class SwitchUserForm

class SwitchUserForm extends FormBase {
  
  /**
   * The csrf token generator.
   */
  protected CsrfTokenGenerator $csrfToken;
  
  /**
   * The user storage.
   */
  protected UserStorageInterface $userStorage;
  
  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) : static {
    $instance = parent::create($container);
    $instance->csrfToken = $container->get('csrf_token');
    $instance->userStorage = $container->get('entity_type.manager')
      ->getStorage('user');
    $instance->stringTranslation = $container->get('string_translation');
    return $instance;
  }
  
  /**
   * {@inheritdoc}
   */
  public function getFormId() : string {
    return 'devel_switchuser_form';
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) : array {
    $form['autocomplete'] = [
      '#type' => 'container',
      '#attributes' => [
        'class' => [
          'container-inline',
        ],
      ],
    ];
    $form['autocomplete']['userid'] = [
      '#type' => 'entity_autocomplete',
      '#title' => $this->t('Username'),
      '#placeholder' => $this->t('Enter username'),
      '#target_type' => 'user',
      '#selection_settings' => [
        'include_anonymous' => FALSE,
      ],
      '#process_default_value' => FALSE,
      '#title_display' => 'invisible',
      '#required' => TRUE,
      '#size' => '28',
    ];
    $form['autocomplete']['actions'] = [
      '#type' => 'actions',
    ];
    $form['autocomplete']['actions']['submit'] = [
      '#type' => 'submit',
      '#value' => $this->t('Switch'),
    ];
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) : void {
    $userId = $form_state->getValue('userid');
    if ($userId === NULL) {
      $form_state->setErrorByName('userid', $this->t('Username not found'));
      return;
    }
    /** @var \Drupal\user\UserInterface|null $account */
    $account = $this->userStorage
      ->load($userId);
    if ($account === NULL) {
      $form_state->setErrorByName('userid', $this->t('Username not found'));
    }
    else {
      $form_state->setValue('username', $account->getAccountName());
    }
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) : void {
    // We cannot rely on automatic token creation, since the csrf seed changes
    // after the redirect and the generated token is not more valid.
    // @todo find another way to do this.
    $url = Url::fromRoute('devel.switch', [
      'name' => $form_state->getValue('username'),
    ]);
    $url->setOption('query', [
      'token' => $this->csrfToken
        ->get($url->getInternalPath()),
    ]);
    $form_state->setRedirectUrl($url);
  }

}