the progressbar style has an inverted property, set it to true and the bar will be filled (1 - progress). that's yet not what you want. the next step is to exchange the bar style and the bar background. so the "background" overlaps the real background as the bar fills, thus making it look as if the bar fills from right to left.
at least this is what i did once a while back. maybe there's an easier way.
|