def on_position_change(obj=nil, &block)
      @on_position_change_obj = obj
      @on_position_change = Proc.new { |device, obj_ptr, index, position|
        yield self, @servos[index], position, object_for(obj_ptr)
      }
      Klass.set_OnPositionChange_Handler(@handle, @on_position_change, pointer_for(obj))
    end
def on_position_change(obj=nil, &block)
      @on_position_change_obj = obj
      @on_position_change = Proc.new { |device, obj_ptr, index, position|
        yield self, @servos[index], position, object_for(obj_ptr)
      }
      Klass.set_OnPositionChange_Handler(@handle, @on_position_change, pointer_for(obj))
    end
Sets a position change handler callback function. This is called when a the servo position has changed.

 @param [String] obj Object to pass to the callback function. This is optional.
 @param [Proc] Block When the callback is executed, the device and object are yielded to this block.
 @example
     servo.on_position_change do |motor, position, obj|
       puts "Moving servo #{motor.index} to #{position}"
     end
 As this runs in it's own thread, be sure that all errors are properly handled or the thread will halt and not fire any more.
 @return [Boolean] returns true or raises an error