class TestAjaxBlock

class TestAjaxBlock extends BlockBase {
    
    /**
     * {@inheritdoc}
     */
    public function blockForm($form, FormStateInterface $form_state) {
        $form['ajax_test'] = [
            '#type' => 'radios',
            '#options' => [
                1 => $this->t('Ajax test option 1'),
                2 => $this->t('Ajax test option 2'),
            ],
            '#prefix' => '<div id="test-ajax-wrapper">',
            '#suffix' => '</div>',
            '#title' => $this->t('Time in this ajax test is @time', [
                '@time' => time(),
            ]),
            '#ajax' => [
                'wrapper' => 'test-ajax-wrapper',
                'callback' => [
                    $this,
                    'ajaxCallback',
                ],
            ],
        ];
        return $form;
    }
    
    /**
     * Ajax callback.
     */
    public function ajaxCallback($form, $form_state) {
        return $form['settings']['ajax_test'];
    }
    
    /**
     * {@inheritdoc}
     */
    public function build() {
        $build['content'] = [
            '#markup' => $this->t('Every word is like an unnecessary stain on silence and nothingness.'),
        ];
        return $build;
    }

}