code: private void updateSliderState(int touchX, int touchY) {
          int distanceX = touchX - mCircleCenterX;
          int distanceY = mCircleCenterY - touchY;
          //noinspection SuspiciousNameCombination
          double c = Math.sqrt(Math.pow(distanceX, 2) + Math.pow(distanceY, 2));
          mAngle = Math.acos(distanceX / c);
          if (distanceY < 0) {
              mAngle = -mAngle;
          }
  
          if (mListener != null) {
              // notify slider moved listener of the new position which should be in [0..1] range
              mListener.onSliderMoved((mAngle - mStartAngle) / (2 * Math.PI));
          }
      }
docs: Invoked when slider starts moving or is currently moving. This method calculates and sets position and angle of the thumb.
  
  @param touchX Where is the touch identifier now on X axis
  @param touchY Where is the touch identifier now on Y axis