Custom GTK widget in Vala not working

Question

I'm trying to make a custom GTK widget in Vala, but I'm already failing at the very first basic attempt, so I'd like some help in knowing where I'm going wrong. I feel like I must be missing something painstakingly obvious, but I just can't see it.

I have three files with the following contents:

start.vala:

using Gtk;

namespace WTF
{
        MainWindow main_window;

        int main(string[] args)
        {
                Gtk.init(ref args);
                main_window = new MainWindow();
                Gtk.main();
                return 0;
        }
}

main_window.vala:

using Gtk;

namespace WTF
{
        public class MainWindow : Window
        {
                public MainWindow()
                {
                        /* */
                        Entry entry = new Entry();
                        entry.set_text("Yo!");
                        this.add(entry);
                        /* */

                        /* 
                        CustomWidget cw = new CustomWidget();
                        this.add(cw);
                        /* */

                        this.window_position = WindowPosition.CENTER;
                        this.set_default_size(400, 200);
                        this.destroy.connect(Gtk.main_quit);

                        this.show_all();
                }
        }
}

custom_widget.vala:

using Gtk;

namespace WTF
{
        public class CustomWidget : Bin
        {
                public CustomWidget()
                {
                        Entry entry = new Entry();
                        entry.set_text("Yo");
                        this.add(entry);
                        this.show_all();
                }
        }
}

As you can see, in main_window.vala, I have two sets of code. One that adds the Entry widget directly, and one that adds my custom widget. If you run the one that adds the Entry widget directly, you get this result:

If you run the one with the custom widget, however, you get this result:

Just for the record, this is the complication command I use:

valac --pkg gtk+-2.0 start.vala main_window.vala custom_widget.vala -o wtf

EDIT:

Following user4815162342's suggestion, I implemented the size_allocate method on my custom Bin widget, like so:

public override void size_allocate(Gdk.Rectangle r)
{
        stdout.printf("Size_allocate: %d,%d ; %d,%d\n", r.x, r.y, r.width, r.height);
        Allocation a = Allocation() { x = r.x, y = r.y, width = r.width, height = r.height };
        this.set_allocation(a);

        stdout.printf("\tHas child: %s\n", this.child != null ? "true" : "false");
        if (this.child != null)
        {
                int border_width = (int)this.border_width;
                Gdk.Rectangle cr = Gdk.Rectangle()
                {
                        x = r.x + border_width,
                        y = r.y + border_width,
                        width = r.width - 2 * border_width,
                        height = r.height - 2 * border_width
                };
                stdout.printf("\tChild size allocate: %d,%d ; %d, %d\n", cr.x, cr.y, cr.width, cr.height);
                this.child.size_allocate(cr);
        }
}

It writes the following in the console:

Size_allocate: 0,0 ; 400,200
        Has child: true
        Child size allocate: 0,0 ; 400, 200

And the window renders thusly:

Answer 1

GtkBin is an abstract single-child container, typically intended to decorate the child widget in some way, or change its visibility or size. Without some added value, a single-child container would be indistinguishable from the widget it contains and therefore not very useful.

Since GtkBin doesn't know what kind of decorations you will draw around the child, it expects you to implement your own size_allocate. A simple implementation is available in gtk_event_area_size_allocate, a more complex one in gtk_button_size_allocate.

This answer shows a minimal size_allocate implementation in PyGTK which should be straightforward to port to Vala. If you do anything more complex than that, you will need to also implement expose, and possibly other methods, but this will get you started.