class DemoBase

abstract class DemoBase extends FormBase {
    
    /**
     * Implements a form submit handler.
     *
     * @param array $form
     *   The render array of the currently built form.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   Object describing the current state of the form.
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        // Find out what was submitted.
        $values = $form_state->getValues();
        foreach ($values as $key => $value) {
            $label = $form[$key]['#title'] ?? $key;
            // Many arrays return 0 for unselected values so lets filter that out.
            if (is_array($value)) {
                $value = array_filter($value);
            }
            // Only display for controls that have titles and values.
            if ($value) {
                $display_value = is_array($value) ? print_r($value, 1) : $value;
                $message = $this->t('Value for %title: %value', [
                    '%title' => $label,
                    '%value' => $display_value,
                ]);
                $this->messenger()
                    ->addMessage($message);
            }
        }
    }

}