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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: gtkada evenhandler on_procedure method , how to access children
 of topwindow in the eventhandler
Date: Sat, 2 Nov 2019 20:21:29 +0100
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Date: 2019-11-02T20:21:29+01:00
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On 2019-11-02 20:07, Alain De Vos wrote:
> On Saturday, November 2, 2019 at 11:22:49 AM UTC+1, Alain De Vos wrote:
>> I have a window with obe button and one label and want to change the text on the label when the button is pressed in the eventhandler, I pass the top window.
>>
>> procedure hello is
>>     Top     : Gtk.Window.Gtk_Window;
>>     Button  : Gtk_Button;
>>     Label   : Gtk_Label;
>>     VBox    : Gtk_Box;
>>     procedure Create_Window is
>>     begin
>>        Gtk.Window.Gtk_New
>>          (Window   => Top,
>>           The_Type => Gtk.Enums.Window_Toplevel);
>>        Gtk.Window.Set_Title (Window => Top, Title  => "Hello from ADA");
>>        Gtk_New(Button);
>>        Button.On_Clicked (Call => On_Button_Click'Access,
>>                           Slot => Top);
>>        Gtk_New(Label);
>>        Gtk_New_Vbox(VBox);
>>        Add (VBox,Button);
>>        Add (VBox,Label);
>>        Add (Top,VBox);
>>        Gtk.Window.Show_All (Top);
>>     end Create_Window;
>> begin
>>     Gtk.Main.Init;
>>     Create_Window;
>>     Gtk.Main.Main;
>> end hello;
>>
>> I pass the topwindow to the eventhandler(slot).
>> In the eventhandler I need go from the Top to the Vbox to the Label and set the text on the label ... button pressed. I probably need to set a property in the main and query it in the handler and cast the correct type ...
>> {The label is not directly visible in the handler ...}
>>
>> package body buttonhandler is
>>     procedure On_Button_Click (Top : access Glib.Object.GObject_Record'Class) is
>>     begin
>>        Put_line ("Hallo");
>>        -- I need to go from Top to Vbox to Label and set text ... ????
>>     end On_Button_Click;
>> end buttonhandler;
> 
> Ok, so now I try to force a casting, but I'm still not there,
> 
> package body buttonhandler is
>     type MyVBoxR is new Gtk_VBox_Record with record
>        Button : Gtk_Button;
>        Label  : Gtk_Label;
>     end record;
>     type MyVBoxT is access all MyVBoxR'Class;
>     procedure On_Button_Click (T : access Glib.Object.GObject_Record'Class) is
>        MyVBox  : MyVBoxT;
>     begin
>        Put_line ("You pressed");
>        MyVBox :=MyVBoxT (T);   -- Dangerous
>        Gtk.Button.Set_Label (MyVBox.Button,"You pressed");
>     end On_Button_Click;
> end buttonhandler;
> 
> No compile time warning but a runtime error on "MyVBox :=MyVBoxT (T);"  ,
> the error is :
> Hallo
> raised CONSTRAINT_ERROR : buttonhandler.adb:15 tag check failed

Which is why the idea is bad.

The proper conversion would look like this:

    MyVBoxR'Class (T.all)'Unchecked_Access

However in Ada it is done rather this way:

    procedure On_Button_Click (T : access GObject_Record'Class) is
       MyVBox : MyVBoxR'Class renames MyVBoxR'Class (T.all);
    begin
       MyVBox.Button.Set_Label ("You pressed");
       ...

Note that in GtkAda you must catch and suppress exceptions. If Ada 
exceptions propagate into C code they kill all application. The right 
way is this:

    with GLib.Messages;  use GLib.Messages;

    procedure On_Button_Click (T : access GObject_Record'Class) is
    begin
       declare
          MyVBox : MyVBoxR'Class renames MyVBoxR'Class (T.all);
       begin
          MyVBox.Button.Set_Label ("You pressed");
          ...
       end;
    exception
       when Error : others =>
          Log
          (  "MyApp",
             Log_Level_Critical,
             (  "Fault: "
             &  Exception_Information (Error)
             &  " in On_Button_Click "
          )  );
    end On_Button_Click;

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