We’re now 4 versions into iOS, and Apple still hasn’t brought UISlider up to speed with NSSlider. OS X gets horizontal, vertical, and rotary sliders while iOS is stuck with only horizontal. There’s probably some UI guideline which states that rotary sliders are no good for touch interfaces but honestly, it is really hard to make an attractive layout with a lot of controls using only horizontal sliders. Music apps tend to need a lot of controls, and making EQ adjustments using a slider reminds me of the bad old days of generic AudioUnit interfaces.
After a cursory Google search, I couldn’t find any code out there for rotary sliders so I wrote my own UIControl subclass. I tried to stick as closely as possible to the UISlider interface so that it won’t be too hard to switch when Apple gets around to making a standard control. There is a precision property that changes how quickly the knob rotates, this lets you make very accurate adjustments.