WvCallback calls a member function inside a class when certain event has happened or when certain task has been finished. The key point of WvCallback is that it can be used in a function that is not in the same class as the function that will be invoked after the event has occurred. WvCallback can operate as:
1. a function pointer when the invoked function is not part of any class or as
2. a member function pointer when the invoked function belongs to a class