Re: Generic positioning

[Armin Burgmeier <armin arbur net>]

On Mon, 2008-10-27 at 11:33 +0000, Damon Chaplin wrote:
> On Tue, 2008-10-21 at 23:55 +0200, Murray Cumming wrote: 
> > We now have a patch for generic positioning in goocanvas. This puts x,
> > y, width, and height properties on the GooCanvasItem base interface,
> > without changing the behavour of existing x, y, width and height
> > properties where they already exist on some items. So it's not an API
> > change - just an API addition or simplification.
> > 
> > This allows applications to move and resize items without special code
> > for each type of item. The generic-position-demo shows how this can be
> > useful.
> > 
> > This is simpler than working with transforms and, unlike transforms, it
> > does not also affect details such as rectangle borders or specified text
> > sizes.
> > 
> > I'd like this patch to be considered, please.
> > http://bugzilla.gnome.org/show_bug.cgi?id=555097
> 
> My main problem with this is that if we are going to add x, y, width &
> height properties to the GooCanvasItem interface then they should have
> well-defined semantics. But they don't - the x and y properties refer to
> arbitrary points in the item and the width & height may or may not be
> the exact width & height of the item.
> 
> I would think that if people see generic x, y, width & height properties
> then they will expect them to refer to the top-left coordinate and the
> exact width & height of the item.

What do you mean with the _exact_ width & height, by the way? Is this
because the width of a rectangle, for example, does not cover the entire
border? If so, then I think changing that would also break
compatibility. Does this matter?

> I wouldn't mind adding x, y, width & height properties to some of the
> items though.
> 
> 
> If I was writing an application that allowed users to drag items around
> and snap them to grids etc. I'd write my own custom items derived from a
> base class that supported drag-handles and all the API necessary to do
> this generically. I wouldn't really expect the builtin canvas items to
> be usable for this, since the functionality desired will probably differ
> from application to application.
> 
> Damon

Armin