class AjaxFormsTestSimpleForm

class AjaxFormsTestSimpleForm extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'ajax_forms_test_simple_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $object = new Callbacks();
        $form = [];
        $form['select'] = [
            '#title' => $this->t('Color'),
            '#type' => 'select',
            '#options' => [
                'red' => 'red',
                'green' => 'green',
                'blue' => 'blue',
            ],
            '#ajax' => [
                'callback' => [
                    $object,
                    'selectCallback',
                ],
            ],
            '#suffix' => '<div id="ajax_selected_color">No color yet selected</div>',
        ];
        $form['checkbox'] = [
            '#type' => 'checkbox',
            '#title' => $this->t('Test checkbox'),
            '#ajax' => [
                'callback' => [
                    $object,
                    'checkboxCallback',
                ],
            ],
            '#suffix' => '<div id="ajax_checkbox_value">No action yet</div>',
        ];
        $form['submit'] = [
            '#type' => 'submit',
            '#value' => $this->t('submit'),
        ];
        // This is for testing invalid callbacks that should return a 500 error in
        // \Drupal\Core\Form\FormAjaxResponseBuilderInterface::buildResponse().
        $invalid_callbacks = [
            'null' => NULL,
            'empty' => '',
            'nonexistent' => 'some_function_that_does_not_exist',
        ];
        foreach ($invalid_callbacks as $key => $value) {
            $form['select_' . $key . '_callback'] = [
                '#type' => 'select',
                '#title' => $this->t('Test %key callbacks', [
                    '%key' => $key,
                ]),
                '#options' => [
                    'red' => 'red',
                    'green' => 'green',
                ],
                '#ajax' => [
                    'callback' => $value,
                ],
            ];
        }
        $form['test_group'] = [
            '#type' => 'details',
            '#title' => $this->t('Test group'),
            '#open' => TRUE,
        ];
        // Test ajax element in a #group.
        $form['checkbox_in_group_wrapper'] = [
            '#type' => 'container',
            '#attributes' => [
                'id' => 'checkbox-wrapper',
            ],
            '#group' => 'test_group',
            'checkbox_in_group' => [
                '#type' => 'checkbox',
                '#title' => $this->t('AJAX checkbox in a group'),
                '#ajax' => [
                    'callback' => [
                        $object,
                        'checkboxGroupCallback',
                    ],
                    'wrapper' => 'checkbox-wrapper',
                ],
            ],
            'nested_group' => [
                '#type' => 'details',
                '#title' => $this->t('Nested group'),
                '#open' => TRUE,
            ],
            'checkbox_in_nested' => [
                '#type' => 'checkbox',
                '#group' => 'nested_group',
                '#title' => $this->t('AJAX checkbox in a nested group'),
                '#ajax' => [
                    'callback' => [
                        $object,
                        'checkboxGroupCallback',
                    ],
                    'wrapper' => 'checkbox-wrapper',
                ],
            ],
        ];
        $form['another_checkbox_in_nested'] = [
            '#type' => 'checkbox',
            '#group' => 'nested_group',
            '#title' => $this->t('Another AJAX checkbox in a nested group'),
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
    }

}