class DispatchForm

class DispatchForm extends FormBase {
    
    /**
     * The event_dispatcher service.
     *
     * @var \Symfony\Contracts\EventDispatcher\EventDispatcherInterface
     */
    protected $dispatcher;
    
    /**
     * Constructor.
     *
     * @param \Symfony\Contracts\EventDispatcher\EventDispatcherInterface $dispatcher
     *   The event_dispatcher service.
     */
    public function __construct(EventDispatcherInterface $dispatcher) {
        $this->dispatcher = $dispatcher;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        return new static($container->get('event_dispatcher'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'rules_test_event.dispatcher_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['event'] = [
            '#type' => 'radios',
            '#options' => [
                0 => $this->t('PlainEvent'),
                1 => $this->t('GenericEvent'),
                2 => $this->t('GetterEvent'),
            ],
            '#description' => $this->t('Choose Event to dispatch for testing.'),
        ];
        $form['submit'] = [
            '#type' => 'submit',
            '#value' => 'Dispatch event',
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $choice = $form_state->getValue('event');
        var_dump($choice);
        switch ($choice) {
            case 0:
                $event = new PlainEvent();
                break;
            case 1:
                $event = new GenericEvent('Test subject', [
                    'publicProperty' => 'public property',
                    'protectedProperty' => 'protected property',
                    'privateProperty' => 'private property',
                ]);
                break;
            case 2:
                $event = new GetterEvent();
                break;
        }
        $this->dispatcher
            ->dispatch($event, $event::EVENT_NAME);
    }

}