A Series of Fortunate Events
Matthias Noback
Twitter: @matthiasnoback

What are events, really?
Things that happen

They trigger actions

Just now...
Attendees arrived,
triggered me to turn on microphone,
which triggered you to stop talking,
which triggered me to start talking

Events in software
Events model what happened in a system

Other parts of the system
can respond to what happened

Imperative programming
Only commands
doThis();
doThat();
updateSomething($something);
return $something;

Extracting events
doThis();
// this was done
doThat();
// that was done
updateSomething($something)
// something was updated
return $something;

Starting position
class PostService
{
...
function addComment($postId, $comment)
{
$post = $this->fetchPost($postId);
$post->addComment($comment);
$this->save($post);
$this->logger->info('New comment');
$this->mailer->send('New comment');
}
}

Starting position
class PostService
{
function __construct(
Mailer $mailer,
Logger $logger
) {
$this->mailer = $mailer;
$this->logger = $logger;
}
function addComment($postId, $comment)
{
...
}
}

Making events explicit
class PostService
{
function addComment($postId, $comment)
{
...
$this->newCommentAdded();
}
function newCommentAdded()
{
$this->logger->info('New comment');
$this->mailer->send('New comment');
}
}

Dependency graph
PPoossttSSeerrvviiccee
Logger
Mailer

Design issues (1)
I don't think the PostService should
know how to use a Mailer and a Logger

Design issues (2)
I want to change the behavior of
PostService without modifying the
class itself

Fix the problems
By introducing events!
(later)

Observer pattern
Notify other parts of the application
when a change occurs
class PostService
{
function newCommentAdded()
{
foreach ($this->observers as $observer) {
$observer->notify();
}
}
}

Observer contract
Subject knows nothing about its
observers, except their very simple
interface
interface Observer
{
function notify();
}

Concrete observers
class LoggingObserver implements Observer
{
function __construct(Logger $logger)
{
$this->logger = $logger;
}
function notify()
{
$this->logger->info('New comment');
}
}

Concrete observers
class NotificationMailObserver implements Observer
{
function __construct(Mailer $mailer)
{
$this->mailer = $mailer;
}
function notify()
{
$this->mailer->send('New comment');
}
}

Configuration
class PostService
{
function __construct(array $observers)
{
$this->observers = $observers;
}
}
$postService = new PostService(
array(
new LoggingObserver($logger),
new NotificationMailObserver($mailer)
)
);

Before
PPoossttSSeerrvviiccee
Logger
Mailer

After
NotificationMailObserver
Observer
Observer
LoggingObserver
Logger
Mailer
PostService

Design Principles Party

Single responsibility
Each class has one small,
well-defined responsibility

Single responsibility
● PostService:
“add comments to posts”
● LoggingObserver:
“write a line to the log”
● NotificationMailObserver:
“send a notification mail”

Single responsibility
When a change is required, it can be
isolated to just a small part of the
application

Single responsibility
● “Capitalize the comment!”:
PostService
● “Use a different logger!”:
LoggerObserver
● “Add a timestamp to the notification mail!”:
NotificationMailObserver

Dependency inversion
Depend on abstractions, not on
concretions

Dependency inversion
First PostService depended on
something concrete: the Mailer, the
Logger.

PostService Logger
Mailer

Dependency inversion
Now it depends on something abstract:
an Observer

PostService
Observer
Observer

Dependency inversion
Only the concrete observers depend on
concrete things like Mailer and Logger

LoggingObserver
NotificationMailObserver
Logger
Mailer

Open/closed
A class should be open for extension and
closed for modification

Open/closed
You don't need to modify the class to
change its behavior

PostService
Observer
Observer Observer

Open/closed
We made it closed for modification,
open for extension

Event data
Mr. Boddy was murdered!
● By Mrs. Peacock
● In the dining room
● With a candle stick

Currently missing!
class LogNewCommentObserver implements Observer
{
function notify()
{
// we'd like to be more specific
$this->logger->info('New comment');
}
}

Event object
class CommentAddedEvent
{
public function __construct($postId, $comment)
{
$this->postId = $postId;
$this->comment = $comment;
}
function comment()
{
return $this->comment;
}
function postId()
{
return $this->postId;
}
}

Event object
We use the event object
to store the context of the event

From observer...
interface Observer
{
function notify();
}

… to event handler
interface CommentAddedEventHandler
{
function handle(CommentAddedEvent $event);
}

Event handlers
class LoggingEventHandler implements
CommentAddedEventHandler
{
function __construct(Logger $logger)
{
$this->logger = $logger;
}
public function handle(CommentAddedEvent $event)
{
$this->logger->info(
'New comment' . $event->comment()
);
}
}

Event handlers
class NotificationMailEventHandler implements
CommentAddedEventHandler
{
function __construct(Mailer $mailer)
{
$this->mailer = $mailer;
}
public function handle(CommentAddedEvent $event)
{
$this->mailer->send(
'New comment: ' . $event->comment();
);
}
}

Configuration
class PostService
{
function __construct(array $eventHandlers)
{
$this->eventHandlers = $eventHandlers;
}
}
$postService = new PostService(
array(
new LoggingEventHandler($logger),
new NotificationMailEventHandler($mailer)
)
);

Looping over event handlers
class PostService
{
public function addComment($postId, $comment)
{
$this->newCommentAdded($postId, $comment);
}
function newCommentAdded($postId, $comment)
{
$event = new CommentAddedEvent(
$postId,
$comment
);
foreach ($this->eventHandlers as $eventHandler) {
$eventHandler->handle($event);
}
}
}

Introducing a Mediator
Instead of talking to the event handlers
Let's leave the talking to a mediator

Mediators for events
● Doctrine, Zend: Event manager
● The PHP League: Event emitter
● Symfony: Event dispatcher

Before
PostService
LoggingEventHandler::handle()
NotificationMailEventHandler::handle()

After
PostService EventDispatcher
LoggingEventHandler::handle()
NotificationMailEventHandler::handle()

In code
class PostService
{
function __construct(EventDispatcherInterface $dispatcher)
{
$this->dispatcher = $dispatcher;
}
function newCommentAdded($postId, $comment)
{
$event = new CommentAddedEvent($postId, $comment);
$this->dispatcher->dispatch(
'comment_added',
$event
);
}
}

Event class
Custom event classes should extend
Symfony Event class:
use SymfonyComponentEventDispatcherEvent;
class CommentAddedEvent extends Event
{
...
}

Configuration
use SymfonyComponentEventDispatcherEvent;
$dispatcher = new EventDispatcher();
$loggingEventHandler = new
LoggingEventHandler($logger);
$dispatcher->addListener(
'comment_added',
array($loggingEventHandler, 'handle')
);
...
$postService = new PostService($dispatcher);

Symfony2
● An event dispatcher is available as the
event_dispatcher service
● You can register event listeners using
service tags

Inject the event dispatcher
# services.yml
services:
post_service:
class: PostService
arguments: [@event_dispatcher]

Register your listeners
# services.yml
services:
...
logging_event_handler:
class: LoggingEventHandler
arguments: [@logger]
tags:
- {
name: kernel.event_listener
event: comment_added
method: handle
}

Events and application flow
Symfony2 uses events to generate
response for any given HTTP request

The HttpKernel
$request = Request::createFromGlobals();
// $kernel is in an instance of HttpKernelInterface
$response = $kernel->handle($request);
$response->send();

Kernel events

kernel.request
● Route matching
● Authentication

kernel.controller
● Replace the controller
● Do some access checks

kernel.view
● Render a template

kernel.response
● Modify the response
● E.g. inject the Symfony toolbar

kernel.exception
● Generate a response
● Render a nice page with the stack trace

Special types of events
● Kernel events are not merely
notifications
● They allow other parts of the
application to step in and modify or
override behavior

Chain of responsibility
Some sort
of request
Handler 1 Handler 2 Handler 3
Some sort
of request
Some sort
of request
Response

Symfony example
I've got an exception!
What should I tell the user?
Listener 1 Listener 2 Listener 3
Exception! Exception!
Response

Propagation
class HandleExceptionListener
{
function onKernelException(
GetResponseForExceptionEvent $event
) {
$event->setResponse(new Response('Error!'));
// this is the best response ever, don't let
// other spoil it!
$event->stopPropagation();
}
}

Priorities
$dispatcher = new EventDispatcher();
$dispatcher->addListener(
'comment_added',
array($object, $method),
// priority
100
);

Concerns

Concern 1: Hard to understand
“Click-through understanding” impossible
$event = new CommentAddedEvent($postId, $comment);
$this->dispatcher->dispatch(
'comment_added',
$event
);

interface EventDispatcherInterface
{
function dispatch($eventName, Event $event = null);
...
}

Solution
Use Xdebug

Concern 2: Out-of-domain concepts
● “Comment”
● “PostId”
● “Add comment to post”
● “Dispatcher” (?!)

We did a good thing
We fixed coupling issues

But this guy, Coupling,
has a sister
She's called Cohesion

Cohesion
● Belonging together
● Concepts like “dispatcher”, “event listener”, even
“event”, don't belong in your code

Solutions (1)
Descriptive, explicit naming:
● NotificationMailEventListener becomes
SendNotificationMailWhenCommentAdded
● CommentAddedEvent becomes CommentAdded
● onCommentAdded becomes
whenCommentAdded

Solutions (1)
This also hides implementation details!

Solutions (2)
Use an event dispatcher for things
that are not naturally cohesive anyway

Solutions (2)
Use something else
when an event dispatcher causes low cohesion

Example: resolving the controller
$event = new GetResponseEvent($request);
$dispatcher->dispatch('kernel.request', $event);
$controller = $request->attributes->get('_controller');
$controller = $controllerResolver->resolve($request);

Concern 3: Loss of control
● You rely on event listeners to do some really
important work
● How do you know if they are in place
and do their job?

Solution
● “Won't fix”
● You have to learn to live with it

It's good

exercise
control
Inversion of control
give up
control!

Just like...
● A router determines the right controller
● The service container injects the right
constructor arguments
● And when you die, someone will bury your
body for you

I admit, inversion of control can be scary

But it will
● lead to better design
● require less change
● make maintenance easier

PresentationFinished
AskQuestionsWhenPresentationFinished
SayThankYouWhenNoMoreQuestions

Symfony, service definitions, kernel events
http://leanpub.com/a-year-with-symfony/c/symfony-camp
Pay even less ;)

Class and package design principles
http://leanpub.com/principles-of-php-package-design/c/symfony-camp
Get a $10 discount!

Design patterns
● Observer
● Mediator
● Chain of responsibility
● ...
Design Patterns by “The Gang of Four”

SOLID principles
● Single responsibility
● Open/closed
● Dependency inversion
● ...
Agile Software Development by Robert C. Martin

Images
● www.ohiseered.com/2011_11_01_archive.html
● Mrs. Peacock, Candlestick:
www.cluecult.com
● Leonardo DiCaprio:
screenrant.com/leonardo-dicaprio-defends-wolf-wall-street-controversy/
● Book covers:
Amazon
● Party:
todesignoffsite.com/events-2/to-do-closing-party-with-love-design/
● Russell Crowe:
malinaelena.wordpress.com/2014/04/18/top-8-filme-cu-russell-crowe/

What did you think?
joind.in/12541
Twitter: @matthiasnoback