[Vala] How to use Gtk.Builder in our custum widgets?

[Sepehr Aryani <sepehr aryani gmail com>]

I've created a UI in glade and I want to use it inside my vala app. I need to derive my own Gtk.Window and 
customize it but the problem is that I don't know how to relate UI to my custom Window class.

class MyWindow : Gtk.Window{
    public Window(){
    }
}

public static int main(string[] args){){
    Gtk.init(ref args);
    try{
        var builder = new Gtk.Builder();
        builder.add_from_file("../ui/main.glade");
        builder.connect_signals(null);
                
        var window = (MyWindow) builder.get_object("gld_top_window"); // !!!!!! does not work correctly
        window.destroy.connect(Gtk.main_quit);
        window.show_all();
        
        Gtk.main();
     }catch(Error e){
        stderr.printf("cannot load the ui: %s\n", e.message);
        return 1;
     }
}

does not work well for me. Besides I need to pass Gtk.builder reference to MyWindow to be able to handle the 
ui inside my custom window class. In C++ we could do this using get_widget_derived. I didn't see the same 
mechanism in Vala's gtk library. would someone please help me?