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);
    }

}