title: Increase/ decrease value with dragging
body: <p>I'd like to have such a feauture in my app: when user drag finger on the screen up to down some parameter will be decreased, when drag down to up, parameter will be increased. I will use is to dynamically change length of drawed element. Could you give me some instrutions how to write such a thing?</p>
  
  <p>EDIT:
  To be more precisely. I think I should use MotionEvent.ACTION_MOVE. But can't figure out how to do something like: depending on move Y length I add or subtract proportional value. Could you help me?</p>