Fixed Async Fibonacci Vala

using Gtk;

public long fibonacci (int n)
{
    if (n < 2) {
        return n;
      } else {
        return (fibonacci(n - 1) + fibonacci(n - 2));
    }
}

/*long process of much processing*/
public async long long_process (int val)
{

    SourceFunc callback = long_process.callback;
    long number = 0;

    ThreadFunc<void*> run = () => {
        number = fibonacci (val);
        Idle.add((owned) callback);
        return null;
    };

    Thread.create<void*>(run, false);

    yield;

    return number;
}

public static int main (string[] args)
{
    Gtk.init (ref args);

    var window = new Window ();
    window.title = "Fibonacci GTK+";
    window.set_border_width (12);
    window.set_default_size (70, 70);
    window.set_position (Gtk.WindowPosition.CENTER);
    window.destroy.connect (Gtk.main_quit);

    var headerbar = new HeaderBar ();
    var entry = new Entry ();
    var button = new Button.with_label ("Calculate");
    var label = new Label ("Result");
    var box = new Box (Orientation.VERTICAL, 0);

    headerbar.title = "Fibonacci GTK+";
    headerbar.show_close_button = true;

    box.pack_start (entry, false, true, 0);
    box.pack_start (label, false, true, 12);
    box.pack_start (button, false, true, 0);

    window.set_titlebar (headerbar);

    window.add (box);
    window.show_all ();

    /*** LOGIC ***/
    button.clicked.connect ( ()=> {

        /*here it called a long process that freezes the user interface*/
        int val = int.parse(entry.get_text());

        var loop = new MainLoop();
        long_process.begin(val, (obj, res) => {
                try {
                    double result = long_process.end(res);
                    label.set_label(result.to_string ());
                } catch (ThreadError e) {
                    string msg = e.message;
                    stderr.printf(@"Thread error: $msg\n");
                }
                loop.quit();
            });
        loop.run();
    });


    Gtk.main ();
    return 0;
}