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.
    }

}