code: def start_notify(self, on_change):
          
          # Tell the device what callback to use for changes to this characteristic.
          self._device._notify_characteristic(self._characteristic, on_change)
          # Turn on notifications of characteristic changes.
          self._device._peripheral.setNotifyValue_forCharacteristic_(True,
              self._characteristic)
docs: Enable notification of changes for this characteristic on the
          specified on_change callback.  on_change should be a function that takes
          one parameter which is the value (as a string of bytes) of the changed
          characteristic value.