public static void registerPayPalNotificationListenerObservers() {
Object observer = PayPalNotificationListener.observer();
_retainer.addObject(observer);
NSNotificationCenter.defaultCenter().addObserver(observer,
new NSSelector("handleDeniedPaymentNotification", NotificationClassArray),
PayPalNotificationListener.DeniedPayPalPaymentReceivedNotification,
null);
NSNotificationCenter.defaultCenter().addObserver(observer,
new NSSelector("handleFailedPaymentNotification", NotificationClassArray),
PayPalNotificationListener.FailedPayPalPaymentReceivedNotification,
null);
NSNotificationCenter.defaultCenter().addObserver(observer,
new NSSelector("handleInvalidPaymentNotification", NotificationClassArray),
PayPalNotificationListener.InvalidPayPalPaymentReceivedNotification,
null);
NSNotificationCenter.defaultCenter().addObserver(observer,
new NSSelector("handlePendingPaymentNotification", NotificationClassArray),
PayPalNotificationListener.PendingPayPalPaymentReceivedNotification,
null);
NSNotificationCenter.defaultCenter().addObserver(observer,
new NSSelector("handleValidPaymentNotification", NotificationClassArray),
PayPalNotificationListener.ValidPayPalPaymentReceivedNotification,
null);
}