Re: on_*_event return

[Murray Cumming <murrayc murrayc com>]

On Sat, 2008-01-12 at 13:49 -0500, JLM wrote:
> Thanks for the information. I think this gives me more questions than it
> answers.
> 
> Murray Cumming wrote:
> > On Sat, 2008-01-12 at 10:02 -0500, JLM wrote:
> >> Some widget event handler functions return a boolean value.
> >> on_expose_event is one. I think the return value is supposed to tell the
> >> caller whether this widget handled the event or not. 
> > 
> > Yes:
> > http://www.gtkmm.org/docs/gtkmm-2.4/docs/tutorial/html/sec-xeventsignals.html
> > 
> >> What exactly does it mean to handle the event?
> > 
> > It means that no further signal handler will be asked to handle the
> > event too.
> > 
> >> For example, I have an on_expose_event function set up for my widget
> >> that is subclassed from DrawingArea.
> > 
> > When overeriding, remember also to call the base class's implementation.
> > Sometimes it does something useful.
> >
> Should the base class's function always be called? If not, when should
> it? I noticed that this isn't followed in the tutorial. The Cairo
> example (first one I think) does not call the base class in the
> on_expose_event function.
> 
> >>  It only does something if the
> >> widget is realized, so I put everything in an if(is_realized()) { ... }
> >> block. So should I return false if the widget is not realized when this
> >> function is called? Or is the widget considered to handle the event, but
> >> it just happens to not do anything?
> >>
> >> Are there any rules of thumb for when to return true versus false in
> >> these cases?
> > 
> > I have often wondered the same thing in similar situations. I don't
> > think I have a general answer, but maybe someone else does. I am
> > generally not a fan of these signals or this technique because I think
> > it requires you to have a knowledge of what the default implementation
> > is.
> If you are not a fan of this technique, then I am curious how you do
> these things.

I use this technique. It's the correct one to use. I just don't like it
much.

>  The recommended techniques in the manual suggests
> subclassing widgets to better organize your code, so I have done that. I
> am not sure how to customize the drawing of my new widget without using
> the on_expose_event function.
> 
> I'm only doing it this way because it's the only way I know how. If you
> have a better way, I'm interested in hearing it.
> > 
> > But that would be the same with the more-traditional C++ technique of
> > override-and-sometimes-call-the-base-handler.

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