Event

... an event is an action or occurrence recognised by software that may be handled by the software.

https://en.wikipedia.org/wiki/Event_(computing)

Event

thing that happens or takes place, especially one of importance.

The NZ Pocket Oxford Dictionary

Lots of things happen in an application

Same terminology

jQuery('#eric').click(function (event) {
  console.log('Hello world!');
});
      

Same pattern

• Hooks
• Actions
• Rules

Event Dispatcher

The EventDispatcher component provides tools that allow your application components to communicate with each other by dispatching events and listening to them.

https://symfony.com/doc/current/components/event_dispatcher.html

Event Dispatcher

Event Dispatcher

Event Dispatcher

Event Dispatcher

Event Dispatcher

Lets build a form

We have a form that collects a name and email.

• Log the submission
• Send an email to the user
• Add the user to our CRM
• Post to slack
• Calculate if the user gets prize

Submission

public function submitForm(array &$form, FormStateInterface $form_state) {
  $name = $form_state->getValue('name');
  $email = $form_state->getValue('email');

  $this->logger('example_form')->notice('Somebody submitted the form...');
}
              

Add another thing

public function submitForm(array &$form, FormStateInterface $form_state) {
  $name = $form_state->getValue('name');
  $email = $form_state->getValue('email');

  $this->logger('example_form')->notice('Somebody submitted the form....');

  $this->mailer->mail('example', 'example_form', $email, $this->currentUser->getPreferredLangcode(), ['name' => $name]);

}
              

Add dependencies for the thing

public function create(ContainerInterface $container) {
  $form = new static();
  $form->setCurrentUser($container->get('current_user'));
  $form->setMailer($container->get('plugin.manager.mail'));
  return $form;
}
              

All behaviours added

public function submitForm(array &$form, FormStateInterface $form_state) {

  $name = $form_state->getValue('name');
  $email = $form_state->getValue('email');

  $this->logger('example_form')->notice('Somebody submitted the form.');

  $this->mailer->mail('example', 'example_form', $email, $this->currentUser->getPreferredLangcode(), ['name' => $name]);

  $this->crmManager->add($email, ['name' => $name]);

  $this->slack->sendMessage('A new user has submitted your awesome form', 'info');

  $date = new \DateTime('@' . $this->time->getRequestTime();
  if ($date->format('N') === 1 && $date->format('i') === 29) {
    drupal_set_message('Congratulations you are a winner...');
  }
}
              

Dependencies

public function create(ContainerInterface $container) {
  $form = new static();
  $form->setCurrentUser($container->get('current_user'));
  $form->setMailer($container->get('plugin.manager.mail'));
  $form->setCrmManager($container->get('custom.crm_manager'));
  $form->setSlack($container->get('slack.manager'));
  $form->setTime($container->get('datetime.time'));
  return $form;
}
              

Problems

• Submit method is doing too many things
• Form class knows too many things
• Hard to maintain
• Hard to reuse

Dependency graph

Dependency graph

Lines of communication

Dispatching an event

Designing an event

• What information are we communicating?
• Can that information change?
• Are we collecting any additional information along the way?

Documentation

final class ExampleModuleEvents {

  /** docBlock */
  const SIGNUP_FORM_SUBMIT = 'module_name.signup_form_submit';
}
              

Extend the event class

use Symfony\Component\EventDispatcher\Event;

class SignupFormEvent extends Event {

  protected $submittedName;
  protected $submittedEmail;

  public function __construct($name, $email) {
    $this->submittedName = $name;
    $this->submittedEmail = $email;
  }

  public function getSubmittedName() {
    return $this->submittedName;
  }

  public function getSubmittedEmail() {
    return $this->submittedEmail;
  }
}
              

Back to the form

public function create(ContainerInterface $container) {
  $form = new static();
  $form->setCurrentUser($container->get('current_user'));
  $form->setMailer($container->get('plugin.manager.mail'));
  $form->setCrmManager($container->get('custom.crm_manager'));
  $form->setSlack($container->get('slack.manager'));
  $form->setTime($container->get('datetime.time'));
  return $form;
}
              

Inject the event dispatcher into the form

public function create(ContainerInterface $container) {
  $form = new static();
  $form->setEventDispatcher($container->get('event_dispatcher'));
  return $form;
}
              

Dispatch the event

public function submitForm(array &$form, FormStateInterface $form_state) {
  $name = $form_state->getValue('name');
  $email = $form_state->getValue('email');

  $event = new SignupFormEvent($name, $email);

  $this->eventDispatcher->dispatch(ExampleModuleEvents::SIGNUP_FORM_SUBMIT, $event);
}
              

Subscribing to events

EventSubscriberInterface

public static function getSubscribedEvents();

Implement EventSubscriberInterface

use Symfony\Component\EventDispatcher\EventSubscriberInterface;

class RegistrationMailer implements EventSubscriberInterface {
  protected $mailManager;
  protected $currentUser;

  public function __construct(MailManagerInterface $mailManager, AccountProxyInterface $currentUser) {
    $this->mailManager = $mailManager;
    $this->currentUser = $currentUser;
  };

  public static function getSubscribedEvents() {
    $events = [];
    $events[ExampleModuleEvents::SIGNUP_FORM_SUBMIT][] = ['onRegister'];
    return $events;
  }

  public function onRegister(SignupFormEvent $event) { ... }
}
              

Within the subscriber

public function onRegister($event) {
  $module = 'event_demo';
  $key = 'register_interest';
  $to = $event->getSubmittedEmail();
  $params = ['name' => $event->getSubmittedName()];
  $language = $this->currentUser->getPreferredLangcode();
  $this->mailManager->mail($module, $key, $to, $language, $params);
}
              

services.yaml

services:
  example_module.registration_mailer:
    class: "Drupal\example_crm_module\EventSubscriber\RegistrationMailer"
    arguments: ["@plugin.manager.mail", "@current_user"]
    tags:
      - { name: event_subscriber }
              

Detour?

Service

Any PHP object that performs some sort of "global" task.

http://symfony.com/doc/current/book/service_container.html

Service container

Dependency Injection

Inject dependencies.

Not Dependency Injection:

public function myMethod() {
  $slack = new SlackClient();
  $slack->sendMessage('This is bad.', 'info');
}
              

Dependency Injection:

protected $slack;

public function __construct(SlackClientInterface $slack) {
  $this->slack = $slack;
}

public function myMethod() {
  $this->slack->sendMessage('This is better.', 'info');
}
              

Step 1: Configuration

Step 2: Magic

When we want this

arguments: ["@plugin.manager.mail"]

Or this

$container->get('@plugin.manager.mail')

Step 3: Service Container

Lines of communication

Dependency graph

Why is this good?

• Single Responsibility Principle
• Open / Closed
• Loose Coupling

Why might this be bad?

• Loose Coupling

Going further with events

• Priorities
• Stopping the event flow

Going futher with events

public static function getSubscribedEvents() {
  $events[ExampleModuleEvents::SIGNUP_FORM_SUBMIT][] = ['onRegister'];
  return $events;
}
              

Priority

public static function getSubscribedEvents() {
  $events[ExampleModuleEvents::SIGNUP_FORM_SUBMIT][] = ['onRegister', 0];
  return $events;
}
              

Priority

public static function getSubscribedEvents() {
  $events[ExampleModuleEvents::SIGNUP_FORM_SUBMIT][] = ['onRegister', 1000];
  return $events;
}
              

Stopping an event

public function onRegister($event) {
  $event->stopPropagation();
}
              

Workflow of a typical request

1. Request recieved
2. Resolve controller
3. Resolve controller arguments
4. Call controller
5. Response

Workflow of a request

1. Request recieved
   • kernel.request
2. Resolve controller
   • kernel.controller
3. Resolve controller arguments
   • kernel.controller_arguments
4. Call controller
   • kernel.view
   • kernel.response
5. Response delivered
   • kernel.terminate

• kernel.exception

KernelEvent

private $kernel;

private $request;

private $requestType;
      

GetResponseEvent Event

private $response;

public function getResponse()
 {
  return $this->response;
}

public function setResponse(Response $response)
 {
  $this->response = $response;
  $this->stopPropagation();
}
      

Chain of responsibility

Chain of responsibility

Chain of responsibility

kernel.request

• RedirectLeadingSlashesSubscriber, 1000
• MaintenanceModeSubscriber, 30

kernel.exception

404 pages

• DefaultExceptionHtmlSubscriber
• Fast404ExceptionHtmlSubscriber

Fast 404 listener

$request = $event->getRequest();

$config = $this->configFactory->get('system.performance');
$exclude_paths = $config->get('fast_404.exclude_paths');
if ($config->get('fast_404.enabled') && $exclude_paths && !preg_match($exclude_paths, $request->getPathInfo())) {
  $fast_paths = $config->get('fast_404.paths');
  if ($fast_paths && preg_match($fast_paths, $request->getPathInfo())) {
    $fast_404_html = strtr($config->get('fast_404.html'), ['@path' => Html::escape($request->getUri())]);
    $response = new Response($fast_404_html, Response::HTTP_NOT_FOUND);
    $event->setResponse($response);
  }
}
      

Default exception listener

$this->makeSubrequest($event, Url::fromRoute('system.404')->toString(), Response::HTTP_NOT_FOUND);