Observer Pattern

Observer Pattern

interface Observer {
	function notify($obj);
}

class ExchangeRate {
    static private $instance = NULL;
    private $observers = array();
    private $exchange_rate;
 
    private function ExchangeRate() {
    }
     static public function getInstance() {
        if (self::$instance == NULL) {
            self::$instance = new ExchangeRate();
        }
        return self::$instance;
    } 
 
    public function getExchangeRate() {
        return $this->$exchange_rate;
    }
     public function setExchangeRate($new_rate) {
        $this->$exchange_rate = $new_rate;
        $this->notifyObservers();
    }
     public function registerObserver($obj) {
        $this->observers[] = $obj;
    }
     function notifyObservers() {
        foreach($this->observers as $obj) {
            $obj->notify($this);
        }
    }
}
 
class ProductItem implements Observer {
    public function __construct() {
        ExchangeRate::getInstance()->registerObserver($this);
    }
     public function notify($obj) {
        if ($obj instanceof ExchangeRate) {
             // Update exchange rate data
             print "Received update!\n";
        }
   }
}
 
$product1 = new ProductItem();
$product2 = new ProductItem();