The context is a mechanism for the interchange of data
between the components of the windows and the services.
The context stores to objects (data) associate to a name.
The context is shared by all the objects of a
window. If from a window we go to another one,
is created a new context son, who contains all the data of the
previous window but he cannot modify them. If we modified a
data, the change is local, and when we return to the previous window,
that modification appears. If on the contrary, we go to a new
window, the local information propagates to the new window.
Also we can define "listeners", that is to say, objects that are
waiting for the changes of variable determining.