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);
    }

}