class SimpleFormBlock

class SimpleFormBlock extends BlockBase implements ContainerFactoryPluginInterface {
    
    /**
     * Form builder service.
     *
     * @var \Drupal\Core\Form\FormBuilderInterface
     */
    protected $formBuilder;
    
    /**
     * {@inheritdoc}
     */
    public function __construct(array $configuration, $plugin_id, $plugin_definition, FormBuilderInterface $form_builder) {
        parent::__construct($configuration, $plugin_id, $plugin_definition);
        $this->formBuilder = $form_builder;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
        return new static($configuration, $plugin_id, $plugin_definition, $container->get('form_builder'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function build() {
        $output = [
            'description' => [
                '#markup' => $this->t('Using form provided by @classname', [
                    '@classname' => SimpleForm::class,
                ]),
            ],
        ];
        // Use the form builder service to retrieve a form by providing the full
        // name of the class that implements the form you want to display. getForm()
        // will return a render array representing the form that can be used
        // anywhere render arrays are used.
        //
        // In this case the build() method of a block plugin is expected to return
        // a render array so we add the form to the existing output and return it.
        $output['form'] = $this->formBuilder
            ->getForm(SimpleForm::class);
        return $output;
    }

}