class SubmitDriven

class SubmitDriven extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'ajax_example_autotextfields';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        // This container wil be replaced by AJAX.
        $form['container'] = [
            '#type' => 'container',
            '#attributes' => [
                'id' => 'box-container',
            ],
        ];
        // The box contains some markup that we can change on a submit request.
        $form['container']['box'] = [
            '#type' => 'markup',
            '#markup' => '<h1>Initial markup for box</h1>',
        ];
        $form['submit'] = [
            '#type' => 'submit',
            // The AJAX handler will call our callback, and will replace whatever page
            // element has id box-container.
'#ajax' => [
                'callback' => '::promptCallback',
                'wrapper' => 'box-container',
            ],
            '#value' => $this->t('Submit'),
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
    }
    
    /**
     * Callback for submit_driven example.
     *
     * Select the 'box' element, change the markup in it, and return it as a
     * renderable array.
     *
     * @return array
     *   Renderable array (the box element)
     */
    public function promptCallback(array &$form, FormStateInterface $form_state) {
        // In most cases, it is recommended that you put this logic in form
        // generation rather than the callback. Submit driven forms are an
        // exception, because you may not want to return the form at all.
        $element = $form['container'];
        $element['box']['#markup'] = "Clicked submit ({$form_state->getValue('op')}): " . date('c');
        return $element;
    }

}