class EventsExampleForm

class EventsExampleForm extends FormBase {
    
    /**
     * The event dispatcher service.
     *
     * @var \Symfony\Component\EventDispatcher\EventDispatcherInterface
     */
    protected $eventDispatcher;
    
    /**
     * Constructs a new UserLoginForm.
     *
     * @param \Symfony\Component\EventDispatcher\EventDispatcherInterface $event_dispatcher
     *   The event dispatcher service.
     */
    public function __construct(EventDispatcherInterface $event_dispatcher) {
        // The event dispatcher service is an implementation of
        // \Symfony\Component\EventDispatcher\EventDispatcherInterface. In Drupal
        // this is generally and instance of the
        // \Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher service.
        // This dispatcher improves performance when dispatching events by compiling
        // a list of subscribers into the service container so that they do not need
        // to be looked up every time.
        $this->eventDispatcher = $event_dispatcher;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        return new static($container->get('event_dispatcher'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['intro'] = [
            '#markup' => '<p>' . $this->t('This form demonstrates subscribing to, and dispatching, events. When the form is submitted an event is dispatched indicating a new report has been submitted. Event subscribers respond to this event with various messages depending on the incident type. Review the code for the events_example module to see how it works.') . '</p>',
        ];
        $form['incident_type'] = [
            '#type' => 'radios',
            '#required' => TRUE,
            '#title' => $this->t('What type of incident do you want to report?'),
            '#options' => [
                'stolen_princess' => $this->t('Missing princess'),
                'cat' => $this->t('Cat stuck in tree'),
                'joker' => $this->t('Something involving the Joker'),
            ],
        ];
        $form['incident'] = [
            '#type' => 'textarea',
            '#required' => FALSE,
            '#title' => $this->t('Incident report'),
            '#description' => $this->t('Describe the incident in detail. This information will be passed along to all crime fighters.'),
            '#cols' => 60,
            '#rows' => 5,
        ];
        $form['actions'] = [
            '#type' => 'actions',
        ];
        $form['actions']['submit'] = [
            '#type' => 'submit',
            '#value' => $this->t('Submit'),
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'events_example_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $type = $form_state->getValue('incident_type');
        $report = $form_state->getValue('incident');
        // When dispatching, or triggering, an event start by constructing a new
        // event object. Then use the event dispatcher service to notify any event
        // subscribers. Event objects are used to transport relevant data to any
        // subscribers, as well as keep track of the current state of an event. It
        // is best practice to create a unique class wrapping
        // \Symfony\Component\EventDispatcher\Event.
        $event = new IncidentReportEvent($type, $report);
        // Dispatch an event by specifying which event, and providing an event
        // object. Rather than hard code the event name you should use a constant
        // to represent the event being dispatched. The constant serves as a
        // location for documentation of the event, and ensures your code is future
        // proofed against event name changes.
        $this->eventDispatcher
            ->dispatch($event, IncidentEvents::NEW_REPORT);
    }

}