In the past I’ve mentioned a few open source iOS controls for creating rotary knobs, but have never gotten into creating a gesture recognizer enabling this type of user control.
Today I came across a tutorial explaining the math needed to create this type of control, and doing so using a gesture recognizer. This approach is also slightly different in that you can’t slide anywhere in the knob area, but rather click a point in the control and move that point within the knob.
The tutorial is from Melle and can be found on his website here.
An interesting read if you’ve wanted to know how to how to create a rotary knob control or use iOS’s gesture recognizer API.