How to emit predefined signals in gtkmm.

[German Diago Gomez <al059365 alumail uji es>]

Hello. My question is quite simple, but I haven't been able to find such
thing in the gtkmm documentation.
 I'm using cairo and a Gtk::DrawingArea derived class to draw inside a
window in Gtk.
The problem is that I get data from three entries in a function, and
after that, I want to update the drawing of the area when a button is clicked.
 To update the drawing, if I'm right, I must do it from the
on_expose_event overriden function in the Gtk::DrawingArea derived
class. Since I want to update immediatly after calling the slot
connected to signal_clicked() in my button, I must emit a signal
signal_expose_event() if I'm not wrong.
  I looked at the documentation and I realized that
signal_expose_event() doesn't return a sigc::signal but a proxy object.
How can I emit a signal so that on_expose_event is called after clicking
the button from the slot called for the button?