Gtk3 Cairo Context 0x0

Gtk3 Cairo Context 0x0

[ldries46]

I am creating a new Drawing_Area.
I have packed the drawing Area in a Viewport which is packed in a scrolled 
window.
Now have the program line:
CR := Create(Get_Window (MainWindow.DrawingArea));
where MainWindow.DrawingArea is type Gtk_Drawing_Area with the value 
0x4694f98 (meaning the value is no null)
created with Gtk_New (MainWindow.DrawingArea);
The Cairo context CR has however a value 0x0 which finally results when 
using Save(CR);
in:
Program received signal SIGSEGV, Segmentation fault.
cairo_save (cr=0x0) at ../../src/src/cairo.c:409
409 ../../src/src/cairo.c: No such file or directory.
Current language: auto
The current source language is "auto; currently c".

In the version 2015 of GtkAda from AdaCore the create function  worked and 
was positioned after the creation of the Drawing Area but before the 
connection to the "draw" event.
On the Gtkada group I do get only the answer that only in the handler I can 
get the context but that means that I cannot create a framework within the 
area before any work with on the area because therefore I need the cairo 
context f.i. to set s line color.

What must I do to retrieve a valid CR?

Bertus

Re: Gtk3 Cairo Context 0x0

[Dmitry A. Kazakov]

On 2016-07-21 16:42, ldries46 wrote:
> I am creating a new Drawing_Area.
> I have packed the drawing Area in a Viewport which is packed in a
> scrolled window.
> Now have the program line:
> CR := Create(Get_Window (MainWindow.DrawingArea));
> where MainWindow.DrawingArea is type Gtk_Drawing_Area with the value
> 0x4694f98 (meaning the value is no null)
> created with Gtk_New (MainWindow.DrawingArea);
> The Cairo context CR has however a value 0x0 which finally results when
> using Save(CR);
> in:
> Program received signal SIGSEGV, Segmentation fault.
> cairo_save (cr=0x0) at ../../src/src/cairo.c:409
> 409 ../../src/src/cairo.c: No such file or directory.
> Current language: auto
> The current source language is "auto; currently c".
>
> In the version 2015 of GtkAda from AdaCore the create function  worked
> and was positioned after the creation of the Drawing Area but before the
> connection to the "draw" event.
> On the Gtkada group I do get only the answer that only in the handler

That is right.

[There are rare cases when memory-mapped contexts used, but drawing area 
presumes a normal window's drawing context.]

> I can get the context but that means that I cannot create a framework
> within the area before any work with on the area because therefore I
> need the cairo context f.i. to set s line color.

Which framework?

Whatever you might do with a context is useless if the context is not 
the one used for drawing onto the actual window. That one you get in the 
handler. When initialization is needed do it upon the first call.

> What must I do to retrieve a valid CR?

It does not work this way, you must reconsider your design.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de