program HelloWorld;

{$mode objfpc}{$H+}

uses
  Glib2, Gdk2, Gtk2;

(* This is a callback function. The data arguments are ignored
 * in this example. More on callbacks below. *)
procedure hello(Widget: PGtkWidget; Data: gpointer); cdecl;
begin
   g_print ('Hello World'#13#10);
end;

function delete_event( Widget: PGtkWidget; Event: PGdkEvent; Data: gpointer):gint; cdecl;
begin
  (* If you return FALSE in the "delete_event" signal handler,
   * GTK will emit the "destroy" signal. Returning TRUE means
   * you don't want the window to be destroyed.
   * This is useful for popping up 'are you sure you want to quit?'
   * type dialogs. *)

  g_print ('delete event occurred'#13#10);

  (* Change TRUE to FALSE and the main window will be destroyed with
   * a "delete_event". *)

  Result:=gTRUE;
end;

(* Another callback *)
procedure destroy(Widget: PGtkWidget; Data: gpointer); cdecl;
begin
  gtk_main_quit;
end;

var
  (* GtkWidget is the storage type for widgets *)
  Window: PGtkWidget;
  Button: PGtkWidget;
begin

  (* This is called in all GTK applications. Arguments are parsed
   * from the command line and are returned to the application. *)
  gtk_init (@argc, @argv);

  (* create a new window *)
  window := gtk_window_new (GTK_WINDOW_TOPLEVEL);

  (* When the window is given the "delete_event" signal (this is given
   * by the window manager, usually by the "close" option, or on the
   * titlebar), we ask it to call the delete_event () function
   * as defined above. The data passed to the callback
   * function is NULL and is ignored in the callback function. *)
  g_signal_connect (G_OBJECT (window), 'delete_event',
                      G_CALLBACK (@delete_event), NULL);

  (* Here we connect the "destroy" event to a signal handler.
   * This event occurs when we call gtk_widget_destroy() on the window,
   * or if we return FALSE in the "delete_event" callback. *)
  g_signal_connect (G_OBJECT (window), 'destroy',
                      G_CALLBACK (@destroy), NULL);

  (* Sets the border width of the window. *)
  gtk_container_set_border_width (GTK_CONTAINER (window), 10);

  (* Creates a new button with the label "Hello World". *)
  button := gtk_button_new_with_label ('Hello World');

  (* When the button receives the "clicked" signal, it will call the
   * function hello() passing it NULL as its argument.  The hello()
   * function is defined above. *)
  g_signal_connect (G_OBJECT (button), 'clicked',
                      G_CALLBACK (@hello), NULL);

  (* This will cause the window to be destroyed by calling
   * gtk_widget_destroy(window) when "clicked".  Again, the destroy
   * signal could come from here, or the window manager. *)
  g_signal_connect_swapped (G_OBJECT (button), 'clicked',
                              G_CALLBACK (@gtk_widget_destroy), window);

  (* This packs the button into the window (a gtk container). *)
  gtk_container_add (GTK_CONTAINER (window), button);

  (* The final step is to display this newly created widget. *)
  gtk_widget_show (button);

  (* and the window *)
  gtk_widget_show (window);

  (* All GTK applications must have a gtk_main(). Control ends here
   * and waits for an event to occur (like a key press or
   * mouse event). *)
  gtk_main ();
end.