class Wizard

class Wizard extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'ajax_example_wizard';
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        // Since FormBase uses service traits, we can inject these services without
        // adding our own __construct() method.
        $form = new static($container);
        $form->setStringTranslation($container->get('string_translation'));
        $form->setMessenger($container->get('messenger'));
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state, $no_js_use = FALSE) {
        $url = Url::fromUri('internal:/examples/ajax-example/wizard-nojs');
        $link = Link::fromTextAndUrl($this->t('examples/ajax-example/wizard-nojs'), $url)
            ->toString();
        // Prepare link for multiple arguments.
        $urltwo = Url::fromUri('internal:/examples/ajax-example/wizard');
        $linktwo = Link::fromTextAndUrl($this->t('examples/ajax-example/wizard'), $urltwo)
            ->toString();
        // We want to deal with hierarchical form values.
        $form['#tree'] = TRUE;
        $form['description'] = [
            '#markup' => $this->t('This example is a step-by-step wizard. The @link does it without page reloads; the @link1 is the same code but simulates a non-javascript environment, showing it with page reloads.', [
                '@link' => $linktwo,
                '@link1' => $link,
            ]),
        ];
        $form['step'] = [
            '#type' => 'value',
            '#value' => !empty($form_state->getValue('step')) ? $form_state->getValue('step') : 1,
        ];
        switch ($form['step']['#value']) {
            case 1:
                $limit_validation_errors = [
                    [
                        'step',
                    ],
                ];
                $form['step1'] = [
                    '#type' => 'fieldset',
                    '#title' => $this->t('Step 1: Personal details'),
                ];
                $form['step1']['name'] = [
                    '#type' => 'textfield',
                    '#title' => $this->t('Your name'),
                    '#default_value' => $form_state->hasValue([
                        'step1',
                        'name',
                    ]) ? $form_state->getValue([
                        'step1',
                        'name',
                    ]) : '',
                    '#required' => TRUE,
                ];
                break;
            case 2:
                $limit_validation_errors = [
                    [
                        'step',
                    ],
                    [
                        'step1',
                    ],
                ];
                $form['step1'] = [
                    '#type' => 'value',
                    '#value' => $form_state->getValue('step1'),
                ];
                $form['step2'] = [
                    '#type' => 'fieldset',
                    '#title' => $this->t('Step 2: Street address info'),
                ];
                $form['step2']['address'] = [
                    '#type' => 'textfield',
                    '#title' => $this->t('Your street address'),
                    '#default_value' => $form_state->hasValue([
                        'step2',
                        'address',
                    ]) ? $form_state->getValue([
                        'step2',
                        'address',
                    ]) : '',
                    '#required' => TRUE,
                ];
                break;
            case 3:
                $limit_validation_errors = [
                    [
                        'step',
                    ],
                    [
                        'step1',
                    ],
                    [
                        'step2',
                    ],
                ];
                $form['step1'] = [
                    '#type' => 'value',
                    '#value' => $form_state->getValue('step1'),
                ];
                $form['step2'] = [
                    '#type' => 'value',
                    '#value' => $form_state->getValue('step2'),
                ];
                $form['step3'] = [
                    '#type' => 'fieldset',
                    '#title' => $this->t('Step 3: City info'),
                ];
                $form['step3']['city'] = [
                    '#type' => 'textfield',
                    '#title' => $this->t('Your city'),
                    '#default_value' => $form_state->hasValue([
                        'step3',
                        'city',
                    ]) ? $form_state->getValue([
                        'step3',
                        'city',
                    ]) : '',
                    '#required' => TRUE,
                ];
                break;
            default:
                $limit_validation_errors = [];
        }
        $form['actions'] = [
            '#type' => 'actions',
        ];
        if ($form['step']['#value'] > 1) {
            $form['actions']['prev'] = [
                '#type' => 'submit',
                '#value' => $this->t('Previous step'),
                '#limit_validation_errors' => $limit_validation_errors,
                '#submit' => [
                    '::prevSubmit',
                ],
                '#ajax' => [
                    'wrapper' => 'ajax-example-wizard-wrapper',
                    'callback' => '::prompt',
                ],
            ];
        }
        if ($form['step']['#value'] != 3) {
            $form['actions']['next'] = [
                '#type' => 'submit',
                '#value' => $this->t('Next step'),
                '#submit' => [
                    '::nextSubmit',
                ],
                '#ajax' => [
                    'wrapper' => 'ajax-example-wizard-wrapper',
                    'callback' => '::prompt',
                ],
            ];
        }
        if ($form['step']['#value'] == 3) {
            $form['actions']['submit'] = [
                '#type' => 'submit',
                '#value' => $this->t("Submit your information"),
            ];
        }
        // This simply allows us to demonstrate no-javascript use without
        // actually turning off javascript in the browser. Removing the #ajax
        // element turns off AJAX behaviors on that element and as a result
        // ajax.js doesn't get loaded.
        // For demonstration only! You don't need this.
        if ($no_js_use) {
            // Remove the #ajax from the above, so ajax.js won't be loaded.
            // For demonstration only.
            unset($form['actions']['next']['#ajax']);
            unset($form['actions']['prev']['#ajax']);
        }
        $form['#prefix'] = '<div id="ajax-example-wizard-wrapper">';
        $form['#suffix'] = '</div>';
        return $form;
    }
    
    /**
     * Wizard callback function.
     *
     * @param array $form
     *   Form API form.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   Form API form.
     *
     * @return array
     *   Form array.
     */
    public function prompt(array $form, FormStateInterface $form_state) {
        return $form;
    }
    
    /**
     * Ajax callback that moves the form to the next step and rebuild the form.
     *
     * @param array $form
     *   The Form API form.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The FormState object.
     *
     * @return array
     *   The Form API form.
     */
    public function nextSubmit(array $form, FormStateInterface $form_state) {
        $form_state->setValue('step', $form_state->getValue('step') + 1);
        $form_state->setRebuild();
        return $form;
    }
    
    /**
     * Ajax callback that moves the form to the previous step.
     *
     * @param array $form
     *   The Form API form.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The FormState object.
     *
     * @return array
     *   The Form API form.
     */
    public function prevSubmit(array $form, FormStateInterface $form_state) {
        $form_state->setValue('step', $form_state->getValue('step') - 1);
        $form_state->setRebuild();
        return $form;
    }
    
    /**
     * Save away the current information.
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $messenger = $this->messenger();
        $messenger->addMessage($this->t('Your information has been submitted:'));
        $messenger->addMessage($this->t('Name: @name', [
            '@name' => $form_state->getValue([
                'step1',
                'name',
            ]),
        ]));
        $messenger->addMessage($this->t('Address: @address', [
            '@address' => $form_state->getValue([
                'step2',
                'address',
            ]),
        ]));
        $messenger->addMessage($this->t('City: @city', [
            '@city' => $form_state->getValue([
                'step3',
                'city',
            ]),
        ]));
    }

}