Matters of State

Matters of State
Using Symfony’s Event Dispatcher to  
turn your application into a simple state machine

Photo by jeff_golden
Bon anniversaire Symfony!

About me
• Portland
• twitter.com/kriswallsmith
• github.com/kriswallsmith
• kriswallsmith.net

var HelloWorld = React.createClass({
render: function() {
return (
<div>Hello World</div>
)
}
})

var HelloWorld = React.createClass({
return (
<div>Hello World</div>
)
}
})
JSX

(everything ﬂows from state)

“React thinks of UIs as simple state machines. By
thinking of a UI as being in various states and rendering
those states, it's easy to keep your UI consistent.”
https://facebook.github.io/react/docs/interactivity-and-dynamic-uis.html#components-are-just-state-machines

<button id="click_button">
0 click(s)
</button>

$('#my_button').click(function() {
var $btn = $(this);
var clicks = $btn.text().match(/d+/)[0];
var text = clicks + ' click';
if (clicks != 1) text += 's';
$btn.text(text);
})

$('#my_button').click(function() {
var $btn = $(this);
var clicks = $btn.text().match(/d+/)[0];
var text = clicks + ' click';
if (clicks != 1) text += 's';
$btn.text(text);
})
oops…

Imperative vs. Declarative Photo by

(code that explains how
to reach your goal)

(code that describes
what your goal is)

Take the next right
• Depress the gas pedal and increase your speed to 10mph
• Engage your right turn signal
• As you approach the intersection, remove your foot from the gas
pedal and gently apply the brake
• Check for pedestrians crossing the intersection
• Check your mirrors and blind spot for bicyclists
• Lift your foot from the brake and slowly increase speed by
depressing the gas pedal
• Turn the steering wheel to the right

Take the next right
declarative

Take the next right
imperative

SELECT * FROM users WHERE username="kriswallsmith";

public function registerAction(Request $request)
{
$form = $this->createForm('user');
$form->handleRequest($request);
if ($form->isValid()) {
$user = $form->getData();
$message = Swift_Message::newInstance()
->setTo($user->getEmail())
->setSubject('Welcome!')
->setBody($this->renderView(
'AppBundle:Email:welcome.html.twig',
['user' => $user]
));
$mailer = $this->get('mailer');
$mailer->send($message);
$em = $this->get('doctrine')->getManager();
$em->persist($user);
$em->flush();
return $this->redirectToRoute('home');
}
return $this->render(
'AppBundle:User:register.html.twig',
['form' => $form->createView()]
);
}

{
$mailer = $this->get('app.mailer');
$mailer->sendWelcomeEmail($user);
$em->flush();
}
);
}

{
$mailer = $this->get('app.mailer');
$mailer->sendWelcomeEmail($user);
$em->flush();
}
);
}
declarative!

{
$em->persist($form->getData());
$em->flush();
}
);
}

Excuse me Kris, 
but what about the email?

Event Dispatcher
Photo by rhodesj

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

public function addListener(
$eventName,
$listener,
$priority = 0
);
public function dispatch(
$eventName,
Event $event = null
);

$dispatcher->dispatch('user.create');

$dispatcher->dispatch(
'user.create',
new UserEvent($user)
);

$dispatcher->dispatch(
UserEvents::CREATE,
);

$dispatcher->addListener(
UserEvents::CREATE,
function(UserEvent $event) {
echo "Welcome, {$event->getUserName()}!";
}
);

$dispatcher->addListener(
UserEvents::CREATE,
[$listener, 'onUserCreate']
);

Something just happened
or is about to happen

Photo by Simon Law
Reactive Listeners

class UserListener
{
public function onUserCreate(UserEvent $event)
{
$user = $event->getUser();
$this->mailer->sendWelcomeEmail($user);
}
}

class ActivityFeedListener
{
public function onUserCreate(UserEvent $event)
{
$this->feed->logRegistration($user);
}
}

class ActivityFeedListener
{
public function onFollow(UserUserEvent $event)
{
$otherUser = $event->getOtherUser();
$this->feed->logFollow($user, $otherUser);
}
}

{
$dispatcher = $this->get('event_dispatcher');
$dispatcher->dispatch(UserEvents::CREATE, new UserEvent($user));
$em->persist();
$em->flush();
}
'AppBundle:Register:register.html.twig',
);
}

class UserManager
{
public function persistUser(User $user)
{
$this->em->persist($user);
$this->dispatcher->dispatch(
UserEvents::CREATE,
);
}
}

class FlushListener
{
public function preFlush(PreFlushEventArgs $event)
{
$em = $event->getEntityManager();
$uow = $em->getUnitOfWork();
foreach ($uow->getIdentityMap() as $class => $objects) {
if (User::class === $class) {
foreach ($objects as $user) {
if ($uow->isScheduledForInsert($user)) {
$this->dispatcher->dispatch(
UserEvents::CREATE,
);
}
}
}
}
}
}

That ﬂush listener is ugly…

What if…
Photo by Jared Cherup
What if…

/** @OnCreate(UserEvents::CREATE) */
class User
{
}

/** @OnChange(UserEvents::CHANGE) */
class User
{
}

/** @OnDestroy(UserEvents::DESTROY) */
class User
{
}

/**
* @OnCreate(UserEvents::CREATE)
* @OnUpdate(UserEvents::UPDATE)
* @OnDestroy(UserEvents::DESTROY)
*/
class User
{
}

class User
{
/** @OnChange(UserEvents::USERNAME_CHANGE) */
private $username;
}

class User
{
/**
* @OnAdd(UserEvents::FAVORITE)
* @OnRemove(UserEvents::UNFAVORITE)
*/
private $favorites;
}

class User
{
/**
* @OnAdd(
* UserEvents::FIRST_FAVORITE,
* unless="old_value()"
* )
*/
private $favorites;
}

class User
{
/**
* @OnAdd(
* UserEvents::FOLLOW,
* class=UserUserEvent::class,
* arguments="[object, added_value()]"
* )
*/
private $follows;
}

class User
{
/**
* @OnAdd(
* UserEvents::FOLLOW,
* before=ActivityEvents::CREATE
* )
*/
private $follows;
}

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