Access data from previously submitted values in buildForm in FormBase plugin

Question

I have a FormBase plugin for an admin configuration page. My desired behavior is to select a series of checkboxes and when you submit the form based on those values a new set of fields will show. However I'm not sure how to build the form after submitting the form with new values.

Here's my current buildForm function

    $form['pages'] = [
        '#type' => 'checkboxes',
        '#title' => 'Pages to clone:',
        '#options' => [
            'about' => 'About',
            'home' => 'Home',
            'blog' => 'Blog',
        ],
    ];

    $form['actions'] = [
        '#type' => 'actions',
    ];
    $form['actions']['submit'] = [
        '#type' => 'submit',
        '#value' => $this->t('Submit'),
    ];

(Sidenote: Not sure if or why the last bit with the 'actions' is necessary) The form is build fine and in my submitForm function I have

public function submitForm(array &$form, FormStateInterface $form_state) {
    $form_state->set('pages', $form_state->getValue());
    $form_state->set('test_parameter', 'test_value');
}

However when I add a var_dump($form_state) to the buildForm function and submit the form like so

public function buildForm(array $form, FormStateInterface $form_state) { var_dump($form_state); $form_state->set('pages', $form_state->getValue()); $form_state->set('test_parameter', 'test_value'); }

I get some information about the form_state but nothing about the parameters I set.

object(Drupal\Core\Form\FormState)#599 (33) {
["complete_form":protected]=> NULL ["build_info":protected]=>
array(4) { ["args"]=> array(0) { } ["files"]=> array(0) { } ["callback_object"]=> object(Drupal\my_plugin\Form\PageCloner)#600 (10) { ["number":protected]=> int(1) ["requestStack":protected]=> NULL ["configFactory":protected]=> NULL ["routeMatch":protected]=> NULL ["_serviceIds":protected]=> array(0) { } ["linkGenerator":protected]=> NULL ["loggerFactory":protected]=> NULL ["redirectDestination":protected]=> NULL ["stringTranslation":protected]=> NULL ["urlGenerator":protected]=> NULL } ["form_id"]=> string(9) "page_pack" } ["rebuild_info":protected]=> array(0) { } ["rebuild":protected]=> bool(false)
["invalidToken":protected]=> bool(false) ["response":protected]=> NULL ["redirect":protected]=> NULL ["no_redirect":protected]=>
NULL ["method":protected]=> string(4) "POST"
["requestMethod":protected]=> string(3) "GET"
["cache":protected]=> bool(false) ["no_cache":protected]=> NULL ["values":protected]=> array(0) { }
["cleanValueKeys":protected]=> array(4) { [0]=> string(7) "form_id" [1]=> string(10) "form_token" [2]=> string(13) "form_build_id" [3]=> string(2) "op" } ["input":protected]=> array(0) { } ["always_process":protected]=> NULL ["must_validate":protected]=> NULL ["programmed":protected]=> bool(false)
["programmed_bypass_access_check":protected]=> bool(true)
["process_input":protected]=> NULL ["submitted":protected]=>
bool(false) ["executed":protected]=> bool(false)
["triggering_element":protected]=> NULL
["has_file_element":protected]=> NULL ["groups":protected]=>
array(0) { } ["storage":protected]=> array(0) { }
["buttons":protected]=> array(0) { } ["temporary":protected]=>
array(0) { } ["validation_complete":protected]=> bool(false)
["errors":protected]=> array(0) { }
["limit_validation_errors":protected]=> NULL
["validate_handlers":protected]=> array(0) { }
["submit_handlers":protected]=> array(0) { } }

What is the best way to get information retrieved in the formSubmit function back to when the buildForm function is fired again after submit? Or is there a better way I'm not considering to create a new form based on these values?

UPDATE: 4k4's solution worked but I ran into another issue where the form state wasn't properly saving. I resolved this by adding

parent::submitForm($form, $form_state);

to the submitForm function before $form_state->setRebuild(); this led to an error saying

Cannot call abstract method Drupal\Core\Form\FormInterface::submitForm()

to resolve that I changed the form from extending FormBase to extending ConfigFormBase instead like so

class MyPlugin extends ConfigFormBase {

Answer 1

If you want to rebuild the form with the values of $form_state you have to set the form to be rebuilt after processing:

public function submitForm(array &$form, FormStateInterface $form_state) {
  $form_state->set('test_parameter', 'test_value');
  $form_state->setRebuild();
}

Otherwise the form redirects to a new form build.