[cairo] Goocanvas: absolute-position animation and "update" confusion

[Carl Worth]

On Thu, 30 Nov 2006 14:43:00 +0000, Damon Chaplin wrote:
> I just committed some changes so you can avoid all the boilerplate code.
> demo/demo-item.c is a lot simpler now.

Thanks, that does look a lot better now. It's still enough
boilerplate, (mostly the gobject inheritance stuff), that I'll
still probably try to share one class for all my program's widgets,
but that should be just fine.

Meanwhile, here are a couple of snags I ran into:

1. I'd like to be able to animate an item to an absolute position,
   rather than a relative position. Would you be interested in having
   two variants for that? [Separate, interesting things to do would
   be to animate along a path, and to animate with something other
   than linear timing along it.]

2. Since I have to compute a relative offset to use the current
   animate functions I have to be able to query the item's current
   position. Unfortunately, in demo-item.c there appear to be two
   copies of the position and size, (an "x,y,width,height" in DemoItem
   and a position/size that the canvas itself manipulates with things
   like animate). Having two copies makes things harder since I have
   to carefully keep them in synch. Is there a good reason to have
   these two copies?

3. As far as keeping the two copies of position/size in synch, I
   haven't succeeded at doing that yet. What are the correct semantics
   of the "update" function? In demo-item I see it copying a position
   from demo_item->x,y to its parent objects simple->bounds.x1,y1.
   This seems to be necessary on the "update" to get the constructed
   "x,y" value to replace a stale (0,0) in simple-bounds. However,
   after doing any animation, subsequent calls to "update" still now
   have stale data in demo_item->x,y and the correct position in
   simple->bounds. Am I just confused? What's the best way out of this
   mess?

Oh, and finally, I accidentally rotated an item during an animation
one time and I think I saw it shrink and grow on the way, (at least
the way I wrote the paint function). It looked like maybe the canvas
was keeping the size of the bounds unchanged and the rotated drawing
was shrinking to always fit within those bounds. Maybe this was my
bug---I didn't look closely since I wasn't actually trying to rotate
anything anyway.

-Carl
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