Understanding the Observer Pattern in PHP

For a while, I’ve been looking at plugin systems, but not really fully understanding the pattern behind them. Don’t get me wrong, I see how they work, but I didn’t know the reason why - the theory or pattern behind it. Well turns out, generally, they’re based upon the observer pattern. I decided to write my own observer pattern demonstration here.

Our example is going to be very simple: post a message to twitter. We’re not going to work with any credentials or anything, just want to post a message. I do want to add an observer that will shorten any url however. In this example, we’ll be making the logic classes stubs (you can create these later), and using [url] to stand for an URL we might replace.

Let’s start in:

Our two logic classes

Remember, we’re just going to have some blank stub logic classes here. They are for demonstration purposes only.

class twitterTransport
{
  public function __construct()
  { /** logic here **/ }

  public function tweet($object)
  { /** logic here **/ print $object->message . ' was just tweeted'; }
}

class urlShortener
{
  public function shorten($message)
  {
    return str_replace('[url]', '[u]', $message);
  }
}

Pretty simple, the first class all it does is post to twitter with a public method called tweet(). This accepts an object of the twitter message (which we’ll list next!). It prints out the message so you know what we would have sent to twitter. The url shortening class - all it is is your logic to shorten urls inside of a message. In this case, pretty simple.

Ok - as promised, here is our twitter message class:

class twitterMessage
{
  public $message = '';

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

The actual launching code

We’re going to jump a head here and show what code we’ll be using to add the url shortener as well as post the message. It’s really short - but it’ll give us an idea of what class we need to create next:

$tweeter = new twitterTransportObservable();
$tweeter->registerObserver('PREPOST', new urlShortenerObserver());
$message = new twitterMessage("this is my message [url]");
$tweeter->postMessage($message);

Ok good. First off, we create a new instance of twitterTransportObservable. By the keyword Observable, we can tell that this class is something that will “do something we can watch” or observe. Any time a class is Observable, it has to have a method to add watchers to itself - or registerObserver(). In our example, we’re sending in a type - PREPOST - so before we post the message, and a new object.

The new object is of type urlShortenerObserver(). We can see that this will be a ‘watcher’ by the name. No more details are given here, so its methods must be used/exposed inside of the Observable class.

Next, we’re just making a new twitter message object, pretty simple.

Finally, we’re calling postMessage() sending in our twitter message. Remember, $tweeter is an instance of twitterTransportObservable, so there must be a method called twitterTransportObservable::postMessage().

So far so good.

Looking at the Observable Class

So now we know we need to build twitterTransportObservable. I’m going to post the code here, but don’t worry, we’ll take it apart, step by step:

class twitterTransportObservable
{
  protected $_observers = array();

  public function registerObserver($type, $observer)
  {
    if (!isset($this->_observers[$type])) $this->_observers[$type] = array();
    $this->_observers[$type][] = $observer;
  }

  public function postMessage($object)
  {
    $this->_notify('PREPOST', $object);

    $sender = new twitterTransport();
    $sender->tweet($object);

    $this->_notify('POSTED', $sender);
  }

  protected function _notify($type, $object)
  {
    if (isset($this->_observers[$type])) {
      foreach ($this->_observers[$type] as $observer) {
        $observer->notify($object);
      }
    }
  }
}

Ok - pretty big - let’s go slow:

First off, we have the $_observers. Since this class is of type observable, we know it has watchers. Well, it has to be aware of its watchers, because it registers them… so we need an array to hold all of our watchers, or $_observers.

The first method is registerObserver(). You’ll see this takes in a variable called $type and a variable called $object. Well, we saw this used in our launching code. It appears that this was started with PREPOST as the $type and a new urlShortenerObserver as the $object. Moving along, the $_observers array is keyed by $type - so we just did some good programming: if the key is not set, set it by creating an empty array. Finally, the next line grabs that array, and adds the passed in $object to the internal array of $_observers. So now, our observable class has its first observer. An important thing to note is that the order you add them using registerObserver(), is the order they will remain in the array in the Observable class.

Quick reminder: Remember, objects are passed by reference!

Next, we have the postMessage() function - which takes in an object of a twitter message. The first thing the function does is notify our self that we’re PREPOST, while passing in the $object to that notify call. Think of this as the ‘hook’ - or someone yelling at the watchers saying “Anyone of type PREPOST, I’ve got this $object for you to deal with!”. Next, this function creates a new twitterTransport and tweets the object. Remember, the $object has now returned from the notify call and may be changed. Finally, there is another call to _notify() with POSTED as the type. This is just for example, our example doesn’t really need this. But, imagine you created an observer which logged the output of twitter’s response to your post? This would be perfect for that hook.

Ok, so the last thing we have to look at is the _notify() function - which we’ve called a few times during our postMessage(). This simply looks to see if there is an observer that we’ve been storing locally keyed on the $type key. If there is a key of this $type, we loop through each observer of that $type, and pass in our object to its notify() function. OK - don’t get confused, that notify() function is different than our _notify() function. It belongs to the observer (in our example, urlShortenerObserver::notify()). So basically, it calls all the observer’s notify() with a reference to the object, and its done.

Whew, that was a lot - but we have one more part left:

The Observer class

We have another class that is used to observe or watch the observable classes. In this case, we wanted to have any URLs shortened before we posted a message to twitter… so we registered this observer with PREPOST. During the Observable’s _notify() function, we called this observerable class’s notify() method. So, let’s finally take a look at the code:

class urlShortenerObserver
{
  public function notify($object)
  {
    $urlShortener = new urlShortener();
    $object->message = $urlShortener->shorten($object->message);
  }
}

Pretty simple class. It has only one method, called notify() which accepts an object - of type twitterMessage. The first line just creates a new urlShortener() - you remember from way up top? Just a quick str_replace type method. Then, the next line accesses the urlShortener::shorten() method - by passing in the public $message variable of the twitterMessage. The return value is assigned to the twitterMessage::$message var. And remember, since objects are passed by reference, when the next line of the the observable’s class is called, the object will now be modified.

Wrapping Up

Ok - well this was a pretty simple example of this behavior. There are definitely more complex ways and more business logic intense scenarios to use the observer in. Another thing we didn’t do is use many of PHP’s OO properties - but we could always refactor and do that in the future.

All the code

In case you want to run it yourself:

class twitterTransport
{
  public function __construct()
  { /** logic here **/ }
    
  public function tweet($object)
  { /** logic here **/ print $object->message . ' was just tweeted'; }
}
    
class urlShortener
{
  public function shorten($message)
  {
    return str_replace('[url]', '[u]', $message);
  }
}

class twitterMessage
{
  public $message = '';

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

class urlShortenerObserver
{
  public function notify($object)
  {
    $urlShortener = new urlShortener();
    $object->message = $urlShortener->shorten($object->message);
  }
}

class twitterTransportObservable
{
  protected $_observers = array();

  public function registerObserver($type, $observer)
  {
    if (!isset($this->_observers[$type])) $this->_observers[$type] = array();
    $this->_observers[$type][] = $observer;
  }

  public function postMessage($object)
  {
    $this->_notify('PREPOST', $object);

    $sender = new twitterTransport();
    $sender->tweet($object);

    $this->_notify('POSTED', $sender);
  }

  protected function _notify($type, $object)
  {
    if (isset($this->_observers[$type])) {
      foreach ($this->_observers[$type] as $observer) {
        $observer->notify($object);
      }
    }
  }
}

$tweeter = new twitterTransportObservable();
$tweeter->registerObserver('PREPOST', new urlShortenerObserver());
$message = new twitterMessage("this is my message [url]");
$tweeter->postMessage($message);