Callbacks are functions that are invoked when
signals are emitted by
widgets.
Callbacks are functions that are set up by the programmer to react to
signals emitted by widgets. You specify the function that should be
called by connecting the
function to the signal.
The callback is also known as the signal handler function. This can be
either the widget's default handler or a
user-defined handler, that is, a function written by
the programmer.
