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Implementing the Observer Design Pattern with PHP

Saturday, March 01, 2008

If you, like me need to implement a light weight Observer framework within your PHP project, and really do not need PEAR's Event_Dispatcher, look no further. ;-)

Below is my light weight implementation of a Observer pattern for PHP.

interface Observerable {
 public function attach(Observer $observer);
 public function detach(Observer $observer);
 public function notify();
}

interface Observer {
 public function update(Payload $payload);
}

class Payload {
 public $data;
 public function __construct($data) {
  $this->data = $data;
 }
}

class Watched implements Observerable {
 
 private $state;
 private $watchers;
 
 public function __construct(){
  $this->watchers = Array();
 }
 
 public function attach (Observer $observer) {
  if (array_search($observer, $this->watchers, true) === false) {
   $this->watchers[] = $observer;
  }
 }
 
 public function detach (Observer $observer) {
  $observerOffset = array_search($observer, $this->watchers, true);
  if ($observerOffset !== false) {
   unset($this->watchers[$observerOffset]);
  }
 }
 
 public function notify () {
  foreach ($this->watchers as $observer) {
   $observer->update(new Payload($this->state));
  }
 }
 
 public function changeState($newState) {
  $this->state = $newState;
  $this->notify();
 }
}

class Watcher implements Observer {
 
 public function update (Payload $payload) {
  echo $payload->data;
 }
}

// Create Objects
$watched = new Watched();
$watcher1 = new Watcher();
$watcher2 = new Watcher();

// Attach Watchers
$watched->attach($watcher1);
$watched->attach($watcher2);

// Change State of Watched
$state = "foo";
$watched->changeState($state); // Causes "foo" to be printed from within Watcher1 & Watcher2

// Remove a Watcher
$watched->detach($watcher1);
$watched->changeState($state); // Causes "foo" to be printed from within Watcher1

I tend to use a slightly different naming system for most my classes. Apart from that, this small snippet of code is pretty much a standard Observer pattern implementation. Very useful when you are writing small scale PHP applications (like a Facebook App. for example).

I also tend to favor the use of a small container class for passing data into the watchers, but of course since this is PHP, you could probably just use a complex array instead.

Labels: Design Patterns, Facebook PHP application, Observer Pattern, PHP