A NotificationCenter is essentially a notification dispatcher. It notifies all observers of notifications meeting specific criteria. This information is encapsulated in Notification objects. Client objects register themselves with the notification center as observers of specific notifications posted by other objects. When an event occurs, an object posts an appropriate notification to the notification center. The notification center invokes the registered method on each matching observer, passing the notification as argument.

The order in which observers receive notifications is undefined. It is possible for the posting object and the observing object to be the same. The NotificationCenter delivers notifications to observers synchronously. In other words the postNotification() method does not return until all observers have received and processed the notification. If an observer throws an exception while handling a notification, the NotificationCenter stops dispatching the notification and postNotification() rethrows the exception.

In a multithreaded scenario, notifications are always delivered in the thread in which the notification was posted, which may not be the same thread in which an observer registered itself.

The NotificationCenter class is basically a C++ implementation of the NSNotificationCenter class found in Apple's Cocoa (or OpenStep).

While handling a notification, an observer can unregister itself from the notification center, or it can register or unregister other observers. Observers added during a dispatch cycle will not receive the current notification.

The method receiving the notification must be implemented as

void handleNotification(MyNotification* pNf);

The handler method gets co-ownership of the Notification object and must release it when done. This is best done with an AutoPtr:

void MyClass::handleNotification(MyNotification* pNf)
{
    AutoPtr<MyNotification> nf(pNf);
    ...
}

Alternatively, the NObserver class template can be used to register a callback method. In this case, the callback method receives the Notification in an AutoPtr and thus does not have to deal with object ownership issues:

void MyClass::handleNotification(const AutoPtr<MyNotification>& pNf)
{
    ...
}

Creates the NotificationCenter.

Destroys the NotificationCenter.

Convenience method for registering an NObserver. Creates an NObserver<C, N> with optional matcher and registers it. Usage:

notificationCenter.addNObserver(*this, &MyClass::handleNotification);
notificationCenter.addNObserver(*this, &MyClass::handleNotification, &MyClass::matchNotification);

Registers an observer with the NotificationCenter. Usage:

NObserver<MyClass, MyNotification> obs(*this, &MyClass::handleNotification);
notificationCenter.addObserver(obs);

Note: Observer<C, N> is deprecated; use NObserver<C, N> instead.

@deprecated This convenience method instantiates the deprecated Observer<C, N> class template, which triggers a compiler warning. The Observer class requires manual memory management of notifications (caller must call release()), while NObserver handles this automatically via AutoPtr.

Use addNObserver() instead, which creates an NObserver that passes notifications as const AutoPtr<N>&, providing automatic memory management.

// Old (deprecated):
nc.addObserver(*this, &MyClass::onEvent);
void MyClass::onEvent(MyNotification* pNf) { pNf->release(); }

// New (recommended):
nc.addNObserver(*this, &MyClass::onEvent);
void MyClass::onEvent(const AutoPtr<MyNotification>& pNf) { /* no release needed */ }