Re: canvas.render doesn't accept None as second parameter

[John Coppens <john jcoppens com>]

On Sun, 27 Mar 2016 20:54:29 +0100
Damon Chaplin <damonachaplin gmail com> wrote:

> > 1) Is this a doc error or a GI/Goocanvas regression?
>
> It looks like there should be an (allow-none) annotation for
> introspection. Though it isn't too important as you usually want to use
> a bounds rectangle. Or just use the entire canvas bounds.

The problem is that passing references to variables (to receive the
actual bounds from get_bounds() is, AFAIK, not possible in *Python*).
Any assignment to a variable inside a function makes that variable 
local. To be sure, I tried to pass variables as arguments, and elements
of a list, but all produce segmentation faults. I do know how to do this
in C with the & operator, of course. 

I suspect the get_bounds() method should be changed to return a 4-tuple
in Python (and not through the parameters, but as a return value).

> > 2) I couldn't find a working demo which uses 'render' and SVG|PNG...surfaces in python. 
>
> I don't think I've ever tried SVG or PNG surfaces.
>
> Can you get it to work with a PDF surface? The write_pdf_clicked()
> function in goocanvas/demo/demo.c shows how to do that in C. (And it
> still seems to work!)

I've found a way to do it, by not using get_bounds() and defining the
bounds manually:

        side = self.theme.canvas_size * self.zoom
        surface = cairo.SVGSurface("demo.svg", side, side)
        cr = cairo.Context(surface)
        cr.translate(side/2, side/2)
        bounds = goo.CanvasBounds()
        bounds.x1 = -side/2; bounds.y1 = -side/2
        bounds.x2 =  side/2; bounds.y2 =  side/2
        self.canvas.render(cr, bounds, 1)

I still hope someone can show me how to use get_bounds() in the
GooCanvas Python binding!

Thanks for the hints,
John

PS: This seems to work with any surface... Tested with SVG, PS, PDF.