Re: Canvas basics

[Roger Leigh <rleigh whinlatter ukfsn org>]

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Jody Goldberg <jody gnome org> writes:

> On Sat, Jan 08, 2005 at 11:26:45PM +0000, Roger Leigh wrote:
> > I'd like to let the mouse wheel and Alt + mouse wheel scroll
> > horizontally and vertically when over any of the canvas widgets.
>
> With Gtk2 this is nice and clean, not like gtk1 where you had to
> look for buttons 4&5.
>
> Just put a handler for "scroll-event" onto the window.
> Have a look at gnumeric/src/workbook-control-gui.c:wbcg_scroll_wheel_support_cb
> for an example.

That works fairly straightforwardly.  I can now scroll and zoom
nicely.  The only problem is the slowness of zooming: I have to signal
all my "CanvasCell" objects to re-set the "font-desc" property; this
turns out to be quite expensive with as few as 256 cells.

> > The other thing I'd like to do is set a GdkCursor to show when the
> > mouse moves over certain items, like the edge of the current selection
> > or the drag handle in the bottom right corner.  Is this simple to do?
>
> You can look at an example in gnumeric/src/item-bar.c:item_bar_event
>       case GDK_ENTER_NOTIFY:
>       case GDK_MOTION_NOTIFY:
>               ib_set_cursor (ib, x, y);

I hadn't thought of setting the cursor on the main GdkWindow!

> > - - gnome_canvas_item_new allows one to set object properties on the
> >   created item.  However, it does not pass them to g_object_new, so
> >   they can't be used as construction parameters.  (Though I can't
> >   remember why I wanted this now.)
> That is indeed irritating.  Likely a throw back to the old
> pre-gobject days that is still visible because canvas doesn't see
> much maintenance.

I had a look at fixing this today (using g_object_new_valist in
gnome_canvas_item_new).  Unfortunately, it's not as straightforward as
I hoped.  At least item.parent and item.canvas must be set as
construction parameters for it to work.  I may get it fixed tomorrow
if I have time.

I also found a bug with setting the zoom factor (pixels per unit).
For example, in an event handler:

   gnome_canvas_c2w(canvas, event->button.x, event->button.y, &ix, &iy);
   gnome_canvas_item_w2i(GNOME_CANVAS_ITEM(self), &ix, &iy);
   ix *= cell_base->cse_canvas->zoom;
   iy *= cell_base->cse_canvas->zoom;;

If I don't multiply the coordinates by the zoom factor, they are
wrong.  I thought the world/item coordinates should be independent of
that.

> If you don't need anti-aliasing you may want to look at foocanvas, a
> smaller lighter canvas used in gnumeric and nautilus.

I may well do.  Does it support alpha transparency fills on
rectangles, for example?  Also, does this fix the construction
parameter problem?


Thanks,
Roger

- -- 
Roger Leigh
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