class FormTestRebuildPreserveValuesForm

class FormTestRebuildPreserveValuesForm extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'form_test_form_rebuild_preserve_values_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        // Start the form with two checkboxes, to test different defaults, and a
        // textfield, to test more than one element type.
        $form = [
            'checkbox_1_default_off' => [
                '#type' => 'checkbox',
                '#title' => t('This checkbox defaults to unchecked'),
                '#default_value' => FALSE,
            ],
            'checkbox_1_default_on' => [
                '#type' => 'checkbox',
                '#title' => t('This checkbox defaults to checked'),
                '#default_value' => TRUE,
            ],
            'text_1' => [
                '#type' => 'textfield',
                '#title' => t('This textfield has a non-empty default value.'),
                '#default_value' => 'DEFAULT 1',
            ],
        ];
        // Provide an 'add more' button that rebuilds the form with an additional two
        // checkboxes and a textfield. The test is to make sure that the rebuild
        // triggered by this button preserves the user input values for the initial
        // elements and initializes the new elements with the correct default values.
        if (!$form_state->has('add_more')) {
            $form['add_more'] = [
                '#type' => 'submit',
                '#value' => 'Add more',
                '#submit' => [
                    '::addMoreSubmitForm',
                ],
            ];
        }
        else {
            $form += [
                'checkbox_2_default_off' => [
                    '#type' => 'checkbox',
                    '#title' => t('This checkbox defaults to unchecked'),
                    '#default_value' => FALSE,
                ],
                'checkbox_2_default_on' => [
                    '#type' => 'checkbox',
                    '#title' => t('This checkbox defaults to checked'),
                    '#default_value' => TRUE,
                ],
                'text_2' => [
                    '#type' => 'textfield',
                    '#title' => t('This textfield has a non-empty default value.'),
                    '#default_value' => 'DEFAULT 2',
                ],
            ];
        }
        // A submit button that finishes the form workflow (does not rebuild).
        $form['submit'] = [
            '#type' => 'submit',
            '#value' => 'Submit',
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function addMoreSubmitForm(array &$form, FormStateInterface $form_state) {
        // Rebuild, to test preservation of input values.
        $form_state->set('add_more', TRUE);
        $form_state->setRebuild();
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        // Finish the workflow. Do not rebuild.
        $this->messenger()
            ->addStatus($this->t('Form values: %values', [
            '%values' => var_export($form_state->getValues(), TRUE),
        ]));
    }

}