Example Imagine an architectural application. There may also be a label attribute which is common, but optional. This could be implemented as a property. However, the standard property accessing mechanism is much slower than accessing an instance variable and the amount of memory required to store a property is greater. The standard Smalltalk event system is implemented using the property mechanism.
Any object can register an interest in an event triggered by any other object.
The triggering object must keep a list of interested parties. Instead, the property mechanism is used. Answer the receiver. Also signal a change for any dependents with the trigger name as the aspect.