Load Images Asynchronously Gtk Vala

Application.vala

// Compile with valac --pkg gtk+-3.0 --pkg=gio-2.0 Application.vala

public class Application : Gtk.Application {

    private const int COVER_SIZE = 170;

    public Application () {
        Object(application_id: "testing.my.application",
                flags: ApplicationFlags.FLAGS_NONE);
    }

    public static File? open_file(string path) {
        if(path.index_of("file:") == 0 || path.index_of("resource:") == 0 || path.index_of("http:") == 0 || path.index_of("https:") == 0) { 
            return GLib.File.new_for_uri(path);
        } else {
            return GLib.File.new_for_path(path);
        }
    }

    public static void create_cover_image (Gtk.Image image, Gdk.Pixbuf pixbuf, int image_size) throws Error {
        Gdk.Pixbuf cover_image = pixbuf;

        if (cover_image.height == cover_image.width)
            cover_image = cover_image.scale_simple (image_size, image_size, Gdk.InterpType.BILINEAR);

        if (cover_image.height > cover_image.width) {

            int new_height = image_size * cover_image.height / cover_image.width;
            int new_width = image_size;
            int offset = (new_height - new_width) / 2;

            cover_image = new Gdk.Pixbuf.subpixbuf(cover_image.scale_simple (new_width, new_height, Gdk.InterpType.BILINEAR), 0, offset, image_size, image_size);

        } else if (cover_image.height < cover_image.width) {

            int new_height = image_size;
            int new_width = image_size * cover_image.width / cover_image.height;
            int offset = (new_width - new_height) / 2;

            cover_image = new Gdk.Pixbuf.subpixbuf(cover_image.scale_simple (new_width, new_height, Gdk.InterpType.BILINEAR), offset, 0, image_size, COVER_SIZE);
        }

        if(image != null) {
            image.clear();

            image.set_from_pixbuf (cover_image);
        }
    }

    public async void set_image(Gtk.Image image, string path) throws Error {
        GLib.File file = open_file(path);

        GLib.InputStream stream = yield file.read_async ();
        Gdk.Pixbuf cover_image = yield new Gdk.Pixbuf.from_stream_async (stream);
        
        create_cover_image(image, cover_image, COVER_SIZE);

        //  image.set_from_pixbuf (cover_image);
    }

    protected override void activate () {
        var window = new Gtk.ApplicationWindow (this);
        window.set_default_size (400, 400);
        window.title = "My Gtk.Application";

        var button = new Gtk.Button.with_label("Click me");
        button.clicked.connect(() => {
            print("Clicked!");
        });

        var path = "https://upload.wikimedia.org/wikipedia/commons/8/8e/Velazquez-The_Surrender_of_Breda.jpg";
        
        var image = new Gtk.Image();
        image.pixel_size = COVER_SIZE;
        set_image.begin (image, path, (obj, res) => {
            window.show_all();
        });

        var image2 = new Gtk.Image();
        set_image.begin (image2, path, (obj, res) => {
            window.show_all();
        });

        var image3 = new Gtk.Image();
        set_image.begin (image3, path, (obj, res) => {
            window.show_all();
        });

        var image4 = new Gtk.Image();
        set_image.begin (image4, path, (obj, res) => {
            window.show_all();
        });

        var image5 = new Gtk.Image();
        set_image.begin (image5, path, (obj, res) => {
            window.show_all();
        });

        var container = new Gtk.Box(Gtk.Orientation.VERTICAL, 5);
        container.add(button);
        container.add(image);
        container.add(image2);
        container.add(image3);
        container.add(image4);
        container.add(image5);
        

        //  var label = new Gtk.Label("<b>Success With God & Psych</b>".replace("&", "&amp;"));
        //  label.wrap = true;
		//  label.use_markup = true;

        window.add(container);
        window.show_all ();
    }



    public static int main (string[] args) {
        Application app = new Application ();
        return app.run (args);
    }
}