The template class for event observers. This class implements the Observer of the observer pattern.
In the observer-based event model, CXlEventObserver is the base class for all event observers (or listeners). An argument of type T* (where T is a class derived from CXlEventArgs) will be passed to the Update() method.
A class derived from CXlEventObserver should implement the abstract virtual function Update(). This method will be called whenever the event which is being observed occurs.