class UserPasswordResetForm

class UserPasswordResetForm extends FormBase {
  
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'user_pass_reset';
  }
  
  /**
   * {@inheritdoc}
   *
   * @param array $form
   *   An associative array containing the structure of the form.
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *   The current state of the form.
   * @param \Drupal\Core\Session\AccountInterface $user
   *   User requesting reset.
   * @param string $expiration_date
   *   Formatted expiration date for the login link, or NULL if the link does
   *   not expire.
   * @param int $timestamp
   *   The current timestamp.
   * @param string $hash
   *   Login link hash.
   */
  public function buildForm(array $form, FormStateInterface $form_state, AccountInterface $user = NULL, $expiration_date = NULL, $timestamp = NULL, $hash = NULL) {
    if ($expiration_date) {
      $form['message'] = [
        '#markup' => $this->t('<p>This is a one-time login for %user_name and will expire on %expiration_date.</p><p>Click on this button to log in to the site and change your password.</p>', [
          '%user_name' => $user->getAccountName(),
          '%expiration_date' => $expiration_date,
        ]),
      ];
      $form['#title'] = $this->t('Reset password');
    }
    else {
      // No expiration for first time login.
      $form['message'] = [
        '#markup' => $this->t('<p>This is a one-time login for %user_name.</p><p>Click on this button to log in to the site and change your password.</p>', [
          '%user_name' => $user->getAccountName(),
        ]),
      ];
      $form['#title'] = $this->t('Set password');
    }
    $form['help'] = [
      '#markup' => '<p>' . $this->t('This login can be used only once.') . '</p>',
    ];
    $form['actions'] = [
      '#type' => 'actions',
    ];
    $form['actions']['submit'] = [
      '#type' => 'submit',
      '#value' => $this->t('Log in'),
    ];
    $form['#action'] = Url::fromRoute('user.reset.login', [
      'uid' => $user->id(),
      'timestamp' => $timestamp,
      'hash' => $hash,
    ])
      ->toString();
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    // This form works by submitting the hash and timestamp to the user.reset
    // route with a 'login' action.
  }

}