Are you looking for a one-touch gesture (like a circular slider), or a two-touch gesture (like grabbing and turning a real-world knob)? If the latter, take a look at UIRotationGestureRecognizer.
If the former, you're pretty much on your own. You can certainly implement your solution as your own custom gesture recognizer: this is expected by Apple, and there's some documentation to get you started (though I haven't seen many working examples out there). See also How to correctly subclass UIGestureRecognizer.
As a general approach, I'd think of the region for the gesture as a donut shape: a zone with center c, inner radius r1, and outer radius r2. When the user touches down you can calculate the distance from c using the Pythagorean theorem, and the angle with your favorite trig function. With that, you can determine whether the touch is within the zone. As the user drags, you can update the control value based on the angle. At some point, they'll either touch up, or drag outside of the zone, and that will end the gesture. I suggest allowing the touch to stray pretty far inside r1 or outside of r2: fingers are imprecise tools.
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
