GTK drawing area is not realized

Question

In my application I am using a Gtk::DrawingArea like this:

Window win;
DrawingArea area;
Box box(ORIENTATION_VERTICAL);
area.signal_realize().connect(sigc::ptr_fun(&on_video_area_realize));
box.pack_start(myWidgets, true, true);
box.pack_start(area, false, false);
win.add(box);
win.show_all();

The problem is, the function on_video_area_realize is not being called and if I query the status of the DrawingArea with area.get_realized(), it is false, so it has not been realized yet.

I do not understand why it has not been realized? As far as I understand, a widget is realized when it is added to a window - which, as far as I think, I am doing already.

Answer 1

The realize signal happens when the window (and its children) is showed. The following code (I tested this with Gtkmm 3.24.20):

#include <iostream>
#include <gtkmm.h>

void on_video_area_realize()
{
    std::cout << "Video drawing area realized!" << std::endl;
}

int main(int argc, char *argv[])
{
    std::cout << "Gtkmm version : " << gtk_get_major_version() << "."
                                    << gtk_get_minor_version() << "."
                                    << gtk_get_micro_version() << std::endl;

    auto app = Gtk::Application::create(argc, argv, "org.gtkmm.examples.base");
  
    Gtk::Window window;
    Gtk::Box layout{Gtk::ORIENTATION_VERTICAL};
    Gtk::Label label{"Example"};
    
    std::cout << "Checkpoint 1" << std::endl;
    
    Gtk::DrawingArea area;

    std::cout << "Checkpoint 2" << std::endl;

    area.signal_realize().connect(sigc::ptr_fun(&on_video_area_realize));
    
    std::cout << "Checkpoint 3" << std::endl;

    layout.pack_start(label, true, true);
    layout.pack_start(area, false, false);
    
    std::cout << "Checkpoint 4" << std::endl;

    window.add(layout);
    window.show_all();
    
    std::cout << "Checkpoint 5" << std::endl;
  
    return app->run(window);
}

Yields the following output:

Gtkmm version : 3.24.20
Checkpoint 1
Checkpoint 2
Checkpoint 3
Checkpoint 4
Video drawing area realized!
Checkpoint 5

From this, we can see that the drawing area is realized, but not when added to the window.