Scrolling DrawingArea [was: Re: Adding a signal to an exixting widget]

[Stefano Salvi <stefano salvi mn it>]

Murray Cumming wrote:
> On Tue, 2005-08-16 at 12:33 +0200, Stefano Salvi wrote:
>
> > I'm using gtkmm-2.6 from Sarge.
> >
> > I need a "scrollable DrawingArea" to insert into a ScrolledWindow widget.
> > To get the adjustmets to scroll it I need to add the 
> > signal_set_scroll_adjustments signal to my widget.
> > I added the SignalProxy with this declaration:
>
>
> The SignalProxy is for wrapping C GTK+ signals and should never be used
> directly. You would normally just patch gtkmm to patch the new/missing
> signal.
>
> However, this "set_scroll_adjustments" thing seems to be some strange
> special case:
> http://developer.gnome.org/doc/API/2.0/gtk/GtkWidget.html#GtkWidgetClass
The "ScrolledWindow" is a nice container to ad scrollbars to another widget.
It's primary working method is to take a great object and show only a 
part of it. This is rather inefficent for large objects that require a 
large amount of time or other operations to render.
Another method is used if a widget is "scroll capable". In this case, 
this container gives the scrollable widgets a couple of adjustments to 
position itself.

To know if a widget is "scrolling capable", the ScrolledWindow widget 
(GtkScrolledWindow) checks a field present in EVERY WIDGET CLASS that is 
"guint set_scroll_adjustments_signal;". This int keeps the signal ID of 
the signal to set the scrollbars, if the widget is scrollable, and 0 
otherwise.

GtkScrolledWindow tests this class field (and GtkMM ScrolledWindow class 
does the same) to know if a widget is scrollable.

I think this is not a corrct programming choice, as you cannot extend an 
existing widget (say a DrawingArea or a VBox) to add scroll capabilities 
to it, but it's not Gtkmm-related, but Dtk+-related.

In conclusion, to solve the problem I had to:

- create a new GTK widget, derived from GtkDrawingArea, adding the 
"scrolling capabilities" to it (copying extensively from GtlLayout Widget)
- modify my original class to be a wrapper of this new widget

I know this is not a straight way, but it's the only I was able to 
figure out (and actually it works).

If you are interested in some (not very clean) piece of code, ask.

Thank you
	Stefano Salvi