gtkada3 callback function & failure : subprogram must not be deeper than access type

gtkada3 callback function & failure : subprogram must not be deeper than access type

[Alain De Vos]

For callback functions there are 3 methods and one with data.
For the first method I try a program with one button
Connecting the button with the handler fails with the error :
>>>>subprogram must not be deeper than access type

The code :
-- TEST : Connecting via the On_* procedures first method
with Ada.Text_IO; use Ada.Text_IO;
with Glib;         use Glib;
with Glib.Object;  use Glib.Object;
with Gtk;          use Gtk;
with Gtk.Box;      use Gtk.Box;
with Gtk.Button;   use Gtk.Button;
with Gtk.Enums;    use Gtk.Enums;
with Gdk.Event;    use Gdk.Event;
with Gtk.Frame;    use Gtk.Frame;
with Gtk.Grid;     use Gtk.Grid;
with Gtk.Handlers; use Gtk.Handlers;
with Gtk.Label;    use Gtk.Label;
with Gtk.Main;     use Gtk.Main;
with Gtk.Widget;   use Gtk.Widget;
with Gtk.Window;   use Gtk.Window;
with Gtk.Main;   use Gtk.Main;

procedure hello_gtk is
   Main_Window : Gtk.Window.Gtk_Window;
   Button  : Gtk_Button;
   procedure On_Button_Click (Button : access Gtk_Button_Record'Class)
   is
   begin
      Put_line ("Hallo");
   end On_Button_Click;

   procedure Create_Window is
   begin
      Gtk.Window.Gtk_New
        (Window   => Main_Window,
         The_Type => Gtk.Enums.Window_Toplevel);
      Gtk.Window.Set_Title (Window => Main_Window, Title  => "Hello from ADA");
      --  Button.On_Clicked (On_Button_Click'Access); fails with subprogram must not be deeper than access type
      Add (Main_Window,Button);
      Gtk.Window.Show_All (Main_Window);
   end Create_Window;
begin
   Gtk.Main.Init;
   Create_Window;
   Gtk.Main.Main;
end hello_gtk;

Re: gtkada3 callback function & failure : subprogram must not be deeper than access type

[Shark8]

Move the On_Button_Click procedure to its own file (on-button-click.ads or something) and add "With On_Button_Click;" to your hello_gtk procedure.

The reason for the error "subprogram must not be deeper than access type" is because the subprogram's access-level is inside the scope of hello_gtk, but the parameter's access-type exceeds that scope: IOW, the access failure is to prevent the dangling-pointer situation where hello_gtk finishes and therefore leaves the scope (thus invalidating the reference to On_Button_Click).

Re: gtkada3 callback function & failure : subprogram must not be deeper than access type

[Dmitry A. Kazakov]

On 2019-11-01 19:30, Alain De Vos wrote:
> For callback functions there are 3 methods and one with data.
> For the first method I try a program with one button
> Connecting the button with the handler fails with the error :
>>>>> subprogram must not be deeper than access type

You can convert the access type. But I will not post how to do this, 
because it is bad style and idea.

Move On_Button_Click to a package and it will compile.

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