To customize the visual look of a UISlider you can set the thumb and track images. Part of the track images gets stretched to the appropriate with. From the documentation:>
I wanted a pattern behind my UISlider. I ended up setting both images to nil and adding a view under the slider since I didn't need it to be different on either side of the thumb. This would be more difficult if you needed to change it based on the position, but quite doable.