class DynamicFormSections

class DynamicFormSections extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'ajax_example_dynamicsectiondegrades';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state, $nojs = NULL) {
        // Add our CSS and tiny JS to hide things when they should be hidden.
        $form['#attached']['library'][] = 'ajax_example/ajax_example.library';
        // Explanatory text with helpful links.
        $form['info'] = [
            '#markup' => $this->t('<p>Like other examples in this module, this form has a path that
        can be modified with /nojs to simulate its behavior without JavaScript.
        </p><ul>
        <li>@try_it_without_ajax</li>
        <li>@try_it_with_ajax</li>
      </ul>', [
                '@try_it_without_ajax' => Link::createFromRoute($this->t('Try it without AJAX'), 'ajax_example.dynamic_form_sections', [
                    'nojs' => 'nojs',
                ])
                    ->toString(),
                '@try_it_with_ajax' => Link::createFromRoute($this->t('Try it with AJAX'), 'ajax_example.dynamic_form_sections')
                    ->toString(),
            ]),
        ];
        $form['question_type_select'] = [
            // This is our select dropdown.
'#type' => 'select',
            '#title' => $this->t('Question style'),
            // We have a variety of form items you can use to get input from the user.
'#options' => [
                'Choose question style' => 'Choose question style',
                'Multiple Choice' => 'Multiple Choice',
                'True/False' => 'True/False',
                'Fill-in-the-blanks' => 'Fill-in-the-blanks',
            ],
            // The #ajax section tells the AJAX system that whenever this dropdown
            // emits an event, it should call the callback and put the resulting
            // content into the wrapper we specify. The questions-fieldset-wrapper is
            // defined below.
'#ajax' => [
                'wrapper' => 'questions-fieldset-wrapper',
                'callback' => '::promptCallback',
            ],
        ];
        // The CSS for this module hides this next button if JS is enabled.
        $form['question_type_submit'] = [
            '#type' => 'submit',
            '#value' => $this->t('Choose'),
            '#attributes' => [
                'class' => [
                    'ajax-example-inline',
                ],
            ],
            // No need to validate when submitting this.
'#limit_validation_errors' => [],
            '#validate' => [],
        ];
        // This section allows us to demonstrate no-AJAX use without turning off
        // javascript in the browser.
        if ($nojs != 'nojs') {
            // Allow JavaScript to hide the choose button if we're using AJAX.
            $form['question_type_submit']['#attributes']['class'][] = 'ajax-example-hide';
        }
        else {
            // Remove #ajax from the above, so it won't perform AJAX behaviors.
            unset($form['question_type_select']['#ajax']);
        }
        // This fieldset just serves as a container for the part of the form
        // that gets rebuilt. It has a nice line around it so you can see it.
        $form['questions_fieldset'] = [
            '#type' => 'details',
            '#title' => $this->t('Stuff will appear here'),
            '#open' => TRUE,
            // We set the ID of this fieldset to questions-fieldset-wrapper so the
            // AJAX command can replace it.
'#attributes' => [
                'id' => 'questions-fieldset-wrapper',
                'class' => [
                    'questions-wrapper',
                ],
            ],
        ];
        // When the AJAX request comes in, or when the user hit 'Submit' if there is
        // no JavaScript, the form state will tell us what the user has selected
        // from the dropdown. We can look at the value of the dropdown to determine
        // which secondary form to display.
        $question_type = $form_state->getValue('question_type_select');
        if (!empty($question_type) && $question_type !== 'Choose question style') {
            $form['questions_fieldset']['question'] = [
                '#markup' => $this->t('Who was the first president of the U.S.?'),
            ];
            // Build up a secondary form, based on the type of question the user
            // chose.
            switch ($question_type) {
                case 'Multiple Choice':
                    $form['questions_fieldset']['question'] = [
                        '#type' => 'radios',
                        '#title' => $this->t('Who was the first president of the United States'),
                        '#options' => [
                            'George Bush' => 'George Bush',
                            'Adam McGuire' => 'Adam McGuire',
                            'Abraham Lincoln' => 'Abraham Lincoln',
                            'George Washington' => 'George Washington',
                        ],
                    ];
                    break;
                case 'True/False':
                    $form['questions_fieldset']['question'] = [
                        '#type' => 'radios',
                        '#title' => $this->t('Was George Washington the first president of the United States?'),
                        '#options' => [
                            'George Washington' => 'True',
                            0 => 'False',
                        ],
                        '#description' => $this->t('Click "True" if you think George Washington was the first president of the United States.'),
                    ];
                    break;
                case 'Fill-in-the-blanks':
                    $form['questions_fieldset']['question'] = [
                        '#type' => 'textfield',
                        '#title' => $this->t('Who was the first president of the United States'),
                        '#description' => $this->t('Please type the correct answer to the question.'),
                    ];
                    break;
            }
            $form['questions_fieldset']['submit'] = [
                '#type' => 'submit',
                '#value' => $this->t('Submit your answer'),
            ];
        }
        return $form;
    }
    
    /**
     * Final submit handler.
     *
     * Reports what values were finally set.
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $messenger = $this->messenger();
        // This is only executed when a button is pressed, not when the AJAXfield
        // select is changed.
        // Now handle the case of the next, previous, and submit buttons.
        // Only submit will result in actual submission, all others rebuild.
        if ($form_state->getValue('question_type_submit') == 'Choose') {
            $form_state->setValue('question_type_select', $form_state->getUserInput()['question_type_select']);
            $form_state->setRebuild();
        }
        if ($form_state->getValue('submit') == 'Submit your answer') {
            $form_state->setRebuild(FALSE);
            $answer = $form_state->getValue('question');
            // Special handling for the checkbox.
            if ($answer == 1 && $form['questions_fieldset']['question']['#type'] == 'checkbox') {
                $answer = $form['questions_fieldset']['question']['#title'];
            }
            if ($answer == $this->t('George Washington')) {
                $messenger->addMessage($this->t('You got the right answer: @answer', [
                    '@answer' => $answer,
                ]));
            }
            else {
                $messenger->addMessage($this->t('Sorry, your answer (@answer) is wrong', [
                    '@answer' => $answer,
                ]));
            }
            return;
        }
        // Sets the form to be rebuilt after processing.
        $form_state->setRebuild();
    }
    
    /**
     * Callback for the select element.
     *
     * Since the questions_fieldset part of the form has already been built during
     * the AJAX request, we can return only that part of the form to the AJAX
     * request, and it will insert that part into questions-fieldset-wrapper.
     *
     * @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 form structure.
     */
    public function promptCallback(array $form, FormStateInterface $form_state) {
        return $form['questions_fieldset'];
    }

}