Simple code to change the wallpaper

wall.vala

/* compile using: valac --pkg gtk+-3.0 wall.vala */
using Gtk;
using Gdk;
using Cairo;

public class AppWin : Gtk.Window {
    construct {
        app_paintable = true;
        decorated = false;
        resizable = false;
        set_keep_above (true);
        skip_taskbar_hint = true; // no taskbar
        skip_pager_hint = true;
        //type = Gtk.WindowType.POPUP;
        set_type_hint (WindowTypeHint.DESKTOP);
        set_visual (get_screen ().get_rgba_visual());
    }
    static const Gtk.ActionEntry[] main_entries = {
            { "Quit", Gtk.Stock.QUIT, N_("Quit"), "<Control>q", N_("Quit"), action },
            { "Test", "Test", N_("Want to change the wallpaper"), "<Control><Shift>c", N_("Copy the selected text"),
              action },
            { "About", "About", N_("elementary team ?"), null, N_("Show about window"), action }
        };
    void action () {
        print ("Action Detected\n");
    }
    public AppWin () {
        var evBox = new Gtk.EventBox ();
        this.add (evBox);
        evBox.draw.connect(on_draw_cb);
        destroy.connect (Gtk.main_quit);
        Gtk.UIManager ui = new Gtk.UIManager ();
        Gtk.ActionGroup main_actions;

        const string ui_string = """
            <ui>
            <popup name="AppMenu">
                <menuitem name="Test" action="Test"/>
                <separator />
                <menuitem name="About" action="About"/>
            </popup>
            </ui>
        """;
        main_actions = new Gtk.ActionGroup ("MainActionGroup");
        main_actions.set_translation_domain ("pantheon-terminal");
        main_actions.add_actions (main_entries, this);

        try {
            ui.add_ui_from_string (ui_string, -1);
        } catch (Error e) {
            error ("Couldn't load the UI: %s", e.message);
        }

        var menu = ui.get_widget ("ui/AppMenu") as Gtk.Menu;
        menu.show_all ();
        ui.insert_action_group (main_actions, 0);
        ui.ensure_update ();
        evBox.button_press_event.connect ((event) => {
            print ("Button Pressed");
                switch (event.button) {
                    case Gdk.BUTTON_PRIMARY:
                        return false;
                    case Gdk.BUTTON_SECONDARY :
                        menu.select_first (false);
                        menu.popup (null, null, null, event.button, event.time);
                        return true;
                }
                return false;
            });
        show_all ();
    }

    bool on_draw_cb (Context cr) {
        Gdk.Rectangle screen = Gdk.Rectangle () { x = 0, y = 0, width = get_screen ().width(), height = get_screen ().height() };
        this.set_size_request (screen.width, screen.height);
        var pixbuf = new Pixbuf.from_file_at_scale ("/home/voldyman/Pictures/wallpaper-1260152.jpg", screen.width,screen.height, true);
        cairo_set_source_pixbuf (cr, pixbuf, 0,0);
        cr.paint ();
        return true;
    }
/*
    protected override bool draw (Context cr) {
        base.draw (cr);

        Gdk.Rectangle screen = Gdk.Rectangle () { x = 0, y = 0, width = get_screen ().width(), height = get_screen ().height() };
        this.set_size_request (screen.width, screen.height);

        var pixbuf = new Pixbuf.from_file_at_scale ("/home/voldyman/Pictures/wallpaper-1260152.jpg", screen.width,screen.height, true);
        cairo_set_source_pixbuf (cr, pixbuf, 0,0);
        cr.paint ();
        return false;
    }*/
}
int main (string[] args) {
    Gtk.init (ref args);

    var win = new AppWin ();
    win.destroy.connect (Gtk.main_quit);
    win.show_all ();
    Gtk.main ();

    return 1;
}