Signals are notifications emitted by widgets in response to some action
As discussed before, it is essential for any application to respond to
some action that is triggered either by the user or automatically. Signals
help the program in knowing when a particular action has taken place so that
you can act on it.
Every widget has a set of signals associated with it. This means that, the
widget is capable of emitting any of those signals depending on the action
it receives. Also remember that signals are inherited across classes, and
hence the child class always derives all of the signals of its parent and
may add a few of its own.
An important concept to know here is that it is possible to emit a signal
without the user performing an action at all. Most widgets have functions
that will emit a particular signal when you use them, irrespective of
whether the user has actually performed an action or not. An example of
such a function is the clicked() method
of GtkButton, which will emit the
clicked signal for the button.