var started = false;

function onSourceDownloadProgressChanged(sender, eventArgs) {

    //start animation on first call only
    if (!started) { sender.findName("loop").begin(); started = true; }

    // get progress
    if (eventArgs.progress)
        var progress = Math.ceil(eventArgs.progress * 100);
    else
        var progress = Math.ceil(eventArgs.get_progress() * 100);

    // update percent loaded
    if (sender.findname("percent") != null) {
        var percentblock = sender.findName("percent");
        percentblock.Text = progress.toString() + "%";
    }  

}