fibonacci.vala

using Gtk;


 public class MyThread : Object{
    public long n { get; private set; }

    public MyThread(long n){
        this.n = n;
    }

    public long run(){
        long number;
        number = this.fibonacci(this.n);

        Thread.exit (42);
        return number;
    }
    public long fibonacci (long i){

        if (i < 2) {
            return i;
      } else {
            stdout.printf("Running! \n");
            return (this.fibonacci(i - 1) + fibonacci(i - 2));
        }
    }

}


public class Application : Gtk.Application {

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

    }

    protected override void activate(){

    Gtk.ApplicationWindow window = new Gtk.ApplicationWindow (this);

    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 loop = new MainLoop();

    var headerbar = new Gtk.HeaderBar ();
    var entry = new Gtk.Entry ();
    var button = new Gtk.Button.with_label ("Calculate");
    var label = new Gtk.Label ("Result");
    var box = new Gtk.Box (Gtk.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 ( ()=> {
        try {
        // Start a thread:
        MyThread my_thread = new MyThread (long.parse (entry.get_text ()));
        Thread<long> thread = new Thread<long>.try ("My fst. thread", my_thread.run);

        // Wait until thread finishes:
        long result = thread.join ();

        label.set_label (result.to_string ());

    } catch (Error e) {
        stdout.printf ("Error: %s\n", e.message);
    }

    });

    }
}

public static int main (string[] args){

    if (Thread.supported () == false) {
        stderr.printf ("Threads are not supported!\n");
        return -1;
    }

    var app = new Application();

    app.run(args);
    Gtk.main ();
    return 0;
}