test_xdg_desktop_portal_wallpaper.vala

// valac --pkg gtk+-3.0 test_xdg_desktop_portal_wallpaper.vala

namespace org.freedesktop.portal {
	[DBus (name = "org.freedesktop.portal.Wallpaper")]
	interface Wallpaper : Object {
		public abstract GLib.ObjectPath ShowWallpaperPreview (string app_id, string uri) throws GLib.Error;
		public abstract GLib.ObjectPath SetWallpaper (string app_id, string uri) throws GLib.Error;
	}
}

public async void show_wallpaper_preview () {
	var bus = yield Bus.get (BusType.SESSION);

	var wallpaper = yield bus.get_proxy <org.freedesktop.portal.Wallpaper> ("org.freedesktop.portal.Desktop", "/org/freedesktop/portal/desktop");

	var response = wallpaper.ShowWallpaperPreview ("org.gnome.MyApp", "file:///home/feborges/bg.png");

  print ("RESPONSE = %s\n", response);
}

public async void set_wallpaper () {
  var bus = yield Bus.get (BusType.SESSION);

  var wallpaper = yield bus.get_proxy <org.freedesktop.portal.Wallpaper> ("org.freedesktop.portal.Desktop", "/org/freedesktop/portal/desktop");

  wallpaper.SetWallpaper ("org.gnome.MyApp", "file:///home/feborges/wp.jpg");
}

public class MyApplication : Gtk.Application {
    public MyApplication () {
        application_id = "org.gnome.MyApp";
    }

    protected override void activate () {
        var window = new Gtk.ApplicationWindow (this);
        var box = new Gtk.Box (Gtk.Orientation.VERTICAL, 10);
      	var button = new Gtk.Button.with_label ("Show Wallpaper Preview");
      	button.clicked.connect (() => { show_wallpaper_preview (); });
        box.add (button);

        var set_wp_button = new Gtk.Button.with_label ("Set Wallpaper");
        set_wp_button.clicked.connect (() => { set_wallpaper (); });
        box.add (set_wp_button);

        window.add (box);
        window.set_title ("Welcome to GNOME");
        window.set_default_size (200, 100);
        window.show_all ();
    }
}
 
public int main (string[] args) {
    GLib.Environment.set_application_name ("Wallpaper APP");
     return new MyApplication ().run (args);
}