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Subject: Re: Where do I get my Cairo context from (in GTK3)?
From: borisgaertner56@gmail.com
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Xref: news.eternal-september.org comp.lang.ada:31294
Date: 2016-08-05T08:56:12-07:00
List-Id: <comp.lang.ada>

Am Sonntag, 24. April 2016 22:44:25 UTC+2 schrieb hreba:
> In GTk2 you got it from a call of Gdk.Cairo.Create.
>=20
> In GTK3 it is passed as a parameter of the callback function for the=20
> "draw" signal with the C prototype:
.....


To make a long story short, you have to add two lines and to modify one lin=
e:

    package DA_Ret_Callback is new Gtk.Handlers.User_Return_Callback
      (Gtk.Widget.Gtk_Widget_Record, Boolean, Handler_DataD);

    -- begin of addition
    package Context_Marshaller is new=20
            DA_Ret_Callback.Marshallers.Generic_Marshaller
                  (Cairo.Cairo_context, Cairo.Get_Context);
    .. end of addition=20

    function Draw_Handler
      (da: access Gtk.Widget.Gtk_Widget_Record'Class;
       cc: Cairo.Cairo_Context;
       hd: Handler_DataD) return Boolean is
    begin
       ...
    end Draw_Handler;


    procedure Connect_Handlers
      (obj: Glib.Object.GObject; hd: Handler_DataD;
       sn_realize, sn_config, sn_draw: Glib.Signal_Name) is
    begin
       ...
       DA_Ret_Callback.Connect
        (Gtk.Widget.Gtk_Widget(obj), sn_draw,
         Context_Marshaller.To_Marshaller(Draw_Handler'Access),
            --modified, was: DA_Ret_Callback.To_Marshaller( .... )
         hd);
    end Connect_Handlers;=20

The point is that, in Gtk-3, a callback function that draws into a widget, =
is
called with a Cairo context,whereeas in Gtk-2, your created the context you=
rself.

In GtkAda for Gtk-3, the instantiations of the generic package Return_Callb=
ack provide a marshaller that can forward a cairo context to a callback fun=
ction.

In generic package User_Return_Callback such a marshaller is still missing.=
 Therefore you have to define it yourself. This is done as shown above. The=
 good thing is that you can do it in your application - there is no need to=
 modify and to recompile the GtkAda sources.
--
Boris Gaertner