Example: ProgressBar

public class Application : Gtk.Window {
public Application () {
// Prepare Gtk.Window:
this.title = "My Gtk.ProgressBar";
this.window_position = Gtk.WindowPosition.CENTER;
this.destroy.connect (Gtk.main_quit);

// The ProgressBar:
Gtk.ProgressBar bar = new Gtk.ProgressBar ();
this.add (bar);

// Display text:
bar.set_text ("My progress");
bar.set_show_text (true);

// Fill the bar:
GLib.Timeout.add (500, () => {
// Get the current progress:
// (0.0 -> 0%; 1.0 -> 100%)
double progress = bar.get_fraction ();

// Update the bar:
progress = progress + 0.2;
bar.set_fraction (progress);

// Repeat until 100%
return progress < 1.0;
});
}

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

Application app = new Application ();
app.show_all ();
Gtk.main ();
return 0;
}
}

 valac --pkg gtk+-3.0 Gtk.ProgressBar.vala

See: