class MultistepForm

class MultistepForm extends FormBase {
  
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'form_api_example_multistep_form';
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    if ($form_state->has('page_num') && $form_state->get('page_num') == 2) {
      return $this->fapiExamplePageTwo($form, $form_state);
    }
    $form_state->set('page_num', 1);
    $form['description'] = [
      '#type' => 'item',
      '#title' => $this->t('A basic multistep form (page 1)'),
    ];
    $form['first_name'] = [
      '#type' => 'textfield',
      '#title' => $this->t('First Name'),
      '#description' => $this->t('Enter your first name.'),
      '#default_value' => $form_state->getValue('first_name', ''),
      '#required' => TRUE,
    ];
    $form['last_name'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Last Name'),
      '#default_value' => $form_state->getValue('last_name', ''),
      '#description' => $this->t('Enter your last name.'),
    ];
    $form['birth_year'] = [
      '#type' => 'number',
      '#title' => $this->t('Birth Year'),
      '#default_value' => $form_state->getValue('birth_year', ''),
      '#description' => $this->t('Format is "YYYY" and value between 1900 and 2000'),
    ];
    // Group submit handlers in an actions element with a key of "actions" so
    // that it gets styled correctly, and so that other modules may add actions
    // to the form. This is not required, but is convention.
    $form['actions'] = [
      '#type' => 'actions',
    ];
    $form['actions']['next'] = [
      '#type' => 'submit',
      '#button_type' => 'primary',
      '#value' => $this->t('Next'),
      // Custom submission handler for page 1.
'#submit' => [
        '::fapiExampleMultistepFormNextSubmit',
      ],
      // Custom validation handler for page 1.
'#validate' => [
        '::fapiExampleMultistepFormNextValidate',
      ],
    ];
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $page_values = $form_state->get('page_values');
    $this->messenger()
      ->addMessage($this->t('The form has been submitted. name="@first @last", year of birth=@year_of_birth', [
      '@first' => $page_values['first_name'],
      '@last' => $page_values['last_name'],
      '@year_of_birth' => $page_values['birth_year'],
    ]));
    $this->messenger()
      ->addMessage($this->t('And the favorite color is @color', [
      '@color' => $form_state->getValue('color'),
    ]));
  }
  
  /**
   * Provides custom validation handler for page 1.
   *
   * @param array $form
   *   An associative array containing the structure of the form.
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *   The current state of the form.
   */
  public function fapiExampleMultistepFormNextValidate(array &$form, FormStateInterface $form_state) {
    $birth_year = $form_state->getValue('birth_year');
    if ($birth_year != '' && ($birth_year < 1900 || $birth_year > 2000)) {
      // Set an error for the form element with a key of "birth_year".
      $form_state->setErrorByName('birth_year', $this->t('Enter a year between 1900 and 2000.'));
    }
  }
  
  /**
   * Provides custom submission handler for page 1.
   *
   * @param array $form
   *   An associative array containing the structure of the form.
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *   The current state of the form.
   */
  public function fapiExampleMultistepFormNextSubmit(array &$form, FormStateInterface $form_state) {
    $form_state->set('page_values', [
      // Keep only first step values to minimize stored data.
'first_name' => $form_state->getValue('first_name'),
      'last_name' => $form_state->getValue('last_name'),
      'birth_year' => $form_state->getValue('birth_year'),
    ])
      ->set('page_num', 2)
      ->setRebuild(TRUE);
  }
  
  /**
   * Builds the second step form (page 2).
   *
   * @param array $form
   *   An associative array containing the structure of the form.
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *   The current state of the form.
   *
   * @return array
   *   The render array defining the elements of the form.
   */
  public function fapiExamplePageTwo(array &$form, FormStateInterface $form_state) {
    $form['description'] = [
      '#type' => 'item',
      '#title' => $this->t('A basic multistep form (page 2)'),
    ];
    $form['color'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Favorite color'),
      '#required' => TRUE,
      '#default_value' => $form_state->getValue('color', ''),
    ];
    $form['actions'] = [
      '#type' => 'actions',
    ];
    $form['actions']['back'] = [
      '#type' => 'submit',
      '#value' => $this->t('Back'),
      // Custom submission handler for 'Back' button.
'#submit' => [
        '::fapiExamplePageTwoBack',
      ],
      // We won't bother validating the required 'color' field, since they
      // have to come back to this page to submit anyway.
'#limit_validation_errors' => [],
    ];
    $form['actions']['submit'] = [
      '#type' => 'submit',
      '#button_type' => 'primary',
      '#value' => $this->t('Submit'),
    ];
    return $form;
  }
  
  /**
   * Provides custom submission handler for 'Back' button (page 2).
   *
   * @param array $form
   *   An associative array containing the structure of the form.
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *   The current state of the form.
   */
  public function fapiExamplePageTwoBack(array &$form, FormStateInterface $form_state) {
    $form_state->setValues($form_state->get('page_values'))
      ->set('page_num', 1)
      ->setRebuild(TRUE);
  }

}