition['value'], $condition['operator']);
                    // Sadly, the Database layer doesn't allow us to build a condition
                    // in the form ':placeholder = :placeholder2', because the 'field'
                    // part of a condition is always escaped.
                    // As a (cheap) workaround, we separately build a condition with no
                    // field, and concatenate the field and the condition separately.
                    $value_part = $this->connection
                        ->condition('AND');
                    $value_part->condition('anonymous_name', $condition['value'], $condition['operator']);
                    $value_part->compile($this->connection, $query);
                    $or->condition($this->connection
                        ->condition('AND')
                        ->where(str_replace($query->escapeField('anonymous_name'), ':anonymous_name', (string) $value_part), $value_part->arguments() + [
                        ':anonymous_name' => \Drupal::config('user.settings')->get('anonymous'),
                    ])
                        ->condition('base_table.uid', 0));
                    $query->condition($or);
                }
            }
        }
    }

}