reposition DrawingArea inside a Scrollbar after enlarging

[Hendrik Tews <tews os inf tu-dresden de>]

Hi,

what is the preferred way to reposition a GtkDrawingArea inside a
GtkScrollbar just after the GtkDrawingArea has been enlarged?

In detail: I have a DrawingArea (in Gtk 2.20 in Debian squeeze)
that gets now and then enlarged when new elements are drawn on
it. For enlarging it I use gtk_widget_set_size_request. The
DrawingArea sits inside a Scrollbar. After enlarging the
DrawingArea I want to make sure that the new element, which I've
just drawn in the new region, is visible. I therefore call
gtk_adjustment_clamp_page on both scroll bars with appropriate
lower and upper values. I then call gtk_widget_queue_clear on the
DrawingArea and let it redraw when the exposure event arrives.

The trouble is that the new element is often outside the visible
portion of the DrawingArea. So gtk_adjustment_clamp_page does not
work in this case. AFAICS the problem is that the size does not
change immediately when I call gtk_widget_set_size_request and
therefore the call of gtk_adjustment_clamp_page works with the
old size. When the adjustments finally get the new size they
forgot about my clamp_page request and the new region with the
new element in it remains invisible.

When and how should I change the adjustments so that the new
region in the DrawingArea becomes visible?

Using the size-allocate signal does not work, because apparently
it is only emitted if my size request is larger then the current
size. (Sometimes I also delete elements and shrink the
DrawingArea. But also in these cases I want to ensure that a
certain current element is visible).

Changing the adjustment in the handler of the exposure event does
also not work reliably. Sometimes the DrawingArea receives
exposure events before the size has changed. Therefore I would
have to set the adjustment on every exposure event, which would
break the function of the scroll bars.

For screen shots of my application visit
http://askra.de/software/prooftree/ .

I posted this already on gtk-app-devel, but got no replies there.

Bye,

Hendrik Tews