class SwitchUserForm

class SwitchUserForm extends FormBase {
    
    /**
     * The csrf token generator.
     *
     * @var \Drupal\Core\Access\CsrfTokenGenerator
     */
    protected $csrfToken;
    
    /**
     * Constructs a new SwitchUserForm object.
     *
     * @param \Drupal\Core\Access\CsrfTokenGenerator $csrf_token_generator
     *   The CSRF token generator.
     */
    public function __construct(CsrfTokenGenerator $csrf_token_generator) {
        $this->csrfToken = $csrf_token_generator;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        return new static($container->get('csrf_token'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'devel_switchuser_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $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,
            '#maxlength' => UserInterface::USERNAME_MAX_LENGTH,
            '#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 $form_state) {
        if (!($account = User::load($form_state->getValue('userid')))) {
            $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) {
        // 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);
    }

}