docs: def _prepare_transition(self, is_on=None, **kwargs):
          """
          Perform pre-transition tasks and construct the destination state.
  
          :param is_on: The on-off state to transition to.
          :param kwargs: The state to transition to.
          :return: The destination state of the transition.
          """
          # Handle transitions with changing on-off-state
          dest_state = kwargs.copy()
          if is_on is not None:
              if is_on and not self.is_on:
                  if 'brightness' not in kwargs or kwargs['brightness'] is None:
                      dest_state['brightness'] = self.brightness
                  self.set(brightness=0, cancel_transition=False)
              elif not is_on and self.is_on:
                  dest_state['brightness'] = 0
  
          return dest_state