actively rotating an image using a Knob

I am trying to use a knob to rotate an image (in my case it's a polarizer) in kivy. My idea is to make a function that is active while the knob is being pressed, so it reads the angle and changes it into the image's canvas. The problem is that I don't know if there is a knob property that gives me its "status", so I can do something like:

While Knob is active: