View difference between Paste ID: MDtMYa7m and V5dMAPud

 // gtk-hello.vala:
 // valac --pkg gtk+-3.0 gtk-hello.vala
 using Gtk;
 
 class HelloWindow : Gtk.Window {
 	public HelloWindow() {
 		this.title = "First GTK+ Program";
 		this.border_width = 10;
 		this.window_position = WindowPosition.CENTER;
 		this.set_default_size (350, 70);
 
 		var button = new Button.with_label ("Click me!");
 	    button.clicked.connect (() => {
         	button.label = "Thank you";
     	});
 
     	this.add (button);
 
     	this.add_tick_callback(() => {
     		DateTime now = new DateTime.now(new TimeZone.utc());
     		//print(now.format("%T - Redrawing\n"));
     		this.queue_draw();
     		return true;
     	});
 	}
 
 	public override bool draw(Cairo.Context cr) {
 		return base.draw(cr);
 	}
 }
 
 int main (string[] args) {
     Gtk.init (ref args);
 
     var window = new HelloWindow ();
     window.destroy.connect (Gtk.main_quit);
     window.show_all ();
 
     Gtk.main ();
     return 0;
 }