Re: The state of the canvas

On Fri, 2005-11-18 at 14:54 +0000, Bill Haneman wrote:
> Alexander Larsson said...
> >In fact, I think its more important to have a simple canvas like that
> >than a really powerful flexible thing to be used for diagram editors and
> >powerpoint apps for the simple reason that I'm not at all sure such apps
> >would use a generic canvas. Each major app is gonna have some
> >requirement that makes them do their own canvas widget anyway. The
> >canvas is after all an extremely core part of an app like dia,
> >illustrator or powerpoint. The question is, is it possible to do a
> >canvas that caters to both usecases?
> >
> I think there's a compelling reason for multiple apps to use a common 
> canvas, despite their 'custom' needs: accessibility.  Getting canvas 
> accessibility right is not trivial, and it makes sense to try and do it 
> in one place, to the extent possible.

This is a no-argument. If we have an overcomplicated canvas the result
will be that the people who just want to do a custom widget instead of a
full-blown main-application-window-content canvas will jut not use it,
so instead of perhaps having to make a simpler canvas accessible you
have to make each apps custom stuff accessible. Of course, maybe we
manage to make a complex canvas that has enough helper functions etc
that it will work for such cases too.

> >>From your page its very obvious that your target for this design is Dia,
> >since you have things like selection and focus handling. This isn't bad
> >per-se, but its likely that apps may want slightly different behaviour
> >with such things, and maybe the core should just be powerful enough that
> >things like that can be implemented in a layer above it.
> >  
> >
> I think not - all canvasses need selection and focus handling for 
> accessibility reasons.  Even if the resulting content is "read only", 
> selection and focus handling are important for blind users.

I can see the need for some sorts of keyboard focus, but the kind of
selection we're talking about here is like the selection in a listview.
i.e. which canvas items are selected. While a drawing app uses such
selections, in many other uses of the canvas there just isn't something
like that. For example, take the evolution calendar view, which uses the
gnome canvas. You don't "select" the canvas item which renders a day. In
fact you don't select any item, they are just used to render the widget

> That would work, perhaps, but I think general-purpose "zoom level" 
> pruning would be a useful general feature nonetheless.  I don't think 
> it's specific to maps at all.

Yes, zooming and scale factors are very interesting. However you need to
be very careful about them in the design since they can easily lead to
conflicts with the model/view split as I said in my earlier mail.

 Alexander Larsson                                            Red Hat, Inc 
                   alexl redhat com    alla lysator liu se 
He's a fiendish albino gangster looking for a cure to the poison coursing 
through his veins. She's an elegant Bolivian mechanic with only herself to 
blame. They fight crime! 

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