php:observer [Goodies]

OBSERVER PATTERN

15.04.2010

The observer pattern gives you another way to avoid tight coupling between components. This pattern is simple: One object makes itself observable by adding a method that allows another object, the observer, to register itself. When the observable object changes, it sends a message to the registered observers. What those observers do with that information isn't relevant or important to the observable object. The result is a way for objects to talk with each other without necessarily understanding why.

Our example is related to an user class. When a new user is added, the observer must be notified (we attached it before this).

interface IObserver { function notify($sender_obj); }
 
class Log_System implements IObserver {
    public function notify($sender_obj) {
        echo "LOG: ".get_class($sender_obj)." Add Name: ".$sender_obj->get_name()."\n";
    }
}
 
class User {
    protected $_observers;
    protected $_name;
 
    public function get_name() { return $this->_name; }
 
    public function add_user($name) {
        $this->_name = $name;
        $this->_notify_observers();
    }
 
    public function attach_observer(IObserver $observer) {
        $this->_observers[] = $observer;
    }
 
    protected function _notify_observers() {
        foreach ($this->_observers as $observer) {
            $observer->notify($this);
        }
    }
}
 
$newuser = new User();
$newuser->attach_observer(new Log_System);
$newuser->add_user("John Doe");

Run it in a terminal (php observerpat.php) and you'll get the echo result. The Observer (Log_System) will receive an User object and extract the information as necessary using object methods (get_name() for example).

Credits/References:

Giorgio Sironi
Five Common PHP Design Patterns