gtkmm3 drawings outside on_draw

I am working on the real-time plot application where a stream of data is to be plotted on screen. Earlier using gtkmm2 I had done this using a custom widget (derived from Gtk::Bin) where I have a member function which creates a cairo context and does the plotting.

Now with gtkmm3 I am unable to plot in any method other than on_draw. Here's what my custom draw method body looks like

Gtk::Allocation oAllocation = get_allocation();
Glib::RefPtr <Gdk::Window> refWindow = get_window();
Cairo::RefPtr <Cairo::Context> refContext =
  refWindow->create_cairo_context();

refWindow->begin_paint_rect(oAllocation); //added later

refContext->save();

refContext->reset_clip();

refContext->set_source_rgba(1,
                            1,
                            1,
                            1);

refContext->move_to(oAllocation.get_x(),
                    oAllocation.get_y());
refContext->line_to(oAllocation.get_x()
                    + oAllocation.get_width(),
                    oAllocation.get_y()
                    + oAllocation.get_height());
refContext->stroke();

refContext->restore();

refWindow->end_paint();

Initially I derived the class from Gtk::DrawingArea then tried with Gtk::Bin while adding the begin_paint_rect call.

Is it forbidden to draw in any place other than on_draw?

Solution

For something like a plot (or anything that is rather complex to draw) I advise to use a buffer; I lost a month of my life because I read that gtkmm3 does buffering so that using "double buffering" isn't needed anymore (as opposed to gtkmm2), but it aint that simple (read: that isn't true).

So, what you should do is just draw to your own surface; and every time you change something call queue_draw_region or queue_draw_area.

Then in on_draw get the list of clip rectangles and copy those from your private surface to the cr that is passed to the on_draw function. Cairo normally does the exact same thing (or so they claim), copying what you just copied again, to the screen; so you should turn that off (this should be possible I read).

The reason you can't use Cairo's buffering is because it doesn't KEEP that buffer; what you get is some corrupted surface, so you are forced to redraw EVERYTHING inside the clip rectangle list. That wouldn't be too bad if you (your application) was the only one making changes (as per your queue_draw_* calls): then you could set a flag, invalidate the part(s) that needs redrawing and simply postpone the draw until you get to on_draw. But sometimes on_draw is called for other reasons, for example, when you open a menu that goes over your drawing area. I think this is a bug (or a design error) but it is the way it is. The result is that you can't know what you have to redraw EXCEPT by looking at the clip rectangle list; which makes it incredibly hard to just draw a part of your area unless your drawing is made up of many separate rectangles (like, say, a chess board). The only feasible way is to keep a full copy of the image in memory (your private surface) and just copy the clip rectangle list from there when in on_draw.