2.4.2 Qualified Notification with Subscribing


In this variation of Qualified Notification, the listener specifies which change it is interested in, and a notification is issued only with the relevant change occurs. The two solutions therefore differ in which entity performs filtering of unnecessary notifications: in Qualified Notification the filtering is performed by the listener, after the notification is delivered, while in Qualified Notification with Subscribing, the filtering is performed by the Model, honoring the listener’s registration instructions.


For a Qualified Notification with Subscribing, the Model must support:

  • an appropriate signature for the register() method, allowing to specify the type of modification a listener is interested in.
  • preservation of the passed information in an internal registry, which will be used during notification.

When the Model experiences a change in its content, it must consult its internal registry to verify if a listener is interested in this information. The Model then informs the listener only if this is the case.

Practical Example: Apple KVO (Key-Value Observer)

Apple KVO (Key-Value Observer) is a form of Qualified Notification with Subscribing. The Model supports an addObserver method, whose signature allows to specify
the property the listener is interested in, and the type of change. For example, a listener registers for changes on the Model property my_property with the following call

[model addObserver:destination

When the my_property value changes, the listener’s method observeValueForKeyPath:ofObject:change:context: will be invoked on the listener. Changes in other Model’s properties will not be delivered.