Better Living Through Events

Effectively decoupling code can be difficult: the user clicks a button and 12 things need to happen, but you don't want those things all tied to your button code. Events to the rescue! In this talk, Bill will discuss events, what they are, patterns for using them, some libraries out there to help you write event-driven code.

Transcript

1. Better Living Through Events
   order.complete
   sendEmail() updateReport() drinkBeer()
   July 9, 2013 @ Nashville PHP
   

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2. @epochblue
   

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3. why are we here?
   

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4. Let’s talk about...
   hot chicken
   

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5. a hot chicken ordering system
   Input order
   Order Heard Order Started Order Checked Order Finished Order Up
   Order delivered
   

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6. a hot chicken ordering system
   Input order
   updateOrder() updateOrder() updateOrder() updateOrder() updateOrder()
   Order delivered
   

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7. sounds simple enough, right?
   function updateOrder() {
   $status = $this->getStatus();
   switch($status) {
   case 'NEW':
   $this->setHeardTime(new \DateTime());
   $this->setStatus('HEARD')->save();
   break;
   // ... repeat for other states
   }
   }
   

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8. the marketing department appears
   

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9. email order updates
   function updateOrder() {
   $status = $this->getStatus();
   switch($status) {
   case 'NEW':
   $this->setHeardTime(new \DateTime());
   $this->setStatus('HEARD')->save();
   break;
   // ... repeat for other states
   }
   $mailer = new Mailer(
   $this->getCustomer()->getEmail(),
   'yourfriend@hattiebs.com',
   "Your order is now ${this->getStatus()}!"
   );
   $mailer->send();
   }
   

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10. more ideas appear
    

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11. and they want metrics
    function updateOrder() {
    $status = $this->getStatus();
    switch($status) {
    case 'NEW':
    $this->setHeardTime(new \DateTime());
    $this->setStatus('HEARD')->save();
    $db = $this->getReportingDB();
    $db->timeToHeard(
    $this->getHeardTime() -
    $this->getCreatedTime()
    );
    break;
    // ... repeat for other states
    }
    $mailer = new Mailer(
    $this->getCustomer()->getEmail(),
    'yourfriend@hattiebs.com',
    "Your order is now ${this->getStatus()}!"
    );
    $mailer->send();
    }
    

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12. even more ideas appear
    Users should receive text messages as the state of their order
    changes
    Update a progress bar (Hi Josh!) on an UI somewhere on the line
    Every 10th order, the customer should be sent a satisfaction survey
    Every 50th order, send the customer a coupon for
    And think of all the metrics:
    Count orders that fail quality check
    Count orders that use coupons and how much they paid
    Send email for orders taking longer than 15 minutes to fill
    etc etc etc
    

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13. AND before long YOU HAVE...
    function updateOrder() {
    $status = $this->getStatus();
    switch($status) {
    case 'NEW':
    $this->setHeardTime(new \DateTime());
    $this->setStatus('ACCEPTED')->save();
    $report = $this->getReportingDB();
    $db->timeToAccepted(
    $this->getAcceptedTime() -
    $this->getCreatedTime()
    );
    break;
    // ... repeat for other states
    case 'HEARD':
    $this->setCheckedTime(new \DateTime());
    $this->setStatus('STARTED')->save();
    $report = $this->getReportingDB();
    $db->timeToStarted(
    $this->getStartedTime() -
    $this->getHeardTime()
    );
    break;
    case 'STARTED':
    $this->setCompletedTime(new \DateTime());
    $this->setStatus('COMPLETED')->save();
    $report = $this->getReportingDB();
    $db->timeToChecked(
    $this->getCheckedTime() -
    $this->getStartedTime()
    );
    break;
    case 'COMPLETED':
    $this->setUpTime(new \DateTime());
    $this->setStatus('UP')->save();
    $report = $this->getReportingDB();
    $db->timeTothisUp(
    $this->getUpTime() -
    $this->getCompletedTime()
    );
    break;
    case 'UP':
    $this->setDeliveredTime(new \DateTime());
    $this->setStatus('DELIVERED')->save();
    $report = $this->getReportingDB();
    $db->timeToDelivered(
    $this->getDeliveredTime() -
    $this->getUpTime()
    );
    break;
    }
    $mailer = new Mailer(
    $this->getCustomer()->getEmail(),
    'yourfriend@hattiebs.com',
    "Your order is now ${this->getStatus()}!"
    );
    $mailer->send();
    }
    lolwut
    

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14. what do we have?
    We have an Order object...
    ...that needs to update as the order progresses...
    ...and when it updates, lots of things need to happen.
    we need EVENTS
    

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15. so what is an event?
    An event is an action detected by the
    system, whose occurrence is
    announced (the event is “fired”), and
    anything listening for the
    announcement performs some action
    (or actions) in response to the
    announcement (the event is
    “handled”).
    Huh?
    

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16. again, with pictures
    order.complete
    Event
    announcement
    doSomething() doSomething() doSomething()
    do something
    

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17. is there a pattern for this?
    yes!
    (two, in fact)
    

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18. observer pattern
    Two parts:
    Subject & Observer
    Simple interface:
    Observers “subscribe” to
    Subjects
    Subjects “notify” Observers
    of changes
    Observers can
    “unsubscribe” from Subjects
    if they no longer want
    updates
    subject observer
    subject observer
    attach()
    subject observer
    update()
    subject observer
    detach()
    

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19. mediator pattern
    Three parts:
    Subject, Observer, Mediator
    How it differs from Observer:
    Observers and Subjects
    communicate through the Mediator
    Has a similar interface:
    Observers “subscribe” to events
    thrown by the Mediator
    Subjects tell the Mediator to
    “publish” an event
    The Mediator “notifies” the
    subscribing Observers of the event
    subject
    observer
    mediator
    attach() update()
    publish()
    

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20. back to the hot chicken.
    

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21. what do we have now?
    We have an Order object...
    ...that needs to update as the order progresses...
    ...and when it updates, things need to happen...
    ...and now we need to change it to use events.
    

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22. splsubject and splobserver
    

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23. splsubject and splobserver
    interface SplSubject {
    public attach(\SplObserver $observer)
    public detach(\SplObserver $observer)
    public notify()
    }
    interface SplObserver {
    public update(\SplSubject $subject)
    }
    

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24. Step 0: what we have now
    

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25. where are we now?
    Order
    email update reporting DB
    

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26. Step 1: order becomes a subject
    

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27. order: now AS A subject
    class Order implements \SplSubject
    {
    private $observers = new \SplObjectStorage();
    function attach(\SplObserver $observer) {
    $this->observers->attach($observer);
    }
    function detach(\SplObserver $observer) {
    $this->observers->detach($observer);
    }
    function notify() {
    foreach($this->observers as $o) {
    $o->update($this);
    }
    }
    // ...snip
    }
    

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28. Pst. it looks the same.
    Order
    email update reporting DB
    

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29. Step 2: email and report observers
    

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30. EMAIL: NOW AS AN OBSERVER
    // EmailNotifier.php
    class EmailNotifier implements \SplObserver
    {
    public function update(\SplSubject $subject) {
    $mailer = new Mailer(
    $subject->getCustomer()->getEmail(),
    'yourfriend@hattiebs.com',
    "Your order is now ${subject->getStatus()}"
    );
    $mailer->send();
    }
    }
    

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31. reporting: NOW AS AN OBSERVER
    // OrderReporter.php
    class OrderReporter implements \SplObserver
    {
    public function update(\SplSubject $subject) {
    $status = $subject->getStatus();
    $db = $this->getReportingDB();
    switch($status) {
    case 'NEW':
    $db->timeToHeard(
    $subject->getHeardTime() -
    $subject->getCreatedTime()
    );
    break;
    // ... repeat for other states
    }
    

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32. step 2
    EmailNotifier OrderReporter
    Order
    email update reporting DB
    

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33. Step 3: wire ‘em up
    

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34. attach those observers
    // bootstrap.php
    $order = new Order();
    $mail_observer = new EmailNotifier(...);
    $report_observer = new OrderReporter(...);
    $order->attach($mail_observer);
    $order->attach($report_observer);
    

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35. step 3
    EmailNotifier OrderReporter
    Order
    email update reporting DB
    

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36. Step 4: simplify order updating
    

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37. updateorder(): before
    function updateOrder() {
    $status = $this->getStatus();
    switch($status) {
    case 'NEW':
    $this->setHeardTime(new \DateTime());
    $this->setStatus('HEARD')->save();
    $report = $this->getReportingDB();
    $db->timeToAccepted(
    $this->getHeardTime() -
    $this->getCreatedTime()
    );
    break;
    // ... repeat for other states
    }
    $mailer = new Mailer(
    $this->getCustomer()->getEmail(),
    'yourfriend@hattiebs.com',
    "Your order is now ${this->getStatus()}!"
    );
    $mailer->send();
    }
    

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38. updateorder(): after
    function updateOrder() {
    $status = $this->getStatus();
    switch($status) {
    case 'NEW':
    $this->setHeardTime(new \DateTime());
    $this->setStatus('HEARD')->save();
    break;
    // ... repeat for other states
    }
    $this->notify($this);
    }
    

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39. side by side
    function updateOrder() {
    $status = $this->getStatus();
    switch($status) {
    case 'NEW':
    $this->setHeardTime(new \DateTime());
    $this->setStatus('ACCEPTED')->save();
    $report = $this->getReportingDB();
    $db->timeToAccepted(
    $this->getAcceptedTime() -
    $this->getCreatedTime()
    );
    break;
    // ... repeat for other states
    case 'HEARD':
    $this->setCheckedTime(new \DateTime());
    $this->setStatus('STARTED')->save();
    $report = $this->getReportingDB();
    $db->timeToStarted(
    $this->getStartedTime() -
    $this->getHeardTime()
    );
    break;
    case 'STARTED':
    $this->setCompletedTime(new \DateTime());
    $this->setStatus('COMPLETED')->save();
    $report = $this->getReportingDB();
    $db->timeToChecked(
    $this->getCheckedTime() -
    $this->getStartedTime()
    );
    break;
    case 'COMPLETED':
    $this->setUpTime(new \DateTime());
    $this->setStatus('UP')->save();
    $report = $this->getReportingDB();
    $db->timeTothisUp(
    $this->getUpTime() -
    $this->getCompletedTime()
    );
    break;
    case 'UP':
    $this->setDeliveredTime(new \DateTime());
    $this->setStatus('DELIVERED')->save();
    $report = $this->getReportingDB();
    $db->timeToDelivered(
    $this->getDeliveredTime() -
    $this->getUpTime()
    );
    break;
    }
    $mailer = new Mailer(
    $this->getCustomer()->getEmail(),
    'yourfriend@hattiebs.com',
    "Your order is now ${this->getStatus()}!"
    );
    $mailer->send();
    }
    function updateOrder() {
    $status = $this->getStatus();
    switch($status) {
    case 'NEW':
    $this->setHeardTime(new \DateTime());
    $this->setStatus('ACCEPTED')->save();
    break;
    case 'HEARD':
    $this->setCheckedTime(new \DateTime());
    $this->setStatus('STARTED')->save();
    break;
    case 'STARTED':
    $this->setCompletedTime(new \DateTime());
    $this->setStatus('COMPLETED')->save();
    break;
    case 'COMPLETED':
    $this->setUpTime(new \DateTime());
    $this->setStatus('UP')->save();
    break;
    case 'UP':
    $this->setDeliveredTime(new \DateTime());
    $this->setStatus('DELIVERED')->save();
    break;
    }
    $this->notify($this);
    }
    

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40. and finally...
    Order
    EmailNotifier OrderReporter
    update()
    

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41. so what do we have now?
    Email notifications and the reporting DB updates are no longer
    coupled to the Order object
    Other objects now have a mechanism for telling an Order that they
    want to know when it updates
    Order now has a mechanism for telling other objects when it
    updates
    And neither need to know that the other exists
    Using events here...
    Made our order update code cleaner, more focused, better
    encapsulated
    Decoupled our objects from each other
    

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42. oh no!
    HERE COMES SCOPE CREEP!
    

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43. “customer loyalty program”
    // FreeChicken.php
    class FreeChicken implements \SplObserver
    {
    public function update(\SplSubject $subject) {
    if ($subject->getStatus !== 'COMPLETED') {
    return;
    }
    $customer = $subject->getCustomer();
    $orders = count($customer->getOrders());
    if ($orders > 0 && $orders % 50 === 0)) {
    $customer->sendCouponForFreeChicken();
    }
    }
    }
    

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44. “customer loyalty program”, cont’d
    // bootstrap.php
    $order = new Order();
    $mail_observer = new EmailNotifier(...);
    $report_observer = new OrderReporter(...);
    $free_chicken = new FreeChicken(...);
    $order->attach($mail_observer);
    $order->attach($report_observer);
    $order->attach($free_chicken);
    

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45. let’s try again...
    Email notifications and the reporting DB updates are no longer
    coupled to the Order object
    Other objects now have a mechanism for telling an Order that they
    want to know when it updates
    Order now has a mechanism for telling other objects when it updates
    And neither needs to know that the other exists
    Using events here...
    Made our order update code cleaner, more focused, better
    encapsulated
    Decoupled our objects from each other
    Made changes/additions easier
    

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46. observer drawbacks?
    

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47. what about A mediator?
    

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48. no more subject
    // Mediator.php
    class Mediator
    {
    private $events = array();
    public function attach($event, $observer) {
    if (isset($this->events[$event])) {
    $this->events[$event]->attach($observer);
    } else {
    $this->events[$event] = new \SplObjectStorage();
    $this->events[$event]->attach($observer);
    }
    }
    // ... wait for it
    

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49. no more subject, CONT’d
    // ... wait for it
    public function detach($event, $observer) {
    if (isset($this->events[$event])) {
    $this->events[$event]->detach($observer);
    }
    }
    public function publish($event, $subject) {
    if (isset($this->events[$event])) {
    foreach($this->events[$event] as $ob) {
    $ob->update($subject);
    }
    }
    }
    }
    

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50. so what happened to the subject?
    

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51. see, there’s still a subject
    class Order
    {
    function updateOrder($order) {
    $status = $order->getStatus();
    switch($status) {
    case 'NEW':
    $order->setAcceptedTime(new \DateTime());
    $order->setStatus('ACCEPTED')->save();
    break;
    // ...repeat for other states
    }
    $this->get('mediator')->publish('order.update', $this);
    }
    }
    

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52. what about the observers?
    actually, they stay the same.
    

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53. BUT HOOKing them UP IS DIFFERENT?
    

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54. mediator hook ups
    // bootstrap.php
    $dispatcher = new Mediator();
    $mail_observer = new EmailNotifier( ... );
    $report_observer = new OrderReporter( ... );
    $free_chicken = new FreeChicken( ... );
    $dispatcher->attach('order.update', $mail_observer);
    $dispatcher->attach('order.update', $report_observer);
    $dispatcher->attach('order.update', $free_chicken);
    

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55. OK. that was a lot. let’s recap.
    

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56. once more, with feeling
    Events help decouple and clean up code when you have many (or an unknown
    number) of objects that need to know when the state of an object changes.
    Think: UI code, state machines, saving objects, beginning/ending a process...
    Observer Pattern
    Simpler, but more limited
    Better for small numbers of objects, persistent object, or singletons
    SplSubject/SplObserver
    Mediator Pattern
    (Slightly) More Complicated, but more flexible
    Typically better for web applications
    Symfony’s EventDispatcher component
    

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57. MORE FUN WITH events
    

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58. more with events
    Task/Message queues
    Event Loops
    IRC Bots
    Web servers
    NodeJS (see also: ReactPHP)
    Event-Driven Architecture
    

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59. That’s it. Questions?
    

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60. THanks!
    twitter & github: @epochblue
    

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61. errata
    http://www.hattieb.com/
    http://iostudio.com/
    http://studionow.com/
    http://en.wikipedia.org/wiki/Observer_pattern
    http://en.wikipedia.org/wiki/Mediator_pattern
    http://www.php.net/manual/en/class.splsubject.php
    http://www.php.net/manual/en/class.splobserver.php
    http://symfony.com/doc/current/components/event_dispatcher/introduction.html
    http://reactphp.org/
    https://github.com/epochblue/philip/
    

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