This class allows you to catch the events raised by an object before its
main event observer gets them.
The observer object will raise the same events as the observed object, and so
you will handle them with the same handlers.
If you cancel an event in the observer event handler, the main event observer of the
observed object will never see it.
See the TableView control source code for an example: it is implemented
by using this method.
The observer object is attached to the observed object, and is freed only when the observed object is freed.
Esta clase es .|